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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 1 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
CONTENTSSection Page
SCOPE 3
REFERENCES 3
BACKGROUND 4
DEFINITIONS 4
SYSTEM TYPES AND APPLICATIONS 6
BASIC DESIGN CONSIDERATIONS 6
RELIABILITY6
FLEXIBILITY 6ENVIRONMENTAL AND SAFETY CONCERNS 7
SYSTEM SELECTION7
COOLING WATER REQUIREMENTS8
LOAD GROWTH AND RESERVE CAPACITY8
COOLING TOWER TYPES8
COOLING TOWER BASIN 9
MATERIALS OF CONSTRUCTION10
WATER TREATMENT FOR ONCE-THROUGH SYSTEMS10
WATER TREATMENT FOR SALT WATER COOLING TOWER SYSTEMS 10
WATER TREATMENT FOR FRESH WATER COOLING TOWER SYSTEMS 10
EXTERNAL TREATMENT 11
INTERNAL TREATMENT11
BLOWDOWN 14
CONTROLS AND MONITORS 14
WASTEWATER REUSE POSSIBILITIES 14
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS 15
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS 19
REVAMP AND EXPANSION PROJECTS 27
COMPUTER PROGRAMS 29
NOMENCLATURE 29
Changes shown by
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Section Page
XXVII 2 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
CONTENTS (Cont)Section Page
TABLESTable 1 Impurities Found in Cooling Water 30Table 2 Allowable Concentration of Impurities in Recirculated Fresh Cooling Water 31Table 3 Method of Chemical Receipt 32Table 4 Water Treatment Analysis Worksheet 33
FIGURESFigure 1 Typical Once-Through Cooling Water System 34Figure 2 Typical Recirculated Cooling Water System 35Figure 3 Cooling Tower Types 36Figure 4 Support Structure for Water Intake Crib 37Figure 5 Water Intake Pipe 38Figure 6 Water Intake Bay 39Figure 7A Sump Dimensions vs Flow Rate Customary Units 40Figure 7B Sump Dimensions vs Flow Rate Metric Units 41Figure 7C Sump Dimensions 42Figure 8 Configurations for Multiple-Pump Pits 43Figure 9A Fan Horsepower Requirements Customary Units 44Figure 9B Fan Horsepower Requirements Metric Units 45Figure 10 Cooling Tower Orientation 46Figure 11 Water Treatment Flow Plan for Recirculating Cooling Water Systems 47Figure 12 Cooling Tower Disengaging Stack 48
Revision Memo
1200 Highlights of this revision are as follows1 Revised contents to reflect design methods and philosophy used on recent projects2 Included learnings from consulting on recent problems experienced3 Corrected references to withdrawn International Practices4 Clarified recommendations regarding cooling water distribution piping5 Included more specific definition regarding recommended cooling tower basin minimum
holdup6 Added a recommended simplified plan for balancing cooling water exchanger water usage in
the Revamp and Expansion Projects section7 Included recent changes in recommended cooling water treatment
EMRE COOLING WATER SYSTEM SPECIALISTS
TECHNOLOGY AREA CONTACT PHONE NO(703) 846-
E-MAIL(AMERICAS)
Water Treatment R G Balmer
C M Havran
3640
3806
RGBALME
CMHAVRASystem Planningand Design
R A LieberwirthG E Stover
7378(281) 834-7599
RALIEB2GESTOVE
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Section Page
COOLING WATER SYSTEMS XXVII 3 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
SCOPE
This section presents the criteria and general procedures for the selection and design of cooling water systems in processplants Major components are described for both once-through and recirculated type cooling systems using either fresh or salt
water This section also includes information on the chemical treatment required to control corrosion scaling and biologicalfouling in cooling water systems Information on automatic equipment used to monitor water condition and to make thenecessary adjustment in chemical treatment is also included
REFERENCES
DESIGN PRACTICES
Section II Design Temperature Design Pressure and Flange RatingSection IX Heat Exchange EquipmentSection X PumpsSection XII InstrumentationSection XIV Fluid Flow
Section XV Safety in Plant DesignENVIRONMENTAL DESIGN PRACTICE
Section XIX-B Water Reuse
OFFSITES DESIGN PRACTICE
Section XXVI-A Boiler Feedwater Treating Systems
INTERNATIONAL PRACTICES
IP 1-1-1 Drawings Diagrams and Line ListsIP 3-10-3 Cement Lined Pipe and FittingsIP 3-10-4 Plastic and Plastic-Lined PipingIP 8-1-1 Cooling TowersIP 10-11-1 Sizing of Drivers and Transmissions for Compressors Fans and PumpsIP 16-1-1 Area Classification and Related Electrical Design for Flammable Liquids Gases or VaporsIP 16-4-1 Grounding and Overvoltage ProtectionIP 19-6-1 Facilities for Corrosion Monitoring in Process Equipment
OTHER LITERATURE
Cooling Tower Water Treatment Guidelines ERampE Report No EE102E78Exxon Cooling System Handbook January 1986Environmentally Acceptable Cooling Water Treatments ERampE Report No EE34E86Karlsruhe Cooling Water System Reliability Study ERampE Report No 85EEEL3268Reuse of Wastewater as Cooling Tower Makeup ERampE Report No EE58E85
Guidelines for the Inspection Preventative Maintenance and Rehabilitation of Cooling Towers ERampE Report No EE47E89Guidelines for Safety Evaluation of Chemical Injection Facilities ERampE Report No EE92E94EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating ServiceEMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased)EMRE Refinery Construction Materials Manual Manual No EETD 028Cooling Tower Institute (CTI) Acceptance Test Codes for Water Cooling Towers (ATC-105)Cooling Tower Institute (CTI) Cooling Tower Performance Curves
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Section Page
XXVII 4 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BACKGROUND
Rejection of heat is a requirement of a great many processes and utility systems Coolers and condensers reject much of thisheat to water The main purpose of a cooling water system is to absorb the heat rejected and dispose of it The system must
be safe reliable and flexible and it must have a minimum impact on the environmentCooling system selection and design is very site specific and cannot be generalized The EMRE COOLING SYSTEMSPECIALISTS are available for consultation on specific requirements
DEFINITIONS
Ambient Wet Bulb Temperature
The wet bulb temperature that exists in the cooling tower area It is the temperature specified in the Design Specification for the design and guarantee basis Note the actual tower inlet wet bulb temperature may be higher than the ambient wet bulbtemperature due to hot air recirculation
Approach
The difference in temperature between cold water leaving the tower and ambient wet bulb temperature
Battery Limits
A boundary which is normally assumed to be at the physical edge of the block in which the facility is located
Blowdown
The continuous or intermittent discharge of a small amount of the circulating water Its purpose is to limit the increase in theconcentration of solids in the water due to evaporation It is expressed in percent of water circulated
Brackish Water
Water which contains 3000 to 5000 wppm of dissolved salts
Cell
The smallest tower subdivision which can function as an independent unit with regard to air and water flow
Cooling Tower Pumping Head
The total pressure at the centerline of the tower inlet to its hot water distribution system plus the difference in elevation betweenthe centerline of the inlet and the top of the cold water basin curb It does not include friction pressure drop in the riser pipeThis is a parameter representing the head required for the cooling tower only It excludes the cooling water distribution system
Counterflow Tower
A cooling tower in which air induced at the bottom of the tower flows up through the fill against the falling water
Crossflow Tower
A cooling tower in which air induced at the sides of the tower flows horizontally across the fill and the falling water
Cycles of Concentration
The ratio of dissolved solids in circulating water to dissolved solids in makeup water
Dissolved Solids
A measure of the total quantity of dissolved salts in water It can be determined by conductivity measurements
Drift
The entrained water carried from the tower by exhaust air expressed in percent of water circulated
Dry Bulb Temperature
The temperature of ambient air read on an ordinary thermometer
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Section Page
COOLING WATER SYSTEMS XXVII 5 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DEFINITIONS (Cont)
Fill
Packing within the cooling tower to provide the required surface area for heat transfer between ambient air and the returncooling water Splash fill breaks the water into small drops by causing it to cascade through successive offset levels of parallel level splash bars Cellular or film fill causes the water to spread into thin films over large vertical areas
Heat Load
Heat removed from the circulation water within the tower It may be calculated from the range and the circulating water flowrate
Interference
The mixing of tower inlet air with the discharge vapors from another tower or other heat source This results in reduced thermalperformance
Makeup
The water required to replace circulating water which is lost by evaporation drift blowdown and leakage It is expressed inpercent of water circulated
Offsites
Support facilities such as utilities (steam power cooling water) tankage waste treating etc for the processing operations
Once-Through Cooling System
A system in which water passes through the heat exchange equipment once and is then discharged
Onsites
Facilities that are a part of the processing operations
Range or Water Cooling Range
The difference in temperature between the hot and cold circulating water or on-tower temperature minus off-tower temperature
Recirculated Cooling System
A system in which water is circulated to coolers in a loop and the heat absorbed from this cooling operation is dissipated by acooling tower or some other type of heat exchanger
Recirculation
An undesirable condition in which part of the tower discharge vapor stream is recirculated through the tower resulting inincreased inlet air wet bulb temperature
Relative Humidity
The ratio of the quantity of water vapor present in air to the quantity which would saturate the air at the existing temperature
Salt Water
Water which contains more than 5000 wppm of dissolved salts
Wet Bulb Temperature
The equilibrium temperature obtained when ambient air is passed over a continuously wetted thermometer bulb evaporatingthe water and cooling the bulb This equilibrium temperature occurs when the heat transferred from the air to the wettedsurface equals the heat loss due to latent heat of evaporation
Wind Rose
A diagram that shows the average frequency and intensity of wind from different directions for a particular location
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Section Page
XXVII 6 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
SYSTEM TYPES AND APPLICATIONS
Cooling water systems in refineries and chemical plants are generally either the once-through or the recirculated type designedfor fresh salt or brackish water
In the once-through system (see Figure 1 ) pumps take suction from a source of supply such as a river or other body of water and deliver the water to process coolers and other consumers After passing through the cooling exchangers the hot coolingwater is discharged through pressurized piping or a gravity flow systemIn a recirculated system (see Figure 2 ) pumps take suction from a cooling tower basin and deliver the water to process coolersand other consumers After passing through the cooling exchangers the hot cooling water is discharged through a pressurizedreturn system to a cooling towerThe choice between a once-through and recirculated cooling water system must be based on (1) availability of water of satisfactory quality (2) process temperatures (3) atmospheric conditions (4) investment and operating costs and (5) effluentwater quality and temperature limitationsThe cooling water is distributed to individual consumers throughout the plant in piping main supply and return headers andseries and parallel branches consisting of laterals and sublaterals The main cooling water distribution headers are either buried in underground trenches or are located above grade on ground-level pipe sleepers or on elevated pipe racksUnderground piping has the advantage of minimizing problems associated with thrust loads thermal expansion and freezingElevated piping is easier and less expensive to install and repair and it makes it easier to locate leaks
BASIC DESIGN CONSIDERATIONS
RELIABILITY
The cooling water system is an important utility and its reliability is critical to successful plant operation The typical systemprovides cooling water to both onsite and offsite consumers It can have a direct impact on the sizing criteria for the emergencyrelief (flare) system (refer to Section XV-C Safety in Plant Design Pressure Relief ) The system therefore normally must becapable of continuous operation so that cooling water is always available to critical consumersThe reliability required to achieve continuous operation is provided in several ways
bull Spare makeup and water circulating pump(s) are provided to assure continuous operation
bull Different types of drivers are specified for the supply or water circulating pumps Motor-driven pumps are supplied fromtwo different buses on a secondary selective electrical distribution system
bull Cooling tower fan drivers are normally supplied from a secondary selective systembull Holdup volume is provided in the cooling tower basin to permit an orderly shutdown of process units if the makeup water
supply is lost or a failure occurs in any part of the recirculated cooling water system
bull Valves blinds and bypasses are provided to permit individual components to be removed from the system for maintenancewhile the system is operating
bull Construction materials provided for cooling water facilities are designed for long-term corrosion resistance sincemaintenance work is often difficult (if not impossible) to schedule without a complete refinery shutdown
FLEXIBILITY
The cooling water system must include sufficient flexibility to cover all present operating requirements plus any future operatingrequirements that are defined in the design basis document For example future operating requirements could be dictated by anear-term increase in plant sizeThe type of flexibility features provided in a system may include the following
bull Makeup water treating facilities should be able to deliver the required water assuming the worst conditions of the water source
bull Waste treating facilities should be designed to handle effluent water containing varying concentrations of oil and chemicalsover the range of expected ambient temperatures
bull Valves and blind flanges should be provided to permit future tie-ins to the system without a shutdown An exception wouldbe on lines where a ldquohot taprdquo could be made with minimum risk of an emergency shutdown or where the cost of requiredvalves is prohibitive
bull Cooling tower cells should be completely partitioned as described in IP 8-1-1 to allow personnel entry to any cell for maintenance without affecting the operation or capacity of the other cells Normally spare cells are not providedMaintenance is carried out during cooler periods when the cooler ambient and wet bulb temperatures enable the coolingtower to handle greater heat loads
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Section Page
COOLING WATER SYSTEMS XXVII 7 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
bull Where multiple cooling tower systems exist consideration should be given to providing valved cross connections betweentheir respective supply mains and between their respective return mains These would be used only for special operations
such as turnarounds or emergencies The impact on flare sizing should be evaluated if these cross connections areprovided
ENVIRONMENTAL AND SAFETY CONCERNS
Once-through cooling water systems usually require controls to minimize the release of hydrocarbons and free halide residualsto the environment As a minimum cooling water discharges should be monitored for free oil total organic carbon or hydrocarbon vapors to determine if system coolers are leaking Free chlorine residuals in the discharge should also bemonitored as required to meet local regulations and to detect process leaks In most cases once-through cooling water flowsare too large to be handled in the plants wastewater treatment system However once-through water that could becomecontaminated with hydrocarbons is often segregated and treated in a large skim pond or special purpose separator prior todischarge Local environmental regulations and permit requirements will dictate the extent of treatment requiredCooling tower blowdown from recirculated systems is normally bypassed around major wastewater treatment and dischargedwith the treated wastewater An oil-in-water detector should be provided to detect oil in the blowdown from process leaks Thiswill enable stopping the blowdown or diverting it to temporary holdup until the leak is corrected A major rupture in a processheat exchanger tube can also result in the cooling tower being the discharge point for hydrocarbon vaporsBecause of the likely presence of flammable vapors the areas around cooling towers are subject to electrical classification inaccordance with IP 16-1-1 The type and amount of chemicals added to the cooling water should be evaluated as to their effects on the plants wastewater treatment process Biocides corrosion inhibitors and antifoulants may result in significant problems for the wastewater treatment plantOther environmental concerns are
bull Cooling tower noise emissions from fans and from the flow of cooling water over the tower may require suppression if near a local community
bull Cooling tower water mist drift can cause fog and ice formation on nearby roads and promote deterioration of nearbyequipment It may also result in damage to sensitive vegetation in the surrounding area
bull Process gas releases to atmosphere resulting from process gas pickup by cooling water including the venting of toxicgases from exchanger tube ruptures must be considered in locating cooling towers or discharge points from once-throughsystems
bull Spills and overflows of toxic and hazardous chemicals used in treating the cooling water must be contained
SYSTEM SELECTION
The selection of a cooling water system is often determined by the availability of water In regions where water is scarce once-through systems are obviously eliminated from consideration and recirculated systems are chosen A once-through coolingsystem may be the economical choice if the plant is located adjacent to a body of water and consumers are elevated no morethan 50 ft (15 m) above the water level However once-through system costs may be increased significantly by environmentallimitations in many locations These may limit maximum discharge water temperature and therefore will reduce the maximumcooling water ∆T This will increase the required flow and cost of once-through systems In addition regulations may requirewaste treatment of the large volume of effluent thus increasing costsThe choice of a cooling water system must be based on comparisons of initial investment and operating costs Relative initialcosts of a system vary with climatic conditions distance from pump to users complexity of inlet structures and corrosive nature
of the water Salt water or brackish water service requires the use of cement mortar lined pipe and alloys for exchangers andpumps Carbon steel is usually satisfactory for fresh water Special attention should be given to the impact of bottom silt mudand seaweed on intake facilities to once-through or makeup to recirculated systems Cribs flumes large bays screens trashracks or other extensive facilities may be requiredDuring the early stages of a project the designer should review with the planning or design groups the heat level and volume of all process streams which require cooling The optimum method of cooling should then be determined ie heat exchangeandor rejection of heat to air or water A break point should be selected above which air cooling is economical and belowwhich trim water cooling is preferred
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XXVII 8 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
COOLING WATER REQUIREMENTS
Cooling water requirements for each process unit and offsite consumer are obtained from the appropriate design groups For existing locations maximum cooling water loads should be based on the design inlet cooling water temperatures given inSection IX-B Design Considerations for All Types of Heat Exchangers For new locations the inlet cooling water temperaturemust be set based on local water temperature data for once-through systems and wet bulb temperature and selected approachfor recirculated systemsThe bulk water temperature at the cooler outlet has been selected as the controllable variable for design and operation of coolers Although the corrosion rate depends on the metal temperature this is difficult to determine in the operation of a coolerBased upon the findings of a Materials Development Section test program maximum design outlet water temperatures havebeen set as indicated below
Fresh Water 130 degF (54 degC)
Treated Fresh Water 130 degF (54 degC)
Brackish Water 125degF (52
degC)
Salt Water 120 degF (49 degC)
When the maximum plant cooling water requirement has been established for new process and utility consumers appropriateLoad Growth and Reserve Capacity Factors (LGF and RCF) should be applied depending on the development stage of theproject Cooling water loads should be updated and system capacity adjusted periodically as design work progresses
LOAD GROWTH AND RESERVE CAPACITY
Utility loads tend to increase as the loads become better defined through successive stages of project definition Thereforeload growth and reserve capacity factors (LGF and RCF) are applied to raw loads as a means of predicting final equipmentsizes and capital costs These load growth factors do not cover basis changes through project development such as project or process unit sizethroughput For revamp or modernization projects these factors are generally applied only to new loads or changes to existing loads on the basis that existing or unchanged loads are well known
Recommended Load Growth Factors are as follows
CONSUMERSPLANNING
(SCREENING THROUGH DBM) DESIGN SPEC
Existing Facilities 0 - 10 0 - 10
New Facilities 20 - 30 5 - 10
Selection of the load growth factor to use should be based on the quality and level of effort associated with development of theraw loadsFinal sizing of the source facility (eg the cooling tower and circulation pumps) should include a reserve capacity factor of 10to cover small post startup changes minor future projects and dynamic conditions not predicted by steady state balances Thesizing basis for distribution facilities should not include the RCF
COOLING TOWER TYPES
The cooling tower is the major piece of equipment specified in recirculating cooling water systems In effect it is the heatexchanger which cools the hot water to a suitable temperature for re-use primarily by evaporation through contact with airThe cooling tower contains fill to break up the water flow into droplets or film on a surface The fill increases the waterair contact area and enhances heat transferCooling towers are classified according to the means by which air is supplied to the tower ie induced or forced mechanicaldraft (fan) vs natural draft and according to the relative movement of the air and water that is counterflow or crossflow (seeFigure 3 ) Most of the towers in the ExxonMobil circuit are mechanical induced-draft type Economics determine whether theyare counterflow or crossflow
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COOLING WATER SYSTEMS XXVII 9 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
In crossflow towers the air travels horizontally across the falling water while in the counterflow design the air travels verticallyup through the falling water Comparisons of plot space fan horsepower and future maintenance costs determine which
design will be more economical Factors which influence this selection are cooling water rate range and approach for aparticular location In addition in some locations materials of construction and associated labor costs will affect the finalselectionIn the past crossflow towers were usually preferred over counterflow towers for extremely cold locations where ice formationcould restrict or block air flow through the louvers However this consideration is usually no longer valid due to the increaseduse of PVC film-type tower fill rather than splash-type fill in counterflow towersMechanical draft designs use one or more fans to provide the air needed for cooling Induced-draft units have the fans locatedon top of the tower Air is pulled through the packing and discharged vertically upward at high velocities Forced-draft unitshave the fans located at the base of the tower and push the air through the fill Forced-draft towers in large industrial sizesexperience difficulties with recirculation and in providing a uniform distribution of air As a result they have been supersededby induced-draft designsFill may be splash or film type Splash fill normally consists of horizontal slats in horizontal rows offset to one another to causethe water to break up into droplets as it falls downward through the cooling tower Splash fill is characterized by reduced air pressure losses It is also less conducive to clogging and easier to clean after a spill However it is very sensitive to
inadequate support and must remain horizontal and level It is made of treated wood or plastics such as PVC or polypropyleneFilm fill comes in various designs but they all cause the water to flow in films over the fill surface and provides more coolingcapacity within a given amount of space than splash fill Because of this increased heat transfer efficiency film fill is generallyused in most new cooling towers However because of its smaller passages it is more sensitive to plugging and is difficult toclean if it does become plugged It may not be appropriate for services cooling mainly heavy fuels or waxy lubes where a leakwould cause fill plugging or for services with a high biological fouling potential PVC is currently the most widely used materialfor film fill The selection of fill type is usually proposed by the vendor and reviewed by the COOLING WATER SYSTEMSSPECIALISTSNatural-draft or hyperbolic cooling towers depend on the natural draft created by the difference in the density of the enteringand leaving air for movement Generally hyperbolic cooling towers have been used in Europe and in the utility industry in theUSA for large capacities [250000 gpm (16000 Ls)] short ranges [10 to 15 degF (6 to 8 degC)] and long approaches [10 to 15 degF (6to 8 degC)] They are most effective where low ambient wet bulb temperatures occur with high relative humidities These towersnormally have diameters in excess of 200 ft (60 m) and are greater than 250 ft (75 m) in height Their major advantages are
bull No operating or maintenance cost for fans
bull Reduction in problems associated with fogging drift and recirculation due to their great height and diameterThe primary reasons why hyperbolic towers are not used in ExxonMobil projects include
bull High initial cost
bull Large plot space requirement and high visual impact
bull Complete dependence on atmospheric conditions
bull Lack of flexibility for future expansions
COOLING TOWER BASIN
The primary function of the cooling tower basin is to collect the cooled water leaving the tower and to provide a reservoir for thecooling water pumps Design aspects to be considered are the basins capacity cleanability draining facilities sump andscreen details To obtain the necessary time for corrective action during emergency conditions a minimum storage capacity of 90 minutes based on loss of makeup water flow should be provided in the basin between the basin low level alarm (LLA) andthe minimum pump submergence (MPS) When the basin water level reaches the MPS point loss of recirculation pumpsuction and cooling water flow is imminent Typical values for basin water levels are as follows
bull High Level Alarm (HLA) 1 ft (300 mm) below top of the basin wall
bull Normal Water Level 15 ft (450 mm) below top of the basin wall
bull Low Level Alarm (LLA) 25 ft (760 mm) below top of the basin wall
bull Distance between LLA and MPS 35 ft (1070 mm)
bull Minimum Pump Submergence Depth 45 ft (1370 mm)
bull Clearance of Pump Suction off Basin Floor 15 ft (450 mm) Actual values may vary depending on pump type selected pump suction line diameter and cooling tower cell plot area
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XXVII 10 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
MATERIALS OF CONSTRUCTION
The EMRE Refinery Construction Materials Manual and IP 8-1-1 list acceptable materials for all components in each type of cooling water system (once-through and recirculating) For salt water systems emphasis must be placed on proper materialsselection throughout the system The external salt water environments in the vicinity of the tower itself and the intake stationsmust be taken into consideration Proper siting of the tower is extremely important to avoid salt water corrosion of adjacentunits or the need to use expensive materials to operate in such an environment
WATER TREATMENT FOR ONCE-THROUGH SYSTEMS
Once-through salt and fresh cooling water systems can develop a variety of problems such as fouling due to seaweed or other debris marine microorganisms such as muscles and shell fish and biological growth Corrosion and scaling problems areminimal in fresh water once-through systems because the water is not concentrated by evaporation In salt water systemscorrosion is minimized by proper materials selection and scaling problems generally do not occurProtection against fouling by seaweed or other debris is provided by careful selection of the water intake location and travelingscreens with washing facilities In some cases additional steps may be required These may include an additional secondaryset of traveling screens and possibly battery limit strainers on the cooling water supply lines to individual unitsProtection against fouling by marine microorganisms and biological growths is achieved primarily by injecting chlorinehypochlorite chlorinebromine or other similar chemicals which kill these growths Chlorine is commonly used and is injectedin sufficient quantities to produce a residual of 1 wppm in the effluent for one hour each day (shock chlorination) Continuouschlorination for 2 - 3 days per month at lower levels (03 wppm residual chlorine in the effluent) may also be used in addition toshock chlorinationThe large water volume discharged from once-through systems usually makes chemical treating for corrosion or deposit controltoo costly These problems are handled by proper materials selection and by keeping water velocities through exchanger tubesgreater than 3 fts (09 ms) Occasionally the water analysis of a fresh water source being considered for a once-throughsystem using carbon steel materials will indicate the need for some corrosion or deposit control In this case the cost of thethreshold treatment with the once-through system should be compared to the costs of a recirculated system before making afinal choice
WATER TREATMENT FOR SALT WATER COOLING TOWER SYSTEMS
A total dissolved salts (TDS) concentration limit of 55000 wppm for sea water in cooling tower systems requires high blowdownrates since the makeup sea water typically has from 35000 to 40000 wppm TDS Because of cost restraints the only water treatment normally specified for these systems is for biological control Chlorine is most commonly used either as directchlorine injection or hypochlorite produced in electrochlorinators Care must be taken in selecting the intake location to assurethat the suspended solids do not exceed 200 wppm in the recirculated water If it is not possible to find sea water with sufficientclarity clarification with polyelectrolytes should be evaluated Corrosion is one of the major problems with the use of sea water in cooling towers This requires proper materials selection
WATER TREATMENT FOR FRESH WATER COOLING TOWER SYSTEMS
Cooling tower water treatment is necessary to minimize or eliminate corrosion scale and biological fouling of process heattransfer surfaces caused by minerals and impurities in the water The difficulties caused by these impurities and the means of treatment are shown in Table 1 Water lost by evaporation results in an increase in total dissolved salts in the recirculated water To indicate the degree of concentration of impurities the term cycles of concentration is used
Corrosion is an electrochemical process that deteriorates metals exposed to water in the presence of corrosive agents such asacids oxygen or bacteria A common form of corrosion is pitting Severe corrosion can lead to equipment failure Corrosionis caused by many conditions including process leaks into the cooling water water flow velocity that is too low (causes depositsand fouling lending to corrosion) or too high (causes erosion and corrosion) low pH and high temperatures Corrosion is lesslikely in water with non-acidic pH values (greater than 7) although scaling is more likely in these pH rangesScaling is characterized by the formation of hard dense deposits on material surfaces These deposits impact heat transfer and can become a site for localized under-deposit corrosion Scaling is influenced by many factors including makeup water composition total dissolved solids in the recirculating water water temperature and pH and flow velocity Calcium carbonateis the main scaling constituent in all waters and it is the least soluble It forms from the hardness (calcium Ca ++ ) andbicarbonate (HCO 3
ndash ) alkalinity present in most waterFouling can be caused either by the settlement of suspended matter or by microbiological growth of algae bacterial slime or fungi Fouling will reduce heat transfer cause plugging and create corrosion sites
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COOLING WATER SYSTEMS XXVII 11 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Treatment of the cooling water is broken down into two major categories external and internal External treatmentencompasses any treatment given to the cooling tower makeup water Internal treatment is the addition of chemicals directly to
the recirculated cooling water
EXTERNAL TREATMENT
External treatment to remove impurities in order to reduce the amount of blowdown should always be evaluated as to costeffectiveness The most frequently used techniques for treating makeup water are shown in Figure 11 Details of availabletreatment processes (see Table 1 for methods available) are covered in Section XXVI-A Boiler Feedwater Treatment For cooling tower systems that meet currently required drift limits in refineries and chemical plants uncontrolled cooling water losses (drift and miscellaneous process uses) normally amount to about 14 of the cooling water evaporation loss Under these conditions the maximum cycles of concentration achievable are 8 with no controlled blowdown Some cooling towersare operating at up to 10 cycles of concentration Additional controlled blowdown required to limit the cycles of concentrationper the water quality criteria is covered later
INTERNAL TREATMENT
The makeup water analyses amount of cooling water losses environmental restraints potential chemical hazards materials of construction and film temperatures of heat transfer surfaces determine the type of internal treatment required for cooling tower systems The internal water treatment program selected must address corrosion scale fouling biological growth andchemical attack of cooling tower componentsNumerous chemicals and treatment programs are continuously being developed for addressing the potential problems Theslightly acidic low pH programs of the past which used chromate based chemicals and usually required acid injection arebeing completely phased out because of environmental restrictions on chromate and the higher costs associated with meetingcurrent practices for handling hazardous chemicals such as acid Non-chromate treatments which use inhibitors such asphosphates zinc and organics along with higher pH have been developed to replace the earlier programs Although not aseffective as chromate these inhibitors operate at higher water pH which is a less corrosive environment The disadvantage of the higher pH is the greater tendency to form scale This tendency is controlled by the use of dispersants or scale inhibitorswhich are normally organic polymers or phosphenate A corrosion inhibitor for copper or brass is also normally required if these metals are present in the systemThe following paragraphs describe techniques and chemicals to control alkalinity (pH) corrosion scale sludge and biologicalgrowth The chemicals mentioned are normally purchased as pre-mixes from vendors and dosed in combination Mostcorrosion and scale control program chemicals are two drum formulations due to the incompatibility of some chemicals in theconcentrated state Therefore separate feed systems are generally provided for chemicals for control of corrosion and scale (2drums) and biological growth (if liquid) If a program includes fixed pH then an additional feed system is required to add acidWater treatment programs are site specific and changing The COOLING WATER SYSTEMS SPECIALISTS should beconsulted for recommendations on the internal water treatment programs when an analysis of the available water is known
Alkalinity (pH) Control
Cooling water treatment programs presently available fall into two basic categories those that do not control pH (floating pH or alkaline treatment programs without pH adjustment) and those that control pH within a specified range by adding acid Theprograms that do not control pH are gaining favor because of the additional cost equipment and precautions required tominimize the risk associated with handling acid and because of improvements in chemicals to control depositsHigh pH in the recirculating cooling water is a result of the alkalinity of the fresh water due to bicarbonate salts This combinedwith carbon dioxide results in the water having a pH range of 6 to 85 When this fresh makeup water is mixed with therecirculated cooling water and passed through the cooling tower the air flow strips out the carbon dioxide Bicarbonate ionsthen convert to carbonate ions raising the pH which can cause both calcium carbonate and calcium phosphate salts toprecipitate out of solution and form deposits Another debit in allowing the cooling water pH to increase is increased chlorineconsumption to control biological growthsIf a program using pH control is justified and chosen acid addition is used under control of a pH analyzer to controlrecirculated cooling water alkalinity and pH to the range required by the treatment program
Corrosion Control
To reduce corrosion to an acceptable level chemical corrosion inhibitors which form protective films on heat transfer surfacesare the most effective protection Inhibiting corrosion is accomplished by phosphates organics zinc nitrites and molybdatesalts Unfortunately the use of chromate which is a reliable corrosion inhibitor is prohibited by environmental constraintsNitrite is not practical in an open system due to atmospheric oxygen converting the nitrite to nitrate
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ExxonMobil Proprietary
Section Page
XXVII 12 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Phosphate inhibitors (orthophosphate polyphosphate phosphonate) are the basic treatment for new towers in todaysenvironment In addition to the phosphate chemistry salts of zinc may be added to aid in inhibiting corrosion These ions
combine with the hydroxide ion forming thin protective films Zinc is used to provide a synergistic effect in combination with thephosphate salts for protecting carbon steel surfaces from corrosion Finally all-organic programs are offered by severalmanufacturers The use of these treatments has been made possible by the development of polymer dispersants which givecalcium salts a much higher solubility and reduce the tendency to scaleWhen a cooling water system contains copper alloys azole based and azole substitutes are required as copper corrosioninhibitors There are several effective inhibitors The most common is tolytriazole (TTA) due to its resistance to chlorineThe corrosion inhibitors most frequently used in ExxonMobil recirculating cooling water systems are shown below indescending order of preference provided their use meets all environmental requirements
bull Stabilized Phosphate (OrthoPolyphosphate) - Environmental restrictions on the use of zinc have resulted in ExxonMobilplants in the USA using a blend of orthophosphate and polyphosphate type corrosion inhibitors with good results Thistreatment forms a thin protective calcium phosphate film on heat transfer surfaces through the use of orthophosphateconcentrations as high as 12 - 17 wppm The use of calcium phosphate dispersants and Hydroxethylidene Diphosphonate(HEDP) stabilizes the treatment and prevents the formation of calcium phosphate scale Concentration of the dispersantsdepends upon the calcium concentration temperature and pH Stabilized Phosphate treatment programs can operate in apH range of 68 to 78 Alternatively an alkaline (floating pH) stabilized phosphate treatment program can be used Thisprogram uses little or no acid addition and is gaining in popularity although not well established yet It operates with pHvalues of 8 - 9 and uses higher quantities of dispersants and lower levels of phosphate A corrosion inhibitor for copper and brass is normally included for all stabilized phosphate programs Several vendors offer these programs Twoseparate chemical feeders are generally required
bull Zinc Alkaline Phosphate - Heritage Exxon plants in Europe where the use of zinc is not restricted have obtained goodcontrol of corrosion and scaling using zinc-polyphosphate treatment programs Low (65 - 72) and high (73 - 80) pHprograms have been used The high pH (zinc alkaline phosphate) programs are the lowest cost and are recommended
As with the Stabilized Phosphate treatment dispersants such as HEDP and polyacrylates are required to prevent theformation of zinc or calcium phosphate scale on heat transfer surfaces A corrosion inhibitor for copper or brass isnormally used with this program which is offered by several vendors
bull Organic Salts - All-organic (polymer) programs are offered by several manufacturers and have been used successfullywithin h Exxon In general these treatments are based on a combination of a phosphonate to control corrosion polymers(dispersants) to prevent scale deposition and a corrosion inhibitor for copper or brass This type of program is mosteffective in conditions of high hardness high alkalinity (M alkalinity gt300 wppm as CaCO 3) and high pH values (gt 85)However some manufacturers have reported success in softer water with alternative polymer component formulations Ingeneral an inorganic corrosion control program is preferred to the all-organic programs which are sensitive to alkalinityand more costly
Detailed control limits for effective use of each of the above inhibitors and materials selection are covered in Report NoEE34E86 Environmentally Acceptable Cooling Water Treatments Report No EE102E78 Cooling Tower Water Treatment Guidelines and in the EMRE Refinery Construction Materials Manual
Scale and Sludge Control
Scale and sludge deposits in cooling water systems cause loss of heat transfer and provide the conditions for pitting corrosionScale consists of inorganic salts which have precipitated from the cooling water forming a coating on heat transfer surfacesSludge consists of organic and inorganic suspended matter that settles in quiescent areas and in heat exchangers The key toprevention of both scale and sludge formation in a cooling system is keeping water velocities greater than 3 fts (09 ms) andthe use of chemical additives in combination with a blowdown rate that keeps the impurity concentration below the level whichcauses deposits (See Table 2 for limits on impurities) Scale control has become increasingly important in higher alkalinityldquofloating pHrdquo water treatment programsSalts most commonly found in scale deposits are calcium carbonate calcium sulfate tricalcium phosphate zinc phosphatemagnesium silicate and silica The control method for preventing scale from each of these salts is discussed below
bull Calcium Carbonate - Where justified using pHalkalinity control in the range of 70 to 75 will normally inhibit the formationof calcium carbonate scale if anti-scalent chemicals (phosphonates acrylates copolymers) are part of the chemicaltreatment (See Report No EE102E78 Cooling Tower Water Treatment Guidelines ) Higher pH water treatmentprograms require greater concentrations of these chemicals along with dispersants and flocculants to inhibit the formationof scale and precipitation of other salts
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 13 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
bull Calcium Sulfate - When heat transfer skin temperatures are below 250 degF (120 degC) and the cooling water bulktemperatures are under 130 degF (54 degC) gypsum scale (CaSO 42H 2O) will form if the product of the calcium and sulfate ionsconcentrations each expressed as wppm CaCO 3 equivalent exceeds 500000 Gypsum scale is prevented bymaintaining bulk water temperature below the indicated limits and controlling calcium and sulfate ions through blowdownor through the use of newer dispersants on the market
bull Tri-Calcium and Zinc Phosphate - Almost all chemical additives for corrosion control contain some type of phosphatesalt These have a tendency to form the orthophosphate ion depending on time pH and temperature Cooling watersusing these corrosion inhibitors may have an orthophosphate concentration in excess of 5 wppm as PO 4
= To preventcalcium phosphate scale Table 2 shows a limit of 1000 wppm of calcium as CaCO 3 equivalent when using a phosphateinhibitor in combination with a chemical dispersant (See Report No EE102E78 Cooling Tower Water Treatment Guidelines for solubility of calcium phosphate) These inhibitors and dispersants generally fall into one of the followingcategories
ndash Sequestrants antiprecipitants and inhibitors such as hydroyethylidene diphosphonate (HEDP) phosphate esters andaminomethylene phosphonate (AMP) function by defyingdistorting the crystal structure of the scale formations toprevent nucleation and growth of the scale
ndash Dispersants based on either anionic or nonionic polymers such as polyacrylamide polyacrylate or polymaleic acidfunction to stabilize colloidal and scale-forming particles in suspension Anionic polymers prevent agglomeration of suspended particles by either charge repulsion or crystal distortion while nonionic polymers reduce the surfacetension of particles
bull Silica Deposits - Quantitative indices for determining limits for silica in combination with other ions are not available due tothe formation of complex polymers Instead the rule of thumb limit of keeping reactive silica concentrations in the coolingwater below 150 wppm as SiO 2 has been successful
Possible alternatives for controlling sludge are
bull Chemical Dispersants - Water treating chemical suppliers now offer many types of dispersants These are mostlypolyelectrolytes Claims for removal of slime silt and mud from cooling tower basins and exchangers are madeHowever satisfactory results using these chemicals as a coagulant aid require very accurate control of the amount fed tothe water and stable operation of the water system Also suspended solids should be kept below 200 wppm for particleslarger than 045 microns Dispersants are used in virtually all fresh water cooling tower systems despite their high cost
bull Sidestream Filtration - The use of a sand pressure filter is the most reliable method for sludge removal Operating andmaintenance costs are minimal for this equipment
bull Higher Blowdown Rates - This may be an economical alternative depending on the cost of makeup water Additionalchemical costs must be included in any evaluation of this method
bull Makeup Clarification - This alternative may require additional raw water treatment facilities to handle seasonal variationsin turbidity andor suspended solids This method is applicable only where suspended solids come from makeup water
Biological Control
Operating conditions in recirculated cooling water are ideal for the growth of biological matter Water temperatures from 70 to120 degF (20 to 50 degC) favorable water pH continuous supply of nutrients including carbon nitrogen and phosphorus inorganicsalts and sunlight all lend themselves to environmental conditions encouraging microbiological growth and plant lifeIf biological growths get out of control and form large sticky agglomerations some of the following operating problems willresult
ndash Fouling of heat transfer surfaces by bacterial slimes resulting in flow restrictions and high process temperatures ndash Reduced cooling tower efficiency resulting from algae fungi and bacterial slime growths in the water distribution basin
and fill area of the cooling tower ndash Excessive plugging of screens and filters ndash Corrosion ndash Wood destruction by fungi ndash Clogging of water distribution nozzles ndash Excessive sludge accumulation in the cooling tower basin
An important part of the water treatment program is to control biological activity by feeding a biocide chemical Oxidizingbiocides such as chlorine sodium hypochlorite chlorine dioxide or bromine-associated compounds are most commonly usedwith good results Plate counts should be maintained below 50000 countsml (bacterial)
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ExxonMobil Proprietary
Section Page
XXVII 14 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Chlorine is the most effective biocide for cooling water because of its low cost extensive period of successful use ease of application and minimum environmental effects if applied under controlled conditions Either a liquid feed (sodium
hypochlorite) or a gas feed (100 chlorine) is added to the cooling tower basin A sufficient dosage is that which obtains a 01to 03 wppm continuous free residual in the hot return water to the cooling tower
Advantages to continuous chlorination over shock treatment based on ExxonMobil experience are ndash Minimizes chlorine degradation of polymers ndash Better reduction in total aerobic bacteria plate count resulting in cleaner heat transfer surfaces ndash Does not cause significant pH depressions ndash Better control results with less chlorine consumption
Details concerning chlorine addition as well as the other alternative biocides are given in Report No EE102E78 Cooling Tower Water Treatment Guidelines and in EMRE Engineering Water and Wastewater Design Guide TMEE 080 DG 11-3-1Gas Chlorinators for Water Treating Service and DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) Gaseous chlorine systems have become more costly in recent years because of the additional precautions equipment andprocedures required to minimize the risk associated with handling this gas Alternatives to gaseous chlorine are available butrequire more attention and have a higher operating cost Sodium hypochlorite (either purchased or generated) and sodiumbromide stabilized solid halide (chlorinebromine) products chlorine dioxide and ozone are all alternatives which may haveapplications Contact the COOLING WATER SYSTEMS SPECIALISTS for guidance in the use of these alternative treatmentschemes
BLOWDOWN
Blowdown control is required to maintain the proper cycles of concentration in the system The most reliable and simplest typeof control for blowdown is specific conductance measurement of the cooling water Electrical signals from the specificconductance measurements of some impurity in the cooling water are used to actuate an automatic blowdown valveImpurities normally used for blowdown control are dissolved solids chloride calcium magnesium and silica
CONTROLS AND MONITORS
Any chemical treatment used to control problems in a cooling water system must be carefully monitored to be completelyeffective
The installation of automatic controls should be considered for recirculated cooling water systems in all plants Designs shouldinclude pH conductivity corrosivity inhibitor and biocide control Automatic controllers are available which continuouslymonitor the condition of the water and automatically make chemical additions to keep the water within compositional limitsTest exchangers and corrosion coupons are also useful in monitoring the effectiveness of the water treatment program
An oil-in-water monitor should be considered to provide early detection of oil leaks into the cooling water This will enablecorrective action to be taken before extensive contamination of the cooling water circuit occurs This is especially important incooling towers with high efficiency cellular fill which can be difficult to clean if significantly fouledManual control systems may be used in locations that have adequate technical personnel to perform routine water analysesand to modify chemical treatment necessitated by sudden variations in water composition process contamination etc
WASTEWATER REUSE POSSIBILITIES
Recirculating cooling water systems normally have the highest plant raw water demand A potential alternative is to use treatedwastewater to supply part or all of the makeup water requirement The treatment required must be determined but will typicallyinclude primary oil removal (eg API Separator) secondary oil and suspended solids removal (eg filtration dissolved or induced air flotation) and soluble carbon removal utilizing biological treatmentThe reuse of treated wastewater as makeup currently exists at only a few locations If this alternative is being considered refer to Report No EE58E85 Reuse of Wastewater as Cooling Tower Makeup and Section XIX-B Water Reuse
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 15 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS
STEP 1 - Select a location for the intake station and pumps
Factors which influence this selection are as followsbull Depth of water and tidal variations
bull Relative location of intake and outfall direction of current
bull Length and size of intake suction piping vs pump discharge piping
STEP 2 - Determine cooling water requirements
Determine the design cold water inlet temperature based on site data covering seasonal water temperature as a function of inlet location and depth (for existing locations refer to Table 2 in Section IX-B )Tabulate the cooling water requirements for each process unit and offsite consumer Apply the proper LGFs to all coolingloads depending on the stage of the project and whether the new units are exact duplicates of existing unitsIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The designer should investigate methods to increase effluent water temperature to the maximumrecommended limit for the particular type of water used One common method is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 3 - Determine the total required capacity for water intake facilities
Process Coolers - The total requirements determined in Step 2 including LGFsFirewater - This is often supplied from the intake bay of a once-through systemFuture Expansions - These should be considered in sizing intake facilitiesTotal Required Capacity - The sum of the above three items
STEP 4 - Provide water intake facilities
Intake Crib - The intake crib with its support structure is located at the extreme end of the water intake line (See Figure 4 )The intake is usually located 200 to 300 ft (60 to 90 m) offshore and at a sufficient depth below the lowest water level to
preclude vortex formation and subsequent pump cavitation during storm conditions Additionally the crib should have 3 to 5 ft(1 to 15 m) clearance above the sea floor to minimize sand and silt pickup A concrete velocity cap should be specified to cover the inlet crib This will minimize the incidence of fish and other foreignmatter entering the system A horizontal inlet flow pattern should be designed with a maximum entrance velocity of 1 fts(03 ms) Experience shows that vertical velocity flow confuses fish and they are drawn into the intake lineThe intake crib support structure protects the crib from adverse effects of storms and tidal motions as well as protecting it fromimpact of floating and submerged objects A self-cleaning screen can be specified to prevent entrance of large objects into thesystem at the cribIntake Line (Figure 5 ) - An intake line is required if the water depth along the shore does not permit water to flow directly intothe intake bay The intake line is usually buried in the sea floor and covered with a minimum required overburden Consultwith the COOLING WATER SYSTEMS SPECIALISTS for required coverage at a specific siteChlorine or sodium hypochlorite is injected at the intake crib to prevent marine growth in the lineNote that an intake crib and line will not be required if water of sufficient depth is available at the supply pump stationIntake Bay (Figure 6 ) - The intake bay is an enclosure which houses the pumps and screens Depending on a particular design a series of screens is used to ensure the supply of clean water Trash racks comprise the first element of the filter system These remove only very coarse material from the water such as bulk seaweed They can be supplied with atraversing raking mechanismLift screens provide secondary cleanup These screens can be lifted out of the intake bay for periodic cleaning Double liftscreens can be used if it is deemed necessary to have one in service at all timesFinal water filtering is usually done by traveling screens Details of the screen dimensions and the mounting are usuallydetermined during detailed engineering Screen net open area is typically approximately 50 of total screen area Thescreens should be sized for a normal water velocity of 12 fts (04 ms) through the open screen area Maximum velocityshould be limited to 24 fts (08 ms) with 50 of the screen area plugged This is the upper velocity limit for efficientscreening
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ExxonMobil Proprietary
Section Page
XXVII 16 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
If high suspended solids greater than 200 mesh (eg sand) are expected to be a continuous problem ejectors may be installedupstream and downstream of the traveling screen to remove accumulated sand and solids from the intake bay in these areas
Traveling screens and ejectors are supplied with wash water from the raw water pumps or from separate wash water pumpsOnoff control using timers should allow optimum scheduling of the ejectors depending on the time of the year
STEP 5 - Specify water intake facilities
Water intake facilities should be designed in accordance with the following guidelines from the Hydraulic Institute Standards The water intake should be designed to provide an evenly distributed flow of water to the pump suction A properly designedintake reduces the possibility of vortex formation and results in better pump performance with less noise Geometry of theintake and the manner in which water is introduced into the intake are important parameters in providing an even distribution of water flow Complete analysis of intake structures is best accomplished by scale model testsThe pump manufacturer will generally render advice on specific problems while the intake design is still preliminary He shouldbe provided with the necessary intake layout drawings showing the physical limitations of the siteFigures 7A and 7B have been developed for single- and multiple-pump arrangements to show suggestions for basic sumpdimensions They are for pumps normally operating in the capacity range of approximately 3000 to 300000 gpm (200 to20000 Ls) Since these values are composite averages from a great many pump types and cover the entire range of specificspeeds they must not be thought of as absolute values but rather as basic guides subject to some possible variations
All the dimensions in Figures 7A and 7B are based on the rated capacity of the pump at the design head Any increase incapacity above these values should be momentary or very limited in time If operation at an increased capacity is to beundertaken for considerable periods of time the maximum capacity should be used for the design value in obtaining sumpdimensionsDimension C is an average value based on an analysis of many pumps Its final value should be specified by the pumpmanufacturerDimension B is a suggested maximum dimension which may be less depending on actual suction bell or bowl diameters in useby the pump manufacturer The edge of the bell should be close to the back wall of the sump When the position of the backwall is determined by the driving equipment or the discharge piping Dimension B may become excessive and a ldquofalserdquo backwall should be installedDimension S is a minimum value for the sump width for a single pump installation This dimension can be increased but if it isto be made smaller the manufacturer should be consulted or a sump model test should be run to determine its adequacy
Motor size may dictate minimum pump spacing be especially aware of this when specifying pumps with unusually high pumpheads eg greater than 80 psig (550 kPa)Dimension H is a minimum value based on the ldquonormal low water levelrdquo at the pump suction bell taking into considerationfriction losses through the inlet screen and approach channel This dimension can be considerably less momentarily or infrequently without excessive damage to the pump It should be remembered however that this does not representldquosubmergencerdquo Submergence is normally quoted as dimension ldquoHrdquo minus ldquoCrdquo This represents the physical height of water level above the bottom of the suction inletThe actual submergence of the pump is something less than this since the impeller eye is some distance above the bottom of the suction bell possibly as much as 3 to 4 ft (1 to 13 m) For the purposes of sump design in connection with this chart it isunderstood that the pump has been selected in accordance with specific speed charts The submergence referred to hereinhas to do only with vortexing and eddy formationsDimensions Y and A are recommended minimum values These dimensions can be as large as desired but should be limitedto the restrictions indicated on the curve If the design does not include a screen Dimension A should be considerably longerThe screen or gate widths should not be substantially less than S and heights should not be less than H If the main stream
velocity is more than 2 fts (06 ms) it may be necessary to construct straightening vanes in the approach channel increaseDimension A conduct a sump model test of the installation or work out some combination of these factorsDimension S becomes the width of an individual pump cell or the center-to-center distance of two pumps if no division walls areusedOn multiple pump installations the recommended dimensions in Figures 7A and 7B also apply as noted above and thefollowing additional determinants should be consideredFigure 8A - Low velocity and straight-line flow to all units simultaneously is the first recommended style of pit Velocities in thepump area should be approximately 1 fts (03 ms) Some pumps with velocities of 2 fts (06 ms) and higher have given goodresults This is particularly true where the design resulted from a model study Not recommended would be an abrupt changein size of inlet pipe to sump or inlet from one side introducing eddying
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 17 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Figure 8B - A number of pumps in the same sump will operate best without separating walls unless all pumps are always inoperation at the same time in which case the use of separating walls may be beneficial If walls must be used for structural
purposes and pumps will operate intermittently one should leave flow space behind each wall from the pit floor up to at leastthe minimum water level and the wall should not extend upstream beyond the rim of the suction bell If walls are used increaseDimension ldquoSrdquo by the thickness of the wall for correct centerline spacing Round off the ends of the walls Not recommended isthe placement of a number of pumps around the edge of a sump with or without dividing wallsFigure 8C - Abrupt changes in size from inlet pipe or channel to pump bay are not desirable A relatively small pipe emptyinginto a large pump pit should connect to the pit with a gradually increasing taper section The angle should be as large aspossible preferably not less than 45 degrees With this arrangement pit velocities much less than one foot per second aredesirable Especially not recommended is a small pipe directly connected to a large pit with pumps close to the inlet Flow willhave excessive change of direction to get to most of the pumps Centering pumps in the pit leave large ldquovortex areasrdquo behindthe pumps which will cause operational troubleFigure 8D - If the pit velocity can be kept low enough [below 1 fts (03 ms)] an abrupt change from inlet pipe to pit can beaccommodated if the length equals or exceeds the values shown It is assumed that as ratio WP increases the inlet velocityat ldquoPrdquo will increase up to an allowed maximum of 8 fts (24 ms) at WP = 10 Pumps ldquoin linerdquo are not recommended unless theratio of pit to pump size is quite large and pumps are separated longitudinally by a generous margin A pit can generally be
constructed at much less cost by use of a recommended designFigure 8E - It is sometimes desirable to install pumps in tunnels or pipelines A drop pipe or false well to house the pump witha vaned inlet well facing upstream will be satisfactory in flows up to 8 fts (24 ms) Without the inlet well the pump section bellshould be positioned at least two pipe diameters vertically above the top of the tunnel not hung into the tunnel flow especiallywith tunnel velocities 2 fts (06 ms) or more There should be no signs of air along the top of the tunnel It may be necessaryto lower the scoop or to insist on a minimum water level in the vertical wellNote The foregoing statements apply to sumps for clear liquid
STEP 6 - Provide chemical treatment
Provide chemical treatment facilities for the once-through cooling water system Normally the only chemical treatment requiredin a once-through cooling system is for biological control Facilities for adding chlorine to once-through cooling water are thesame as those described for the recirculated cooling water system The design capacity of the chlorinator should be based onachieving a chlorine residual of 1 wppm in the effluent The chlorine required by the organics and other reducing agents in thewater must be satisfied before any residual chlorine will appear in the effluent The design capacity of the chlorinator should be
based on the total water chlorine demand plus 1 wppm in the effluent In the absence of water data the chlorination systemdesign capacity should be based on 10 wppm minimum instantaneous dosage for maximum cooling water flow For very largeonce-through cooling water systems it will be impractical to design for shock rates at this level In this case a continuousdosage of 1 - 2 wppm should be used (Refer to Step 5 of the DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS )
STEP 7 - Specify pumps and distribution system
Specify other major facilities in the once-through cooling water system The recommended materials for pumps valves andpiping are shown in the EMRErsquos Refinery Construction Materials Manual
Pumps1 Type - These pumps are usually vertical centrifugal pumps However horizontal centrifugal pumps have been used An
economic study andor consideration of existing facilities will determine which type should be specified In the event thathorizontal centrifugal pumps are specified facilities for pump priming will likely be required For large pumps in salt water service there is a very limited selection of pumps because of the special materials required Consult with the DISCIPLINESPECIALISTS early in the design effort to determine what specific pumps are currently available
2 Total Required Capacity - Firm capacity (excluding spares) of the supply pumps should equal the maximum cooling water requirements including LGF and RCF
3 NumberIndividual Pump Capacity - Normally one spare pump is provided for up to six operating pumps two spares for up to twelve operating pumps etc The most common arrangements for water supply pumps are
bull Two 100 capacity pumps
bull Three 50 capacity pumps
bull Four 33-13 capacity pumps
bull Five 25 capacity pumps
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ExxonMobil Proprietary
Section Page
XXVII 18 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Maximum commercially available pump capacity ranges up to 300000 gpm (20000 Ls) at discharge pressures up to 100psi (700 kPa) Maximum capacity will be less for higher discharge pressures Other factors which may influence pump
selection arebull Selecting capacity to match existing pumps
bull Selecting capacity for future requirements
bull Selecting capacity corresponding to manufacturers available standardVendor information should be obtained on the availability size and cost of the pumps for several configurations Acomparison should be made of the investments and operating costs as well as other advantages and disadvantages toselect a pump configuration
4 Drivers - These pumps are usually motor-driven Because of the remote location of the intake facilities the steamdistribution costs are normally too high to justify the use of a steam turbine driver However use of a secondary selectivesubstation provides a reliable source of power to the pump drivers
5 Pump ∆∆∆∆P - Pump ∆P is based on supplying the maximum cooling water requirement at the battery limit design pressureestablished for the system The ∆P should include the total circulating system pressure drop plus any elevation differencebetween the low water level at the intake and the discharge point of contact with air
6 Pump Instrumentation - The remote location of the intake facilities makes remote control and monitoring of the water supply operation desirable Pump controls located in the control house provide for minimum manning Local controlbackup should be provided The Owners operating and manning philosophy determine the basis for instrumentation andcontrolThe spare pump is controlled to automatically start on low pressure in the supply header Concurrently alarms should beactuated in the control room to indicate the low-pressure situation and spare pump operation
Distribution Network1 Configuration - The piping network is a once-through system with a return line from each consumer or consumer group to
the cooling water effluent line2 Layout - The routing of the piping network should connect all cooling water consumers in the most economical manner
permitted by the plant layout Careful consideration should be given to the turnaround philosophy of the plant Thenetwork layout should preclude the necessity of having above-ground water lines that are in service running through anarea that is being turned around Underground piping or an extra lateral may be required to accomplish this objective
3 Valves and Blinds - Battery limit valves should be provided in the network to isolate individual process units from thesystem Valves should be provided to permit onstream removal and maintenance of spared equipment
4 Basket Strainers - Consideration should be given to the installation of basket strainers in the laterals serving each of theconsumer loops These provide added insurance against entrance of foreign material into the cooling equipment The useof intake screens with 025 in (63 mm) openings may eliminate the need for basket strainers in the laterals
Line Sizing1 Design Basis - The network is sized so that water is delivered to the most distant consumer at a pressure equal to the
required process design (battery limit) pressure2 Flow Rate - Each segment of the distribution network is sized for the simultaneous maximum flow in that segment for any
operating condition The main supply and discharge headers are sized for the simultaneous maximum flow of all segments(not necessarily the sum of the simultaneous maximum flows of the individual segments)
3 Pressure Drop
Line Sizing Basis - The distribution system should be sized according to the guidelines established in Section XIV The use of an ldquoold piperdquo friction factor of C = 90 - 100 (Hazen-Williams factors) in the main lines is recommended to assuregood operation in later years when the system experiences fouling For large diameter lines [greater than 48 in (1200mm)] maximum water velocity should be limited to 10 fts (3 ms) based on civilmechanical considerations (supports andanchoring required to handle potential pressure surges) For salt water systems utilizing cement-lined pipe consider thereduced inner diameter in the friction loss calculations A study may be necessary to determine the economic pump ∆P vsline diameters
4 Equivalent Length Factors - Equivalent length factors are used in pressure drop calculations for sizing the distributionsystem The piping length scaled from the plot plan is multiplied by an equivalent length factor to compensate for pipingelbows expansion loops and other piping variations
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Section Page
COOLING WATER SYSTEMS XXVII 19 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
The equivalent length factors are selected based on the expected variation from a straight piping run Typical equivalentlength factors suitable for planning purposes for different piping applications are as follows
TYPE OF PIPING EQUIVALENT LENGTH FACTORS
Long piping runs [gt 1000 ft (300 m)] 12 to 14
Onsite distribution piping 15 to 20
For design purposes the actual piping layout should be evaluated to ensure that the equivalent length factors areapplicable to the specific situation Particular attention is required for revamp work and saltwater systems utilizing cement-lined pipe
STEP 8 - Prepare a system flow plan similar to the one shown in Figure 1
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS
STEP 1 - Determine cooling water requirements
Tabulate the cooling water requirements and cooling water discharge temperature for each process unit and offsite consumer Apply the proper LGFs to all cooling loads depending on the stage of the project and whether the new units are exactduplicates of existing units Determine the total cooling water flow required including LGFs and the bulk temperature (T 1) of thecombined cooling water flow from all coolers to the cooling towerIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The onsite and offsite designers should investigate methods to increase effluent water temperature to themaximum recommended limit for the particular type of water used This will result in a lower cost cooling tower for a given heatrejection rate One common method of maximizing return water temperature is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 2 - Select cooling tower design parameters
Select the proper design conditions of ambient wet bulb temperature approach temperature range and flow rate for specifyingthe cooling tower Based on these conditions the cooling tower vendor will size the tower and provide the tower characteristicand performance curves which serve as the basis for the performance guarantee Required performance test and evaluationprocedures are described in IP 8-1-1 Economic studies may be required to select the ambient wet bulb temperature andapproach temperature for new locations or for expansion projects with special cooling requirements
STEP 3 - Prepare cooling tower specification sheet
Include the following information in the heat exchange section of the design specificationTower Flow Capacity - This was determined in Step 1Type of Tower - The tower usually specified is an induced draft multicell type suitable for fresh salt or brackish water depending on the water available Additional tower characteristics are usually developed and proposed by the tower vendor (eg crossflow versus counterflow type of tower fill etc) The vendor should be made aware of conditions such as thelikelihood of heavy oil leakage into the cooling water or high fouling potential which may influence the tower type or fill
selectionNumber of Cells - The minimum number of cells required (normally not less than 2) may be specified The maximum flow per cell is a function of the cooling tower design conditions selected in Step 2 Typical values range up to 10000 gpm (600 Ls) for crossflow designs and 8000 gpm (500 Ls) for counterflow designs Normally spare cells are not provided Maintenance iscarried out during cooler periods or load is shed
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Section Page
XXVII 20 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Ambient Wet Bulb Temperature1 The ambient wet bulb temperature selected for specifying the cooling tower design is generally that temperature which is
equaled or exceeded 5 of the time during the four hottest months (5 of the hourly readings taken during the four hottestmonths will exceed this value) Specifying the 5 temperature should give satisfactory results since the time that the wetbulb temperature is exceeded is for short duration and the system inertia is usually sufficient to avoid problems The tower specification should make it clear that the wet bulb temperature specified is an ambient value as opposed to the enteringvalue which may be higher because of recirculation The tower manufacturer should adjust the ambient value upward totake into account his estimates of recirculation Currently cooling tower test codes provide procedures for measuringperformance in the case of either entering or ambient wet-bulb temperature specifications An increase of 2 degF (11 degC) isgenerally added to ambient wet bulb temperature to estimate entering wet bulb temperature for large cooling towers
2 If plant production will peak during the four summer months when the 5 ambient wet bulb temperature may be exceededconsideration should be given to specifying a higher design ambient wet bulb temperature (one that will be exceeded 212 of the time for example) However the incremental cost of the resulting closer approach and larger cooling tower must be justified by the value of the incremental products made
On-Tower Temperature (T1) - This temperature cannot be accurately determined until the design of all process coolers iscompleted and the volume of cooling water and outlet temperature are known for each (see Step 1) In the planning stage or early design stage of a project this temperature must be estimated and is usually set equal to the maximum design cooler outlet temperature shown under COOLING WATER REQUIREMENTS in the BASIC DESIGN CONSIDERATIONS portion of this sectionApproach Temperature - The approach temperature has the most pronounced effect on cooling tower size and cost For example reducing the approach from 10 to 5 degF (55 to 3 degC) can increase the cooling tower investment by 50 percent and thefan operating cost by 65 percent Thus various approach temperatures should be evaluated to determine the minimum cost of the total system including the process exchanger area Approaches used in the past designs have been from 5 to 12 degF (3 to7degC) Note that it is not customary in the cooling tower industry to guarantee approaches of less than 5 degF to (3 degC) to coolingtower design inlet wet bulb temperature A typical approach for a new grass roots cooling tower is 10 degF (6 degC)Off-Tower Temperature (T2) - This is the sum of the approach temperature and the ambient air wet bulb temperature For existing locations Table 2 in Section IX-B may be used
Range and Flow Rate1 The range for a given heat load is related to the circulating water rate as follows
Q = M C p ∆Tcong (500) (gpm) (range) Eq (1)
where Q = Heat load on the tower BtuhM = Water circulating rate lbhCp = Specific heat capacity of water at average temperature Btulb degF∆T = (T 1 ndash T 2) degF = (water cooling range)gpm = Circulation rate in US gallons per minute
For calculation in Metric units
Q = M C p ∆Tcong (1162 x 10 -3) (Ls) (range) Eq (1M)
where Q = Heat load on the tower kJh
M = Water circulating rate kghCp = Specific heat capacity of water at average temperature kJkg degC∆T = (T 1 ndash T 2) degC = (water cooling range)Ls = Circulation rate in liters per second
2 Generally lower tower investment fan horsepower and pump horsepower result from maximizing the range andminimizing the flow rate However based on fouling considerations of process coolers the maximum allowable hot water temperatures must be kept below recommended limits Also while increasing the range minimizes cooling tower investment and operating cost the cost of exchanger surface requirements is adversely affected by the LMTD and acompromise must be made The range is bounded by the maximum allowable hot water temperature and the Off-Tower temperature (T 2) For established locations refer to Section IX-B for design cold water temperatures to assist in selectingthe approach and range
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Section Page
COOLING WATER SYSTEMS XXVII 21 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Makeup Water Requirements - Calculate the various losses in the cooling water system to determine makeup requirementsThese losses include
1 Evaporation Loss (E) - Allow 1 of the circulation rate for each 10 degF (55 degC) temperature drop through the tower Notethat the actual evaporation loss will be somewhat less depending on ambient air temperature
( )sL555
TT)gpm(
1000TT
E 2121
minus=
minus= Eq (2)
2 Drift Loss (D) - IP 8-1-1 limits the maximum allowable drift loss to 0005 of the design water circulation rate for freshbrackish or salt water
( )sL1000050
)gpm(1000050
D
=
= Eq (3)
This low drift level contributes little to solids removal from the tower with the spray This allows higher cycles of concentration to be achieved in cooling towers which use clarified softened water for makeup and thus have low requiredblowdown rates Higher drift loss percentages in older or poorly maintained towers will limit the maximum cycles of
concentration achievable3 Miscellaneous Losses (O) - Miscellaneous losses are estimated to be one-seventh of the difference between evaporation
loss and drift loss
minus=
7DE
O Eq (4)
4 Blowdown Rate (B) ndash Guidelines for maximum concentrations of impurities allowed in recirculated fresh cooling water areshown in Table 2 For salt water the blowdown is based on keeping the total dissolved salts below 55000 wppm Obtainthe chemical analysis of the makeup water and divide each individual component concentration into its counterpart inTable 2 The result is the maximum allowable cycles of concentration for that constituent (a worksheet is provided inTable 4 ) The impurity with the lowest factor will be the control and will determine the required blowdown rate
)OD(1Cycles
EB +minus
minus= Eq (5)
The makeup water requirement is the sum of all losses shown above
MakeUp = E + D + O + B Eq (6)
STEP 4 - Specify the controls necessary for proper cooling tower operation
Air Side - Cooling towers are provided with motor-driven manually adjusted pitch multiple-bladed propeller fans Figures9A amp B may be used to estimate the normal operating horsepower requirements Since the fan motor and gear reducer arelocated at the top of the tower [generally 40 to 50 ft (12 to 15 m) above grade] the need for equipment to handle thesecomponents should be discussed with the affiliateControl of the air flow through the tower to maintain reasonably constant cold water temperatures can be accomplished byturning off individual fans in multiple-cell installations adjusting the blade pitch or using variable speed motors To turn off individual fans remote at grade start-stop buttons for the motors are usually specified
Whenever the ambient air temperature is 32 degF (0 degC) or below icing of the tower louvers and structural members in contact withthe incoming air will occur If the heat load on the tower is relatively free of seasonal variations two-speed fans should provideadequate ice control down to approximately 0 degF (-18 degC) ambient temperatures When sustained temperatures of 0 degF (-18 degC)and below occur two-speed fans capable of two-speed reverse operation with appropriate time delays for change of directionand speed should be specified to periodically eliminate ice which will build up on the louvers Other methods of winterizationare available but in general are more expensive and require greater operator attention In very cold climates steam heatingshould be provided to prevent freezing in the cold water basin during shutdown periodsWater Side - The water circulation rate is maintained by operating one or more pumps of sufficient capacity to meet the systemdemand Flow-measuring facilities local temperature indicators and a high-temperature alarm are provided on the coolingtower water supply and return headers Hand-operated flow control valves are required for distributing the water equally to theindividual cells On counterflow towers provisions for measuring the flow to each cell (pitot tubes) are required They are notrequired on crossflow towers since the open distribution basin provides visual aid in balancing flows Shutoff valves should beprovided for servicing the flow control valves without shutting down the entire tower
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Section Page
XXVII 22 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
STEP 5 - Provide chemical treatment facilities for the fresh water system
Alkalinity (pH) Control (if used)1 In cases where a controlled pH corrosionscale control program is being used the alkalinity (pH) of the recirculated cooling
water should be controlled in accordance with the program requirements Normally acid addition is required to accomplishthis This is due to free carbon dioxide being stripped from the cooling water in the tower to approximately 5 ppm For example if the alkalinity has to be controlled to between 25 and 50 ppm for a pH of 70 to 75 the volume of 98 sulfuricacid (66 degBeacute) required to treat the makeup water is calculated using the following equation
[Note 93 sulfuric acid should be used if ambient temperatures will be less than 35 degF (2 degC)]
GallonsHour 66 degBeacute H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate gpm) (326 x 10 -5)
For calculation in Metric limits
kgh of 100 (66 degBeacute) H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate Ls) (793 x 10 -3)
2 The metering pump should be designed for a maximum feed rate of two to three times the normal feed rate A glass woolfilter is required on the pump suction piping to minimize plugging Normally two - 100 capacity pumps are provided
Positive displacement metering pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumps with flow control areused for higher rates
3 Hydrochloric acid may also be used for alkalinity control The gallons per hour of 20 degBeacute hydrochloric acid required is36 times the amount of 66 degBeacute sulfuric acid In Metric units the weight of 32 (20 degBeacute) hydrochloric acid required is227 times the weight of 100 (66 degBeacute) sulfuric acid
4 Specifications for chemical feeders may be found in EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased)
5 A pH analyzer with high and low alarm is provided to control the amount of acid feed The acid should be added to thecooling tower makeup water (Figure 2 ) The lag time between the point of acid injection and the sample outlet to the pHcell should be less than two minutes for effective control
6 Spills or overflows from the sulfuric acid tank should be contained
Corrosion Inhibitors and Anti-Scalents1 Two chemical feeders are generally provided The following guides in sizing the equipment will allow the use of any of the
proprietary products from the vendors when final selection by the owner is made See Report No EE102E78 Cooling Tower Water Treatment Guidelines EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and Report No EE34E86 Environmentally - Acceptable Cooling Water Treatments for detailed discussion of acceptable commercial chemicals
bull If the water treatment program to be used is known design for the chemical rates specified by the vendor or COOLING WATER SYSTEMS SPECIALIST
bull Otherwise design for 100 wppm of each commercial liquid product in the recirculated cooling water The design feedrate should be based on the design total blowdown flow (both uncontrolled and controlled) since this is where theproduct is lost For 100 wppm concentration the chemical rates will be as follows
GallonsHour Chemical = Blowdown (gpm) x (0006)
LitersHour Chemical = Blowdown (Ls) x (036)
2 The design capacity of the metering pump should be two to three times this calculated number Normally 2 - 100capacity pumps are provided Positive displacement pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumpswith flow control are used for higher rates
3 Specifications for chemical feeders may be found in EMRErsquos Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased) Reference should also be made to the checklists containedin Report No EE92E94 Guidelines for Safety Evaluation of Chemical Injection Facilities
4 A final check should be made with the owner and the COOLING WATER SYSTEMS SPECIALISTS as to the treatmentprogram selected and if there is a need for an additional chemical feeder
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COOLING WATER SYSTEMS XXVII 23 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Biological ControlFacilities should be provided for feeding the biocide into the back of the cooling tower basin If chlorine is used it must bepresent in the cooling water as free residual to be effective However the following amounts of chlorine are first required toreact with the listed impurities when present in the cooling water before a free residual can be established
IMPURITIESAMOUNT OF CHLORINE REQUIRED (wppm)
WITH EACH WPPM OF IMPURITY
Ammonia (NH 3)
Hydrogen Sulfide (H 2S)
Sodium Sulfite (Na 2SO 3)
Ferrous Iron (Fe ++)
10
22
056
064
The design capacity of the chlorine feeder shall be based on achieving a 01 wppm free chlorine residual in the recirculatingwater flow or a minimum dosage of 7 wppm at maximum design makeup water flow rate and conditions (see EMRE Water and
Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service )Chlorine may be fed either as the commercial liquid sodium hypochlorite (124 wt Cl 2) or liquid chlorine (100 Cl 2) Liquidhypochlorite is being used increasingly because of the higher costs associated with precautions equipment and proceduresrequired to minimize the risks associated with handling gaseous chlorine The hypochlorite solution is fed using a meteringpump which should be sized to feed an equivalent of 15 ppm 100 Cl 2 (Refer to EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) ) In case of spills or an overflow from thestorage tank the sodium hypochlorite may be sent to the cooling tower basinGas chlorinators are covered by EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service The use of gas chlorination is being reduced but may still be appropriate in some locations subject tolocal management approval The gas from either commercial cylinders or an evaporator is dissolved in a water slip stream bymeans of an eductor This results in a chlorine solution strength of approximately 01 This solution should be evenlydistributed below the water surface by means of a header lateral pipe arrangement at the rear of the cooling tower basinopposite the cooling water pumpsSome additional guides on the use of gas chlorinators are given below It is also recommended that the safety guidelines
published by the Manufacturing Chemists Association and the Chlorine Institute be followedbull Dilution water for the chlorinator must not be less than 50 degF (10 degC) or ice formation may cause plugging problems
bull Continuous rates of chlorine gas withdrawal should not exceed 400 lb (180 kg)24 hours from each one ton cylinder atroom temperature [70 degF (21 degC)] This rate may be exceeded (up to 50) for periods not exceeding 2 hours Rule of thumb is not to design for more than 4 cylinders operating in parallel When feed rate exceeds 3000 lb (1360 kg) per daythe liquid chlorine should be fed to an evaporator upstream of the chlorinator
bull If the ambient temperature can be expected to be below 50 degF (10 degC) for extended periods a building should be providedfor the chlorinator and cylinders
bull Weighing scales should be provided to determine when shipping containers should be replaced and to verify chlorine feedrates
bull Analyzers to measure the free chlorine residual in the circulated cooling water should be provided
bull Chlorine leak detectors and a catch basin for major spills should be provided
Control SystemProvide automatic controllers to continuously monitor water condition and to add chemicals to the system (complete systemsmay be available through the water treatment program vendor as a lease or purchase)pH Control - This controller adds acid to correct water alkalinity a timer on the acid pump sounds an alarm if the pump doesnot shut off in a predetermined length of time Backup controls are required to prevent the cooling system from beingpermanently damaged in case the pH electrodes become fouledCorrosivity and Inhibitor Control - Systems are available which give a direct readout of corrosion rate in milsyear If thecorrosion rate exceeds a set point the controller shuts off the acid increases the inhibitor addition opens the blowdown valveand sounds an alarm Electrodes measure the completeness of the inhibitor film by the value of the potential differencebetween the electrodes The larger the difference the more incomplete the film and a potential reading exceeding a set levelactivates the inhibitor feed
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XXVII 24 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Free Residual Chlorine Control - Free residual chlorine should be controlled automatically for biocide treatment Severalgood analyzers are currently available These units can be incorporated into an automatic control system
Oil in Water DetectionMonitoring Corrosion and Deposit FormationThe following types of devices for measuring corrosion rates and deposit formation should be provided to allow the plant tomonitor the effectiveness of the cooling water treatment before scheduled turnaroundsCoupons - A corrosion coupon rack should be specified for the hot return water to the tower in accordance with IP 19-6-1 Corrosion Monitor - On-line corrosion analyzer if specified should monitor the hot return water to the tower in accordance withIP 19-6-1
On-Line Test Exchanger Although there are a variety of deposit monitors from the water treatment chemical vendors they do not necessarily representactual heat transfer conditions in process exchangers For this reason a test exchanger should be installed for evaluatingcorrosion and deposit formation Details are covered in Report No EE102E78 Cooling Tower Water Treatment Guidelines
Alternatively a critical process exchanger can be instrumented such that its heat transfer coefficient can be monitored
Instrumentation would include flows and inlet and outlet temperatures of process fluid and cooling waterSTEP 6 - Specify cooling tower location
Select a location for the cooling tower The following factors influence this selection
Location Factors1 The location should be within a reasonable distance of the units served and consistent with future refinery plans Piping
costs to and from battery limits can be a significant portion of the total cooling system cost and can be the location-determining factor particularly for salt water systems While piping costs can be easily determined other costs affectinglocation cannot be estimated readily Therefore judgment is required in selecting the location
2 Safety spacing requirements (see Section XV-G Equipment Spacing )3 Cooling tower contribution to plant boundary noise4 For salt water cooling towers wind direction and distance to other facilities is of major concern because of potential
corrosion problems
5 Factors affecting cooling tower performance ie orientation with respect to wind site congestion etc
Plot RequirementsIn estimating plot requirements (basin area) one should allow 5 to 6 ft of tower length for each 1000 gpm (2 m for each 60 Ls)of circulating capacity This figure is based on a typical counterflow design with a width of 50 ft (12 m) and does not includepump suction pit space Exact requirements can be obtained from cooling tower vendors after the tower capacity has beenestimated
Fogging and DriftTwo distinct problems are associated with the discharge air from a cooling tower (1) fogging which can be a nuisance or ahazard and (2) drift which is generally a nuisance because of increased corrosion of surrounding equipment Fogging fromcooling towers occurs during periods of combined high ambient humidity and low temperature as a result of mixing the warmmoist cooling tower exhaust air with the cooler ambient air The ambient air cannot absorb all moisture as vapor and thus tinyfog droplets are formed by condensation Fogging and resulting icing in cold climates has produced hazardous drivingconditions on highways up to 850 ft (250 m) awayProprietary designs are available to combat potential fogging problems Basically these designs incorporate heating of thedischarge air with hot return water precooling coils (similar to an air cooler) to eliminate any visible plume However thesedesigns are expensive and relatively unproven Therefore proper location based on reliable weather data is the preferredsolutionDrift from a cooling tower represents a more difficult problem since it does not depend on definable climatic conditions andoccurs for all wind directions The effects of drift on nearby equipment and vegetation are particularly severe and generally of concern only for salt water systems because of the high solids content of the drift (concentrated salt water) For salt water cooling systems electric lines substations and instrumentation should be located upwind in the direction of the predominantwind They should be provided with adequate shelter or materials when within 450 ft (140 m) of the cooling tower In additionspecifications for all equipment within 150 ft (45 m) of the tower should include surface treatment suitable for a marineenvironment
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COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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DESIGN PRACTICES December 2000
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TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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DESIGN PRACTICES December 2000
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FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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Section Page
COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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Section Page
XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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Section Page
XXVII 2 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
CONTENTS (Cont)Section Page
TABLESTable 1 Impurities Found in Cooling Water 30Table 2 Allowable Concentration of Impurities in Recirculated Fresh Cooling Water 31Table 3 Method of Chemical Receipt 32Table 4 Water Treatment Analysis Worksheet 33
FIGURESFigure 1 Typical Once-Through Cooling Water System 34Figure 2 Typical Recirculated Cooling Water System 35Figure 3 Cooling Tower Types 36Figure 4 Support Structure for Water Intake Crib 37Figure 5 Water Intake Pipe 38Figure 6 Water Intake Bay 39Figure 7A Sump Dimensions vs Flow Rate Customary Units 40Figure 7B Sump Dimensions vs Flow Rate Metric Units 41Figure 7C Sump Dimensions 42Figure 8 Configurations for Multiple-Pump Pits 43Figure 9A Fan Horsepower Requirements Customary Units 44Figure 9B Fan Horsepower Requirements Metric Units 45Figure 10 Cooling Tower Orientation 46Figure 11 Water Treatment Flow Plan for Recirculating Cooling Water Systems 47Figure 12 Cooling Tower Disengaging Stack 48
Revision Memo
1200 Highlights of this revision are as follows1 Revised contents to reflect design methods and philosophy used on recent projects2 Included learnings from consulting on recent problems experienced3 Corrected references to withdrawn International Practices4 Clarified recommendations regarding cooling water distribution piping5 Included more specific definition regarding recommended cooling tower basin minimum
holdup6 Added a recommended simplified plan for balancing cooling water exchanger water usage in
the Revamp and Expansion Projects section7 Included recent changes in recommended cooling water treatment
EMRE COOLING WATER SYSTEM SPECIALISTS
TECHNOLOGY AREA CONTACT PHONE NO(703) 846-
E-MAIL(AMERICAS)
Water Treatment R G Balmer
C M Havran
3640
3806
RGBALME
CMHAVRASystem Planningand Design
R A LieberwirthG E Stover
7378(281) 834-7599
RALIEB2GESTOVE
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Section Page
COOLING WATER SYSTEMS XXVII 3 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
SCOPE
This section presents the criteria and general procedures for the selection and design of cooling water systems in processplants Major components are described for both once-through and recirculated type cooling systems using either fresh or salt
water This section also includes information on the chemical treatment required to control corrosion scaling and biologicalfouling in cooling water systems Information on automatic equipment used to monitor water condition and to make thenecessary adjustment in chemical treatment is also included
REFERENCES
DESIGN PRACTICES
Section II Design Temperature Design Pressure and Flange RatingSection IX Heat Exchange EquipmentSection X PumpsSection XII InstrumentationSection XIV Fluid Flow
Section XV Safety in Plant DesignENVIRONMENTAL DESIGN PRACTICE
Section XIX-B Water Reuse
OFFSITES DESIGN PRACTICE
Section XXVI-A Boiler Feedwater Treating Systems
INTERNATIONAL PRACTICES
IP 1-1-1 Drawings Diagrams and Line ListsIP 3-10-3 Cement Lined Pipe and FittingsIP 3-10-4 Plastic and Plastic-Lined PipingIP 8-1-1 Cooling TowersIP 10-11-1 Sizing of Drivers and Transmissions for Compressors Fans and PumpsIP 16-1-1 Area Classification and Related Electrical Design for Flammable Liquids Gases or VaporsIP 16-4-1 Grounding and Overvoltage ProtectionIP 19-6-1 Facilities for Corrosion Monitoring in Process Equipment
OTHER LITERATURE
Cooling Tower Water Treatment Guidelines ERampE Report No EE102E78Exxon Cooling System Handbook January 1986Environmentally Acceptable Cooling Water Treatments ERampE Report No EE34E86Karlsruhe Cooling Water System Reliability Study ERampE Report No 85EEEL3268Reuse of Wastewater as Cooling Tower Makeup ERampE Report No EE58E85
Guidelines for the Inspection Preventative Maintenance and Rehabilitation of Cooling Towers ERampE Report No EE47E89Guidelines for Safety Evaluation of Chemical Injection Facilities ERampE Report No EE92E94EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating ServiceEMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased)EMRE Refinery Construction Materials Manual Manual No EETD 028Cooling Tower Institute (CTI) Acceptance Test Codes for Water Cooling Towers (ATC-105)Cooling Tower Institute (CTI) Cooling Tower Performance Curves
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Section Page
XXVII 4 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BACKGROUND
Rejection of heat is a requirement of a great many processes and utility systems Coolers and condensers reject much of thisheat to water The main purpose of a cooling water system is to absorb the heat rejected and dispose of it The system must
be safe reliable and flexible and it must have a minimum impact on the environmentCooling system selection and design is very site specific and cannot be generalized The EMRE COOLING SYSTEMSPECIALISTS are available for consultation on specific requirements
DEFINITIONS
Ambient Wet Bulb Temperature
The wet bulb temperature that exists in the cooling tower area It is the temperature specified in the Design Specification for the design and guarantee basis Note the actual tower inlet wet bulb temperature may be higher than the ambient wet bulbtemperature due to hot air recirculation
Approach
The difference in temperature between cold water leaving the tower and ambient wet bulb temperature
Battery Limits
A boundary which is normally assumed to be at the physical edge of the block in which the facility is located
Blowdown
The continuous or intermittent discharge of a small amount of the circulating water Its purpose is to limit the increase in theconcentration of solids in the water due to evaporation It is expressed in percent of water circulated
Brackish Water
Water which contains 3000 to 5000 wppm of dissolved salts
Cell
The smallest tower subdivision which can function as an independent unit with regard to air and water flow
Cooling Tower Pumping Head
The total pressure at the centerline of the tower inlet to its hot water distribution system plus the difference in elevation betweenthe centerline of the inlet and the top of the cold water basin curb It does not include friction pressure drop in the riser pipeThis is a parameter representing the head required for the cooling tower only It excludes the cooling water distribution system
Counterflow Tower
A cooling tower in which air induced at the bottom of the tower flows up through the fill against the falling water
Crossflow Tower
A cooling tower in which air induced at the sides of the tower flows horizontally across the fill and the falling water
Cycles of Concentration
The ratio of dissolved solids in circulating water to dissolved solids in makeup water
Dissolved Solids
A measure of the total quantity of dissolved salts in water It can be determined by conductivity measurements
Drift
The entrained water carried from the tower by exhaust air expressed in percent of water circulated
Dry Bulb Temperature
The temperature of ambient air read on an ordinary thermometer
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COOLING WATER SYSTEMS XXVII 5 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DEFINITIONS (Cont)
Fill
Packing within the cooling tower to provide the required surface area for heat transfer between ambient air and the returncooling water Splash fill breaks the water into small drops by causing it to cascade through successive offset levels of parallel level splash bars Cellular or film fill causes the water to spread into thin films over large vertical areas
Heat Load
Heat removed from the circulation water within the tower It may be calculated from the range and the circulating water flowrate
Interference
The mixing of tower inlet air with the discharge vapors from another tower or other heat source This results in reduced thermalperformance
Makeup
The water required to replace circulating water which is lost by evaporation drift blowdown and leakage It is expressed inpercent of water circulated
Offsites
Support facilities such as utilities (steam power cooling water) tankage waste treating etc for the processing operations
Once-Through Cooling System
A system in which water passes through the heat exchange equipment once and is then discharged
Onsites
Facilities that are a part of the processing operations
Range or Water Cooling Range
The difference in temperature between the hot and cold circulating water or on-tower temperature minus off-tower temperature
Recirculated Cooling System
A system in which water is circulated to coolers in a loop and the heat absorbed from this cooling operation is dissipated by acooling tower or some other type of heat exchanger
Recirculation
An undesirable condition in which part of the tower discharge vapor stream is recirculated through the tower resulting inincreased inlet air wet bulb temperature
Relative Humidity
The ratio of the quantity of water vapor present in air to the quantity which would saturate the air at the existing temperature
Salt Water
Water which contains more than 5000 wppm of dissolved salts
Wet Bulb Temperature
The equilibrium temperature obtained when ambient air is passed over a continuously wetted thermometer bulb evaporatingthe water and cooling the bulb This equilibrium temperature occurs when the heat transferred from the air to the wettedsurface equals the heat loss due to latent heat of evaporation
Wind Rose
A diagram that shows the average frequency and intensity of wind from different directions for a particular location
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XXVII 6 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
SYSTEM TYPES AND APPLICATIONS
Cooling water systems in refineries and chemical plants are generally either the once-through or the recirculated type designedfor fresh salt or brackish water
In the once-through system (see Figure 1 ) pumps take suction from a source of supply such as a river or other body of water and deliver the water to process coolers and other consumers After passing through the cooling exchangers the hot coolingwater is discharged through pressurized piping or a gravity flow systemIn a recirculated system (see Figure 2 ) pumps take suction from a cooling tower basin and deliver the water to process coolersand other consumers After passing through the cooling exchangers the hot cooling water is discharged through a pressurizedreturn system to a cooling towerThe choice between a once-through and recirculated cooling water system must be based on (1) availability of water of satisfactory quality (2) process temperatures (3) atmospheric conditions (4) investment and operating costs and (5) effluentwater quality and temperature limitationsThe cooling water is distributed to individual consumers throughout the plant in piping main supply and return headers andseries and parallel branches consisting of laterals and sublaterals The main cooling water distribution headers are either buried in underground trenches or are located above grade on ground-level pipe sleepers or on elevated pipe racksUnderground piping has the advantage of minimizing problems associated with thrust loads thermal expansion and freezingElevated piping is easier and less expensive to install and repair and it makes it easier to locate leaks
BASIC DESIGN CONSIDERATIONS
RELIABILITY
The cooling water system is an important utility and its reliability is critical to successful plant operation The typical systemprovides cooling water to both onsite and offsite consumers It can have a direct impact on the sizing criteria for the emergencyrelief (flare) system (refer to Section XV-C Safety in Plant Design Pressure Relief ) The system therefore normally must becapable of continuous operation so that cooling water is always available to critical consumersThe reliability required to achieve continuous operation is provided in several ways
bull Spare makeup and water circulating pump(s) are provided to assure continuous operation
bull Different types of drivers are specified for the supply or water circulating pumps Motor-driven pumps are supplied fromtwo different buses on a secondary selective electrical distribution system
bull Cooling tower fan drivers are normally supplied from a secondary selective systembull Holdup volume is provided in the cooling tower basin to permit an orderly shutdown of process units if the makeup water
supply is lost or a failure occurs in any part of the recirculated cooling water system
bull Valves blinds and bypasses are provided to permit individual components to be removed from the system for maintenancewhile the system is operating
bull Construction materials provided for cooling water facilities are designed for long-term corrosion resistance sincemaintenance work is often difficult (if not impossible) to schedule without a complete refinery shutdown
FLEXIBILITY
The cooling water system must include sufficient flexibility to cover all present operating requirements plus any future operatingrequirements that are defined in the design basis document For example future operating requirements could be dictated by anear-term increase in plant sizeThe type of flexibility features provided in a system may include the following
bull Makeup water treating facilities should be able to deliver the required water assuming the worst conditions of the water source
bull Waste treating facilities should be designed to handle effluent water containing varying concentrations of oil and chemicalsover the range of expected ambient temperatures
bull Valves and blind flanges should be provided to permit future tie-ins to the system without a shutdown An exception wouldbe on lines where a ldquohot taprdquo could be made with minimum risk of an emergency shutdown or where the cost of requiredvalves is prohibitive
bull Cooling tower cells should be completely partitioned as described in IP 8-1-1 to allow personnel entry to any cell for maintenance without affecting the operation or capacity of the other cells Normally spare cells are not providedMaintenance is carried out during cooler periods when the cooler ambient and wet bulb temperatures enable the coolingtower to handle greater heat loads
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Section Page
COOLING WATER SYSTEMS XXVII 7 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
bull Where multiple cooling tower systems exist consideration should be given to providing valved cross connections betweentheir respective supply mains and between their respective return mains These would be used only for special operations
such as turnarounds or emergencies The impact on flare sizing should be evaluated if these cross connections areprovided
ENVIRONMENTAL AND SAFETY CONCERNS
Once-through cooling water systems usually require controls to minimize the release of hydrocarbons and free halide residualsto the environment As a minimum cooling water discharges should be monitored for free oil total organic carbon or hydrocarbon vapors to determine if system coolers are leaking Free chlorine residuals in the discharge should also bemonitored as required to meet local regulations and to detect process leaks In most cases once-through cooling water flowsare too large to be handled in the plants wastewater treatment system However once-through water that could becomecontaminated with hydrocarbons is often segregated and treated in a large skim pond or special purpose separator prior todischarge Local environmental regulations and permit requirements will dictate the extent of treatment requiredCooling tower blowdown from recirculated systems is normally bypassed around major wastewater treatment and dischargedwith the treated wastewater An oil-in-water detector should be provided to detect oil in the blowdown from process leaks Thiswill enable stopping the blowdown or diverting it to temporary holdup until the leak is corrected A major rupture in a processheat exchanger tube can also result in the cooling tower being the discharge point for hydrocarbon vaporsBecause of the likely presence of flammable vapors the areas around cooling towers are subject to electrical classification inaccordance with IP 16-1-1 The type and amount of chemicals added to the cooling water should be evaluated as to their effects on the plants wastewater treatment process Biocides corrosion inhibitors and antifoulants may result in significant problems for the wastewater treatment plantOther environmental concerns are
bull Cooling tower noise emissions from fans and from the flow of cooling water over the tower may require suppression if near a local community
bull Cooling tower water mist drift can cause fog and ice formation on nearby roads and promote deterioration of nearbyequipment It may also result in damage to sensitive vegetation in the surrounding area
bull Process gas releases to atmosphere resulting from process gas pickup by cooling water including the venting of toxicgases from exchanger tube ruptures must be considered in locating cooling towers or discharge points from once-throughsystems
bull Spills and overflows of toxic and hazardous chemicals used in treating the cooling water must be contained
SYSTEM SELECTION
The selection of a cooling water system is often determined by the availability of water In regions where water is scarce once-through systems are obviously eliminated from consideration and recirculated systems are chosen A once-through coolingsystem may be the economical choice if the plant is located adjacent to a body of water and consumers are elevated no morethan 50 ft (15 m) above the water level However once-through system costs may be increased significantly by environmentallimitations in many locations These may limit maximum discharge water temperature and therefore will reduce the maximumcooling water ∆T This will increase the required flow and cost of once-through systems In addition regulations may requirewaste treatment of the large volume of effluent thus increasing costsThe choice of a cooling water system must be based on comparisons of initial investment and operating costs Relative initialcosts of a system vary with climatic conditions distance from pump to users complexity of inlet structures and corrosive nature
of the water Salt water or brackish water service requires the use of cement mortar lined pipe and alloys for exchangers andpumps Carbon steel is usually satisfactory for fresh water Special attention should be given to the impact of bottom silt mudand seaweed on intake facilities to once-through or makeup to recirculated systems Cribs flumes large bays screens trashracks or other extensive facilities may be requiredDuring the early stages of a project the designer should review with the planning or design groups the heat level and volume of all process streams which require cooling The optimum method of cooling should then be determined ie heat exchangeandor rejection of heat to air or water A break point should be selected above which air cooling is economical and belowwhich trim water cooling is preferred
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XXVII 8 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
COOLING WATER REQUIREMENTS
Cooling water requirements for each process unit and offsite consumer are obtained from the appropriate design groups For existing locations maximum cooling water loads should be based on the design inlet cooling water temperatures given inSection IX-B Design Considerations for All Types of Heat Exchangers For new locations the inlet cooling water temperaturemust be set based on local water temperature data for once-through systems and wet bulb temperature and selected approachfor recirculated systemsThe bulk water temperature at the cooler outlet has been selected as the controllable variable for design and operation of coolers Although the corrosion rate depends on the metal temperature this is difficult to determine in the operation of a coolerBased upon the findings of a Materials Development Section test program maximum design outlet water temperatures havebeen set as indicated below
Fresh Water 130 degF (54 degC)
Treated Fresh Water 130 degF (54 degC)
Brackish Water 125degF (52
degC)
Salt Water 120 degF (49 degC)
When the maximum plant cooling water requirement has been established for new process and utility consumers appropriateLoad Growth and Reserve Capacity Factors (LGF and RCF) should be applied depending on the development stage of theproject Cooling water loads should be updated and system capacity adjusted periodically as design work progresses
LOAD GROWTH AND RESERVE CAPACITY
Utility loads tend to increase as the loads become better defined through successive stages of project definition Thereforeload growth and reserve capacity factors (LGF and RCF) are applied to raw loads as a means of predicting final equipmentsizes and capital costs These load growth factors do not cover basis changes through project development such as project or process unit sizethroughput For revamp or modernization projects these factors are generally applied only to new loads or changes to existing loads on the basis that existing or unchanged loads are well known
Recommended Load Growth Factors are as follows
CONSUMERSPLANNING
(SCREENING THROUGH DBM) DESIGN SPEC
Existing Facilities 0 - 10 0 - 10
New Facilities 20 - 30 5 - 10
Selection of the load growth factor to use should be based on the quality and level of effort associated with development of theraw loadsFinal sizing of the source facility (eg the cooling tower and circulation pumps) should include a reserve capacity factor of 10to cover small post startup changes minor future projects and dynamic conditions not predicted by steady state balances Thesizing basis for distribution facilities should not include the RCF
COOLING TOWER TYPES
The cooling tower is the major piece of equipment specified in recirculating cooling water systems In effect it is the heatexchanger which cools the hot water to a suitable temperature for re-use primarily by evaporation through contact with airThe cooling tower contains fill to break up the water flow into droplets or film on a surface The fill increases the waterair contact area and enhances heat transferCooling towers are classified according to the means by which air is supplied to the tower ie induced or forced mechanicaldraft (fan) vs natural draft and according to the relative movement of the air and water that is counterflow or crossflow (seeFigure 3 ) Most of the towers in the ExxonMobil circuit are mechanical induced-draft type Economics determine whether theyare counterflow or crossflow
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COOLING WATER SYSTEMS XXVII 9 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
In crossflow towers the air travels horizontally across the falling water while in the counterflow design the air travels verticallyup through the falling water Comparisons of plot space fan horsepower and future maintenance costs determine which
design will be more economical Factors which influence this selection are cooling water rate range and approach for aparticular location In addition in some locations materials of construction and associated labor costs will affect the finalselectionIn the past crossflow towers were usually preferred over counterflow towers for extremely cold locations where ice formationcould restrict or block air flow through the louvers However this consideration is usually no longer valid due to the increaseduse of PVC film-type tower fill rather than splash-type fill in counterflow towersMechanical draft designs use one or more fans to provide the air needed for cooling Induced-draft units have the fans locatedon top of the tower Air is pulled through the packing and discharged vertically upward at high velocities Forced-draft unitshave the fans located at the base of the tower and push the air through the fill Forced-draft towers in large industrial sizesexperience difficulties with recirculation and in providing a uniform distribution of air As a result they have been supersededby induced-draft designsFill may be splash or film type Splash fill normally consists of horizontal slats in horizontal rows offset to one another to causethe water to break up into droplets as it falls downward through the cooling tower Splash fill is characterized by reduced air pressure losses It is also less conducive to clogging and easier to clean after a spill However it is very sensitive to
inadequate support and must remain horizontal and level It is made of treated wood or plastics such as PVC or polypropyleneFilm fill comes in various designs but they all cause the water to flow in films over the fill surface and provides more coolingcapacity within a given amount of space than splash fill Because of this increased heat transfer efficiency film fill is generallyused in most new cooling towers However because of its smaller passages it is more sensitive to plugging and is difficult toclean if it does become plugged It may not be appropriate for services cooling mainly heavy fuels or waxy lubes where a leakwould cause fill plugging or for services with a high biological fouling potential PVC is currently the most widely used materialfor film fill The selection of fill type is usually proposed by the vendor and reviewed by the COOLING WATER SYSTEMSSPECIALISTSNatural-draft or hyperbolic cooling towers depend on the natural draft created by the difference in the density of the enteringand leaving air for movement Generally hyperbolic cooling towers have been used in Europe and in the utility industry in theUSA for large capacities [250000 gpm (16000 Ls)] short ranges [10 to 15 degF (6 to 8 degC)] and long approaches [10 to 15 degF (6to 8 degC)] They are most effective where low ambient wet bulb temperatures occur with high relative humidities These towersnormally have diameters in excess of 200 ft (60 m) and are greater than 250 ft (75 m) in height Their major advantages are
bull No operating or maintenance cost for fans
bull Reduction in problems associated with fogging drift and recirculation due to their great height and diameterThe primary reasons why hyperbolic towers are not used in ExxonMobil projects include
bull High initial cost
bull Large plot space requirement and high visual impact
bull Complete dependence on atmospheric conditions
bull Lack of flexibility for future expansions
COOLING TOWER BASIN
The primary function of the cooling tower basin is to collect the cooled water leaving the tower and to provide a reservoir for thecooling water pumps Design aspects to be considered are the basins capacity cleanability draining facilities sump andscreen details To obtain the necessary time for corrective action during emergency conditions a minimum storage capacity of 90 minutes based on loss of makeup water flow should be provided in the basin between the basin low level alarm (LLA) andthe minimum pump submergence (MPS) When the basin water level reaches the MPS point loss of recirculation pumpsuction and cooling water flow is imminent Typical values for basin water levels are as follows
bull High Level Alarm (HLA) 1 ft (300 mm) below top of the basin wall
bull Normal Water Level 15 ft (450 mm) below top of the basin wall
bull Low Level Alarm (LLA) 25 ft (760 mm) below top of the basin wall
bull Distance between LLA and MPS 35 ft (1070 mm)
bull Minimum Pump Submergence Depth 45 ft (1370 mm)
bull Clearance of Pump Suction off Basin Floor 15 ft (450 mm) Actual values may vary depending on pump type selected pump suction line diameter and cooling tower cell plot area
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XXVII 10 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
MATERIALS OF CONSTRUCTION
The EMRE Refinery Construction Materials Manual and IP 8-1-1 list acceptable materials for all components in each type of cooling water system (once-through and recirculating) For salt water systems emphasis must be placed on proper materialsselection throughout the system The external salt water environments in the vicinity of the tower itself and the intake stationsmust be taken into consideration Proper siting of the tower is extremely important to avoid salt water corrosion of adjacentunits or the need to use expensive materials to operate in such an environment
WATER TREATMENT FOR ONCE-THROUGH SYSTEMS
Once-through salt and fresh cooling water systems can develop a variety of problems such as fouling due to seaweed or other debris marine microorganisms such as muscles and shell fish and biological growth Corrosion and scaling problems areminimal in fresh water once-through systems because the water is not concentrated by evaporation In salt water systemscorrosion is minimized by proper materials selection and scaling problems generally do not occurProtection against fouling by seaweed or other debris is provided by careful selection of the water intake location and travelingscreens with washing facilities In some cases additional steps may be required These may include an additional secondaryset of traveling screens and possibly battery limit strainers on the cooling water supply lines to individual unitsProtection against fouling by marine microorganisms and biological growths is achieved primarily by injecting chlorinehypochlorite chlorinebromine or other similar chemicals which kill these growths Chlorine is commonly used and is injectedin sufficient quantities to produce a residual of 1 wppm in the effluent for one hour each day (shock chlorination) Continuouschlorination for 2 - 3 days per month at lower levels (03 wppm residual chlorine in the effluent) may also be used in addition toshock chlorinationThe large water volume discharged from once-through systems usually makes chemical treating for corrosion or deposit controltoo costly These problems are handled by proper materials selection and by keeping water velocities through exchanger tubesgreater than 3 fts (09 ms) Occasionally the water analysis of a fresh water source being considered for a once-throughsystem using carbon steel materials will indicate the need for some corrosion or deposit control In this case the cost of thethreshold treatment with the once-through system should be compared to the costs of a recirculated system before making afinal choice
WATER TREATMENT FOR SALT WATER COOLING TOWER SYSTEMS
A total dissolved salts (TDS) concentration limit of 55000 wppm for sea water in cooling tower systems requires high blowdownrates since the makeup sea water typically has from 35000 to 40000 wppm TDS Because of cost restraints the only water treatment normally specified for these systems is for biological control Chlorine is most commonly used either as directchlorine injection or hypochlorite produced in electrochlorinators Care must be taken in selecting the intake location to assurethat the suspended solids do not exceed 200 wppm in the recirculated water If it is not possible to find sea water with sufficientclarity clarification with polyelectrolytes should be evaluated Corrosion is one of the major problems with the use of sea water in cooling towers This requires proper materials selection
WATER TREATMENT FOR FRESH WATER COOLING TOWER SYSTEMS
Cooling tower water treatment is necessary to minimize or eliminate corrosion scale and biological fouling of process heattransfer surfaces caused by minerals and impurities in the water The difficulties caused by these impurities and the means of treatment are shown in Table 1 Water lost by evaporation results in an increase in total dissolved salts in the recirculated water To indicate the degree of concentration of impurities the term cycles of concentration is used
Corrosion is an electrochemical process that deteriorates metals exposed to water in the presence of corrosive agents such asacids oxygen or bacteria A common form of corrosion is pitting Severe corrosion can lead to equipment failure Corrosionis caused by many conditions including process leaks into the cooling water water flow velocity that is too low (causes depositsand fouling lending to corrosion) or too high (causes erosion and corrosion) low pH and high temperatures Corrosion is lesslikely in water with non-acidic pH values (greater than 7) although scaling is more likely in these pH rangesScaling is characterized by the formation of hard dense deposits on material surfaces These deposits impact heat transfer and can become a site for localized under-deposit corrosion Scaling is influenced by many factors including makeup water composition total dissolved solids in the recirculating water water temperature and pH and flow velocity Calcium carbonateis the main scaling constituent in all waters and it is the least soluble It forms from the hardness (calcium Ca ++ ) andbicarbonate (HCO 3
ndash ) alkalinity present in most waterFouling can be caused either by the settlement of suspended matter or by microbiological growth of algae bacterial slime or fungi Fouling will reduce heat transfer cause plugging and create corrosion sites
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COOLING WATER SYSTEMS XXVII 11 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Treatment of the cooling water is broken down into two major categories external and internal External treatmentencompasses any treatment given to the cooling tower makeup water Internal treatment is the addition of chemicals directly to
the recirculated cooling water
EXTERNAL TREATMENT
External treatment to remove impurities in order to reduce the amount of blowdown should always be evaluated as to costeffectiveness The most frequently used techniques for treating makeup water are shown in Figure 11 Details of availabletreatment processes (see Table 1 for methods available) are covered in Section XXVI-A Boiler Feedwater Treatment For cooling tower systems that meet currently required drift limits in refineries and chemical plants uncontrolled cooling water losses (drift and miscellaneous process uses) normally amount to about 14 of the cooling water evaporation loss Under these conditions the maximum cycles of concentration achievable are 8 with no controlled blowdown Some cooling towersare operating at up to 10 cycles of concentration Additional controlled blowdown required to limit the cycles of concentrationper the water quality criteria is covered later
INTERNAL TREATMENT
The makeup water analyses amount of cooling water losses environmental restraints potential chemical hazards materials of construction and film temperatures of heat transfer surfaces determine the type of internal treatment required for cooling tower systems The internal water treatment program selected must address corrosion scale fouling biological growth andchemical attack of cooling tower componentsNumerous chemicals and treatment programs are continuously being developed for addressing the potential problems Theslightly acidic low pH programs of the past which used chromate based chemicals and usually required acid injection arebeing completely phased out because of environmental restrictions on chromate and the higher costs associated with meetingcurrent practices for handling hazardous chemicals such as acid Non-chromate treatments which use inhibitors such asphosphates zinc and organics along with higher pH have been developed to replace the earlier programs Although not aseffective as chromate these inhibitors operate at higher water pH which is a less corrosive environment The disadvantage of the higher pH is the greater tendency to form scale This tendency is controlled by the use of dispersants or scale inhibitorswhich are normally organic polymers or phosphenate A corrosion inhibitor for copper or brass is also normally required if these metals are present in the systemThe following paragraphs describe techniques and chemicals to control alkalinity (pH) corrosion scale sludge and biologicalgrowth The chemicals mentioned are normally purchased as pre-mixes from vendors and dosed in combination Mostcorrosion and scale control program chemicals are two drum formulations due to the incompatibility of some chemicals in theconcentrated state Therefore separate feed systems are generally provided for chemicals for control of corrosion and scale (2drums) and biological growth (if liquid) If a program includes fixed pH then an additional feed system is required to add acidWater treatment programs are site specific and changing The COOLING WATER SYSTEMS SPECIALISTS should beconsulted for recommendations on the internal water treatment programs when an analysis of the available water is known
Alkalinity (pH) Control
Cooling water treatment programs presently available fall into two basic categories those that do not control pH (floating pH or alkaline treatment programs without pH adjustment) and those that control pH within a specified range by adding acid Theprograms that do not control pH are gaining favor because of the additional cost equipment and precautions required tominimize the risk associated with handling acid and because of improvements in chemicals to control depositsHigh pH in the recirculating cooling water is a result of the alkalinity of the fresh water due to bicarbonate salts This combinedwith carbon dioxide results in the water having a pH range of 6 to 85 When this fresh makeup water is mixed with therecirculated cooling water and passed through the cooling tower the air flow strips out the carbon dioxide Bicarbonate ionsthen convert to carbonate ions raising the pH which can cause both calcium carbonate and calcium phosphate salts toprecipitate out of solution and form deposits Another debit in allowing the cooling water pH to increase is increased chlorineconsumption to control biological growthsIf a program using pH control is justified and chosen acid addition is used under control of a pH analyzer to controlrecirculated cooling water alkalinity and pH to the range required by the treatment program
Corrosion Control
To reduce corrosion to an acceptable level chemical corrosion inhibitors which form protective films on heat transfer surfacesare the most effective protection Inhibiting corrosion is accomplished by phosphates organics zinc nitrites and molybdatesalts Unfortunately the use of chromate which is a reliable corrosion inhibitor is prohibited by environmental constraintsNitrite is not practical in an open system due to atmospheric oxygen converting the nitrite to nitrate
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ExxonMobil Proprietary
Section Page
XXVII 12 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Phosphate inhibitors (orthophosphate polyphosphate phosphonate) are the basic treatment for new towers in todaysenvironment In addition to the phosphate chemistry salts of zinc may be added to aid in inhibiting corrosion These ions
combine with the hydroxide ion forming thin protective films Zinc is used to provide a synergistic effect in combination with thephosphate salts for protecting carbon steel surfaces from corrosion Finally all-organic programs are offered by severalmanufacturers The use of these treatments has been made possible by the development of polymer dispersants which givecalcium salts a much higher solubility and reduce the tendency to scaleWhen a cooling water system contains copper alloys azole based and azole substitutes are required as copper corrosioninhibitors There are several effective inhibitors The most common is tolytriazole (TTA) due to its resistance to chlorineThe corrosion inhibitors most frequently used in ExxonMobil recirculating cooling water systems are shown below indescending order of preference provided their use meets all environmental requirements
bull Stabilized Phosphate (OrthoPolyphosphate) - Environmental restrictions on the use of zinc have resulted in ExxonMobilplants in the USA using a blend of orthophosphate and polyphosphate type corrosion inhibitors with good results Thistreatment forms a thin protective calcium phosphate film on heat transfer surfaces through the use of orthophosphateconcentrations as high as 12 - 17 wppm The use of calcium phosphate dispersants and Hydroxethylidene Diphosphonate(HEDP) stabilizes the treatment and prevents the formation of calcium phosphate scale Concentration of the dispersantsdepends upon the calcium concentration temperature and pH Stabilized Phosphate treatment programs can operate in apH range of 68 to 78 Alternatively an alkaline (floating pH) stabilized phosphate treatment program can be used Thisprogram uses little or no acid addition and is gaining in popularity although not well established yet It operates with pHvalues of 8 - 9 and uses higher quantities of dispersants and lower levels of phosphate A corrosion inhibitor for copper and brass is normally included for all stabilized phosphate programs Several vendors offer these programs Twoseparate chemical feeders are generally required
bull Zinc Alkaline Phosphate - Heritage Exxon plants in Europe where the use of zinc is not restricted have obtained goodcontrol of corrosion and scaling using zinc-polyphosphate treatment programs Low (65 - 72) and high (73 - 80) pHprograms have been used The high pH (zinc alkaline phosphate) programs are the lowest cost and are recommended
As with the Stabilized Phosphate treatment dispersants such as HEDP and polyacrylates are required to prevent theformation of zinc or calcium phosphate scale on heat transfer surfaces A corrosion inhibitor for copper or brass isnormally used with this program which is offered by several vendors
bull Organic Salts - All-organic (polymer) programs are offered by several manufacturers and have been used successfullywithin h Exxon In general these treatments are based on a combination of a phosphonate to control corrosion polymers(dispersants) to prevent scale deposition and a corrosion inhibitor for copper or brass This type of program is mosteffective in conditions of high hardness high alkalinity (M alkalinity gt300 wppm as CaCO 3) and high pH values (gt 85)However some manufacturers have reported success in softer water with alternative polymer component formulations Ingeneral an inorganic corrosion control program is preferred to the all-organic programs which are sensitive to alkalinityand more costly
Detailed control limits for effective use of each of the above inhibitors and materials selection are covered in Report NoEE34E86 Environmentally Acceptable Cooling Water Treatments Report No EE102E78 Cooling Tower Water Treatment Guidelines and in the EMRE Refinery Construction Materials Manual
Scale and Sludge Control
Scale and sludge deposits in cooling water systems cause loss of heat transfer and provide the conditions for pitting corrosionScale consists of inorganic salts which have precipitated from the cooling water forming a coating on heat transfer surfacesSludge consists of organic and inorganic suspended matter that settles in quiescent areas and in heat exchangers The key toprevention of both scale and sludge formation in a cooling system is keeping water velocities greater than 3 fts (09 ms) andthe use of chemical additives in combination with a blowdown rate that keeps the impurity concentration below the level whichcauses deposits (See Table 2 for limits on impurities) Scale control has become increasingly important in higher alkalinityldquofloating pHrdquo water treatment programsSalts most commonly found in scale deposits are calcium carbonate calcium sulfate tricalcium phosphate zinc phosphatemagnesium silicate and silica The control method for preventing scale from each of these salts is discussed below
bull Calcium Carbonate - Where justified using pHalkalinity control in the range of 70 to 75 will normally inhibit the formationof calcium carbonate scale if anti-scalent chemicals (phosphonates acrylates copolymers) are part of the chemicaltreatment (See Report No EE102E78 Cooling Tower Water Treatment Guidelines ) Higher pH water treatmentprograms require greater concentrations of these chemicals along with dispersants and flocculants to inhibit the formationof scale and precipitation of other salts
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 13 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
bull Calcium Sulfate - When heat transfer skin temperatures are below 250 degF (120 degC) and the cooling water bulktemperatures are under 130 degF (54 degC) gypsum scale (CaSO 42H 2O) will form if the product of the calcium and sulfate ionsconcentrations each expressed as wppm CaCO 3 equivalent exceeds 500000 Gypsum scale is prevented bymaintaining bulk water temperature below the indicated limits and controlling calcium and sulfate ions through blowdownor through the use of newer dispersants on the market
bull Tri-Calcium and Zinc Phosphate - Almost all chemical additives for corrosion control contain some type of phosphatesalt These have a tendency to form the orthophosphate ion depending on time pH and temperature Cooling watersusing these corrosion inhibitors may have an orthophosphate concentration in excess of 5 wppm as PO 4
= To preventcalcium phosphate scale Table 2 shows a limit of 1000 wppm of calcium as CaCO 3 equivalent when using a phosphateinhibitor in combination with a chemical dispersant (See Report No EE102E78 Cooling Tower Water Treatment Guidelines for solubility of calcium phosphate) These inhibitors and dispersants generally fall into one of the followingcategories
ndash Sequestrants antiprecipitants and inhibitors such as hydroyethylidene diphosphonate (HEDP) phosphate esters andaminomethylene phosphonate (AMP) function by defyingdistorting the crystal structure of the scale formations toprevent nucleation and growth of the scale
ndash Dispersants based on either anionic or nonionic polymers such as polyacrylamide polyacrylate or polymaleic acidfunction to stabilize colloidal and scale-forming particles in suspension Anionic polymers prevent agglomeration of suspended particles by either charge repulsion or crystal distortion while nonionic polymers reduce the surfacetension of particles
bull Silica Deposits - Quantitative indices for determining limits for silica in combination with other ions are not available due tothe formation of complex polymers Instead the rule of thumb limit of keeping reactive silica concentrations in the coolingwater below 150 wppm as SiO 2 has been successful
Possible alternatives for controlling sludge are
bull Chemical Dispersants - Water treating chemical suppliers now offer many types of dispersants These are mostlypolyelectrolytes Claims for removal of slime silt and mud from cooling tower basins and exchangers are madeHowever satisfactory results using these chemicals as a coagulant aid require very accurate control of the amount fed tothe water and stable operation of the water system Also suspended solids should be kept below 200 wppm for particleslarger than 045 microns Dispersants are used in virtually all fresh water cooling tower systems despite their high cost
bull Sidestream Filtration - The use of a sand pressure filter is the most reliable method for sludge removal Operating andmaintenance costs are minimal for this equipment
bull Higher Blowdown Rates - This may be an economical alternative depending on the cost of makeup water Additionalchemical costs must be included in any evaluation of this method
bull Makeup Clarification - This alternative may require additional raw water treatment facilities to handle seasonal variationsin turbidity andor suspended solids This method is applicable only where suspended solids come from makeup water
Biological Control
Operating conditions in recirculated cooling water are ideal for the growth of biological matter Water temperatures from 70 to120 degF (20 to 50 degC) favorable water pH continuous supply of nutrients including carbon nitrogen and phosphorus inorganicsalts and sunlight all lend themselves to environmental conditions encouraging microbiological growth and plant lifeIf biological growths get out of control and form large sticky agglomerations some of the following operating problems willresult
ndash Fouling of heat transfer surfaces by bacterial slimes resulting in flow restrictions and high process temperatures ndash Reduced cooling tower efficiency resulting from algae fungi and bacterial slime growths in the water distribution basin
and fill area of the cooling tower ndash Excessive plugging of screens and filters ndash Corrosion ndash Wood destruction by fungi ndash Clogging of water distribution nozzles ndash Excessive sludge accumulation in the cooling tower basin
An important part of the water treatment program is to control biological activity by feeding a biocide chemical Oxidizingbiocides such as chlorine sodium hypochlorite chlorine dioxide or bromine-associated compounds are most commonly usedwith good results Plate counts should be maintained below 50000 countsml (bacterial)
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ExxonMobil Proprietary
Section Page
XXVII 14 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Chlorine is the most effective biocide for cooling water because of its low cost extensive period of successful use ease of application and minimum environmental effects if applied under controlled conditions Either a liquid feed (sodium
hypochlorite) or a gas feed (100 chlorine) is added to the cooling tower basin A sufficient dosage is that which obtains a 01to 03 wppm continuous free residual in the hot return water to the cooling tower
Advantages to continuous chlorination over shock treatment based on ExxonMobil experience are ndash Minimizes chlorine degradation of polymers ndash Better reduction in total aerobic bacteria plate count resulting in cleaner heat transfer surfaces ndash Does not cause significant pH depressions ndash Better control results with less chlorine consumption
Details concerning chlorine addition as well as the other alternative biocides are given in Report No EE102E78 Cooling Tower Water Treatment Guidelines and in EMRE Engineering Water and Wastewater Design Guide TMEE 080 DG 11-3-1Gas Chlorinators for Water Treating Service and DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) Gaseous chlorine systems have become more costly in recent years because of the additional precautions equipment andprocedures required to minimize the risk associated with handling this gas Alternatives to gaseous chlorine are available butrequire more attention and have a higher operating cost Sodium hypochlorite (either purchased or generated) and sodiumbromide stabilized solid halide (chlorinebromine) products chlorine dioxide and ozone are all alternatives which may haveapplications Contact the COOLING WATER SYSTEMS SPECIALISTS for guidance in the use of these alternative treatmentschemes
BLOWDOWN
Blowdown control is required to maintain the proper cycles of concentration in the system The most reliable and simplest typeof control for blowdown is specific conductance measurement of the cooling water Electrical signals from the specificconductance measurements of some impurity in the cooling water are used to actuate an automatic blowdown valveImpurities normally used for blowdown control are dissolved solids chloride calcium magnesium and silica
CONTROLS AND MONITORS
Any chemical treatment used to control problems in a cooling water system must be carefully monitored to be completelyeffective
The installation of automatic controls should be considered for recirculated cooling water systems in all plants Designs shouldinclude pH conductivity corrosivity inhibitor and biocide control Automatic controllers are available which continuouslymonitor the condition of the water and automatically make chemical additions to keep the water within compositional limitsTest exchangers and corrosion coupons are also useful in monitoring the effectiveness of the water treatment program
An oil-in-water monitor should be considered to provide early detection of oil leaks into the cooling water This will enablecorrective action to be taken before extensive contamination of the cooling water circuit occurs This is especially important incooling towers with high efficiency cellular fill which can be difficult to clean if significantly fouledManual control systems may be used in locations that have adequate technical personnel to perform routine water analysesand to modify chemical treatment necessitated by sudden variations in water composition process contamination etc
WASTEWATER REUSE POSSIBILITIES
Recirculating cooling water systems normally have the highest plant raw water demand A potential alternative is to use treatedwastewater to supply part or all of the makeup water requirement The treatment required must be determined but will typicallyinclude primary oil removal (eg API Separator) secondary oil and suspended solids removal (eg filtration dissolved or induced air flotation) and soluble carbon removal utilizing biological treatmentThe reuse of treated wastewater as makeup currently exists at only a few locations If this alternative is being considered refer to Report No EE58E85 Reuse of Wastewater as Cooling Tower Makeup and Section XIX-B Water Reuse
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 15 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS
STEP 1 - Select a location for the intake station and pumps
Factors which influence this selection are as followsbull Depth of water and tidal variations
bull Relative location of intake and outfall direction of current
bull Length and size of intake suction piping vs pump discharge piping
STEP 2 - Determine cooling water requirements
Determine the design cold water inlet temperature based on site data covering seasonal water temperature as a function of inlet location and depth (for existing locations refer to Table 2 in Section IX-B )Tabulate the cooling water requirements for each process unit and offsite consumer Apply the proper LGFs to all coolingloads depending on the stage of the project and whether the new units are exact duplicates of existing unitsIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The designer should investigate methods to increase effluent water temperature to the maximumrecommended limit for the particular type of water used One common method is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 3 - Determine the total required capacity for water intake facilities
Process Coolers - The total requirements determined in Step 2 including LGFsFirewater - This is often supplied from the intake bay of a once-through systemFuture Expansions - These should be considered in sizing intake facilitiesTotal Required Capacity - The sum of the above three items
STEP 4 - Provide water intake facilities
Intake Crib - The intake crib with its support structure is located at the extreme end of the water intake line (See Figure 4 )The intake is usually located 200 to 300 ft (60 to 90 m) offshore and at a sufficient depth below the lowest water level to
preclude vortex formation and subsequent pump cavitation during storm conditions Additionally the crib should have 3 to 5 ft(1 to 15 m) clearance above the sea floor to minimize sand and silt pickup A concrete velocity cap should be specified to cover the inlet crib This will minimize the incidence of fish and other foreignmatter entering the system A horizontal inlet flow pattern should be designed with a maximum entrance velocity of 1 fts(03 ms) Experience shows that vertical velocity flow confuses fish and they are drawn into the intake lineThe intake crib support structure protects the crib from adverse effects of storms and tidal motions as well as protecting it fromimpact of floating and submerged objects A self-cleaning screen can be specified to prevent entrance of large objects into thesystem at the cribIntake Line (Figure 5 ) - An intake line is required if the water depth along the shore does not permit water to flow directly intothe intake bay The intake line is usually buried in the sea floor and covered with a minimum required overburden Consultwith the COOLING WATER SYSTEMS SPECIALISTS for required coverage at a specific siteChlorine or sodium hypochlorite is injected at the intake crib to prevent marine growth in the lineNote that an intake crib and line will not be required if water of sufficient depth is available at the supply pump stationIntake Bay (Figure 6 ) - The intake bay is an enclosure which houses the pumps and screens Depending on a particular design a series of screens is used to ensure the supply of clean water Trash racks comprise the first element of the filter system These remove only very coarse material from the water such as bulk seaweed They can be supplied with atraversing raking mechanismLift screens provide secondary cleanup These screens can be lifted out of the intake bay for periodic cleaning Double liftscreens can be used if it is deemed necessary to have one in service at all timesFinal water filtering is usually done by traveling screens Details of the screen dimensions and the mounting are usuallydetermined during detailed engineering Screen net open area is typically approximately 50 of total screen area Thescreens should be sized for a normal water velocity of 12 fts (04 ms) through the open screen area Maximum velocityshould be limited to 24 fts (08 ms) with 50 of the screen area plugged This is the upper velocity limit for efficientscreening
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ExxonMobil Proprietary
Section Page
XXVII 16 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
If high suspended solids greater than 200 mesh (eg sand) are expected to be a continuous problem ejectors may be installedupstream and downstream of the traveling screen to remove accumulated sand and solids from the intake bay in these areas
Traveling screens and ejectors are supplied with wash water from the raw water pumps or from separate wash water pumpsOnoff control using timers should allow optimum scheduling of the ejectors depending on the time of the year
STEP 5 - Specify water intake facilities
Water intake facilities should be designed in accordance with the following guidelines from the Hydraulic Institute Standards The water intake should be designed to provide an evenly distributed flow of water to the pump suction A properly designedintake reduces the possibility of vortex formation and results in better pump performance with less noise Geometry of theintake and the manner in which water is introduced into the intake are important parameters in providing an even distribution of water flow Complete analysis of intake structures is best accomplished by scale model testsThe pump manufacturer will generally render advice on specific problems while the intake design is still preliminary He shouldbe provided with the necessary intake layout drawings showing the physical limitations of the siteFigures 7A and 7B have been developed for single- and multiple-pump arrangements to show suggestions for basic sumpdimensions They are for pumps normally operating in the capacity range of approximately 3000 to 300000 gpm (200 to20000 Ls) Since these values are composite averages from a great many pump types and cover the entire range of specificspeeds they must not be thought of as absolute values but rather as basic guides subject to some possible variations
All the dimensions in Figures 7A and 7B are based on the rated capacity of the pump at the design head Any increase incapacity above these values should be momentary or very limited in time If operation at an increased capacity is to beundertaken for considerable periods of time the maximum capacity should be used for the design value in obtaining sumpdimensionsDimension C is an average value based on an analysis of many pumps Its final value should be specified by the pumpmanufacturerDimension B is a suggested maximum dimension which may be less depending on actual suction bell or bowl diameters in useby the pump manufacturer The edge of the bell should be close to the back wall of the sump When the position of the backwall is determined by the driving equipment or the discharge piping Dimension B may become excessive and a ldquofalserdquo backwall should be installedDimension S is a minimum value for the sump width for a single pump installation This dimension can be increased but if it isto be made smaller the manufacturer should be consulted or a sump model test should be run to determine its adequacy
Motor size may dictate minimum pump spacing be especially aware of this when specifying pumps with unusually high pumpheads eg greater than 80 psig (550 kPa)Dimension H is a minimum value based on the ldquonormal low water levelrdquo at the pump suction bell taking into considerationfriction losses through the inlet screen and approach channel This dimension can be considerably less momentarily or infrequently without excessive damage to the pump It should be remembered however that this does not representldquosubmergencerdquo Submergence is normally quoted as dimension ldquoHrdquo minus ldquoCrdquo This represents the physical height of water level above the bottom of the suction inletThe actual submergence of the pump is something less than this since the impeller eye is some distance above the bottom of the suction bell possibly as much as 3 to 4 ft (1 to 13 m) For the purposes of sump design in connection with this chart it isunderstood that the pump has been selected in accordance with specific speed charts The submergence referred to hereinhas to do only with vortexing and eddy formationsDimensions Y and A are recommended minimum values These dimensions can be as large as desired but should be limitedto the restrictions indicated on the curve If the design does not include a screen Dimension A should be considerably longerThe screen or gate widths should not be substantially less than S and heights should not be less than H If the main stream
velocity is more than 2 fts (06 ms) it may be necessary to construct straightening vanes in the approach channel increaseDimension A conduct a sump model test of the installation or work out some combination of these factorsDimension S becomes the width of an individual pump cell or the center-to-center distance of two pumps if no division walls areusedOn multiple pump installations the recommended dimensions in Figures 7A and 7B also apply as noted above and thefollowing additional determinants should be consideredFigure 8A - Low velocity and straight-line flow to all units simultaneously is the first recommended style of pit Velocities in thepump area should be approximately 1 fts (03 ms) Some pumps with velocities of 2 fts (06 ms) and higher have given goodresults This is particularly true where the design resulted from a model study Not recommended would be an abrupt changein size of inlet pipe to sump or inlet from one side introducing eddying
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 17 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Figure 8B - A number of pumps in the same sump will operate best without separating walls unless all pumps are always inoperation at the same time in which case the use of separating walls may be beneficial If walls must be used for structural
purposes and pumps will operate intermittently one should leave flow space behind each wall from the pit floor up to at leastthe minimum water level and the wall should not extend upstream beyond the rim of the suction bell If walls are used increaseDimension ldquoSrdquo by the thickness of the wall for correct centerline spacing Round off the ends of the walls Not recommended isthe placement of a number of pumps around the edge of a sump with or without dividing wallsFigure 8C - Abrupt changes in size from inlet pipe or channel to pump bay are not desirable A relatively small pipe emptyinginto a large pump pit should connect to the pit with a gradually increasing taper section The angle should be as large aspossible preferably not less than 45 degrees With this arrangement pit velocities much less than one foot per second aredesirable Especially not recommended is a small pipe directly connected to a large pit with pumps close to the inlet Flow willhave excessive change of direction to get to most of the pumps Centering pumps in the pit leave large ldquovortex areasrdquo behindthe pumps which will cause operational troubleFigure 8D - If the pit velocity can be kept low enough [below 1 fts (03 ms)] an abrupt change from inlet pipe to pit can beaccommodated if the length equals or exceeds the values shown It is assumed that as ratio WP increases the inlet velocityat ldquoPrdquo will increase up to an allowed maximum of 8 fts (24 ms) at WP = 10 Pumps ldquoin linerdquo are not recommended unless theratio of pit to pump size is quite large and pumps are separated longitudinally by a generous margin A pit can generally be
constructed at much less cost by use of a recommended designFigure 8E - It is sometimes desirable to install pumps in tunnels or pipelines A drop pipe or false well to house the pump witha vaned inlet well facing upstream will be satisfactory in flows up to 8 fts (24 ms) Without the inlet well the pump section bellshould be positioned at least two pipe diameters vertically above the top of the tunnel not hung into the tunnel flow especiallywith tunnel velocities 2 fts (06 ms) or more There should be no signs of air along the top of the tunnel It may be necessaryto lower the scoop or to insist on a minimum water level in the vertical wellNote The foregoing statements apply to sumps for clear liquid
STEP 6 - Provide chemical treatment
Provide chemical treatment facilities for the once-through cooling water system Normally the only chemical treatment requiredin a once-through cooling system is for biological control Facilities for adding chlorine to once-through cooling water are thesame as those described for the recirculated cooling water system The design capacity of the chlorinator should be based onachieving a chlorine residual of 1 wppm in the effluent The chlorine required by the organics and other reducing agents in thewater must be satisfied before any residual chlorine will appear in the effluent The design capacity of the chlorinator should be
based on the total water chlorine demand plus 1 wppm in the effluent In the absence of water data the chlorination systemdesign capacity should be based on 10 wppm minimum instantaneous dosage for maximum cooling water flow For very largeonce-through cooling water systems it will be impractical to design for shock rates at this level In this case a continuousdosage of 1 - 2 wppm should be used (Refer to Step 5 of the DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS )
STEP 7 - Specify pumps and distribution system
Specify other major facilities in the once-through cooling water system The recommended materials for pumps valves andpiping are shown in the EMRErsquos Refinery Construction Materials Manual
Pumps1 Type - These pumps are usually vertical centrifugal pumps However horizontal centrifugal pumps have been used An
economic study andor consideration of existing facilities will determine which type should be specified In the event thathorizontal centrifugal pumps are specified facilities for pump priming will likely be required For large pumps in salt water service there is a very limited selection of pumps because of the special materials required Consult with the DISCIPLINESPECIALISTS early in the design effort to determine what specific pumps are currently available
2 Total Required Capacity - Firm capacity (excluding spares) of the supply pumps should equal the maximum cooling water requirements including LGF and RCF
3 NumberIndividual Pump Capacity - Normally one spare pump is provided for up to six operating pumps two spares for up to twelve operating pumps etc The most common arrangements for water supply pumps are
bull Two 100 capacity pumps
bull Three 50 capacity pumps
bull Four 33-13 capacity pumps
bull Five 25 capacity pumps
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ExxonMobil Proprietary
Section Page
XXVII 18 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Maximum commercially available pump capacity ranges up to 300000 gpm (20000 Ls) at discharge pressures up to 100psi (700 kPa) Maximum capacity will be less for higher discharge pressures Other factors which may influence pump
selection arebull Selecting capacity to match existing pumps
bull Selecting capacity for future requirements
bull Selecting capacity corresponding to manufacturers available standardVendor information should be obtained on the availability size and cost of the pumps for several configurations Acomparison should be made of the investments and operating costs as well as other advantages and disadvantages toselect a pump configuration
4 Drivers - These pumps are usually motor-driven Because of the remote location of the intake facilities the steamdistribution costs are normally too high to justify the use of a steam turbine driver However use of a secondary selectivesubstation provides a reliable source of power to the pump drivers
5 Pump ∆∆∆∆P - Pump ∆P is based on supplying the maximum cooling water requirement at the battery limit design pressureestablished for the system The ∆P should include the total circulating system pressure drop plus any elevation differencebetween the low water level at the intake and the discharge point of contact with air
6 Pump Instrumentation - The remote location of the intake facilities makes remote control and monitoring of the water supply operation desirable Pump controls located in the control house provide for minimum manning Local controlbackup should be provided The Owners operating and manning philosophy determine the basis for instrumentation andcontrolThe spare pump is controlled to automatically start on low pressure in the supply header Concurrently alarms should beactuated in the control room to indicate the low-pressure situation and spare pump operation
Distribution Network1 Configuration - The piping network is a once-through system with a return line from each consumer or consumer group to
the cooling water effluent line2 Layout - The routing of the piping network should connect all cooling water consumers in the most economical manner
permitted by the plant layout Careful consideration should be given to the turnaround philosophy of the plant Thenetwork layout should preclude the necessity of having above-ground water lines that are in service running through anarea that is being turned around Underground piping or an extra lateral may be required to accomplish this objective
3 Valves and Blinds - Battery limit valves should be provided in the network to isolate individual process units from thesystem Valves should be provided to permit onstream removal and maintenance of spared equipment
4 Basket Strainers - Consideration should be given to the installation of basket strainers in the laterals serving each of theconsumer loops These provide added insurance against entrance of foreign material into the cooling equipment The useof intake screens with 025 in (63 mm) openings may eliminate the need for basket strainers in the laterals
Line Sizing1 Design Basis - The network is sized so that water is delivered to the most distant consumer at a pressure equal to the
required process design (battery limit) pressure2 Flow Rate - Each segment of the distribution network is sized for the simultaneous maximum flow in that segment for any
operating condition The main supply and discharge headers are sized for the simultaneous maximum flow of all segments(not necessarily the sum of the simultaneous maximum flows of the individual segments)
3 Pressure Drop
Line Sizing Basis - The distribution system should be sized according to the guidelines established in Section XIV The use of an ldquoold piperdquo friction factor of C = 90 - 100 (Hazen-Williams factors) in the main lines is recommended to assuregood operation in later years when the system experiences fouling For large diameter lines [greater than 48 in (1200mm)] maximum water velocity should be limited to 10 fts (3 ms) based on civilmechanical considerations (supports andanchoring required to handle potential pressure surges) For salt water systems utilizing cement-lined pipe consider thereduced inner diameter in the friction loss calculations A study may be necessary to determine the economic pump ∆P vsline diameters
4 Equivalent Length Factors - Equivalent length factors are used in pressure drop calculations for sizing the distributionsystem The piping length scaled from the plot plan is multiplied by an equivalent length factor to compensate for pipingelbows expansion loops and other piping variations
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COOLING WATER SYSTEMS XXVII 19 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
The equivalent length factors are selected based on the expected variation from a straight piping run Typical equivalentlength factors suitable for planning purposes for different piping applications are as follows
TYPE OF PIPING EQUIVALENT LENGTH FACTORS
Long piping runs [gt 1000 ft (300 m)] 12 to 14
Onsite distribution piping 15 to 20
For design purposes the actual piping layout should be evaluated to ensure that the equivalent length factors areapplicable to the specific situation Particular attention is required for revamp work and saltwater systems utilizing cement-lined pipe
STEP 8 - Prepare a system flow plan similar to the one shown in Figure 1
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS
STEP 1 - Determine cooling water requirements
Tabulate the cooling water requirements and cooling water discharge temperature for each process unit and offsite consumer Apply the proper LGFs to all cooling loads depending on the stage of the project and whether the new units are exactduplicates of existing units Determine the total cooling water flow required including LGFs and the bulk temperature (T 1) of thecombined cooling water flow from all coolers to the cooling towerIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The onsite and offsite designers should investigate methods to increase effluent water temperature to themaximum recommended limit for the particular type of water used This will result in a lower cost cooling tower for a given heatrejection rate One common method of maximizing return water temperature is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 2 - Select cooling tower design parameters
Select the proper design conditions of ambient wet bulb temperature approach temperature range and flow rate for specifyingthe cooling tower Based on these conditions the cooling tower vendor will size the tower and provide the tower characteristicand performance curves which serve as the basis for the performance guarantee Required performance test and evaluationprocedures are described in IP 8-1-1 Economic studies may be required to select the ambient wet bulb temperature andapproach temperature for new locations or for expansion projects with special cooling requirements
STEP 3 - Prepare cooling tower specification sheet
Include the following information in the heat exchange section of the design specificationTower Flow Capacity - This was determined in Step 1Type of Tower - The tower usually specified is an induced draft multicell type suitable for fresh salt or brackish water depending on the water available Additional tower characteristics are usually developed and proposed by the tower vendor (eg crossflow versus counterflow type of tower fill etc) The vendor should be made aware of conditions such as thelikelihood of heavy oil leakage into the cooling water or high fouling potential which may influence the tower type or fill
selectionNumber of Cells - The minimum number of cells required (normally not less than 2) may be specified The maximum flow per cell is a function of the cooling tower design conditions selected in Step 2 Typical values range up to 10000 gpm (600 Ls) for crossflow designs and 8000 gpm (500 Ls) for counterflow designs Normally spare cells are not provided Maintenance iscarried out during cooler periods or load is shed
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XXVII 20 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Ambient Wet Bulb Temperature1 The ambient wet bulb temperature selected for specifying the cooling tower design is generally that temperature which is
equaled or exceeded 5 of the time during the four hottest months (5 of the hourly readings taken during the four hottestmonths will exceed this value) Specifying the 5 temperature should give satisfactory results since the time that the wetbulb temperature is exceeded is for short duration and the system inertia is usually sufficient to avoid problems The tower specification should make it clear that the wet bulb temperature specified is an ambient value as opposed to the enteringvalue which may be higher because of recirculation The tower manufacturer should adjust the ambient value upward totake into account his estimates of recirculation Currently cooling tower test codes provide procedures for measuringperformance in the case of either entering or ambient wet-bulb temperature specifications An increase of 2 degF (11 degC) isgenerally added to ambient wet bulb temperature to estimate entering wet bulb temperature for large cooling towers
2 If plant production will peak during the four summer months when the 5 ambient wet bulb temperature may be exceededconsideration should be given to specifying a higher design ambient wet bulb temperature (one that will be exceeded 212 of the time for example) However the incremental cost of the resulting closer approach and larger cooling tower must be justified by the value of the incremental products made
On-Tower Temperature (T1) - This temperature cannot be accurately determined until the design of all process coolers iscompleted and the volume of cooling water and outlet temperature are known for each (see Step 1) In the planning stage or early design stage of a project this temperature must be estimated and is usually set equal to the maximum design cooler outlet temperature shown under COOLING WATER REQUIREMENTS in the BASIC DESIGN CONSIDERATIONS portion of this sectionApproach Temperature - The approach temperature has the most pronounced effect on cooling tower size and cost For example reducing the approach from 10 to 5 degF (55 to 3 degC) can increase the cooling tower investment by 50 percent and thefan operating cost by 65 percent Thus various approach temperatures should be evaluated to determine the minimum cost of the total system including the process exchanger area Approaches used in the past designs have been from 5 to 12 degF (3 to7degC) Note that it is not customary in the cooling tower industry to guarantee approaches of less than 5 degF to (3 degC) to coolingtower design inlet wet bulb temperature A typical approach for a new grass roots cooling tower is 10 degF (6 degC)Off-Tower Temperature (T2) - This is the sum of the approach temperature and the ambient air wet bulb temperature For existing locations Table 2 in Section IX-B may be used
Range and Flow Rate1 The range for a given heat load is related to the circulating water rate as follows
Q = M C p ∆Tcong (500) (gpm) (range) Eq (1)
where Q = Heat load on the tower BtuhM = Water circulating rate lbhCp = Specific heat capacity of water at average temperature Btulb degF∆T = (T 1 ndash T 2) degF = (water cooling range)gpm = Circulation rate in US gallons per minute
For calculation in Metric units
Q = M C p ∆Tcong (1162 x 10 -3) (Ls) (range) Eq (1M)
where Q = Heat load on the tower kJh
M = Water circulating rate kghCp = Specific heat capacity of water at average temperature kJkg degC∆T = (T 1 ndash T 2) degC = (water cooling range)Ls = Circulation rate in liters per second
2 Generally lower tower investment fan horsepower and pump horsepower result from maximizing the range andminimizing the flow rate However based on fouling considerations of process coolers the maximum allowable hot water temperatures must be kept below recommended limits Also while increasing the range minimizes cooling tower investment and operating cost the cost of exchanger surface requirements is adversely affected by the LMTD and acompromise must be made The range is bounded by the maximum allowable hot water temperature and the Off-Tower temperature (T 2) For established locations refer to Section IX-B for design cold water temperatures to assist in selectingthe approach and range
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COOLING WATER SYSTEMS XXVII 21 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Makeup Water Requirements - Calculate the various losses in the cooling water system to determine makeup requirementsThese losses include
1 Evaporation Loss (E) - Allow 1 of the circulation rate for each 10 degF (55 degC) temperature drop through the tower Notethat the actual evaporation loss will be somewhat less depending on ambient air temperature
( )sL555
TT)gpm(
1000TT
E 2121
minus=
minus= Eq (2)
2 Drift Loss (D) - IP 8-1-1 limits the maximum allowable drift loss to 0005 of the design water circulation rate for freshbrackish or salt water
( )sL1000050
)gpm(1000050
D
=
= Eq (3)
This low drift level contributes little to solids removal from the tower with the spray This allows higher cycles of concentration to be achieved in cooling towers which use clarified softened water for makeup and thus have low requiredblowdown rates Higher drift loss percentages in older or poorly maintained towers will limit the maximum cycles of
concentration achievable3 Miscellaneous Losses (O) - Miscellaneous losses are estimated to be one-seventh of the difference between evaporation
loss and drift loss
minus=
7DE
O Eq (4)
4 Blowdown Rate (B) ndash Guidelines for maximum concentrations of impurities allowed in recirculated fresh cooling water areshown in Table 2 For salt water the blowdown is based on keeping the total dissolved salts below 55000 wppm Obtainthe chemical analysis of the makeup water and divide each individual component concentration into its counterpart inTable 2 The result is the maximum allowable cycles of concentration for that constituent (a worksheet is provided inTable 4 ) The impurity with the lowest factor will be the control and will determine the required blowdown rate
)OD(1Cycles
EB +minus
minus= Eq (5)
The makeup water requirement is the sum of all losses shown above
MakeUp = E + D + O + B Eq (6)
STEP 4 - Specify the controls necessary for proper cooling tower operation
Air Side - Cooling towers are provided with motor-driven manually adjusted pitch multiple-bladed propeller fans Figures9A amp B may be used to estimate the normal operating horsepower requirements Since the fan motor and gear reducer arelocated at the top of the tower [generally 40 to 50 ft (12 to 15 m) above grade] the need for equipment to handle thesecomponents should be discussed with the affiliateControl of the air flow through the tower to maintain reasonably constant cold water temperatures can be accomplished byturning off individual fans in multiple-cell installations adjusting the blade pitch or using variable speed motors To turn off individual fans remote at grade start-stop buttons for the motors are usually specified
Whenever the ambient air temperature is 32 degF (0 degC) or below icing of the tower louvers and structural members in contact withthe incoming air will occur If the heat load on the tower is relatively free of seasonal variations two-speed fans should provideadequate ice control down to approximately 0 degF (-18 degC) ambient temperatures When sustained temperatures of 0 degF (-18 degC)and below occur two-speed fans capable of two-speed reverse operation with appropriate time delays for change of directionand speed should be specified to periodically eliminate ice which will build up on the louvers Other methods of winterizationare available but in general are more expensive and require greater operator attention In very cold climates steam heatingshould be provided to prevent freezing in the cold water basin during shutdown periodsWater Side - The water circulation rate is maintained by operating one or more pumps of sufficient capacity to meet the systemdemand Flow-measuring facilities local temperature indicators and a high-temperature alarm are provided on the coolingtower water supply and return headers Hand-operated flow control valves are required for distributing the water equally to theindividual cells On counterflow towers provisions for measuring the flow to each cell (pitot tubes) are required They are notrequired on crossflow towers since the open distribution basin provides visual aid in balancing flows Shutoff valves should beprovided for servicing the flow control valves without shutting down the entire tower
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XXVII 22 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
STEP 5 - Provide chemical treatment facilities for the fresh water system
Alkalinity (pH) Control (if used)1 In cases where a controlled pH corrosionscale control program is being used the alkalinity (pH) of the recirculated cooling
water should be controlled in accordance with the program requirements Normally acid addition is required to accomplishthis This is due to free carbon dioxide being stripped from the cooling water in the tower to approximately 5 ppm For example if the alkalinity has to be controlled to between 25 and 50 ppm for a pH of 70 to 75 the volume of 98 sulfuricacid (66 degBeacute) required to treat the makeup water is calculated using the following equation
[Note 93 sulfuric acid should be used if ambient temperatures will be less than 35 degF (2 degC)]
GallonsHour 66 degBeacute H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate gpm) (326 x 10 -5)
For calculation in Metric limits
kgh of 100 (66 degBeacute) H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate Ls) (793 x 10 -3)
2 The metering pump should be designed for a maximum feed rate of two to three times the normal feed rate A glass woolfilter is required on the pump suction piping to minimize plugging Normally two - 100 capacity pumps are provided
Positive displacement metering pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumps with flow control areused for higher rates
3 Hydrochloric acid may also be used for alkalinity control The gallons per hour of 20 degBeacute hydrochloric acid required is36 times the amount of 66 degBeacute sulfuric acid In Metric units the weight of 32 (20 degBeacute) hydrochloric acid required is227 times the weight of 100 (66 degBeacute) sulfuric acid
4 Specifications for chemical feeders may be found in EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased)
5 A pH analyzer with high and low alarm is provided to control the amount of acid feed The acid should be added to thecooling tower makeup water (Figure 2 ) The lag time between the point of acid injection and the sample outlet to the pHcell should be less than two minutes for effective control
6 Spills or overflows from the sulfuric acid tank should be contained
Corrosion Inhibitors and Anti-Scalents1 Two chemical feeders are generally provided The following guides in sizing the equipment will allow the use of any of the
proprietary products from the vendors when final selection by the owner is made See Report No EE102E78 Cooling Tower Water Treatment Guidelines EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and Report No EE34E86 Environmentally - Acceptable Cooling Water Treatments for detailed discussion of acceptable commercial chemicals
bull If the water treatment program to be used is known design for the chemical rates specified by the vendor or COOLING WATER SYSTEMS SPECIALIST
bull Otherwise design for 100 wppm of each commercial liquid product in the recirculated cooling water The design feedrate should be based on the design total blowdown flow (both uncontrolled and controlled) since this is where theproduct is lost For 100 wppm concentration the chemical rates will be as follows
GallonsHour Chemical = Blowdown (gpm) x (0006)
LitersHour Chemical = Blowdown (Ls) x (036)
2 The design capacity of the metering pump should be two to three times this calculated number Normally 2 - 100capacity pumps are provided Positive displacement pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumpswith flow control are used for higher rates
3 Specifications for chemical feeders may be found in EMRErsquos Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased) Reference should also be made to the checklists containedin Report No EE92E94 Guidelines for Safety Evaluation of Chemical Injection Facilities
4 A final check should be made with the owner and the COOLING WATER SYSTEMS SPECIALISTS as to the treatmentprogram selected and if there is a need for an additional chemical feeder
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COOLING WATER SYSTEMS XXVII 23 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Biological ControlFacilities should be provided for feeding the biocide into the back of the cooling tower basin If chlorine is used it must bepresent in the cooling water as free residual to be effective However the following amounts of chlorine are first required toreact with the listed impurities when present in the cooling water before a free residual can be established
IMPURITIESAMOUNT OF CHLORINE REQUIRED (wppm)
WITH EACH WPPM OF IMPURITY
Ammonia (NH 3)
Hydrogen Sulfide (H 2S)
Sodium Sulfite (Na 2SO 3)
Ferrous Iron (Fe ++)
10
22
056
064
The design capacity of the chlorine feeder shall be based on achieving a 01 wppm free chlorine residual in the recirculatingwater flow or a minimum dosage of 7 wppm at maximum design makeup water flow rate and conditions (see EMRE Water and
Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service )Chlorine may be fed either as the commercial liquid sodium hypochlorite (124 wt Cl 2) or liquid chlorine (100 Cl 2) Liquidhypochlorite is being used increasingly because of the higher costs associated with precautions equipment and proceduresrequired to minimize the risks associated with handling gaseous chlorine The hypochlorite solution is fed using a meteringpump which should be sized to feed an equivalent of 15 ppm 100 Cl 2 (Refer to EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) ) In case of spills or an overflow from thestorage tank the sodium hypochlorite may be sent to the cooling tower basinGas chlorinators are covered by EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service The use of gas chlorination is being reduced but may still be appropriate in some locations subject tolocal management approval The gas from either commercial cylinders or an evaporator is dissolved in a water slip stream bymeans of an eductor This results in a chlorine solution strength of approximately 01 This solution should be evenlydistributed below the water surface by means of a header lateral pipe arrangement at the rear of the cooling tower basinopposite the cooling water pumpsSome additional guides on the use of gas chlorinators are given below It is also recommended that the safety guidelines
published by the Manufacturing Chemists Association and the Chlorine Institute be followedbull Dilution water for the chlorinator must not be less than 50 degF (10 degC) or ice formation may cause plugging problems
bull Continuous rates of chlorine gas withdrawal should not exceed 400 lb (180 kg)24 hours from each one ton cylinder atroom temperature [70 degF (21 degC)] This rate may be exceeded (up to 50) for periods not exceeding 2 hours Rule of thumb is not to design for more than 4 cylinders operating in parallel When feed rate exceeds 3000 lb (1360 kg) per daythe liquid chlorine should be fed to an evaporator upstream of the chlorinator
bull If the ambient temperature can be expected to be below 50 degF (10 degC) for extended periods a building should be providedfor the chlorinator and cylinders
bull Weighing scales should be provided to determine when shipping containers should be replaced and to verify chlorine feedrates
bull Analyzers to measure the free chlorine residual in the circulated cooling water should be provided
bull Chlorine leak detectors and a catch basin for major spills should be provided
Control SystemProvide automatic controllers to continuously monitor water condition and to add chemicals to the system (complete systemsmay be available through the water treatment program vendor as a lease or purchase)pH Control - This controller adds acid to correct water alkalinity a timer on the acid pump sounds an alarm if the pump doesnot shut off in a predetermined length of time Backup controls are required to prevent the cooling system from beingpermanently damaged in case the pH electrodes become fouledCorrosivity and Inhibitor Control - Systems are available which give a direct readout of corrosion rate in milsyear If thecorrosion rate exceeds a set point the controller shuts off the acid increases the inhibitor addition opens the blowdown valveand sounds an alarm Electrodes measure the completeness of the inhibitor film by the value of the potential differencebetween the electrodes The larger the difference the more incomplete the film and a potential reading exceeding a set levelactivates the inhibitor feed
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XXVII 24 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Free Residual Chlorine Control - Free residual chlorine should be controlled automatically for biocide treatment Severalgood analyzers are currently available These units can be incorporated into an automatic control system
Oil in Water DetectionMonitoring Corrosion and Deposit FormationThe following types of devices for measuring corrosion rates and deposit formation should be provided to allow the plant tomonitor the effectiveness of the cooling water treatment before scheduled turnaroundsCoupons - A corrosion coupon rack should be specified for the hot return water to the tower in accordance with IP 19-6-1 Corrosion Monitor - On-line corrosion analyzer if specified should monitor the hot return water to the tower in accordance withIP 19-6-1
On-Line Test Exchanger Although there are a variety of deposit monitors from the water treatment chemical vendors they do not necessarily representactual heat transfer conditions in process exchangers For this reason a test exchanger should be installed for evaluatingcorrosion and deposit formation Details are covered in Report No EE102E78 Cooling Tower Water Treatment Guidelines
Alternatively a critical process exchanger can be instrumented such that its heat transfer coefficient can be monitored
Instrumentation would include flows and inlet and outlet temperatures of process fluid and cooling waterSTEP 6 - Specify cooling tower location
Select a location for the cooling tower The following factors influence this selection
Location Factors1 The location should be within a reasonable distance of the units served and consistent with future refinery plans Piping
costs to and from battery limits can be a significant portion of the total cooling system cost and can be the location-determining factor particularly for salt water systems While piping costs can be easily determined other costs affectinglocation cannot be estimated readily Therefore judgment is required in selecting the location
2 Safety spacing requirements (see Section XV-G Equipment Spacing )3 Cooling tower contribution to plant boundary noise4 For salt water cooling towers wind direction and distance to other facilities is of major concern because of potential
corrosion problems
5 Factors affecting cooling tower performance ie orientation with respect to wind site congestion etc
Plot RequirementsIn estimating plot requirements (basin area) one should allow 5 to 6 ft of tower length for each 1000 gpm (2 m for each 60 Ls)of circulating capacity This figure is based on a typical counterflow design with a width of 50 ft (12 m) and does not includepump suction pit space Exact requirements can be obtained from cooling tower vendors after the tower capacity has beenestimated
Fogging and DriftTwo distinct problems are associated with the discharge air from a cooling tower (1) fogging which can be a nuisance or ahazard and (2) drift which is generally a nuisance because of increased corrosion of surrounding equipment Fogging fromcooling towers occurs during periods of combined high ambient humidity and low temperature as a result of mixing the warmmoist cooling tower exhaust air with the cooler ambient air The ambient air cannot absorb all moisture as vapor and thus tinyfog droplets are formed by condensation Fogging and resulting icing in cold climates has produced hazardous drivingconditions on highways up to 850 ft (250 m) awayProprietary designs are available to combat potential fogging problems Basically these designs incorporate heating of thedischarge air with hot return water precooling coils (similar to an air cooler) to eliminate any visible plume However thesedesigns are expensive and relatively unproven Therefore proper location based on reliable weather data is the preferredsolutionDrift from a cooling tower represents a more difficult problem since it does not depend on definable climatic conditions andoccurs for all wind directions The effects of drift on nearby equipment and vegetation are particularly severe and generally of concern only for salt water systems because of the high solids content of the drift (concentrated salt water) For salt water cooling systems electric lines substations and instrumentation should be located upwind in the direction of the predominantwind They should be provided with adequate shelter or materials when within 450 ft (140 m) of the cooling tower In additionspecifications for all equipment within 150 ft (45 m) of the tower should include surface treatment suitable for a marineenvironment
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COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
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COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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DESIGN PRACTICES December 2000
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TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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DESIGN PRACTICES December 2000
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FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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Section Page
XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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Section Page
COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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Section Page
XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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Section Page
COOLING WATER SYSTEMS XXVII 3 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
SCOPE
This section presents the criteria and general procedures for the selection and design of cooling water systems in processplants Major components are described for both once-through and recirculated type cooling systems using either fresh or salt
water This section also includes information on the chemical treatment required to control corrosion scaling and biologicalfouling in cooling water systems Information on automatic equipment used to monitor water condition and to make thenecessary adjustment in chemical treatment is also included
REFERENCES
DESIGN PRACTICES
Section II Design Temperature Design Pressure and Flange RatingSection IX Heat Exchange EquipmentSection X PumpsSection XII InstrumentationSection XIV Fluid Flow
Section XV Safety in Plant DesignENVIRONMENTAL DESIGN PRACTICE
Section XIX-B Water Reuse
OFFSITES DESIGN PRACTICE
Section XXVI-A Boiler Feedwater Treating Systems
INTERNATIONAL PRACTICES
IP 1-1-1 Drawings Diagrams and Line ListsIP 3-10-3 Cement Lined Pipe and FittingsIP 3-10-4 Plastic and Plastic-Lined PipingIP 8-1-1 Cooling TowersIP 10-11-1 Sizing of Drivers and Transmissions for Compressors Fans and PumpsIP 16-1-1 Area Classification and Related Electrical Design for Flammable Liquids Gases or VaporsIP 16-4-1 Grounding and Overvoltage ProtectionIP 19-6-1 Facilities for Corrosion Monitoring in Process Equipment
OTHER LITERATURE
Cooling Tower Water Treatment Guidelines ERampE Report No EE102E78Exxon Cooling System Handbook January 1986Environmentally Acceptable Cooling Water Treatments ERampE Report No EE34E86Karlsruhe Cooling Water System Reliability Study ERampE Report No 85EEEL3268Reuse of Wastewater as Cooling Tower Makeup ERampE Report No EE58E85
Guidelines for the Inspection Preventative Maintenance and Rehabilitation of Cooling Towers ERampE Report No EE47E89Guidelines for Safety Evaluation of Chemical Injection Facilities ERampE Report No EE92E94EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating ServiceEMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased)EMRE Refinery Construction Materials Manual Manual No EETD 028Cooling Tower Institute (CTI) Acceptance Test Codes for Water Cooling Towers (ATC-105)Cooling Tower Institute (CTI) Cooling Tower Performance Curves
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XXVII 4 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BACKGROUND
Rejection of heat is a requirement of a great many processes and utility systems Coolers and condensers reject much of thisheat to water The main purpose of a cooling water system is to absorb the heat rejected and dispose of it The system must
be safe reliable and flexible and it must have a minimum impact on the environmentCooling system selection and design is very site specific and cannot be generalized The EMRE COOLING SYSTEMSPECIALISTS are available for consultation on specific requirements
DEFINITIONS
Ambient Wet Bulb Temperature
The wet bulb temperature that exists in the cooling tower area It is the temperature specified in the Design Specification for the design and guarantee basis Note the actual tower inlet wet bulb temperature may be higher than the ambient wet bulbtemperature due to hot air recirculation
Approach
The difference in temperature between cold water leaving the tower and ambient wet bulb temperature
Battery Limits
A boundary which is normally assumed to be at the physical edge of the block in which the facility is located
Blowdown
The continuous or intermittent discharge of a small amount of the circulating water Its purpose is to limit the increase in theconcentration of solids in the water due to evaporation It is expressed in percent of water circulated
Brackish Water
Water which contains 3000 to 5000 wppm of dissolved salts
Cell
The smallest tower subdivision which can function as an independent unit with regard to air and water flow
Cooling Tower Pumping Head
The total pressure at the centerline of the tower inlet to its hot water distribution system plus the difference in elevation betweenthe centerline of the inlet and the top of the cold water basin curb It does not include friction pressure drop in the riser pipeThis is a parameter representing the head required for the cooling tower only It excludes the cooling water distribution system
Counterflow Tower
A cooling tower in which air induced at the bottom of the tower flows up through the fill against the falling water
Crossflow Tower
A cooling tower in which air induced at the sides of the tower flows horizontally across the fill and the falling water
Cycles of Concentration
The ratio of dissolved solids in circulating water to dissolved solids in makeup water
Dissolved Solids
A measure of the total quantity of dissolved salts in water It can be determined by conductivity measurements
Drift
The entrained water carried from the tower by exhaust air expressed in percent of water circulated
Dry Bulb Temperature
The temperature of ambient air read on an ordinary thermometer
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Section Page
COOLING WATER SYSTEMS XXVII 5 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DEFINITIONS (Cont)
Fill
Packing within the cooling tower to provide the required surface area for heat transfer between ambient air and the returncooling water Splash fill breaks the water into small drops by causing it to cascade through successive offset levels of parallel level splash bars Cellular or film fill causes the water to spread into thin films over large vertical areas
Heat Load
Heat removed from the circulation water within the tower It may be calculated from the range and the circulating water flowrate
Interference
The mixing of tower inlet air with the discharge vapors from another tower or other heat source This results in reduced thermalperformance
Makeup
The water required to replace circulating water which is lost by evaporation drift blowdown and leakage It is expressed inpercent of water circulated
Offsites
Support facilities such as utilities (steam power cooling water) tankage waste treating etc for the processing operations
Once-Through Cooling System
A system in which water passes through the heat exchange equipment once and is then discharged
Onsites
Facilities that are a part of the processing operations
Range or Water Cooling Range
The difference in temperature between the hot and cold circulating water or on-tower temperature minus off-tower temperature
Recirculated Cooling System
A system in which water is circulated to coolers in a loop and the heat absorbed from this cooling operation is dissipated by acooling tower or some other type of heat exchanger
Recirculation
An undesirable condition in which part of the tower discharge vapor stream is recirculated through the tower resulting inincreased inlet air wet bulb temperature
Relative Humidity
The ratio of the quantity of water vapor present in air to the quantity which would saturate the air at the existing temperature
Salt Water
Water which contains more than 5000 wppm of dissolved salts
Wet Bulb Temperature
The equilibrium temperature obtained when ambient air is passed over a continuously wetted thermometer bulb evaporatingthe water and cooling the bulb This equilibrium temperature occurs when the heat transferred from the air to the wettedsurface equals the heat loss due to latent heat of evaporation
Wind Rose
A diagram that shows the average frequency and intensity of wind from different directions for a particular location
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XXVII 6 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
SYSTEM TYPES AND APPLICATIONS
Cooling water systems in refineries and chemical plants are generally either the once-through or the recirculated type designedfor fresh salt or brackish water
In the once-through system (see Figure 1 ) pumps take suction from a source of supply such as a river or other body of water and deliver the water to process coolers and other consumers After passing through the cooling exchangers the hot coolingwater is discharged through pressurized piping or a gravity flow systemIn a recirculated system (see Figure 2 ) pumps take suction from a cooling tower basin and deliver the water to process coolersand other consumers After passing through the cooling exchangers the hot cooling water is discharged through a pressurizedreturn system to a cooling towerThe choice between a once-through and recirculated cooling water system must be based on (1) availability of water of satisfactory quality (2) process temperatures (3) atmospheric conditions (4) investment and operating costs and (5) effluentwater quality and temperature limitationsThe cooling water is distributed to individual consumers throughout the plant in piping main supply and return headers andseries and parallel branches consisting of laterals and sublaterals The main cooling water distribution headers are either buried in underground trenches or are located above grade on ground-level pipe sleepers or on elevated pipe racksUnderground piping has the advantage of minimizing problems associated with thrust loads thermal expansion and freezingElevated piping is easier and less expensive to install and repair and it makes it easier to locate leaks
BASIC DESIGN CONSIDERATIONS
RELIABILITY
The cooling water system is an important utility and its reliability is critical to successful plant operation The typical systemprovides cooling water to both onsite and offsite consumers It can have a direct impact on the sizing criteria for the emergencyrelief (flare) system (refer to Section XV-C Safety in Plant Design Pressure Relief ) The system therefore normally must becapable of continuous operation so that cooling water is always available to critical consumersThe reliability required to achieve continuous operation is provided in several ways
bull Spare makeup and water circulating pump(s) are provided to assure continuous operation
bull Different types of drivers are specified for the supply or water circulating pumps Motor-driven pumps are supplied fromtwo different buses on a secondary selective electrical distribution system
bull Cooling tower fan drivers are normally supplied from a secondary selective systembull Holdup volume is provided in the cooling tower basin to permit an orderly shutdown of process units if the makeup water
supply is lost or a failure occurs in any part of the recirculated cooling water system
bull Valves blinds and bypasses are provided to permit individual components to be removed from the system for maintenancewhile the system is operating
bull Construction materials provided for cooling water facilities are designed for long-term corrosion resistance sincemaintenance work is often difficult (if not impossible) to schedule without a complete refinery shutdown
FLEXIBILITY
The cooling water system must include sufficient flexibility to cover all present operating requirements plus any future operatingrequirements that are defined in the design basis document For example future operating requirements could be dictated by anear-term increase in plant sizeThe type of flexibility features provided in a system may include the following
bull Makeup water treating facilities should be able to deliver the required water assuming the worst conditions of the water source
bull Waste treating facilities should be designed to handle effluent water containing varying concentrations of oil and chemicalsover the range of expected ambient temperatures
bull Valves and blind flanges should be provided to permit future tie-ins to the system without a shutdown An exception wouldbe on lines where a ldquohot taprdquo could be made with minimum risk of an emergency shutdown or where the cost of requiredvalves is prohibitive
bull Cooling tower cells should be completely partitioned as described in IP 8-1-1 to allow personnel entry to any cell for maintenance without affecting the operation or capacity of the other cells Normally spare cells are not providedMaintenance is carried out during cooler periods when the cooler ambient and wet bulb temperatures enable the coolingtower to handle greater heat loads
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Section Page
COOLING WATER SYSTEMS XXVII 7 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
bull Where multiple cooling tower systems exist consideration should be given to providing valved cross connections betweentheir respective supply mains and between their respective return mains These would be used only for special operations
such as turnarounds or emergencies The impact on flare sizing should be evaluated if these cross connections areprovided
ENVIRONMENTAL AND SAFETY CONCERNS
Once-through cooling water systems usually require controls to minimize the release of hydrocarbons and free halide residualsto the environment As a minimum cooling water discharges should be monitored for free oil total organic carbon or hydrocarbon vapors to determine if system coolers are leaking Free chlorine residuals in the discharge should also bemonitored as required to meet local regulations and to detect process leaks In most cases once-through cooling water flowsare too large to be handled in the plants wastewater treatment system However once-through water that could becomecontaminated with hydrocarbons is often segregated and treated in a large skim pond or special purpose separator prior todischarge Local environmental regulations and permit requirements will dictate the extent of treatment requiredCooling tower blowdown from recirculated systems is normally bypassed around major wastewater treatment and dischargedwith the treated wastewater An oil-in-water detector should be provided to detect oil in the blowdown from process leaks Thiswill enable stopping the blowdown or diverting it to temporary holdup until the leak is corrected A major rupture in a processheat exchanger tube can also result in the cooling tower being the discharge point for hydrocarbon vaporsBecause of the likely presence of flammable vapors the areas around cooling towers are subject to electrical classification inaccordance with IP 16-1-1 The type and amount of chemicals added to the cooling water should be evaluated as to their effects on the plants wastewater treatment process Biocides corrosion inhibitors and antifoulants may result in significant problems for the wastewater treatment plantOther environmental concerns are
bull Cooling tower noise emissions from fans and from the flow of cooling water over the tower may require suppression if near a local community
bull Cooling tower water mist drift can cause fog and ice formation on nearby roads and promote deterioration of nearbyequipment It may also result in damage to sensitive vegetation in the surrounding area
bull Process gas releases to atmosphere resulting from process gas pickup by cooling water including the venting of toxicgases from exchanger tube ruptures must be considered in locating cooling towers or discharge points from once-throughsystems
bull Spills and overflows of toxic and hazardous chemicals used in treating the cooling water must be contained
SYSTEM SELECTION
The selection of a cooling water system is often determined by the availability of water In regions where water is scarce once-through systems are obviously eliminated from consideration and recirculated systems are chosen A once-through coolingsystem may be the economical choice if the plant is located adjacent to a body of water and consumers are elevated no morethan 50 ft (15 m) above the water level However once-through system costs may be increased significantly by environmentallimitations in many locations These may limit maximum discharge water temperature and therefore will reduce the maximumcooling water ∆T This will increase the required flow and cost of once-through systems In addition regulations may requirewaste treatment of the large volume of effluent thus increasing costsThe choice of a cooling water system must be based on comparisons of initial investment and operating costs Relative initialcosts of a system vary with climatic conditions distance from pump to users complexity of inlet structures and corrosive nature
of the water Salt water or brackish water service requires the use of cement mortar lined pipe and alloys for exchangers andpumps Carbon steel is usually satisfactory for fresh water Special attention should be given to the impact of bottom silt mudand seaweed on intake facilities to once-through or makeup to recirculated systems Cribs flumes large bays screens trashracks or other extensive facilities may be requiredDuring the early stages of a project the designer should review with the planning or design groups the heat level and volume of all process streams which require cooling The optimum method of cooling should then be determined ie heat exchangeandor rejection of heat to air or water A break point should be selected above which air cooling is economical and belowwhich trim water cooling is preferred
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XXVII 8 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
COOLING WATER REQUIREMENTS
Cooling water requirements for each process unit and offsite consumer are obtained from the appropriate design groups For existing locations maximum cooling water loads should be based on the design inlet cooling water temperatures given inSection IX-B Design Considerations for All Types of Heat Exchangers For new locations the inlet cooling water temperaturemust be set based on local water temperature data for once-through systems and wet bulb temperature and selected approachfor recirculated systemsThe bulk water temperature at the cooler outlet has been selected as the controllable variable for design and operation of coolers Although the corrosion rate depends on the metal temperature this is difficult to determine in the operation of a coolerBased upon the findings of a Materials Development Section test program maximum design outlet water temperatures havebeen set as indicated below
Fresh Water 130 degF (54 degC)
Treated Fresh Water 130 degF (54 degC)
Brackish Water 125degF (52
degC)
Salt Water 120 degF (49 degC)
When the maximum plant cooling water requirement has been established for new process and utility consumers appropriateLoad Growth and Reserve Capacity Factors (LGF and RCF) should be applied depending on the development stage of theproject Cooling water loads should be updated and system capacity adjusted periodically as design work progresses
LOAD GROWTH AND RESERVE CAPACITY
Utility loads tend to increase as the loads become better defined through successive stages of project definition Thereforeload growth and reserve capacity factors (LGF and RCF) are applied to raw loads as a means of predicting final equipmentsizes and capital costs These load growth factors do not cover basis changes through project development such as project or process unit sizethroughput For revamp or modernization projects these factors are generally applied only to new loads or changes to existing loads on the basis that existing or unchanged loads are well known
Recommended Load Growth Factors are as follows
CONSUMERSPLANNING
(SCREENING THROUGH DBM) DESIGN SPEC
Existing Facilities 0 - 10 0 - 10
New Facilities 20 - 30 5 - 10
Selection of the load growth factor to use should be based on the quality and level of effort associated with development of theraw loadsFinal sizing of the source facility (eg the cooling tower and circulation pumps) should include a reserve capacity factor of 10to cover small post startup changes minor future projects and dynamic conditions not predicted by steady state balances Thesizing basis for distribution facilities should not include the RCF
COOLING TOWER TYPES
The cooling tower is the major piece of equipment specified in recirculating cooling water systems In effect it is the heatexchanger which cools the hot water to a suitable temperature for re-use primarily by evaporation through contact with airThe cooling tower contains fill to break up the water flow into droplets or film on a surface The fill increases the waterair contact area and enhances heat transferCooling towers are classified according to the means by which air is supplied to the tower ie induced or forced mechanicaldraft (fan) vs natural draft and according to the relative movement of the air and water that is counterflow or crossflow (seeFigure 3 ) Most of the towers in the ExxonMobil circuit are mechanical induced-draft type Economics determine whether theyare counterflow or crossflow
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COOLING WATER SYSTEMS XXVII 9 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
In crossflow towers the air travels horizontally across the falling water while in the counterflow design the air travels verticallyup through the falling water Comparisons of plot space fan horsepower and future maintenance costs determine which
design will be more economical Factors which influence this selection are cooling water rate range and approach for aparticular location In addition in some locations materials of construction and associated labor costs will affect the finalselectionIn the past crossflow towers were usually preferred over counterflow towers for extremely cold locations where ice formationcould restrict or block air flow through the louvers However this consideration is usually no longer valid due to the increaseduse of PVC film-type tower fill rather than splash-type fill in counterflow towersMechanical draft designs use one or more fans to provide the air needed for cooling Induced-draft units have the fans locatedon top of the tower Air is pulled through the packing and discharged vertically upward at high velocities Forced-draft unitshave the fans located at the base of the tower and push the air through the fill Forced-draft towers in large industrial sizesexperience difficulties with recirculation and in providing a uniform distribution of air As a result they have been supersededby induced-draft designsFill may be splash or film type Splash fill normally consists of horizontal slats in horizontal rows offset to one another to causethe water to break up into droplets as it falls downward through the cooling tower Splash fill is characterized by reduced air pressure losses It is also less conducive to clogging and easier to clean after a spill However it is very sensitive to
inadequate support and must remain horizontal and level It is made of treated wood or plastics such as PVC or polypropyleneFilm fill comes in various designs but they all cause the water to flow in films over the fill surface and provides more coolingcapacity within a given amount of space than splash fill Because of this increased heat transfer efficiency film fill is generallyused in most new cooling towers However because of its smaller passages it is more sensitive to plugging and is difficult toclean if it does become plugged It may not be appropriate for services cooling mainly heavy fuels or waxy lubes where a leakwould cause fill plugging or for services with a high biological fouling potential PVC is currently the most widely used materialfor film fill The selection of fill type is usually proposed by the vendor and reviewed by the COOLING WATER SYSTEMSSPECIALISTSNatural-draft or hyperbolic cooling towers depend on the natural draft created by the difference in the density of the enteringand leaving air for movement Generally hyperbolic cooling towers have been used in Europe and in the utility industry in theUSA for large capacities [250000 gpm (16000 Ls)] short ranges [10 to 15 degF (6 to 8 degC)] and long approaches [10 to 15 degF (6to 8 degC)] They are most effective where low ambient wet bulb temperatures occur with high relative humidities These towersnormally have diameters in excess of 200 ft (60 m) and are greater than 250 ft (75 m) in height Their major advantages are
bull No operating or maintenance cost for fans
bull Reduction in problems associated with fogging drift and recirculation due to their great height and diameterThe primary reasons why hyperbolic towers are not used in ExxonMobil projects include
bull High initial cost
bull Large plot space requirement and high visual impact
bull Complete dependence on atmospheric conditions
bull Lack of flexibility for future expansions
COOLING TOWER BASIN
The primary function of the cooling tower basin is to collect the cooled water leaving the tower and to provide a reservoir for thecooling water pumps Design aspects to be considered are the basins capacity cleanability draining facilities sump andscreen details To obtain the necessary time for corrective action during emergency conditions a minimum storage capacity of 90 minutes based on loss of makeup water flow should be provided in the basin between the basin low level alarm (LLA) andthe minimum pump submergence (MPS) When the basin water level reaches the MPS point loss of recirculation pumpsuction and cooling water flow is imminent Typical values for basin water levels are as follows
bull High Level Alarm (HLA) 1 ft (300 mm) below top of the basin wall
bull Normal Water Level 15 ft (450 mm) below top of the basin wall
bull Low Level Alarm (LLA) 25 ft (760 mm) below top of the basin wall
bull Distance between LLA and MPS 35 ft (1070 mm)
bull Minimum Pump Submergence Depth 45 ft (1370 mm)
bull Clearance of Pump Suction off Basin Floor 15 ft (450 mm) Actual values may vary depending on pump type selected pump suction line diameter and cooling tower cell plot area
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XXVII 10 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
MATERIALS OF CONSTRUCTION
The EMRE Refinery Construction Materials Manual and IP 8-1-1 list acceptable materials for all components in each type of cooling water system (once-through and recirculating) For salt water systems emphasis must be placed on proper materialsselection throughout the system The external salt water environments in the vicinity of the tower itself and the intake stationsmust be taken into consideration Proper siting of the tower is extremely important to avoid salt water corrosion of adjacentunits or the need to use expensive materials to operate in such an environment
WATER TREATMENT FOR ONCE-THROUGH SYSTEMS
Once-through salt and fresh cooling water systems can develop a variety of problems such as fouling due to seaweed or other debris marine microorganisms such as muscles and shell fish and biological growth Corrosion and scaling problems areminimal in fresh water once-through systems because the water is not concentrated by evaporation In salt water systemscorrosion is minimized by proper materials selection and scaling problems generally do not occurProtection against fouling by seaweed or other debris is provided by careful selection of the water intake location and travelingscreens with washing facilities In some cases additional steps may be required These may include an additional secondaryset of traveling screens and possibly battery limit strainers on the cooling water supply lines to individual unitsProtection against fouling by marine microorganisms and biological growths is achieved primarily by injecting chlorinehypochlorite chlorinebromine or other similar chemicals which kill these growths Chlorine is commonly used and is injectedin sufficient quantities to produce a residual of 1 wppm in the effluent for one hour each day (shock chlorination) Continuouschlorination for 2 - 3 days per month at lower levels (03 wppm residual chlorine in the effluent) may also be used in addition toshock chlorinationThe large water volume discharged from once-through systems usually makes chemical treating for corrosion or deposit controltoo costly These problems are handled by proper materials selection and by keeping water velocities through exchanger tubesgreater than 3 fts (09 ms) Occasionally the water analysis of a fresh water source being considered for a once-throughsystem using carbon steel materials will indicate the need for some corrosion or deposit control In this case the cost of thethreshold treatment with the once-through system should be compared to the costs of a recirculated system before making afinal choice
WATER TREATMENT FOR SALT WATER COOLING TOWER SYSTEMS
A total dissolved salts (TDS) concentration limit of 55000 wppm for sea water in cooling tower systems requires high blowdownrates since the makeup sea water typically has from 35000 to 40000 wppm TDS Because of cost restraints the only water treatment normally specified for these systems is for biological control Chlorine is most commonly used either as directchlorine injection or hypochlorite produced in electrochlorinators Care must be taken in selecting the intake location to assurethat the suspended solids do not exceed 200 wppm in the recirculated water If it is not possible to find sea water with sufficientclarity clarification with polyelectrolytes should be evaluated Corrosion is one of the major problems with the use of sea water in cooling towers This requires proper materials selection
WATER TREATMENT FOR FRESH WATER COOLING TOWER SYSTEMS
Cooling tower water treatment is necessary to minimize or eliminate corrosion scale and biological fouling of process heattransfer surfaces caused by minerals and impurities in the water The difficulties caused by these impurities and the means of treatment are shown in Table 1 Water lost by evaporation results in an increase in total dissolved salts in the recirculated water To indicate the degree of concentration of impurities the term cycles of concentration is used
Corrosion is an electrochemical process that deteriorates metals exposed to water in the presence of corrosive agents such asacids oxygen or bacteria A common form of corrosion is pitting Severe corrosion can lead to equipment failure Corrosionis caused by many conditions including process leaks into the cooling water water flow velocity that is too low (causes depositsand fouling lending to corrosion) or too high (causes erosion and corrosion) low pH and high temperatures Corrosion is lesslikely in water with non-acidic pH values (greater than 7) although scaling is more likely in these pH rangesScaling is characterized by the formation of hard dense deposits on material surfaces These deposits impact heat transfer and can become a site for localized under-deposit corrosion Scaling is influenced by many factors including makeup water composition total dissolved solids in the recirculating water water temperature and pH and flow velocity Calcium carbonateis the main scaling constituent in all waters and it is the least soluble It forms from the hardness (calcium Ca ++ ) andbicarbonate (HCO 3
ndash ) alkalinity present in most waterFouling can be caused either by the settlement of suspended matter or by microbiological growth of algae bacterial slime or fungi Fouling will reduce heat transfer cause plugging and create corrosion sites
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COOLING WATER SYSTEMS XXVII 11 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Treatment of the cooling water is broken down into two major categories external and internal External treatmentencompasses any treatment given to the cooling tower makeup water Internal treatment is the addition of chemicals directly to
the recirculated cooling water
EXTERNAL TREATMENT
External treatment to remove impurities in order to reduce the amount of blowdown should always be evaluated as to costeffectiveness The most frequently used techniques for treating makeup water are shown in Figure 11 Details of availabletreatment processes (see Table 1 for methods available) are covered in Section XXVI-A Boiler Feedwater Treatment For cooling tower systems that meet currently required drift limits in refineries and chemical plants uncontrolled cooling water losses (drift and miscellaneous process uses) normally amount to about 14 of the cooling water evaporation loss Under these conditions the maximum cycles of concentration achievable are 8 with no controlled blowdown Some cooling towersare operating at up to 10 cycles of concentration Additional controlled blowdown required to limit the cycles of concentrationper the water quality criteria is covered later
INTERNAL TREATMENT
The makeup water analyses amount of cooling water losses environmental restraints potential chemical hazards materials of construction and film temperatures of heat transfer surfaces determine the type of internal treatment required for cooling tower systems The internal water treatment program selected must address corrosion scale fouling biological growth andchemical attack of cooling tower componentsNumerous chemicals and treatment programs are continuously being developed for addressing the potential problems Theslightly acidic low pH programs of the past which used chromate based chemicals and usually required acid injection arebeing completely phased out because of environmental restrictions on chromate and the higher costs associated with meetingcurrent practices for handling hazardous chemicals such as acid Non-chromate treatments which use inhibitors such asphosphates zinc and organics along with higher pH have been developed to replace the earlier programs Although not aseffective as chromate these inhibitors operate at higher water pH which is a less corrosive environment The disadvantage of the higher pH is the greater tendency to form scale This tendency is controlled by the use of dispersants or scale inhibitorswhich are normally organic polymers or phosphenate A corrosion inhibitor for copper or brass is also normally required if these metals are present in the systemThe following paragraphs describe techniques and chemicals to control alkalinity (pH) corrosion scale sludge and biologicalgrowth The chemicals mentioned are normally purchased as pre-mixes from vendors and dosed in combination Mostcorrosion and scale control program chemicals are two drum formulations due to the incompatibility of some chemicals in theconcentrated state Therefore separate feed systems are generally provided for chemicals for control of corrosion and scale (2drums) and biological growth (if liquid) If a program includes fixed pH then an additional feed system is required to add acidWater treatment programs are site specific and changing The COOLING WATER SYSTEMS SPECIALISTS should beconsulted for recommendations on the internal water treatment programs when an analysis of the available water is known
Alkalinity (pH) Control
Cooling water treatment programs presently available fall into two basic categories those that do not control pH (floating pH or alkaline treatment programs without pH adjustment) and those that control pH within a specified range by adding acid Theprograms that do not control pH are gaining favor because of the additional cost equipment and precautions required tominimize the risk associated with handling acid and because of improvements in chemicals to control depositsHigh pH in the recirculating cooling water is a result of the alkalinity of the fresh water due to bicarbonate salts This combinedwith carbon dioxide results in the water having a pH range of 6 to 85 When this fresh makeup water is mixed with therecirculated cooling water and passed through the cooling tower the air flow strips out the carbon dioxide Bicarbonate ionsthen convert to carbonate ions raising the pH which can cause both calcium carbonate and calcium phosphate salts toprecipitate out of solution and form deposits Another debit in allowing the cooling water pH to increase is increased chlorineconsumption to control biological growthsIf a program using pH control is justified and chosen acid addition is used under control of a pH analyzer to controlrecirculated cooling water alkalinity and pH to the range required by the treatment program
Corrosion Control
To reduce corrosion to an acceptable level chemical corrosion inhibitors which form protective films on heat transfer surfacesare the most effective protection Inhibiting corrosion is accomplished by phosphates organics zinc nitrites and molybdatesalts Unfortunately the use of chromate which is a reliable corrosion inhibitor is prohibited by environmental constraintsNitrite is not practical in an open system due to atmospheric oxygen converting the nitrite to nitrate
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ExxonMobil Proprietary
Section Page
XXVII 12 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Phosphate inhibitors (orthophosphate polyphosphate phosphonate) are the basic treatment for new towers in todaysenvironment In addition to the phosphate chemistry salts of zinc may be added to aid in inhibiting corrosion These ions
combine with the hydroxide ion forming thin protective films Zinc is used to provide a synergistic effect in combination with thephosphate salts for protecting carbon steel surfaces from corrosion Finally all-organic programs are offered by severalmanufacturers The use of these treatments has been made possible by the development of polymer dispersants which givecalcium salts a much higher solubility and reduce the tendency to scaleWhen a cooling water system contains copper alloys azole based and azole substitutes are required as copper corrosioninhibitors There are several effective inhibitors The most common is tolytriazole (TTA) due to its resistance to chlorineThe corrosion inhibitors most frequently used in ExxonMobil recirculating cooling water systems are shown below indescending order of preference provided their use meets all environmental requirements
bull Stabilized Phosphate (OrthoPolyphosphate) - Environmental restrictions on the use of zinc have resulted in ExxonMobilplants in the USA using a blend of orthophosphate and polyphosphate type corrosion inhibitors with good results Thistreatment forms a thin protective calcium phosphate film on heat transfer surfaces through the use of orthophosphateconcentrations as high as 12 - 17 wppm The use of calcium phosphate dispersants and Hydroxethylidene Diphosphonate(HEDP) stabilizes the treatment and prevents the formation of calcium phosphate scale Concentration of the dispersantsdepends upon the calcium concentration temperature and pH Stabilized Phosphate treatment programs can operate in apH range of 68 to 78 Alternatively an alkaline (floating pH) stabilized phosphate treatment program can be used Thisprogram uses little or no acid addition and is gaining in popularity although not well established yet It operates with pHvalues of 8 - 9 and uses higher quantities of dispersants and lower levels of phosphate A corrosion inhibitor for copper and brass is normally included for all stabilized phosphate programs Several vendors offer these programs Twoseparate chemical feeders are generally required
bull Zinc Alkaline Phosphate - Heritage Exxon plants in Europe where the use of zinc is not restricted have obtained goodcontrol of corrosion and scaling using zinc-polyphosphate treatment programs Low (65 - 72) and high (73 - 80) pHprograms have been used The high pH (zinc alkaline phosphate) programs are the lowest cost and are recommended
As with the Stabilized Phosphate treatment dispersants such as HEDP and polyacrylates are required to prevent theformation of zinc or calcium phosphate scale on heat transfer surfaces A corrosion inhibitor for copper or brass isnormally used with this program which is offered by several vendors
bull Organic Salts - All-organic (polymer) programs are offered by several manufacturers and have been used successfullywithin h Exxon In general these treatments are based on a combination of a phosphonate to control corrosion polymers(dispersants) to prevent scale deposition and a corrosion inhibitor for copper or brass This type of program is mosteffective in conditions of high hardness high alkalinity (M alkalinity gt300 wppm as CaCO 3) and high pH values (gt 85)However some manufacturers have reported success in softer water with alternative polymer component formulations Ingeneral an inorganic corrosion control program is preferred to the all-organic programs which are sensitive to alkalinityand more costly
Detailed control limits for effective use of each of the above inhibitors and materials selection are covered in Report NoEE34E86 Environmentally Acceptable Cooling Water Treatments Report No EE102E78 Cooling Tower Water Treatment Guidelines and in the EMRE Refinery Construction Materials Manual
Scale and Sludge Control
Scale and sludge deposits in cooling water systems cause loss of heat transfer and provide the conditions for pitting corrosionScale consists of inorganic salts which have precipitated from the cooling water forming a coating on heat transfer surfacesSludge consists of organic and inorganic suspended matter that settles in quiescent areas and in heat exchangers The key toprevention of both scale and sludge formation in a cooling system is keeping water velocities greater than 3 fts (09 ms) andthe use of chemical additives in combination with a blowdown rate that keeps the impurity concentration below the level whichcauses deposits (See Table 2 for limits on impurities) Scale control has become increasingly important in higher alkalinityldquofloating pHrdquo water treatment programsSalts most commonly found in scale deposits are calcium carbonate calcium sulfate tricalcium phosphate zinc phosphatemagnesium silicate and silica The control method for preventing scale from each of these salts is discussed below
bull Calcium Carbonate - Where justified using pHalkalinity control in the range of 70 to 75 will normally inhibit the formationof calcium carbonate scale if anti-scalent chemicals (phosphonates acrylates copolymers) are part of the chemicaltreatment (See Report No EE102E78 Cooling Tower Water Treatment Guidelines ) Higher pH water treatmentprograms require greater concentrations of these chemicals along with dispersants and flocculants to inhibit the formationof scale and precipitation of other salts
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 13 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
bull Calcium Sulfate - When heat transfer skin temperatures are below 250 degF (120 degC) and the cooling water bulktemperatures are under 130 degF (54 degC) gypsum scale (CaSO 42H 2O) will form if the product of the calcium and sulfate ionsconcentrations each expressed as wppm CaCO 3 equivalent exceeds 500000 Gypsum scale is prevented bymaintaining bulk water temperature below the indicated limits and controlling calcium and sulfate ions through blowdownor through the use of newer dispersants on the market
bull Tri-Calcium and Zinc Phosphate - Almost all chemical additives for corrosion control contain some type of phosphatesalt These have a tendency to form the orthophosphate ion depending on time pH and temperature Cooling watersusing these corrosion inhibitors may have an orthophosphate concentration in excess of 5 wppm as PO 4
= To preventcalcium phosphate scale Table 2 shows a limit of 1000 wppm of calcium as CaCO 3 equivalent when using a phosphateinhibitor in combination with a chemical dispersant (See Report No EE102E78 Cooling Tower Water Treatment Guidelines for solubility of calcium phosphate) These inhibitors and dispersants generally fall into one of the followingcategories
ndash Sequestrants antiprecipitants and inhibitors such as hydroyethylidene diphosphonate (HEDP) phosphate esters andaminomethylene phosphonate (AMP) function by defyingdistorting the crystal structure of the scale formations toprevent nucleation and growth of the scale
ndash Dispersants based on either anionic or nonionic polymers such as polyacrylamide polyacrylate or polymaleic acidfunction to stabilize colloidal and scale-forming particles in suspension Anionic polymers prevent agglomeration of suspended particles by either charge repulsion or crystal distortion while nonionic polymers reduce the surfacetension of particles
bull Silica Deposits - Quantitative indices for determining limits for silica in combination with other ions are not available due tothe formation of complex polymers Instead the rule of thumb limit of keeping reactive silica concentrations in the coolingwater below 150 wppm as SiO 2 has been successful
Possible alternatives for controlling sludge are
bull Chemical Dispersants - Water treating chemical suppliers now offer many types of dispersants These are mostlypolyelectrolytes Claims for removal of slime silt and mud from cooling tower basins and exchangers are madeHowever satisfactory results using these chemicals as a coagulant aid require very accurate control of the amount fed tothe water and stable operation of the water system Also suspended solids should be kept below 200 wppm for particleslarger than 045 microns Dispersants are used in virtually all fresh water cooling tower systems despite their high cost
bull Sidestream Filtration - The use of a sand pressure filter is the most reliable method for sludge removal Operating andmaintenance costs are minimal for this equipment
bull Higher Blowdown Rates - This may be an economical alternative depending on the cost of makeup water Additionalchemical costs must be included in any evaluation of this method
bull Makeup Clarification - This alternative may require additional raw water treatment facilities to handle seasonal variationsin turbidity andor suspended solids This method is applicable only where suspended solids come from makeup water
Biological Control
Operating conditions in recirculated cooling water are ideal for the growth of biological matter Water temperatures from 70 to120 degF (20 to 50 degC) favorable water pH continuous supply of nutrients including carbon nitrogen and phosphorus inorganicsalts and sunlight all lend themselves to environmental conditions encouraging microbiological growth and plant lifeIf biological growths get out of control and form large sticky agglomerations some of the following operating problems willresult
ndash Fouling of heat transfer surfaces by bacterial slimes resulting in flow restrictions and high process temperatures ndash Reduced cooling tower efficiency resulting from algae fungi and bacterial slime growths in the water distribution basin
and fill area of the cooling tower ndash Excessive plugging of screens and filters ndash Corrosion ndash Wood destruction by fungi ndash Clogging of water distribution nozzles ndash Excessive sludge accumulation in the cooling tower basin
An important part of the water treatment program is to control biological activity by feeding a biocide chemical Oxidizingbiocides such as chlorine sodium hypochlorite chlorine dioxide or bromine-associated compounds are most commonly usedwith good results Plate counts should be maintained below 50000 countsml (bacterial)
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ExxonMobil Proprietary
Section Page
XXVII 14 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Chlorine is the most effective biocide for cooling water because of its low cost extensive period of successful use ease of application and minimum environmental effects if applied under controlled conditions Either a liquid feed (sodium
hypochlorite) or a gas feed (100 chlorine) is added to the cooling tower basin A sufficient dosage is that which obtains a 01to 03 wppm continuous free residual in the hot return water to the cooling tower
Advantages to continuous chlorination over shock treatment based on ExxonMobil experience are ndash Minimizes chlorine degradation of polymers ndash Better reduction in total aerobic bacteria plate count resulting in cleaner heat transfer surfaces ndash Does not cause significant pH depressions ndash Better control results with less chlorine consumption
Details concerning chlorine addition as well as the other alternative biocides are given in Report No EE102E78 Cooling Tower Water Treatment Guidelines and in EMRE Engineering Water and Wastewater Design Guide TMEE 080 DG 11-3-1Gas Chlorinators for Water Treating Service and DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) Gaseous chlorine systems have become more costly in recent years because of the additional precautions equipment andprocedures required to minimize the risk associated with handling this gas Alternatives to gaseous chlorine are available butrequire more attention and have a higher operating cost Sodium hypochlorite (either purchased or generated) and sodiumbromide stabilized solid halide (chlorinebromine) products chlorine dioxide and ozone are all alternatives which may haveapplications Contact the COOLING WATER SYSTEMS SPECIALISTS for guidance in the use of these alternative treatmentschemes
BLOWDOWN
Blowdown control is required to maintain the proper cycles of concentration in the system The most reliable and simplest typeof control for blowdown is specific conductance measurement of the cooling water Electrical signals from the specificconductance measurements of some impurity in the cooling water are used to actuate an automatic blowdown valveImpurities normally used for blowdown control are dissolved solids chloride calcium magnesium and silica
CONTROLS AND MONITORS
Any chemical treatment used to control problems in a cooling water system must be carefully monitored to be completelyeffective
The installation of automatic controls should be considered for recirculated cooling water systems in all plants Designs shouldinclude pH conductivity corrosivity inhibitor and biocide control Automatic controllers are available which continuouslymonitor the condition of the water and automatically make chemical additions to keep the water within compositional limitsTest exchangers and corrosion coupons are also useful in monitoring the effectiveness of the water treatment program
An oil-in-water monitor should be considered to provide early detection of oil leaks into the cooling water This will enablecorrective action to be taken before extensive contamination of the cooling water circuit occurs This is especially important incooling towers with high efficiency cellular fill which can be difficult to clean if significantly fouledManual control systems may be used in locations that have adequate technical personnel to perform routine water analysesand to modify chemical treatment necessitated by sudden variations in water composition process contamination etc
WASTEWATER REUSE POSSIBILITIES
Recirculating cooling water systems normally have the highest plant raw water demand A potential alternative is to use treatedwastewater to supply part or all of the makeup water requirement The treatment required must be determined but will typicallyinclude primary oil removal (eg API Separator) secondary oil and suspended solids removal (eg filtration dissolved or induced air flotation) and soluble carbon removal utilizing biological treatmentThe reuse of treated wastewater as makeup currently exists at only a few locations If this alternative is being considered refer to Report No EE58E85 Reuse of Wastewater as Cooling Tower Makeup and Section XIX-B Water Reuse
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 15 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS
STEP 1 - Select a location for the intake station and pumps
Factors which influence this selection are as followsbull Depth of water and tidal variations
bull Relative location of intake and outfall direction of current
bull Length and size of intake suction piping vs pump discharge piping
STEP 2 - Determine cooling water requirements
Determine the design cold water inlet temperature based on site data covering seasonal water temperature as a function of inlet location and depth (for existing locations refer to Table 2 in Section IX-B )Tabulate the cooling water requirements for each process unit and offsite consumer Apply the proper LGFs to all coolingloads depending on the stage of the project and whether the new units are exact duplicates of existing unitsIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The designer should investigate methods to increase effluent water temperature to the maximumrecommended limit for the particular type of water used One common method is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 3 - Determine the total required capacity for water intake facilities
Process Coolers - The total requirements determined in Step 2 including LGFsFirewater - This is often supplied from the intake bay of a once-through systemFuture Expansions - These should be considered in sizing intake facilitiesTotal Required Capacity - The sum of the above three items
STEP 4 - Provide water intake facilities
Intake Crib - The intake crib with its support structure is located at the extreme end of the water intake line (See Figure 4 )The intake is usually located 200 to 300 ft (60 to 90 m) offshore and at a sufficient depth below the lowest water level to
preclude vortex formation and subsequent pump cavitation during storm conditions Additionally the crib should have 3 to 5 ft(1 to 15 m) clearance above the sea floor to minimize sand and silt pickup A concrete velocity cap should be specified to cover the inlet crib This will minimize the incidence of fish and other foreignmatter entering the system A horizontal inlet flow pattern should be designed with a maximum entrance velocity of 1 fts(03 ms) Experience shows that vertical velocity flow confuses fish and they are drawn into the intake lineThe intake crib support structure protects the crib from adverse effects of storms and tidal motions as well as protecting it fromimpact of floating and submerged objects A self-cleaning screen can be specified to prevent entrance of large objects into thesystem at the cribIntake Line (Figure 5 ) - An intake line is required if the water depth along the shore does not permit water to flow directly intothe intake bay The intake line is usually buried in the sea floor and covered with a minimum required overburden Consultwith the COOLING WATER SYSTEMS SPECIALISTS for required coverage at a specific siteChlorine or sodium hypochlorite is injected at the intake crib to prevent marine growth in the lineNote that an intake crib and line will not be required if water of sufficient depth is available at the supply pump stationIntake Bay (Figure 6 ) - The intake bay is an enclosure which houses the pumps and screens Depending on a particular design a series of screens is used to ensure the supply of clean water Trash racks comprise the first element of the filter system These remove only very coarse material from the water such as bulk seaweed They can be supplied with atraversing raking mechanismLift screens provide secondary cleanup These screens can be lifted out of the intake bay for periodic cleaning Double liftscreens can be used if it is deemed necessary to have one in service at all timesFinal water filtering is usually done by traveling screens Details of the screen dimensions and the mounting are usuallydetermined during detailed engineering Screen net open area is typically approximately 50 of total screen area Thescreens should be sized for a normal water velocity of 12 fts (04 ms) through the open screen area Maximum velocityshould be limited to 24 fts (08 ms) with 50 of the screen area plugged This is the upper velocity limit for efficientscreening
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ExxonMobil Proprietary
Section Page
XXVII 16 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
If high suspended solids greater than 200 mesh (eg sand) are expected to be a continuous problem ejectors may be installedupstream and downstream of the traveling screen to remove accumulated sand and solids from the intake bay in these areas
Traveling screens and ejectors are supplied with wash water from the raw water pumps or from separate wash water pumpsOnoff control using timers should allow optimum scheduling of the ejectors depending on the time of the year
STEP 5 - Specify water intake facilities
Water intake facilities should be designed in accordance with the following guidelines from the Hydraulic Institute Standards The water intake should be designed to provide an evenly distributed flow of water to the pump suction A properly designedintake reduces the possibility of vortex formation and results in better pump performance with less noise Geometry of theintake and the manner in which water is introduced into the intake are important parameters in providing an even distribution of water flow Complete analysis of intake structures is best accomplished by scale model testsThe pump manufacturer will generally render advice on specific problems while the intake design is still preliminary He shouldbe provided with the necessary intake layout drawings showing the physical limitations of the siteFigures 7A and 7B have been developed for single- and multiple-pump arrangements to show suggestions for basic sumpdimensions They are for pumps normally operating in the capacity range of approximately 3000 to 300000 gpm (200 to20000 Ls) Since these values are composite averages from a great many pump types and cover the entire range of specificspeeds they must not be thought of as absolute values but rather as basic guides subject to some possible variations
All the dimensions in Figures 7A and 7B are based on the rated capacity of the pump at the design head Any increase incapacity above these values should be momentary or very limited in time If operation at an increased capacity is to beundertaken for considerable periods of time the maximum capacity should be used for the design value in obtaining sumpdimensionsDimension C is an average value based on an analysis of many pumps Its final value should be specified by the pumpmanufacturerDimension B is a suggested maximum dimension which may be less depending on actual suction bell or bowl diameters in useby the pump manufacturer The edge of the bell should be close to the back wall of the sump When the position of the backwall is determined by the driving equipment or the discharge piping Dimension B may become excessive and a ldquofalserdquo backwall should be installedDimension S is a minimum value for the sump width for a single pump installation This dimension can be increased but if it isto be made smaller the manufacturer should be consulted or a sump model test should be run to determine its adequacy
Motor size may dictate minimum pump spacing be especially aware of this when specifying pumps with unusually high pumpheads eg greater than 80 psig (550 kPa)Dimension H is a minimum value based on the ldquonormal low water levelrdquo at the pump suction bell taking into considerationfriction losses through the inlet screen and approach channel This dimension can be considerably less momentarily or infrequently without excessive damage to the pump It should be remembered however that this does not representldquosubmergencerdquo Submergence is normally quoted as dimension ldquoHrdquo minus ldquoCrdquo This represents the physical height of water level above the bottom of the suction inletThe actual submergence of the pump is something less than this since the impeller eye is some distance above the bottom of the suction bell possibly as much as 3 to 4 ft (1 to 13 m) For the purposes of sump design in connection with this chart it isunderstood that the pump has been selected in accordance with specific speed charts The submergence referred to hereinhas to do only with vortexing and eddy formationsDimensions Y and A are recommended minimum values These dimensions can be as large as desired but should be limitedto the restrictions indicated on the curve If the design does not include a screen Dimension A should be considerably longerThe screen or gate widths should not be substantially less than S and heights should not be less than H If the main stream
velocity is more than 2 fts (06 ms) it may be necessary to construct straightening vanes in the approach channel increaseDimension A conduct a sump model test of the installation or work out some combination of these factorsDimension S becomes the width of an individual pump cell or the center-to-center distance of two pumps if no division walls areusedOn multiple pump installations the recommended dimensions in Figures 7A and 7B also apply as noted above and thefollowing additional determinants should be consideredFigure 8A - Low velocity and straight-line flow to all units simultaneously is the first recommended style of pit Velocities in thepump area should be approximately 1 fts (03 ms) Some pumps with velocities of 2 fts (06 ms) and higher have given goodresults This is particularly true where the design resulted from a model study Not recommended would be an abrupt changein size of inlet pipe to sump or inlet from one side introducing eddying
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 17 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Figure 8B - A number of pumps in the same sump will operate best without separating walls unless all pumps are always inoperation at the same time in which case the use of separating walls may be beneficial If walls must be used for structural
purposes and pumps will operate intermittently one should leave flow space behind each wall from the pit floor up to at leastthe minimum water level and the wall should not extend upstream beyond the rim of the suction bell If walls are used increaseDimension ldquoSrdquo by the thickness of the wall for correct centerline spacing Round off the ends of the walls Not recommended isthe placement of a number of pumps around the edge of a sump with or without dividing wallsFigure 8C - Abrupt changes in size from inlet pipe or channel to pump bay are not desirable A relatively small pipe emptyinginto a large pump pit should connect to the pit with a gradually increasing taper section The angle should be as large aspossible preferably not less than 45 degrees With this arrangement pit velocities much less than one foot per second aredesirable Especially not recommended is a small pipe directly connected to a large pit with pumps close to the inlet Flow willhave excessive change of direction to get to most of the pumps Centering pumps in the pit leave large ldquovortex areasrdquo behindthe pumps which will cause operational troubleFigure 8D - If the pit velocity can be kept low enough [below 1 fts (03 ms)] an abrupt change from inlet pipe to pit can beaccommodated if the length equals or exceeds the values shown It is assumed that as ratio WP increases the inlet velocityat ldquoPrdquo will increase up to an allowed maximum of 8 fts (24 ms) at WP = 10 Pumps ldquoin linerdquo are not recommended unless theratio of pit to pump size is quite large and pumps are separated longitudinally by a generous margin A pit can generally be
constructed at much less cost by use of a recommended designFigure 8E - It is sometimes desirable to install pumps in tunnels or pipelines A drop pipe or false well to house the pump witha vaned inlet well facing upstream will be satisfactory in flows up to 8 fts (24 ms) Without the inlet well the pump section bellshould be positioned at least two pipe diameters vertically above the top of the tunnel not hung into the tunnel flow especiallywith tunnel velocities 2 fts (06 ms) or more There should be no signs of air along the top of the tunnel It may be necessaryto lower the scoop or to insist on a minimum water level in the vertical wellNote The foregoing statements apply to sumps for clear liquid
STEP 6 - Provide chemical treatment
Provide chemical treatment facilities for the once-through cooling water system Normally the only chemical treatment requiredin a once-through cooling system is for biological control Facilities for adding chlorine to once-through cooling water are thesame as those described for the recirculated cooling water system The design capacity of the chlorinator should be based onachieving a chlorine residual of 1 wppm in the effluent The chlorine required by the organics and other reducing agents in thewater must be satisfied before any residual chlorine will appear in the effluent The design capacity of the chlorinator should be
based on the total water chlorine demand plus 1 wppm in the effluent In the absence of water data the chlorination systemdesign capacity should be based on 10 wppm minimum instantaneous dosage for maximum cooling water flow For very largeonce-through cooling water systems it will be impractical to design for shock rates at this level In this case a continuousdosage of 1 - 2 wppm should be used (Refer to Step 5 of the DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS )
STEP 7 - Specify pumps and distribution system
Specify other major facilities in the once-through cooling water system The recommended materials for pumps valves andpiping are shown in the EMRErsquos Refinery Construction Materials Manual
Pumps1 Type - These pumps are usually vertical centrifugal pumps However horizontal centrifugal pumps have been used An
economic study andor consideration of existing facilities will determine which type should be specified In the event thathorizontal centrifugal pumps are specified facilities for pump priming will likely be required For large pumps in salt water service there is a very limited selection of pumps because of the special materials required Consult with the DISCIPLINESPECIALISTS early in the design effort to determine what specific pumps are currently available
2 Total Required Capacity - Firm capacity (excluding spares) of the supply pumps should equal the maximum cooling water requirements including LGF and RCF
3 NumberIndividual Pump Capacity - Normally one spare pump is provided for up to six operating pumps two spares for up to twelve operating pumps etc The most common arrangements for water supply pumps are
bull Two 100 capacity pumps
bull Three 50 capacity pumps
bull Four 33-13 capacity pumps
bull Five 25 capacity pumps
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ExxonMobil Proprietary
Section Page
XXVII 18 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Maximum commercially available pump capacity ranges up to 300000 gpm (20000 Ls) at discharge pressures up to 100psi (700 kPa) Maximum capacity will be less for higher discharge pressures Other factors which may influence pump
selection arebull Selecting capacity to match existing pumps
bull Selecting capacity for future requirements
bull Selecting capacity corresponding to manufacturers available standardVendor information should be obtained on the availability size and cost of the pumps for several configurations Acomparison should be made of the investments and operating costs as well as other advantages and disadvantages toselect a pump configuration
4 Drivers - These pumps are usually motor-driven Because of the remote location of the intake facilities the steamdistribution costs are normally too high to justify the use of a steam turbine driver However use of a secondary selectivesubstation provides a reliable source of power to the pump drivers
5 Pump ∆∆∆∆P - Pump ∆P is based on supplying the maximum cooling water requirement at the battery limit design pressureestablished for the system The ∆P should include the total circulating system pressure drop plus any elevation differencebetween the low water level at the intake and the discharge point of contact with air
6 Pump Instrumentation - The remote location of the intake facilities makes remote control and monitoring of the water supply operation desirable Pump controls located in the control house provide for minimum manning Local controlbackup should be provided The Owners operating and manning philosophy determine the basis for instrumentation andcontrolThe spare pump is controlled to automatically start on low pressure in the supply header Concurrently alarms should beactuated in the control room to indicate the low-pressure situation and spare pump operation
Distribution Network1 Configuration - The piping network is a once-through system with a return line from each consumer or consumer group to
the cooling water effluent line2 Layout - The routing of the piping network should connect all cooling water consumers in the most economical manner
permitted by the plant layout Careful consideration should be given to the turnaround philosophy of the plant Thenetwork layout should preclude the necessity of having above-ground water lines that are in service running through anarea that is being turned around Underground piping or an extra lateral may be required to accomplish this objective
3 Valves and Blinds - Battery limit valves should be provided in the network to isolate individual process units from thesystem Valves should be provided to permit onstream removal and maintenance of spared equipment
4 Basket Strainers - Consideration should be given to the installation of basket strainers in the laterals serving each of theconsumer loops These provide added insurance against entrance of foreign material into the cooling equipment The useof intake screens with 025 in (63 mm) openings may eliminate the need for basket strainers in the laterals
Line Sizing1 Design Basis - The network is sized so that water is delivered to the most distant consumer at a pressure equal to the
required process design (battery limit) pressure2 Flow Rate - Each segment of the distribution network is sized for the simultaneous maximum flow in that segment for any
operating condition The main supply and discharge headers are sized for the simultaneous maximum flow of all segments(not necessarily the sum of the simultaneous maximum flows of the individual segments)
3 Pressure Drop
Line Sizing Basis - The distribution system should be sized according to the guidelines established in Section XIV The use of an ldquoold piperdquo friction factor of C = 90 - 100 (Hazen-Williams factors) in the main lines is recommended to assuregood operation in later years when the system experiences fouling For large diameter lines [greater than 48 in (1200mm)] maximum water velocity should be limited to 10 fts (3 ms) based on civilmechanical considerations (supports andanchoring required to handle potential pressure surges) For salt water systems utilizing cement-lined pipe consider thereduced inner diameter in the friction loss calculations A study may be necessary to determine the economic pump ∆P vsline diameters
4 Equivalent Length Factors - Equivalent length factors are used in pressure drop calculations for sizing the distributionsystem The piping length scaled from the plot plan is multiplied by an equivalent length factor to compensate for pipingelbows expansion loops and other piping variations
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Section Page
COOLING WATER SYSTEMS XXVII 19 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
The equivalent length factors are selected based on the expected variation from a straight piping run Typical equivalentlength factors suitable for planning purposes for different piping applications are as follows
TYPE OF PIPING EQUIVALENT LENGTH FACTORS
Long piping runs [gt 1000 ft (300 m)] 12 to 14
Onsite distribution piping 15 to 20
For design purposes the actual piping layout should be evaluated to ensure that the equivalent length factors areapplicable to the specific situation Particular attention is required for revamp work and saltwater systems utilizing cement-lined pipe
STEP 8 - Prepare a system flow plan similar to the one shown in Figure 1
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS
STEP 1 - Determine cooling water requirements
Tabulate the cooling water requirements and cooling water discharge temperature for each process unit and offsite consumer Apply the proper LGFs to all cooling loads depending on the stage of the project and whether the new units are exactduplicates of existing units Determine the total cooling water flow required including LGFs and the bulk temperature (T 1) of thecombined cooling water flow from all coolers to the cooling towerIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The onsite and offsite designers should investigate methods to increase effluent water temperature to themaximum recommended limit for the particular type of water used This will result in a lower cost cooling tower for a given heatrejection rate One common method of maximizing return water temperature is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 2 - Select cooling tower design parameters
Select the proper design conditions of ambient wet bulb temperature approach temperature range and flow rate for specifyingthe cooling tower Based on these conditions the cooling tower vendor will size the tower and provide the tower characteristicand performance curves which serve as the basis for the performance guarantee Required performance test and evaluationprocedures are described in IP 8-1-1 Economic studies may be required to select the ambient wet bulb temperature andapproach temperature for new locations or for expansion projects with special cooling requirements
STEP 3 - Prepare cooling tower specification sheet
Include the following information in the heat exchange section of the design specificationTower Flow Capacity - This was determined in Step 1Type of Tower - The tower usually specified is an induced draft multicell type suitable for fresh salt or brackish water depending on the water available Additional tower characteristics are usually developed and proposed by the tower vendor (eg crossflow versus counterflow type of tower fill etc) The vendor should be made aware of conditions such as thelikelihood of heavy oil leakage into the cooling water or high fouling potential which may influence the tower type or fill
selectionNumber of Cells - The minimum number of cells required (normally not less than 2) may be specified The maximum flow per cell is a function of the cooling tower design conditions selected in Step 2 Typical values range up to 10000 gpm (600 Ls) for crossflow designs and 8000 gpm (500 Ls) for counterflow designs Normally spare cells are not provided Maintenance iscarried out during cooler periods or load is shed
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Section Page
XXVII 20 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Ambient Wet Bulb Temperature1 The ambient wet bulb temperature selected for specifying the cooling tower design is generally that temperature which is
equaled or exceeded 5 of the time during the four hottest months (5 of the hourly readings taken during the four hottestmonths will exceed this value) Specifying the 5 temperature should give satisfactory results since the time that the wetbulb temperature is exceeded is for short duration and the system inertia is usually sufficient to avoid problems The tower specification should make it clear that the wet bulb temperature specified is an ambient value as opposed to the enteringvalue which may be higher because of recirculation The tower manufacturer should adjust the ambient value upward totake into account his estimates of recirculation Currently cooling tower test codes provide procedures for measuringperformance in the case of either entering or ambient wet-bulb temperature specifications An increase of 2 degF (11 degC) isgenerally added to ambient wet bulb temperature to estimate entering wet bulb temperature for large cooling towers
2 If plant production will peak during the four summer months when the 5 ambient wet bulb temperature may be exceededconsideration should be given to specifying a higher design ambient wet bulb temperature (one that will be exceeded 212 of the time for example) However the incremental cost of the resulting closer approach and larger cooling tower must be justified by the value of the incremental products made
On-Tower Temperature (T1) - This temperature cannot be accurately determined until the design of all process coolers iscompleted and the volume of cooling water and outlet temperature are known for each (see Step 1) In the planning stage or early design stage of a project this temperature must be estimated and is usually set equal to the maximum design cooler outlet temperature shown under COOLING WATER REQUIREMENTS in the BASIC DESIGN CONSIDERATIONS portion of this sectionApproach Temperature - The approach temperature has the most pronounced effect on cooling tower size and cost For example reducing the approach from 10 to 5 degF (55 to 3 degC) can increase the cooling tower investment by 50 percent and thefan operating cost by 65 percent Thus various approach temperatures should be evaluated to determine the minimum cost of the total system including the process exchanger area Approaches used in the past designs have been from 5 to 12 degF (3 to7degC) Note that it is not customary in the cooling tower industry to guarantee approaches of less than 5 degF to (3 degC) to coolingtower design inlet wet bulb temperature A typical approach for a new grass roots cooling tower is 10 degF (6 degC)Off-Tower Temperature (T2) - This is the sum of the approach temperature and the ambient air wet bulb temperature For existing locations Table 2 in Section IX-B may be used
Range and Flow Rate1 The range for a given heat load is related to the circulating water rate as follows
Q = M C p ∆Tcong (500) (gpm) (range) Eq (1)
where Q = Heat load on the tower BtuhM = Water circulating rate lbhCp = Specific heat capacity of water at average temperature Btulb degF∆T = (T 1 ndash T 2) degF = (water cooling range)gpm = Circulation rate in US gallons per minute
For calculation in Metric units
Q = M C p ∆Tcong (1162 x 10 -3) (Ls) (range) Eq (1M)
where Q = Heat load on the tower kJh
M = Water circulating rate kghCp = Specific heat capacity of water at average temperature kJkg degC∆T = (T 1 ndash T 2) degC = (water cooling range)Ls = Circulation rate in liters per second
2 Generally lower tower investment fan horsepower and pump horsepower result from maximizing the range andminimizing the flow rate However based on fouling considerations of process coolers the maximum allowable hot water temperatures must be kept below recommended limits Also while increasing the range minimizes cooling tower investment and operating cost the cost of exchanger surface requirements is adversely affected by the LMTD and acompromise must be made The range is bounded by the maximum allowable hot water temperature and the Off-Tower temperature (T 2) For established locations refer to Section IX-B for design cold water temperatures to assist in selectingthe approach and range
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Section Page
COOLING WATER SYSTEMS XXVII 21 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Makeup Water Requirements - Calculate the various losses in the cooling water system to determine makeup requirementsThese losses include
1 Evaporation Loss (E) - Allow 1 of the circulation rate for each 10 degF (55 degC) temperature drop through the tower Notethat the actual evaporation loss will be somewhat less depending on ambient air temperature
( )sL555
TT)gpm(
1000TT
E 2121
minus=
minus= Eq (2)
2 Drift Loss (D) - IP 8-1-1 limits the maximum allowable drift loss to 0005 of the design water circulation rate for freshbrackish or salt water
( )sL1000050
)gpm(1000050
D
=
= Eq (3)
This low drift level contributes little to solids removal from the tower with the spray This allows higher cycles of concentration to be achieved in cooling towers which use clarified softened water for makeup and thus have low requiredblowdown rates Higher drift loss percentages in older or poorly maintained towers will limit the maximum cycles of
concentration achievable3 Miscellaneous Losses (O) - Miscellaneous losses are estimated to be one-seventh of the difference between evaporation
loss and drift loss
minus=
7DE
O Eq (4)
4 Blowdown Rate (B) ndash Guidelines for maximum concentrations of impurities allowed in recirculated fresh cooling water areshown in Table 2 For salt water the blowdown is based on keeping the total dissolved salts below 55000 wppm Obtainthe chemical analysis of the makeup water and divide each individual component concentration into its counterpart inTable 2 The result is the maximum allowable cycles of concentration for that constituent (a worksheet is provided inTable 4 ) The impurity with the lowest factor will be the control and will determine the required blowdown rate
)OD(1Cycles
EB +minus
minus= Eq (5)
The makeup water requirement is the sum of all losses shown above
MakeUp = E + D + O + B Eq (6)
STEP 4 - Specify the controls necessary for proper cooling tower operation
Air Side - Cooling towers are provided with motor-driven manually adjusted pitch multiple-bladed propeller fans Figures9A amp B may be used to estimate the normal operating horsepower requirements Since the fan motor and gear reducer arelocated at the top of the tower [generally 40 to 50 ft (12 to 15 m) above grade] the need for equipment to handle thesecomponents should be discussed with the affiliateControl of the air flow through the tower to maintain reasonably constant cold water temperatures can be accomplished byturning off individual fans in multiple-cell installations adjusting the blade pitch or using variable speed motors To turn off individual fans remote at grade start-stop buttons for the motors are usually specified
Whenever the ambient air temperature is 32 degF (0 degC) or below icing of the tower louvers and structural members in contact withthe incoming air will occur If the heat load on the tower is relatively free of seasonal variations two-speed fans should provideadequate ice control down to approximately 0 degF (-18 degC) ambient temperatures When sustained temperatures of 0 degF (-18 degC)and below occur two-speed fans capable of two-speed reverse operation with appropriate time delays for change of directionand speed should be specified to periodically eliminate ice which will build up on the louvers Other methods of winterizationare available but in general are more expensive and require greater operator attention In very cold climates steam heatingshould be provided to prevent freezing in the cold water basin during shutdown periodsWater Side - The water circulation rate is maintained by operating one or more pumps of sufficient capacity to meet the systemdemand Flow-measuring facilities local temperature indicators and a high-temperature alarm are provided on the coolingtower water supply and return headers Hand-operated flow control valves are required for distributing the water equally to theindividual cells On counterflow towers provisions for measuring the flow to each cell (pitot tubes) are required They are notrequired on crossflow towers since the open distribution basin provides visual aid in balancing flows Shutoff valves should beprovided for servicing the flow control valves without shutting down the entire tower
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Section Page
XXVII 22 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
STEP 5 - Provide chemical treatment facilities for the fresh water system
Alkalinity (pH) Control (if used)1 In cases where a controlled pH corrosionscale control program is being used the alkalinity (pH) of the recirculated cooling
water should be controlled in accordance with the program requirements Normally acid addition is required to accomplishthis This is due to free carbon dioxide being stripped from the cooling water in the tower to approximately 5 ppm For example if the alkalinity has to be controlled to between 25 and 50 ppm for a pH of 70 to 75 the volume of 98 sulfuricacid (66 degBeacute) required to treat the makeup water is calculated using the following equation
[Note 93 sulfuric acid should be used if ambient temperatures will be less than 35 degF (2 degC)]
GallonsHour 66 degBeacute H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate gpm) (326 x 10 -5)
For calculation in Metric limits
kgh of 100 (66 degBeacute) H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate Ls) (793 x 10 -3)
2 The metering pump should be designed for a maximum feed rate of two to three times the normal feed rate A glass woolfilter is required on the pump suction piping to minimize plugging Normally two - 100 capacity pumps are provided
Positive displacement metering pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumps with flow control areused for higher rates
3 Hydrochloric acid may also be used for alkalinity control The gallons per hour of 20 degBeacute hydrochloric acid required is36 times the amount of 66 degBeacute sulfuric acid In Metric units the weight of 32 (20 degBeacute) hydrochloric acid required is227 times the weight of 100 (66 degBeacute) sulfuric acid
4 Specifications for chemical feeders may be found in EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased)
5 A pH analyzer with high and low alarm is provided to control the amount of acid feed The acid should be added to thecooling tower makeup water (Figure 2 ) The lag time between the point of acid injection and the sample outlet to the pHcell should be less than two minutes for effective control
6 Spills or overflows from the sulfuric acid tank should be contained
Corrosion Inhibitors and Anti-Scalents1 Two chemical feeders are generally provided The following guides in sizing the equipment will allow the use of any of the
proprietary products from the vendors when final selection by the owner is made See Report No EE102E78 Cooling Tower Water Treatment Guidelines EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and Report No EE34E86 Environmentally - Acceptable Cooling Water Treatments for detailed discussion of acceptable commercial chemicals
bull If the water treatment program to be used is known design for the chemical rates specified by the vendor or COOLING WATER SYSTEMS SPECIALIST
bull Otherwise design for 100 wppm of each commercial liquid product in the recirculated cooling water The design feedrate should be based on the design total blowdown flow (both uncontrolled and controlled) since this is where theproduct is lost For 100 wppm concentration the chemical rates will be as follows
GallonsHour Chemical = Blowdown (gpm) x (0006)
LitersHour Chemical = Blowdown (Ls) x (036)
2 The design capacity of the metering pump should be two to three times this calculated number Normally 2 - 100capacity pumps are provided Positive displacement pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumpswith flow control are used for higher rates
3 Specifications for chemical feeders may be found in EMRErsquos Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased) Reference should also be made to the checklists containedin Report No EE92E94 Guidelines for Safety Evaluation of Chemical Injection Facilities
4 A final check should be made with the owner and the COOLING WATER SYSTEMS SPECIALISTS as to the treatmentprogram selected and if there is a need for an additional chemical feeder
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COOLING WATER SYSTEMS XXVII 23 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Biological ControlFacilities should be provided for feeding the biocide into the back of the cooling tower basin If chlorine is used it must bepresent in the cooling water as free residual to be effective However the following amounts of chlorine are first required toreact with the listed impurities when present in the cooling water before a free residual can be established
IMPURITIESAMOUNT OF CHLORINE REQUIRED (wppm)
WITH EACH WPPM OF IMPURITY
Ammonia (NH 3)
Hydrogen Sulfide (H 2S)
Sodium Sulfite (Na 2SO 3)
Ferrous Iron (Fe ++)
10
22
056
064
The design capacity of the chlorine feeder shall be based on achieving a 01 wppm free chlorine residual in the recirculatingwater flow or a minimum dosage of 7 wppm at maximum design makeup water flow rate and conditions (see EMRE Water and
Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service )Chlorine may be fed either as the commercial liquid sodium hypochlorite (124 wt Cl 2) or liquid chlorine (100 Cl 2) Liquidhypochlorite is being used increasingly because of the higher costs associated with precautions equipment and proceduresrequired to minimize the risks associated with handling gaseous chlorine The hypochlorite solution is fed using a meteringpump which should be sized to feed an equivalent of 15 ppm 100 Cl 2 (Refer to EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) ) In case of spills or an overflow from thestorage tank the sodium hypochlorite may be sent to the cooling tower basinGas chlorinators are covered by EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service The use of gas chlorination is being reduced but may still be appropriate in some locations subject tolocal management approval The gas from either commercial cylinders or an evaporator is dissolved in a water slip stream bymeans of an eductor This results in a chlorine solution strength of approximately 01 This solution should be evenlydistributed below the water surface by means of a header lateral pipe arrangement at the rear of the cooling tower basinopposite the cooling water pumpsSome additional guides on the use of gas chlorinators are given below It is also recommended that the safety guidelines
published by the Manufacturing Chemists Association and the Chlorine Institute be followedbull Dilution water for the chlorinator must not be less than 50 degF (10 degC) or ice formation may cause plugging problems
bull Continuous rates of chlorine gas withdrawal should not exceed 400 lb (180 kg)24 hours from each one ton cylinder atroom temperature [70 degF (21 degC)] This rate may be exceeded (up to 50) for periods not exceeding 2 hours Rule of thumb is not to design for more than 4 cylinders operating in parallel When feed rate exceeds 3000 lb (1360 kg) per daythe liquid chlorine should be fed to an evaporator upstream of the chlorinator
bull If the ambient temperature can be expected to be below 50 degF (10 degC) for extended periods a building should be providedfor the chlorinator and cylinders
bull Weighing scales should be provided to determine when shipping containers should be replaced and to verify chlorine feedrates
bull Analyzers to measure the free chlorine residual in the circulated cooling water should be provided
bull Chlorine leak detectors and a catch basin for major spills should be provided
Control SystemProvide automatic controllers to continuously monitor water condition and to add chemicals to the system (complete systemsmay be available through the water treatment program vendor as a lease or purchase)pH Control - This controller adds acid to correct water alkalinity a timer on the acid pump sounds an alarm if the pump doesnot shut off in a predetermined length of time Backup controls are required to prevent the cooling system from beingpermanently damaged in case the pH electrodes become fouledCorrosivity and Inhibitor Control - Systems are available which give a direct readout of corrosion rate in milsyear If thecorrosion rate exceeds a set point the controller shuts off the acid increases the inhibitor addition opens the blowdown valveand sounds an alarm Electrodes measure the completeness of the inhibitor film by the value of the potential differencebetween the electrodes The larger the difference the more incomplete the film and a potential reading exceeding a set levelactivates the inhibitor feed
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XXVII 24 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Free Residual Chlorine Control - Free residual chlorine should be controlled automatically for biocide treatment Severalgood analyzers are currently available These units can be incorporated into an automatic control system
Oil in Water DetectionMonitoring Corrosion and Deposit FormationThe following types of devices for measuring corrosion rates and deposit formation should be provided to allow the plant tomonitor the effectiveness of the cooling water treatment before scheduled turnaroundsCoupons - A corrosion coupon rack should be specified for the hot return water to the tower in accordance with IP 19-6-1 Corrosion Monitor - On-line corrosion analyzer if specified should monitor the hot return water to the tower in accordance withIP 19-6-1
On-Line Test Exchanger Although there are a variety of deposit monitors from the water treatment chemical vendors they do not necessarily representactual heat transfer conditions in process exchangers For this reason a test exchanger should be installed for evaluatingcorrosion and deposit formation Details are covered in Report No EE102E78 Cooling Tower Water Treatment Guidelines
Alternatively a critical process exchanger can be instrumented such that its heat transfer coefficient can be monitored
Instrumentation would include flows and inlet and outlet temperatures of process fluid and cooling waterSTEP 6 - Specify cooling tower location
Select a location for the cooling tower The following factors influence this selection
Location Factors1 The location should be within a reasonable distance of the units served and consistent with future refinery plans Piping
costs to and from battery limits can be a significant portion of the total cooling system cost and can be the location-determining factor particularly for salt water systems While piping costs can be easily determined other costs affectinglocation cannot be estimated readily Therefore judgment is required in selecting the location
2 Safety spacing requirements (see Section XV-G Equipment Spacing )3 Cooling tower contribution to plant boundary noise4 For salt water cooling towers wind direction and distance to other facilities is of major concern because of potential
corrosion problems
5 Factors affecting cooling tower performance ie orientation with respect to wind site congestion etc
Plot RequirementsIn estimating plot requirements (basin area) one should allow 5 to 6 ft of tower length for each 1000 gpm (2 m for each 60 Ls)of circulating capacity This figure is based on a typical counterflow design with a width of 50 ft (12 m) and does not includepump suction pit space Exact requirements can be obtained from cooling tower vendors after the tower capacity has beenestimated
Fogging and DriftTwo distinct problems are associated with the discharge air from a cooling tower (1) fogging which can be a nuisance or ahazard and (2) drift which is generally a nuisance because of increased corrosion of surrounding equipment Fogging fromcooling towers occurs during periods of combined high ambient humidity and low temperature as a result of mixing the warmmoist cooling tower exhaust air with the cooler ambient air The ambient air cannot absorb all moisture as vapor and thus tinyfog droplets are formed by condensation Fogging and resulting icing in cold climates has produced hazardous drivingconditions on highways up to 850 ft (250 m) awayProprietary designs are available to combat potential fogging problems Basically these designs incorporate heating of thedischarge air with hot return water precooling coils (similar to an air cooler) to eliminate any visible plume However thesedesigns are expensive and relatively unproven Therefore proper location based on reliable weather data is the preferredsolutionDrift from a cooling tower represents a more difficult problem since it does not depend on definable climatic conditions andoccurs for all wind directions The effects of drift on nearby equipment and vegetation are particularly severe and generally of concern only for salt water systems because of the high solids content of the drift (concentrated salt water) For salt water cooling systems electric lines substations and instrumentation should be located upwind in the direction of the predominantwind They should be provided with adequate shelter or materials when within 450 ft (140 m) of the cooling tower In additionspecifications for all equipment within 150 ft (45 m) of the tower should include surface treatment suitable for a marineenvironment
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COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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DESIGN PRACTICES December 2000
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TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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DESIGN PRACTICES December 2000
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FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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Section Page
COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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ExxonMobil Proprietary
Section Page
XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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Section Page
XXVII 4 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BACKGROUND
Rejection of heat is a requirement of a great many processes and utility systems Coolers and condensers reject much of thisheat to water The main purpose of a cooling water system is to absorb the heat rejected and dispose of it The system must
be safe reliable and flexible and it must have a minimum impact on the environmentCooling system selection and design is very site specific and cannot be generalized The EMRE COOLING SYSTEMSPECIALISTS are available for consultation on specific requirements
DEFINITIONS
Ambient Wet Bulb Temperature
The wet bulb temperature that exists in the cooling tower area It is the temperature specified in the Design Specification for the design and guarantee basis Note the actual tower inlet wet bulb temperature may be higher than the ambient wet bulbtemperature due to hot air recirculation
Approach
The difference in temperature between cold water leaving the tower and ambient wet bulb temperature
Battery Limits
A boundary which is normally assumed to be at the physical edge of the block in which the facility is located
Blowdown
The continuous or intermittent discharge of a small amount of the circulating water Its purpose is to limit the increase in theconcentration of solids in the water due to evaporation It is expressed in percent of water circulated
Brackish Water
Water which contains 3000 to 5000 wppm of dissolved salts
Cell
The smallest tower subdivision which can function as an independent unit with regard to air and water flow
Cooling Tower Pumping Head
The total pressure at the centerline of the tower inlet to its hot water distribution system plus the difference in elevation betweenthe centerline of the inlet and the top of the cold water basin curb It does not include friction pressure drop in the riser pipeThis is a parameter representing the head required for the cooling tower only It excludes the cooling water distribution system
Counterflow Tower
A cooling tower in which air induced at the bottom of the tower flows up through the fill against the falling water
Crossflow Tower
A cooling tower in which air induced at the sides of the tower flows horizontally across the fill and the falling water
Cycles of Concentration
The ratio of dissolved solids in circulating water to dissolved solids in makeup water
Dissolved Solids
A measure of the total quantity of dissolved salts in water It can be determined by conductivity measurements
Drift
The entrained water carried from the tower by exhaust air expressed in percent of water circulated
Dry Bulb Temperature
The temperature of ambient air read on an ordinary thermometer
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Section Page
COOLING WATER SYSTEMS XXVII 5 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DEFINITIONS (Cont)
Fill
Packing within the cooling tower to provide the required surface area for heat transfer between ambient air and the returncooling water Splash fill breaks the water into small drops by causing it to cascade through successive offset levels of parallel level splash bars Cellular or film fill causes the water to spread into thin films over large vertical areas
Heat Load
Heat removed from the circulation water within the tower It may be calculated from the range and the circulating water flowrate
Interference
The mixing of tower inlet air with the discharge vapors from another tower or other heat source This results in reduced thermalperformance
Makeup
The water required to replace circulating water which is lost by evaporation drift blowdown and leakage It is expressed inpercent of water circulated
Offsites
Support facilities such as utilities (steam power cooling water) tankage waste treating etc for the processing operations
Once-Through Cooling System
A system in which water passes through the heat exchange equipment once and is then discharged
Onsites
Facilities that are a part of the processing operations
Range or Water Cooling Range
The difference in temperature between the hot and cold circulating water or on-tower temperature minus off-tower temperature
Recirculated Cooling System
A system in which water is circulated to coolers in a loop and the heat absorbed from this cooling operation is dissipated by acooling tower or some other type of heat exchanger
Recirculation
An undesirable condition in which part of the tower discharge vapor stream is recirculated through the tower resulting inincreased inlet air wet bulb temperature
Relative Humidity
The ratio of the quantity of water vapor present in air to the quantity which would saturate the air at the existing temperature
Salt Water
Water which contains more than 5000 wppm of dissolved salts
Wet Bulb Temperature
The equilibrium temperature obtained when ambient air is passed over a continuously wetted thermometer bulb evaporatingthe water and cooling the bulb This equilibrium temperature occurs when the heat transferred from the air to the wettedsurface equals the heat loss due to latent heat of evaporation
Wind Rose
A diagram that shows the average frequency and intensity of wind from different directions for a particular location
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Section Page
XXVII 6 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
SYSTEM TYPES AND APPLICATIONS
Cooling water systems in refineries and chemical plants are generally either the once-through or the recirculated type designedfor fresh salt or brackish water
In the once-through system (see Figure 1 ) pumps take suction from a source of supply such as a river or other body of water and deliver the water to process coolers and other consumers After passing through the cooling exchangers the hot coolingwater is discharged through pressurized piping or a gravity flow systemIn a recirculated system (see Figure 2 ) pumps take suction from a cooling tower basin and deliver the water to process coolersand other consumers After passing through the cooling exchangers the hot cooling water is discharged through a pressurizedreturn system to a cooling towerThe choice between a once-through and recirculated cooling water system must be based on (1) availability of water of satisfactory quality (2) process temperatures (3) atmospheric conditions (4) investment and operating costs and (5) effluentwater quality and temperature limitationsThe cooling water is distributed to individual consumers throughout the plant in piping main supply and return headers andseries and parallel branches consisting of laterals and sublaterals The main cooling water distribution headers are either buried in underground trenches or are located above grade on ground-level pipe sleepers or on elevated pipe racksUnderground piping has the advantage of minimizing problems associated with thrust loads thermal expansion and freezingElevated piping is easier and less expensive to install and repair and it makes it easier to locate leaks
BASIC DESIGN CONSIDERATIONS
RELIABILITY
The cooling water system is an important utility and its reliability is critical to successful plant operation The typical systemprovides cooling water to both onsite and offsite consumers It can have a direct impact on the sizing criteria for the emergencyrelief (flare) system (refer to Section XV-C Safety in Plant Design Pressure Relief ) The system therefore normally must becapable of continuous operation so that cooling water is always available to critical consumersThe reliability required to achieve continuous operation is provided in several ways
bull Spare makeup and water circulating pump(s) are provided to assure continuous operation
bull Different types of drivers are specified for the supply or water circulating pumps Motor-driven pumps are supplied fromtwo different buses on a secondary selective electrical distribution system
bull Cooling tower fan drivers are normally supplied from a secondary selective systembull Holdup volume is provided in the cooling tower basin to permit an orderly shutdown of process units if the makeup water
supply is lost or a failure occurs in any part of the recirculated cooling water system
bull Valves blinds and bypasses are provided to permit individual components to be removed from the system for maintenancewhile the system is operating
bull Construction materials provided for cooling water facilities are designed for long-term corrosion resistance sincemaintenance work is often difficult (if not impossible) to schedule without a complete refinery shutdown
FLEXIBILITY
The cooling water system must include sufficient flexibility to cover all present operating requirements plus any future operatingrequirements that are defined in the design basis document For example future operating requirements could be dictated by anear-term increase in plant sizeThe type of flexibility features provided in a system may include the following
bull Makeup water treating facilities should be able to deliver the required water assuming the worst conditions of the water source
bull Waste treating facilities should be designed to handle effluent water containing varying concentrations of oil and chemicalsover the range of expected ambient temperatures
bull Valves and blind flanges should be provided to permit future tie-ins to the system without a shutdown An exception wouldbe on lines where a ldquohot taprdquo could be made with minimum risk of an emergency shutdown or where the cost of requiredvalves is prohibitive
bull Cooling tower cells should be completely partitioned as described in IP 8-1-1 to allow personnel entry to any cell for maintenance without affecting the operation or capacity of the other cells Normally spare cells are not providedMaintenance is carried out during cooler periods when the cooler ambient and wet bulb temperatures enable the coolingtower to handle greater heat loads
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Section Page
COOLING WATER SYSTEMS XXVII 7 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
bull Where multiple cooling tower systems exist consideration should be given to providing valved cross connections betweentheir respective supply mains and between their respective return mains These would be used only for special operations
such as turnarounds or emergencies The impact on flare sizing should be evaluated if these cross connections areprovided
ENVIRONMENTAL AND SAFETY CONCERNS
Once-through cooling water systems usually require controls to minimize the release of hydrocarbons and free halide residualsto the environment As a minimum cooling water discharges should be monitored for free oil total organic carbon or hydrocarbon vapors to determine if system coolers are leaking Free chlorine residuals in the discharge should also bemonitored as required to meet local regulations and to detect process leaks In most cases once-through cooling water flowsare too large to be handled in the plants wastewater treatment system However once-through water that could becomecontaminated with hydrocarbons is often segregated and treated in a large skim pond or special purpose separator prior todischarge Local environmental regulations and permit requirements will dictate the extent of treatment requiredCooling tower blowdown from recirculated systems is normally bypassed around major wastewater treatment and dischargedwith the treated wastewater An oil-in-water detector should be provided to detect oil in the blowdown from process leaks Thiswill enable stopping the blowdown or diverting it to temporary holdup until the leak is corrected A major rupture in a processheat exchanger tube can also result in the cooling tower being the discharge point for hydrocarbon vaporsBecause of the likely presence of flammable vapors the areas around cooling towers are subject to electrical classification inaccordance with IP 16-1-1 The type and amount of chemicals added to the cooling water should be evaluated as to their effects on the plants wastewater treatment process Biocides corrosion inhibitors and antifoulants may result in significant problems for the wastewater treatment plantOther environmental concerns are
bull Cooling tower noise emissions from fans and from the flow of cooling water over the tower may require suppression if near a local community
bull Cooling tower water mist drift can cause fog and ice formation on nearby roads and promote deterioration of nearbyequipment It may also result in damage to sensitive vegetation in the surrounding area
bull Process gas releases to atmosphere resulting from process gas pickup by cooling water including the venting of toxicgases from exchanger tube ruptures must be considered in locating cooling towers or discharge points from once-throughsystems
bull Spills and overflows of toxic and hazardous chemicals used in treating the cooling water must be contained
SYSTEM SELECTION
The selection of a cooling water system is often determined by the availability of water In regions where water is scarce once-through systems are obviously eliminated from consideration and recirculated systems are chosen A once-through coolingsystem may be the economical choice if the plant is located adjacent to a body of water and consumers are elevated no morethan 50 ft (15 m) above the water level However once-through system costs may be increased significantly by environmentallimitations in many locations These may limit maximum discharge water temperature and therefore will reduce the maximumcooling water ∆T This will increase the required flow and cost of once-through systems In addition regulations may requirewaste treatment of the large volume of effluent thus increasing costsThe choice of a cooling water system must be based on comparisons of initial investment and operating costs Relative initialcosts of a system vary with climatic conditions distance from pump to users complexity of inlet structures and corrosive nature
of the water Salt water or brackish water service requires the use of cement mortar lined pipe and alloys for exchangers andpumps Carbon steel is usually satisfactory for fresh water Special attention should be given to the impact of bottom silt mudand seaweed on intake facilities to once-through or makeup to recirculated systems Cribs flumes large bays screens trashracks or other extensive facilities may be requiredDuring the early stages of a project the designer should review with the planning or design groups the heat level and volume of all process streams which require cooling The optimum method of cooling should then be determined ie heat exchangeandor rejection of heat to air or water A break point should be selected above which air cooling is economical and belowwhich trim water cooling is preferred
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Section Page
XXVII 8 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
COOLING WATER REQUIREMENTS
Cooling water requirements for each process unit and offsite consumer are obtained from the appropriate design groups For existing locations maximum cooling water loads should be based on the design inlet cooling water temperatures given inSection IX-B Design Considerations for All Types of Heat Exchangers For new locations the inlet cooling water temperaturemust be set based on local water temperature data for once-through systems and wet bulb temperature and selected approachfor recirculated systemsThe bulk water temperature at the cooler outlet has been selected as the controllable variable for design and operation of coolers Although the corrosion rate depends on the metal temperature this is difficult to determine in the operation of a coolerBased upon the findings of a Materials Development Section test program maximum design outlet water temperatures havebeen set as indicated below
Fresh Water 130 degF (54 degC)
Treated Fresh Water 130 degF (54 degC)
Brackish Water 125degF (52
degC)
Salt Water 120 degF (49 degC)
When the maximum plant cooling water requirement has been established for new process and utility consumers appropriateLoad Growth and Reserve Capacity Factors (LGF and RCF) should be applied depending on the development stage of theproject Cooling water loads should be updated and system capacity adjusted periodically as design work progresses
LOAD GROWTH AND RESERVE CAPACITY
Utility loads tend to increase as the loads become better defined through successive stages of project definition Thereforeload growth and reserve capacity factors (LGF and RCF) are applied to raw loads as a means of predicting final equipmentsizes and capital costs These load growth factors do not cover basis changes through project development such as project or process unit sizethroughput For revamp or modernization projects these factors are generally applied only to new loads or changes to existing loads on the basis that existing or unchanged loads are well known
Recommended Load Growth Factors are as follows
CONSUMERSPLANNING
(SCREENING THROUGH DBM) DESIGN SPEC
Existing Facilities 0 - 10 0 - 10
New Facilities 20 - 30 5 - 10
Selection of the load growth factor to use should be based on the quality and level of effort associated with development of theraw loadsFinal sizing of the source facility (eg the cooling tower and circulation pumps) should include a reserve capacity factor of 10to cover small post startup changes minor future projects and dynamic conditions not predicted by steady state balances Thesizing basis for distribution facilities should not include the RCF
COOLING TOWER TYPES
The cooling tower is the major piece of equipment specified in recirculating cooling water systems In effect it is the heatexchanger which cools the hot water to a suitable temperature for re-use primarily by evaporation through contact with airThe cooling tower contains fill to break up the water flow into droplets or film on a surface The fill increases the waterair contact area and enhances heat transferCooling towers are classified according to the means by which air is supplied to the tower ie induced or forced mechanicaldraft (fan) vs natural draft and according to the relative movement of the air and water that is counterflow or crossflow (seeFigure 3 ) Most of the towers in the ExxonMobil circuit are mechanical induced-draft type Economics determine whether theyare counterflow or crossflow
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COOLING WATER SYSTEMS XXVII 9 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
In crossflow towers the air travels horizontally across the falling water while in the counterflow design the air travels verticallyup through the falling water Comparisons of plot space fan horsepower and future maintenance costs determine which
design will be more economical Factors which influence this selection are cooling water rate range and approach for aparticular location In addition in some locations materials of construction and associated labor costs will affect the finalselectionIn the past crossflow towers were usually preferred over counterflow towers for extremely cold locations where ice formationcould restrict or block air flow through the louvers However this consideration is usually no longer valid due to the increaseduse of PVC film-type tower fill rather than splash-type fill in counterflow towersMechanical draft designs use one or more fans to provide the air needed for cooling Induced-draft units have the fans locatedon top of the tower Air is pulled through the packing and discharged vertically upward at high velocities Forced-draft unitshave the fans located at the base of the tower and push the air through the fill Forced-draft towers in large industrial sizesexperience difficulties with recirculation and in providing a uniform distribution of air As a result they have been supersededby induced-draft designsFill may be splash or film type Splash fill normally consists of horizontal slats in horizontal rows offset to one another to causethe water to break up into droplets as it falls downward through the cooling tower Splash fill is characterized by reduced air pressure losses It is also less conducive to clogging and easier to clean after a spill However it is very sensitive to
inadequate support and must remain horizontal and level It is made of treated wood or plastics such as PVC or polypropyleneFilm fill comes in various designs but they all cause the water to flow in films over the fill surface and provides more coolingcapacity within a given amount of space than splash fill Because of this increased heat transfer efficiency film fill is generallyused in most new cooling towers However because of its smaller passages it is more sensitive to plugging and is difficult toclean if it does become plugged It may not be appropriate for services cooling mainly heavy fuels or waxy lubes where a leakwould cause fill plugging or for services with a high biological fouling potential PVC is currently the most widely used materialfor film fill The selection of fill type is usually proposed by the vendor and reviewed by the COOLING WATER SYSTEMSSPECIALISTSNatural-draft or hyperbolic cooling towers depend on the natural draft created by the difference in the density of the enteringand leaving air for movement Generally hyperbolic cooling towers have been used in Europe and in the utility industry in theUSA for large capacities [250000 gpm (16000 Ls)] short ranges [10 to 15 degF (6 to 8 degC)] and long approaches [10 to 15 degF (6to 8 degC)] They are most effective where low ambient wet bulb temperatures occur with high relative humidities These towersnormally have diameters in excess of 200 ft (60 m) and are greater than 250 ft (75 m) in height Their major advantages are
bull No operating or maintenance cost for fans
bull Reduction in problems associated with fogging drift and recirculation due to their great height and diameterThe primary reasons why hyperbolic towers are not used in ExxonMobil projects include
bull High initial cost
bull Large plot space requirement and high visual impact
bull Complete dependence on atmospheric conditions
bull Lack of flexibility for future expansions
COOLING TOWER BASIN
The primary function of the cooling tower basin is to collect the cooled water leaving the tower and to provide a reservoir for thecooling water pumps Design aspects to be considered are the basins capacity cleanability draining facilities sump andscreen details To obtain the necessary time for corrective action during emergency conditions a minimum storage capacity of 90 minutes based on loss of makeup water flow should be provided in the basin between the basin low level alarm (LLA) andthe minimum pump submergence (MPS) When the basin water level reaches the MPS point loss of recirculation pumpsuction and cooling water flow is imminent Typical values for basin water levels are as follows
bull High Level Alarm (HLA) 1 ft (300 mm) below top of the basin wall
bull Normal Water Level 15 ft (450 mm) below top of the basin wall
bull Low Level Alarm (LLA) 25 ft (760 mm) below top of the basin wall
bull Distance between LLA and MPS 35 ft (1070 mm)
bull Minimum Pump Submergence Depth 45 ft (1370 mm)
bull Clearance of Pump Suction off Basin Floor 15 ft (450 mm) Actual values may vary depending on pump type selected pump suction line diameter and cooling tower cell plot area
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Section Page
XXVII 10 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
MATERIALS OF CONSTRUCTION
The EMRE Refinery Construction Materials Manual and IP 8-1-1 list acceptable materials for all components in each type of cooling water system (once-through and recirculating) For salt water systems emphasis must be placed on proper materialsselection throughout the system The external salt water environments in the vicinity of the tower itself and the intake stationsmust be taken into consideration Proper siting of the tower is extremely important to avoid salt water corrosion of adjacentunits or the need to use expensive materials to operate in such an environment
WATER TREATMENT FOR ONCE-THROUGH SYSTEMS
Once-through salt and fresh cooling water systems can develop a variety of problems such as fouling due to seaweed or other debris marine microorganisms such as muscles and shell fish and biological growth Corrosion and scaling problems areminimal in fresh water once-through systems because the water is not concentrated by evaporation In salt water systemscorrosion is minimized by proper materials selection and scaling problems generally do not occurProtection against fouling by seaweed or other debris is provided by careful selection of the water intake location and travelingscreens with washing facilities In some cases additional steps may be required These may include an additional secondaryset of traveling screens and possibly battery limit strainers on the cooling water supply lines to individual unitsProtection against fouling by marine microorganisms and biological growths is achieved primarily by injecting chlorinehypochlorite chlorinebromine or other similar chemicals which kill these growths Chlorine is commonly used and is injectedin sufficient quantities to produce a residual of 1 wppm in the effluent for one hour each day (shock chlorination) Continuouschlorination for 2 - 3 days per month at lower levels (03 wppm residual chlorine in the effluent) may also be used in addition toshock chlorinationThe large water volume discharged from once-through systems usually makes chemical treating for corrosion or deposit controltoo costly These problems are handled by proper materials selection and by keeping water velocities through exchanger tubesgreater than 3 fts (09 ms) Occasionally the water analysis of a fresh water source being considered for a once-throughsystem using carbon steel materials will indicate the need for some corrosion or deposit control In this case the cost of thethreshold treatment with the once-through system should be compared to the costs of a recirculated system before making afinal choice
WATER TREATMENT FOR SALT WATER COOLING TOWER SYSTEMS
A total dissolved salts (TDS) concentration limit of 55000 wppm for sea water in cooling tower systems requires high blowdownrates since the makeup sea water typically has from 35000 to 40000 wppm TDS Because of cost restraints the only water treatment normally specified for these systems is for biological control Chlorine is most commonly used either as directchlorine injection or hypochlorite produced in electrochlorinators Care must be taken in selecting the intake location to assurethat the suspended solids do not exceed 200 wppm in the recirculated water If it is not possible to find sea water with sufficientclarity clarification with polyelectrolytes should be evaluated Corrosion is one of the major problems with the use of sea water in cooling towers This requires proper materials selection
WATER TREATMENT FOR FRESH WATER COOLING TOWER SYSTEMS
Cooling tower water treatment is necessary to minimize or eliminate corrosion scale and biological fouling of process heattransfer surfaces caused by minerals and impurities in the water The difficulties caused by these impurities and the means of treatment are shown in Table 1 Water lost by evaporation results in an increase in total dissolved salts in the recirculated water To indicate the degree of concentration of impurities the term cycles of concentration is used
Corrosion is an electrochemical process that deteriorates metals exposed to water in the presence of corrosive agents such asacids oxygen or bacteria A common form of corrosion is pitting Severe corrosion can lead to equipment failure Corrosionis caused by many conditions including process leaks into the cooling water water flow velocity that is too low (causes depositsand fouling lending to corrosion) or too high (causes erosion and corrosion) low pH and high temperatures Corrosion is lesslikely in water with non-acidic pH values (greater than 7) although scaling is more likely in these pH rangesScaling is characterized by the formation of hard dense deposits on material surfaces These deposits impact heat transfer and can become a site for localized under-deposit corrosion Scaling is influenced by many factors including makeup water composition total dissolved solids in the recirculating water water temperature and pH and flow velocity Calcium carbonateis the main scaling constituent in all waters and it is the least soluble It forms from the hardness (calcium Ca ++ ) andbicarbonate (HCO 3
ndash ) alkalinity present in most waterFouling can be caused either by the settlement of suspended matter or by microbiological growth of algae bacterial slime or fungi Fouling will reduce heat transfer cause plugging and create corrosion sites
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Section Page
COOLING WATER SYSTEMS XXVII 11 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Treatment of the cooling water is broken down into two major categories external and internal External treatmentencompasses any treatment given to the cooling tower makeup water Internal treatment is the addition of chemicals directly to
the recirculated cooling water
EXTERNAL TREATMENT
External treatment to remove impurities in order to reduce the amount of blowdown should always be evaluated as to costeffectiveness The most frequently used techniques for treating makeup water are shown in Figure 11 Details of availabletreatment processes (see Table 1 for methods available) are covered in Section XXVI-A Boiler Feedwater Treatment For cooling tower systems that meet currently required drift limits in refineries and chemical plants uncontrolled cooling water losses (drift and miscellaneous process uses) normally amount to about 14 of the cooling water evaporation loss Under these conditions the maximum cycles of concentration achievable are 8 with no controlled blowdown Some cooling towersare operating at up to 10 cycles of concentration Additional controlled blowdown required to limit the cycles of concentrationper the water quality criteria is covered later
INTERNAL TREATMENT
The makeup water analyses amount of cooling water losses environmental restraints potential chemical hazards materials of construction and film temperatures of heat transfer surfaces determine the type of internal treatment required for cooling tower systems The internal water treatment program selected must address corrosion scale fouling biological growth andchemical attack of cooling tower componentsNumerous chemicals and treatment programs are continuously being developed for addressing the potential problems Theslightly acidic low pH programs of the past which used chromate based chemicals and usually required acid injection arebeing completely phased out because of environmental restrictions on chromate and the higher costs associated with meetingcurrent practices for handling hazardous chemicals such as acid Non-chromate treatments which use inhibitors such asphosphates zinc and organics along with higher pH have been developed to replace the earlier programs Although not aseffective as chromate these inhibitors operate at higher water pH which is a less corrosive environment The disadvantage of the higher pH is the greater tendency to form scale This tendency is controlled by the use of dispersants or scale inhibitorswhich are normally organic polymers or phosphenate A corrosion inhibitor for copper or brass is also normally required if these metals are present in the systemThe following paragraphs describe techniques and chemicals to control alkalinity (pH) corrosion scale sludge and biologicalgrowth The chemicals mentioned are normally purchased as pre-mixes from vendors and dosed in combination Mostcorrosion and scale control program chemicals are two drum formulations due to the incompatibility of some chemicals in theconcentrated state Therefore separate feed systems are generally provided for chemicals for control of corrosion and scale (2drums) and biological growth (if liquid) If a program includes fixed pH then an additional feed system is required to add acidWater treatment programs are site specific and changing The COOLING WATER SYSTEMS SPECIALISTS should beconsulted for recommendations on the internal water treatment programs when an analysis of the available water is known
Alkalinity (pH) Control
Cooling water treatment programs presently available fall into two basic categories those that do not control pH (floating pH or alkaline treatment programs without pH adjustment) and those that control pH within a specified range by adding acid Theprograms that do not control pH are gaining favor because of the additional cost equipment and precautions required tominimize the risk associated with handling acid and because of improvements in chemicals to control depositsHigh pH in the recirculating cooling water is a result of the alkalinity of the fresh water due to bicarbonate salts This combinedwith carbon dioxide results in the water having a pH range of 6 to 85 When this fresh makeup water is mixed with therecirculated cooling water and passed through the cooling tower the air flow strips out the carbon dioxide Bicarbonate ionsthen convert to carbonate ions raising the pH which can cause both calcium carbonate and calcium phosphate salts toprecipitate out of solution and form deposits Another debit in allowing the cooling water pH to increase is increased chlorineconsumption to control biological growthsIf a program using pH control is justified and chosen acid addition is used under control of a pH analyzer to controlrecirculated cooling water alkalinity and pH to the range required by the treatment program
Corrosion Control
To reduce corrosion to an acceptable level chemical corrosion inhibitors which form protective films on heat transfer surfacesare the most effective protection Inhibiting corrosion is accomplished by phosphates organics zinc nitrites and molybdatesalts Unfortunately the use of chromate which is a reliable corrosion inhibitor is prohibited by environmental constraintsNitrite is not practical in an open system due to atmospheric oxygen converting the nitrite to nitrate
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XXVII 12 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Phosphate inhibitors (orthophosphate polyphosphate phosphonate) are the basic treatment for new towers in todaysenvironment In addition to the phosphate chemistry salts of zinc may be added to aid in inhibiting corrosion These ions
combine with the hydroxide ion forming thin protective films Zinc is used to provide a synergistic effect in combination with thephosphate salts for protecting carbon steel surfaces from corrosion Finally all-organic programs are offered by severalmanufacturers The use of these treatments has been made possible by the development of polymer dispersants which givecalcium salts a much higher solubility and reduce the tendency to scaleWhen a cooling water system contains copper alloys azole based and azole substitutes are required as copper corrosioninhibitors There are several effective inhibitors The most common is tolytriazole (TTA) due to its resistance to chlorineThe corrosion inhibitors most frequently used in ExxonMobil recirculating cooling water systems are shown below indescending order of preference provided their use meets all environmental requirements
bull Stabilized Phosphate (OrthoPolyphosphate) - Environmental restrictions on the use of zinc have resulted in ExxonMobilplants in the USA using a blend of orthophosphate and polyphosphate type corrosion inhibitors with good results Thistreatment forms a thin protective calcium phosphate film on heat transfer surfaces through the use of orthophosphateconcentrations as high as 12 - 17 wppm The use of calcium phosphate dispersants and Hydroxethylidene Diphosphonate(HEDP) stabilizes the treatment and prevents the formation of calcium phosphate scale Concentration of the dispersantsdepends upon the calcium concentration temperature and pH Stabilized Phosphate treatment programs can operate in apH range of 68 to 78 Alternatively an alkaline (floating pH) stabilized phosphate treatment program can be used Thisprogram uses little or no acid addition and is gaining in popularity although not well established yet It operates with pHvalues of 8 - 9 and uses higher quantities of dispersants and lower levels of phosphate A corrosion inhibitor for copper and brass is normally included for all stabilized phosphate programs Several vendors offer these programs Twoseparate chemical feeders are generally required
bull Zinc Alkaline Phosphate - Heritage Exxon plants in Europe where the use of zinc is not restricted have obtained goodcontrol of corrosion and scaling using zinc-polyphosphate treatment programs Low (65 - 72) and high (73 - 80) pHprograms have been used The high pH (zinc alkaline phosphate) programs are the lowest cost and are recommended
As with the Stabilized Phosphate treatment dispersants such as HEDP and polyacrylates are required to prevent theformation of zinc or calcium phosphate scale on heat transfer surfaces A corrosion inhibitor for copper or brass isnormally used with this program which is offered by several vendors
bull Organic Salts - All-organic (polymer) programs are offered by several manufacturers and have been used successfullywithin h Exxon In general these treatments are based on a combination of a phosphonate to control corrosion polymers(dispersants) to prevent scale deposition and a corrosion inhibitor for copper or brass This type of program is mosteffective in conditions of high hardness high alkalinity (M alkalinity gt300 wppm as CaCO 3) and high pH values (gt 85)However some manufacturers have reported success in softer water with alternative polymer component formulations Ingeneral an inorganic corrosion control program is preferred to the all-organic programs which are sensitive to alkalinityand more costly
Detailed control limits for effective use of each of the above inhibitors and materials selection are covered in Report NoEE34E86 Environmentally Acceptable Cooling Water Treatments Report No EE102E78 Cooling Tower Water Treatment Guidelines and in the EMRE Refinery Construction Materials Manual
Scale and Sludge Control
Scale and sludge deposits in cooling water systems cause loss of heat transfer and provide the conditions for pitting corrosionScale consists of inorganic salts which have precipitated from the cooling water forming a coating on heat transfer surfacesSludge consists of organic and inorganic suspended matter that settles in quiescent areas and in heat exchangers The key toprevention of both scale and sludge formation in a cooling system is keeping water velocities greater than 3 fts (09 ms) andthe use of chemical additives in combination with a blowdown rate that keeps the impurity concentration below the level whichcauses deposits (See Table 2 for limits on impurities) Scale control has become increasingly important in higher alkalinityldquofloating pHrdquo water treatment programsSalts most commonly found in scale deposits are calcium carbonate calcium sulfate tricalcium phosphate zinc phosphatemagnesium silicate and silica The control method for preventing scale from each of these salts is discussed below
bull Calcium Carbonate - Where justified using pHalkalinity control in the range of 70 to 75 will normally inhibit the formationof calcium carbonate scale if anti-scalent chemicals (phosphonates acrylates copolymers) are part of the chemicaltreatment (See Report No EE102E78 Cooling Tower Water Treatment Guidelines ) Higher pH water treatmentprograms require greater concentrations of these chemicals along with dispersants and flocculants to inhibit the formationof scale and precipitation of other salts
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 13 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
bull Calcium Sulfate - When heat transfer skin temperatures are below 250 degF (120 degC) and the cooling water bulktemperatures are under 130 degF (54 degC) gypsum scale (CaSO 42H 2O) will form if the product of the calcium and sulfate ionsconcentrations each expressed as wppm CaCO 3 equivalent exceeds 500000 Gypsum scale is prevented bymaintaining bulk water temperature below the indicated limits and controlling calcium and sulfate ions through blowdownor through the use of newer dispersants on the market
bull Tri-Calcium and Zinc Phosphate - Almost all chemical additives for corrosion control contain some type of phosphatesalt These have a tendency to form the orthophosphate ion depending on time pH and temperature Cooling watersusing these corrosion inhibitors may have an orthophosphate concentration in excess of 5 wppm as PO 4
= To preventcalcium phosphate scale Table 2 shows a limit of 1000 wppm of calcium as CaCO 3 equivalent when using a phosphateinhibitor in combination with a chemical dispersant (See Report No EE102E78 Cooling Tower Water Treatment Guidelines for solubility of calcium phosphate) These inhibitors and dispersants generally fall into one of the followingcategories
ndash Sequestrants antiprecipitants and inhibitors such as hydroyethylidene diphosphonate (HEDP) phosphate esters andaminomethylene phosphonate (AMP) function by defyingdistorting the crystal structure of the scale formations toprevent nucleation and growth of the scale
ndash Dispersants based on either anionic or nonionic polymers such as polyacrylamide polyacrylate or polymaleic acidfunction to stabilize colloidal and scale-forming particles in suspension Anionic polymers prevent agglomeration of suspended particles by either charge repulsion or crystal distortion while nonionic polymers reduce the surfacetension of particles
bull Silica Deposits - Quantitative indices for determining limits for silica in combination with other ions are not available due tothe formation of complex polymers Instead the rule of thumb limit of keeping reactive silica concentrations in the coolingwater below 150 wppm as SiO 2 has been successful
Possible alternatives for controlling sludge are
bull Chemical Dispersants - Water treating chemical suppliers now offer many types of dispersants These are mostlypolyelectrolytes Claims for removal of slime silt and mud from cooling tower basins and exchangers are madeHowever satisfactory results using these chemicals as a coagulant aid require very accurate control of the amount fed tothe water and stable operation of the water system Also suspended solids should be kept below 200 wppm for particleslarger than 045 microns Dispersants are used in virtually all fresh water cooling tower systems despite their high cost
bull Sidestream Filtration - The use of a sand pressure filter is the most reliable method for sludge removal Operating andmaintenance costs are minimal for this equipment
bull Higher Blowdown Rates - This may be an economical alternative depending on the cost of makeup water Additionalchemical costs must be included in any evaluation of this method
bull Makeup Clarification - This alternative may require additional raw water treatment facilities to handle seasonal variationsin turbidity andor suspended solids This method is applicable only where suspended solids come from makeup water
Biological Control
Operating conditions in recirculated cooling water are ideal for the growth of biological matter Water temperatures from 70 to120 degF (20 to 50 degC) favorable water pH continuous supply of nutrients including carbon nitrogen and phosphorus inorganicsalts and sunlight all lend themselves to environmental conditions encouraging microbiological growth and plant lifeIf biological growths get out of control and form large sticky agglomerations some of the following operating problems willresult
ndash Fouling of heat transfer surfaces by bacterial slimes resulting in flow restrictions and high process temperatures ndash Reduced cooling tower efficiency resulting from algae fungi and bacterial slime growths in the water distribution basin
and fill area of the cooling tower ndash Excessive plugging of screens and filters ndash Corrosion ndash Wood destruction by fungi ndash Clogging of water distribution nozzles ndash Excessive sludge accumulation in the cooling tower basin
An important part of the water treatment program is to control biological activity by feeding a biocide chemical Oxidizingbiocides such as chlorine sodium hypochlorite chlorine dioxide or bromine-associated compounds are most commonly usedwith good results Plate counts should be maintained below 50000 countsml (bacterial)
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ExxonMobil Proprietary
Section Page
XXVII 14 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Chlorine is the most effective biocide for cooling water because of its low cost extensive period of successful use ease of application and minimum environmental effects if applied under controlled conditions Either a liquid feed (sodium
hypochlorite) or a gas feed (100 chlorine) is added to the cooling tower basin A sufficient dosage is that which obtains a 01to 03 wppm continuous free residual in the hot return water to the cooling tower
Advantages to continuous chlorination over shock treatment based on ExxonMobil experience are ndash Minimizes chlorine degradation of polymers ndash Better reduction in total aerobic bacteria plate count resulting in cleaner heat transfer surfaces ndash Does not cause significant pH depressions ndash Better control results with less chlorine consumption
Details concerning chlorine addition as well as the other alternative biocides are given in Report No EE102E78 Cooling Tower Water Treatment Guidelines and in EMRE Engineering Water and Wastewater Design Guide TMEE 080 DG 11-3-1Gas Chlorinators for Water Treating Service and DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) Gaseous chlorine systems have become more costly in recent years because of the additional precautions equipment andprocedures required to minimize the risk associated with handling this gas Alternatives to gaseous chlorine are available butrequire more attention and have a higher operating cost Sodium hypochlorite (either purchased or generated) and sodiumbromide stabilized solid halide (chlorinebromine) products chlorine dioxide and ozone are all alternatives which may haveapplications Contact the COOLING WATER SYSTEMS SPECIALISTS for guidance in the use of these alternative treatmentschemes
BLOWDOWN
Blowdown control is required to maintain the proper cycles of concentration in the system The most reliable and simplest typeof control for blowdown is specific conductance measurement of the cooling water Electrical signals from the specificconductance measurements of some impurity in the cooling water are used to actuate an automatic blowdown valveImpurities normally used for blowdown control are dissolved solids chloride calcium magnesium and silica
CONTROLS AND MONITORS
Any chemical treatment used to control problems in a cooling water system must be carefully monitored to be completelyeffective
The installation of automatic controls should be considered for recirculated cooling water systems in all plants Designs shouldinclude pH conductivity corrosivity inhibitor and biocide control Automatic controllers are available which continuouslymonitor the condition of the water and automatically make chemical additions to keep the water within compositional limitsTest exchangers and corrosion coupons are also useful in monitoring the effectiveness of the water treatment program
An oil-in-water monitor should be considered to provide early detection of oil leaks into the cooling water This will enablecorrective action to be taken before extensive contamination of the cooling water circuit occurs This is especially important incooling towers with high efficiency cellular fill which can be difficult to clean if significantly fouledManual control systems may be used in locations that have adequate technical personnel to perform routine water analysesand to modify chemical treatment necessitated by sudden variations in water composition process contamination etc
WASTEWATER REUSE POSSIBILITIES
Recirculating cooling water systems normally have the highest plant raw water demand A potential alternative is to use treatedwastewater to supply part or all of the makeup water requirement The treatment required must be determined but will typicallyinclude primary oil removal (eg API Separator) secondary oil and suspended solids removal (eg filtration dissolved or induced air flotation) and soluble carbon removal utilizing biological treatmentThe reuse of treated wastewater as makeup currently exists at only a few locations If this alternative is being considered refer to Report No EE58E85 Reuse of Wastewater as Cooling Tower Makeup and Section XIX-B Water Reuse
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 15 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS
STEP 1 - Select a location for the intake station and pumps
Factors which influence this selection are as followsbull Depth of water and tidal variations
bull Relative location of intake and outfall direction of current
bull Length and size of intake suction piping vs pump discharge piping
STEP 2 - Determine cooling water requirements
Determine the design cold water inlet temperature based on site data covering seasonal water temperature as a function of inlet location and depth (for existing locations refer to Table 2 in Section IX-B )Tabulate the cooling water requirements for each process unit and offsite consumer Apply the proper LGFs to all coolingloads depending on the stage of the project and whether the new units are exact duplicates of existing unitsIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The designer should investigate methods to increase effluent water temperature to the maximumrecommended limit for the particular type of water used One common method is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 3 - Determine the total required capacity for water intake facilities
Process Coolers - The total requirements determined in Step 2 including LGFsFirewater - This is often supplied from the intake bay of a once-through systemFuture Expansions - These should be considered in sizing intake facilitiesTotal Required Capacity - The sum of the above three items
STEP 4 - Provide water intake facilities
Intake Crib - The intake crib with its support structure is located at the extreme end of the water intake line (See Figure 4 )The intake is usually located 200 to 300 ft (60 to 90 m) offshore and at a sufficient depth below the lowest water level to
preclude vortex formation and subsequent pump cavitation during storm conditions Additionally the crib should have 3 to 5 ft(1 to 15 m) clearance above the sea floor to minimize sand and silt pickup A concrete velocity cap should be specified to cover the inlet crib This will minimize the incidence of fish and other foreignmatter entering the system A horizontal inlet flow pattern should be designed with a maximum entrance velocity of 1 fts(03 ms) Experience shows that vertical velocity flow confuses fish and they are drawn into the intake lineThe intake crib support structure protects the crib from adverse effects of storms and tidal motions as well as protecting it fromimpact of floating and submerged objects A self-cleaning screen can be specified to prevent entrance of large objects into thesystem at the cribIntake Line (Figure 5 ) - An intake line is required if the water depth along the shore does not permit water to flow directly intothe intake bay The intake line is usually buried in the sea floor and covered with a minimum required overburden Consultwith the COOLING WATER SYSTEMS SPECIALISTS for required coverage at a specific siteChlorine or sodium hypochlorite is injected at the intake crib to prevent marine growth in the lineNote that an intake crib and line will not be required if water of sufficient depth is available at the supply pump stationIntake Bay (Figure 6 ) - The intake bay is an enclosure which houses the pumps and screens Depending on a particular design a series of screens is used to ensure the supply of clean water Trash racks comprise the first element of the filter system These remove only very coarse material from the water such as bulk seaweed They can be supplied with atraversing raking mechanismLift screens provide secondary cleanup These screens can be lifted out of the intake bay for periodic cleaning Double liftscreens can be used if it is deemed necessary to have one in service at all timesFinal water filtering is usually done by traveling screens Details of the screen dimensions and the mounting are usuallydetermined during detailed engineering Screen net open area is typically approximately 50 of total screen area Thescreens should be sized for a normal water velocity of 12 fts (04 ms) through the open screen area Maximum velocityshould be limited to 24 fts (08 ms) with 50 of the screen area plugged This is the upper velocity limit for efficientscreening
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Section Page
XXVII 16 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
If high suspended solids greater than 200 mesh (eg sand) are expected to be a continuous problem ejectors may be installedupstream and downstream of the traveling screen to remove accumulated sand and solids from the intake bay in these areas
Traveling screens and ejectors are supplied with wash water from the raw water pumps or from separate wash water pumpsOnoff control using timers should allow optimum scheduling of the ejectors depending on the time of the year
STEP 5 - Specify water intake facilities
Water intake facilities should be designed in accordance with the following guidelines from the Hydraulic Institute Standards The water intake should be designed to provide an evenly distributed flow of water to the pump suction A properly designedintake reduces the possibility of vortex formation and results in better pump performance with less noise Geometry of theintake and the manner in which water is introduced into the intake are important parameters in providing an even distribution of water flow Complete analysis of intake structures is best accomplished by scale model testsThe pump manufacturer will generally render advice on specific problems while the intake design is still preliminary He shouldbe provided with the necessary intake layout drawings showing the physical limitations of the siteFigures 7A and 7B have been developed for single- and multiple-pump arrangements to show suggestions for basic sumpdimensions They are for pumps normally operating in the capacity range of approximately 3000 to 300000 gpm (200 to20000 Ls) Since these values are composite averages from a great many pump types and cover the entire range of specificspeeds they must not be thought of as absolute values but rather as basic guides subject to some possible variations
All the dimensions in Figures 7A and 7B are based on the rated capacity of the pump at the design head Any increase incapacity above these values should be momentary or very limited in time If operation at an increased capacity is to beundertaken for considerable periods of time the maximum capacity should be used for the design value in obtaining sumpdimensionsDimension C is an average value based on an analysis of many pumps Its final value should be specified by the pumpmanufacturerDimension B is a suggested maximum dimension which may be less depending on actual suction bell or bowl diameters in useby the pump manufacturer The edge of the bell should be close to the back wall of the sump When the position of the backwall is determined by the driving equipment or the discharge piping Dimension B may become excessive and a ldquofalserdquo backwall should be installedDimension S is a minimum value for the sump width for a single pump installation This dimension can be increased but if it isto be made smaller the manufacturer should be consulted or a sump model test should be run to determine its adequacy
Motor size may dictate minimum pump spacing be especially aware of this when specifying pumps with unusually high pumpheads eg greater than 80 psig (550 kPa)Dimension H is a minimum value based on the ldquonormal low water levelrdquo at the pump suction bell taking into considerationfriction losses through the inlet screen and approach channel This dimension can be considerably less momentarily or infrequently without excessive damage to the pump It should be remembered however that this does not representldquosubmergencerdquo Submergence is normally quoted as dimension ldquoHrdquo minus ldquoCrdquo This represents the physical height of water level above the bottom of the suction inletThe actual submergence of the pump is something less than this since the impeller eye is some distance above the bottom of the suction bell possibly as much as 3 to 4 ft (1 to 13 m) For the purposes of sump design in connection with this chart it isunderstood that the pump has been selected in accordance with specific speed charts The submergence referred to hereinhas to do only with vortexing and eddy formationsDimensions Y and A are recommended minimum values These dimensions can be as large as desired but should be limitedto the restrictions indicated on the curve If the design does not include a screen Dimension A should be considerably longerThe screen or gate widths should not be substantially less than S and heights should not be less than H If the main stream
velocity is more than 2 fts (06 ms) it may be necessary to construct straightening vanes in the approach channel increaseDimension A conduct a sump model test of the installation or work out some combination of these factorsDimension S becomes the width of an individual pump cell or the center-to-center distance of two pumps if no division walls areusedOn multiple pump installations the recommended dimensions in Figures 7A and 7B also apply as noted above and thefollowing additional determinants should be consideredFigure 8A - Low velocity and straight-line flow to all units simultaneously is the first recommended style of pit Velocities in thepump area should be approximately 1 fts (03 ms) Some pumps with velocities of 2 fts (06 ms) and higher have given goodresults This is particularly true where the design resulted from a model study Not recommended would be an abrupt changein size of inlet pipe to sump or inlet from one side introducing eddying
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Section Page
COOLING WATER SYSTEMS XXVII 17 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Figure 8B - A number of pumps in the same sump will operate best without separating walls unless all pumps are always inoperation at the same time in which case the use of separating walls may be beneficial If walls must be used for structural
purposes and pumps will operate intermittently one should leave flow space behind each wall from the pit floor up to at leastthe minimum water level and the wall should not extend upstream beyond the rim of the suction bell If walls are used increaseDimension ldquoSrdquo by the thickness of the wall for correct centerline spacing Round off the ends of the walls Not recommended isthe placement of a number of pumps around the edge of a sump with or without dividing wallsFigure 8C - Abrupt changes in size from inlet pipe or channel to pump bay are not desirable A relatively small pipe emptyinginto a large pump pit should connect to the pit with a gradually increasing taper section The angle should be as large aspossible preferably not less than 45 degrees With this arrangement pit velocities much less than one foot per second aredesirable Especially not recommended is a small pipe directly connected to a large pit with pumps close to the inlet Flow willhave excessive change of direction to get to most of the pumps Centering pumps in the pit leave large ldquovortex areasrdquo behindthe pumps which will cause operational troubleFigure 8D - If the pit velocity can be kept low enough [below 1 fts (03 ms)] an abrupt change from inlet pipe to pit can beaccommodated if the length equals or exceeds the values shown It is assumed that as ratio WP increases the inlet velocityat ldquoPrdquo will increase up to an allowed maximum of 8 fts (24 ms) at WP = 10 Pumps ldquoin linerdquo are not recommended unless theratio of pit to pump size is quite large and pumps are separated longitudinally by a generous margin A pit can generally be
constructed at much less cost by use of a recommended designFigure 8E - It is sometimes desirable to install pumps in tunnels or pipelines A drop pipe or false well to house the pump witha vaned inlet well facing upstream will be satisfactory in flows up to 8 fts (24 ms) Without the inlet well the pump section bellshould be positioned at least two pipe diameters vertically above the top of the tunnel not hung into the tunnel flow especiallywith tunnel velocities 2 fts (06 ms) or more There should be no signs of air along the top of the tunnel It may be necessaryto lower the scoop or to insist on a minimum water level in the vertical wellNote The foregoing statements apply to sumps for clear liquid
STEP 6 - Provide chemical treatment
Provide chemical treatment facilities for the once-through cooling water system Normally the only chemical treatment requiredin a once-through cooling system is for biological control Facilities for adding chlorine to once-through cooling water are thesame as those described for the recirculated cooling water system The design capacity of the chlorinator should be based onachieving a chlorine residual of 1 wppm in the effluent The chlorine required by the organics and other reducing agents in thewater must be satisfied before any residual chlorine will appear in the effluent The design capacity of the chlorinator should be
based on the total water chlorine demand plus 1 wppm in the effluent In the absence of water data the chlorination systemdesign capacity should be based on 10 wppm minimum instantaneous dosage for maximum cooling water flow For very largeonce-through cooling water systems it will be impractical to design for shock rates at this level In this case a continuousdosage of 1 - 2 wppm should be used (Refer to Step 5 of the DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS )
STEP 7 - Specify pumps and distribution system
Specify other major facilities in the once-through cooling water system The recommended materials for pumps valves andpiping are shown in the EMRErsquos Refinery Construction Materials Manual
Pumps1 Type - These pumps are usually vertical centrifugal pumps However horizontal centrifugal pumps have been used An
economic study andor consideration of existing facilities will determine which type should be specified In the event thathorizontal centrifugal pumps are specified facilities for pump priming will likely be required For large pumps in salt water service there is a very limited selection of pumps because of the special materials required Consult with the DISCIPLINESPECIALISTS early in the design effort to determine what specific pumps are currently available
2 Total Required Capacity - Firm capacity (excluding spares) of the supply pumps should equal the maximum cooling water requirements including LGF and RCF
3 NumberIndividual Pump Capacity - Normally one spare pump is provided for up to six operating pumps two spares for up to twelve operating pumps etc The most common arrangements for water supply pumps are
bull Two 100 capacity pumps
bull Three 50 capacity pumps
bull Four 33-13 capacity pumps
bull Five 25 capacity pumps
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Section Page
XXVII 18 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Maximum commercially available pump capacity ranges up to 300000 gpm (20000 Ls) at discharge pressures up to 100psi (700 kPa) Maximum capacity will be less for higher discharge pressures Other factors which may influence pump
selection arebull Selecting capacity to match existing pumps
bull Selecting capacity for future requirements
bull Selecting capacity corresponding to manufacturers available standardVendor information should be obtained on the availability size and cost of the pumps for several configurations Acomparison should be made of the investments and operating costs as well as other advantages and disadvantages toselect a pump configuration
4 Drivers - These pumps are usually motor-driven Because of the remote location of the intake facilities the steamdistribution costs are normally too high to justify the use of a steam turbine driver However use of a secondary selectivesubstation provides a reliable source of power to the pump drivers
5 Pump ∆∆∆∆P - Pump ∆P is based on supplying the maximum cooling water requirement at the battery limit design pressureestablished for the system The ∆P should include the total circulating system pressure drop plus any elevation differencebetween the low water level at the intake and the discharge point of contact with air
6 Pump Instrumentation - The remote location of the intake facilities makes remote control and monitoring of the water supply operation desirable Pump controls located in the control house provide for minimum manning Local controlbackup should be provided The Owners operating and manning philosophy determine the basis for instrumentation andcontrolThe spare pump is controlled to automatically start on low pressure in the supply header Concurrently alarms should beactuated in the control room to indicate the low-pressure situation and spare pump operation
Distribution Network1 Configuration - The piping network is a once-through system with a return line from each consumer or consumer group to
the cooling water effluent line2 Layout - The routing of the piping network should connect all cooling water consumers in the most economical manner
permitted by the plant layout Careful consideration should be given to the turnaround philosophy of the plant Thenetwork layout should preclude the necessity of having above-ground water lines that are in service running through anarea that is being turned around Underground piping or an extra lateral may be required to accomplish this objective
3 Valves and Blinds - Battery limit valves should be provided in the network to isolate individual process units from thesystem Valves should be provided to permit onstream removal and maintenance of spared equipment
4 Basket Strainers - Consideration should be given to the installation of basket strainers in the laterals serving each of theconsumer loops These provide added insurance against entrance of foreign material into the cooling equipment The useof intake screens with 025 in (63 mm) openings may eliminate the need for basket strainers in the laterals
Line Sizing1 Design Basis - The network is sized so that water is delivered to the most distant consumer at a pressure equal to the
required process design (battery limit) pressure2 Flow Rate - Each segment of the distribution network is sized for the simultaneous maximum flow in that segment for any
operating condition The main supply and discharge headers are sized for the simultaneous maximum flow of all segments(not necessarily the sum of the simultaneous maximum flows of the individual segments)
3 Pressure Drop
Line Sizing Basis - The distribution system should be sized according to the guidelines established in Section XIV The use of an ldquoold piperdquo friction factor of C = 90 - 100 (Hazen-Williams factors) in the main lines is recommended to assuregood operation in later years when the system experiences fouling For large diameter lines [greater than 48 in (1200mm)] maximum water velocity should be limited to 10 fts (3 ms) based on civilmechanical considerations (supports andanchoring required to handle potential pressure surges) For salt water systems utilizing cement-lined pipe consider thereduced inner diameter in the friction loss calculations A study may be necessary to determine the economic pump ∆P vsline diameters
4 Equivalent Length Factors - Equivalent length factors are used in pressure drop calculations for sizing the distributionsystem The piping length scaled from the plot plan is multiplied by an equivalent length factor to compensate for pipingelbows expansion loops and other piping variations
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 19 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
The equivalent length factors are selected based on the expected variation from a straight piping run Typical equivalentlength factors suitable for planning purposes for different piping applications are as follows
TYPE OF PIPING EQUIVALENT LENGTH FACTORS
Long piping runs [gt 1000 ft (300 m)] 12 to 14
Onsite distribution piping 15 to 20
For design purposes the actual piping layout should be evaluated to ensure that the equivalent length factors areapplicable to the specific situation Particular attention is required for revamp work and saltwater systems utilizing cement-lined pipe
STEP 8 - Prepare a system flow plan similar to the one shown in Figure 1
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS
STEP 1 - Determine cooling water requirements
Tabulate the cooling water requirements and cooling water discharge temperature for each process unit and offsite consumer Apply the proper LGFs to all cooling loads depending on the stage of the project and whether the new units are exactduplicates of existing units Determine the total cooling water flow required including LGFs and the bulk temperature (T 1) of thecombined cooling water flow from all coolers to the cooling towerIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The onsite and offsite designers should investigate methods to increase effluent water temperature to themaximum recommended limit for the particular type of water used This will result in a lower cost cooling tower for a given heatrejection rate One common method of maximizing return water temperature is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 2 - Select cooling tower design parameters
Select the proper design conditions of ambient wet bulb temperature approach temperature range and flow rate for specifyingthe cooling tower Based on these conditions the cooling tower vendor will size the tower and provide the tower characteristicand performance curves which serve as the basis for the performance guarantee Required performance test and evaluationprocedures are described in IP 8-1-1 Economic studies may be required to select the ambient wet bulb temperature andapproach temperature for new locations or for expansion projects with special cooling requirements
STEP 3 - Prepare cooling tower specification sheet
Include the following information in the heat exchange section of the design specificationTower Flow Capacity - This was determined in Step 1Type of Tower - The tower usually specified is an induced draft multicell type suitable for fresh salt or brackish water depending on the water available Additional tower characteristics are usually developed and proposed by the tower vendor (eg crossflow versus counterflow type of tower fill etc) The vendor should be made aware of conditions such as thelikelihood of heavy oil leakage into the cooling water or high fouling potential which may influence the tower type or fill
selectionNumber of Cells - The minimum number of cells required (normally not less than 2) may be specified The maximum flow per cell is a function of the cooling tower design conditions selected in Step 2 Typical values range up to 10000 gpm (600 Ls) for crossflow designs and 8000 gpm (500 Ls) for counterflow designs Normally spare cells are not provided Maintenance iscarried out during cooler periods or load is shed
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ExxonMobil Proprietary
Section Page
XXVII 20 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Ambient Wet Bulb Temperature1 The ambient wet bulb temperature selected for specifying the cooling tower design is generally that temperature which is
equaled or exceeded 5 of the time during the four hottest months (5 of the hourly readings taken during the four hottestmonths will exceed this value) Specifying the 5 temperature should give satisfactory results since the time that the wetbulb temperature is exceeded is for short duration and the system inertia is usually sufficient to avoid problems The tower specification should make it clear that the wet bulb temperature specified is an ambient value as opposed to the enteringvalue which may be higher because of recirculation The tower manufacturer should adjust the ambient value upward totake into account his estimates of recirculation Currently cooling tower test codes provide procedures for measuringperformance in the case of either entering or ambient wet-bulb temperature specifications An increase of 2 degF (11 degC) isgenerally added to ambient wet bulb temperature to estimate entering wet bulb temperature for large cooling towers
2 If plant production will peak during the four summer months when the 5 ambient wet bulb temperature may be exceededconsideration should be given to specifying a higher design ambient wet bulb temperature (one that will be exceeded 212 of the time for example) However the incremental cost of the resulting closer approach and larger cooling tower must be justified by the value of the incremental products made
On-Tower Temperature (T1) - This temperature cannot be accurately determined until the design of all process coolers iscompleted and the volume of cooling water and outlet temperature are known for each (see Step 1) In the planning stage or early design stage of a project this temperature must be estimated and is usually set equal to the maximum design cooler outlet temperature shown under COOLING WATER REQUIREMENTS in the BASIC DESIGN CONSIDERATIONS portion of this sectionApproach Temperature - The approach temperature has the most pronounced effect on cooling tower size and cost For example reducing the approach from 10 to 5 degF (55 to 3 degC) can increase the cooling tower investment by 50 percent and thefan operating cost by 65 percent Thus various approach temperatures should be evaluated to determine the minimum cost of the total system including the process exchanger area Approaches used in the past designs have been from 5 to 12 degF (3 to7degC) Note that it is not customary in the cooling tower industry to guarantee approaches of less than 5 degF to (3 degC) to coolingtower design inlet wet bulb temperature A typical approach for a new grass roots cooling tower is 10 degF (6 degC)Off-Tower Temperature (T2) - This is the sum of the approach temperature and the ambient air wet bulb temperature For existing locations Table 2 in Section IX-B may be used
Range and Flow Rate1 The range for a given heat load is related to the circulating water rate as follows
Q = M C p ∆Tcong (500) (gpm) (range) Eq (1)
where Q = Heat load on the tower BtuhM = Water circulating rate lbhCp = Specific heat capacity of water at average temperature Btulb degF∆T = (T 1 ndash T 2) degF = (water cooling range)gpm = Circulation rate in US gallons per minute
For calculation in Metric units
Q = M C p ∆Tcong (1162 x 10 -3) (Ls) (range) Eq (1M)
where Q = Heat load on the tower kJh
M = Water circulating rate kghCp = Specific heat capacity of water at average temperature kJkg degC∆T = (T 1 ndash T 2) degC = (water cooling range)Ls = Circulation rate in liters per second
2 Generally lower tower investment fan horsepower and pump horsepower result from maximizing the range andminimizing the flow rate However based on fouling considerations of process coolers the maximum allowable hot water temperatures must be kept below recommended limits Also while increasing the range minimizes cooling tower investment and operating cost the cost of exchanger surface requirements is adversely affected by the LMTD and acompromise must be made The range is bounded by the maximum allowable hot water temperature and the Off-Tower temperature (T 2) For established locations refer to Section IX-B for design cold water temperatures to assist in selectingthe approach and range
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Section Page
COOLING WATER SYSTEMS XXVII 21 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Makeup Water Requirements - Calculate the various losses in the cooling water system to determine makeup requirementsThese losses include
1 Evaporation Loss (E) - Allow 1 of the circulation rate for each 10 degF (55 degC) temperature drop through the tower Notethat the actual evaporation loss will be somewhat less depending on ambient air temperature
( )sL555
TT)gpm(
1000TT
E 2121
minus=
minus= Eq (2)
2 Drift Loss (D) - IP 8-1-1 limits the maximum allowable drift loss to 0005 of the design water circulation rate for freshbrackish or salt water
( )sL1000050
)gpm(1000050
D
=
= Eq (3)
This low drift level contributes little to solids removal from the tower with the spray This allows higher cycles of concentration to be achieved in cooling towers which use clarified softened water for makeup and thus have low requiredblowdown rates Higher drift loss percentages in older or poorly maintained towers will limit the maximum cycles of
concentration achievable3 Miscellaneous Losses (O) - Miscellaneous losses are estimated to be one-seventh of the difference between evaporation
loss and drift loss
minus=
7DE
O Eq (4)
4 Blowdown Rate (B) ndash Guidelines for maximum concentrations of impurities allowed in recirculated fresh cooling water areshown in Table 2 For salt water the blowdown is based on keeping the total dissolved salts below 55000 wppm Obtainthe chemical analysis of the makeup water and divide each individual component concentration into its counterpart inTable 2 The result is the maximum allowable cycles of concentration for that constituent (a worksheet is provided inTable 4 ) The impurity with the lowest factor will be the control and will determine the required blowdown rate
)OD(1Cycles
EB +minus
minus= Eq (5)
The makeup water requirement is the sum of all losses shown above
MakeUp = E + D + O + B Eq (6)
STEP 4 - Specify the controls necessary for proper cooling tower operation
Air Side - Cooling towers are provided with motor-driven manually adjusted pitch multiple-bladed propeller fans Figures9A amp B may be used to estimate the normal operating horsepower requirements Since the fan motor and gear reducer arelocated at the top of the tower [generally 40 to 50 ft (12 to 15 m) above grade] the need for equipment to handle thesecomponents should be discussed with the affiliateControl of the air flow through the tower to maintain reasonably constant cold water temperatures can be accomplished byturning off individual fans in multiple-cell installations adjusting the blade pitch or using variable speed motors To turn off individual fans remote at grade start-stop buttons for the motors are usually specified
Whenever the ambient air temperature is 32 degF (0 degC) or below icing of the tower louvers and structural members in contact withthe incoming air will occur If the heat load on the tower is relatively free of seasonal variations two-speed fans should provideadequate ice control down to approximately 0 degF (-18 degC) ambient temperatures When sustained temperatures of 0 degF (-18 degC)and below occur two-speed fans capable of two-speed reverse operation with appropriate time delays for change of directionand speed should be specified to periodically eliminate ice which will build up on the louvers Other methods of winterizationare available but in general are more expensive and require greater operator attention In very cold climates steam heatingshould be provided to prevent freezing in the cold water basin during shutdown periodsWater Side - The water circulation rate is maintained by operating one or more pumps of sufficient capacity to meet the systemdemand Flow-measuring facilities local temperature indicators and a high-temperature alarm are provided on the coolingtower water supply and return headers Hand-operated flow control valves are required for distributing the water equally to theindividual cells On counterflow towers provisions for measuring the flow to each cell (pitot tubes) are required They are notrequired on crossflow towers since the open distribution basin provides visual aid in balancing flows Shutoff valves should beprovided for servicing the flow control valves without shutting down the entire tower
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XXVII 22 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
STEP 5 - Provide chemical treatment facilities for the fresh water system
Alkalinity (pH) Control (if used)1 In cases where a controlled pH corrosionscale control program is being used the alkalinity (pH) of the recirculated cooling
water should be controlled in accordance with the program requirements Normally acid addition is required to accomplishthis This is due to free carbon dioxide being stripped from the cooling water in the tower to approximately 5 ppm For example if the alkalinity has to be controlled to between 25 and 50 ppm for a pH of 70 to 75 the volume of 98 sulfuricacid (66 degBeacute) required to treat the makeup water is calculated using the following equation
[Note 93 sulfuric acid should be used if ambient temperatures will be less than 35 degF (2 degC)]
GallonsHour 66 degBeacute H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate gpm) (326 x 10 -5)
For calculation in Metric limits
kgh of 100 (66 degBeacute) H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate Ls) (793 x 10 -3)
2 The metering pump should be designed for a maximum feed rate of two to three times the normal feed rate A glass woolfilter is required on the pump suction piping to minimize plugging Normally two - 100 capacity pumps are provided
Positive displacement metering pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumps with flow control areused for higher rates
3 Hydrochloric acid may also be used for alkalinity control The gallons per hour of 20 degBeacute hydrochloric acid required is36 times the amount of 66 degBeacute sulfuric acid In Metric units the weight of 32 (20 degBeacute) hydrochloric acid required is227 times the weight of 100 (66 degBeacute) sulfuric acid
4 Specifications for chemical feeders may be found in EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased)
5 A pH analyzer with high and low alarm is provided to control the amount of acid feed The acid should be added to thecooling tower makeup water (Figure 2 ) The lag time between the point of acid injection and the sample outlet to the pHcell should be less than two minutes for effective control
6 Spills or overflows from the sulfuric acid tank should be contained
Corrosion Inhibitors and Anti-Scalents1 Two chemical feeders are generally provided The following guides in sizing the equipment will allow the use of any of the
proprietary products from the vendors when final selection by the owner is made See Report No EE102E78 Cooling Tower Water Treatment Guidelines EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and Report No EE34E86 Environmentally - Acceptable Cooling Water Treatments for detailed discussion of acceptable commercial chemicals
bull If the water treatment program to be used is known design for the chemical rates specified by the vendor or COOLING WATER SYSTEMS SPECIALIST
bull Otherwise design for 100 wppm of each commercial liquid product in the recirculated cooling water The design feedrate should be based on the design total blowdown flow (both uncontrolled and controlled) since this is where theproduct is lost For 100 wppm concentration the chemical rates will be as follows
GallonsHour Chemical = Blowdown (gpm) x (0006)
LitersHour Chemical = Blowdown (Ls) x (036)
2 The design capacity of the metering pump should be two to three times this calculated number Normally 2 - 100capacity pumps are provided Positive displacement pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumpswith flow control are used for higher rates
3 Specifications for chemical feeders may be found in EMRErsquos Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased) Reference should also be made to the checklists containedin Report No EE92E94 Guidelines for Safety Evaluation of Chemical Injection Facilities
4 A final check should be made with the owner and the COOLING WATER SYSTEMS SPECIALISTS as to the treatmentprogram selected and if there is a need for an additional chemical feeder
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COOLING WATER SYSTEMS XXVII 23 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Biological ControlFacilities should be provided for feeding the biocide into the back of the cooling tower basin If chlorine is used it must bepresent in the cooling water as free residual to be effective However the following amounts of chlorine are first required toreact with the listed impurities when present in the cooling water before a free residual can be established
IMPURITIESAMOUNT OF CHLORINE REQUIRED (wppm)
WITH EACH WPPM OF IMPURITY
Ammonia (NH 3)
Hydrogen Sulfide (H 2S)
Sodium Sulfite (Na 2SO 3)
Ferrous Iron (Fe ++)
10
22
056
064
The design capacity of the chlorine feeder shall be based on achieving a 01 wppm free chlorine residual in the recirculatingwater flow or a minimum dosage of 7 wppm at maximum design makeup water flow rate and conditions (see EMRE Water and
Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service )Chlorine may be fed either as the commercial liquid sodium hypochlorite (124 wt Cl 2) or liquid chlorine (100 Cl 2) Liquidhypochlorite is being used increasingly because of the higher costs associated with precautions equipment and proceduresrequired to minimize the risks associated with handling gaseous chlorine The hypochlorite solution is fed using a meteringpump which should be sized to feed an equivalent of 15 ppm 100 Cl 2 (Refer to EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) ) In case of spills or an overflow from thestorage tank the sodium hypochlorite may be sent to the cooling tower basinGas chlorinators are covered by EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service The use of gas chlorination is being reduced but may still be appropriate in some locations subject tolocal management approval The gas from either commercial cylinders or an evaporator is dissolved in a water slip stream bymeans of an eductor This results in a chlorine solution strength of approximately 01 This solution should be evenlydistributed below the water surface by means of a header lateral pipe arrangement at the rear of the cooling tower basinopposite the cooling water pumpsSome additional guides on the use of gas chlorinators are given below It is also recommended that the safety guidelines
published by the Manufacturing Chemists Association and the Chlorine Institute be followedbull Dilution water for the chlorinator must not be less than 50 degF (10 degC) or ice formation may cause plugging problems
bull Continuous rates of chlorine gas withdrawal should not exceed 400 lb (180 kg)24 hours from each one ton cylinder atroom temperature [70 degF (21 degC)] This rate may be exceeded (up to 50) for periods not exceeding 2 hours Rule of thumb is not to design for more than 4 cylinders operating in parallel When feed rate exceeds 3000 lb (1360 kg) per daythe liquid chlorine should be fed to an evaporator upstream of the chlorinator
bull If the ambient temperature can be expected to be below 50 degF (10 degC) for extended periods a building should be providedfor the chlorinator and cylinders
bull Weighing scales should be provided to determine when shipping containers should be replaced and to verify chlorine feedrates
bull Analyzers to measure the free chlorine residual in the circulated cooling water should be provided
bull Chlorine leak detectors and a catch basin for major spills should be provided
Control SystemProvide automatic controllers to continuously monitor water condition and to add chemicals to the system (complete systemsmay be available through the water treatment program vendor as a lease or purchase)pH Control - This controller adds acid to correct water alkalinity a timer on the acid pump sounds an alarm if the pump doesnot shut off in a predetermined length of time Backup controls are required to prevent the cooling system from beingpermanently damaged in case the pH electrodes become fouledCorrosivity and Inhibitor Control - Systems are available which give a direct readout of corrosion rate in milsyear If thecorrosion rate exceeds a set point the controller shuts off the acid increases the inhibitor addition opens the blowdown valveand sounds an alarm Electrodes measure the completeness of the inhibitor film by the value of the potential differencebetween the electrodes The larger the difference the more incomplete the film and a potential reading exceeding a set levelactivates the inhibitor feed
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XXVII 24 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Free Residual Chlorine Control - Free residual chlorine should be controlled automatically for biocide treatment Severalgood analyzers are currently available These units can be incorporated into an automatic control system
Oil in Water DetectionMonitoring Corrosion and Deposit FormationThe following types of devices for measuring corrosion rates and deposit formation should be provided to allow the plant tomonitor the effectiveness of the cooling water treatment before scheduled turnaroundsCoupons - A corrosion coupon rack should be specified for the hot return water to the tower in accordance with IP 19-6-1 Corrosion Monitor - On-line corrosion analyzer if specified should monitor the hot return water to the tower in accordance withIP 19-6-1
On-Line Test Exchanger Although there are a variety of deposit monitors from the water treatment chemical vendors they do not necessarily representactual heat transfer conditions in process exchangers For this reason a test exchanger should be installed for evaluatingcorrosion and deposit formation Details are covered in Report No EE102E78 Cooling Tower Water Treatment Guidelines
Alternatively a critical process exchanger can be instrumented such that its heat transfer coefficient can be monitored
Instrumentation would include flows and inlet and outlet temperatures of process fluid and cooling waterSTEP 6 - Specify cooling tower location
Select a location for the cooling tower The following factors influence this selection
Location Factors1 The location should be within a reasonable distance of the units served and consistent with future refinery plans Piping
costs to and from battery limits can be a significant portion of the total cooling system cost and can be the location-determining factor particularly for salt water systems While piping costs can be easily determined other costs affectinglocation cannot be estimated readily Therefore judgment is required in selecting the location
2 Safety spacing requirements (see Section XV-G Equipment Spacing )3 Cooling tower contribution to plant boundary noise4 For salt water cooling towers wind direction and distance to other facilities is of major concern because of potential
corrosion problems
5 Factors affecting cooling tower performance ie orientation with respect to wind site congestion etc
Plot RequirementsIn estimating plot requirements (basin area) one should allow 5 to 6 ft of tower length for each 1000 gpm (2 m for each 60 Ls)of circulating capacity This figure is based on a typical counterflow design with a width of 50 ft (12 m) and does not includepump suction pit space Exact requirements can be obtained from cooling tower vendors after the tower capacity has beenestimated
Fogging and DriftTwo distinct problems are associated with the discharge air from a cooling tower (1) fogging which can be a nuisance or ahazard and (2) drift which is generally a nuisance because of increased corrosion of surrounding equipment Fogging fromcooling towers occurs during periods of combined high ambient humidity and low temperature as a result of mixing the warmmoist cooling tower exhaust air with the cooler ambient air The ambient air cannot absorb all moisture as vapor and thus tinyfog droplets are formed by condensation Fogging and resulting icing in cold climates has produced hazardous drivingconditions on highways up to 850 ft (250 m) awayProprietary designs are available to combat potential fogging problems Basically these designs incorporate heating of thedischarge air with hot return water precooling coils (similar to an air cooler) to eliminate any visible plume However thesedesigns are expensive and relatively unproven Therefore proper location based on reliable weather data is the preferredsolutionDrift from a cooling tower represents a more difficult problem since it does not depend on definable climatic conditions andoccurs for all wind directions The effects of drift on nearby equipment and vegetation are particularly severe and generally of concern only for salt water systems because of the high solids content of the drift (concentrated salt water) For salt water cooling systems electric lines substations and instrumentation should be located upwind in the direction of the predominantwind They should be provided with adequate shelter or materials when within 450 ft (140 m) of the cooling tower In additionspecifications for all equipment within 150 ft (45 m) of the tower should include surface treatment suitable for a marineenvironment
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COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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XXVII 28 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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DESIGN PRACTICES December 2000
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TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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DESIGN PRACTICES December 2000
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TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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DESIGN PRACTICES December 2000
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FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 5 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DEFINITIONS (Cont)
Fill
Packing within the cooling tower to provide the required surface area for heat transfer between ambient air and the returncooling water Splash fill breaks the water into small drops by causing it to cascade through successive offset levels of parallel level splash bars Cellular or film fill causes the water to spread into thin films over large vertical areas
Heat Load
Heat removed from the circulation water within the tower It may be calculated from the range and the circulating water flowrate
Interference
The mixing of tower inlet air with the discharge vapors from another tower or other heat source This results in reduced thermalperformance
Makeup
The water required to replace circulating water which is lost by evaporation drift blowdown and leakage It is expressed inpercent of water circulated
Offsites
Support facilities such as utilities (steam power cooling water) tankage waste treating etc for the processing operations
Once-Through Cooling System
A system in which water passes through the heat exchange equipment once and is then discharged
Onsites
Facilities that are a part of the processing operations
Range or Water Cooling Range
The difference in temperature between the hot and cold circulating water or on-tower temperature minus off-tower temperature
Recirculated Cooling System
A system in which water is circulated to coolers in a loop and the heat absorbed from this cooling operation is dissipated by acooling tower or some other type of heat exchanger
Recirculation
An undesirable condition in which part of the tower discharge vapor stream is recirculated through the tower resulting inincreased inlet air wet bulb temperature
Relative Humidity
The ratio of the quantity of water vapor present in air to the quantity which would saturate the air at the existing temperature
Salt Water
Water which contains more than 5000 wppm of dissolved salts
Wet Bulb Temperature
The equilibrium temperature obtained when ambient air is passed over a continuously wetted thermometer bulb evaporatingthe water and cooling the bulb This equilibrium temperature occurs when the heat transferred from the air to the wettedsurface equals the heat loss due to latent heat of evaporation
Wind Rose
A diagram that shows the average frequency and intensity of wind from different directions for a particular location
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Section Page
XXVII 6 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
SYSTEM TYPES AND APPLICATIONS
Cooling water systems in refineries and chemical plants are generally either the once-through or the recirculated type designedfor fresh salt or brackish water
In the once-through system (see Figure 1 ) pumps take suction from a source of supply such as a river or other body of water and deliver the water to process coolers and other consumers After passing through the cooling exchangers the hot coolingwater is discharged through pressurized piping or a gravity flow systemIn a recirculated system (see Figure 2 ) pumps take suction from a cooling tower basin and deliver the water to process coolersand other consumers After passing through the cooling exchangers the hot cooling water is discharged through a pressurizedreturn system to a cooling towerThe choice between a once-through and recirculated cooling water system must be based on (1) availability of water of satisfactory quality (2) process temperatures (3) atmospheric conditions (4) investment and operating costs and (5) effluentwater quality and temperature limitationsThe cooling water is distributed to individual consumers throughout the plant in piping main supply and return headers andseries and parallel branches consisting of laterals and sublaterals The main cooling water distribution headers are either buried in underground trenches or are located above grade on ground-level pipe sleepers or on elevated pipe racksUnderground piping has the advantage of minimizing problems associated with thrust loads thermal expansion and freezingElevated piping is easier and less expensive to install and repair and it makes it easier to locate leaks
BASIC DESIGN CONSIDERATIONS
RELIABILITY
The cooling water system is an important utility and its reliability is critical to successful plant operation The typical systemprovides cooling water to both onsite and offsite consumers It can have a direct impact on the sizing criteria for the emergencyrelief (flare) system (refer to Section XV-C Safety in Plant Design Pressure Relief ) The system therefore normally must becapable of continuous operation so that cooling water is always available to critical consumersThe reliability required to achieve continuous operation is provided in several ways
bull Spare makeup and water circulating pump(s) are provided to assure continuous operation
bull Different types of drivers are specified for the supply or water circulating pumps Motor-driven pumps are supplied fromtwo different buses on a secondary selective electrical distribution system
bull Cooling tower fan drivers are normally supplied from a secondary selective systembull Holdup volume is provided in the cooling tower basin to permit an orderly shutdown of process units if the makeup water
supply is lost or a failure occurs in any part of the recirculated cooling water system
bull Valves blinds and bypasses are provided to permit individual components to be removed from the system for maintenancewhile the system is operating
bull Construction materials provided for cooling water facilities are designed for long-term corrosion resistance sincemaintenance work is often difficult (if not impossible) to schedule without a complete refinery shutdown
FLEXIBILITY
The cooling water system must include sufficient flexibility to cover all present operating requirements plus any future operatingrequirements that are defined in the design basis document For example future operating requirements could be dictated by anear-term increase in plant sizeThe type of flexibility features provided in a system may include the following
bull Makeup water treating facilities should be able to deliver the required water assuming the worst conditions of the water source
bull Waste treating facilities should be designed to handle effluent water containing varying concentrations of oil and chemicalsover the range of expected ambient temperatures
bull Valves and blind flanges should be provided to permit future tie-ins to the system without a shutdown An exception wouldbe on lines where a ldquohot taprdquo could be made with minimum risk of an emergency shutdown or where the cost of requiredvalves is prohibitive
bull Cooling tower cells should be completely partitioned as described in IP 8-1-1 to allow personnel entry to any cell for maintenance without affecting the operation or capacity of the other cells Normally spare cells are not providedMaintenance is carried out during cooler periods when the cooler ambient and wet bulb temperatures enable the coolingtower to handle greater heat loads
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 7 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
bull Where multiple cooling tower systems exist consideration should be given to providing valved cross connections betweentheir respective supply mains and between their respective return mains These would be used only for special operations
such as turnarounds or emergencies The impact on flare sizing should be evaluated if these cross connections areprovided
ENVIRONMENTAL AND SAFETY CONCERNS
Once-through cooling water systems usually require controls to minimize the release of hydrocarbons and free halide residualsto the environment As a minimum cooling water discharges should be monitored for free oil total organic carbon or hydrocarbon vapors to determine if system coolers are leaking Free chlorine residuals in the discharge should also bemonitored as required to meet local regulations and to detect process leaks In most cases once-through cooling water flowsare too large to be handled in the plants wastewater treatment system However once-through water that could becomecontaminated with hydrocarbons is often segregated and treated in a large skim pond or special purpose separator prior todischarge Local environmental regulations and permit requirements will dictate the extent of treatment requiredCooling tower blowdown from recirculated systems is normally bypassed around major wastewater treatment and dischargedwith the treated wastewater An oil-in-water detector should be provided to detect oil in the blowdown from process leaks Thiswill enable stopping the blowdown or diverting it to temporary holdup until the leak is corrected A major rupture in a processheat exchanger tube can also result in the cooling tower being the discharge point for hydrocarbon vaporsBecause of the likely presence of flammable vapors the areas around cooling towers are subject to electrical classification inaccordance with IP 16-1-1 The type and amount of chemicals added to the cooling water should be evaluated as to their effects on the plants wastewater treatment process Biocides corrosion inhibitors and antifoulants may result in significant problems for the wastewater treatment plantOther environmental concerns are
bull Cooling tower noise emissions from fans and from the flow of cooling water over the tower may require suppression if near a local community
bull Cooling tower water mist drift can cause fog and ice formation on nearby roads and promote deterioration of nearbyequipment It may also result in damage to sensitive vegetation in the surrounding area
bull Process gas releases to atmosphere resulting from process gas pickup by cooling water including the venting of toxicgases from exchanger tube ruptures must be considered in locating cooling towers or discharge points from once-throughsystems
bull Spills and overflows of toxic and hazardous chemicals used in treating the cooling water must be contained
SYSTEM SELECTION
The selection of a cooling water system is often determined by the availability of water In regions where water is scarce once-through systems are obviously eliminated from consideration and recirculated systems are chosen A once-through coolingsystem may be the economical choice if the plant is located adjacent to a body of water and consumers are elevated no morethan 50 ft (15 m) above the water level However once-through system costs may be increased significantly by environmentallimitations in many locations These may limit maximum discharge water temperature and therefore will reduce the maximumcooling water ∆T This will increase the required flow and cost of once-through systems In addition regulations may requirewaste treatment of the large volume of effluent thus increasing costsThe choice of a cooling water system must be based on comparisons of initial investment and operating costs Relative initialcosts of a system vary with climatic conditions distance from pump to users complexity of inlet structures and corrosive nature
of the water Salt water or brackish water service requires the use of cement mortar lined pipe and alloys for exchangers andpumps Carbon steel is usually satisfactory for fresh water Special attention should be given to the impact of bottom silt mudand seaweed on intake facilities to once-through or makeup to recirculated systems Cribs flumes large bays screens trashracks or other extensive facilities may be requiredDuring the early stages of a project the designer should review with the planning or design groups the heat level and volume of all process streams which require cooling The optimum method of cooling should then be determined ie heat exchangeandor rejection of heat to air or water A break point should be selected above which air cooling is economical and belowwhich trim water cooling is preferred
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ExxonMobil Proprietary
Section Page
XXVII 8 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
COOLING WATER REQUIREMENTS
Cooling water requirements for each process unit and offsite consumer are obtained from the appropriate design groups For existing locations maximum cooling water loads should be based on the design inlet cooling water temperatures given inSection IX-B Design Considerations for All Types of Heat Exchangers For new locations the inlet cooling water temperaturemust be set based on local water temperature data for once-through systems and wet bulb temperature and selected approachfor recirculated systemsThe bulk water temperature at the cooler outlet has been selected as the controllable variable for design and operation of coolers Although the corrosion rate depends on the metal temperature this is difficult to determine in the operation of a coolerBased upon the findings of a Materials Development Section test program maximum design outlet water temperatures havebeen set as indicated below
Fresh Water 130 degF (54 degC)
Treated Fresh Water 130 degF (54 degC)
Brackish Water 125degF (52
degC)
Salt Water 120 degF (49 degC)
When the maximum plant cooling water requirement has been established for new process and utility consumers appropriateLoad Growth and Reserve Capacity Factors (LGF and RCF) should be applied depending on the development stage of theproject Cooling water loads should be updated and system capacity adjusted periodically as design work progresses
LOAD GROWTH AND RESERVE CAPACITY
Utility loads tend to increase as the loads become better defined through successive stages of project definition Thereforeload growth and reserve capacity factors (LGF and RCF) are applied to raw loads as a means of predicting final equipmentsizes and capital costs These load growth factors do not cover basis changes through project development such as project or process unit sizethroughput For revamp or modernization projects these factors are generally applied only to new loads or changes to existing loads on the basis that existing or unchanged loads are well known
Recommended Load Growth Factors are as follows
CONSUMERSPLANNING
(SCREENING THROUGH DBM) DESIGN SPEC
Existing Facilities 0 - 10 0 - 10
New Facilities 20 - 30 5 - 10
Selection of the load growth factor to use should be based on the quality and level of effort associated with development of theraw loadsFinal sizing of the source facility (eg the cooling tower and circulation pumps) should include a reserve capacity factor of 10to cover small post startup changes minor future projects and dynamic conditions not predicted by steady state balances Thesizing basis for distribution facilities should not include the RCF
COOLING TOWER TYPES
The cooling tower is the major piece of equipment specified in recirculating cooling water systems In effect it is the heatexchanger which cools the hot water to a suitable temperature for re-use primarily by evaporation through contact with airThe cooling tower contains fill to break up the water flow into droplets or film on a surface The fill increases the waterair contact area and enhances heat transferCooling towers are classified according to the means by which air is supplied to the tower ie induced or forced mechanicaldraft (fan) vs natural draft and according to the relative movement of the air and water that is counterflow or crossflow (seeFigure 3 ) Most of the towers in the ExxonMobil circuit are mechanical induced-draft type Economics determine whether theyare counterflow or crossflow
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Section Page
COOLING WATER SYSTEMS XXVII 9 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
In crossflow towers the air travels horizontally across the falling water while in the counterflow design the air travels verticallyup through the falling water Comparisons of plot space fan horsepower and future maintenance costs determine which
design will be more economical Factors which influence this selection are cooling water rate range and approach for aparticular location In addition in some locations materials of construction and associated labor costs will affect the finalselectionIn the past crossflow towers were usually preferred over counterflow towers for extremely cold locations where ice formationcould restrict or block air flow through the louvers However this consideration is usually no longer valid due to the increaseduse of PVC film-type tower fill rather than splash-type fill in counterflow towersMechanical draft designs use one or more fans to provide the air needed for cooling Induced-draft units have the fans locatedon top of the tower Air is pulled through the packing and discharged vertically upward at high velocities Forced-draft unitshave the fans located at the base of the tower and push the air through the fill Forced-draft towers in large industrial sizesexperience difficulties with recirculation and in providing a uniform distribution of air As a result they have been supersededby induced-draft designsFill may be splash or film type Splash fill normally consists of horizontal slats in horizontal rows offset to one another to causethe water to break up into droplets as it falls downward through the cooling tower Splash fill is characterized by reduced air pressure losses It is also less conducive to clogging and easier to clean after a spill However it is very sensitive to
inadequate support and must remain horizontal and level It is made of treated wood or plastics such as PVC or polypropyleneFilm fill comes in various designs but they all cause the water to flow in films over the fill surface and provides more coolingcapacity within a given amount of space than splash fill Because of this increased heat transfer efficiency film fill is generallyused in most new cooling towers However because of its smaller passages it is more sensitive to plugging and is difficult toclean if it does become plugged It may not be appropriate for services cooling mainly heavy fuels or waxy lubes where a leakwould cause fill plugging or for services with a high biological fouling potential PVC is currently the most widely used materialfor film fill The selection of fill type is usually proposed by the vendor and reviewed by the COOLING WATER SYSTEMSSPECIALISTSNatural-draft or hyperbolic cooling towers depend on the natural draft created by the difference in the density of the enteringand leaving air for movement Generally hyperbolic cooling towers have been used in Europe and in the utility industry in theUSA for large capacities [250000 gpm (16000 Ls)] short ranges [10 to 15 degF (6 to 8 degC)] and long approaches [10 to 15 degF (6to 8 degC)] They are most effective where low ambient wet bulb temperatures occur with high relative humidities These towersnormally have diameters in excess of 200 ft (60 m) and are greater than 250 ft (75 m) in height Their major advantages are
bull No operating or maintenance cost for fans
bull Reduction in problems associated with fogging drift and recirculation due to their great height and diameterThe primary reasons why hyperbolic towers are not used in ExxonMobil projects include
bull High initial cost
bull Large plot space requirement and high visual impact
bull Complete dependence on atmospheric conditions
bull Lack of flexibility for future expansions
COOLING TOWER BASIN
The primary function of the cooling tower basin is to collect the cooled water leaving the tower and to provide a reservoir for thecooling water pumps Design aspects to be considered are the basins capacity cleanability draining facilities sump andscreen details To obtain the necessary time for corrective action during emergency conditions a minimum storage capacity of 90 minutes based on loss of makeup water flow should be provided in the basin between the basin low level alarm (LLA) andthe minimum pump submergence (MPS) When the basin water level reaches the MPS point loss of recirculation pumpsuction and cooling water flow is imminent Typical values for basin water levels are as follows
bull High Level Alarm (HLA) 1 ft (300 mm) below top of the basin wall
bull Normal Water Level 15 ft (450 mm) below top of the basin wall
bull Low Level Alarm (LLA) 25 ft (760 mm) below top of the basin wall
bull Distance between LLA and MPS 35 ft (1070 mm)
bull Minimum Pump Submergence Depth 45 ft (1370 mm)
bull Clearance of Pump Suction off Basin Floor 15 ft (450 mm) Actual values may vary depending on pump type selected pump suction line diameter and cooling tower cell plot area
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Section Page
XXVII 10 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
MATERIALS OF CONSTRUCTION
The EMRE Refinery Construction Materials Manual and IP 8-1-1 list acceptable materials for all components in each type of cooling water system (once-through and recirculating) For salt water systems emphasis must be placed on proper materialsselection throughout the system The external salt water environments in the vicinity of the tower itself and the intake stationsmust be taken into consideration Proper siting of the tower is extremely important to avoid salt water corrosion of adjacentunits or the need to use expensive materials to operate in such an environment
WATER TREATMENT FOR ONCE-THROUGH SYSTEMS
Once-through salt and fresh cooling water systems can develop a variety of problems such as fouling due to seaweed or other debris marine microorganisms such as muscles and shell fish and biological growth Corrosion and scaling problems areminimal in fresh water once-through systems because the water is not concentrated by evaporation In salt water systemscorrosion is minimized by proper materials selection and scaling problems generally do not occurProtection against fouling by seaweed or other debris is provided by careful selection of the water intake location and travelingscreens with washing facilities In some cases additional steps may be required These may include an additional secondaryset of traveling screens and possibly battery limit strainers on the cooling water supply lines to individual unitsProtection against fouling by marine microorganisms and biological growths is achieved primarily by injecting chlorinehypochlorite chlorinebromine or other similar chemicals which kill these growths Chlorine is commonly used and is injectedin sufficient quantities to produce a residual of 1 wppm in the effluent for one hour each day (shock chlorination) Continuouschlorination for 2 - 3 days per month at lower levels (03 wppm residual chlorine in the effluent) may also be used in addition toshock chlorinationThe large water volume discharged from once-through systems usually makes chemical treating for corrosion or deposit controltoo costly These problems are handled by proper materials selection and by keeping water velocities through exchanger tubesgreater than 3 fts (09 ms) Occasionally the water analysis of a fresh water source being considered for a once-throughsystem using carbon steel materials will indicate the need for some corrosion or deposit control In this case the cost of thethreshold treatment with the once-through system should be compared to the costs of a recirculated system before making afinal choice
WATER TREATMENT FOR SALT WATER COOLING TOWER SYSTEMS
A total dissolved salts (TDS) concentration limit of 55000 wppm for sea water in cooling tower systems requires high blowdownrates since the makeup sea water typically has from 35000 to 40000 wppm TDS Because of cost restraints the only water treatment normally specified for these systems is for biological control Chlorine is most commonly used either as directchlorine injection or hypochlorite produced in electrochlorinators Care must be taken in selecting the intake location to assurethat the suspended solids do not exceed 200 wppm in the recirculated water If it is not possible to find sea water with sufficientclarity clarification with polyelectrolytes should be evaluated Corrosion is one of the major problems with the use of sea water in cooling towers This requires proper materials selection
WATER TREATMENT FOR FRESH WATER COOLING TOWER SYSTEMS
Cooling tower water treatment is necessary to minimize or eliminate corrosion scale and biological fouling of process heattransfer surfaces caused by minerals and impurities in the water The difficulties caused by these impurities and the means of treatment are shown in Table 1 Water lost by evaporation results in an increase in total dissolved salts in the recirculated water To indicate the degree of concentration of impurities the term cycles of concentration is used
Corrosion is an electrochemical process that deteriorates metals exposed to water in the presence of corrosive agents such asacids oxygen or bacteria A common form of corrosion is pitting Severe corrosion can lead to equipment failure Corrosionis caused by many conditions including process leaks into the cooling water water flow velocity that is too low (causes depositsand fouling lending to corrosion) or too high (causes erosion and corrosion) low pH and high temperatures Corrosion is lesslikely in water with non-acidic pH values (greater than 7) although scaling is more likely in these pH rangesScaling is characterized by the formation of hard dense deposits on material surfaces These deposits impact heat transfer and can become a site for localized under-deposit corrosion Scaling is influenced by many factors including makeup water composition total dissolved solids in the recirculating water water temperature and pH and flow velocity Calcium carbonateis the main scaling constituent in all waters and it is the least soluble It forms from the hardness (calcium Ca ++ ) andbicarbonate (HCO 3
ndash ) alkalinity present in most waterFouling can be caused either by the settlement of suspended matter or by microbiological growth of algae bacterial slime or fungi Fouling will reduce heat transfer cause plugging and create corrosion sites
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Section Page
COOLING WATER SYSTEMS XXVII 11 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Treatment of the cooling water is broken down into two major categories external and internal External treatmentencompasses any treatment given to the cooling tower makeup water Internal treatment is the addition of chemicals directly to
the recirculated cooling water
EXTERNAL TREATMENT
External treatment to remove impurities in order to reduce the amount of blowdown should always be evaluated as to costeffectiveness The most frequently used techniques for treating makeup water are shown in Figure 11 Details of availabletreatment processes (see Table 1 for methods available) are covered in Section XXVI-A Boiler Feedwater Treatment For cooling tower systems that meet currently required drift limits in refineries and chemical plants uncontrolled cooling water losses (drift and miscellaneous process uses) normally amount to about 14 of the cooling water evaporation loss Under these conditions the maximum cycles of concentration achievable are 8 with no controlled blowdown Some cooling towersare operating at up to 10 cycles of concentration Additional controlled blowdown required to limit the cycles of concentrationper the water quality criteria is covered later
INTERNAL TREATMENT
The makeup water analyses amount of cooling water losses environmental restraints potential chemical hazards materials of construction and film temperatures of heat transfer surfaces determine the type of internal treatment required for cooling tower systems The internal water treatment program selected must address corrosion scale fouling biological growth andchemical attack of cooling tower componentsNumerous chemicals and treatment programs are continuously being developed for addressing the potential problems Theslightly acidic low pH programs of the past which used chromate based chemicals and usually required acid injection arebeing completely phased out because of environmental restrictions on chromate and the higher costs associated with meetingcurrent practices for handling hazardous chemicals such as acid Non-chromate treatments which use inhibitors such asphosphates zinc and organics along with higher pH have been developed to replace the earlier programs Although not aseffective as chromate these inhibitors operate at higher water pH which is a less corrosive environment The disadvantage of the higher pH is the greater tendency to form scale This tendency is controlled by the use of dispersants or scale inhibitorswhich are normally organic polymers or phosphenate A corrosion inhibitor for copper or brass is also normally required if these metals are present in the systemThe following paragraphs describe techniques and chemicals to control alkalinity (pH) corrosion scale sludge and biologicalgrowth The chemicals mentioned are normally purchased as pre-mixes from vendors and dosed in combination Mostcorrosion and scale control program chemicals are two drum formulations due to the incompatibility of some chemicals in theconcentrated state Therefore separate feed systems are generally provided for chemicals for control of corrosion and scale (2drums) and biological growth (if liquid) If a program includes fixed pH then an additional feed system is required to add acidWater treatment programs are site specific and changing The COOLING WATER SYSTEMS SPECIALISTS should beconsulted for recommendations on the internal water treatment programs when an analysis of the available water is known
Alkalinity (pH) Control
Cooling water treatment programs presently available fall into two basic categories those that do not control pH (floating pH or alkaline treatment programs without pH adjustment) and those that control pH within a specified range by adding acid Theprograms that do not control pH are gaining favor because of the additional cost equipment and precautions required tominimize the risk associated with handling acid and because of improvements in chemicals to control depositsHigh pH in the recirculating cooling water is a result of the alkalinity of the fresh water due to bicarbonate salts This combinedwith carbon dioxide results in the water having a pH range of 6 to 85 When this fresh makeup water is mixed with therecirculated cooling water and passed through the cooling tower the air flow strips out the carbon dioxide Bicarbonate ionsthen convert to carbonate ions raising the pH which can cause both calcium carbonate and calcium phosphate salts toprecipitate out of solution and form deposits Another debit in allowing the cooling water pH to increase is increased chlorineconsumption to control biological growthsIf a program using pH control is justified and chosen acid addition is used under control of a pH analyzer to controlrecirculated cooling water alkalinity and pH to the range required by the treatment program
Corrosion Control
To reduce corrosion to an acceptable level chemical corrosion inhibitors which form protective films on heat transfer surfacesare the most effective protection Inhibiting corrosion is accomplished by phosphates organics zinc nitrites and molybdatesalts Unfortunately the use of chromate which is a reliable corrosion inhibitor is prohibited by environmental constraintsNitrite is not practical in an open system due to atmospheric oxygen converting the nitrite to nitrate
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Section Page
XXVII 12 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Phosphate inhibitors (orthophosphate polyphosphate phosphonate) are the basic treatment for new towers in todaysenvironment In addition to the phosphate chemistry salts of zinc may be added to aid in inhibiting corrosion These ions
combine with the hydroxide ion forming thin protective films Zinc is used to provide a synergistic effect in combination with thephosphate salts for protecting carbon steel surfaces from corrosion Finally all-organic programs are offered by severalmanufacturers The use of these treatments has been made possible by the development of polymer dispersants which givecalcium salts a much higher solubility and reduce the tendency to scaleWhen a cooling water system contains copper alloys azole based and azole substitutes are required as copper corrosioninhibitors There are several effective inhibitors The most common is tolytriazole (TTA) due to its resistance to chlorineThe corrosion inhibitors most frequently used in ExxonMobil recirculating cooling water systems are shown below indescending order of preference provided their use meets all environmental requirements
bull Stabilized Phosphate (OrthoPolyphosphate) - Environmental restrictions on the use of zinc have resulted in ExxonMobilplants in the USA using a blend of orthophosphate and polyphosphate type corrosion inhibitors with good results Thistreatment forms a thin protective calcium phosphate film on heat transfer surfaces through the use of orthophosphateconcentrations as high as 12 - 17 wppm The use of calcium phosphate dispersants and Hydroxethylidene Diphosphonate(HEDP) stabilizes the treatment and prevents the formation of calcium phosphate scale Concentration of the dispersantsdepends upon the calcium concentration temperature and pH Stabilized Phosphate treatment programs can operate in apH range of 68 to 78 Alternatively an alkaline (floating pH) stabilized phosphate treatment program can be used Thisprogram uses little or no acid addition and is gaining in popularity although not well established yet It operates with pHvalues of 8 - 9 and uses higher quantities of dispersants and lower levels of phosphate A corrosion inhibitor for copper and brass is normally included for all stabilized phosphate programs Several vendors offer these programs Twoseparate chemical feeders are generally required
bull Zinc Alkaline Phosphate - Heritage Exxon plants in Europe where the use of zinc is not restricted have obtained goodcontrol of corrosion and scaling using zinc-polyphosphate treatment programs Low (65 - 72) and high (73 - 80) pHprograms have been used The high pH (zinc alkaline phosphate) programs are the lowest cost and are recommended
As with the Stabilized Phosphate treatment dispersants such as HEDP and polyacrylates are required to prevent theformation of zinc or calcium phosphate scale on heat transfer surfaces A corrosion inhibitor for copper or brass isnormally used with this program which is offered by several vendors
bull Organic Salts - All-organic (polymer) programs are offered by several manufacturers and have been used successfullywithin h Exxon In general these treatments are based on a combination of a phosphonate to control corrosion polymers(dispersants) to prevent scale deposition and a corrosion inhibitor for copper or brass This type of program is mosteffective in conditions of high hardness high alkalinity (M alkalinity gt300 wppm as CaCO 3) and high pH values (gt 85)However some manufacturers have reported success in softer water with alternative polymer component formulations Ingeneral an inorganic corrosion control program is preferred to the all-organic programs which are sensitive to alkalinityand more costly
Detailed control limits for effective use of each of the above inhibitors and materials selection are covered in Report NoEE34E86 Environmentally Acceptable Cooling Water Treatments Report No EE102E78 Cooling Tower Water Treatment Guidelines and in the EMRE Refinery Construction Materials Manual
Scale and Sludge Control
Scale and sludge deposits in cooling water systems cause loss of heat transfer and provide the conditions for pitting corrosionScale consists of inorganic salts which have precipitated from the cooling water forming a coating on heat transfer surfacesSludge consists of organic and inorganic suspended matter that settles in quiescent areas and in heat exchangers The key toprevention of both scale and sludge formation in a cooling system is keeping water velocities greater than 3 fts (09 ms) andthe use of chemical additives in combination with a blowdown rate that keeps the impurity concentration below the level whichcauses deposits (See Table 2 for limits on impurities) Scale control has become increasingly important in higher alkalinityldquofloating pHrdquo water treatment programsSalts most commonly found in scale deposits are calcium carbonate calcium sulfate tricalcium phosphate zinc phosphatemagnesium silicate and silica The control method for preventing scale from each of these salts is discussed below
bull Calcium Carbonate - Where justified using pHalkalinity control in the range of 70 to 75 will normally inhibit the formationof calcium carbonate scale if anti-scalent chemicals (phosphonates acrylates copolymers) are part of the chemicaltreatment (See Report No EE102E78 Cooling Tower Water Treatment Guidelines ) Higher pH water treatmentprograms require greater concentrations of these chemicals along with dispersants and flocculants to inhibit the formationof scale and precipitation of other salts
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Section Page
COOLING WATER SYSTEMS XXVII 13 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
bull Calcium Sulfate - When heat transfer skin temperatures are below 250 degF (120 degC) and the cooling water bulktemperatures are under 130 degF (54 degC) gypsum scale (CaSO 42H 2O) will form if the product of the calcium and sulfate ionsconcentrations each expressed as wppm CaCO 3 equivalent exceeds 500000 Gypsum scale is prevented bymaintaining bulk water temperature below the indicated limits and controlling calcium and sulfate ions through blowdownor through the use of newer dispersants on the market
bull Tri-Calcium and Zinc Phosphate - Almost all chemical additives for corrosion control contain some type of phosphatesalt These have a tendency to form the orthophosphate ion depending on time pH and temperature Cooling watersusing these corrosion inhibitors may have an orthophosphate concentration in excess of 5 wppm as PO 4
= To preventcalcium phosphate scale Table 2 shows a limit of 1000 wppm of calcium as CaCO 3 equivalent when using a phosphateinhibitor in combination with a chemical dispersant (See Report No EE102E78 Cooling Tower Water Treatment Guidelines for solubility of calcium phosphate) These inhibitors and dispersants generally fall into one of the followingcategories
ndash Sequestrants antiprecipitants and inhibitors such as hydroyethylidene diphosphonate (HEDP) phosphate esters andaminomethylene phosphonate (AMP) function by defyingdistorting the crystal structure of the scale formations toprevent nucleation and growth of the scale
ndash Dispersants based on either anionic or nonionic polymers such as polyacrylamide polyacrylate or polymaleic acidfunction to stabilize colloidal and scale-forming particles in suspension Anionic polymers prevent agglomeration of suspended particles by either charge repulsion or crystal distortion while nonionic polymers reduce the surfacetension of particles
bull Silica Deposits - Quantitative indices for determining limits for silica in combination with other ions are not available due tothe formation of complex polymers Instead the rule of thumb limit of keeping reactive silica concentrations in the coolingwater below 150 wppm as SiO 2 has been successful
Possible alternatives for controlling sludge are
bull Chemical Dispersants - Water treating chemical suppliers now offer many types of dispersants These are mostlypolyelectrolytes Claims for removal of slime silt and mud from cooling tower basins and exchangers are madeHowever satisfactory results using these chemicals as a coagulant aid require very accurate control of the amount fed tothe water and stable operation of the water system Also suspended solids should be kept below 200 wppm for particleslarger than 045 microns Dispersants are used in virtually all fresh water cooling tower systems despite their high cost
bull Sidestream Filtration - The use of a sand pressure filter is the most reliable method for sludge removal Operating andmaintenance costs are minimal for this equipment
bull Higher Blowdown Rates - This may be an economical alternative depending on the cost of makeup water Additionalchemical costs must be included in any evaluation of this method
bull Makeup Clarification - This alternative may require additional raw water treatment facilities to handle seasonal variationsin turbidity andor suspended solids This method is applicable only where suspended solids come from makeup water
Biological Control
Operating conditions in recirculated cooling water are ideal for the growth of biological matter Water temperatures from 70 to120 degF (20 to 50 degC) favorable water pH continuous supply of nutrients including carbon nitrogen and phosphorus inorganicsalts and sunlight all lend themselves to environmental conditions encouraging microbiological growth and plant lifeIf biological growths get out of control and form large sticky agglomerations some of the following operating problems willresult
ndash Fouling of heat transfer surfaces by bacterial slimes resulting in flow restrictions and high process temperatures ndash Reduced cooling tower efficiency resulting from algae fungi and bacterial slime growths in the water distribution basin
and fill area of the cooling tower ndash Excessive plugging of screens and filters ndash Corrosion ndash Wood destruction by fungi ndash Clogging of water distribution nozzles ndash Excessive sludge accumulation in the cooling tower basin
An important part of the water treatment program is to control biological activity by feeding a biocide chemical Oxidizingbiocides such as chlorine sodium hypochlorite chlorine dioxide or bromine-associated compounds are most commonly usedwith good results Plate counts should be maintained below 50000 countsml (bacterial)
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ExxonMobil Proprietary
Section Page
XXVII 14 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Chlorine is the most effective biocide for cooling water because of its low cost extensive period of successful use ease of application and minimum environmental effects if applied under controlled conditions Either a liquid feed (sodium
hypochlorite) or a gas feed (100 chlorine) is added to the cooling tower basin A sufficient dosage is that which obtains a 01to 03 wppm continuous free residual in the hot return water to the cooling tower
Advantages to continuous chlorination over shock treatment based on ExxonMobil experience are ndash Minimizes chlorine degradation of polymers ndash Better reduction in total aerobic bacteria plate count resulting in cleaner heat transfer surfaces ndash Does not cause significant pH depressions ndash Better control results with less chlorine consumption
Details concerning chlorine addition as well as the other alternative biocides are given in Report No EE102E78 Cooling Tower Water Treatment Guidelines and in EMRE Engineering Water and Wastewater Design Guide TMEE 080 DG 11-3-1Gas Chlorinators for Water Treating Service and DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) Gaseous chlorine systems have become more costly in recent years because of the additional precautions equipment andprocedures required to minimize the risk associated with handling this gas Alternatives to gaseous chlorine are available butrequire more attention and have a higher operating cost Sodium hypochlorite (either purchased or generated) and sodiumbromide stabilized solid halide (chlorinebromine) products chlorine dioxide and ozone are all alternatives which may haveapplications Contact the COOLING WATER SYSTEMS SPECIALISTS for guidance in the use of these alternative treatmentschemes
BLOWDOWN
Blowdown control is required to maintain the proper cycles of concentration in the system The most reliable and simplest typeof control for blowdown is specific conductance measurement of the cooling water Electrical signals from the specificconductance measurements of some impurity in the cooling water are used to actuate an automatic blowdown valveImpurities normally used for blowdown control are dissolved solids chloride calcium magnesium and silica
CONTROLS AND MONITORS
Any chemical treatment used to control problems in a cooling water system must be carefully monitored to be completelyeffective
The installation of automatic controls should be considered for recirculated cooling water systems in all plants Designs shouldinclude pH conductivity corrosivity inhibitor and biocide control Automatic controllers are available which continuouslymonitor the condition of the water and automatically make chemical additions to keep the water within compositional limitsTest exchangers and corrosion coupons are also useful in monitoring the effectiveness of the water treatment program
An oil-in-water monitor should be considered to provide early detection of oil leaks into the cooling water This will enablecorrective action to be taken before extensive contamination of the cooling water circuit occurs This is especially important incooling towers with high efficiency cellular fill which can be difficult to clean if significantly fouledManual control systems may be used in locations that have adequate technical personnel to perform routine water analysesand to modify chemical treatment necessitated by sudden variations in water composition process contamination etc
WASTEWATER REUSE POSSIBILITIES
Recirculating cooling water systems normally have the highest plant raw water demand A potential alternative is to use treatedwastewater to supply part or all of the makeup water requirement The treatment required must be determined but will typicallyinclude primary oil removal (eg API Separator) secondary oil and suspended solids removal (eg filtration dissolved or induced air flotation) and soluble carbon removal utilizing biological treatmentThe reuse of treated wastewater as makeup currently exists at only a few locations If this alternative is being considered refer to Report No EE58E85 Reuse of Wastewater as Cooling Tower Makeup and Section XIX-B Water Reuse
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 15 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS
STEP 1 - Select a location for the intake station and pumps
Factors which influence this selection are as followsbull Depth of water and tidal variations
bull Relative location of intake and outfall direction of current
bull Length and size of intake suction piping vs pump discharge piping
STEP 2 - Determine cooling water requirements
Determine the design cold water inlet temperature based on site data covering seasonal water temperature as a function of inlet location and depth (for existing locations refer to Table 2 in Section IX-B )Tabulate the cooling water requirements for each process unit and offsite consumer Apply the proper LGFs to all coolingloads depending on the stage of the project and whether the new units are exact duplicates of existing unitsIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The designer should investigate methods to increase effluent water temperature to the maximumrecommended limit for the particular type of water used One common method is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 3 - Determine the total required capacity for water intake facilities
Process Coolers - The total requirements determined in Step 2 including LGFsFirewater - This is often supplied from the intake bay of a once-through systemFuture Expansions - These should be considered in sizing intake facilitiesTotal Required Capacity - The sum of the above three items
STEP 4 - Provide water intake facilities
Intake Crib - The intake crib with its support structure is located at the extreme end of the water intake line (See Figure 4 )The intake is usually located 200 to 300 ft (60 to 90 m) offshore and at a sufficient depth below the lowest water level to
preclude vortex formation and subsequent pump cavitation during storm conditions Additionally the crib should have 3 to 5 ft(1 to 15 m) clearance above the sea floor to minimize sand and silt pickup A concrete velocity cap should be specified to cover the inlet crib This will minimize the incidence of fish and other foreignmatter entering the system A horizontal inlet flow pattern should be designed with a maximum entrance velocity of 1 fts(03 ms) Experience shows that vertical velocity flow confuses fish and they are drawn into the intake lineThe intake crib support structure protects the crib from adverse effects of storms and tidal motions as well as protecting it fromimpact of floating and submerged objects A self-cleaning screen can be specified to prevent entrance of large objects into thesystem at the cribIntake Line (Figure 5 ) - An intake line is required if the water depth along the shore does not permit water to flow directly intothe intake bay The intake line is usually buried in the sea floor and covered with a minimum required overburden Consultwith the COOLING WATER SYSTEMS SPECIALISTS for required coverage at a specific siteChlorine or sodium hypochlorite is injected at the intake crib to prevent marine growth in the lineNote that an intake crib and line will not be required if water of sufficient depth is available at the supply pump stationIntake Bay (Figure 6 ) - The intake bay is an enclosure which houses the pumps and screens Depending on a particular design a series of screens is used to ensure the supply of clean water Trash racks comprise the first element of the filter system These remove only very coarse material from the water such as bulk seaweed They can be supplied with atraversing raking mechanismLift screens provide secondary cleanup These screens can be lifted out of the intake bay for periodic cleaning Double liftscreens can be used if it is deemed necessary to have one in service at all timesFinal water filtering is usually done by traveling screens Details of the screen dimensions and the mounting are usuallydetermined during detailed engineering Screen net open area is typically approximately 50 of total screen area Thescreens should be sized for a normal water velocity of 12 fts (04 ms) through the open screen area Maximum velocityshould be limited to 24 fts (08 ms) with 50 of the screen area plugged This is the upper velocity limit for efficientscreening
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Section Page
XXVII 16 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
If high suspended solids greater than 200 mesh (eg sand) are expected to be a continuous problem ejectors may be installedupstream and downstream of the traveling screen to remove accumulated sand and solids from the intake bay in these areas
Traveling screens and ejectors are supplied with wash water from the raw water pumps or from separate wash water pumpsOnoff control using timers should allow optimum scheduling of the ejectors depending on the time of the year
STEP 5 - Specify water intake facilities
Water intake facilities should be designed in accordance with the following guidelines from the Hydraulic Institute Standards The water intake should be designed to provide an evenly distributed flow of water to the pump suction A properly designedintake reduces the possibility of vortex formation and results in better pump performance with less noise Geometry of theintake and the manner in which water is introduced into the intake are important parameters in providing an even distribution of water flow Complete analysis of intake structures is best accomplished by scale model testsThe pump manufacturer will generally render advice on specific problems while the intake design is still preliminary He shouldbe provided with the necessary intake layout drawings showing the physical limitations of the siteFigures 7A and 7B have been developed for single- and multiple-pump arrangements to show suggestions for basic sumpdimensions They are for pumps normally operating in the capacity range of approximately 3000 to 300000 gpm (200 to20000 Ls) Since these values are composite averages from a great many pump types and cover the entire range of specificspeeds they must not be thought of as absolute values but rather as basic guides subject to some possible variations
All the dimensions in Figures 7A and 7B are based on the rated capacity of the pump at the design head Any increase incapacity above these values should be momentary or very limited in time If operation at an increased capacity is to beundertaken for considerable periods of time the maximum capacity should be used for the design value in obtaining sumpdimensionsDimension C is an average value based on an analysis of many pumps Its final value should be specified by the pumpmanufacturerDimension B is a suggested maximum dimension which may be less depending on actual suction bell or bowl diameters in useby the pump manufacturer The edge of the bell should be close to the back wall of the sump When the position of the backwall is determined by the driving equipment or the discharge piping Dimension B may become excessive and a ldquofalserdquo backwall should be installedDimension S is a minimum value for the sump width for a single pump installation This dimension can be increased but if it isto be made smaller the manufacturer should be consulted or a sump model test should be run to determine its adequacy
Motor size may dictate minimum pump spacing be especially aware of this when specifying pumps with unusually high pumpheads eg greater than 80 psig (550 kPa)Dimension H is a minimum value based on the ldquonormal low water levelrdquo at the pump suction bell taking into considerationfriction losses through the inlet screen and approach channel This dimension can be considerably less momentarily or infrequently without excessive damage to the pump It should be remembered however that this does not representldquosubmergencerdquo Submergence is normally quoted as dimension ldquoHrdquo minus ldquoCrdquo This represents the physical height of water level above the bottom of the suction inletThe actual submergence of the pump is something less than this since the impeller eye is some distance above the bottom of the suction bell possibly as much as 3 to 4 ft (1 to 13 m) For the purposes of sump design in connection with this chart it isunderstood that the pump has been selected in accordance with specific speed charts The submergence referred to hereinhas to do only with vortexing and eddy formationsDimensions Y and A are recommended minimum values These dimensions can be as large as desired but should be limitedto the restrictions indicated on the curve If the design does not include a screen Dimension A should be considerably longerThe screen or gate widths should not be substantially less than S and heights should not be less than H If the main stream
velocity is more than 2 fts (06 ms) it may be necessary to construct straightening vanes in the approach channel increaseDimension A conduct a sump model test of the installation or work out some combination of these factorsDimension S becomes the width of an individual pump cell or the center-to-center distance of two pumps if no division walls areusedOn multiple pump installations the recommended dimensions in Figures 7A and 7B also apply as noted above and thefollowing additional determinants should be consideredFigure 8A - Low velocity and straight-line flow to all units simultaneously is the first recommended style of pit Velocities in thepump area should be approximately 1 fts (03 ms) Some pumps with velocities of 2 fts (06 ms) and higher have given goodresults This is particularly true where the design resulted from a model study Not recommended would be an abrupt changein size of inlet pipe to sump or inlet from one side introducing eddying
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Section Page
COOLING WATER SYSTEMS XXVII 17 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Figure 8B - A number of pumps in the same sump will operate best without separating walls unless all pumps are always inoperation at the same time in which case the use of separating walls may be beneficial If walls must be used for structural
purposes and pumps will operate intermittently one should leave flow space behind each wall from the pit floor up to at leastthe minimum water level and the wall should not extend upstream beyond the rim of the suction bell If walls are used increaseDimension ldquoSrdquo by the thickness of the wall for correct centerline spacing Round off the ends of the walls Not recommended isthe placement of a number of pumps around the edge of a sump with or without dividing wallsFigure 8C - Abrupt changes in size from inlet pipe or channel to pump bay are not desirable A relatively small pipe emptyinginto a large pump pit should connect to the pit with a gradually increasing taper section The angle should be as large aspossible preferably not less than 45 degrees With this arrangement pit velocities much less than one foot per second aredesirable Especially not recommended is a small pipe directly connected to a large pit with pumps close to the inlet Flow willhave excessive change of direction to get to most of the pumps Centering pumps in the pit leave large ldquovortex areasrdquo behindthe pumps which will cause operational troubleFigure 8D - If the pit velocity can be kept low enough [below 1 fts (03 ms)] an abrupt change from inlet pipe to pit can beaccommodated if the length equals or exceeds the values shown It is assumed that as ratio WP increases the inlet velocityat ldquoPrdquo will increase up to an allowed maximum of 8 fts (24 ms) at WP = 10 Pumps ldquoin linerdquo are not recommended unless theratio of pit to pump size is quite large and pumps are separated longitudinally by a generous margin A pit can generally be
constructed at much less cost by use of a recommended designFigure 8E - It is sometimes desirable to install pumps in tunnels or pipelines A drop pipe or false well to house the pump witha vaned inlet well facing upstream will be satisfactory in flows up to 8 fts (24 ms) Without the inlet well the pump section bellshould be positioned at least two pipe diameters vertically above the top of the tunnel not hung into the tunnel flow especiallywith tunnel velocities 2 fts (06 ms) or more There should be no signs of air along the top of the tunnel It may be necessaryto lower the scoop or to insist on a minimum water level in the vertical wellNote The foregoing statements apply to sumps for clear liquid
STEP 6 - Provide chemical treatment
Provide chemical treatment facilities for the once-through cooling water system Normally the only chemical treatment requiredin a once-through cooling system is for biological control Facilities for adding chlorine to once-through cooling water are thesame as those described for the recirculated cooling water system The design capacity of the chlorinator should be based onachieving a chlorine residual of 1 wppm in the effluent The chlorine required by the organics and other reducing agents in thewater must be satisfied before any residual chlorine will appear in the effluent The design capacity of the chlorinator should be
based on the total water chlorine demand plus 1 wppm in the effluent In the absence of water data the chlorination systemdesign capacity should be based on 10 wppm minimum instantaneous dosage for maximum cooling water flow For very largeonce-through cooling water systems it will be impractical to design for shock rates at this level In this case a continuousdosage of 1 - 2 wppm should be used (Refer to Step 5 of the DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS )
STEP 7 - Specify pumps and distribution system
Specify other major facilities in the once-through cooling water system The recommended materials for pumps valves andpiping are shown in the EMRErsquos Refinery Construction Materials Manual
Pumps1 Type - These pumps are usually vertical centrifugal pumps However horizontal centrifugal pumps have been used An
economic study andor consideration of existing facilities will determine which type should be specified In the event thathorizontal centrifugal pumps are specified facilities for pump priming will likely be required For large pumps in salt water service there is a very limited selection of pumps because of the special materials required Consult with the DISCIPLINESPECIALISTS early in the design effort to determine what specific pumps are currently available
2 Total Required Capacity - Firm capacity (excluding spares) of the supply pumps should equal the maximum cooling water requirements including LGF and RCF
3 NumberIndividual Pump Capacity - Normally one spare pump is provided for up to six operating pumps two spares for up to twelve operating pumps etc The most common arrangements for water supply pumps are
bull Two 100 capacity pumps
bull Three 50 capacity pumps
bull Four 33-13 capacity pumps
bull Five 25 capacity pumps
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Section Page
XXVII 18 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Maximum commercially available pump capacity ranges up to 300000 gpm (20000 Ls) at discharge pressures up to 100psi (700 kPa) Maximum capacity will be less for higher discharge pressures Other factors which may influence pump
selection arebull Selecting capacity to match existing pumps
bull Selecting capacity for future requirements
bull Selecting capacity corresponding to manufacturers available standardVendor information should be obtained on the availability size and cost of the pumps for several configurations Acomparison should be made of the investments and operating costs as well as other advantages and disadvantages toselect a pump configuration
4 Drivers - These pumps are usually motor-driven Because of the remote location of the intake facilities the steamdistribution costs are normally too high to justify the use of a steam turbine driver However use of a secondary selectivesubstation provides a reliable source of power to the pump drivers
5 Pump ∆∆∆∆P - Pump ∆P is based on supplying the maximum cooling water requirement at the battery limit design pressureestablished for the system The ∆P should include the total circulating system pressure drop plus any elevation differencebetween the low water level at the intake and the discharge point of contact with air
6 Pump Instrumentation - The remote location of the intake facilities makes remote control and monitoring of the water supply operation desirable Pump controls located in the control house provide for minimum manning Local controlbackup should be provided The Owners operating and manning philosophy determine the basis for instrumentation andcontrolThe spare pump is controlled to automatically start on low pressure in the supply header Concurrently alarms should beactuated in the control room to indicate the low-pressure situation and spare pump operation
Distribution Network1 Configuration - The piping network is a once-through system with a return line from each consumer or consumer group to
the cooling water effluent line2 Layout - The routing of the piping network should connect all cooling water consumers in the most economical manner
permitted by the plant layout Careful consideration should be given to the turnaround philosophy of the plant Thenetwork layout should preclude the necessity of having above-ground water lines that are in service running through anarea that is being turned around Underground piping or an extra lateral may be required to accomplish this objective
3 Valves and Blinds - Battery limit valves should be provided in the network to isolate individual process units from thesystem Valves should be provided to permit onstream removal and maintenance of spared equipment
4 Basket Strainers - Consideration should be given to the installation of basket strainers in the laterals serving each of theconsumer loops These provide added insurance against entrance of foreign material into the cooling equipment The useof intake screens with 025 in (63 mm) openings may eliminate the need for basket strainers in the laterals
Line Sizing1 Design Basis - The network is sized so that water is delivered to the most distant consumer at a pressure equal to the
required process design (battery limit) pressure2 Flow Rate - Each segment of the distribution network is sized for the simultaneous maximum flow in that segment for any
operating condition The main supply and discharge headers are sized for the simultaneous maximum flow of all segments(not necessarily the sum of the simultaneous maximum flows of the individual segments)
3 Pressure Drop
Line Sizing Basis - The distribution system should be sized according to the guidelines established in Section XIV The use of an ldquoold piperdquo friction factor of C = 90 - 100 (Hazen-Williams factors) in the main lines is recommended to assuregood operation in later years when the system experiences fouling For large diameter lines [greater than 48 in (1200mm)] maximum water velocity should be limited to 10 fts (3 ms) based on civilmechanical considerations (supports andanchoring required to handle potential pressure surges) For salt water systems utilizing cement-lined pipe consider thereduced inner diameter in the friction loss calculations A study may be necessary to determine the economic pump ∆P vsline diameters
4 Equivalent Length Factors - Equivalent length factors are used in pressure drop calculations for sizing the distributionsystem The piping length scaled from the plot plan is multiplied by an equivalent length factor to compensate for pipingelbows expansion loops and other piping variations
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Section Page
COOLING WATER SYSTEMS XXVII 19 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
The equivalent length factors are selected based on the expected variation from a straight piping run Typical equivalentlength factors suitable for planning purposes for different piping applications are as follows
TYPE OF PIPING EQUIVALENT LENGTH FACTORS
Long piping runs [gt 1000 ft (300 m)] 12 to 14
Onsite distribution piping 15 to 20
For design purposes the actual piping layout should be evaluated to ensure that the equivalent length factors areapplicable to the specific situation Particular attention is required for revamp work and saltwater systems utilizing cement-lined pipe
STEP 8 - Prepare a system flow plan similar to the one shown in Figure 1
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS
STEP 1 - Determine cooling water requirements
Tabulate the cooling water requirements and cooling water discharge temperature for each process unit and offsite consumer Apply the proper LGFs to all cooling loads depending on the stage of the project and whether the new units are exactduplicates of existing units Determine the total cooling water flow required including LGFs and the bulk temperature (T 1) of thecombined cooling water flow from all coolers to the cooling towerIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The onsite and offsite designers should investigate methods to increase effluent water temperature to themaximum recommended limit for the particular type of water used This will result in a lower cost cooling tower for a given heatrejection rate One common method of maximizing return water temperature is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 2 - Select cooling tower design parameters
Select the proper design conditions of ambient wet bulb temperature approach temperature range and flow rate for specifyingthe cooling tower Based on these conditions the cooling tower vendor will size the tower and provide the tower characteristicand performance curves which serve as the basis for the performance guarantee Required performance test and evaluationprocedures are described in IP 8-1-1 Economic studies may be required to select the ambient wet bulb temperature andapproach temperature for new locations or for expansion projects with special cooling requirements
STEP 3 - Prepare cooling tower specification sheet
Include the following information in the heat exchange section of the design specificationTower Flow Capacity - This was determined in Step 1Type of Tower - The tower usually specified is an induced draft multicell type suitable for fresh salt or brackish water depending on the water available Additional tower characteristics are usually developed and proposed by the tower vendor (eg crossflow versus counterflow type of tower fill etc) The vendor should be made aware of conditions such as thelikelihood of heavy oil leakage into the cooling water or high fouling potential which may influence the tower type or fill
selectionNumber of Cells - The minimum number of cells required (normally not less than 2) may be specified The maximum flow per cell is a function of the cooling tower design conditions selected in Step 2 Typical values range up to 10000 gpm (600 Ls) for crossflow designs and 8000 gpm (500 Ls) for counterflow designs Normally spare cells are not provided Maintenance iscarried out during cooler periods or load is shed
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Section Page
XXVII 20 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Ambient Wet Bulb Temperature1 The ambient wet bulb temperature selected for specifying the cooling tower design is generally that temperature which is
equaled or exceeded 5 of the time during the four hottest months (5 of the hourly readings taken during the four hottestmonths will exceed this value) Specifying the 5 temperature should give satisfactory results since the time that the wetbulb temperature is exceeded is for short duration and the system inertia is usually sufficient to avoid problems The tower specification should make it clear that the wet bulb temperature specified is an ambient value as opposed to the enteringvalue which may be higher because of recirculation The tower manufacturer should adjust the ambient value upward totake into account his estimates of recirculation Currently cooling tower test codes provide procedures for measuringperformance in the case of either entering or ambient wet-bulb temperature specifications An increase of 2 degF (11 degC) isgenerally added to ambient wet bulb temperature to estimate entering wet bulb temperature for large cooling towers
2 If plant production will peak during the four summer months when the 5 ambient wet bulb temperature may be exceededconsideration should be given to specifying a higher design ambient wet bulb temperature (one that will be exceeded 212 of the time for example) However the incremental cost of the resulting closer approach and larger cooling tower must be justified by the value of the incremental products made
On-Tower Temperature (T1) - This temperature cannot be accurately determined until the design of all process coolers iscompleted and the volume of cooling water and outlet temperature are known for each (see Step 1) In the planning stage or early design stage of a project this temperature must be estimated and is usually set equal to the maximum design cooler outlet temperature shown under COOLING WATER REQUIREMENTS in the BASIC DESIGN CONSIDERATIONS portion of this sectionApproach Temperature - The approach temperature has the most pronounced effect on cooling tower size and cost For example reducing the approach from 10 to 5 degF (55 to 3 degC) can increase the cooling tower investment by 50 percent and thefan operating cost by 65 percent Thus various approach temperatures should be evaluated to determine the minimum cost of the total system including the process exchanger area Approaches used in the past designs have been from 5 to 12 degF (3 to7degC) Note that it is not customary in the cooling tower industry to guarantee approaches of less than 5 degF to (3 degC) to coolingtower design inlet wet bulb temperature A typical approach for a new grass roots cooling tower is 10 degF (6 degC)Off-Tower Temperature (T2) - This is the sum of the approach temperature and the ambient air wet bulb temperature For existing locations Table 2 in Section IX-B may be used
Range and Flow Rate1 The range for a given heat load is related to the circulating water rate as follows
Q = M C p ∆Tcong (500) (gpm) (range) Eq (1)
where Q = Heat load on the tower BtuhM = Water circulating rate lbhCp = Specific heat capacity of water at average temperature Btulb degF∆T = (T 1 ndash T 2) degF = (water cooling range)gpm = Circulation rate in US gallons per minute
For calculation in Metric units
Q = M C p ∆Tcong (1162 x 10 -3) (Ls) (range) Eq (1M)
where Q = Heat load on the tower kJh
M = Water circulating rate kghCp = Specific heat capacity of water at average temperature kJkg degC∆T = (T 1 ndash T 2) degC = (water cooling range)Ls = Circulation rate in liters per second
2 Generally lower tower investment fan horsepower and pump horsepower result from maximizing the range andminimizing the flow rate However based on fouling considerations of process coolers the maximum allowable hot water temperatures must be kept below recommended limits Also while increasing the range minimizes cooling tower investment and operating cost the cost of exchanger surface requirements is adversely affected by the LMTD and acompromise must be made The range is bounded by the maximum allowable hot water temperature and the Off-Tower temperature (T 2) For established locations refer to Section IX-B for design cold water temperatures to assist in selectingthe approach and range
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Section Page
COOLING WATER SYSTEMS XXVII 21 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Makeup Water Requirements - Calculate the various losses in the cooling water system to determine makeup requirementsThese losses include
1 Evaporation Loss (E) - Allow 1 of the circulation rate for each 10 degF (55 degC) temperature drop through the tower Notethat the actual evaporation loss will be somewhat less depending on ambient air temperature
( )sL555
TT)gpm(
1000TT
E 2121
minus=
minus= Eq (2)
2 Drift Loss (D) - IP 8-1-1 limits the maximum allowable drift loss to 0005 of the design water circulation rate for freshbrackish or salt water
( )sL1000050
)gpm(1000050
D
=
= Eq (3)
This low drift level contributes little to solids removal from the tower with the spray This allows higher cycles of concentration to be achieved in cooling towers which use clarified softened water for makeup and thus have low requiredblowdown rates Higher drift loss percentages in older or poorly maintained towers will limit the maximum cycles of
concentration achievable3 Miscellaneous Losses (O) - Miscellaneous losses are estimated to be one-seventh of the difference between evaporation
loss and drift loss
minus=
7DE
O Eq (4)
4 Blowdown Rate (B) ndash Guidelines for maximum concentrations of impurities allowed in recirculated fresh cooling water areshown in Table 2 For salt water the blowdown is based on keeping the total dissolved salts below 55000 wppm Obtainthe chemical analysis of the makeup water and divide each individual component concentration into its counterpart inTable 2 The result is the maximum allowable cycles of concentration for that constituent (a worksheet is provided inTable 4 ) The impurity with the lowest factor will be the control and will determine the required blowdown rate
)OD(1Cycles
EB +minus
minus= Eq (5)
The makeup water requirement is the sum of all losses shown above
MakeUp = E + D + O + B Eq (6)
STEP 4 - Specify the controls necessary for proper cooling tower operation
Air Side - Cooling towers are provided with motor-driven manually adjusted pitch multiple-bladed propeller fans Figures9A amp B may be used to estimate the normal operating horsepower requirements Since the fan motor and gear reducer arelocated at the top of the tower [generally 40 to 50 ft (12 to 15 m) above grade] the need for equipment to handle thesecomponents should be discussed with the affiliateControl of the air flow through the tower to maintain reasonably constant cold water temperatures can be accomplished byturning off individual fans in multiple-cell installations adjusting the blade pitch or using variable speed motors To turn off individual fans remote at grade start-stop buttons for the motors are usually specified
Whenever the ambient air temperature is 32 degF (0 degC) or below icing of the tower louvers and structural members in contact withthe incoming air will occur If the heat load on the tower is relatively free of seasonal variations two-speed fans should provideadequate ice control down to approximately 0 degF (-18 degC) ambient temperatures When sustained temperatures of 0 degF (-18 degC)and below occur two-speed fans capable of two-speed reverse operation with appropriate time delays for change of directionand speed should be specified to periodically eliminate ice which will build up on the louvers Other methods of winterizationare available but in general are more expensive and require greater operator attention In very cold climates steam heatingshould be provided to prevent freezing in the cold water basin during shutdown periodsWater Side - The water circulation rate is maintained by operating one or more pumps of sufficient capacity to meet the systemdemand Flow-measuring facilities local temperature indicators and a high-temperature alarm are provided on the coolingtower water supply and return headers Hand-operated flow control valves are required for distributing the water equally to theindividual cells On counterflow towers provisions for measuring the flow to each cell (pitot tubes) are required They are notrequired on crossflow towers since the open distribution basin provides visual aid in balancing flows Shutoff valves should beprovided for servicing the flow control valves without shutting down the entire tower
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XXVII 22 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
STEP 5 - Provide chemical treatment facilities for the fresh water system
Alkalinity (pH) Control (if used)1 In cases where a controlled pH corrosionscale control program is being used the alkalinity (pH) of the recirculated cooling
water should be controlled in accordance with the program requirements Normally acid addition is required to accomplishthis This is due to free carbon dioxide being stripped from the cooling water in the tower to approximately 5 ppm For example if the alkalinity has to be controlled to between 25 and 50 ppm for a pH of 70 to 75 the volume of 98 sulfuricacid (66 degBeacute) required to treat the makeup water is calculated using the following equation
[Note 93 sulfuric acid should be used if ambient temperatures will be less than 35 degF (2 degC)]
GallonsHour 66 degBeacute H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate gpm) (326 x 10 -5)
For calculation in Metric limits
kgh of 100 (66 degBeacute) H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate Ls) (793 x 10 -3)
2 The metering pump should be designed for a maximum feed rate of two to three times the normal feed rate A glass woolfilter is required on the pump suction piping to minimize plugging Normally two - 100 capacity pumps are provided
Positive displacement metering pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumps with flow control areused for higher rates
3 Hydrochloric acid may also be used for alkalinity control The gallons per hour of 20 degBeacute hydrochloric acid required is36 times the amount of 66 degBeacute sulfuric acid In Metric units the weight of 32 (20 degBeacute) hydrochloric acid required is227 times the weight of 100 (66 degBeacute) sulfuric acid
4 Specifications for chemical feeders may be found in EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased)
5 A pH analyzer with high and low alarm is provided to control the amount of acid feed The acid should be added to thecooling tower makeup water (Figure 2 ) The lag time between the point of acid injection and the sample outlet to the pHcell should be less than two minutes for effective control
6 Spills or overflows from the sulfuric acid tank should be contained
Corrosion Inhibitors and Anti-Scalents1 Two chemical feeders are generally provided The following guides in sizing the equipment will allow the use of any of the
proprietary products from the vendors when final selection by the owner is made See Report No EE102E78 Cooling Tower Water Treatment Guidelines EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and Report No EE34E86 Environmentally - Acceptable Cooling Water Treatments for detailed discussion of acceptable commercial chemicals
bull If the water treatment program to be used is known design for the chemical rates specified by the vendor or COOLING WATER SYSTEMS SPECIALIST
bull Otherwise design for 100 wppm of each commercial liquid product in the recirculated cooling water The design feedrate should be based on the design total blowdown flow (both uncontrolled and controlled) since this is where theproduct is lost For 100 wppm concentration the chemical rates will be as follows
GallonsHour Chemical = Blowdown (gpm) x (0006)
LitersHour Chemical = Blowdown (Ls) x (036)
2 The design capacity of the metering pump should be two to three times this calculated number Normally 2 - 100capacity pumps are provided Positive displacement pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumpswith flow control are used for higher rates
3 Specifications for chemical feeders may be found in EMRErsquos Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased) Reference should also be made to the checklists containedin Report No EE92E94 Guidelines for Safety Evaluation of Chemical Injection Facilities
4 A final check should be made with the owner and the COOLING WATER SYSTEMS SPECIALISTS as to the treatmentprogram selected and if there is a need for an additional chemical feeder
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COOLING WATER SYSTEMS XXVII 23 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Biological ControlFacilities should be provided for feeding the biocide into the back of the cooling tower basin If chlorine is used it must bepresent in the cooling water as free residual to be effective However the following amounts of chlorine are first required toreact with the listed impurities when present in the cooling water before a free residual can be established
IMPURITIESAMOUNT OF CHLORINE REQUIRED (wppm)
WITH EACH WPPM OF IMPURITY
Ammonia (NH 3)
Hydrogen Sulfide (H 2S)
Sodium Sulfite (Na 2SO 3)
Ferrous Iron (Fe ++)
10
22
056
064
The design capacity of the chlorine feeder shall be based on achieving a 01 wppm free chlorine residual in the recirculatingwater flow or a minimum dosage of 7 wppm at maximum design makeup water flow rate and conditions (see EMRE Water and
Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service )Chlorine may be fed either as the commercial liquid sodium hypochlorite (124 wt Cl 2) or liquid chlorine (100 Cl 2) Liquidhypochlorite is being used increasingly because of the higher costs associated with precautions equipment and proceduresrequired to minimize the risks associated with handling gaseous chlorine The hypochlorite solution is fed using a meteringpump which should be sized to feed an equivalent of 15 ppm 100 Cl 2 (Refer to EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) ) In case of spills or an overflow from thestorage tank the sodium hypochlorite may be sent to the cooling tower basinGas chlorinators are covered by EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service The use of gas chlorination is being reduced but may still be appropriate in some locations subject tolocal management approval The gas from either commercial cylinders or an evaporator is dissolved in a water slip stream bymeans of an eductor This results in a chlorine solution strength of approximately 01 This solution should be evenlydistributed below the water surface by means of a header lateral pipe arrangement at the rear of the cooling tower basinopposite the cooling water pumpsSome additional guides on the use of gas chlorinators are given below It is also recommended that the safety guidelines
published by the Manufacturing Chemists Association and the Chlorine Institute be followedbull Dilution water for the chlorinator must not be less than 50 degF (10 degC) or ice formation may cause plugging problems
bull Continuous rates of chlorine gas withdrawal should not exceed 400 lb (180 kg)24 hours from each one ton cylinder atroom temperature [70 degF (21 degC)] This rate may be exceeded (up to 50) for periods not exceeding 2 hours Rule of thumb is not to design for more than 4 cylinders operating in parallel When feed rate exceeds 3000 lb (1360 kg) per daythe liquid chlorine should be fed to an evaporator upstream of the chlorinator
bull If the ambient temperature can be expected to be below 50 degF (10 degC) for extended periods a building should be providedfor the chlorinator and cylinders
bull Weighing scales should be provided to determine when shipping containers should be replaced and to verify chlorine feedrates
bull Analyzers to measure the free chlorine residual in the circulated cooling water should be provided
bull Chlorine leak detectors and a catch basin for major spills should be provided
Control SystemProvide automatic controllers to continuously monitor water condition and to add chemicals to the system (complete systemsmay be available through the water treatment program vendor as a lease or purchase)pH Control - This controller adds acid to correct water alkalinity a timer on the acid pump sounds an alarm if the pump doesnot shut off in a predetermined length of time Backup controls are required to prevent the cooling system from beingpermanently damaged in case the pH electrodes become fouledCorrosivity and Inhibitor Control - Systems are available which give a direct readout of corrosion rate in milsyear If thecorrosion rate exceeds a set point the controller shuts off the acid increases the inhibitor addition opens the blowdown valveand sounds an alarm Electrodes measure the completeness of the inhibitor film by the value of the potential differencebetween the electrodes The larger the difference the more incomplete the film and a potential reading exceeding a set levelactivates the inhibitor feed
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XXVII 24 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Free Residual Chlorine Control - Free residual chlorine should be controlled automatically for biocide treatment Severalgood analyzers are currently available These units can be incorporated into an automatic control system
Oil in Water DetectionMonitoring Corrosion and Deposit FormationThe following types of devices for measuring corrosion rates and deposit formation should be provided to allow the plant tomonitor the effectiveness of the cooling water treatment before scheduled turnaroundsCoupons - A corrosion coupon rack should be specified for the hot return water to the tower in accordance with IP 19-6-1 Corrosion Monitor - On-line corrosion analyzer if specified should monitor the hot return water to the tower in accordance withIP 19-6-1
On-Line Test Exchanger Although there are a variety of deposit monitors from the water treatment chemical vendors they do not necessarily representactual heat transfer conditions in process exchangers For this reason a test exchanger should be installed for evaluatingcorrosion and deposit formation Details are covered in Report No EE102E78 Cooling Tower Water Treatment Guidelines
Alternatively a critical process exchanger can be instrumented such that its heat transfer coefficient can be monitored
Instrumentation would include flows and inlet and outlet temperatures of process fluid and cooling waterSTEP 6 - Specify cooling tower location
Select a location for the cooling tower The following factors influence this selection
Location Factors1 The location should be within a reasonable distance of the units served and consistent with future refinery plans Piping
costs to and from battery limits can be a significant portion of the total cooling system cost and can be the location-determining factor particularly for salt water systems While piping costs can be easily determined other costs affectinglocation cannot be estimated readily Therefore judgment is required in selecting the location
2 Safety spacing requirements (see Section XV-G Equipment Spacing )3 Cooling tower contribution to plant boundary noise4 For salt water cooling towers wind direction and distance to other facilities is of major concern because of potential
corrosion problems
5 Factors affecting cooling tower performance ie orientation with respect to wind site congestion etc
Plot RequirementsIn estimating plot requirements (basin area) one should allow 5 to 6 ft of tower length for each 1000 gpm (2 m for each 60 Ls)of circulating capacity This figure is based on a typical counterflow design with a width of 50 ft (12 m) and does not includepump suction pit space Exact requirements can be obtained from cooling tower vendors after the tower capacity has beenestimated
Fogging and DriftTwo distinct problems are associated with the discharge air from a cooling tower (1) fogging which can be a nuisance or ahazard and (2) drift which is generally a nuisance because of increased corrosion of surrounding equipment Fogging fromcooling towers occurs during periods of combined high ambient humidity and low temperature as a result of mixing the warmmoist cooling tower exhaust air with the cooler ambient air The ambient air cannot absorb all moisture as vapor and thus tinyfog droplets are formed by condensation Fogging and resulting icing in cold climates has produced hazardous drivingconditions on highways up to 850 ft (250 m) awayProprietary designs are available to combat potential fogging problems Basically these designs incorporate heating of thedischarge air with hot return water precooling coils (similar to an air cooler) to eliminate any visible plume However thesedesigns are expensive and relatively unproven Therefore proper location based on reliable weather data is the preferredsolutionDrift from a cooling tower represents a more difficult problem since it does not depend on definable climatic conditions andoccurs for all wind directions The effects of drift on nearby equipment and vegetation are particularly severe and generally of concern only for salt water systems because of the high solids content of the drift (concentrated salt water) For salt water cooling systems electric lines substations and instrumentation should be located upwind in the direction of the predominantwind They should be provided with adequate shelter or materials when within 450 ft (140 m) of the cooling tower In additionspecifications for all equipment within 150 ft (45 m) of the tower should include surface treatment suitable for a marineenvironment
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COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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XXVII 28 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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XXVII 30 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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COOLING WATER SYSTEMS XXVII 31 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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XXVII 32 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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COOLING WATER SYSTEMS XXVII 33 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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COOLING WATER SYSTEMS XXVII 35 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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Section Page
XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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COOLING WATER SYSTEMS XXVII 37 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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XXVII 6 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
SYSTEM TYPES AND APPLICATIONS
Cooling water systems in refineries and chemical plants are generally either the once-through or the recirculated type designedfor fresh salt or brackish water
In the once-through system (see Figure 1 ) pumps take suction from a source of supply such as a river or other body of water and deliver the water to process coolers and other consumers After passing through the cooling exchangers the hot coolingwater is discharged through pressurized piping or a gravity flow systemIn a recirculated system (see Figure 2 ) pumps take suction from a cooling tower basin and deliver the water to process coolersand other consumers After passing through the cooling exchangers the hot cooling water is discharged through a pressurizedreturn system to a cooling towerThe choice between a once-through and recirculated cooling water system must be based on (1) availability of water of satisfactory quality (2) process temperatures (3) atmospheric conditions (4) investment and operating costs and (5) effluentwater quality and temperature limitationsThe cooling water is distributed to individual consumers throughout the plant in piping main supply and return headers andseries and parallel branches consisting of laterals and sublaterals The main cooling water distribution headers are either buried in underground trenches or are located above grade on ground-level pipe sleepers or on elevated pipe racksUnderground piping has the advantage of minimizing problems associated with thrust loads thermal expansion and freezingElevated piping is easier and less expensive to install and repair and it makes it easier to locate leaks
BASIC DESIGN CONSIDERATIONS
RELIABILITY
The cooling water system is an important utility and its reliability is critical to successful plant operation The typical systemprovides cooling water to both onsite and offsite consumers It can have a direct impact on the sizing criteria for the emergencyrelief (flare) system (refer to Section XV-C Safety in Plant Design Pressure Relief ) The system therefore normally must becapable of continuous operation so that cooling water is always available to critical consumersThe reliability required to achieve continuous operation is provided in several ways
bull Spare makeup and water circulating pump(s) are provided to assure continuous operation
bull Different types of drivers are specified for the supply or water circulating pumps Motor-driven pumps are supplied fromtwo different buses on a secondary selective electrical distribution system
bull Cooling tower fan drivers are normally supplied from a secondary selective systembull Holdup volume is provided in the cooling tower basin to permit an orderly shutdown of process units if the makeup water
supply is lost or a failure occurs in any part of the recirculated cooling water system
bull Valves blinds and bypasses are provided to permit individual components to be removed from the system for maintenancewhile the system is operating
bull Construction materials provided for cooling water facilities are designed for long-term corrosion resistance sincemaintenance work is often difficult (if not impossible) to schedule without a complete refinery shutdown
FLEXIBILITY
The cooling water system must include sufficient flexibility to cover all present operating requirements plus any future operatingrequirements that are defined in the design basis document For example future operating requirements could be dictated by anear-term increase in plant sizeThe type of flexibility features provided in a system may include the following
bull Makeup water treating facilities should be able to deliver the required water assuming the worst conditions of the water source
bull Waste treating facilities should be designed to handle effluent water containing varying concentrations of oil and chemicalsover the range of expected ambient temperatures
bull Valves and blind flanges should be provided to permit future tie-ins to the system without a shutdown An exception wouldbe on lines where a ldquohot taprdquo could be made with minimum risk of an emergency shutdown or where the cost of requiredvalves is prohibitive
bull Cooling tower cells should be completely partitioned as described in IP 8-1-1 to allow personnel entry to any cell for maintenance without affecting the operation or capacity of the other cells Normally spare cells are not providedMaintenance is carried out during cooler periods when the cooler ambient and wet bulb temperatures enable the coolingtower to handle greater heat loads
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COOLING WATER SYSTEMS XXVII 7 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
bull Where multiple cooling tower systems exist consideration should be given to providing valved cross connections betweentheir respective supply mains and between their respective return mains These would be used only for special operations
such as turnarounds or emergencies The impact on flare sizing should be evaluated if these cross connections areprovided
ENVIRONMENTAL AND SAFETY CONCERNS
Once-through cooling water systems usually require controls to minimize the release of hydrocarbons and free halide residualsto the environment As a minimum cooling water discharges should be monitored for free oil total organic carbon or hydrocarbon vapors to determine if system coolers are leaking Free chlorine residuals in the discharge should also bemonitored as required to meet local regulations and to detect process leaks In most cases once-through cooling water flowsare too large to be handled in the plants wastewater treatment system However once-through water that could becomecontaminated with hydrocarbons is often segregated and treated in a large skim pond or special purpose separator prior todischarge Local environmental regulations and permit requirements will dictate the extent of treatment requiredCooling tower blowdown from recirculated systems is normally bypassed around major wastewater treatment and dischargedwith the treated wastewater An oil-in-water detector should be provided to detect oil in the blowdown from process leaks Thiswill enable stopping the blowdown or diverting it to temporary holdup until the leak is corrected A major rupture in a processheat exchanger tube can also result in the cooling tower being the discharge point for hydrocarbon vaporsBecause of the likely presence of flammable vapors the areas around cooling towers are subject to electrical classification inaccordance with IP 16-1-1 The type and amount of chemicals added to the cooling water should be evaluated as to their effects on the plants wastewater treatment process Biocides corrosion inhibitors and antifoulants may result in significant problems for the wastewater treatment plantOther environmental concerns are
bull Cooling tower noise emissions from fans and from the flow of cooling water over the tower may require suppression if near a local community
bull Cooling tower water mist drift can cause fog and ice formation on nearby roads and promote deterioration of nearbyequipment It may also result in damage to sensitive vegetation in the surrounding area
bull Process gas releases to atmosphere resulting from process gas pickup by cooling water including the venting of toxicgases from exchanger tube ruptures must be considered in locating cooling towers or discharge points from once-throughsystems
bull Spills and overflows of toxic and hazardous chemicals used in treating the cooling water must be contained
SYSTEM SELECTION
The selection of a cooling water system is often determined by the availability of water In regions where water is scarce once-through systems are obviously eliminated from consideration and recirculated systems are chosen A once-through coolingsystem may be the economical choice if the plant is located adjacent to a body of water and consumers are elevated no morethan 50 ft (15 m) above the water level However once-through system costs may be increased significantly by environmentallimitations in many locations These may limit maximum discharge water temperature and therefore will reduce the maximumcooling water ∆T This will increase the required flow and cost of once-through systems In addition regulations may requirewaste treatment of the large volume of effluent thus increasing costsThe choice of a cooling water system must be based on comparisons of initial investment and operating costs Relative initialcosts of a system vary with climatic conditions distance from pump to users complexity of inlet structures and corrosive nature
of the water Salt water or brackish water service requires the use of cement mortar lined pipe and alloys for exchangers andpumps Carbon steel is usually satisfactory for fresh water Special attention should be given to the impact of bottom silt mudand seaweed on intake facilities to once-through or makeup to recirculated systems Cribs flumes large bays screens trashracks or other extensive facilities may be requiredDuring the early stages of a project the designer should review with the planning or design groups the heat level and volume of all process streams which require cooling The optimum method of cooling should then be determined ie heat exchangeandor rejection of heat to air or water A break point should be selected above which air cooling is economical and belowwhich trim water cooling is preferred
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Section Page
XXVII 8 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
COOLING WATER REQUIREMENTS
Cooling water requirements for each process unit and offsite consumer are obtained from the appropriate design groups For existing locations maximum cooling water loads should be based on the design inlet cooling water temperatures given inSection IX-B Design Considerations for All Types of Heat Exchangers For new locations the inlet cooling water temperaturemust be set based on local water temperature data for once-through systems and wet bulb temperature and selected approachfor recirculated systemsThe bulk water temperature at the cooler outlet has been selected as the controllable variable for design and operation of coolers Although the corrosion rate depends on the metal temperature this is difficult to determine in the operation of a coolerBased upon the findings of a Materials Development Section test program maximum design outlet water temperatures havebeen set as indicated below
Fresh Water 130 degF (54 degC)
Treated Fresh Water 130 degF (54 degC)
Brackish Water 125degF (52
degC)
Salt Water 120 degF (49 degC)
When the maximum plant cooling water requirement has been established for new process and utility consumers appropriateLoad Growth and Reserve Capacity Factors (LGF and RCF) should be applied depending on the development stage of theproject Cooling water loads should be updated and system capacity adjusted periodically as design work progresses
LOAD GROWTH AND RESERVE CAPACITY
Utility loads tend to increase as the loads become better defined through successive stages of project definition Thereforeload growth and reserve capacity factors (LGF and RCF) are applied to raw loads as a means of predicting final equipmentsizes and capital costs These load growth factors do not cover basis changes through project development such as project or process unit sizethroughput For revamp or modernization projects these factors are generally applied only to new loads or changes to existing loads on the basis that existing or unchanged loads are well known
Recommended Load Growth Factors are as follows
CONSUMERSPLANNING
(SCREENING THROUGH DBM) DESIGN SPEC
Existing Facilities 0 - 10 0 - 10
New Facilities 20 - 30 5 - 10
Selection of the load growth factor to use should be based on the quality and level of effort associated with development of theraw loadsFinal sizing of the source facility (eg the cooling tower and circulation pumps) should include a reserve capacity factor of 10to cover small post startup changes minor future projects and dynamic conditions not predicted by steady state balances Thesizing basis for distribution facilities should not include the RCF
COOLING TOWER TYPES
The cooling tower is the major piece of equipment specified in recirculating cooling water systems In effect it is the heatexchanger which cools the hot water to a suitable temperature for re-use primarily by evaporation through contact with airThe cooling tower contains fill to break up the water flow into droplets or film on a surface The fill increases the waterair contact area and enhances heat transferCooling towers are classified according to the means by which air is supplied to the tower ie induced or forced mechanicaldraft (fan) vs natural draft and according to the relative movement of the air and water that is counterflow or crossflow (seeFigure 3 ) Most of the towers in the ExxonMobil circuit are mechanical induced-draft type Economics determine whether theyare counterflow or crossflow
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Section Page
COOLING WATER SYSTEMS XXVII 9 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
In crossflow towers the air travels horizontally across the falling water while in the counterflow design the air travels verticallyup through the falling water Comparisons of plot space fan horsepower and future maintenance costs determine which
design will be more economical Factors which influence this selection are cooling water rate range and approach for aparticular location In addition in some locations materials of construction and associated labor costs will affect the finalselectionIn the past crossflow towers were usually preferred over counterflow towers for extremely cold locations where ice formationcould restrict or block air flow through the louvers However this consideration is usually no longer valid due to the increaseduse of PVC film-type tower fill rather than splash-type fill in counterflow towersMechanical draft designs use one or more fans to provide the air needed for cooling Induced-draft units have the fans locatedon top of the tower Air is pulled through the packing and discharged vertically upward at high velocities Forced-draft unitshave the fans located at the base of the tower and push the air through the fill Forced-draft towers in large industrial sizesexperience difficulties with recirculation and in providing a uniform distribution of air As a result they have been supersededby induced-draft designsFill may be splash or film type Splash fill normally consists of horizontal slats in horizontal rows offset to one another to causethe water to break up into droplets as it falls downward through the cooling tower Splash fill is characterized by reduced air pressure losses It is also less conducive to clogging and easier to clean after a spill However it is very sensitive to
inadequate support and must remain horizontal and level It is made of treated wood or plastics such as PVC or polypropyleneFilm fill comes in various designs but they all cause the water to flow in films over the fill surface and provides more coolingcapacity within a given amount of space than splash fill Because of this increased heat transfer efficiency film fill is generallyused in most new cooling towers However because of its smaller passages it is more sensitive to plugging and is difficult toclean if it does become plugged It may not be appropriate for services cooling mainly heavy fuels or waxy lubes where a leakwould cause fill plugging or for services with a high biological fouling potential PVC is currently the most widely used materialfor film fill The selection of fill type is usually proposed by the vendor and reviewed by the COOLING WATER SYSTEMSSPECIALISTSNatural-draft or hyperbolic cooling towers depend on the natural draft created by the difference in the density of the enteringand leaving air for movement Generally hyperbolic cooling towers have been used in Europe and in the utility industry in theUSA for large capacities [250000 gpm (16000 Ls)] short ranges [10 to 15 degF (6 to 8 degC)] and long approaches [10 to 15 degF (6to 8 degC)] They are most effective where low ambient wet bulb temperatures occur with high relative humidities These towersnormally have diameters in excess of 200 ft (60 m) and are greater than 250 ft (75 m) in height Their major advantages are
bull No operating or maintenance cost for fans
bull Reduction in problems associated with fogging drift and recirculation due to their great height and diameterThe primary reasons why hyperbolic towers are not used in ExxonMobil projects include
bull High initial cost
bull Large plot space requirement and high visual impact
bull Complete dependence on atmospheric conditions
bull Lack of flexibility for future expansions
COOLING TOWER BASIN
The primary function of the cooling tower basin is to collect the cooled water leaving the tower and to provide a reservoir for thecooling water pumps Design aspects to be considered are the basins capacity cleanability draining facilities sump andscreen details To obtain the necessary time for corrective action during emergency conditions a minimum storage capacity of 90 minutes based on loss of makeup water flow should be provided in the basin between the basin low level alarm (LLA) andthe minimum pump submergence (MPS) When the basin water level reaches the MPS point loss of recirculation pumpsuction and cooling water flow is imminent Typical values for basin water levels are as follows
bull High Level Alarm (HLA) 1 ft (300 mm) below top of the basin wall
bull Normal Water Level 15 ft (450 mm) below top of the basin wall
bull Low Level Alarm (LLA) 25 ft (760 mm) below top of the basin wall
bull Distance between LLA and MPS 35 ft (1070 mm)
bull Minimum Pump Submergence Depth 45 ft (1370 mm)
bull Clearance of Pump Suction off Basin Floor 15 ft (450 mm) Actual values may vary depending on pump type selected pump suction line diameter and cooling tower cell plot area
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XXVII 10 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
MATERIALS OF CONSTRUCTION
The EMRE Refinery Construction Materials Manual and IP 8-1-1 list acceptable materials for all components in each type of cooling water system (once-through and recirculating) For salt water systems emphasis must be placed on proper materialsselection throughout the system The external salt water environments in the vicinity of the tower itself and the intake stationsmust be taken into consideration Proper siting of the tower is extremely important to avoid salt water corrosion of adjacentunits or the need to use expensive materials to operate in such an environment
WATER TREATMENT FOR ONCE-THROUGH SYSTEMS
Once-through salt and fresh cooling water systems can develop a variety of problems such as fouling due to seaweed or other debris marine microorganisms such as muscles and shell fish and biological growth Corrosion and scaling problems areminimal in fresh water once-through systems because the water is not concentrated by evaporation In salt water systemscorrosion is minimized by proper materials selection and scaling problems generally do not occurProtection against fouling by seaweed or other debris is provided by careful selection of the water intake location and travelingscreens with washing facilities In some cases additional steps may be required These may include an additional secondaryset of traveling screens and possibly battery limit strainers on the cooling water supply lines to individual unitsProtection against fouling by marine microorganisms and biological growths is achieved primarily by injecting chlorinehypochlorite chlorinebromine or other similar chemicals which kill these growths Chlorine is commonly used and is injectedin sufficient quantities to produce a residual of 1 wppm in the effluent for one hour each day (shock chlorination) Continuouschlorination for 2 - 3 days per month at lower levels (03 wppm residual chlorine in the effluent) may also be used in addition toshock chlorinationThe large water volume discharged from once-through systems usually makes chemical treating for corrosion or deposit controltoo costly These problems are handled by proper materials selection and by keeping water velocities through exchanger tubesgreater than 3 fts (09 ms) Occasionally the water analysis of a fresh water source being considered for a once-throughsystem using carbon steel materials will indicate the need for some corrosion or deposit control In this case the cost of thethreshold treatment with the once-through system should be compared to the costs of a recirculated system before making afinal choice
WATER TREATMENT FOR SALT WATER COOLING TOWER SYSTEMS
A total dissolved salts (TDS) concentration limit of 55000 wppm for sea water in cooling tower systems requires high blowdownrates since the makeup sea water typically has from 35000 to 40000 wppm TDS Because of cost restraints the only water treatment normally specified for these systems is for biological control Chlorine is most commonly used either as directchlorine injection or hypochlorite produced in electrochlorinators Care must be taken in selecting the intake location to assurethat the suspended solids do not exceed 200 wppm in the recirculated water If it is not possible to find sea water with sufficientclarity clarification with polyelectrolytes should be evaluated Corrosion is one of the major problems with the use of sea water in cooling towers This requires proper materials selection
WATER TREATMENT FOR FRESH WATER COOLING TOWER SYSTEMS
Cooling tower water treatment is necessary to minimize or eliminate corrosion scale and biological fouling of process heattransfer surfaces caused by minerals and impurities in the water The difficulties caused by these impurities and the means of treatment are shown in Table 1 Water lost by evaporation results in an increase in total dissolved salts in the recirculated water To indicate the degree of concentration of impurities the term cycles of concentration is used
Corrosion is an electrochemical process that deteriorates metals exposed to water in the presence of corrosive agents such asacids oxygen or bacteria A common form of corrosion is pitting Severe corrosion can lead to equipment failure Corrosionis caused by many conditions including process leaks into the cooling water water flow velocity that is too low (causes depositsand fouling lending to corrosion) or too high (causes erosion and corrosion) low pH and high temperatures Corrosion is lesslikely in water with non-acidic pH values (greater than 7) although scaling is more likely in these pH rangesScaling is characterized by the formation of hard dense deposits on material surfaces These deposits impact heat transfer and can become a site for localized under-deposit corrosion Scaling is influenced by many factors including makeup water composition total dissolved solids in the recirculating water water temperature and pH and flow velocity Calcium carbonateis the main scaling constituent in all waters and it is the least soluble It forms from the hardness (calcium Ca ++ ) andbicarbonate (HCO 3
ndash ) alkalinity present in most waterFouling can be caused either by the settlement of suspended matter or by microbiological growth of algae bacterial slime or fungi Fouling will reduce heat transfer cause plugging and create corrosion sites
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COOLING WATER SYSTEMS XXVII 11 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Treatment of the cooling water is broken down into two major categories external and internal External treatmentencompasses any treatment given to the cooling tower makeup water Internal treatment is the addition of chemicals directly to
the recirculated cooling water
EXTERNAL TREATMENT
External treatment to remove impurities in order to reduce the amount of blowdown should always be evaluated as to costeffectiveness The most frequently used techniques for treating makeup water are shown in Figure 11 Details of availabletreatment processes (see Table 1 for methods available) are covered in Section XXVI-A Boiler Feedwater Treatment For cooling tower systems that meet currently required drift limits in refineries and chemical plants uncontrolled cooling water losses (drift and miscellaneous process uses) normally amount to about 14 of the cooling water evaporation loss Under these conditions the maximum cycles of concentration achievable are 8 with no controlled blowdown Some cooling towersare operating at up to 10 cycles of concentration Additional controlled blowdown required to limit the cycles of concentrationper the water quality criteria is covered later
INTERNAL TREATMENT
The makeup water analyses amount of cooling water losses environmental restraints potential chemical hazards materials of construction and film temperatures of heat transfer surfaces determine the type of internal treatment required for cooling tower systems The internal water treatment program selected must address corrosion scale fouling biological growth andchemical attack of cooling tower componentsNumerous chemicals and treatment programs are continuously being developed for addressing the potential problems Theslightly acidic low pH programs of the past which used chromate based chemicals and usually required acid injection arebeing completely phased out because of environmental restrictions on chromate and the higher costs associated with meetingcurrent practices for handling hazardous chemicals such as acid Non-chromate treatments which use inhibitors such asphosphates zinc and organics along with higher pH have been developed to replace the earlier programs Although not aseffective as chromate these inhibitors operate at higher water pH which is a less corrosive environment The disadvantage of the higher pH is the greater tendency to form scale This tendency is controlled by the use of dispersants or scale inhibitorswhich are normally organic polymers or phosphenate A corrosion inhibitor for copper or brass is also normally required if these metals are present in the systemThe following paragraphs describe techniques and chemicals to control alkalinity (pH) corrosion scale sludge and biologicalgrowth The chemicals mentioned are normally purchased as pre-mixes from vendors and dosed in combination Mostcorrosion and scale control program chemicals are two drum formulations due to the incompatibility of some chemicals in theconcentrated state Therefore separate feed systems are generally provided for chemicals for control of corrosion and scale (2drums) and biological growth (if liquid) If a program includes fixed pH then an additional feed system is required to add acidWater treatment programs are site specific and changing The COOLING WATER SYSTEMS SPECIALISTS should beconsulted for recommendations on the internal water treatment programs when an analysis of the available water is known
Alkalinity (pH) Control
Cooling water treatment programs presently available fall into two basic categories those that do not control pH (floating pH or alkaline treatment programs without pH adjustment) and those that control pH within a specified range by adding acid Theprograms that do not control pH are gaining favor because of the additional cost equipment and precautions required tominimize the risk associated with handling acid and because of improvements in chemicals to control depositsHigh pH in the recirculating cooling water is a result of the alkalinity of the fresh water due to bicarbonate salts This combinedwith carbon dioxide results in the water having a pH range of 6 to 85 When this fresh makeup water is mixed with therecirculated cooling water and passed through the cooling tower the air flow strips out the carbon dioxide Bicarbonate ionsthen convert to carbonate ions raising the pH which can cause both calcium carbonate and calcium phosphate salts toprecipitate out of solution and form deposits Another debit in allowing the cooling water pH to increase is increased chlorineconsumption to control biological growthsIf a program using pH control is justified and chosen acid addition is used under control of a pH analyzer to controlrecirculated cooling water alkalinity and pH to the range required by the treatment program
Corrosion Control
To reduce corrosion to an acceptable level chemical corrosion inhibitors which form protective films on heat transfer surfacesare the most effective protection Inhibiting corrosion is accomplished by phosphates organics zinc nitrites and molybdatesalts Unfortunately the use of chromate which is a reliable corrosion inhibitor is prohibited by environmental constraintsNitrite is not practical in an open system due to atmospheric oxygen converting the nitrite to nitrate
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XXVII 12 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Phosphate inhibitors (orthophosphate polyphosphate phosphonate) are the basic treatment for new towers in todaysenvironment In addition to the phosphate chemistry salts of zinc may be added to aid in inhibiting corrosion These ions
combine with the hydroxide ion forming thin protective films Zinc is used to provide a synergistic effect in combination with thephosphate salts for protecting carbon steel surfaces from corrosion Finally all-organic programs are offered by severalmanufacturers The use of these treatments has been made possible by the development of polymer dispersants which givecalcium salts a much higher solubility and reduce the tendency to scaleWhen a cooling water system contains copper alloys azole based and azole substitutes are required as copper corrosioninhibitors There are several effective inhibitors The most common is tolytriazole (TTA) due to its resistance to chlorineThe corrosion inhibitors most frequently used in ExxonMobil recirculating cooling water systems are shown below indescending order of preference provided their use meets all environmental requirements
bull Stabilized Phosphate (OrthoPolyphosphate) - Environmental restrictions on the use of zinc have resulted in ExxonMobilplants in the USA using a blend of orthophosphate and polyphosphate type corrosion inhibitors with good results Thistreatment forms a thin protective calcium phosphate film on heat transfer surfaces through the use of orthophosphateconcentrations as high as 12 - 17 wppm The use of calcium phosphate dispersants and Hydroxethylidene Diphosphonate(HEDP) stabilizes the treatment and prevents the formation of calcium phosphate scale Concentration of the dispersantsdepends upon the calcium concentration temperature and pH Stabilized Phosphate treatment programs can operate in apH range of 68 to 78 Alternatively an alkaline (floating pH) stabilized phosphate treatment program can be used Thisprogram uses little or no acid addition and is gaining in popularity although not well established yet It operates with pHvalues of 8 - 9 and uses higher quantities of dispersants and lower levels of phosphate A corrosion inhibitor for copper and brass is normally included for all stabilized phosphate programs Several vendors offer these programs Twoseparate chemical feeders are generally required
bull Zinc Alkaline Phosphate - Heritage Exxon plants in Europe where the use of zinc is not restricted have obtained goodcontrol of corrosion and scaling using zinc-polyphosphate treatment programs Low (65 - 72) and high (73 - 80) pHprograms have been used The high pH (zinc alkaline phosphate) programs are the lowest cost and are recommended
As with the Stabilized Phosphate treatment dispersants such as HEDP and polyacrylates are required to prevent theformation of zinc or calcium phosphate scale on heat transfer surfaces A corrosion inhibitor for copper or brass isnormally used with this program which is offered by several vendors
bull Organic Salts - All-organic (polymer) programs are offered by several manufacturers and have been used successfullywithin h Exxon In general these treatments are based on a combination of a phosphonate to control corrosion polymers(dispersants) to prevent scale deposition and a corrosion inhibitor for copper or brass This type of program is mosteffective in conditions of high hardness high alkalinity (M alkalinity gt300 wppm as CaCO 3) and high pH values (gt 85)However some manufacturers have reported success in softer water with alternative polymer component formulations Ingeneral an inorganic corrosion control program is preferred to the all-organic programs which are sensitive to alkalinityand more costly
Detailed control limits for effective use of each of the above inhibitors and materials selection are covered in Report NoEE34E86 Environmentally Acceptable Cooling Water Treatments Report No EE102E78 Cooling Tower Water Treatment Guidelines and in the EMRE Refinery Construction Materials Manual
Scale and Sludge Control
Scale and sludge deposits in cooling water systems cause loss of heat transfer and provide the conditions for pitting corrosionScale consists of inorganic salts which have precipitated from the cooling water forming a coating on heat transfer surfacesSludge consists of organic and inorganic suspended matter that settles in quiescent areas and in heat exchangers The key toprevention of both scale and sludge formation in a cooling system is keeping water velocities greater than 3 fts (09 ms) andthe use of chemical additives in combination with a blowdown rate that keeps the impurity concentration below the level whichcauses deposits (See Table 2 for limits on impurities) Scale control has become increasingly important in higher alkalinityldquofloating pHrdquo water treatment programsSalts most commonly found in scale deposits are calcium carbonate calcium sulfate tricalcium phosphate zinc phosphatemagnesium silicate and silica The control method for preventing scale from each of these salts is discussed below
bull Calcium Carbonate - Where justified using pHalkalinity control in the range of 70 to 75 will normally inhibit the formationof calcium carbonate scale if anti-scalent chemicals (phosphonates acrylates copolymers) are part of the chemicaltreatment (See Report No EE102E78 Cooling Tower Water Treatment Guidelines ) Higher pH water treatmentprograms require greater concentrations of these chemicals along with dispersants and flocculants to inhibit the formationof scale and precipitation of other salts
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COOLING WATER SYSTEMS XXVII 13 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
bull Calcium Sulfate - When heat transfer skin temperatures are below 250 degF (120 degC) and the cooling water bulktemperatures are under 130 degF (54 degC) gypsum scale (CaSO 42H 2O) will form if the product of the calcium and sulfate ionsconcentrations each expressed as wppm CaCO 3 equivalent exceeds 500000 Gypsum scale is prevented bymaintaining bulk water temperature below the indicated limits and controlling calcium and sulfate ions through blowdownor through the use of newer dispersants on the market
bull Tri-Calcium and Zinc Phosphate - Almost all chemical additives for corrosion control contain some type of phosphatesalt These have a tendency to form the orthophosphate ion depending on time pH and temperature Cooling watersusing these corrosion inhibitors may have an orthophosphate concentration in excess of 5 wppm as PO 4
= To preventcalcium phosphate scale Table 2 shows a limit of 1000 wppm of calcium as CaCO 3 equivalent when using a phosphateinhibitor in combination with a chemical dispersant (See Report No EE102E78 Cooling Tower Water Treatment Guidelines for solubility of calcium phosphate) These inhibitors and dispersants generally fall into one of the followingcategories
ndash Sequestrants antiprecipitants and inhibitors such as hydroyethylidene diphosphonate (HEDP) phosphate esters andaminomethylene phosphonate (AMP) function by defyingdistorting the crystal structure of the scale formations toprevent nucleation and growth of the scale
ndash Dispersants based on either anionic or nonionic polymers such as polyacrylamide polyacrylate or polymaleic acidfunction to stabilize colloidal and scale-forming particles in suspension Anionic polymers prevent agglomeration of suspended particles by either charge repulsion or crystal distortion while nonionic polymers reduce the surfacetension of particles
bull Silica Deposits - Quantitative indices for determining limits for silica in combination with other ions are not available due tothe formation of complex polymers Instead the rule of thumb limit of keeping reactive silica concentrations in the coolingwater below 150 wppm as SiO 2 has been successful
Possible alternatives for controlling sludge are
bull Chemical Dispersants - Water treating chemical suppliers now offer many types of dispersants These are mostlypolyelectrolytes Claims for removal of slime silt and mud from cooling tower basins and exchangers are madeHowever satisfactory results using these chemicals as a coagulant aid require very accurate control of the amount fed tothe water and stable operation of the water system Also suspended solids should be kept below 200 wppm for particleslarger than 045 microns Dispersants are used in virtually all fresh water cooling tower systems despite their high cost
bull Sidestream Filtration - The use of a sand pressure filter is the most reliable method for sludge removal Operating andmaintenance costs are minimal for this equipment
bull Higher Blowdown Rates - This may be an economical alternative depending on the cost of makeup water Additionalchemical costs must be included in any evaluation of this method
bull Makeup Clarification - This alternative may require additional raw water treatment facilities to handle seasonal variationsin turbidity andor suspended solids This method is applicable only where suspended solids come from makeup water
Biological Control
Operating conditions in recirculated cooling water are ideal for the growth of biological matter Water temperatures from 70 to120 degF (20 to 50 degC) favorable water pH continuous supply of nutrients including carbon nitrogen and phosphorus inorganicsalts and sunlight all lend themselves to environmental conditions encouraging microbiological growth and plant lifeIf biological growths get out of control and form large sticky agglomerations some of the following operating problems willresult
ndash Fouling of heat transfer surfaces by bacterial slimes resulting in flow restrictions and high process temperatures ndash Reduced cooling tower efficiency resulting from algae fungi and bacterial slime growths in the water distribution basin
and fill area of the cooling tower ndash Excessive plugging of screens and filters ndash Corrosion ndash Wood destruction by fungi ndash Clogging of water distribution nozzles ndash Excessive sludge accumulation in the cooling tower basin
An important part of the water treatment program is to control biological activity by feeding a biocide chemical Oxidizingbiocides such as chlorine sodium hypochlorite chlorine dioxide or bromine-associated compounds are most commonly usedwith good results Plate counts should be maintained below 50000 countsml (bacterial)
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XXVII 14 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Chlorine is the most effective biocide for cooling water because of its low cost extensive period of successful use ease of application and minimum environmental effects if applied under controlled conditions Either a liquid feed (sodium
hypochlorite) or a gas feed (100 chlorine) is added to the cooling tower basin A sufficient dosage is that which obtains a 01to 03 wppm continuous free residual in the hot return water to the cooling tower
Advantages to continuous chlorination over shock treatment based on ExxonMobil experience are ndash Minimizes chlorine degradation of polymers ndash Better reduction in total aerobic bacteria plate count resulting in cleaner heat transfer surfaces ndash Does not cause significant pH depressions ndash Better control results with less chlorine consumption
Details concerning chlorine addition as well as the other alternative biocides are given in Report No EE102E78 Cooling Tower Water Treatment Guidelines and in EMRE Engineering Water and Wastewater Design Guide TMEE 080 DG 11-3-1Gas Chlorinators for Water Treating Service and DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) Gaseous chlorine systems have become more costly in recent years because of the additional precautions equipment andprocedures required to minimize the risk associated with handling this gas Alternatives to gaseous chlorine are available butrequire more attention and have a higher operating cost Sodium hypochlorite (either purchased or generated) and sodiumbromide stabilized solid halide (chlorinebromine) products chlorine dioxide and ozone are all alternatives which may haveapplications Contact the COOLING WATER SYSTEMS SPECIALISTS for guidance in the use of these alternative treatmentschemes
BLOWDOWN
Blowdown control is required to maintain the proper cycles of concentration in the system The most reliable and simplest typeof control for blowdown is specific conductance measurement of the cooling water Electrical signals from the specificconductance measurements of some impurity in the cooling water are used to actuate an automatic blowdown valveImpurities normally used for blowdown control are dissolved solids chloride calcium magnesium and silica
CONTROLS AND MONITORS
Any chemical treatment used to control problems in a cooling water system must be carefully monitored to be completelyeffective
The installation of automatic controls should be considered for recirculated cooling water systems in all plants Designs shouldinclude pH conductivity corrosivity inhibitor and biocide control Automatic controllers are available which continuouslymonitor the condition of the water and automatically make chemical additions to keep the water within compositional limitsTest exchangers and corrosion coupons are also useful in monitoring the effectiveness of the water treatment program
An oil-in-water monitor should be considered to provide early detection of oil leaks into the cooling water This will enablecorrective action to be taken before extensive contamination of the cooling water circuit occurs This is especially important incooling towers with high efficiency cellular fill which can be difficult to clean if significantly fouledManual control systems may be used in locations that have adequate technical personnel to perform routine water analysesand to modify chemical treatment necessitated by sudden variations in water composition process contamination etc
WASTEWATER REUSE POSSIBILITIES
Recirculating cooling water systems normally have the highest plant raw water demand A potential alternative is to use treatedwastewater to supply part or all of the makeup water requirement The treatment required must be determined but will typicallyinclude primary oil removal (eg API Separator) secondary oil and suspended solids removal (eg filtration dissolved or induced air flotation) and soluble carbon removal utilizing biological treatmentThe reuse of treated wastewater as makeup currently exists at only a few locations If this alternative is being considered refer to Report No EE58E85 Reuse of Wastewater as Cooling Tower Makeup and Section XIX-B Water Reuse
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COOLING WATER SYSTEMS XXVII 15 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS
STEP 1 - Select a location for the intake station and pumps
Factors which influence this selection are as followsbull Depth of water and tidal variations
bull Relative location of intake and outfall direction of current
bull Length and size of intake suction piping vs pump discharge piping
STEP 2 - Determine cooling water requirements
Determine the design cold water inlet temperature based on site data covering seasonal water temperature as a function of inlet location and depth (for existing locations refer to Table 2 in Section IX-B )Tabulate the cooling water requirements for each process unit and offsite consumer Apply the proper LGFs to all coolingloads depending on the stage of the project and whether the new units are exact duplicates of existing unitsIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The designer should investigate methods to increase effluent water temperature to the maximumrecommended limit for the particular type of water used One common method is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 3 - Determine the total required capacity for water intake facilities
Process Coolers - The total requirements determined in Step 2 including LGFsFirewater - This is often supplied from the intake bay of a once-through systemFuture Expansions - These should be considered in sizing intake facilitiesTotal Required Capacity - The sum of the above three items
STEP 4 - Provide water intake facilities
Intake Crib - The intake crib with its support structure is located at the extreme end of the water intake line (See Figure 4 )The intake is usually located 200 to 300 ft (60 to 90 m) offshore and at a sufficient depth below the lowest water level to
preclude vortex formation and subsequent pump cavitation during storm conditions Additionally the crib should have 3 to 5 ft(1 to 15 m) clearance above the sea floor to minimize sand and silt pickup A concrete velocity cap should be specified to cover the inlet crib This will minimize the incidence of fish and other foreignmatter entering the system A horizontal inlet flow pattern should be designed with a maximum entrance velocity of 1 fts(03 ms) Experience shows that vertical velocity flow confuses fish and they are drawn into the intake lineThe intake crib support structure protects the crib from adverse effects of storms and tidal motions as well as protecting it fromimpact of floating and submerged objects A self-cleaning screen can be specified to prevent entrance of large objects into thesystem at the cribIntake Line (Figure 5 ) - An intake line is required if the water depth along the shore does not permit water to flow directly intothe intake bay The intake line is usually buried in the sea floor and covered with a minimum required overburden Consultwith the COOLING WATER SYSTEMS SPECIALISTS for required coverage at a specific siteChlorine or sodium hypochlorite is injected at the intake crib to prevent marine growth in the lineNote that an intake crib and line will not be required if water of sufficient depth is available at the supply pump stationIntake Bay (Figure 6 ) - The intake bay is an enclosure which houses the pumps and screens Depending on a particular design a series of screens is used to ensure the supply of clean water Trash racks comprise the first element of the filter system These remove only very coarse material from the water such as bulk seaweed They can be supplied with atraversing raking mechanismLift screens provide secondary cleanup These screens can be lifted out of the intake bay for periodic cleaning Double liftscreens can be used if it is deemed necessary to have one in service at all timesFinal water filtering is usually done by traveling screens Details of the screen dimensions and the mounting are usuallydetermined during detailed engineering Screen net open area is typically approximately 50 of total screen area Thescreens should be sized for a normal water velocity of 12 fts (04 ms) through the open screen area Maximum velocityshould be limited to 24 fts (08 ms) with 50 of the screen area plugged This is the upper velocity limit for efficientscreening
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ExxonMobil Proprietary
Section Page
XXVII 16 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
If high suspended solids greater than 200 mesh (eg sand) are expected to be a continuous problem ejectors may be installedupstream and downstream of the traveling screen to remove accumulated sand and solids from the intake bay in these areas
Traveling screens and ejectors are supplied with wash water from the raw water pumps or from separate wash water pumpsOnoff control using timers should allow optimum scheduling of the ejectors depending on the time of the year
STEP 5 - Specify water intake facilities
Water intake facilities should be designed in accordance with the following guidelines from the Hydraulic Institute Standards The water intake should be designed to provide an evenly distributed flow of water to the pump suction A properly designedintake reduces the possibility of vortex formation and results in better pump performance with less noise Geometry of theintake and the manner in which water is introduced into the intake are important parameters in providing an even distribution of water flow Complete analysis of intake structures is best accomplished by scale model testsThe pump manufacturer will generally render advice on specific problems while the intake design is still preliminary He shouldbe provided with the necessary intake layout drawings showing the physical limitations of the siteFigures 7A and 7B have been developed for single- and multiple-pump arrangements to show suggestions for basic sumpdimensions They are for pumps normally operating in the capacity range of approximately 3000 to 300000 gpm (200 to20000 Ls) Since these values are composite averages from a great many pump types and cover the entire range of specificspeeds they must not be thought of as absolute values but rather as basic guides subject to some possible variations
All the dimensions in Figures 7A and 7B are based on the rated capacity of the pump at the design head Any increase incapacity above these values should be momentary or very limited in time If operation at an increased capacity is to beundertaken for considerable periods of time the maximum capacity should be used for the design value in obtaining sumpdimensionsDimension C is an average value based on an analysis of many pumps Its final value should be specified by the pumpmanufacturerDimension B is a suggested maximum dimension which may be less depending on actual suction bell or bowl diameters in useby the pump manufacturer The edge of the bell should be close to the back wall of the sump When the position of the backwall is determined by the driving equipment or the discharge piping Dimension B may become excessive and a ldquofalserdquo backwall should be installedDimension S is a minimum value for the sump width for a single pump installation This dimension can be increased but if it isto be made smaller the manufacturer should be consulted or a sump model test should be run to determine its adequacy
Motor size may dictate minimum pump spacing be especially aware of this when specifying pumps with unusually high pumpheads eg greater than 80 psig (550 kPa)Dimension H is a minimum value based on the ldquonormal low water levelrdquo at the pump suction bell taking into considerationfriction losses through the inlet screen and approach channel This dimension can be considerably less momentarily or infrequently without excessive damage to the pump It should be remembered however that this does not representldquosubmergencerdquo Submergence is normally quoted as dimension ldquoHrdquo minus ldquoCrdquo This represents the physical height of water level above the bottom of the suction inletThe actual submergence of the pump is something less than this since the impeller eye is some distance above the bottom of the suction bell possibly as much as 3 to 4 ft (1 to 13 m) For the purposes of sump design in connection with this chart it isunderstood that the pump has been selected in accordance with specific speed charts The submergence referred to hereinhas to do only with vortexing and eddy formationsDimensions Y and A are recommended minimum values These dimensions can be as large as desired but should be limitedto the restrictions indicated on the curve If the design does not include a screen Dimension A should be considerably longerThe screen or gate widths should not be substantially less than S and heights should not be less than H If the main stream
velocity is more than 2 fts (06 ms) it may be necessary to construct straightening vanes in the approach channel increaseDimension A conduct a sump model test of the installation or work out some combination of these factorsDimension S becomes the width of an individual pump cell or the center-to-center distance of two pumps if no division walls areusedOn multiple pump installations the recommended dimensions in Figures 7A and 7B also apply as noted above and thefollowing additional determinants should be consideredFigure 8A - Low velocity and straight-line flow to all units simultaneously is the first recommended style of pit Velocities in thepump area should be approximately 1 fts (03 ms) Some pumps with velocities of 2 fts (06 ms) and higher have given goodresults This is particularly true where the design resulted from a model study Not recommended would be an abrupt changein size of inlet pipe to sump or inlet from one side introducing eddying
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COOLING WATER SYSTEMS XXVII 17 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Figure 8B - A number of pumps in the same sump will operate best without separating walls unless all pumps are always inoperation at the same time in which case the use of separating walls may be beneficial If walls must be used for structural
purposes and pumps will operate intermittently one should leave flow space behind each wall from the pit floor up to at leastthe minimum water level and the wall should not extend upstream beyond the rim of the suction bell If walls are used increaseDimension ldquoSrdquo by the thickness of the wall for correct centerline spacing Round off the ends of the walls Not recommended isthe placement of a number of pumps around the edge of a sump with or without dividing wallsFigure 8C - Abrupt changes in size from inlet pipe or channel to pump bay are not desirable A relatively small pipe emptyinginto a large pump pit should connect to the pit with a gradually increasing taper section The angle should be as large aspossible preferably not less than 45 degrees With this arrangement pit velocities much less than one foot per second aredesirable Especially not recommended is a small pipe directly connected to a large pit with pumps close to the inlet Flow willhave excessive change of direction to get to most of the pumps Centering pumps in the pit leave large ldquovortex areasrdquo behindthe pumps which will cause operational troubleFigure 8D - If the pit velocity can be kept low enough [below 1 fts (03 ms)] an abrupt change from inlet pipe to pit can beaccommodated if the length equals or exceeds the values shown It is assumed that as ratio WP increases the inlet velocityat ldquoPrdquo will increase up to an allowed maximum of 8 fts (24 ms) at WP = 10 Pumps ldquoin linerdquo are not recommended unless theratio of pit to pump size is quite large and pumps are separated longitudinally by a generous margin A pit can generally be
constructed at much less cost by use of a recommended designFigure 8E - It is sometimes desirable to install pumps in tunnels or pipelines A drop pipe or false well to house the pump witha vaned inlet well facing upstream will be satisfactory in flows up to 8 fts (24 ms) Without the inlet well the pump section bellshould be positioned at least two pipe diameters vertically above the top of the tunnel not hung into the tunnel flow especiallywith tunnel velocities 2 fts (06 ms) or more There should be no signs of air along the top of the tunnel It may be necessaryto lower the scoop or to insist on a minimum water level in the vertical wellNote The foregoing statements apply to sumps for clear liquid
STEP 6 - Provide chemical treatment
Provide chemical treatment facilities for the once-through cooling water system Normally the only chemical treatment requiredin a once-through cooling system is for biological control Facilities for adding chlorine to once-through cooling water are thesame as those described for the recirculated cooling water system The design capacity of the chlorinator should be based onachieving a chlorine residual of 1 wppm in the effluent The chlorine required by the organics and other reducing agents in thewater must be satisfied before any residual chlorine will appear in the effluent The design capacity of the chlorinator should be
based on the total water chlorine demand plus 1 wppm in the effluent In the absence of water data the chlorination systemdesign capacity should be based on 10 wppm minimum instantaneous dosage for maximum cooling water flow For very largeonce-through cooling water systems it will be impractical to design for shock rates at this level In this case a continuousdosage of 1 - 2 wppm should be used (Refer to Step 5 of the DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS )
STEP 7 - Specify pumps and distribution system
Specify other major facilities in the once-through cooling water system The recommended materials for pumps valves andpiping are shown in the EMRErsquos Refinery Construction Materials Manual
Pumps1 Type - These pumps are usually vertical centrifugal pumps However horizontal centrifugal pumps have been used An
economic study andor consideration of existing facilities will determine which type should be specified In the event thathorizontal centrifugal pumps are specified facilities for pump priming will likely be required For large pumps in salt water service there is a very limited selection of pumps because of the special materials required Consult with the DISCIPLINESPECIALISTS early in the design effort to determine what specific pumps are currently available
2 Total Required Capacity - Firm capacity (excluding spares) of the supply pumps should equal the maximum cooling water requirements including LGF and RCF
3 NumberIndividual Pump Capacity - Normally one spare pump is provided for up to six operating pumps two spares for up to twelve operating pumps etc The most common arrangements for water supply pumps are
bull Two 100 capacity pumps
bull Three 50 capacity pumps
bull Four 33-13 capacity pumps
bull Five 25 capacity pumps
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Section Page
XXVII 18 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Maximum commercially available pump capacity ranges up to 300000 gpm (20000 Ls) at discharge pressures up to 100psi (700 kPa) Maximum capacity will be less for higher discharge pressures Other factors which may influence pump
selection arebull Selecting capacity to match existing pumps
bull Selecting capacity for future requirements
bull Selecting capacity corresponding to manufacturers available standardVendor information should be obtained on the availability size and cost of the pumps for several configurations Acomparison should be made of the investments and operating costs as well as other advantages and disadvantages toselect a pump configuration
4 Drivers - These pumps are usually motor-driven Because of the remote location of the intake facilities the steamdistribution costs are normally too high to justify the use of a steam turbine driver However use of a secondary selectivesubstation provides a reliable source of power to the pump drivers
5 Pump ∆∆∆∆P - Pump ∆P is based on supplying the maximum cooling water requirement at the battery limit design pressureestablished for the system The ∆P should include the total circulating system pressure drop plus any elevation differencebetween the low water level at the intake and the discharge point of contact with air
6 Pump Instrumentation - The remote location of the intake facilities makes remote control and monitoring of the water supply operation desirable Pump controls located in the control house provide for minimum manning Local controlbackup should be provided The Owners operating and manning philosophy determine the basis for instrumentation andcontrolThe spare pump is controlled to automatically start on low pressure in the supply header Concurrently alarms should beactuated in the control room to indicate the low-pressure situation and spare pump operation
Distribution Network1 Configuration - The piping network is a once-through system with a return line from each consumer or consumer group to
the cooling water effluent line2 Layout - The routing of the piping network should connect all cooling water consumers in the most economical manner
permitted by the plant layout Careful consideration should be given to the turnaround philosophy of the plant Thenetwork layout should preclude the necessity of having above-ground water lines that are in service running through anarea that is being turned around Underground piping or an extra lateral may be required to accomplish this objective
3 Valves and Blinds - Battery limit valves should be provided in the network to isolate individual process units from thesystem Valves should be provided to permit onstream removal and maintenance of spared equipment
4 Basket Strainers - Consideration should be given to the installation of basket strainers in the laterals serving each of theconsumer loops These provide added insurance against entrance of foreign material into the cooling equipment The useof intake screens with 025 in (63 mm) openings may eliminate the need for basket strainers in the laterals
Line Sizing1 Design Basis - The network is sized so that water is delivered to the most distant consumer at a pressure equal to the
required process design (battery limit) pressure2 Flow Rate - Each segment of the distribution network is sized for the simultaneous maximum flow in that segment for any
operating condition The main supply and discharge headers are sized for the simultaneous maximum flow of all segments(not necessarily the sum of the simultaneous maximum flows of the individual segments)
3 Pressure Drop
Line Sizing Basis - The distribution system should be sized according to the guidelines established in Section XIV The use of an ldquoold piperdquo friction factor of C = 90 - 100 (Hazen-Williams factors) in the main lines is recommended to assuregood operation in later years when the system experiences fouling For large diameter lines [greater than 48 in (1200mm)] maximum water velocity should be limited to 10 fts (3 ms) based on civilmechanical considerations (supports andanchoring required to handle potential pressure surges) For salt water systems utilizing cement-lined pipe consider thereduced inner diameter in the friction loss calculations A study may be necessary to determine the economic pump ∆P vsline diameters
4 Equivalent Length Factors - Equivalent length factors are used in pressure drop calculations for sizing the distributionsystem The piping length scaled from the plot plan is multiplied by an equivalent length factor to compensate for pipingelbows expansion loops and other piping variations
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Section Page
COOLING WATER SYSTEMS XXVII 19 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
The equivalent length factors are selected based on the expected variation from a straight piping run Typical equivalentlength factors suitable for planning purposes for different piping applications are as follows
TYPE OF PIPING EQUIVALENT LENGTH FACTORS
Long piping runs [gt 1000 ft (300 m)] 12 to 14
Onsite distribution piping 15 to 20
For design purposes the actual piping layout should be evaluated to ensure that the equivalent length factors areapplicable to the specific situation Particular attention is required for revamp work and saltwater systems utilizing cement-lined pipe
STEP 8 - Prepare a system flow plan similar to the one shown in Figure 1
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS
STEP 1 - Determine cooling water requirements
Tabulate the cooling water requirements and cooling water discharge temperature for each process unit and offsite consumer Apply the proper LGFs to all cooling loads depending on the stage of the project and whether the new units are exactduplicates of existing units Determine the total cooling water flow required including LGFs and the bulk temperature (T 1) of thecombined cooling water flow from all coolers to the cooling towerIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The onsite and offsite designers should investigate methods to increase effluent water temperature to themaximum recommended limit for the particular type of water used This will result in a lower cost cooling tower for a given heatrejection rate One common method of maximizing return water temperature is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 2 - Select cooling tower design parameters
Select the proper design conditions of ambient wet bulb temperature approach temperature range and flow rate for specifyingthe cooling tower Based on these conditions the cooling tower vendor will size the tower and provide the tower characteristicand performance curves which serve as the basis for the performance guarantee Required performance test and evaluationprocedures are described in IP 8-1-1 Economic studies may be required to select the ambient wet bulb temperature andapproach temperature for new locations or for expansion projects with special cooling requirements
STEP 3 - Prepare cooling tower specification sheet
Include the following information in the heat exchange section of the design specificationTower Flow Capacity - This was determined in Step 1Type of Tower - The tower usually specified is an induced draft multicell type suitable for fresh salt or brackish water depending on the water available Additional tower characteristics are usually developed and proposed by the tower vendor (eg crossflow versus counterflow type of tower fill etc) The vendor should be made aware of conditions such as thelikelihood of heavy oil leakage into the cooling water or high fouling potential which may influence the tower type or fill
selectionNumber of Cells - The minimum number of cells required (normally not less than 2) may be specified The maximum flow per cell is a function of the cooling tower design conditions selected in Step 2 Typical values range up to 10000 gpm (600 Ls) for crossflow designs and 8000 gpm (500 Ls) for counterflow designs Normally spare cells are not provided Maintenance iscarried out during cooler periods or load is shed
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XXVII 20 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Ambient Wet Bulb Temperature1 The ambient wet bulb temperature selected for specifying the cooling tower design is generally that temperature which is
equaled or exceeded 5 of the time during the four hottest months (5 of the hourly readings taken during the four hottestmonths will exceed this value) Specifying the 5 temperature should give satisfactory results since the time that the wetbulb temperature is exceeded is for short duration and the system inertia is usually sufficient to avoid problems The tower specification should make it clear that the wet bulb temperature specified is an ambient value as opposed to the enteringvalue which may be higher because of recirculation The tower manufacturer should adjust the ambient value upward totake into account his estimates of recirculation Currently cooling tower test codes provide procedures for measuringperformance in the case of either entering or ambient wet-bulb temperature specifications An increase of 2 degF (11 degC) isgenerally added to ambient wet bulb temperature to estimate entering wet bulb temperature for large cooling towers
2 If plant production will peak during the four summer months when the 5 ambient wet bulb temperature may be exceededconsideration should be given to specifying a higher design ambient wet bulb temperature (one that will be exceeded 212 of the time for example) However the incremental cost of the resulting closer approach and larger cooling tower must be justified by the value of the incremental products made
On-Tower Temperature (T1) - This temperature cannot be accurately determined until the design of all process coolers iscompleted and the volume of cooling water and outlet temperature are known for each (see Step 1) In the planning stage or early design stage of a project this temperature must be estimated and is usually set equal to the maximum design cooler outlet temperature shown under COOLING WATER REQUIREMENTS in the BASIC DESIGN CONSIDERATIONS portion of this sectionApproach Temperature - The approach temperature has the most pronounced effect on cooling tower size and cost For example reducing the approach from 10 to 5 degF (55 to 3 degC) can increase the cooling tower investment by 50 percent and thefan operating cost by 65 percent Thus various approach temperatures should be evaluated to determine the minimum cost of the total system including the process exchanger area Approaches used in the past designs have been from 5 to 12 degF (3 to7degC) Note that it is not customary in the cooling tower industry to guarantee approaches of less than 5 degF to (3 degC) to coolingtower design inlet wet bulb temperature A typical approach for a new grass roots cooling tower is 10 degF (6 degC)Off-Tower Temperature (T2) - This is the sum of the approach temperature and the ambient air wet bulb temperature For existing locations Table 2 in Section IX-B may be used
Range and Flow Rate1 The range for a given heat load is related to the circulating water rate as follows
Q = M C p ∆Tcong (500) (gpm) (range) Eq (1)
where Q = Heat load on the tower BtuhM = Water circulating rate lbhCp = Specific heat capacity of water at average temperature Btulb degF∆T = (T 1 ndash T 2) degF = (water cooling range)gpm = Circulation rate in US gallons per minute
For calculation in Metric units
Q = M C p ∆Tcong (1162 x 10 -3) (Ls) (range) Eq (1M)
where Q = Heat load on the tower kJh
M = Water circulating rate kghCp = Specific heat capacity of water at average temperature kJkg degC∆T = (T 1 ndash T 2) degC = (water cooling range)Ls = Circulation rate in liters per second
2 Generally lower tower investment fan horsepower and pump horsepower result from maximizing the range andminimizing the flow rate However based on fouling considerations of process coolers the maximum allowable hot water temperatures must be kept below recommended limits Also while increasing the range minimizes cooling tower investment and operating cost the cost of exchanger surface requirements is adversely affected by the LMTD and acompromise must be made The range is bounded by the maximum allowable hot water temperature and the Off-Tower temperature (T 2) For established locations refer to Section IX-B for design cold water temperatures to assist in selectingthe approach and range
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COOLING WATER SYSTEMS XXVII 21 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Makeup Water Requirements - Calculate the various losses in the cooling water system to determine makeup requirementsThese losses include
1 Evaporation Loss (E) - Allow 1 of the circulation rate for each 10 degF (55 degC) temperature drop through the tower Notethat the actual evaporation loss will be somewhat less depending on ambient air temperature
( )sL555
TT)gpm(
1000TT
E 2121
minus=
minus= Eq (2)
2 Drift Loss (D) - IP 8-1-1 limits the maximum allowable drift loss to 0005 of the design water circulation rate for freshbrackish or salt water
( )sL1000050
)gpm(1000050
D
=
= Eq (3)
This low drift level contributes little to solids removal from the tower with the spray This allows higher cycles of concentration to be achieved in cooling towers which use clarified softened water for makeup and thus have low requiredblowdown rates Higher drift loss percentages in older or poorly maintained towers will limit the maximum cycles of
concentration achievable3 Miscellaneous Losses (O) - Miscellaneous losses are estimated to be one-seventh of the difference between evaporation
loss and drift loss
minus=
7DE
O Eq (4)
4 Blowdown Rate (B) ndash Guidelines for maximum concentrations of impurities allowed in recirculated fresh cooling water areshown in Table 2 For salt water the blowdown is based on keeping the total dissolved salts below 55000 wppm Obtainthe chemical analysis of the makeup water and divide each individual component concentration into its counterpart inTable 2 The result is the maximum allowable cycles of concentration for that constituent (a worksheet is provided inTable 4 ) The impurity with the lowest factor will be the control and will determine the required blowdown rate
)OD(1Cycles
EB +minus
minus= Eq (5)
The makeup water requirement is the sum of all losses shown above
MakeUp = E + D + O + B Eq (6)
STEP 4 - Specify the controls necessary for proper cooling tower operation
Air Side - Cooling towers are provided with motor-driven manually adjusted pitch multiple-bladed propeller fans Figures9A amp B may be used to estimate the normal operating horsepower requirements Since the fan motor and gear reducer arelocated at the top of the tower [generally 40 to 50 ft (12 to 15 m) above grade] the need for equipment to handle thesecomponents should be discussed with the affiliateControl of the air flow through the tower to maintain reasonably constant cold water temperatures can be accomplished byturning off individual fans in multiple-cell installations adjusting the blade pitch or using variable speed motors To turn off individual fans remote at grade start-stop buttons for the motors are usually specified
Whenever the ambient air temperature is 32 degF (0 degC) or below icing of the tower louvers and structural members in contact withthe incoming air will occur If the heat load on the tower is relatively free of seasonal variations two-speed fans should provideadequate ice control down to approximately 0 degF (-18 degC) ambient temperatures When sustained temperatures of 0 degF (-18 degC)and below occur two-speed fans capable of two-speed reverse operation with appropriate time delays for change of directionand speed should be specified to periodically eliminate ice which will build up on the louvers Other methods of winterizationare available but in general are more expensive and require greater operator attention In very cold climates steam heatingshould be provided to prevent freezing in the cold water basin during shutdown periodsWater Side - The water circulation rate is maintained by operating one or more pumps of sufficient capacity to meet the systemdemand Flow-measuring facilities local temperature indicators and a high-temperature alarm are provided on the coolingtower water supply and return headers Hand-operated flow control valves are required for distributing the water equally to theindividual cells On counterflow towers provisions for measuring the flow to each cell (pitot tubes) are required They are notrequired on crossflow towers since the open distribution basin provides visual aid in balancing flows Shutoff valves should beprovided for servicing the flow control valves without shutting down the entire tower
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XXVII 22 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
STEP 5 - Provide chemical treatment facilities for the fresh water system
Alkalinity (pH) Control (if used)1 In cases where a controlled pH corrosionscale control program is being used the alkalinity (pH) of the recirculated cooling
water should be controlled in accordance with the program requirements Normally acid addition is required to accomplishthis This is due to free carbon dioxide being stripped from the cooling water in the tower to approximately 5 ppm For example if the alkalinity has to be controlled to between 25 and 50 ppm for a pH of 70 to 75 the volume of 98 sulfuricacid (66 degBeacute) required to treat the makeup water is calculated using the following equation
[Note 93 sulfuric acid should be used if ambient temperatures will be less than 35 degF (2 degC)]
GallonsHour 66 degBeacute H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate gpm) (326 x 10 -5)
For calculation in Metric limits
kgh of 100 (66 degBeacute) H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate Ls) (793 x 10 -3)
2 The metering pump should be designed for a maximum feed rate of two to three times the normal feed rate A glass woolfilter is required on the pump suction piping to minimize plugging Normally two - 100 capacity pumps are provided
Positive displacement metering pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumps with flow control areused for higher rates
3 Hydrochloric acid may also be used for alkalinity control The gallons per hour of 20 degBeacute hydrochloric acid required is36 times the amount of 66 degBeacute sulfuric acid In Metric units the weight of 32 (20 degBeacute) hydrochloric acid required is227 times the weight of 100 (66 degBeacute) sulfuric acid
4 Specifications for chemical feeders may be found in EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased)
5 A pH analyzer with high and low alarm is provided to control the amount of acid feed The acid should be added to thecooling tower makeup water (Figure 2 ) The lag time between the point of acid injection and the sample outlet to the pHcell should be less than two minutes for effective control
6 Spills or overflows from the sulfuric acid tank should be contained
Corrosion Inhibitors and Anti-Scalents1 Two chemical feeders are generally provided The following guides in sizing the equipment will allow the use of any of the
proprietary products from the vendors when final selection by the owner is made See Report No EE102E78 Cooling Tower Water Treatment Guidelines EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and Report No EE34E86 Environmentally - Acceptable Cooling Water Treatments for detailed discussion of acceptable commercial chemicals
bull If the water treatment program to be used is known design for the chemical rates specified by the vendor or COOLING WATER SYSTEMS SPECIALIST
bull Otherwise design for 100 wppm of each commercial liquid product in the recirculated cooling water The design feedrate should be based on the design total blowdown flow (both uncontrolled and controlled) since this is where theproduct is lost For 100 wppm concentration the chemical rates will be as follows
GallonsHour Chemical = Blowdown (gpm) x (0006)
LitersHour Chemical = Blowdown (Ls) x (036)
2 The design capacity of the metering pump should be two to three times this calculated number Normally 2 - 100capacity pumps are provided Positive displacement pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumpswith flow control are used for higher rates
3 Specifications for chemical feeders may be found in EMRErsquos Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased) Reference should also be made to the checklists containedin Report No EE92E94 Guidelines for Safety Evaluation of Chemical Injection Facilities
4 A final check should be made with the owner and the COOLING WATER SYSTEMS SPECIALISTS as to the treatmentprogram selected and if there is a need for an additional chemical feeder
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Section Page
COOLING WATER SYSTEMS XXVII 23 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Biological ControlFacilities should be provided for feeding the biocide into the back of the cooling tower basin If chlorine is used it must bepresent in the cooling water as free residual to be effective However the following amounts of chlorine are first required toreact with the listed impurities when present in the cooling water before a free residual can be established
IMPURITIESAMOUNT OF CHLORINE REQUIRED (wppm)
WITH EACH WPPM OF IMPURITY
Ammonia (NH 3)
Hydrogen Sulfide (H 2S)
Sodium Sulfite (Na 2SO 3)
Ferrous Iron (Fe ++)
10
22
056
064
The design capacity of the chlorine feeder shall be based on achieving a 01 wppm free chlorine residual in the recirculatingwater flow or a minimum dosage of 7 wppm at maximum design makeup water flow rate and conditions (see EMRE Water and
Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service )Chlorine may be fed either as the commercial liquid sodium hypochlorite (124 wt Cl 2) or liquid chlorine (100 Cl 2) Liquidhypochlorite is being used increasingly because of the higher costs associated with precautions equipment and proceduresrequired to minimize the risks associated with handling gaseous chlorine The hypochlorite solution is fed using a meteringpump which should be sized to feed an equivalent of 15 ppm 100 Cl 2 (Refer to EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) ) In case of spills or an overflow from thestorage tank the sodium hypochlorite may be sent to the cooling tower basinGas chlorinators are covered by EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service The use of gas chlorination is being reduced but may still be appropriate in some locations subject tolocal management approval The gas from either commercial cylinders or an evaporator is dissolved in a water slip stream bymeans of an eductor This results in a chlorine solution strength of approximately 01 This solution should be evenlydistributed below the water surface by means of a header lateral pipe arrangement at the rear of the cooling tower basinopposite the cooling water pumpsSome additional guides on the use of gas chlorinators are given below It is also recommended that the safety guidelines
published by the Manufacturing Chemists Association and the Chlorine Institute be followedbull Dilution water for the chlorinator must not be less than 50 degF (10 degC) or ice formation may cause plugging problems
bull Continuous rates of chlorine gas withdrawal should not exceed 400 lb (180 kg)24 hours from each one ton cylinder atroom temperature [70 degF (21 degC)] This rate may be exceeded (up to 50) for periods not exceeding 2 hours Rule of thumb is not to design for more than 4 cylinders operating in parallel When feed rate exceeds 3000 lb (1360 kg) per daythe liquid chlorine should be fed to an evaporator upstream of the chlorinator
bull If the ambient temperature can be expected to be below 50 degF (10 degC) for extended periods a building should be providedfor the chlorinator and cylinders
bull Weighing scales should be provided to determine when shipping containers should be replaced and to verify chlorine feedrates
bull Analyzers to measure the free chlorine residual in the circulated cooling water should be provided
bull Chlorine leak detectors and a catch basin for major spills should be provided
Control SystemProvide automatic controllers to continuously monitor water condition and to add chemicals to the system (complete systemsmay be available through the water treatment program vendor as a lease or purchase)pH Control - This controller adds acid to correct water alkalinity a timer on the acid pump sounds an alarm if the pump doesnot shut off in a predetermined length of time Backup controls are required to prevent the cooling system from beingpermanently damaged in case the pH electrodes become fouledCorrosivity and Inhibitor Control - Systems are available which give a direct readout of corrosion rate in milsyear If thecorrosion rate exceeds a set point the controller shuts off the acid increases the inhibitor addition opens the blowdown valveand sounds an alarm Electrodes measure the completeness of the inhibitor film by the value of the potential differencebetween the electrodes The larger the difference the more incomplete the film and a potential reading exceeding a set levelactivates the inhibitor feed
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XXVII 24 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Free Residual Chlorine Control - Free residual chlorine should be controlled automatically for biocide treatment Severalgood analyzers are currently available These units can be incorporated into an automatic control system
Oil in Water DetectionMonitoring Corrosion and Deposit FormationThe following types of devices for measuring corrosion rates and deposit formation should be provided to allow the plant tomonitor the effectiveness of the cooling water treatment before scheduled turnaroundsCoupons - A corrosion coupon rack should be specified for the hot return water to the tower in accordance with IP 19-6-1 Corrosion Monitor - On-line corrosion analyzer if specified should monitor the hot return water to the tower in accordance withIP 19-6-1
On-Line Test Exchanger Although there are a variety of deposit monitors from the water treatment chemical vendors they do not necessarily representactual heat transfer conditions in process exchangers For this reason a test exchanger should be installed for evaluatingcorrosion and deposit formation Details are covered in Report No EE102E78 Cooling Tower Water Treatment Guidelines
Alternatively a critical process exchanger can be instrumented such that its heat transfer coefficient can be monitored
Instrumentation would include flows and inlet and outlet temperatures of process fluid and cooling waterSTEP 6 - Specify cooling tower location
Select a location for the cooling tower The following factors influence this selection
Location Factors1 The location should be within a reasonable distance of the units served and consistent with future refinery plans Piping
costs to and from battery limits can be a significant portion of the total cooling system cost and can be the location-determining factor particularly for salt water systems While piping costs can be easily determined other costs affectinglocation cannot be estimated readily Therefore judgment is required in selecting the location
2 Safety spacing requirements (see Section XV-G Equipment Spacing )3 Cooling tower contribution to plant boundary noise4 For salt water cooling towers wind direction and distance to other facilities is of major concern because of potential
corrosion problems
5 Factors affecting cooling tower performance ie orientation with respect to wind site congestion etc
Plot RequirementsIn estimating plot requirements (basin area) one should allow 5 to 6 ft of tower length for each 1000 gpm (2 m for each 60 Ls)of circulating capacity This figure is based on a typical counterflow design with a width of 50 ft (12 m) and does not includepump suction pit space Exact requirements can be obtained from cooling tower vendors after the tower capacity has beenestimated
Fogging and DriftTwo distinct problems are associated with the discharge air from a cooling tower (1) fogging which can be a nuisance or ahazard and (2) drift which is generally a nuisance because of increased corrosion of surrounding equipment Fogging fromcooling towers occurs during periods of combined high ambient humidity and low temperature as a result of mixing the warmmoist cooling tower exhaust air with the cooler ambient air The ambient air cannot absorb all moisture as vapor and thus tinyfog droplets are formed by condensation Fogging and resulting icing in cold climates has produced hazardous drivingconditions on highways up to 850 ft (250 m) awayProprietary designs are available to combat potential fogging problems Basically these designs incorporate heating of thedischarge air with hot return water precooling coils (similar to an air cooler) to eliminate any visible plume However thesedesigns are expensive and relatively unproven Therefore proper location based on reliable weather data is the preferredsolutionDrift from a cooling tower represents a more difficult problem since it does not depend on definable climatic conditions andoccurs for all wind directions The effects of drift on nearby equipment and vegetation are particularly severe and generally of concern only for salt water systems because of the high solids content of the drift (concentrated salt water) For salt water cooling systems electric lines substations and instrumentation should be located upwind in the direction of the predominantwind They should be provided with adequate shelter or materials when within 450 ft (140 m) of the cooling tower In additionspecifications for all equipment within 150 ft (45 m) of the tower should include surface treatment suitable for a marineenvironment
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COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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XXVII 28 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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XXVII 30 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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COOLING WATER SYSTEMS XXVII 31 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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XXVII 32 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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COOLING WATER SYSTEMS XXVII 33 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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COOLING WATER SYSTEMS XXVII 35 of 48
DESIGN PRACTICES December 2000
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FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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COOLING WATER SYSTEMS XXVII 37 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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COOLING WATER SYSTEMS XXVII 7 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
bull Where multiple cooling tower systems exist consideration should be given to providing valved cross connections betweentheir respective supply mains and between their respective return mains These would be used only for special operations
such as turnarounds or emergencies The impact on flare sizing should be evaluated if these cross connections areprovided
ENVIRONMENTAL AND SAFETY CONCERNS
Once-through cooling water systems usually require controls to minimize the release of hydrocarbons and free halide residualsto the environment As a minimum cooling water discharges should be monitored for free oil total organic carbon or hydrocarbon vapors to determine if system coolers are leaking Free chlorine residuals in the discharge should also bemonitored as required to meet local regulations and to detect process leaks In most cases once-through cooling water flowsare too large to be handled in the plants wastewater treatment system However once-through water that could becomecontaminated with hydrocarbons is often segregated and treated in a large skim pond or special purpose separator prior todischarge Local environmental regulations and permit requirements will dictate the extent of treatment requiredCooling tower blowdown from recirculated systems is normally bypassed around major wastewater treatment and dischargedwith the treated wastewater An oil-in-water detector should be provided to detect oil in the blowdown from process leaks Thiswill enable stopping the blowdown or diverting it to temporary holdup until the leak is corrected A major rupture in a processheat exchanger tube can also result in the cooling tower being the discharge point for hydrocarbon vaporsBecause of the likely presence of flammable vapors the areas around cooling towers are subject to electrical classification inaccordance with IP 16-1-1 The type and amount of chemicals added to the cooling water should be evaluated as to their effects on the plants wastewater treatment process Biocides corrosion inhibitors and antifoulants may result in significant problems for the wastewater treatment plantOther environmental concerns are
bull Cooling tower noise emissions from fans and from the flow of cooling water over the tower may require suppression if near a local community
bull Cooling tower water mist drift can cause fog and ice formation on nearby roads and promote deterioration of nearbyequipment It may also result in damage to sensitive vegetation in the surrounding area
bull Process gas releases to atmosphere resulting from process gas pickup by cooling water including the venting of toxicgases from exchanger tube ruptures must be considered in locating cooling towers or discharge points from once-throughsystems
bull Spills and overflows of toxic and hazardous chemicals used in treating the cooling water must be contained
SYSTEM SELECTION
The selection of a cooling water system is often determined by the availability of water In regions where water is scarce once-through systems are obviously eliminated from consideration and recirculated systems are chosen A once-through coolingsystem may be the economical choice if the plant is located adjacent to a body of water and consumers are elevated no morethan 50 ft (15 m) above the water level However once-through system costs may be increased significantly by environmentallimitations in many locations These may limit maximum discharge water temperature and therefore will reduce the maximumcooling water ∆T This will increase the required flow and cost of once-through systems In addition regulations may requirewaste treatment of the large volume of effluent thus increasing costsThe choice of a cooling water system must be based on comparisons of initial investment and operating costs Relative initialcosts of a system vary with climatic conditions distance from pump to users complexity of inlet structures and corrosive nature
of the water Salt water or brackish water service requires the use of cement mortar lined pipe and alloys for exchangers andpumps Carbon steel is usually satisfactory for fresh water Special attention should be given to the impact of bottom silt mudand seaweed on intake facilities to once-through or makeup to recirculated systems Cribs flumes large bays screens trashracks or other extensive facilities may be requiredDuring the early stages of a project the designer should review with the planning or design groups the heat level and volume of all process streams which require cooling The optimum method of cooling should then be determined ie heat exchangeandor rejection of heat to air or water A break point should be selected above which air cooling is economical and belowwhich trim water cooling is preferred
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XXVII 8 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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BASIC DESIGN CONSIDERATIONS (Cont)
COOLING WATER REQUIREMENTS
Cooling water requirements for each process unit and offsite consumer are obtained from the appropriate design groups For existing locations maximum cooling water loads should be based on the design inlet cooling water temperatures given inSection IX-B Design Considerations for All Types of Heat Exchangers For new locations the inlet cooling water temperaturemust be set based on local water temperature data for once-through systems and wet bulb temperature and selected approachfor recirculated systemsThe bulk water temperature at the cooler outlet has been selected as the controllable variable for design and operation of coolers Although the corrosion rate depends on the metal temperature this is difficult to determine in the operation of a coolerBased upon the findings of a Materials Development Section test program maximum design outlet water temperatures havebeen set as indicated below
Fresh Water 130 degF (54 degC)
Treated Fresh Water 130 degF (54 degC)
Brackish Water 125degF (52
degC)
Salt Water 120 degF (49 degC)
When the maximum plant cooling water requirement has been established for new process and utility consumers appropriateLoad Growth and Reserve Capacity Factors (LGF and RCF) should be applied depending on the development stage of theproject Cooling water loads should be updated and system capacity adjusted periodically as design work progresses
LOAD GROWTH AND RESERVE CAPACITY
Utility loads tend to increase as the loads become better defined through successive stages of project definition Thereforeload growth and reserve capacity factors (LGF and RCF) are applied to raw loads as a means of predicting final equipmentsizes and capital costs These load growth factors do not cover basis changes through project development such as project or process unit sizethroughput For revamp or modernization projects these factors are generally applied only to new loads or changes to existing loads on the basis that existing or unchanged loads are well known
Recommended Load Growth Factors are as follows
CONSUMERSPLANNING
(SCREENING THROUGH DBM) DESIGN SPEC
Existing Facilities 0 - 10 0 - 10
New Facilities 20 - 30 5 - 10
Selection of the load growth factor to use should be based on the quality and level of effort associated with development of theraw loadsFinal sizing of the source facility (eg the cooling tower and circulation pumps) should include a reserve capacity factor of 10to cover small post startup changes minor future projects and dynamic conditions not predicted by steady state balances Thesizing basis for distribution facilities should not include the RCF
COOLING TOWER TYPES
The cooling tower is the major piece of equipment specified in recirculating cooling water systems In effect it is the heatexchanger which cools the hot water to a suitable temperature for re-use primarily by evaporation through contact with airThe cooling tower contains fill to break up the water flow into droplets or film on a surface The fill increases the waterair contact area and enhances heat transferCooling towers are classified according to the means by which air is supplied to the tower ie induced or forced mechanicaldraft (fan) vs natural draft and according to the relative movement of the air and water that is counterflow or crossflow (seeFigure 3 ) Most of the towers in the ExxonMobil circuit are mechanical induced-draft type Economics determine whether theyare counterflow or crossflow
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COOLING WATER SYSTEMS XXVII 9 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
In crossflow towers the air travels horizontally across the falling water while in the counterflow design the air travels verticallyup through the falling water Comparisons of plot space fan horsepower and future maintenance costs determine which
design will be more economical Factors which influence this selection are cooling water rate range and approach for aparticular location In addition in some locations materials of construction and associated labor costs will affect the finalselectionIn the past crossflow towers were usually preferred over counterflow towers for extremely cold locations where ice formationcould restrict or block air flow through the louvers However this consideration is usually no longer valid due to the increaseduse of PVC film-type tower fill rather than splash-type fill in counterflow towersMechanical draft designs use one or more fans to provide the air needed for cooling Induced-draft units have the fans locatedon top of the tower Air is pulled through the packing and discharged vertically upward at high velocities Forced-draft unitshave the fans located at the base of the tower and push the air through the fill Forced-draft towers in large industrial sizesexperience difficulties with recirculation and in providing a uniform distribution of air As a result they have been supersededby induced-draft designsFill may be splash or film type Splash fill normally consists of horizontal slats in horizontal rows offset to one another to causethe water to break up into droplets as it falls downward through the cooling tower Splash fill is characterized by reduced air pressure losses It is also less conducive to clogging and easier to clean after a spill However it is very sensitive to
inadequate support and must remain horizontal and level It is made of treated wood or plastics such as PVC or polypropyleneFilm fill comes in various designs but they all cause the water to flow in films over the fill surface and provides more coolingcapacity within a given amount of space than splash fill Because of this increased heat transfer efficiency film fill is generallyused in most new cooling towers However because of its smaller passages it is more sensitive to plugging and is difficult toclean if it does become plugged It may not be appropriate for services cooling mainly heavy fuels or waxy lubes where a leakwould cause fill plugging or for services with a high biological fouling potential PVC is currently the most widely used materialfor film fill The selection of fill type is usually proposed by the vendor and reviewed by the COOLING WATER SYSTEMSSPECIALISTSNatural-draft or hyperbolic cooling towers depend on the natural draft created by the difference in the density of the enteringand leaving air for movement Generally hyperbolic cooling towers have been used in Europe and in the utility industry in theUSA for large capacities [250000 gpm (16000 Ls)] short ranges [10 to 15 degF (6 to 8 degC)] and long approaches [10 to 15 degF (6to 8 degC)] They are most effective where low ambient wet bulb temperatures occur with high relative humidities These towersnormally have diameters in excess of 200 ft (60 m) and are greater than 250 ft (75 m) in height Their major advantages are
bull No operating or maintenance cost for fans
bull Reduction in problems associated with fogging drift and recirculation due to their great height and diameterThe primary reasons why hyperbolic towers are not used in ExxonMobil projects include
bull High initial cost
bull Large plot space requirement and high visual impact
bull Complete dependence on atmospheric conditions
bull Lack of flexibility for future expansions
COOLING TOWER BASIN
The primary function of the cooling tower basin is to collect the cooled water leaving the tower and to provide a reservoir for thecooling water pumps Design aspects to be considered are the basins capacity cleanability draining facilities sump andscreen details To obtain the necessary time for corrective action during emergency conditions a minimum storage capacity of 90 minutes based on loss of makeup water flow should be provided in the basin between the basin low level alarm (LLA) andthe minimum pump submergence (MPS) When the basin water level reaches the MPS point loss of recirculation pumpsuction and cooling water flow is imminent Typical values for basin water levels are as follows
bull High Level Alarm (HLA) 1 ft (300 mm) below top of the basin wall
bull Normal Water Level 15 ft (450 mm) below top of the basin wall
bull Low Level Alarm (LLA) 25 ft (760 mm) below top of the basin wall
bull Distance between LLA and MPS 35 ft (1070 mm)
bull Minimum Pump Submergence Depth 45 ft (1370 mm)
bull Clearance of Pump Suction off Basin Floor 15 ft (450 mm) Actual values may vary depending on pump type selected pump suction line diameter and cooling tower cell plot area
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XXVII 10 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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BASIC DESIGN CONSIDERATIONS (Cont)
MATERIALS OF CONSTRUCTION
The EMRE Refinery Construction Materials Manual and IP 8-1-1 list acceptable materials for all components in each type of cooling water system (once-through and recirculating) For salt water systems emphasis must be placed on proper materialsselection throughout the system The external salt water environments in the vicinity of the tower itself and the intake stationsmust be taken into consideration Proper siting of the tower is extremely important to avoid salt water corrosion of adjacentunits or the need to use expensive materials to operate in such an environment
WATER TREATMENT FOR ONCE-THROUGH SYSTEMS
Once-through salt and fresh cooling water systems can develop a variety of problems such as fouling due to seaweed or other debris marine microorganisms such as muscles and shell fish and biological growth Corrosion and scaling problems areminimal in fresh water once-through systems because the water is not concentrated by evaporation In salt water systemscorrosion is minimized by proper materials selection and scaling problems generally do not occurProtection against fouling by seaweed or other debris is provided by careful selection of the water intake location and travelingscreens with washing facilities In some cases additional steps may be required These may include an additional secondaryset of traveling screens and possibly battery limit strainers on the cooling water supply lines to individual unitsProtection against fouling by marine microorganisms and biological growths is achieved primarily by injecting chlorinehypochlorite chlorinebromine or other similar chemicals which kill these growths Chlorine is commonly used and is injectedin sufficient quantities to produce a residual of 1 wppm in the effluent for one hour each day (shock chlorination) Continuouschlorination for 2 - 3 days per month at lower levels (03 wppm residual chlorine in the effluent) may also be used in addition toshock chlorinationThe large water volume discharged from once-through systems usually makes chemical treating for corrosion or deposit controltoo costly These problems are handled by proper materials selection and by keeping water velocities through exchanger tubesgreater than 3 fts (09 ms) Occasionally the water analysis of a fresh water source being considered for a once-throughsystem using carbon steel materials will indicate the need for some corrosion or deposit control In this case the cost of thethreshold treatment with the once-through system should be compared to the costs of a recirculated system before making afinal choice
WATER TREATMENT FOR SALT WATER COOLING TOWER SYSTEMS
A total dissolved salts (TDS) concentration limit of 55000 wppm for sea water in cooling tower systems requires high blowdownrates since the makeup sea water typically has from 35000 to 40000 wppm TDS Because of cost restraints the only water treatment normally specified for these systems is for biological control Chlorine is most commonly used either as directchlorine injection or hypochlorite produced in electrochlorinators Care must be taken in selecting the intake location to assurethat the suspended solids do not exceed 200 wppm in the recirculated water If it is not possible to find sea water with sufficientclarity clarification with polyelectrolytes should be evaluated Corrosion is one of the major problems with the use of sea water in cooling towers This requires proper materials selection
WATER TREATMENT FOR FRESH WATER COOLING TOWER SYSTEMS
Cooling tower water treatment is necessary to minimize or eliminate corrosion scale and biological fouling of process heattransfer surfaces caused by minerals and impurities in the water The difficulties caused by these impurities and the means of treatment are shown in Table 1 Water lost by evaporation results in an increase in total dissolved salts in the recirculated water To indicate the degree of concentration of impurities the term cycles of concentration is used
Corrosion is an electrochemical process that deteriorates metals exposed to water in the presence of corrosive agents such asacids oxygen or bacteria A common form of corrosion is pitting Severe corrosion can lead to equipment failure Corrosionis caused by many conditions including process leaks into the cooling water water flow velocity that is too low (causes depositsand fouling lending to corrosion) or too high (causes erosion and corrosion) low pH and high temperatures Corrosion is lesslikely in water with non-acidic pH values (greater than 7) although scaling is more likely in these pH rangesScaling is characterized by the formation of hard dense deposits on material surfaces These deposits impact heat transfer and can become a site for localized under-deposit corrosion Scaling is influenced by many factors including makeup water composition total dissolved solids in the recirculating water water temperature and pH and flow velocity Calcium carbonateis the main scaling constituent in all waters and it is the least soluble It forms from the hardness (calcium Ca ++ ) andbicarbonate (HCO 3
ndash ) alkalinity present in most waterFouling can be caused either by the settlement of suspended matter or by microbiological growth of algae bacterial slime or fungi Fouling will reduce heat transfer cause plugging and create corrosion sites
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 11 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Treatment of the cooling water is broken down into two major categories external and internal External treatmentencompasses any treatment given to the cooling tower makeup water Internal treatment is the addition of chemicals directly to
the recirculated cooling water
EXTERNAL TREATMENT
External treatment to remove impurities in order to reduce the amount of blowdown should always be evaluated as to costeffectiveness The most frequently used techniques for treating makeup water are shown in Figure 11 Details of availabletreatment processes (see Table 1 for methods available) are covered in Section XXVI-A Boiler Feedwater Treatment For cooling tower systems that meet currently required drift limits in refineries and chemical plants uncontrolled cooling water losses (drift and miscellaneous process uses) normally amount to about 14 of the cooling water evaporation loss Under these conditions the maximum cycles of concentration achievable are 8 with no controlled blowdown Some cooling towersare operating at up to 10 cycles of concentration Additional controlled blowdown required to limit the cycles of concentrationper the water quality criteria is covered later
INTERNAL TREATMENT
The makeup water analyses amount of cooling water losses environmental restraints potential chemical hazards materials of construction and film temperatures of heat transfer surfaces determine the type of internal treatment required for cooling tower systems The internal water treatment program selected must address corrosion scale fouling biological growth andchemical attack of cooling tower componentsNumerous chemicals and treatment programs are continuously being developed for addressing the potential problems Theslightly acidic low pH programs of the past which used chromate based chemicals and usually required acid injection arebeing completely phased out because of environmental restrictions on chromate and the higher costs associated with meetingcurrent practices for handling hazardous chemicals such as acid Non-chromate treatments which use inhibitors such asphosphates zinc and organics along with higher pH have been developed to replace the earlier programs Although not aseffective as chromate these inhibitors operate at higher water pH which is a less corrosive environment The disadvantage of the higher pH is the greater tendency to form scale This tendency is controlled by the use of dispersants or scale inhibitorswhich are normally organic polymers or phosphenate A corrosion inhibitor for copper or brass is also normally required if these metals are present in the systemThe following paragraphs describe techniques and chemicals to control alkalinity (pH) corrosion scale sludge and biologicalgrowth The chemicals mentioned are normally purchased as pre-mixes from vendors and dosed in combination Mostcorrosion and scale control program chemicals are two drum formulations due to the incompatibility of some chemicals in theconcentrated state Therefore separate feed systems are generally provided for chemicals for control of corrosion and scale (2drums) and biological growth (if liquid) If a program includes fixed pH then an additional feed system is required to add acidWater treatment programs are site specific and changing The COOLING WATER SYSTEMS SPECIALISTS should beconsulted for recommendations on the internal water treatment programs when an analysis of the available water is known
Alkalinity (pH) Control
Cooling water treatment programs presently available fall into two basic categories those that do not control pH (floating pH or alkaline treatment programs without pH adjustment) and those that control pH within a specified range by adding acid Theprograms that do not control pH are gaining favor because of the additional cost equipment and precautions required tominimize the risk associated with handling acid and because of improvements in chemicals to control depositsHigh pH in the recirculating cooling water is a result of the alkalinity of the fresh water due to bicarbonate salts This combinedwith carbon dioxide results in the water having a pH range of 6 to 85 When this fresh makeup water is mixed with therecirculated cooling water and passed through the cooling tower the air flow strips out the carbon dioxide Bicarbonate ionsthen convert to carbonate ions raising the pH which can cause both calcium carbonate and calcium phosphate salts toprecipitate out of solution and form deposits Another debit in allowing the cooling water pH to increase is increased chlorineconsumption to control biological growthsIf a program using pH control is justified and chosen acid addition is used under control of a pH analyzer to controlrecirculated cooling water alkalinity and pH to the range required by the treatment program
Corrosion Control
To reduce corrosion to an acceptable level chemical corrosion inhibitors which form protective films on heat transfer surfacesare the most effective protection Inhibiting corrosion is accomplished by phosphates organics zinc nitrites and molybdatesalts Unfortunately the use of chromate which is a reliable corrosion inhibitor is prohibited by environmental constraintsNitrite is not practical in an open system due to atmospheric oxygen converting the nitrite to nitrate
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ExxonMobil Proprietary
Section Page
XXVII 12 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Phosphate inhibitors (orthophosphate polyphosphate phosphonate) are the basic treatment for new towers in todaysenvironment In addition to the phosphate chemistry salts of zinc may be added to aid in inhibiting corrosion These ions
combine with the hydroxide ion forming thin protective films Zinc is used to provide a synergistic effect in combination with thephosphate salts for protecting carbon steel surfaces from corrosion Finally all-organic programs are offered by severalmanufacturers The use of these treatments has been made possible by the development of polymer dispersants which givecalcium salts a much higher solubility and reduce the tendency to scaleWhen a cooling water system contains copper alloys azole based and azole substitutes are required as copper corrosioninhibitors There are several effective inhibitors The most common is tolytriazole (TTA) due to its resistance to chlorineThe corrosion inhibitors most frequently used in ExxonMobil recirculating cooling water systems are shown below indescending order of preference provided their use meets all environmental requirements
bull Stabilized Phosphate (OrthoPolyphosphate) - Environmental restrictions on the use of zinc have resulted in ExxonMobilplants in the USA using a blend of orthophosphate and polyphosphate type corrosion inhibitors with good results Thistreatment forms a thin protective calcium phosphate film on heat transfer surfaces through the use of orthophosphateconcentrations as high as 12 - 17 wppm The use of calcium phosphate dispersants and Hydroxethylidene Diphosphonate(HEDP) stabilizes the treatment and prevents the formation of calcium phosphate scale Concentration of the dispersantsdepends upon the calcium concentration temperature and pH Stabilized Phosphate treatment programs can operate in apH range of 68 to 78 Alternatively an alkaline (floating pH) stabilized phosphate treatment program can be used Thisprogram uses little or no acid addition and is gaining in popularity although not well established yet It operates with pHvalues of 8 - 9 and uses higher quantities of dispersants and lower levels of phosphate A corrosion inhibitor for copper and brass is normally included for all stabilized phosphate programs Several vendors offer these programs Twoseparate chemical feeders are generally required
bull Zinc Alkaline Phosphate - Heritage Exxon plants in Europe where the use of zinc is not restricted have obtained goodcontrol of corrosion and scaling using zinc-polyphosphate treatment programs Low (65 - 72) and high (73 - 80) pHprograms have been used The high pH (zinc alkaline phosphate) programs are the lowest cost and are recommended
As with the Stabilized Phosphate treatment dispersants such as HEDP and polyacrylates are required to prevent theformation of zinc or calcium phosphate scale on heat transfer surfaces A corrosion inhibitor for copper or brass isnormally used with this program which is offered by several vendors
bull Organic Salts - All-organic (polymer) programs are offered by several manufacturers and have been used successfullywithin h Exxon In general these treatments are based on a combination of a phosphonate to control corrosion polymers(dispersants) to prevent scale deposition and a corrosion inhibitor for copper or brass This type of program is mosteffective in conditions of high hardness high alkalinity (M alkalinity gt300 wppm as CaCO 3) and high pH values (gt 85)However some manufacturers have reported success in softer water with alternative polymer component formulations Ingeneral an inorganic corrosion control program is preferred to the all-organic programs which are sensitive to alkalinityand more costly
Detailed control limits for effective use of each of the above inhibitors and materials selection are covered in Report NoEE34E86 Environmentally Acceptable Cooling Water Treatments Report No EE102E78 Cooling Tower Water Treatment Guidelines and in the EMRE Refinery Construction Materials Manual
Scale and Sludge Control
Scale and sludge deposits in cooling water systems cause loss of heat transfer and provide the conditions for pitting corrosionScale consists of inorganic salts which have precipitated from the cooling water forming a coating on heat transfer surfacesSludge consists of organic and inorganic suspended matter that settles in quiescent areas and in heat exchangers The key toprevention of both scale and sludge formation in a cooling system is keeping water velocities greater than 3 fts (09 ms) andthe use of chemical additives in combination with a blowdown rate that keeps the impurity concentration below the level whichcauses deposits (See Table 2 for limits on impurities) Scale control has become increasingly important in higher alkalinityldquofloating pHrdquo water treatment programsSalts most commonly found in scale deposits are calcium carbonate calcium sulfate tricalcium phosphate zinc phosphatemagnesium silicate and silica The control method for preventing scale from each of these salts is discussed below
bull Calcium Carbonate - Where justified using pHalkalinity control in the range of 70 to 75 will normally inhibit the formationof calcium carbonate scale if anti-scalent chemicals (phosphonates acrylates copolymers) are part of the chemicaltreatment (See Report No EE102E78 Cooling Tower Water Treatment Guidelines ) Higher pH water treatmentprograms require greater concentrations of these chemicals along with dispersants and flocculants to inhibit the formationof scale and precipitation of other salts
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 13 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
bull Calcium Sulfate - When heat transfer skin temperatures are below 250 degF (120 degC) and the cooling water bulktemperatures are under 130 degF (54 degC) gypsum scale (CaSO 42H 2O) will form if the product of the calcium and sulfate ionsconcentrations each expressed as wppm CaCO 3 equivalent exceeds 500000 Gypsum scale is prevented bymaintaining bulk water temperature below the indicated limits and controlling calcium and sulfate ions through blowdownor through the use of newer dispersants on the market
bull Tri-Calcium and Zinc Phosphate - Almost all chemical additives for corrosion control contain some type of phosphatesalt These have a tendency to form the orthophosphate ion depending on time pH and temperature Cooling watersusing these corrosion inhibitors may have an orthophosphate concentration in excess of 5 wppm as PO 4
= To preventcalcium phosphate scale Table 2 shows a limit of 1000 wppm of calcium as CaCO 3 equivalent when using a phosphateinhibitor in combination with a chemical dispersant (See Report No EE102E78 Cooling Tower Water Treatment Guidelines for solubility of calcium phosphate) These inhibitors and dispersants generally fall into one of the followingcategories
ndash Sequestrants antiprecipitants and inhibitors such as hydroyethylidene diphosphonate (HEDP) phosphate esters andaminomethylene phosphonate (AMP) function by defyingdistorting the crystal structure of the scale formations toprevent nucleation and growth of the scale
ndash Dispersants based on either anionic or nonionic polymers such as polyacrylamide polyacrylate or polymaleic acidfunction to stabilize colloidal and scale-forming particles in suspension Anionic polymers prevent agglomeration of suspended particles by either charge repulsion or crystal distortion while nonionic polymers reduce the surfacetension of particles
bull Silica Deposits - Quantitative indices for determining limits for silica in combination with other ions are not available due tothe formation of complex polymers Instead the rule of thumb limit of keeping reactive silica concentrations in the coolingwater below 150 wppm as SiO 2 has been successful
Possible alternatives for controlling sludge are
bull Chemical Dispersants - Water treating chemical suppliers now offer many types of dispersants These are mostlypolyelectrolytes Claims for removal of slime silt and mud from cooling tower basins and exchangers are madeHowever satisfactory results using these chemicals as a coagulant aid require very accurate control of the amount fed tothe water and stable operation of the water system Also suspended solids should be kept below 200 wppm for particleslarger than 045 microns Dispersants are used in virtually all fresh water cooling tower systems despite their high cost
bull Sidestream Filtration - The use of a sand pressure filter is the most reliable method for sludge removal Operating andmaintenance costs are minimal for this equipment
bull Higher Blowdown Rates - This may be an economical alternative depending on the cost of makeup water Additionalchemical costs must be included in any evaluation of this method
bull Makeup Clarification - This alternative may require additional raw water treatment facilities to handle seasonal variationsin turbidity andor suspended solids This method is applicable only where suspended solids come from makeup water
Biological Control
Operating conditions in recirculated cooling water are ideal for the growth of biological matter Water temperatures from 70 to120 degF (20 to 50 degC) favorable water pH continuous supply of nutrients including carbon nitrogen and phosphorus inorganicsalts and sunlight all lend themselves to environmental conditions encouraging microbiological growth and plant lifeIf biological growths get out of control and form large sticky agglomerations some of the following operating problems willresult
ndash Fouling of heat transfer surfaces by bacterial slimes resulting in flow restrictions and high process temperatures ndash Reduced cooling tower efficiency resulting from algae fungi and bacterial slime growths in the water distribution basin
and fill area of the cooling tower ndash Excessive plugging of screens and filters ndash Corrosion ndash Wood destruction by fungi ndash Clogging of water distribution nozzles ndash Excessive sludge accumulation in the cooling tower basin
An important part of the water treatment program is to control biological activity by feeding a biocide chemical Oxidizingbiocides such as chlorine sodium hypochlorite chlorine dioxide or bromine-associated compounds are most commonly usedwith good results Plate counts should be maintained below 50000 countsml (bacterial)
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ExxonMobil Proprietary
Section Page
XXVII 14 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Chlorine is the most effective biocide for cooling water because of its low cost extensive period of successful use ease of application and minimum environmental effects if applied under controlled conditions Either a liquid feed (sodium
hypochlorite) or a gas feed (100 chlorine) is added to the cooling tower basin A sufficient dosage is that which obtains a 01to 03 wppm continuous free residual in the hot return water to the cooling tower
Advantages to continuous chlorination over shock treatment based on ExxonMobil experience are ndash Minimizes chlorine degradation of polymers ndash Better reduction in total aerobic bacteria plate count resulting in cleaner heat transfer surfaces ndash Does not cause significant pH depressions ndash Better control results with less chlorine consumption
Details concerning chlorine addition as well as the other alternative biocides are given in Report No EE102E78 Cooling Tower Water Treatment Guidelines and in EMRE Engineering Water and Wastewater Design Guide TMEE 080 DG 11-3-1Gas Chlorinators for Water Treating Service and DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) Gaseous chlorine systems have become more costly in recent years because of the additional precautions equipment andprocedures required to minimize the risk associated with handling this gas Alternatives to gaseous chlorine are available butrequire more attention and have a higher operating cost Sodium hypochlorite (either purchased or generated) and sodiumbromide stabilized solid halide (chlorinebromine) products chlorine dioxide and ozone are all alternatives which may haveapplications Contact the COOLING WATER SYSTEMS SPECIALISTS for guidance in the use of these alternative treatmentschemes
BLOWDOWN
Blowdown control is required to maintain the proper cycles of concentration in the system The most reliable and simplest typeof control for blowdown is specific conductance measurement of the cooling water Electrical signals from the specificconductance measurements of some impurity in the cooling water are used to actuate an automatic blowdown valveImpurities normally used for blowdown control are dissolved solids chloride calcium magnesium and silica
CONTROLS AND MONITORS
Any chemical treatment used to control problems in a cooling water system must be carefully monitored to be completelyeffective
The installation of automatic controls should be considered for recirculated cooling water systems in all plants Designs shouldinclude pH conductivity corrosivity inhibitor and biocide control Automatic controllers are available which continuouslymonitor the condition of the water and automatically make chemical additions to keep the water within compositional limitsTest exchangers and corrosion coupons are also useful in monitoring the effectiveness of the water treatment program
An oil-in-water monitor should be considered to provide early detection of oil leaks into the cooling water This will enablecorrective action to be taken before extensive contamination of the cooling water circuit occurs This is especially important incooling towers with high efficiency cellular fill which can be difficult to clean if significantly fouledManual control systems may be used in locations that have adequate technical personnel to perform routine water analysesand to modify chemical treatment necessitated by sudden variations in water composition process contamination etc
WASTEWATER REUSE POSSIBILITIES
Recirculating cooling water systems normally have the highest plant raw water demand A potential alternative is to use treatedwastewater to supply part or all of the makeup water requirement The treatment required must be determined but will typicallyinclude primary oil removal (eg API Separator) secondary oil and suspended solids removal (eg filtration dissolved or induced air flotation) and soluble carbon removal utilizing biological treatmentThe reuse of treated wastewater as makeup currently exists at only a few locations If this alternative is being considered refer to Report No EE58E85 Reuse of Wastewater as Cooling Tower Makeup and Section XIX-B Water Reuse
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 15 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS
STEP 1 - Select a location for the intake station and pumps
Factors which influence this selection are as followsbull Depth of water and tidal variations
bull Relative location of intake and outfall direction of current
bull Length and size of intake suction piping vs pump discharge piping
STEP 2 - Determine cooling water requirements
Determine the design cold water inlet temperature based on site data covering seasonal water temperature as a function of inlet location and depth (for existing locations refer to Table 2 in Section IX-B )Tabulate the cooling water requirements for each process unit and offsite consumer Apply the proper LGFs to all coolingloads depending on the stage of the project and whether the new units are exact duplicates of existing unitsIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The designer should investigate methods to increase effluent water temperature to the maximumrecommended limit for the particular type of water used One common method is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 3 - Determine the total required capacity for water intake facilities
Process Coolers - The total requirements determined in Step 2 including LGFsFirewater - This is often supplied from the intake bay of a once-through systemFuture Expansions - These should be considered in sizing intake facilitiesTotal Required Capacity - The sum of the above three items
STEP 4 - Provide water intake facilities
Intake Crib - The intake crib with its support structure is located at the extreme end of the water intake line (See Figure 4 )The intake is usually located 200 to 300 ft (60 to 90 m) offshore and at a sufficient depth below the lowest water level to
preclude vortex formation and subsequent pump cavitation during storm conditions Additionally the crib should have 3 to 5 ft(1 to 15 m) clearance above the sea floor to minimize sand and silt pickup A concrete velocity cap should be specified to cover the inlet crib This will minimize the incidence of fish and other foreignmatter entering the system A horizontal inlet flow pattern should be designed with a maximum entrance velocity of 1 fts(03 ms) Experience shows that vertical velocity flow confuses fish and they are drawn into the intake lineThe intake crib support structure protects the crib from adverse effects of storms and tidal motions as well as protecting it fromimpact of floating and submerged objects A self-cleaning screen can be specified to prevent entrance of large objects into thesystem at the cribIntake Line (Figure 5 ) - An intake line is required if the water depth along the shore does not permit water to flow directly intothe intake bay The intake line is usually buried in the sea floor and covered with a minimum required overburden Consultwith the COOLING WATER SYSTEMS SPECIALISTS for required coverage at a specific siteChlorine or sodium hypochlorite is injected at the intake crib to prevent marine growth in the lineNote that an intake crib and line will not be required if water of sufficient depth is available at the supply pump stationIntake Bay (Figure 6 ) - The intake bay is an enclosure which houses the pumps and screens Depending on a particular design a series of screens is used to ensure the supply of clean water Trash racks comprise the first element of the filter system These remove only very coarse material from the water such as bulk seaweed They can be supplied with atraversing raking mechanismLift screens provide secondary cleanup These screens can be lifted out of the intake bay for periodic cleaning Double liftscreens can be used if it is deemed necessary to have one in service at all timesFinal water filtering is usually done by traveling screens Details of the screen dimensions and the mounting are usuallydetermined during detailed engineering Screen net open area is typically approximately 50 of total screen area Thescreens should be sized for a normal water velocity of 12 fts (04 ms) through the open screen area Maximum velocityshould be limited to 24 fts (08 ms) with 50 of the screen area plugged This is the upper velocity limit for efficientscreening
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ExxonMobil Proprietary
Section Page
XXVII 16 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
If high suspended solids greater than 200 mesh (eg sand) are expected to be a continuous problem ejectors may be installedupstream and downstream of the traveling screen to remove accumulated sand and solids from the intake bay in these areas
Traveling screens and ejectors are supplied with wash water from the raw water pumps or from separate wash water pumpsOnoff control using timers should allow optimum scheduling of the ejectors depending on the time of the year
STEP 5 - Specify water intake facilities
Water intake facilities should be designed in accordance with the following guidelines from the Hydraulic Institute Standards The water intake should be designed to provide an evenly distributed flow of water to the pump suction A properly designedintake reduces the possibility of vortex formation and results in better pump performance with less noise Geometry of theintake and the manner in which water is introduced into the intake are important parameters in providing an even distribution of water flow Complete analysis of intake structures is best accomplished by scale model testsThe pump manufacturer will generally render advice on specific problems while the intake design is still preliminary He shouldbe provided with the necessary intake layout drawings showing the physical limitations of the siteFigures 7A and 7B have been developed for single- and multiple-pump arrangements to show suggestions for basic sumpdimensions They are for pumps normally operating in the capacity range of approximately 3000 to 300000 gpm (200 to20000 Ls) Since these values are composite averages from a great many pump types and cover the entire range of specificspeeds they must not be thought of as absolute values but rather as basic guides subject to some possible variations
All the dimensions in Figures 7A and 7B are based on the rated capacity of the pump at the design head Any increase incapacity above these values should be momentary or very limited in time If operation at an increased capacity is to beundertaken for considerable periods of time the maximum capacity should be used for the design value in obtaining sumpdimensionsDimension C is an average value based on an analysis of many pumps Its final value should be specified by the pumpmanufacturerDimension B is a suggested maximum dimension which may be less depending on actual suction bell or bowl diameters in useby the pump manufacturer The edge of the bell should be close to the back wall of the sump When the position of the backwall is determined by the driving equipment or the discharge piping Dimension B may become excessive and a ldquofalserdquo backwall should be installedDimension S is a minimum value for the sump width for a single pump installation This dimension can be increased but if it isto be made smaller the manufacturer should be consulted or a sump model test should be run to determine its adequacy
Motor size may dictate minimum pump spacing be especially aware of this when specifying pumps with unusually high pumpheads eg greater than 80 psig (550 kPa)Dimension H is a minimum value based on the ldquonormal low water levelrdquo at the pump suction bell taking into considerationfriction losses through the inlet screen and approach channel This dimension can be considerably less momentarily or infrequently without excessive damage to the pump It should be remembered however that this does not representldquosubmergencerdquo Submergence is normally quoted as dimension ldquoHrdquo minus ldquoCrdquo This represents the physical height of water level above the bottom of the suction inletThe actual submergence of the pump is something less than this since the impeller eye is some distance above the bottom of the suction bell possibly as much as 3 to 4 ft (1 to 13 m) For the purposes of sump design in connection with this chart it isunderstood that the pump has been selected in accordance with specific speed charts The submergence referred to hereinhas to do only with vortexing and eddy formationsDimensions Y and A are recommended minimum values These dimensions can be as large as desired but should be limitedto the restrictions indicated on the curve If the design does not include a screen Dimension A should be considerably longerThe screen or gate widths should not be substantially less than S and heights should not be less than H If the main stream
velocity is more than 2 fts (06 ms) it may be necessary to construct straightening vanes in the approach channel increaseDimension A conduct a sump model test of the installation or work out some combination of these factorsDimension S becomes the width of an individual pump cell or the center-to-center distance of two pumps if no division walls areusedOn multiple pump installations the recommended dimensions in Figures 7A and 7B also apply as noted above and thefollowing additional determinants should be consideredFigure 8A - Low velocity and straight-line flow to all units simultaneously is the first recommended style of pit Velocities in thepump area should be approximately 1 fts (03 ms) Some pumps with velocities of 2 fts (06 ms) and higher have given goodresults This is particularly true where the design resulted from a model study Not recommended would be an abrupt changein size of inlet pipe to sump or inlet from one side introducing eddying
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 17 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Figure 8B - A number of pumps in the same sump will operate best without separating walls unless all pumps are always inoperation at the same time in which case the use of separating walls may be beneficial If walls must be used for structural
purposes and pumps will operate intermittently one should leave flow space behind each wall from the pit floor up to at leastthe minimum water level and the wall should not extend upstream beyond the rim of the suction bell If walls are used increaseDimension ldquoSrdquo by the thickness of the wall for correct centerline spacing Round off the ends of the walls Not recommended isthe placement of a number of pumps around the edge of a sump with or without dividing wallsFigure 8C - Abrupt changes in size from inlet pipe or channel to pump bay are not desirable A relatively small pipe emptyinginto a large pump pit should connect to the pit with a gradually increasing taper section The angle should be as large aspossible preferably not less than 45 degrees With this arrangement pit velocities much less than one foot per second aredesirable Especially not recommended is a small pipe directly connected to a large pit with pumps close to the inlet Flow willhave excessive change of direction to get to most of the pumps Centering pumps in the pit leave large ldquovortex areasrdquo behindthe pumps which will cause operational troubleFigure 8D - If the pit velocity can be kept low enough [below 1 fts (03 ms)] an abrupt change from inlet pipe to pit can beaccommodated if the length equals or exceeds the values shown It is assumed that as ratio WP increases the inlet velocityat ldquoPrdquo will increase up to an allowed maximum of 8 fts (24 ms) at WP = 10 Pumps ldquoin linerdquo are not recommended unless theratio of pit to pump size is quite large and pumps are separated longitudinally by a generous margin A pit can generally be
constructed at much less cost by use of a recommended designFigure 8E - It is sometimes desirable to install pumps in tunnels or pipelines A drop pipe or false well to house the pump witha vaned inlet well facing upstream will be satisfactory in flows up to 8 fts (24 ms) Without the inlet well the pump section bellshould be positioned at least two pipe diameters vertically above the top of the tunnel not hung into the tunnel flow especiallywith tunnel velocities 2 fts (06 ms) or more There should be no signs of air along the top of the tunnel It may be necessaryto lower the scoop or to insist on a minimum water level in the vertical wellNote The foregoing statements apply to sumps for clear liquid
STEP 6 - Provide chemical treatment
Provide chemical treatment facilities for the once-through cooling water system Normally the only chemical treatment requiredin a once-through cooling system is for biological control Facilities for adding chlorine to once-through cooling water are thesame as those described for the recirculated cooling water system The design capacity of the chlorinator should be based onachieving a chlorine residual of 1 wppm in the effluent The chlorine required by the organics and other reducing agents in thewater must be satisfied before any residual chlorine will appear in the effluent The design capacity of the chlorinator should be
based on the total water chlorine demand plus 1 wppm in the effluent In the absence of water data the chlorination systemdesign capacity should be based on 10 wppm minimum instantaneous dosage for maximum cooling water flow For very largeonce-through cooling water systems it will be impractical to design for shock rates at this level In this case a continuousdosage of 1 - 2 wppm should be used (Refer to Step 5 of the DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS )
STEP 7 - Specify pumps and distribution system
Specify other major facilities in the once-through cooling water system The recommended materials for pumps valves andpiping are shown in the EMRErsquos Refinery Construction Materials Manual
Pumps1 Type - These pumps are usually vertical centrifugal pumps However horizontal centrifugal pumps have been used An
economic study andor consideration of existing facilities will determine which type should be specified In the event thathorizontal centrifugal pumps are specified facilities for pump priming will likely be required For large pumps in salt water service there is a very limited selection of pumps because of the special materials required Consult with the DISCIPLINESPECIALISTS early in the design effort to determine what specific pumps are currently available
2 Total Required Capacity - Firm capacity (excluding spares) of the supply pumps should equal the maximum cooling water requirements including LGF and RCF
3 NumberIndividual Pump Capacity - Normally one spare pump is provided for up to six operating pumps two spares for up to twelve operating pumps etc The most common arrangements for water supply pumps are
bull Two 100 capacity pumps
bull Three 50 capacity pumps
bull Four 33-13 capacity pumps
bull Five 25 capacity pumps
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ExxonMobil Proprietary
Section Page
XXVII 18 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Maximum commercially available pump capacity ranges up to 300000 gpm (20000 Ls) at discharge pressures up to 100psi (700 kPa) Maximum capacity will be less for higher discharge pressures Other factors which may influence pump
selection arebull Selecting capacity to match existing pumps
bull Selecting capacity for future requirements
bull Selecting capacity corresponding to manufacturers available standardVendor information should be obtained on the availability size and cost of the pumps for several configurations Acomparison should be made of the investments and operating costs as well as other advantages and disadvantages toselect a pump configuration
4 Drivers - These pumps are usually motor-driven Because of the remote location of the intake facilities the steamdistribution costs are normally too high to justify the use of a steam turbine driver However use of a secondary selectivesubstation provides a reliable source of power to the pump drivers
5 Pump ∆∆∆∆P - Pump ∆P is based on supplying the maximum cooling water requirement at the battery limit design pressureestablished for the system The ∆P should include the total circulating system pressure drop plus any elevation differencebetween the low water level at the intake and the discharge point of contact with air
6 Pump Instrumentation - The remote location of the intake facilities makes remote control and monitoring of the water supply operation desirable Pump controls located in the control house provide for minimum manning Local controlbackup should be provided The Owners operating and manning philosophy determine the basis for instrumentation andcontrolThe spare pump is controlled to automatically start on low pressure in the supply header Concurrently alarms should beactuated in the control room to indicate the low-pressure situation and spare pump operation
Distribution Network1 Configuration - The piping network is a once-through system with a return line from each consumer or consumer group to
the cooling water effluent line2 Layout - The routing of the piping network should connect all cooling water consumers in the most economical manner
permitted by the plant layout Careful consideration should be given to the turnaround philosophy of the plant Thenetwork layout should preclude the necessity of having above-ground water lines that are in service running through anarea that is being turned around Underground piping or an extra lateral may be required to accomplish this objective
3 Valves and Blinds - Battery limit valves should be provided in the network to isolate individual process units from thesystem Valves should be provided to permit onstream removal and maintenance of spared equipment
4 Basket Strainers - Consideration should be given to the installation of basket strainers in the laterals serving each of theconsumer loops These provide added insurance against entrance of foreign material into the cooling equipment The useof intake screens with 025 in (63 mm) openings may eliminate the need for basket strainers in the laterals
Line Sizing1 Design Basis - The network is sized so that water is delivered to the most distant consumer at a pressure equal to the
required process design (battery limit) pressure2 Flow Rate - Each segment of the distribution network is sized for the simultaneous maximum flow in that segment for any
operating condition The main supply and discharge headers are sized for the simultaneous maximum flow of all segments(not necessarily the sum of the simultaneous maximum flows of the individual segments)
3 Pressure Drop
Line Sizing Basis - The distribution system should be sized according to the guidelines established in Section XIV The use of an ldquoold piperdquo friction factor of C = 90 - 100 (Hazen-Williams factors) in the main lines is recommended to assuregood operation in later years when the system experiences fouling For large diameter lines [greater than 48 in (1200mm)] maximum water velocity should be limited to 10 fts (3 ms) based on civilmechanical considerations (supports andanchoring required to handle potential pressure surges) For salt water systems utilizing cement-lined pipe consider thereduced inner diameter in the friction loss calculations A study may be necessary to determine the economic pump ∆P vsline diameters
4 Equivalent Length Factors - Equivalent length factors are used in pressure drop calculations for sizing the distributionsystem The piping length scaled from the plot plan is multiplied by an equivalent length factor to compensate for pipingelbows expansion loops and other piping variations
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Section Page
COOLING WATER SYSTEMS XXVII 19 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
The equivalent length factors are selected based on the expected variation from a straight piping run Typical equivalentlength factors suitable for planning purposes for different piping applications are as follows
TYPE OF PIPING EQUIVALENT LENGTH FACTORS
Long piping runs [gt 1000 ft (300 m)] 12 to 14
Onsite distribution piping 15 to 20
For design purposes the actual piping layout should be evaluated to ensure that the equivalent length factors areapplicable to the specific situation Particular attention is required for revamp work and saltwater systems utilizing cement-lined pipe
STEP 8 - Prepare a system flow plan similar to the one shown in Figure 1
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS
STEP 1 - Determine cooling water requirements
Tabulate the cooling water requirements and cooling water discharge temperature for each process unit and offsite consumer Apply the proper LGFs to all cooling loads depending on the stage of the project and whether the new units are exactduplicates of existing units Determine the total cooling water flow required including LGFs and the bulk temperature (T 1) of thecombined cooling water flow from all coolers to the cooling towerIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The onsite and offsite designers should investigate methods to increase effluent water temperature to themaximum recommended limit for the particular type of water used This will result in a lower cost cooling tower for a given heatrejection rate One common method of maximizing return water temperature is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 2 - Select cooling tower design parameters
Select the proper design conditions of ambient wet bulb temperature approach temperature range and flow rate for specifyingthe cooling tower Based on these conditions the cooling tower vendor will size the tower and provide the tower characteristicand performance curves which serve as the basis for the performance guarantee Required performance test and evaluationprocedures are described in IP 8-1-1 Economic studies may be required to select the ambient wet bulb temperature andapproach temperature for new locations or for expansion projects with special cooling requirements
STEP 3 - Prepare cooling tower specification sheet
Include the following information in the heat exchange section of the design specificationTower Flow Capacity - This was determined in Step 1Type of Tower - The tower usually specified is an induced draft multicell type suitable for fresh salt or brackish water depending on the water available Additional tower characteristics are usually developed and proposed by the tower vendor (eg crossflow versus counterflow type of tower fill etc) The vendor should be made aware of conditions such as thelikelihood of heavy oil leakage into the cooling water or high fouling potential which may influence the tower type or fill
selectionNumber of Cells - The minimum number of cells required (normally not less than 2) may be specified The maximum flow per cell is a function of the cooling tower design conditions selected in Step 2 Typical values range up to 10000 gpm (600 Ls) for crossflow designs and 8000 gpm (500 Ls) for counterflow designs Normally spare cells are not provided Maintenance iscarried out during cooler periods or load is shed
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XXVII 20 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Ambient Wet Bulb Temperature1 The ambient wet bulb temperature selected for specifying the cooling tower design is generally that temperature which is
equaled or exceeded 5 of the time during the four hottest months (5 of the hourly readings taken during the four hottestmonths will exceed this value) Specifying the 5 temperature should give satisfactory results since the time that the wetbulb temperature is exceeded is for short duration and the system inertia is usually sufficient to avoid problems The tower specification should make it clear that the wet bulb temperature specified is an ambient value as opposed to the enteringvalue which may be higher because of recirculation The tower manufacturer should adjust the ambient value upward totake into account his estimates of recirculation Currently cooling tower test codes provide procedures for measuringperformance in the case of either entering or ambient wet-bulb temperature specifications An increase of 2 degF (11 degC) isgenerally added to ambient wet bulb temperature to estimate entering wet bulb temperature for large cooling towers
2 If plant production will peak during the four summer months when the 5 ambient wet bulb temperature may be exceededconsideration should be given to specifying a higher design ambient wet bulb temperature (one that will be exceeded 212 of the time for example) However the incremental cost of the resulting closer approach and larger cooling tower must be justified by the value of the incremental products made
On-Tower Temperature (T1) - This temperature cannot be accurately determined until the design of all process coolers iscompleted and the volume of cooling water and outlet temperature are known for each (see Step 1) In the planning stage or early design stage of a project this temperature must be estimated and is usually set equal to the maximum design cooler outlet temperature shown under COOLING WATER REQUIREMENTS in the BASIC DESIGN CONSIDERATIONS portion of this sectionApproach Temperature - The approach temperature has the most pronounced effect on cooling tower size and cost For example reducing the approach from 10 to 5 degF (55 to 3 degC) can increase the cooling tower investment by 50 percent and thefan operating cost by 65 percent Thus various approach temperatures should be evaluated to determine the minimum cost of the total system including the process exchanger area Approaches used in the past designs have been from 5 to 12 degF (3 to7degC) Note that it is not customary in the cooling tower industry to guarantee approaches of less than 5 degF to (3 degC) to coolingtower design inlet wet bulb temperature A typical approach for a new grass roots cooling tower is 10 degF (6 degC)Off-Tower Temperature (T2) - This is the sum of the approach temperature and the ambient air wet bulb temperature For existing locations Table 2 in Section IX-B may be used
Range and Flow Rate1 The range for a given heat load is related to the circulating water rate as follows
Q = M C p ∆Tcong (500) (gpm) (range) Eq (1)
where Q = Heat load on the tower BtuhM = Water circulating rate lbhCp = Specific heat capacity of water at average temperature Btulb degF∆T = (T 1 ndash T 2) degF = (water cooling range)gpm = Circulation rate in US gallons per minute
For calculation in Metric units
Q = M C p ∆Tcong (1162 x 10 -3) (Ls) (range) Eq (1M)
where Q = Heat load on the tower kJh
M = Water circulating rate kghCp = Specific heat capacity of water at average temperature kJkg degC∆T = (T 1 ndash T 2) degC = (water cooling range)Ls = Circulation rate in liters per second
2 Generally lower tower investment fan horsepower and pump horsepower result from maximizing the range andminimizing the flow rate However based on fouling considerations of process coolers the maximum allowable hot water temperatures must be kept below recommended limits Also while increasing the range minimizes cooling tower investment and operating cost the cost of exchanger surface requirements is adversely affected by the LMTD and acompromise must be made The range is bounded by the maximum allowable hot water temperature and the Off-Tower temperature (T 2) For established locations refer to Section IX-B for design cold water temperatures to assist in selectingthe approach and range
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Section Page
COOLING WATER SYSTEMS XXVII 21 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Makeup Water Requirements - Calculate the various losses in the cooling water system to determine makeup requirementsThese losses include
1 Evaporation Loss (E) - Allow 1 of the circulation rate for each 10 degF (55 degC) temperature drop through the tower Notethat the actual evaporation loss will be somewhat less depending on ambient air temperature
( )sL555
TT)gpm(
1000TT
E 2121
minus=
minus= Eq (2)
2 Drift Loss (D) - IP 8-1-1 limits the maximum allowable drift loss to 0005 of the design water circulation rate for freshbrackish or salt water
( )sL1000050
)gpm(1000050
D
=
= Eq (3)
This low drift level contributes little to solids removal from the tower with the spray This allows higher cycles of concentration to be achieved in cooling towers which use clarified softened water for makeup and thus have low requiredblowdown rates Higher drift loss percentages in older or poorly maintained towers will limit the maximum cycles of
concentration achievable3 Miscellaneous Losses (O) - Miscellaneous losses are estimated to be one-seventh of the difference between evaporation
loss and drift loss
minus=
7DE
O Eq (4)
4 Blowdown Rate (B) ndash Guidelines for maximum concentrations of impurities allowed in recirculated fresh cooling water areshown in Table 2 For salt water the blowdown is based on keeping the total dissolved salts below 55000 wppm Obtainthe chemical analysis of the makeup water and divide each individual component concentration into its counterpart inTable 2 The result is the maximum allowable cycles of concentration for that constituent (a worksheet is provided inTable 4 ) The impurity with the lowest factor will be the control and will determine the required blowdown rate
)OD(1Cycles
EB +minus
minus= Eq (5)
The makeup water requirement is the sum of all losses shown above
MakeUp = E + D + O + B Eq (6)
STEP 4 - Specify the controls necessary for proper cooling tower operation
Air Side - Cooling towers are provided with motor-driven manually adjusted pitch multiple-bladed propeller fans Figures9A amp B may be used to estimate the normal operating horsepower requirements Since the fan motor and gear reducer arelocated at the top of the tower [generally 40 to 50 ft (12 to 15 m) above grade] the need for equipment to handle thesecomponents should be discussed with the affiliateControl of the air flow through the tower to maintain reasonably constant cold water temperatures can be accomplished byturning off individual fans in multiple-cell installations adjusting the blade pitch or using variable speed motors To turn off individual fans remote at grade start-stop buttons for the motors are usually specified
Whenever the ambient air temperature is 32 degF (0 degC) or below icing of the tower louvers and structural members in contact withthe incoming air will occur If the heat load on the tower is relatively free of seasonal variations two-speed fans should provideadequate ice control down to approximately 0 degF (-18 degC) ambient temperatures When sustained temperatures of 0 degF (-18 degC)and below occur two-speed fans capable of two-speed reverse operation with appropriate time delays for change of directionand speed should be specified to periodically eliminate ice which will build up on the louvers Other methods of winterizationare available but in general are more expensive and require greater operator attention In very cold climates steam heatingshould be provided to prevent freezing in the cold water basin during shutdown periodsWater Side - The water circulation rate is maintained by operating one or more pumps of sufficient capacity to meet the systemdemand Flow-measuring facilities local temperature indicators and a high-temperature alarm are provided on the coolingtower water supply and return headers Hand-operated flow control valves are required for distributing the water equally to theindividual cells On counterflow towers provisions for measuring the flow to each cell (pitot tubes) are required They are notrequired on crossflow towers since the open distribution basin provides visual aid in balancing flows Shutoff valves should beprovided for servicing the flow control valves without shutting down the entire tower
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XXVII 22 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
STEP 5 - Provide chemical treatment facilities for the fresh water system
Alkalinity (pH) Control (if used)1 In cases where a controlled pH corrosionscale control program is being used the alkalinity (pH) of the recirculated cooling
water should be controlled in accordance with the program requirements Normally acid addition is required to accomplishthis This is due to free carbon dioxide being stripped from the cooling water in the tower to approximately 5 ppm For example if the alkalinity has to be controlled to between 25 and 50 ppm for a pH of 70 to 75 the volume of 98 sulfuricacid (66 degBeacute) required to treat the makeup water is calculated using the following equation
[Note 93 sulfuric acid should be used if ambient temperatures will be less than 35 degF (2 degC)]
GallonsHour 66 degBeacute H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate gpm) (326 x 10 -5)
For calculation in Metric limits
kgh of 100 (66 degBeacute) H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate Ls) (793 x 10 -3)
2 The metering pump should be designed for a maximum feed rate of two to three times the normal feed rate A glass woolfilter is required on the pump suction piping to minimize plugging Normally two - 100 capacity pumps are provided
Positive displacement metering pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumps with flow control areused for higher rates
3 Hydrochloric acid may also be used for alkalinity control The gallons per hour of 20 degBeacute hydrochloric acid required is36 times the amount of 66 degBeacute sulfuric acid In Metric units the weight of 32 (20 degBeacute) hydrochloric acid required is227 times the weight of 100 (66 degBeacute) sulfuric acid
4 Specifications for chemical feeders may be found in EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased)
5 A pH analyzer with high and low alarm is provided to control the amount of acid feed The acid should be added to thecooling tower makeup water (Figure 2 ) The lag time between the point of acid injection and the sample outlet to the pHcell should be less than two minutes for effective control
6 Spills or overflows from the sulfuric acid tank should be contained
Corrosion Inhibitors and Anti-Scalents1 Two chemical feeders are generally provided The following guides in sizing the equipment will allow the use of any of the
proprietary products from the vendors when final selection by the owner is made See Report No EE102E78 Cooling Tower Water Treatment Guidelines EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and Report No EE34E86 Environmentally - Acceptable Cooling Water Treatments for detailed discussion of acceptable commercial chemicals
bull If the water treatment program to be used is known design for the chemical rates specified by the vendor or COOLING WATER SYSTEMS SPECIALIST
bull Otherwise design for 100 wppm of each commercial liquid product in the recirculated cooling water The design feedrate should be based on the design total blowdown flow (both uncontrolled and controlled) since this is where theproduct is lost For 100 wppm concentration the chemical rates will be as follows
GallonsHour Chemical = Blowdown (gpm) x (0006)
LitersHour Chemical = Blowdown (Ls) x (036)
2 The design capacity of the metering pump should be two to three times this calculated number Normally 2 - 100capacity pumps are provided Positive displacement pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumpswith flow control are used for higher rates
3 Specifications for chemical feeders may be found in EMRErsquos Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased) Reference should also be made to the checklists containedin Report No EE92E94 Guidelines for Safety Evaluation of Chemical Injection Facilities
4 A final check should be made with the owner and the COOLING WATER SYSTEMS SPECIALISTS as to the treatmentprogram selected and if there is a need for an additional chemical feeder
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COOLING WATER SYSTEMS XXVII 23 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Biological ControlFacilities should be provided for feeding the biocide into the back of the cooling tower basin If chlorine is used it must bepresent in the cooling water as free residual to be effective However the following amounts of chlorine are first required toreact with the listed impurities when present in the cooling water before a free residual can be established
IMPURITIESAMOUNT OF CHLORINE REQUIRED (wppm)
WITH EACH WPPM OF IMPURITY
Ammonia (NH 3)
Hydrogen Sulfide (H 2S)
Sodium Sulfite (Na 2SO 3)
Ferrous Iron (Fe ++)
10
22
056
064
The design capacity of the chlorine feeder shall be based on achieving a 01 wppm free chlorine residual in the recirculatingwater flow or a minimum dosage of 7 wppm at maximum design makeup water flow rate and conditions (see EMRE Water and
Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service )Chlorine may be fed either as the commercial liquid sodium hypochlorite (124 wt Cl 2) or liquid chlorine (100 Cl 2) Liquidhypochlorite is being used increasingly because of the higher costs associated with precautions equipment and proceduresrequired to minimize the risks associated with handling gaseous chlorine The hypochlorite solution is fed using a meteringpump which should be sized to feed an equivalent of 15 ppm 100 Cl 2 (Refer to EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) ) In case of spills or an overflow from thestorage tank the sodium hypochlorite may be sent to the cooling tower basinGas chlorinators are covered by EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service The use of gas chlorination is being reduced but may still be appropriate in some locations subject tolocal management approval The gas from either commercial cylinders or an evaporator is dissolved in a water slip stream bymeans of an eductor This results in a chlorine solution strength of approximately 01 This solution should be evenlydistributed below the water surface by means of a header lateral pipe arrangement at the rear of the cooling tower basinopposite the cooling water pumpsSome additional guides on the use of gas chlorinators are given below It is also recommended that the safety guidelines
published by the Manufacturing Chemists Association and the Chlorine Institute be followedbull Dilution water for the chlorinator must not be less than 50 degF (10 degC) or ice formation may cause plugging problems
bull Continuous rates of chlorine gas withdrawal should not exceed 400 lb (180 kg)24 hours from each one ton cylinder atroom temperature [70 degF (21 degC)] This rate may be exceeded (up to 50) for periods not exceeding 2 hours Rule of thumb is not to design for more than 4 cylinders operating in parallel When feed rate exceeds 3000 lb (1360 kg) per daythe liquid chlorine should be fed to an evaporator upstream of the chlorinator
bull If the ambient temperature can be expected to be below 50 degF (10 degC) for extended periods a building should be providedfor the chlorinator and cylinders
bull Weighing scales should be provided to determine when shipping containers should be replaced and to verify chlorine feedrates
bull Analyzers to measure the free chlorine residual in the circulated cooling water should be provided
bull Chlorine leak detectors and a catch basin for major spills should be provided
Control SystemProvide automatic controllers to continuously monitor water condition and to add chemicals to the system (complete systemsmay be available through the water treatment program vendor as a lease or purchase)pH Control - This controller adds acid to correct water alkalinity a timer on the acid pump sounds an alarm if the pump doesnot shut off in a predetermined length of time Backup controls are required to prevent the cooling system from beingpermanently damaged in case the pH electrodes become fouledCorrosivity and Inhibitor Control - Systems are available which give a direct readout of corrosion rate in milsyear If thecorrosion rate exceeds a set point the controller shuts off the acid increases the inhibitor addition opens the blowdown valveand sounds an alarm Electrodes measure the completeness of the inhibitor film by the value of the potential differencebetween the electrodes The larger the difference the more incomplete the film and a potential reading exceeding a set levelactivates the inhibitor feed
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XXVII 24 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Free Residual Chlorine Control - Free residual chlorine should be controlled automatically for biocide treatment Severalgood analyzers are currently available These units can be incorporated into an automatic control system
Oil in Water DetectionMonitoring Corrosion and Deposit FormationThe following types of devices for measuring corrosion rates and deposit formation should be provided to allow the plant tomonitor the effectiveness of the cooling water treatment before scheduled turnaroundsCoupons - A corrosion coupon rack should be specified for the hot return water to the tower in accordance with IP 19-6-1 Corrosion Monitor - On-line corrosion analyzer if specified should monitor the hot return water to the tower in accordance withIP 19-6-1
On-Line Test Exchanger Although there are a variety of deposit monitors from the water treatment chemical vendors they do not necessarily representactual heat transfer conditions in process exchangers For this reason a test exchanger should be installed for evaluatingcorrosion and deposit formation Details are covered in Report No EE102E78 Cooling Tower Water Treatment Guidelines
Alternatively a critical process exchanger can be instrumented such that its heat transfer coefficient can be monitored
Instrumentation would include flows and inlet and outlet temperatures of process fluid and cooling waterSTEP 6 - Specify cooling tower location
Select a location for the cooling tower The following factors influence this selection
Location Factors1 The location should be within a reasonable distance of the units served and consistent with future refinery plans Piping
costs to and from battery limits can be a significant portion of the total cooling system cost and can be the location-determining factor particularly for salt water systems While piping costs can be easily determined other costs affectinglocation cannot be estimated readily Therefore judgment is required in selecting the location
2 Safety spacing requirements (see Section XV-G Equipment Spacing )3 Cooling tower contribution to plant boundary noise4 For salt water cooling towers wind direction and distance to other facilities is of major concern because of potential
corrosion problems
5 Factors affecting cooling tower performance ie orientation with respect to wind site congestion etc
Plot RequirementsIn estimating plot requirements (basin area) one should allow 5 to 6 ft of tower length for each 1000 gpm (2 m for each 60 Ls)of circulating capacity This figure is based on a typical counterflow design with a width of 50 ft (12 m) and does not includepump suction pit space Exact requirements can be obtained from cooling tower vendors after the tower capacity has beenestimated
Fogging and DriftTwo distinct problems are associated with the discharge air from a cooling tower (1) fogging which can be a nuisance or ahazard and (2) drift which is generally a nuisance because of increased corrosion of surrounding equipment Fogging fromcooling towers occurs during periods of combined high ambient humidity and low temperature as a result of mixing the warmmoist cooling tower exhaust air with the cooler ambient air The ambient air cannot absorb all moisture as vapor and thus tinyfog droplets are formed by condensation Fogging and resulting icing in cold climates has produced hazardous drivingconditions on highways up to 850 ft (250 m) awayProprietary designs are available to combat potential fogging problems Basically these designs incorporate heating of thedischarge air with hot return water precooling coils (similar to an air cooler) to eliminate any visible plume However thesedesigns are expensive and relatively unproven Therefore proper location based on reliable weather data is the preferredsolutionDrift from a cooling tower represents a more difficult problem since it does not depend on definable climatic conditions andoccurs for all wind directions The effects of drift on nearby equipment and vegetation are particularly severe and generally of concern only for salt water systems because of the high solids content of the drift (concentrated salt water) For salt water cooling systems electric lines substations and instrumentation should be located upwind in the direction of the predominantwind They should be provided with adequate shelter or materials when within 450 ft (140 m) of the cooling tower In additionspecifications for all equipment within 150 ft (45 m) of the tower should include surface treatment suitable for a marineenvironment
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COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
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DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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XXVII 28 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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XXVII 30 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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DESIGN PRACTICES December 2000
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TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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DESIGN PRACTICES December 2000
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FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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DESIGN PRACTICES December 2000
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FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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DESIGN PRACTICES December 2000
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FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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XXVII 8 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
COOLING WATER REQUIREMENTS
Cooling water requirements for each process unit and offsite consumer are obtained from the appropriate design groups For existing locations maximum cooling water loads should be based on the design inlet cooling water temperatures given inSection IX-B Design Considerations for All Types of Heat Exchangers For new locations the inlet cooling water temperaturemust be set based on local water temperature data for once-through systems and wet bulb temperature and selected approachfor recirculated systemsThe bulk water temperature at the cooler outlet has been selected as the controllable variable for design and operation of coolers Although the corrosion rate depends on the metal temperature this is difficult to determine in the operation of a coolerBased upon the findings of a Materials Development Section test program maximum design outlet water temperatures havebeen set as indicated below
Fresh Water 130 degF (54 degC)
Treated Fresh Water 130 degF (54 degC)
Brackish Water 125degF (52
degC)
Salt Water 120 degF (49 degC)
When the maximum plant cooling water requirement has been established for new process and utility consumers appropriateLoad Growth and Reserve Capacity Factors (LGF and RCF) should be applied depending on the development stage of theproject Cooling water loads should be updated and system capacity adjusted periodically as design work progresses
LOAD GROWTH AND RESERVE CAPACITY
Utility loads tend to increase as the loads become better defined through successive stages of project definition Thereforeload growth and reserve capacity factors (LGF and RCF) are applied to raw loads as a means of predicting final equipmentsizes and capital costs These load growth factors do not cover basis changes through project development such as project or process unit sizethroughput For revamp or modernization projects these factors are generally applied only to new loads or changes to existing loads on the basis that existing or unchanged loads are well known
Recommended Load Growth Factors are as follows
CONSUMERSPLANNING
(SCREENING THROUGH DBM) DESIGN SPEC
Existing Facilities 0 - 10 0 - 10
New Facilities 20 - 30 5 - 10
Selection of the load growth factor to use should be based on the quality and level of effort associated with development of theraw loadsFinal sizing of the source facility (eg the cooling tower and circulation pumps) should include a reserve capacity factor of 10to cover small post startup changes minor future projects and dynamic conditions not predicted by steady state balances Thesizing basis for distribution facilities should not include the RCF
COOLING TOWER TYPES
The cooling tower is the major piece of equipment specified in recirculating cooling water systems In effect it is the heatexchanger which cools the hot water to a suitable temperature for re-use primarily by evaporation through contact with airThe cooling tower contains fill to break up the water flow into droplets or film on a surface The fill increases the waterair contact area and enhances heat transferCooling towers are classified according to the means by which air is supplied to the tower ie induced or forced mechanicaldraft (fan) vs natural draft and according to the relative movement of the air and water that is counterflow or crossflow (seeFigure 3 ) Most of the towers in the ExxonMobil circuit are mechanical induced-draft type Economics determine whether theyare counterflow or crossflow
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COOLING WATER SYSTEMS XXVII 9 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
In crossflow towers the air travels horizontally across the falling water while in the counterflow design the air travels verticallyup through the falling water Comparisons of plot space fan horsepower and future maintenance costs determine which
design will be more economical Factors which influence this selection are cooling water rate range and approach for aparticular location In addition in some locations materials of construction and associated labor costs will affect the finalselectionIn the past crossflow towers were usually preferred over counterflow towers for extremely cold locations where ice formationcould restrict or block air flow through the louvers However this consideration is usually no longer valid due to the increaseduse of PVC film-type tower fill rather than splash-type fill in counterflow towersMechanical draft designs use one or more fans to provide the air needed for cooling Induced-draft units have the fans locatedon top of the tower Air is pulled through the packing and discharged vertically upward at high velocities Forced-draft unitshave the fans located at the base of the tower and push the air through the fill Forced-draft towers in large industrial sizesexperience difficulties with recirculation and in providing a uniform distribution of air As a result they have been supersededby induced-draft designsFill may be splash or film type Splash fill normally consists of horizontal slats in horizontal rows offset to one another to causethe water to break up into droplets as it falls downward through the cooling tower Splash fill is characterized by reduced air pressure losses It is also less conducive to clogging and easier to clean after a spill However it is very sensitive to
inadequate support and must remain horizontal and level It is made of treated wood or plastics such as PVC or polypropyleneFilm fill comes in various designs but they all cause the water to flow in films over the fill surface and provides more coolingcapacity within a given amount of space than splash fill Because of this increased heat transfer efficiency film fill is generallyused in most new cooling towers However because of its smaller passages it is more sensitive to plugging and is difficult toclean if it does become plugged It may not be appropriate for services cooling mainly heavy fuels or waxy lubes where a leakwould cause fill plugging or for services with a high biological fouling potential PVC is currently the most widely used materialfor film fill The selection of fill type is usually proposed by the vendor and reviewed by the COOLING WATER SYSTEMSSPECIALISTSNatural-draft or hyperbolic cooling towers depend on the natural draft created by the difference in the density of the enteringand leaving air for movement Generally hyperbolic cooling towers have been used in Europe and in the utility industry in theUSA for large capacities [250000 gpm (16000 Ls)] short ranges [10 to 15 degF (6 to 8 degC)] and long approaches [10 to 15 degF (6to 8 degC)] They are most effective where low ambient wet bulb temperatures occur with high relative humidities These towersnormally have diameters in excess of 200 ft (60 m) and are greater than 250 ft (75 m) in height Their major advantages are
bull No operating or maintenance cost for fans
bull Reduction in problems associated with fogging drift and recirculation due to their great height and diameterThe primary reasons why hyperbolic towers are not used in ExxonMobil projects include
bull High initial cost
bull Large plot space requirement and high visual impact
bull Complete dependence on atmospheric conditions
bull Lack of flexibility for future expansions
COOLING TOWER BASIN
The primary function of the cooling tower basin is to collect the cooled water leaving the tower and to provide a reservoir for thecooling water pumps Design aspects to be considered are the basins capacity cleanability draining facilities sump andscreen details To obtain the necessary time for corrective action during emergency conditions a minimum storage capacity of 90 minutes based on loss of makeup water flow should be provided in the basin between the basin low level alarm (LLA) andthe minimum pump submergence (MPS) When the basin water level reaches the MPS point loss of recirculation pumpsuction and cooling water flow is imminent Typical values for basin water levels are as follows
bull High Level Alarm (HLA) 1 ft (300 mm) below top of the basin wall
bull Normal Water Level 15 ft (450 mm) below top of the basin wall
bull Low Level Alarm (LLA) 25 ft (760 mm) below top of the basin wall
bull Distance between LLA and MPS 35 ft (1070 mm)
bull Minimum Pump Submergence Depth 45 ft (1370 mm)
bull Clearance of Pump Suction off Basin Floor 15 ft (450 mm) Actual values may vary depending on pump type selected pump suction line diameter and cooling tower cell plot area
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XXVII 10 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
MATERIALS OF CONSTRUCTION
The EMRE Refinery Construction Materials Manual and IP 8-1-1 list acceptable materials for all components in each type of cooling water system (once-through and recirculating) For salt water systems emphasis must be placed on proper materialsselection throughout the system The external salt water environments in the vicinity of the tower itself and the intake stationsmust be taken into consideration Proper siting of the tower is extremely important to avoid salt water corrosion of adjacentunits or the need to use expensive materials to operate in such an environment
WATER TREATMENT FOR ONCE-THROUGH SYSTEMS
Once-through salt and fresh cooling water systems can develop a variety of problems such as fouling due to seaweed or other debris marine microorganisms such as muscles and shell fish and biological growth Corrosion and scaling problems areminimal in fresh water once-through systems because the water is not concentrated by evaporation In salt water systemscorrosion is minimized by proper materials selection and scaling problems generally do not occurProtection against fouling by seaweed or other debris is provided by careful selection of the water intake location and travelingscreens with washing facilities In some cases additional steps may be required These may include an additional secondaryset of traveling screens and possibly battery limit strainers on the cooling water supply lines to individual unitsProtection against fouling by marine microorganisms and biological growths is achieved primarily by injecting chlorinehypochlorite chlorinebromine or other similar chemicals which kill these growths Chlorine is commonly used and is injectedin sufficient quantities to produce a residual of 1 wppm in the effluent for one hour each day (shock chlorination) Continuouschlorination for 2 - 3 days per month at lower levels (03 wppm residual chlorine in the effluent) may also be used in addition toshock chlorinationThe large water volume discharged from once-through systems usually makes chemical treating for corrosion or deposit controltoo costly These problems are handled by proper materials selection and by keeping water velocities through exchanger tubesgreater than 3 fts (09 ms) Occasionally the water analysis of a fresh water source being considered for a once-throughsystem using carbon steel materials will indicate the need for some corrosion or deposit control In this case the cost of thethreshold treatment with the once-through system should be compared to the costs of a recirculated system before making afinal choice
WATER TREATMENT FOR SALT WATER COOLING TOWER SYSTEMS
A total dissolved salts (TDS) concentration limit of 55000 wppm for sea water in cooling tower systems requires high blowdownrates since the makeup sea water typically has from 35000 to 40000 wppm TDS Because of cost restraints the only water treatment normally specified for these systems is for biological control Chlorine is most commonly used either as directchlorine injection or hypochlorite produced in electrochlorinators Care must be taken in selecting the intake location to assurethat the suspended solids do not exceed 200 wppm in the recirculated water If it is not possible to find sea water with sufficientclarity clarification with polyelectrolytes should be evaluated Corrosion is one of the major problems with the use of sea water in cooling towers This requires proper materials selection
WATER TREATMENT FOR FRESH WATER COOLING TOWER SYSTEMS
Cooling tower water treatment is necessary to minimize or eliminate corrosion scale and biological fouling of process heattransfer surfaces caused by minerals and impurities in the water The difficulties caused by these impurities and the means of treatment are shown in Table 1 Water lost by evaporation results in an increase in total dissolved salts in the recirculated water To indicate the degree of concentration of impurities the term cycles of concentration is used
Corrosion is an electrochemical process that deteriorates metals exposed to water in the presence of corrosive agents such asacids oxygen or bacteria A common form of corrosion is pitting Severe corrosion can lead to equipment failure Corrosionis caused by many conditions including process leaks into the cooling water water flow velocity that is too low (causes depositsand fouling lending to corrosion) or too high (causes erosion and corrosion) low pH and high temperatures Corrosion is lesslikely in water with non-acidic pH values (greater than 7) although scaling is more likely in these pH rangesScaling is characterized by the formation of hard dense deposits on material surfaces These deposits impact heat transfer and can become a site for localized under-deposit corrosion Scaling is influenced by many factors including makeup water composition total dissolved solids in the recirculating water water temperature and pH and flow velocity Calcium carbonateis the main scaling constituent in all waters and it is the least soluble It forms from the hardness (calcium Ca ++ ) andbicarbonate (HCO 3
ndash ) alkalinity present in most waterFouling can be caused either by the settlement of suspended matter or by microbiological growth of algae bacterial slime or fungi Fouling will reduce heat transfer cause plugging and create corrosion sites
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COOLING WATER SYSTEMS XXVII 11 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Treatment of the cooling water is broken down into two major categories external and internal External treatmentencompasses any treatment given to the cooling tower makeup water Internal treatment is the addition of chemicals directly to
the recirculated cooling water
EXTERNAL TREATMENT
External treatment to remove impurities in order to reduce the amount of blowdown should always be evaluated as to costeffectiveness The most frequently used techniques for treating makeup water are shown in Figure 11 Details of availabletreatment processes (see Table 1 for methods available) are covered in Section XXVI-A Boiler Feedwater Treatment For cooling tower systems that meet currently required drift limits in refineries and chemical plants uncontrolled cooling water losses (drift and miscellaneous process uses) normally amount to about 14 of the cooling water evaporation loss Under these conditions the maximum cycles of concentration achievable are 8 with no controlled blowdown Some cooling towersare operating at up to 10 cycles of concentration Additional controlled blowdown required to limit the cycles of concentrationper the water quality criteria is covered later
INTERNAL TREATMENT
The makeup water analyses amount of cooling water losses environmental restraints potential chemical hazards materials of construction and film temperatures of heat transfer surfaces determine the type of internal treatment required for cooling tower systems The internal water treatment program selected must address corrosion scale fouling biological growth andchemical attack of cooling tower componentsNumerous chemicals and treatment programs are continuously being developed for addressing the potential problems Theslightly acidic low pH programs of the past which used chromate based chemicals and usually required acid injection arebeing completely phased out because of environmental restrictions on chromate and the higher costs associated with meetingcurrent practices for handling hazardous chemicals such as acid Non-chromate treatments which use inhibitors such asphosphates zinc and organics along with higher pH have been developed to replace the earlier programs Although not aseffective as chromate these inhibitors operate at higher water pH which is a less corrosive environment The disadvantage of the higher pH is the greater tendency to form scale This tendency is controlled by the use of dispersants or scale inhibitorswhich are normally organic polymers or phosphenate A corrosion inhibitor for copper or brass is also normally required if these metals are present in the systemThe following paragraphs describe techniques and chemicals to control alkalinity (pH) corrosion scale sludge and biologicalgrowth The chemicals mentioned are normally purchased as pre-mixes from vendors and dosed in combination Mostcorrosion and scale control program chemicals are two drum formulations due to the incompatibility of some chemicals in theconcentrated state Therefore separate feed systems are generally provided for chemicals for control of corrosion and scale (2drums) and biological growth (if liquid) If a program includes fixed pH then an additional feed system is required to add acidWater treatment programs are site specific and changing The COOLING WATER SYSTEMS SPECIALISTS should beconsulted for recommendations on the internal water treatment programs when an analysis of the available water is known
Alkalinity (pH) Control
Cooling water treatment programs presently available fall into two basic categories those that do not control pH (floating pH or alkaline treatment programs without pH adjustment) and those that control pH within a specified range by adding acid Theprograms that do not control pH are gaining favor because of the additional cost equipment and precautions required tominimize the risk associated with handling acid and because of improvements in chemicals to control depositsHigh pH in the recirculating cooling water is a result of the alkalinity of the fresh water due to bicarbonate salts This combinedwith carbon dioxide results in the water having a pH range of 6 to 85 When this fresh makeup water is mixed with therecirculated cooling water and passed through the cooling tower the air flow strips out the carbon dioxide Bicarbonate ionsthen convert to carbonate ions raising the pH which can cause both calcium carbonate and calcium phosphate salts toprecipitate out of solution and form deposits Another debit in allowing the cooling water pH to increase is increased chlorineconsumption to control biological growthsIf a program using pH control is justified and chosen acid addition is used under control of a pH analyzer to controlrecirculated cooling water alkalinity and pH to the range required by the treatment program
Corrosion Control
To reduce corrosion to an acceptable level chemical corrosion inhibitors which form protective films on heat transfer surfacesare the most effective protection Inhibiting corrosion is accomplished by phosphates organics zinc nitrites and molybdatesalts Unfortunately the use of chromate which is a reliable corrosion inhibitor is prohibited by environmental constraintsNitrite is not practical in an open system due to atmospheric oxygen converting the nitrite to nitrate
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XXVII 12 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Phosphate inhibitors (orthophosphate polyphosphate phosphonate) are the basic treatment for new towers in todaysenvironment In addition to the phosphate chemistry salts of zinc may be added to aid in inhibiting corrosion These ions
combine with the hydroxide ion forming thin protective films Zinc is used to provide a synergistic effect in combination with thephosphate salts for protecting carbon steel surfaces from corrosion Finally all-organic programs are offered by severalmanufacturers The use of these treatments has been made possible by the development of polymer dispersants which givecalcium salts a much higher solubility and reduce the tendency to scaleWhen a cooling water system contains copper alloys azole based and azole substitutes are required as copper corrosioninhibitors There are several effective inhibitors The most common is tolytriazole (TTA) due to its resistance to chlorineThe corrosion inhibitors most frequently used in ExxonMobil recirculating cooling water systems are shown below indescending order of preference provided their use meets all environmental requirements
bull Stabilized Phosphate (OrthoPolyphosphate) - Environmental restrictions on the use of zinc have resulted in ExxonMobilplants in the USA using a blend of orthophosphate and polyphosphate type corrosion inhibitors with good results Thistreatment forms a thin protective calcium phosphate film on heat transfer surfaces through the use of orthophosphateconcentrations as high as 12 - 17 wppm The use of calcium phosphate dispersants and Hydroxethylidene Diphosphonate(HEDP) stabilizes the treatment and prevents the formation of calcium phosphate scale Concentration of the dispersantsdepends upon the calcium concentration temperature and pH Stabilized Phosphate treatment programs can operate in apH range of 68 to 78 Alternatively an alkaline (floating pH) stabilized phosphate treatment program can be used Thisprogram uses little or no acid addition and is gaining in popularity although not well established yet It operates with pHvalues of 8 - 9 and uses higher quantities of dispersants and lower levels of phosphate A corrosion inhibitor for copper and brass is normally included for all stabilized phosphate programs Several vendors offer these programs Twoseparate chemical feeders are generally required
bull Zinc Alkaline Phosphate - Heritage Exxon plants in Europe where the use of zinc is not restricted have obtained goodcontrol of corrosion and scaling using zinc-polyphosphate treatment programs Low (65 - 72) and high (73 - 80) pHprograms have been used The high pH (zinc alkaline phosphate) programs are the lowest cost and are recommended
As with the Stabilized Phosphate treatment dispersants such as HEDP and polyacrylates are required to prevent theformation of zinc or calcium phosphate scale on heat transfer surfaces A corrosion inhibitor for copper or brass isnormally used with this program which is offered by several vendors
bull Organic Salts - All-organic (polymer) programs are offered by several manufacturers and have been used successfullywithin h Exxon In general these treatments are based on a combination of a phosphonate to control corrosion polymers(dispersants) to prevent scale deposition and a corrosion inhibitor for copper or brass This type of program is mosteffective in conditions of high hardness high alkalinity (M alkalinity gt300 wppm as CaCO 3) and high pH values (gt 85)However some manufacturers have reported success in softer water with alternative polymer component formulations Ingeneral an inorganic corrosion control program is preferred to the all-organic programs which are sensitive to alkalinityand more costly
Detailed control limits for effective use of each of the above inhibitors and materials selection are covered in Report NoEE34E86 Environmentally Acceptable Cooling Water Treatments Report No EE102E78 Cooling Tower Water Treatment Guidelines and in the EMRE Refinery Construction Materials Manual
Scale and Sludge Control
Scale and sludge deposits in cooling water systems cause loss of heat transfer and provide the conditions for pitting corrosionScale consists of inorganic salts which have precipitated from the cooling water forming a coating on heat transfer surfacesSludge consists of organic and inorganic suspended matter that settles in quiescent areas and in heat exchangers The key toprevention of both scale and sludge formation in a cooling system is keeping water velocities greater than 3 fts (09 ms) andthe use of chemical additives in combination with a blowdown rate that keeps the impurity concentration below the level whichcauses deposits (See Table 2 for limits on impurities) Scale control has become increasingly important in higher alkalinityldquofloating pHrdquo water treatment programsSalts most commonly found in scale deposits are calcium carbonate calcium sulfate tricalcium phosphate zinc phosphatemagnesium silicate and silica The control method for preventing scale from each of these salts is discussed below
bull Calcium Carbonate - Where justified using pHalkalinity control in the range of 70 to 75 will normally inhibit the formationof calcium carbonate scale if anti-scalent chemicals (phosphonates acrylates copolymers) are part of the chemicaltreatment (See Report No EE102E78 Cooling Tower Water Treatment Guidelines ) Higher pH water treatmentprograms require greater concentrations of these chemicals along with dispersants and flocculants to inhibit the formationof scale and precipitation of other salts
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COOLING WATER SYSTEMS XXVII 13 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
bull Calcium Sulfate - When heat transfer skin temperatures are below 250 degF (120 degC) and the cooling water bulktemperatures are under 130 degF (54 degC) gypsum scale (CaSO 42H 2O) will form if the product of the calcium and sulfate ionsconcentrations each expressed as wppm CaCO 3 equivalent exceeds 500000 Gypsum scale is prevented bymaintaining bulk water temperature below the indicated limits and controlling calcium and sulfate ions through blowdownor through the use of newer dispersants on the market
bull Tri-Calcium and Zinc Phosphate - Almost all chemical additives for corrosion control contain some type of phosphatesalt These have a tendency to form the orthophosphate ion depending on time pH and temperature Cooling watersusing these corrosion inhibitors may have an orthophosphate concentration in excess of 5 wppm as PO 4
= To preventcalcium phosphate scale Table 2 shows a limit of 1000 wppm of calcium as CaCO 3 equivalent when using a phosphateinhibitor in combination with a chemical dispersant (See Report No EE102E78 Cooling Tower Water Treatment Guidelines for solubility of calcium phosphate) These inhibitors and dispersants generally fall into one of the followingcategories
ndash Sequestrants antiprecipitants and inhibitors such as hydroyethylidene diphosphonate (HEDP) phosphate esters andaminomethylene phosphonate (AMP) function by defyingdistorting the crystal structure of the scale formations toprevent nucleation and growth of the scale
ndash Dispersants based on either anionic or nonionic polymers such as polyacrylamide polyacrylate or polymaleic acidfunction to stabilize colloidal and scale-forming particles in suspension Anionic polymers prevent agglomeration of suspended particles by either charge repulsion or crystal distortion while nonionic polymers reduce the surfacetension of particles
bull Silica Deposits - Quantitative indices for determining limits for silica in combination with other ions are not available due tothe formation of complex polymers Instead the rule of thumb limit of keeping reactive silica concentrations in the coolingwater below 150 wppm as SiO 2 has been successful
Possible alternatives for controlling sludge are
bull Chemical Dispersants - Water treating chemical suppliers now offer many types of dispersants These are mostlypolyelectrolytes Claims for removal of slime silt and mud from cooling tower basins and exchangers are madeHowever satisfactory results using these chemicals as a coagulant aid require very accurate control of the amount fed tothe water and stable operation of the water system Also suspended solids should be kept below 200 wppm for particleslarger than 045 microns Dispersants are used in virtually all fresh water cooling tower systems despite their high cost
bull Sidestream Filtration - The use of a sand pressure filter is the most reliable method for sludge removal Operating andmaintenance costs are minimal for this equipment
bull Higher Blowdown Rates - This may be an economical alternative depending on the cost of makeup water Additionalchemical costs must be included in any evaluation of this method
bull Makeup Clarification - This alternative may require additional raw water treatment facilities to handle seasonal variationsin turbidity andor suspended solids This method is applicable only where suspended solids come from makeup water
Biological Control
Operating conditions in recirculated cooling water are ideal for the growth of biological matter Water temperatures from 70 to120 degF (20 to 50 degC) favorable water pH continuous supply of nutrients including carbon nitrogen and phosphorus inorganicsalts and sunlight all lend themselves to environmental conditions encouraging microbiological growth and plant lifeIf biological growths get out of control and form large sticky agglomerations some of the following operating problems willresult
ndash Fouling of heat transfer surfaces by bacterial slimes resulting in flow restrictions and high process temperatures ndash Reduced cooling tower efficiency resulting from algae fungi and bacterial slime growths in the water distribution basin
and fill area of the cooling tower ndash Excessive plugging of screens and filters ndash Corrosion ndash Wood destruction by fungi ndash Clogging of water distribution nozzles ndash Excessive sludge accumulation in the cooling tower basin
An important part of the water treatment program is to control biological activity by feeding a biocide chemical Oxidizingbiocides such as chlorine sodium hypochlorite chlorine dioxide or bromine-associated compounds are most commonly usedwith good results Plate counts should be maintained below 50000 countsml (bacterial)
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ExxonMobil Proprietary
Section Page
XXVII 14 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Chlorine is the most effective biocide for cooling water because of its low cost extensive period of successful use ease of application and minimum environmental effects if applied under controlled conditions Either a liquid feed (sodium
hypochlorite) or a gas feed (100 chlorine) is added to the cooling tower basin A sufficient dosage is that which obtains a 01to 03 wppm continuous free residual in the hot return water to the cooling tower
Advantages to continuous chlorination over shock treatment based on ExxonMobil experience are ndash Minimizes chlorine degradation of polymers ndash Better reduction in total aerobic bacteria plate count resulting in cleaner heat transfer surfaces ndash Does not cause significant pH depressions ndash Better control results with less chlorine consumption
Details concerning chlorine addition as well as the other alternative biocides are given in Report No EE102E78 Cooling Tower Water Treatment Guidelines and in EMRE Engineering Water and Wastewater Design Guide TMEE 080 DG 11-3-1Gas Chlorinators for Water Treating Service and DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) Gaseous chlorine systems have become more costly in recent years because of the additional precautions equipment andprocedures required to minimize the risk associated with handling this gas Alternatives to gaseous chlorine are available butrequire more attention and have a higher operating cost Sodium hypochlorite (either purchased or generated) and sodiumbromide stabilized solid halide (chlorinebromine) products chlorine dioxide and ozone are all alternatives which may haveapplications Contact the COOLING WATER SYSTEMS SPECIALISTS for guidance in the use of these alternative treatmentschemes
BLOWDOWN
Blowdown control is required to maintain the proper cycles of concentration in the system The most reliable and simplest typeof control for blowdown is specific conductance measurement of the cooling water Electrical signals from the specificconductance measurements of some impurity in the cooling water are used to actuate an automatic blowdown valveImpurities normally used for blowdown control are dissolved solids chloride calcium magnesium and silica
CONTROLS AND MONITORS
Any chemical treatment used to control problems in a cooling water system must be carefully monitored to be completelyeffective
The installation of automatic controls should be considered for recirculated cooling water systems in all plants Designs shouldinclude pH conductivity corrosivity inhibitor and biocide control Automatic controllers are available which continuouslymonitor the condition of the water and automatically make chemical additions to keep the water within compositional limitsTest exchangers and corrosion coupons are also useful in monitoring the effectiveness of the water treatment program
An oil-in-water monitor should be considered to provide early detection of oil leaks into the cooling water This will enablecorrective action to be taken before extensive contamination of the cooling water circuit occurs This is especially important incooling towers with high efficiency cellular fill which can be difficult to clean if significantly fouledManual control systems may be used in locations that have adequate technical personnel to perform routine water analysesand to modify chemical treatment necessitated by sudden variations in water composition process contamination etc
WASTEWATER REUSE POSSIBILITIES
Recirculating cooling water systems normally have the highest plant raw water demand A potential alternative is to use treatedwastewater to supply part or all of the makeup water requirement The treatment required must be determined but will typicallyinclude primary oil removal (eg API Separator) secondary oil and suspended solids removal (eg filtration dissolved or induced air flotation) and soluble carbon removal utilizing biological treatmentThe reuse of treated wastewater as makeup currently exists at only a few locations If this alternative is being considered refer to Report No EE58E85 Reuse of Wastewater as Cooling Tower Makeup and Section XIX-B Water Reuse
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 15 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS
STEP 1 - Select a location for the intake station and pumps
Factors which influence this selection are as followsbull Depth of water and tidal variations
bull Relative location of intake and outfall direction of current
bull Length and size of intake suction piping vs pump discharge piping
STEP 2 - Determine cooling water requirements
Determine the design cold water inlet temperature based on site data covering seasonal water temperature as a function of inlet location and depth (for existing locations refer to Table 2 in Section IX-B )Tabulate the cooling water requirements for each process unit and offsite consumer Apply the proper LGFs to all coolingloads depending on the stage of the project and whether the new units are exact duplicates of existing unitsIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The designer should investigate methods to increase effluent water temperature to the maximumrecommended limit for the particular type of water used One common method is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 3 - Determine the total required capacity for water intake facilities
Process Coolers - The total requirements determined in Step 2 including LGFsFirewater - This is often supplied from the intake bay of a once-through systemFuture Expansions - These should be considered in sizing intake facilitiesTotal Required Capacity - The sum of the above three items
STEP 4 - Provide water intake facilities
Intake Crib - The intake crib with its support structure is located at the extreme end of the water intake line (See Figure 4 )The intake is usually located 200 to 300 ft (60 to 90 m) offshore and at a sufficient depth below the lowest water level to
preclude vortex formation and subsequent pump cavitation during storm conditions Additionally the crib should have 3 to 5 ft(1 to 15 m) clearance above the sea floor to minimize sand and silt pickup A concrete velocity cap should be specified to cover the inlet crib This will minimize the incidence of fish and other foreignmatter entering the system A horizontal inlet flow pattern should be designed with a maximum entrance velocity of 1 fts(03 ms) Experience shows that vertical velocity flow confuses fish and they are drawn into the intake lineThe intake crib support structure protects the crib from adverse effects of storms and tidal motions as well as protecting it fromimpact of floating and submerged objects A self-cleaning screen can be specified to prevent entrance of large objects into thesystem at the cribIntake Line (Figure 5 ) - An intake line is required if the water depth along the shore does not permit water to flow directly intothe intake bay The intake line is usually buried in the sea floor and covered with a minimum required overburden Consultwith the COOLING WATER SYSTEMS SPECIALISTS for required coverage at a specific siteChlorine or sodium hypochlorite is injected at the intake crib to prevent marine growth in the lineNote that an intake crib and line will not be required if water of sufficient depth is available at the supply pump stationIntake Bay (Figure 6 ) - The intake bay is an enclosure which houses the pumps and screens Depending on a particular design a series of screens is used to ensure the supply of clean water Trash racks comprise the first element of the filter system These remove only very coarse material from the water such as bulk seaweed They can be supplied with atraversing raking mechanismLift screens provide secondary cleanup These screens can be lifted out of the intake bay for periodic cleaning Double liftscreens can be used if it is deemed necessary to have one in service at all timesFinal water filtering is usually done by traveling screens Details of the screen dimensions and the mounting are usuallydetermined during detailed engineering Screen net open area is typically approximately 50 of total screen area Thescreens should be sized for a normal water velocity of 12 fts (04 ms) through the open screen area Maximum velocityshould be limited to 24 fts (08 ms) with 50 of the screen area plugged This is the upper velocity limit for efficientscreening
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Section Page
XXVII 16 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
If high suspended solids greater than 200 mesh (eg sand) are expected to be a continuous problem ejectors may be installedupstream and downstream of the traveling screen to remove accumulated sand and solids from the intake bay in these areas
Traveling screens and ejectors are supplied with wash water from the raw water pumps or from separate wash water pumpsOnoff control using timers should allow optimum scheduling of the ejectors depending on the time of the year
STEP 5 - Specify water intake facilities
Water intake facilities should be designed in accordance with the following guidelines from the Hydraulic Institute Standards The water intake should be designed to provide an evenly distributed flow of water to the pump suction A properly designedintake reduces the possibility of vortex formation and results in better pump performance with less noise Geometry of theintake and the manner in which water is introduced into the intake are important parameters in providing an even distribution of water flow Complete analysis of intake structures is best accomplished by scale model testsThe pump manufacturer will generally render advice on specific problems while the intake design is still preliminary He shouldbe provided with the necessary intake layout drawings showing the physical limitations of the siteFigures 7A and 7B have been developed for single- and multiple-pump arrangements to show suggestions for basic sumpdimensions They are for pumps normally operating in the capacity range of approximately 3000 to 300000 gpm (200 to20000 Ls) Since these values are composite averages from a great many pump types and cover the entire range of specificspeeds they must not be thought of as absolute values but rather as basic guides subject to some possible variations
All the dimensions in Figures 7A and 7B are based on the rated capacity of the pump at the design head Any increase incapacity above these values should be momentary or very limited in time If operation at an increased capacity is to beundertaken for considerable periods of time the maximum capacity should be used for the design value in obtaining sumpdimensionsDimension C is an average value based on an analysis of many pumps Its final value should be specified by the pumpmanufacturerDimension B is a suggested maximum dimension which may be less depending on actual suction bell or bowl diameters in useby the pump manufacturer The edge of the bell should be close to the back wall of the sump When the position of the backwall is determined by the driving equipment or the discharge piping Dimension B may become excessive and a ldquofalserdquo backwall should be installedDimension S is a minimum value for the sump width for a single pump installation This dimension can be increased but if it isto be made smaller the manufacturer should be consulted or a sump model test should be run to determine its adequacy
Motor size may dictate minimum pump spacing be especially aware of this when specifying pumps with unusually high pumpheads eg greater than 80 psig (550 kPa)Dimension H is a minimum value based on the ldquonormal low water levelrdquo at the pump suction bell taking into considerationfriction losses through the inlet screen and approach channel This dimension can be considerably less momentarily or infrequently without excessive damage to the pump It should be remembered however that this does not representldquosubmergencerdquo Submergence is normally quoted as dimension ldquoHrdquo minus ldquoCrdquo This represents the physical height of water level above the bottom of the suction inletThe actual submergence of the pump is something less than this since the impeller eye is some distance above the bottom of the suction bell possibly as much as 3 to 4 ft (1 to 13 m) For the purposes of sump design in connection with this chart it isunderstood that the pump has been selected in accordance with specific speed charts The submergence referred to hereinhas to do only with vortexing and eddy formationsDimensions Y and A are recommended minimum values These dimensions can be as large as desired but should be limitedto the restrictions indicated on the curve If the design does not include a screen Dimension A should be considerably longerThe screen or gate widths should not be substantially less than S and heights should not be less than H If the main stream
velocity is more than 2 fts (06 ms) it may be necessary to construct straightening vanes in the approach channel increaseDimension A conduct a sump model test of the installation or work out some combination of these factorsDimension S becomes the width of an individual pump cell or the center-to-center distance of two pumps if no division walls areusedOn multiple pump installations the recommended dimensions in Figures 7A and 7B also apply as noted above and thefollowing additional determinants should be consideredFigure 8A - Low velocity and straight-line flow to all units simultaneously is the first recommended style of pit Velocities in thepump area should be approximately 1 fts (03 ms) Some pumps with velocities of 2 fts (06 ms) and higher have given goodresults This is particularly true where the design resulted from a model study Not recommended would be an abrupt changein size of inlet pipe to sump or inlet from one side introducing eddying
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Section Page
COOLING WATER SYSTEMS XXVII 17 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Figure 8B - A number of pumps in the same sump will operate best without separating walls unless all pumps are always inoperation at the same time in which case the use of separating walls may be beneficial If walls must be used for structural
purposes and pumps will operate intermittently one should leave flow space behind each wall from the pit floor up to at leastthe minimum water level and the wall should not extend upstream beyond the rim of the suction bell If walls are used increaseDimension ldquoSrdquo by the thickness of the wall for correct centerline spacing Round off the ends of the walls Not recommended isthe placement of a number of pumps around the edge of a sump with or without dividing wallsFigure 8C - Abrupt changes in size from inlet pipe or channel to pump bay are not desirable A relatively small pipe emptyinginto a large pump pit should connect to the pit with a gradually increasing taper section The angle should be as large aspossible preferably not less than 45 degrees With this arrangement pit velocities much less than one foot per second aredesirable Especially not recommended is a small pipe directly connected to a large pit with pumps close to the inlet Flow willhave excessive change of direction to get to most of the pumps Centering pumps in the pit leave large ldquovortex areasrdquo behindthe pumps which will cause operational troubleFigure 8D - If the pit velocity can be kept low enough [below 1 fts (03 ms)] an abrupt change from inlet pipe to pit can beaccommodated if the length equals or exceeds the values shown It is assumed that as ratio WP increases the inlet velocityat ldquoPrdquo will increase up to an allowed maximum of 8 fts (24 ms) at WP = 10 Pumps ldquoin linerdquo are not recommended unless theratio of pit to pump size is quite large and pumps are separated longitudinally by a generous margin A pit can generally be
constructed at much less cost by use of a recommended designFigure 8E - It is sometimes desirable to install pumps in tunnels or pipelines A drop pipe or false well to house the pump witha vaned inlet well facing upstream will be satisfactory in flows up to 8 fts (24 ms) Without the inlet well the pump section bellshould be positioned at least two pipe diameters vertically above the top of the tunnel not hung into the tunnel flow especiallywith tunnel velocities 2 fts (06 ms) or more There should be no signs of air along the top of the tunnel It may be necessaryto lower the scoop or to insist on a minimum water level in the vertical wellNote The foregoing statements apply to sumps for clear liquid
STEP 6 - Provide chemical treatment
Provide chemical treatment facilities for the once-through cooling water system Normally the only chemical treatment requiredin a once-through cooling system is for biological control Facilities for adding chlorine to once-through cooling water are thesame as those described for the recirculated cooling water system The design capacity of the chlorinator should be based onachieving a chlorine residual of 1 wppm in the effluent The chlorine required by the organics and other reducing agents in thewater must be satisfied before any residual chlorine will appear in the effluent The design capacity of the chlorinator should be
based on the total water chlorine demand plus 1 wppm in the effluent In the absence of water data the chlorination systemdesign capacity should be based on 10 wppm minimum instantaneous dosage for maximum cooling water flow For very largeonce-through cooling water systems it will be impractical to design for shock rates at this level In this case a continuousdosage of 1 - 2 wppm should be used (Refer to Step 5 of the DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS )
STEP 7 - Specify pumps and distribution system
Specify other major facilities in the once-through cooling water system The recommended materials for pumps valves andpiping are shown in the EMRErsquos Refinery Construction Materials Manual
Pumps1 Type - These pumps are usually vertical centrifugal pumps However horizontal centrifugal pumps have been used An
economic study andor consideration of existing facilities will determine which type should be specified In the event thathorizontal centrifugal pumps are specified facilities for pump priming will likely be required For large pumps in salt water service there is a very limited selection of pumps because of the special materials required Consult with the DISCIPLINESPECIALISTS early in the design effort to determine what specific pumps are currently available
2 Total Required Capacity - Firm capacity (excluding spares) of the supply pumps should equal the maximum cooling water requirements including LGF and RCF
3 NumberIndividual Pump Capacity - Normally one spare pump is provided for up to six operating pumps two spares for up to twelve operating pumps etc The most common arrangements for water supply pumps are
bull Two 100 capacity pumps
bull Three 50 capacity pumps
bull Four 33-13 capacity pumps
bull Five 25 capacity pumps
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Section Page
XXVII 18 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Maximum commercially available pump capacity ranges up to 300000 gpm (20000 Ls) at discharge pressures up to 100psi (700 kPa) Maximum capacity will be less for higher discharge pressures Other factors which may influence pump
selection arebull Selecting capacity to match existing pumps
bull Selecting capacity for future requirements
bull Selecting capacity corresponding to manufacturers available standardVendor information should be obtained on the availability size and cost of the pumps for several configurations Acomparison should be made of the investments and operating costs as well as other advantages and disadvantages toselect a pump configuration
4 Drivers - These pumps are usually motor-driven Because of the remote location of the intake facilities the steamdistribution costs are normally too high to justify the use of a steam turbine driver However use of a secondary selectivesubstation provides a reliable source of power to the pump drivers
5 Pump ∆∆∆∆P - Pump ∆P is based on supplying the maximum cooling water requirement at the battery limit design pressureestablished for the system The ∆P should include the total circulating system pressure drop plus any elevation differencebetween the low water level at the intake and the discharge point of contact with air
6 Pump Instrumentation - The remote location of the intake facilities makes remote control and monitoring of the water supply operation desirable Pump controls located in the control house provide for minimum manning Local controlbackup should be provided The Owners operating and manning philosophy determine the basis for instrumentation andcontrolThe spare pump is controlled to automatically start on low pressure in the supply header Concurrently alarms should beactuated in the control room to indicate the low-pressure situation and spare pump operation
Distribution Network1 Configuration - The piping network is a once-through system with a return line from each consumer or consumer group to
the cooling water effluent line2 Layout - The routing of the piping network should connect all cooling water consumers in the most economical manner
permitted by the plant layout Careful consideration should be given to the turnaround philosophy of the plant Thenetwork layout should preclude the necessity of having above-ground water lines that are in service running through anarea that is being turned around Underground piping or an extra lateral may be required to accomplish this objective
3 Valves and Blinds - Battery limit valves should be provided in the network to isolate individual process units from thesystem Valves should be provided to permit onstream removal and maintenance of spared equipment
4 Basket Strainers - Consideration should be given to the installation of basket strainers in the laterals serving each of theconsumer loops These provide added insurance against entrance of foreign material into the cooling equipment The useof intake screens with 025 in (63 mm) openings may eliminate the need for basket strainers in the laterals
Line Sizing1 Design Basis - The network is sized so that water is delivered to the most distant consumer at a pressure equal to the
required process design (battery limit) pressure2 Flow Rate - Each segment of the distribution network is sized for the simultaneous maximum flow in that segment for any
operating condition The main supply and discharge headers are sized for the simultaneous maximum flow of all segments(not necessarily the sum of the simultaneous maximum flows of the individual segments)
3 Pressure Drop
Line Sizing Basis - The distribution system should be sized according to the guidelines established in Section XIV The use of an ldquoold piperdquo friction factor of C = 90 - 100 (Hazen-Williams factors) in the main lines is recommended to assuregood operation in later years when the system experiences fouling For large diameter lines [greater than 48 in (1200mm)] maximum water velocity should be limited to 10 fts (3 ms) based on civilmechanical considerations (supports andanchoring required to handle potential pressure surges) For salt water systems utilizing cement-lined pipe consider thereduced inner diameter in the friction loss calculations A study may be necessary to determine the economic pump ∆P vsline diameters
4 Equivalent Length Factors - Equivalent length factors are used in pressure drop calculations for sizing the distributionsystem The piping length scaled from the plot plan is multiplied by an equivalent length factor to compensate for pipingelbows expansion loops and other piping variations
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Section Page
COOLING WATER SYSTEMS XXVII 19 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
The equivalent length factors are selected based on the expected variation from a straight piping run Typical equivalentlength factors suitable for planning purposes for different piping applications are as follows
TYPE OF PIPING EQUIVALENT LENGTH FACTORS
Long piping runs [gt 1000 ft (300 m)] 12 to 14
Onsite distribution piping 15 to 20
For design purposes the actual piping layout should be evaluated to ensure that the equivalent length factors areapplicable to the specific situation Particular attention is required for revamp work and saltwater systems utilizing cement-lined pipe
STEP 8 - Prepare a system flow plan similar to the one shown in Figure 1
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS
STEP 1 - Determine cooling water requirements
Tabulate the cooling water requirements and cooling water discharge temperature for each process unit and offsite consumer Apply the proper LGFs to all cooling loads depending on the stage of the project and whether the new units are exactduplicates of existing units Determine the total cooling water flow required including LGFs and the bulk temperature (T 1) of thecombined cooling water flow from all coolers to the cooling towerIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The onsite and offsite designers should investigate methods to increase effluent water temperature to themaximum recommended limit for the particular type of water used This will result in a lower cost cooling tower for a given heatrejection rate One common method of maximizing return water temperature is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 2 - Select cooling tower design parameters
Select the proper design conditions of ambient wet bulb temperature approach temperature range and flow rate for specifyingthe cooling tower Based on these conditions the cooling tower vendor will size the tower and provide the tower characteristicand performance curves which serve as the basis for the performance guarantee Required performance test and evaluationprocedures are described in IP 8-1-1 Economic studies may be required to select the ambient wet bulb temperature andapproach temperature for new locations or for expansion projects with special cooling requirements
STEP 3 - Prepare cooling tower specification sheet
Include the following information in the heat exchange section of the design specificationTower Flow Capacity - This was determined in Step 1Type of Tower - The tower usually specified is an induced draft multicell type suitable for fresh salt or brackish water depending on the water available Additional tower characteristics are usually developed and proposed by the tower vendor (eg crossflow versus counterflow type of tower fill etc) The vendor should be made aware of conditions such as thelikelihood of heavy oil leakage into the cooling water or high fouling potential which may influence the tower type or fill
selectionNumber of Cells - The minimum number of cells required (normally not less than 2) may be specified The maximum flow per cell is a function of the cooling tower design conditions selected in Step 2 Typical values range up to 10000 gpm (600 Ls) for crossflow designs and 8000 gpm (500 Ls) for counterflow designs Normally spare cells are not provided Maintenance iscarried out during cooler periods or load is shed
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Section Page
XXVII 20 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Ambient Wet Bulb Temperature1 The ambient wet bulb temperature selected for specifying the cooling tower design is generally that temperature which is
equaled or exceeded 5 of the time during the four hottest months (5 of the hourly readings taken during the four hottestmonths will exceed this value) Specifying the 5 temperature should give satisfactory results since the time that the wetbulb temperature is exceeded is for short duration and the system inertia is usually sufficient to avoid problems The tower specification should make it clear that the wet bulb temperature specified is an ambient value as opposed to the enteringvalue which may be higher because of recirculation The tower manufacturer should adjust the ambient value upward totake into account his estimates of recirculation Currently cooling tower test codes provide procedures for measuringperformance in the case of either entering or ambient wet-bulb temperature specifications An increase of 2 degF (11 degC) isgenerally added to ambient wet bulb temperature to estimate entering wet bulb temperature for large cooling towers
2 If plant production will peak during the four summer months when the 5 ambient wet bulb temperature may be exceededconsideration should be given to specifying a higher design ambient wet bulb temperature (one that will be exceeded 212 of the time for example) However the incremental cost of the resulting closer approach and larger cooling tower must be justified by the value of the incremental products made
On-Tower Temperature (T1) - This temperature cannot be accurately determined until the design of all process coolers iscompleted and the volume of cooling water and outlet temperature are known for each (see Step 1) In the planning stage or early design stage of a project this temperature must be estimated and is usually set equal to the maximum design cooler outlet temperature shown under COOLING WATER REQUIREMENTS in the BASIC DESIGN CONSIDERATIONS portion of this sectionApproach Temperature - The approach temperature has the most pronounced effect on cooling tower size and cost For example reducing the approach from 10 to 5 degF (55 to 3 degC) can increase the cooling tower investment by 50 percent and thefan operating cost by 65 percent Thus various approach temperatures should be evaluated to determine the minimum cost of the total system including the process exchanger area Approaches used in the past designs have been from 5 to 12 degF (3 to7degC) Note that it is not customary in the cooling tower industry to guarantee approaches of less than 5 degF to (3 degC) to coolingtower design inlet wet bulb temperature A typical approach for a new grass roots cooling tower is 10 degF (6 degC)Off-Tower Temperature (T2) - This is the sum of the approach temperature and the ambient air wet bulb temperature For existing locations Table 2 in Section IX-B may be used
Range and Flow Rate1 The range for a given heat load is related to the circulating water rate as follows
Q = M C p ∆Tcong (500) (gpm) (range) Eq (1)
where Q = Heat load on the tower BtuhM = Water circulating rate lbhCp = Specific heat capacity of water at average temperature Btulb degF∆T = (T 1 ndash T 2) degF = (water cooling range)gpm = Circulation rate in US gallons per minute
For calculation in Metric units
Q = M C p ∆Tcong (1162 x 10 -3) (Ls) (range) Eq (1M)
where Q = Heat load on the tower kJh
M = Water circulating rate kghCp = Specific heat capacity of water at average temperature kJkg degC∆T = (T 1 ndash T 2) degC = (water cooling range)Ls = Circulation rate in liters per second
2 Generally lower tower investment fan horsepower and pump horsepower result from maximizing the range andminimizing the flow rate However based on fouling considerations of process coolers the maximum allowable hot water temperatures must be kept below recommended limits Also while increasing the range minimizes cooling tower investment and operating cost the cost of exchanger surface requirements is adversely affected by the LMTD and acompromise must be made The range is bounded by the maximum allowable hot water temperature and the Off-Tower temperature (T 2) For established locations refer to Section IX-B for design cold water temperatures to assist in selectingthe approach and range
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COOLING WATER SYSTEMS XXVII 21 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Makeup Water Requirements - Calculate the various losses in the cooling water system to determine makeup requirementsThese losses include
1 Evaporation Loss (E) - Allow 1 of the circulation rate for each 10 degF (55 degC) temperature drop through the tower Notethat the actual evaporation loss will be somewhat less depending on ambient air temperature
( )sL555
TT)gpm(
1000TT
E 2121
minus=
minus= Eq (2)
2 Drift Loss (D) - IP 8-1-1 limits the maximum allowable drift loss to 0005 of the design water circulation rate for freshbrackish or salt water
( )sL1000050
)gpm(1000050
D
=
= Eq (3)
This low drift level contributes little to solids removal from the tower with the spray This allows higher cycles of concentration to be achieved in cooling towers which use clarified softened water for makeup and thus have low requiredblowdown rates Higher drift loss percentages in older or poorly maintained towers will limit the maximum cycles of
concentration achievable3 Miscellaneous Losses (O) - Miscellaneous losses are estimated to be one-seventh of the difference between evaporation
loss and drift loss
minus=
7DE
O Eq (4)
4 Blowdown Rate (B) ndash Guidelines for maximum concentrations of impurities allowed in recirculated fresh cooling water areshown in Table 2 For salt water the blowdown is based on keeping the total dissolved salts below 55000 wppm Obtainthe chemical analysis of the makeup water and divide each individual component concentration into its counterpart inTable 2 The result is the maximum allowable cycles of concentration for that constituent (a worksheet is provided inTable 4 ) The impurity with the lowest factor will be the control and will determine the required blowdown rate
)OD(1Cycles
EB +minus
minus= Eq (5)
The makeup water requirement is the sum of all losses shown above
MakeUp = E + D + O + B Eq (6)
STEP 4 - Specify the controls necessary for proper cooling tower operation
Air Side - Cooling towers are provided with motor-driven manually adjusted pitch multiple-bladed propeller fans Figures9A amp B may be used to estimate the normal operating horsepower requirements Since the fan motor and gear reducer arelocated at the top of the tower [generally 40 to 50 ft (12 to 15 m) above grade] the need for equipment to handle thesecomponents should be discussed with the affiliateControl of the air flow through the tower to maintain reasonably constant cold water temperatures can be accomplished byturning off individual fans in multiple-cell installations adjusting the blade pitch or using variable speed motors To turn off individual fans remote at grade start-stop buttons for the motors are usually specified
Whenever the ambient air temperature is 32 degF (0 degC) or below icing of the tower louvers and structural members in contact withthe incoming air will occur If the heat load on the tower is relatively free of seasonal variations two-speed fans should provideadequate ice control down to approximately 0 degF (-18 degC) ambient temperatures When sustained temperatures of 0 degF (-18 degC)and below occur two-speed fans capable of two-speed reverse operation with appropriate time delays for change of directionand speed should be specified to periodically eliminate ice which will build up on the louvers Other methods of winterizationare available but in general are more expensive and require greater operator attention In very cold climates steam heatingshould be provided to prevent freezing in the cold water basin during shutdown periodsWater Side - The water circulation rate is maintained by operating one or more pumps of sufficient capacity to meet the systemdemand Flow-measuring facilities local temperature indicators and a high-temperature alarm are provided on the coolingtower water supply and return headers Hand-operated flow control valves are required for distributing the water equally to theindividual cells On counterflow towers provisions for measuring the flow to each cell (pitot tubes) are required They are notrequired on crossflow towers since the open distribution basin provides visual aid in balancing flows Shutoff valves should beprovided for servicing the flow control valves without shutting down the entire tower
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XXVII 22 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
STEP 5 - Provide chemical treatment facilities for the fresh water system
Alkalinity (pH) Control (if used)1 In cases where a controlled pH corrosionscale control program is being used the alkalinity (pH) of the recirculated cooling
water should be controlled in accordance with the program requirements Normally acid addition is required to accomplishthis This is due to free carbon dioxide being stripped from the cooling water in the tower to approximately 5 ppm For example if the alkalinity has to be controlled to between 25 and 50 ppm for a pH of 70 to 75 the volume of 98 sulfuricacid (66 degBeacute) required to treat the makeup water is calculated using the following equation
[Note 93 sulfuric acid should be used if ambient temperatures will be less than 35 degF (2 degC)]
GallonsHour 66 degBeacute H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate gpm) (326 x 10 -5)
For calculation in Metric limits
kgh of 100 (66 degBeacute) H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate Ls) (793 x 10 -3)
2 The metering pump should be designed for a maximum feed rate of two to three times the normal feed rate A glass woolfilter is required on the pump suction piping to minimize plugging Normally two - 100 capacity pumps are provided
Positive displacement metering pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumps with flow control areused for higher rates
3 Hydrochloric acid may also be used for alkalinity control The gallons per hour of 20 degBeacute hydrochloric acid required is36 times the amount of 66 degBeacute sulfuric acid In Metric units the weight of 32 (20 degBeacute) hydrochloric acid required is227 times the weight of 100 (66 degBeacute) sulfuric acid
4 Specifications for chemical feeders may be found in EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased)
5 A pH analyzer with high and low alarm is provided to control the amount of acid feed The acid should be added to thecooling tower makeup water (Figure 2 ) The lag time between the point of acid injection and the sample outlet to the pHcell should be less than two minutes for effective control
6 Spills or overflows from the sulfuric acid tank should be contained
Corrosion Inhibitors and Anti-Scalents1 Two chemical feeders are generally provided The following guides in sizing the equipment will allow the use of any of the
proprietary products from the vendors when final selection by the owner is made See Report No EE102E78 Cooling Tower Water Treatment Guidelines EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and Report No EE34E86 Environmentally - Acceptable Cooling Water Treatments for detailed discussion of acceptable commercial chemicals
bull If the water treatment program to be used is known design for the chemical rates specified by the vendor or COOLING WATER SYSTEMS SPECIALIST
bull Otherwise design for 100 wppm of each commercial liquid product in the recirculated cooling water The design feedrate should be based on the design total blowdown flow (both uncontrolled and controlled) since this is where theproduct is lost For 100 wppm concentration the chemical rates will be as follows
GallonsHour Chemical = Blowdown (gpm) x (0006)
LitersHour Chemical = Blowdown (Ls) x (036)
2 The design capacity of the metering pump should be two to three times this calculated number Normally 2 - 100capacity pumps are provided Positive displacement pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumpswith flow control are used for higher rates
3 Specifications for chemical feeders may be found in EMRErsquos Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased) Reference should also be made to the checklists containedin Report No EE92E94 Guidelines for Safety Evaluation of Chemical Injection Facilities
4 A final check should be made with the owner and the COOLING WATER SYSTEMS SPECIALISTS as to the treatmentprogram selected and if there is a need for an additional chemical feeder
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COOLING WATER SYSTEMS XXVII 23 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Biological ControlFacilities should be provided for feeding the biocide into the back of the cooling tower basin If chlorine is used it must bepresent in the cooling water as free residual to be effective However the following amounts of chlorine are first required toreact with the listed impurities when present in the cooling water before a free residual can be established
IMPURITIESAMOUNT OF CHLORINE REQUIRED (wppm)
WITH EACH WPPM OF IMPURITY
Ammonia (NH 3)
Hydrogen Sulfide (H 2S)
Sodium Sulfite (Na 2SO 3)
Ferrous Iron (Fe ++)
10
22
056
064
The design capacity of the chlorine feeder shall be based on achieving a 01 wppm free chlorine residual in the recirculatingwater flow or a minimum dosage of 7 wppm at maximum design makeup water flow rate and conditions (see EMRE Water and
Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service )Chlorine may be fed either as the commercial liquid sodium hypochlorite (124 wt Cl 2) or liquid chlorine (100 Cl 2) Liquidhypochlorite is being used increasingly because of the higher costs associated with precautions equipment and proceduresrequired to minimize the risks associated with handling gaseous chlorine The hypochlorite solution is fed using a meteringpump which should be sized to feed an equivalent of 15 ppm 100 Cl 2 (Refer to EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) ) In case of spills or an overflow from thestorage tank the sodium hypochlorite may be sent to the cooling tower basinGas chlorinators are covered by EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service The use of gas chlorination is being reduced but may still be appropriate in some locations subject tolocal management approval The gas from either commercial cylinders or an evaporator is dissolved in a water slip stream bymeans of an eductor This results in a chlorine solution strength of approximately 01 This solution should be evenlydistributed below the water surface by means of a header lateral pipe arrangement at the rear of the cooling tower basinopposite the cooling water pumpsSome additional guides on the use of gas chlorinators are given below It is also recommended that the safety guidelines
published by the Manufacturing Chemists Association and the Chlorine Institute be followedbull Dilution water for the chlorinator must not be less than 50 degF (10 degC) or ice formation may cause plugging problems
bull Continuous rates of chlorine gas withdrawal should not exceed 400 lb (180 kg)24 hours from each one ton cylinder atroom temperature [70 degF (21 degC)] This rate may be exceeded (up to 50) for periods not exceeding 2 hours Rule of thumb is not to design for more than 4 cylinders operating in parallel When feed rate exceeds 3000 lb (1360 kg) per daythe liquid chlorine should be fed to an evaporator upstream of the chlorinator
bull If the ambient temperature can be expected to be below 50 degF (10 degC) for extended periods a building should be providedfor the chlorinator and cylinders
bull Weighing scales should be provided to determine when shipping containers should be replaced and to verify chlorine feedrates
bull Analyzers to measure the free chlorine residual in the circulated cooling water should be provided
bull Chlorine leak detectors and a catch basin for major spills should be provided
Control SystemProvide automatic controllers to continuously monitor water condition and to add chemicals to the system (complete systemsmay be available through the water treatment program vendor as a lease or purchase)pH Control - This controller adds acid to correct water alkalinity a timer on the acid pump sounds an alarm if the pump doesnot shut off in a predetermined length of time Backup controls are required to prevent the cooling system from beingpermanently damaged in case the pH electrodes become fouledCorrosivity and Inhibitor Control - Systems are available which give a direct readout of corrosion rate in milsyear If thecorrosion rate exceeds a set point the controller shuts off the acid increases the inhibitor addition opens the blowdown valveand sounds an alarm Electrodes measure the completeness of the inhibitor film by the value of the potential differencebetween the electrodes The larger the difference the more incomplete the film and a potential reading exceeding a set levelactivates the inhibitor feed
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XXVII 24 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Free Residual Chlorine Control - Free residual chlorine should be controlled automatically for biocide treatment Severalgood analyzers are currently available These units can be incorporated into an automatic control system
Oil in Water DetectionMonitoring Corrosion and Deposit FormationThe following types of devices for measuring corrosion rates and deposit formation should be provided to allow the plant tomonitor the effectiveness of the cooling water treatment before scheduled turnaroundsCoupons - A corrosion coupon rack should be specified for the hot return water to the tower in accordance with IP 19-6-1 Corrosion Monitor - On-line corrosion analyzer if specified should monitor the hot return water to the tower in accordance withIP 19-6-1
On-Line Test Exchanger Although there are a variety of deposit monitors from the water treatment chemical vendors they do not necessarily representactual heat transfer conditions in process exchangers For this reason a test exchanger should be installed for evaluatingcorrosion and deposit formation Details are covered in Report No EE102E78 Cooling Tower Water Treatment Guidelines
Alternatively a critical process exchanger can be instrumented such that its heat transfer coefficient can be monitored
Instrumentation would include flows and inlet and outlet temperatures of process fluid and cooling waterSTEP 6 - Specify cooling tower location
Select a location for the cooling tower The following factors influence this selection
Location Factors1 The location should be within a reasonable distance of the units served and consistent with future refinery plans Piping
costs to and from battery limits can be a significant portion of the total cooling system cost and can be the location-determining factor particularly for salt water systems While piping costs can be easily determined other costs affectinglocation cannot be estimated readily Therefore judgment is required in selecting the location
2 Safety spacing requirements (see Section XV-G Equipment Spacing )3 Cooling tower contribution to plant boundary noise4 For salt water cooling towers wind direction and distance to other facilities is of major concern because of potential
corrosion problems
5 Factors affecting cooling tower performance ie orientation with respect to wind site congestion etc
Plot RequirementsIn estimating plot requirements (basin area) one should allow 5 to 6 ft of tower length for each 1000 gpm (2 m for each 60 Ls)of circulating capacity This figure is based on a typical counterflow design with a width of 50 ft (12 m) and does not includepump suction pit space Exact requirements can be obtained from cooling tower vendors after the tower capacity has beenestimated
Fogging and DriftTwo distinct problems are associated with the discharge air from a cooling tower (1) fogging which can be a nuisance or ahazard and (2) drift which is generally a nuisance because of increased corrosion of surrounding equipment Fogging fromcooling towers occurs during periods of combined high ambient humidity and low temperature as a result of mixing the warmmoist cooling tower exhaust air with the cooler ambient air The ambient air cannot absorb all moisture as vapor and thus tinyfog droplets are formed by condensation Fogging and resulting icing in cold climates has produced hazardous drivingconditions on highways up to 850 ft (250 m) awayProprietary designs are available to combat potential fogging problems Basically these designs incorporate heating of thedischarge air with hot return water precooling coils (similar to an air cooler) to eliminate any visible plume However thesedesigns are expensive and relatively unproven Therefore proper location based on reliable weather data is the preferredsolutionDrift from a cooling tower represents a more difficult problem since it does not depend on definable climatic conditions andoccurs for all wind directions The effects of drift on nearby equipment and vegetation are particularly severe and generally of concern only for salt water systems because of the high solids content of the drift (concentrated salt water) For salt water cooling systems electric lines substations and instrumentation should be located upwind in the direction of the predominantwind They should be provided with adequate shelter or materials when within 450 ft (140 m) of the cooling tower In additionspecifications for all equipment within 150 ft (45 m) of the tower should include surface treatment suitable for a marineenvironment
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COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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XXVII 28 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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XXVII 30 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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COOLING WATER SYSTEMS XXVII 31 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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XXVII 32 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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COOLING WATER SYSTEMS XXVII 35 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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COOLING WATER SYSTEMS XXVII 37 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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COOLING WATER SYSTEMS XXVII 9 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
In crossflow towers the air travels horizontally across the falling water while in the counterflow design the air travels verticallyup through the falling water Comparisons of plot space fan horsepower and future maintenance costs determine which
design will be more economical Factors which influence this selection are cooling water rate range and approach for aparticular location In addition in some locations materials of construction and associated labor costs will affect the finalselectionIn the past crossflow towers were usually preferred over counterflow towers for extremely cold locations where ice formationcould restrict or block air flow through the louvers However this consideration is usually no longer valid due to the increaseduse of PVC film-type tower fill rather than splash-type fill in counterflow towersMechanical draft designs use one or more fans to provide the air needed for cooling Induced-draft units have the fans locatedon top of the tower Air is pulled through the packing and discharged vertically upward at high velocities Forced-draft unitshave the fans located at the base of the tower and push the air through the fill Forced-draft towers in large industrial sizesexperience difficulties with recirculation and in providing a uniform distribution of air As a result they have been supersededby induced-draft designsFill may be splash or film type Splash fill normally consists of horizontal slats in horizontal rows offset to one another to causethe water to break up into droplets as it falls downward through the cooling tower Splash fill is characterized by reduced air pressure losses It is also less conducive to clogging and easier to clean after a spill However it is very sensitive to
inadequate support and must remain horizontal and level It is made of treated wood or plastics such as PVC or polypropyleneFilm fill comes in various designs but they all cause the water to flow in films over the fill surface and provides more coolingcapacity within a given amount of space than splash fill Because of this increased heat transfer efficiency film fill is generallyused in most new cooling towers However because of its smaller passages it is more sensitive to plugging and is difficult toclean if it does become plugged It may not be appropriate for services cooling mainly heavy fuels or waxy lubes where a leakwould cause fill plugging or for services with a high biological fouling potential PVC is currently the most widely used materialfor film fill The selection of fill type is usually proposed by the vendor and reviewed by the COOLING WATER SYSTEMSSPECIALISTSNatural-draft or hyperbolic cooling towers depend on the natural draft created by the difference in the density of the enteringand leaving air for movement Generally hyperbolic cooling towers have been used in Europe and in the utility industry in theUSA for large capacities [250000 gpm (16000 Ls)] short ranges [10 to 15 degF (6 to 8 degC)] and long approaches [10 to 15 degF (6to 8 degC)] They are most effective where low ambient wet bulb temperatures occur with high relative humidities These towersnormally have diameters in excess of 200 ft (60 m) and are greater than 250 ft (75 m) in height Their major advantages are
bull No operating or maintenance cost for fans
bull Reduction in problems associated with fogging drift and recirculation due to their great height and diameterThe primary reasons why hyperbolic towers are not used in ExxonMobil projects include
bull High initial cost
bull Large plot space requirement and high visual impact
bull Complete dependence on atmospheric conditions
bull Lack of flexibility for future expansions
COOLING TOWER BASIN
The primary function of the cooling tower basin is to collect the cooled water leaving the tower and to provide a reservoir for thecooling water pumps Design aspects to be considered are the basins capacity cleanability draining facilities sump andscreen details To obtain the necessary time for corrective action during emergency conditions a minimum storage capacity of 90 minutes based on loss of makeup water flow should be provided in the basin between the basin low level alarm (LLA) andthe minimum pump submergence (MPS) When the basin water level reaches the MPS point loss of recirculation pumpsuction and cooling water flow is imminent Typical values for basin water levels are as follows
bull High Level Alarm (HLA) 1 ft (300 mm) below top of the basin wall
bull Normal Water Level 15 ft (450 mm) below top of the basin wall
bull Low Level Alarm (LLA) 25 ft (760 mm) below top of the basin wall
bull Distance between LLA and MPS 35 ft (1070 mm)
bull Minimum Pump Submergence Depth 45 ft (1370 mm)
bull Clearance of Pump Suction off Basin Floor 15 ft (450 mm) Actual values may vary depending on pump type selected pump suction line diameter and cooling tower cell plot area
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XXVII 10 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
MATERIALS OF CONSTRUCTION
The EMRE Refinery Construction Materials Manual and IP 8-1-1 list acceptable materials for all components in each type of cooling water system (once-through and recirculating) For salt water systems emphasis must be placed on proper materialsselection throughout the system The external salt water environments in the vicinity of the tower itself and the intake stationsmust be taken into consideration Proper siting of the tower is extremely important to avoid salt water corrosion of adjacentunits or the need to use expensive materials to operate in such an environment
WATER TREATMENT FOR ONCE-THROUGH SYSTEMS
Once-through salt and fresh cooling water systems can develop a variety of problems such as fouling due to seaweed or other debris marine microorganisms such as muscles and shell fish and biological growth Corrosion and scaling problems areminimal in fresh water once-through systems because the water is not concentrated by evaporation In salt water systemscorrosion is minimized by proper materials selection and scaling problems generally do not occurProtection against fouling by seaweed or other debris is provided by careful selection of the water intake location and travelingscreens with washing facilities In some cases additional steps may be required These may include an additional secondaryset of traveling screens and possibly battery limit strainers on the cooling water supply lines to individual unitsProtection against fouling by marine microorganisms and biological growths is achieved primarily by injecting chlorinehypochlorite chlorinebromine or other similar chemicals which kill these growths Chlorine is commonly used and is injectedin sufficient quantities to produce a residual of 1 wppm in the effluent for one hour each day (shock chlorination) Continuouschlorination for 2 - 3 days per month at lower levels (03 wppm residual chlorine in the effluent) may also be used in addition toshock chlorinationThe large water volume discharged from once-through systems usually makes chemical treating for corrosion or deposit controltoo costly These problems are handled by proper materials selection and by keeping water velocities through exchanger tubesgreater than 3 fts (09 ms) Occasionally the water analysis of a fresh water source being considered for a once-throughsystem using carbon steel materials will indicate the need for some corrosion or deposit control In this case the cost of thethreshold treatment with the once-through system should be compared to the costs of a recirculated system before making afinal choice
WATER TREATMENT FOR SALT WATER COOLING TOWER SYSTEMS
A total dissolved salts (TDS) concentration limit of 55000 wppm for sea water in cooling tower systems requires high blowdownrates since the makeup sea water typically has from 35000 to 40000 wppm TDS Because of cost restraints the only water treatment normally specified for these systems is for biological control Chlorine is most commonly used either as directchlorine injection or hypochlorite produced in electrochlorinators Care must be taken in selecting the intake location to assurethat the suspended solids do not exceed 200 wppm in the recirculated water If it is not possible to find sea water with sufficientclarity clarification with polyelectrolytes should be evaluated Corrosion is one of the major problems with the use of sea water in cooling towers This requires proper materials selection
WATER TREATMENT FOR FRESH WATER COOLING TOWER SYSTEMS
Cooling tower water treatment is necessary to minimize or eliminate corrosion scale and biological fouling of process heattransfer surfaces caused by minerals and impurities in the water The difficulties caused by these impurities and the means of treatment are shown in Table 1 Water lost by evaporation results in an increase in total dissolved salts in the recirculated water To indicate the degree of concentration of impurities the term cycles of concentration is used
Corrosion is an electrochemical process that deteriorates metals exposed to water in the presence of corrosive agents such asacids oxygen or bacteria A common form of corrosion is pitting Severe corrosion can lead to equipment failure Corrosionis caused by many conditions including process leaks into the cooling water water flow velocity that is too low (causes depositsand fouling lending to corrosion) or too high (causes erosion and corrosion) low pH and high temperatures Corrosion is lesslikely in water with non-acidic pH values (greater than 7) although scaling is more likely in these pH rangesScaling is characterized by the formation of hard dense deposits on material surfaces These deposits impact heat transfer and can become a site for localized under-deposit corrosion Scaling is influenced by many factors including makeup water composition total dissolved solids in the recirculating water water temperature and pH and flow velocity Calcium carbonateis the main scaling constituent in all waters and it is the least soluble It forms from the hardness (calcium Ca ++ ) andbicarbonate (HCO 3
ndash ) alkalinity present in most waterFouling can be caused either by the settlement of suspended matter or by microbiological growth of algae bacterial slime or fungi Fouling will reduce heat transfer cause plugging and create corrosion sites
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 11 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Treatment of the cooling water is broken down into two major categories external and internal External treatmentencompasses any treatment given to the cooling tower makeup water Internal treatment is the addition of chemicals directly to
the recirculated cooling water
EXTERNAL TREATMENT
External treatment to remove impurities in order to reduce the amount of blowdown should always be evaluated as to costeffectiveness The most frequently used techniques for treating makeup water are shown in Figure 11 Details of availabletreatment processes (see Table 1 for methods available) are covered in Section XXVI-A Boiler Feedwater Treatment For cooling tower systems that meet currently required drift limits in refineries and chemical plants uncontrolled cooling water losses (drift and miscellaneous process uses) normally amount to about 14 of the cooling water evaporation loss Under these conditions the maximum cycles of concentration achievable are 8 with no controlled blowdown Some cooling towersare operating at up to 10 cycles of concentration Additional controlled blowdown required to limit the cycles of concentrationper the water quality criteria is covered later
INTERNAL TREATMENT
The makeup water analyses amount of cooling water losses environmental restraints potential chemical hazards materials of construction and film temperatures of heat transfer surfaces determine the type of internal treatment required for cooling tower systems The internal water treatment program selected must address corrosion scale fouling biological growth andchemical attack of cooling tower componentsNumerous chemicals and treatment programs are continuously being developed for addressing the potential problems Theslightly acidic low pH programs of the past which used chromate based chemicals and usually required acid injection arebeing completely phased out because of environmental restrictions on chromate and the higher costs associated with meetingcurrent practices for handling hazardous chemicals such as acid Non-chromate treatments which use inhibitors such asphosphates zinc and organics along with higher pH have been developed to replace the earlier programs Although not aseffective as chromate these inhibitors operate at higher water pH which is a less corrosive environment The disadvantage of the higher pH is the greater tendency to form scale This tendency is controlled by the use of dispersants or scale inhibitorswhich are normally organic polymers or phosphenate A corrosion inhibitor for copper or brass is also normally required if these metals are present in the systemThe following paragraphs describe techniques and chemicals to control alkalinity (pH) corrosion scale sludge and biologicalgrowth The chemicals mentioned are normally purchased as pre-mixes from vendors and dosed in combination Mostcorrosion and scale control program chemicals are two drum formulations due to the incompatibility of some chemicals in theconcentrated state Therefore separate feed systems are generally provided for chemicals for control of corrosion and scale (2drums) and biological growth (if liquid) If a program includes fixed pH then an additional feed system is required to add acidWater treatment programs are site specific and changing The COOLING WATER SYSTEMS SPECIALISTS should beconsulted for recommendations on the internal water treatment programs when an analysis of the available water is known
Alkalinity (pH) Control
Cooling water treatment programs presently available fall into two basic categories those that do not control pH (floating pH or alkaline treatment programs without pH adjustment) and those that control pH within a specified range by adding acid Theprograms that do not control pH are gaining favor because of the additional cost equipment and precautions required tominimize the risk associated with handling acid and because of improvements in chemicals to control depositsHigh pH in the recirculating cooling water is a result of the alkalinity of the fresh water due to bicarbonate salts This combinedwith carbon dioxide results in the water having a pH range of 6 to 85 When this fresh makeup water is mixed with therecirculated cooling water and passed through the cooling tower the air flow strips out the carbon dioxide Bicarbonate ionsthen convert to carbonate ions raising the pH which can cause both calcium carbonate and calcium phosphate salts toprecipitate out of solution and form deposits Another debit in allowing the cooling water pH to increase is increased chlorineconsumption to control biological growthsIf a program using pH control is justified and chosen acid addition is used under control of a pH analyzer to controlrecirculated cooling water alkalinity and pH to the range required by the treatment program
Corrosion Control
To reduce corrosion to an acceptable level chemical corrosion inhibitors which form protective films on heat transfer surfacesare the most effective protection Inhibiting corrosion is accomplished by phosphates organics zinc nitrites and molybdatesalts Unfortunately the use of chromate which is a reliable corrosion inhibitor is prohibited by environmental constraintsNitrite is not practical in an open system due to atmospheric oxygen converting the nitrite to nitrate
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ExxonMobil Proprietary
Section Page
XXVII 12 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Phosphate inhibitors (orthophosphate polyphosphate phosphonate) are the basic treatment for new towers in todaysenvironment In addition to the phosphate chemistry salts of zinc may be added to aid in inhibiting corrosion These ions
combine with the hydroxide ion forming thin protective films Zinc is used to provide a synergistic effect in combination with thephosphate salts for protecting carbon steel surfaces from corrosion Finally all-organic programs are offered by severalmanufacturers The use of these treatments has been made possible by the development of polymer dispersants which givecalcium salts a much higher solubility and reduce the tendency to scaleWhen a cooling water system contains copper alloys azole based and azole substitutes are required as copper corrosioninhibitors There are several effective inhibitors The most common is tolytriazole (TTA) due to its resistance to chlorineThe corrosion inhibitors most frequently used in ExxonMobil recirculating cooling water systems are shown below indescending order of preference provided their use meets all environmental requirements
bull Stabilized Phosphate (OrthoPolyphosphate) - Environmental restrictions on the use of zinc have resulted in ExxonMobilplants in the USA using a blend of orthophosphate and polyphosphate type corrosion inhibitors with good results Thistreatment forms a thin protective calcium phosphate film on heat transfer surfaces through the use of orthophosphateconcentrations as high as 12 - 17 wppm The use of calcium phosphate dispersants and Hydroxethylidene Diphosphonate(HEDP) stabilizes the treatment and prevents the formation of calcium phosphate scale Concentration of the dispersantsdepends upon the calcium concentration temperature and pH Stabilized Phosphate treatment programs can operate in apH range of 68 to 78 Alternatively an alkaline (floating pH) stabilized phosphate treatment program can be used Thisprogram uses little or no acid addition and is gaining in popularity although not well established yet It operates with pHvalues of 8 - 9 and uses higher quantities of dispersants and lower levels of phosphate A corrosion inhibitor for copper and brass is normally included for all stabilized phosphate programs Several vendors offer these programs Twoseparate chemical feeders are generally required
bull Zinc Alkaline Phosphate - Heritage Exxon plants in Europe where the use of zinc is not restricted have obtained goodcontrol of corrosion and scaling using zinc-polyphosphate treatment programs Low (65 - 72) and high (73 - 80) pHprograms have been used The high pH (zinc alkaline phosphate) programs are the lowest cost and are recommended
As with the Stabilized Phosphate treatment dispersants such as HEDP and polyacrylates are required to prevent theformation of zinc or calcium phosphate scale on heat transfer surfaces A corrosion inhibitor for copper or brass isnormally used with this program which is offered by several vendors
bull Organic Salts - All-organic (polymer) programs are offered by several manufacturers and have been used successfullywithin h Exxon In general these treatments are based on a combination of a phosphonate to control corrosion polymers(dispersants) to prevent scale deposition and a corrosion inhibitor for copper or brass This type of program is mosteffective in conditions of high hardness high alkalinity (M alkalinity gt300 wppm as CaCO 3) and high pH values (gt 85)However some manufacturers have reported success in softer water with alternative polymer component formulations Ingeneral an inorganic corrosion control program is preferred to the all-organic programs which are sensitive to alkalinityand more costly
Detailed control limits for effective use of each of the above inhibitors and materials selection are covered in Report NoEE34E86 Environmentally Acceptable Cooling Water Treatments Report No EE102E78 Cooling Tower Water Treatment Guidelines and in the EMRE Refinery Construction Materials Manual
Scale and Sludge Control
Scale and sludge deposits in cooling water systems cause loss of heat transfer and provide the conditions for pitting corrosionScale consists of inorganic salts which have precipitated from the cooling water forming a coating on heat transfer surfacesSludge consists of organic and inorganic suspended matter that settles in quiescent areas and in heat exchangers The key toprevention of both scale and sludge formation in a cooling system is keeping water velocities greater than 3 fts (09 ms) andthe use of chemical additives in combination with a blowdown rate that keeps the impurity concentration below the level whichcauses deposits (See Table 2 for limits on impurities) Scale control has become increasingly important in higher alkalinityldquofloating pHrdquo water treatment programsSalts most commonly found in scale deposits are calcium carbonate calcium sulfate tricalcium phosphate zinc phosphatemagnesium silicate and silica The control method for preventing scale from each of these salts is discussed below
bull Calcium Carbonate - Where justified using pHalkalinity control in the range of 70 to 75 will normally inhibit the formationof calcium carbonate scale if anti-scalent chemicals (phosphonates acrylates copolymers) are part of the chemicaltreatment (See Report No EE102E78 Cooling Tower Water Treatment Guidelines ) Higher pH water treatmentprograms require greater concentrations of these chemicals along with dispersants and flocculants to inhibit the formationof scale and precipitation of other salts
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 13 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
bull Calcium Sulfate - When heat transfer skin temperatures are below 250 degF (120 degC) and the cooling water bulktemperatures are under 130 degF (54 degC) gypsum scale (CaSO 42H 2O) will form if the product of the calcium and sulfate ionsconcentrations each expressed as wppm CaCO 3 equivalent exceeds 500000 Gypsum scale is prevented bymaintaining bulk water temperature below the indicated limits and controlling calcium and sulfate ions through blowdownor through the use of newer dispersants on the market
bull Tri-Calcium and Zinc Phosphate - Almost all chemical additives for corrosion control contain some type of phosphatesalt These have a tendency to form the orthophosphate ion depending on time pH and temperature Cooling watersusing these corrosion inhibitors may have an orthophosphate concentration in excess of 5 wppm as PO 4
= To preventcalcium phosphate scale Table 2 shows a limit of 1000 wppm of calcium as CaCO 3 equivalent when using a phosphateinhibitor in combination with a chemical dispersant (See Report No EE102E78 Cooling Tower Water Treatment Guidelines for solubility of calcium phosphate) These inhibitors and dispersants generally fall into one of the followingcategories
ndash Sequestrants antiprecipitants and inhibitors such as hydroyethylidene diphosphonate (HEDP) phosphate esters andaminomethylene phosphonate (AMP) function by defyingdistorting the crystal structure of the scale formations toprevent nucleation and growth of the scale
ndash Dispersants based on either anionic or nonionic polymers such as polyacrylamide polyacrylate or polymaleic acidfunction to stabilize colloidal and scale-forming particles in suspension Anionic polymers prevent agglomeration of suspended particles by either charge repulsion or crystal distortion while nonionic polymers reduce the surfacetension of particles
bull Silica Deposits - Quantitative indices for determining limits for silica in combination with other ions are not available due tothe formation of complex polymers Instead the rule of thumb limit of keeping reactive silica concentrations in the coolingwater below 150 wppm as SiO 2 has been successful
Possible alternatives for controlling sludge are
bull Chemical Dispersants - Water treating chemical suppliers now offer many types of dispersants These are mostlypolyelectrolytes Claims for removal of slime silt and mud from cooling tower basins and exchangers are madeHowever satisfactory results using these chemicals as a coagulant aid require very accurate control of the amount fed tothe water and stable operation of the water system Also suspended solids should be kept below 200 wppm for particleslarger than 045 microns Dispersants are used in virtually all fresh water cooling tower systems despite their high cost
bull Sidestream Filtration - The use of a sand pressure filter is the most reliable method for sludge removal Operating andmaintenance costs are minimal for this equipment
bull Higher Blowdown Rates - This may be an economical alternative depending on the cost of makeup water Additionalchemical costs must be included in any evaluation of this method
bull Makeup Clarification - This alternative may require additional raw water treatment facilities to handle seasonal variationsin turbidity andor suspended solids This method is applicable only where suspended solids come from makeup water
Biological Control
Operating conditions in recirculated cooling water are ideal for the growth of biological matter Water temperatures from 70 to120 degF (20 to 50 degC) favorable water pH continuous supply of nutrients including carbon nitrogen and phosphorus inorganicsalts and sunlight all lend themselves to environmental conditions encouraging microbiological growth and plant lifeIf biological growths get out of control and form large sticky agglomerations some of the following operating problems willresult
ndash Fouling of heat transfer surfaces by bacterial slimes resulting in flow restrictions and high process temperatures ndash Reduced cooling tower efficiency resulting from algae fungi and bacterial slime growths in the water distribution basin
and fill area of the cooling tower ndash Excessive plugging of screens and filters ndash Corrosion ndash Wood destruction by fungi ndash Clogging of water distribution nozzles ndash Excessive sludge accumulation in the cooling tower basin
An important part of the water treatment program is to control biological activity by feeding a biocide chemical Oxidizingbiocides such as chlorine sodium hypochlorite chlorine dioxide or bromine-associated compounds are most commonly usedwith good results Plate counts should be maintained below 50000 countsml (bacterial)
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ExxonMobil Proprietary
Section Page
XXVII 14 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Chlorine is the most effective biocide for cooling water because of its low cost extensive period of successful use ease of application and minimum environmental effects if applied under controlled conditions Either a liquid feed (sodium
hypochlorite) or a gas feed (100 chlorine) is added to the cooling tower basin A sufficient dosage is that which obtains a 01to 03 wppm continuous free residual in the hot return water to the cooling tower
Advantages to continuous chlorination over shock treatment based on ExxonMobil experience are ndash Minimizes chlorine degradation of polymers ndash Better reduction in total aerobic bacteria plate count resulting in cleaner heat transfer surfaces ndash Does not cause significant pH depressions ndash Better control results with less chlorine consumption
Details concerning chlorine addition as well as the other alternative biocides are given in Report No EE102E78 Cooling Tower Water Treatment Guidelines and in EMRE Engineering Water and Wastewater Design Guide TMEE 080 DG 11-3-1Gas Chlorinators for Water Treating Service and DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) Gaseous chlorine systems have become more costly in recent years because of the additional precautions equipment andprocedures required to minimize the risk associated with handling this gas Alternatives to gaseous chlorine are available butrequire more attention and have a higher operating cost Sodium hypochlorite (either purchased or generated) and sodiumbromide stabilized solid halide (chlorinebromine) products chlorine dioxide and ozone are all alternatives which may haveapplications Contact the COOLING WATER SYSTEMS SPECIALISTS for guidance in the use of these alternative treatmentschemes
BLOWDOWN
Blowdown control is required to maintain the proper cycles of concentration in the system The most reliable and simplest typeof control for blowdown is specific conductance measurement of the cooling water Electrical signals from the specificconductance measurements of some impurity in the cooling water are used to actuate an automatic blowdown valveImpurities normally used for blowdown control are dissolved solids chloride calcium magnesium and silica
CONTROLS AND MONITORS
Any chemical treatment used to control problems in a cooling water system must be carefully monitored to be completelyeffective
The installation of automatic controls should be considered for recirculated cooling water systems in all plants Designs shouldinclude pH conductivity corrosivity inhibitor and biocide control Automatic controllers are available which continuouslymonitor the condition of the water and automatically make chemical additions to keep the water within compositional limitsTest exchangers and corrosion coupons are also useful in monitoring the effectiveness of the water treatment program
An oil-in-water monitor should be considered to provide early detection of oil leaks into the cooling water This will enablecorrective action to be taken before extensive contamination of the cooling water circuit occurs This is especially important incooling towers with high efficiency cellular fill which can be difficult to clean if significantly fouledManual control systems may be used in locations that have adequate technical personnel to perform routine water analysesand to modify chemical treatment necessitated by sudden variations in water composition process contamination etc
WASTEWATER REUSE POSSIBILITIES
Recirculating cooling water systems normally have the highest plant raw water demand A potential alternative is to use treatedwastewater to supply part or all of the makeup water requirement The treatment required must be determined but will typicallyinclude primary oil removal (eg API Separator) secondary oil and suspended solids removal (eg filtration dissolved or induced air flotation) and soluble carbon removal utilizing biological treatmentThe reuse of treated wastewater as makeup currently exists at only a few locations If this alternative is being considered refer to Report No EE58E85 Reuse of Wastewater as Cooling Tower Makeup and Section XIX-B Water Reuse
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 15 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS
STEP 1 - Select a location for the intake station and pumps
Factors which influence this selection are as followsbull Depth of water and tidal variations
bull Relative location of intake and outfall direction of current
bull Length and size of intake suction piping vs pump discharge piping
STEP 2 - Determine cooling water requirements
Determine the design cold water inlet temperature based on site data covering seasonal water temperature as a function of inlet location and depth (for existing locations refer to Table 2 in Section IX-B )Tabulate the cooling water requirements for each process unit and offsite consumer Apply the proper LGFs to all coolingloads depending on the stage of the project and whether the new units are exact duplicates of existing unitsIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The designer should investigate methods to increase effluent water temperature to the maximumrecommended limit for the particular type of water used One common method is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 3 - Determine the total required capacity for water intake facilities
Process Coolers - The total requirements determined in Step 2 including LGFsFirewater - This is often supplied from the intake bay of a once-through systemFuture Expansions - These should be considered in sizing intake facilitiesTotal Required Capacity - The sum of the above three items
STEP 4 - Provide water intake facilities
Intake Crib - The intake crib with its support structure is located at the extreme end of the water intake line (See Figure 4 )The intake is usually located 200 to 300 ft (60 to 90 m) offshore and at a sufficient depth below the lowest water level to
preclude vortex formation and subsequent pump cavitation during storm conditions Additionally the crib should have 3 to 5 ft(1 to 15 m) clearance above the sea floor to minimize sand and silt pickup A concrete velocity cap should be specified to cover the inlet crib This will minimize the incidence of fish and other foreignmatter entering the system A horizontal inlet flow pattern should be designed with a maximum entrance velocity of 1 fts(03 ms) Experience shows that vertical velocity flow confuses fish and they are drawn into the intake lineThe intake crib support structure protects the crib from adverse effects of storms and tidal motions as well as protecting it fromimpact of floating and submerged objects A self-cleaning screen can be specified to prevent entrance of large objects into thesystem at the cribIntake Line (Figure 5 ) - An intake line is required if the water depth along the shore does not permit water to flow directly intothe intake bay The intake line is usually buried in the sea floor and covered with a minimum required overburden Consultwith the COOLING WATER SYSTEMS SPECIALISTS for required coverage at a specific siteChlorine or sodium hypochlorite is injected at the intake crib to prevent marine growth in the lineNote that an intake crib and line will not be required if water of sufficient depth is available at the supply pump stationIntake Bay (Figure 6 ) - The intake bay is an enclosure which houses the pumps and screens Depending on a particular design a series of screens is used to ensure the supply of clean water Trash racks comprise the first element of the filter system These remove only very coarse material from the water such as bulk seaweed They can be supplied with atraversing raking mechanismLift screens provide secondary cleanup These screens can be lifted out of the intake bay for periodic cleaning Double liftscreens can be used if it is deemed necessary to have one in service at all timesFinal water filtering is usually done by traveling screens Details of the screen dimensions and the mounting are usuallydetermined during detailed engineering Screen net open area is typically approximately 50 of total screen area Thescreens should be sized for a normal water velocity of 12 fts (04 ms) through the open screen area Maximum velocityshould be limited to 24 fts (08 ms) with 50 of the screen area plugged This is the upper velocity limit for efficientscreening
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ExxonMobil Proprietary
Section Page
XXVII 16 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
If high suspended solids greater than 200 mesh (eg sand) are expected to be a continuous problem ejectors may be installedupstream and downstream of the traveling screen to remove accumulated sand and solids from the intake bay in these areas
Traveling screens and ejectors are supplied with wash water from the raw water pumps or from separate wash water pumpsOnoff control using timers should allow optimum scheduling of the ejectors depending on the time of the year
STEP 5 - Specify water intake facilities
Water intake facilities should be designed in accordance with the following guidelines from the Hydraulic Institute Standards The water intake should be designed to provide an evenly distributed flow of water to the pump suction A properly designedintake reduces the possibility of vortex formation and results in better pump performance with less noise Geometry of theintake and the manner in which water is introduced into the intake are important parameters in providing an even distribution of water flow Complete analysis of intake structures is best accomplished by scale model testsThe pump manufacturer will generally render advice on specific problems while the intake design is still preliminary He shouldbe provided with the necessary intake layout drawings showing the physical limitations of the siteFigures 7A and 7B have been developed for single- and multiple-pump arrangements to show suggestions for basic sumpdimensions They are for pumps normally operating in the capacity range of approximately 3000 to 300000 gpm (200 to20000 Ls) Since these values are composite averages from a great many pump types and cover the entire range of specificspeeds they must not be thought of as absolute values but rather as basic guides subject to some possible variations
All the dimensions in Figures 7A and 7B are based on the rated capacity of the pump at the design head Any increase incapacity above these values should be momentary or very limited in time If operation at an increased capacity is to beundertaken for considerable periods of time the maximum capacity should be used for the design value in obtaining sumpdimensionsDimension C is an average value based on an analysis of many pumps Its final value should be specified by the pumpmanufacturerDimension B is a suggested maximum dimension which may be less depending on actual suction bell or bowl diameters in useby the pump manufacturer The edge of the bell should be close to the back wall of the sump When the position of the backwall is determined by the driving equipment or the discharge piping Dimension B may become excessive and a ldquofalserdquo backwall should be installedDimension S is a minimum value for the sump width for a single pump installation This dimension can be increased but if it isto be made smaller the manufacturer should be consulted or a sump model test should be run to determine its adequacy
Motor size may dictate minimum pump spacing be especially aware of this when specifying pumps with unusually high pumpheads eg greater than 80 psig (550 kPa)Dimension H is a minimum value based on the ldquonormal low water levelrdquo at the pump suction bell taking into considerationfriction losses through the inlet screen and approach channel This dimension can be considerably less momentarily or infrequently without excessive damage to the pump It should be remembered however that this does not representldquosubmergencerdquo Submergence is normally quoted as dimension ldquoHrdquo minus ldquoCrdquo This represents the physical height of water level above the bottom of the suction inletThe actual submergence of the pump is something less than this since the impeller eye is some distance above the bottom of the suction bell possibly as much as 3 to 4 ft (1 to 13 m) For the purposes of sump design in connection with this chart it isunderstood that the pump has been selected in accordance with specific speed charts The submergence referred to hereinhas to do only with vortexing and eddy formationsDimensions Y and A are recommended minimum values These dimensions can be as large as desired but should be limitedto the restrictions indicated on the curve If the design does not include a screen Dimension A should be considerably longerThe screen or gate widths should not be substantially less than S and heights should not be less than H If the main stream
velocity is more than 2 fts (06 ms) it may be necessary to construct straightening vanes in the approach channel increaseDimension A conduct a sump model test of the installation or work out some combination of these factorsDimension S becomes the width of an individual pump cell or the center-to-center distance of two pumps if no division walls areusedOn multiple pump installations the recommended dimensions in Figures 7A and 7B also apply as noted above and thefollowing additional determinants should be consideredFigure 8A - Low velocity and straight-line flow to all units simultaneously is the first recommended style of pit Velocities in thepump area should be approximately 1 fts (03 ms) Some pumps with velocities of 2 fts (06 ms) and higher have given goodresults This is particularly true where the design resulted from a model study Not recommended would be an abrupt changein size of inlet pipe to sump or inlet from one side introducing eddying
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 17 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Figure 8B - A number of pumps in the same sump will operate best without separating walls unless all pumps are always inoperation at the same time in which case the use of separating walls may be beneficial If walls must be used for structural
purposes and pumps will operate intermittently one should leave flow space behind each wall from the pit floor up to at leastthe minimum water level and the wall should not extend upstream beyond the rim of the suction bell If walls are used increaseDimension ldquoSrdquo by the thickness of the wall for correct centerline spacing Round off the ends of the walls Not recommended isthe placement of a number of pumps around the edge of a sump with or without dividing wallsFigure 8C - Abrupt changes in size from inlet pipe or channel to pump bay are not desirable A relatively small pipe emptyinginto a large pump pit should connect to the pit with a gradually increasing taper section The angle should be as large aspossible preferably not less than 45 degrees With this arrangement pit velocities much less than one foot per second aredesirable Especially not recommended is a small pipe directly connected to a large pit with pumps close to the inlet Flow willhave excessive change of direction to get to most of the pumps Centering pumps in the pit leave large ldquovortex areasrdquo behindthe pumps which will cause operational troubleFigure 8D - If the pit velocity can be kept low enough [below 1 fts (03 ms)] an abrupt change from inlet pipe to pit can beaccommodated if the length equals or exceeds the values shown It is assumed that as ratio WP increases the inlet velocityat ldquoPrdquo will increase up to an allowed maximum of 8 fts (24 ms) at WP = 10 Pumps ldquoin linerdquo are not recommended unless theratio of pit to pump size is quite large and pumps are separated longitudinally by a generous margin A pit can generally be
constructed at much less cost by use of a recommended designFigure 8E - It is sometimes desirable to install pumps in tunnels or pipelines A drop pipe or false well to house the pump witha vaned inlet well facing upstream will be satisfactory in flows up to 8 fts (24 ms) Without the inlet well the pump section bellshould be positioned at least two pipe diameters vertically above the top of the tunnel not hung into the tunnel flow especiallywith tunnel velocities 2 fts (06 ms) or more There should be no signs of air along the top of the tunnel It may be necessaryto lower the scoop or to insist on a minimum water level in the vertical wellNote The foregoing statements apply to sumps for clear liquid
STEP 6 - Provide chemical treatment
Provide chemical treatment facilities for the once-through cooling water system Normally the only chemical treatment requiredin a once-through cooling system is for biological control Facilities for adding chlorine to once-through cooling water are thesame as those described for the recirculated cooling water system The design capacity of the chlorinator should be based onachieving a chlorine residual of 1 wppm in the effluent The chlorine required by the organics and other reducing agents in thewater must be satisfied before any residual chlorine will appear in the effluent The design capacity of the chlorinator should be
based on the total water chlorine demand plus 1 wppm in the effluent In the absence of water data the chlorination systemdesign capacity should be based on 10 wppm minimum instantaneous dosage for maximum cooling water flow For very largeonce-through cooling water systems it will be impractical to design for shock rates at this level In this case a continuousdosage of 1 - 2 wppm should be used (Refer to Step 5 of the DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS )
STEP 7 - Specify pumps and distribution system
Specify other major facilities in the once-through cooling water system The recommended materials for pumps valves andpiping are shown in the EMRErsquos Refinery Construction Materials Manual
Pumps1 Type - These pumps are usually vertical centrifugal pumps However horizontal centrifugal pumps have been used An
economic study andor consideration of existing facilities will determine which type should be specified In the event thathorizontal centrifugal pumps are specified facilities for pump priming will likely be required For large pumps in salt water service there is a very limited selection of pumps because of the special materials required Consult with the DISCIPLINESPECIALISTS early in the design effort to determine what specific pumps are currently available
2 Total Required Capacity - Firm capacity (excluding spares) of the supply pumps should equal the maximum cooling water requirements including LGF and RCF
3 NumberIndividual Pump Capacity - Normally one spare pump is provided for up to six operating pumps two spares for up to twelve operating pumps etc The most common arrangements for water supply pumps are
bull Two 100 capacity pumps
bull Three 50 capacity pumps
bull Four 33-13 capacity pumps
bull Five 25 capacity pumps
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ExxonMobil Proprietary
Section Page
XXVII 18 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Maximum commercially available pump capacity ranges up to 300000 gpm (20000 Ls) at discharge pressures up to 100psi (700 kPa) Maximum capacity will be less for higher discharge pressures Other factors which may influence pump
selection arebull Selecting capacity to match existing pumps
bull Selecting capacity for future requirements
bull Selecting capacity corresponding to manufacturers available standardVendor information should be obtained on the availability size and cost of the pumps for several configurations Acomparison should be made of the investments and operating costs as well as other advantages and disadvantages toselect a pump configuration
4 Drivers - These pumps are usually motor-driven Because of the remote location of the intake facilities the steamdistribution costs are normally too high to justify the use of a steam turbine driver However use of a secondary selectivesubstation provides a reliable source of power to the pump drivers
5 Pump ∆∆∆∆P - Pump ∆P is based on supplying the maximum cooling water requirement at the battery limit design pressureestablished for the system The ∆P should include the total circulating system pressure drop plus any elevation differencebetween the low water level at the intake and the discharge point of contact with air
6 Pump Instrumentation - The remote location of the intake facilities makes remote control and monitoring of the water supply operation desirable Pump controls located in the control house provide for minimum manning Local controlbackup should be provided The Owners operating and manning philosophy determine the basis for instrumentation andcontrolThe spare pump is controlled to automatically start on low pressure in the supply header Concurrently alarms should beactuated in the control room to indicate the low-pressure situation and spare pump operation
Distribution Network1 Configuration - The piping network is a once-through system with a return line from each consumer or consumer group to
the cooling water effluent line2 Layout - The routing of the piping network should connect all cooling water consumers in the most economical manner
permitted by the plant layout Careful consideration should be given to the turnaround philosophy of the plant Thenetwork layout should preclude the necessity of having above-ground water lines that are in service running through anarea that is being turned around Underground piping or an extra lateral may be required to accomplish this objective
3 Valves and Blinds - Battery limit valves should be provided in the network to isolate individual process units from thesystem Valves should be provided to permit onstream removal and maintenance of spared equipment
4 Basket Strainers - Consideration should be given to the installation of basket strainers in the laterals serving each of theconsumer loops These provide added insurance against entrance of foreign material into the cooling equipment The useof intake screens with 025 in (63 mm) openings may eliminate the need for basket strainers in the laterals
Line Sizing1 Design Basis - The network is sized so that water is delivered to the most distant consumer at a pressure equal to the
required process design (battery limit) pressure2 Flow Rate - Each segment of the distribution network is sized for the simultaneous maximum flow in that segment for any
operating condition The main supply and discharge headers are sized for the simultaneous maximum flow of all segments(not necessarily the sum of the simultaneous maximum flows of the individual segments)
3 Pressure Drop
Line Sizing Basis - The distribution system should be sized according to the guidelines established in Section XIV The use of an ldquoold piperdquo friction factor of C = 90 - 100 (Hazen-Williams factors) in the main lines is recommended to assuregood operation in later years when the system experiences fouling For large diameter lines [greater than 48 in (1200mm)] maximum water velocity should be limited to 10 fts (3 ms) based on civilmechanical considerations (supports andanchoring required to handle potential pressure surges) For salt water systems utilizing cement-lined pipe consider thereduced inner diameter in the friction loss calculations A study may be necessary to determine the economic pump ∆P vsline diameters
4 Equivalent Length Factors - Equivalent length factors are used in pressure drop calculations for sizing the distributionsystem The piping length scaled from the plot plan is multiplied by an equivalent length factor to compensate for pipingelbows expansion loops and other piping variations
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Section Page
COOLING WATER SYSTEMS XXVII 19 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
The equivalent length factors are selected based on the expected variation from a straight piping run Typical equivalentlength factors suitable for planning purposes for different piping applications are as follows
TYPE OF PIPING EQUIVALENT LENGTH FACTORS
Long piping runs [gt 1000 ft (300 m)] 12 to 14
Onsite distribution piping 15 to 20
For design purposes the actual piping layout should be evaluated to ensure that the equivalent length factors areapplicable to the specific situation Particular attention is required for revamp work and saltwater systems utilizing cement-lined pipe
STEP 8 - Prepare a system flow plan similar to the one shown in Figure 1
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS
STEP 1 - Determine cooling water requirements
Tabulate the cooling water requirements and cooling water discharge temperature for each process unit and offsite consumer Apply the proper LGFs to all cooling loads depending on the stage of the project and whether the new units are exactduplicates of existing units Determine the total cooling water flow required including LGFs and the bulk temperature (T 1) of thecombined cooling water flow from all coolers to the cooling towerIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The onsite and offsite designers should investigate methods to increase effluent water temperature to themaximum recommended limit for the particular type of water used This will result in a lower cost cooling tower for a given heatrejection rate One common method of maximizing return water temperature is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 2 - Select cooling tower design parameters
Select the proper design conditions of ambient wet bulb temperature approach temperature range and flow rate for specifyingthe cooling tower Based on these conditions the cooling tower vendor will size the tower and provide the tower characteristicand performance curves which serve as the basis for the performance guarantee Required performance test and evaluationprocedures are described in IP 8-1-1 Economic studies may be required to select the ambient wet bulb temperature andapproach temperature for new locations or for expansion projects with special cooling requirements
STEP 3 - Prepare cooling tower specification sheet
Include the following information in the heat exchange section of the design specificationTower Flow Capacity - This was determined in Step 1Type of Tower - The tower usually specified is an induced draft multicell type suitable for fresh salt or brackish water depending on the water available Additional tower characteristics are usually developed and proposed by the tower vendor (eg crossflow versus counterflow type of tower fill etc) The vendor should be made aware of conditions such as thelikelihood of heavy oil leakage into the cooling water or high fouling potential which may influence the tower type or fill
selectionNumber of Cells - The minimum number of cells required (normally not less than 2) may be specified The maximum flow per cell is a function of the cooling tower design conditions selected in Step 2 Typical values range up to 10000 gpm (600 Ls) for crossflow designs and 8000 gpm (500 Ls) for counterflow designs Normally spare cells are not provided Maintenance iscarried out during cooler periods or load is shed
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XXVII 20 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Ambient Wet Bulb Temperature1 The ambient wet bulb temperature selected for specifying the cooling tower design is generally that temperature which is
equaled or exceeded 5 of the time during the four hottest months (5 of the hourly readings taken during the four hottestmonths will exceed this value) Specifying the 5 temperature should give satisfactory results since the time that the wetbulb temperature is exceeded is for short duration and the system inertia is usually sufficient to avoid problems The tower specification should make it clear that the wet bulb temperature specified is an ambient value as opposed to the enteringvalue which may be higher because of recirculation The tower manufacturer should adjust the ambient value upward totake into account his estimates of recirculation Currently cooling tower test codes provide procedures for measuringperformance in the case of either entering or ambient wet-bulb temperature specifications An increase of 2 degF (11 degC) isgenerally added to ambient wet bulb temperature to estimate entering wet bulb temperature for large cooling towers
2 If plant production will peak during the four summer months when the 5 ambient wet bulb temperature may be exceededconsideration should be given to specifying a higher design ambient wet bulb temperature (one that will be exceeded 212 of the time for example) However the incremental cost of the resulting closer approach and larger cooling tower must be justified by the value of the incremental products made
On-Tower Temperature (T1) - This temperature cannot be accurately determined until the design of all process coolers iscompleted and the volume of cooling water and outlet temperature are known for each (see Step 1) In the planning stage or early design stage of a project this temperature must be estimated and is usually set equal to the maximum design cooler outlet temperature shown under COOLING WATER REQUIREMENTS in the BASIC DESIGN CONSIDERATIONS portion of this sectionApproach Temperature - The approach temperature has the most pronounced effect on cooling tower size and cost For example reducing the approach from 10 to 5 degF (55 to 3 degC) can increase the cooling tower investment by 50 percent and thefan operating cost by 65 percent Thus various approach temperatures should be evaluated to determine the minimum cost of the total system including the process exchanger area Approaches used in the past designs have been from 5 to 12 degF (3 to7degC) Note that it is not customary in the cooling tower industry to guarantee approaches of less than 5 degF to (3 degC) to coolingtower design inlet wet bulb temperature A typical approach for a new grass roots cooling tower is 10 degF (6 degC)Off-Tower Temperature (T2) - This is the sum of the approach temperature and the ambient air wet bulb temperature For existing locations Table 2 in Section IX-B may be used
Range and Flow Rate1 The range for a given heat load is related to the circulating water rate as follows
Q = M C p ∆Tcong (500) (gpm) (range) Eq (1)
where Q = Heat load on the tower BtuhM = Water circulating rate lbhCp = Specific heat capacity of water at average temperature Btulb degF∆T = (T 1 ndash T 2) degF = (water cooling range)gpm = Circulation rate in US gallons per minute
For calculation in Metric units
Q = M C p ∆Tcong (1162 x 10 -3) (Ls) (range) Eq (1M)
where Q = Heat load on the tower kJh
M = Water circulating rate kghCp = Specific heat capacity of water at average temperature kJkg degC∆T = (T 1 ndash T 2) degC = (water cooling range)Ls = Circulation rate in liters per second
2 Generally lower tower investment fan horsepower and pump horsepower result from maximizing the range andminimizing the flow rate However based on fouling considerations of process coolers the maximum allowable hot water temperatures must be kept below recommended limits Also while increasing the range minimizes cooling tower investment and operating cost the cost of exchanger surface requirements is adversely affected by the LMTD and acompromise must be made The range is bounded by the maximum allowable hot water temperature and the Off-Tower temperature (T 2) For established locations refer to Section IX-B for design cold water temperatures to assist in selectingthe approach and range
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Section Page
COOLING WATER SYSTEMS XXVII 21 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Makeup Water Requirements - Calculate the various losses in the cooling water system to determine makeup requirementsThese losses include
1 Evaporation Loss (E) - Allow 1 of the circulation rate for each 10 degF (55 degC) temperature drop through the tower Notethat the actual evaporation loss will be somewhat less depending on ambient air temperature
( )sL555
TT)gpm(
1000TT
E 2121
minus=
minus= Eq (2)
2 Drift Loss (D) - IP 8-1-1 limits the maximum allowable drift loss to 0005 of the design water circulation rate for freshbrackish or salt water
( )sL1000050
)gpm(1000050
D
=
= Eq (3)
This low drift level contributes little to solids removal from the tower with the spray This allows higher cycles of concentration to be achieved in cooling towers which use clarified softened water for makeup and thus have low requiredblowdown rates Higher drift loss percentages in older or poorly maintained towers will limit the maximum cycles of
concentration achievable3 Miscellaneous Losses (O) - Miscellaneous losses are estimated to be one-seventh of the difference between evaporation
loss and drift loss
minus=
7DE
O Eq (4)
4 Blowdown Rate (B) ndash Guidelines for maximum concentrations of impurities allowed in recirculated fresh cooling water areshown in Table 2 For salt water the blowdown is based on keeping the total dissolved salts below 55000 wppm Obtainthe chemical analysis of the makeup water and divide each individual component concentration into its counterpart inTable 2 The result is the maximum allowable cycles of concentration for that constituent (a worksheet is provided inTable 4 ) The impurity with the lowest factor will be the control and will determine the required blowdown rate
)OD(1Cycles
EB +minus
minus= Eq (5)
The makeup water requirement is the sum of all losses shown above
MakeUp = E + D + O + B Eq (6)
STEP 4 - Specify the controls necessary for proper cooling tower operation
Air Side - Cooling towers are provided with motor-driven manually adjusted pitch multiple-bladed propeller fans Figures9A amp B may be used to estimate the normal operating horsepower requirements Since the fan motor and gear reducer arelocated at the top of the tower [generally 40 to 50 ft (12 to 15 m) above grade] the need for equipment to handle thesecomponents should be discussed with the affiliateControl of the air flow through the tower to maintain reasonably constant cold water temperatures can be accomplished byturning off individual fans in multiple-cell installations adjusting the blade pitch or using variable speed motors To turn off individual fans remote at grade start-stop buttons for the motors are usually specified
Whenever the ambient air temperature is 32 degF (0 degC) or below icing of the tower louvers and structural members in contact withthe incoming air will occur If the heat load on the tower is relatively free of seasonal variations two-speed fans should provideadequate ice control down to approximately 0 degF (-18 degC) ambient temperatures When sustained temperatures of 0 degF (-18 degC)and below occur two-speed fans capable of two-speed reverse operation with appropriate time delays for change of directionand speed should be specified to periodically eliminate ice which will build up on the louvers Other methods of winterizationare available but in general are more expensive and require greater operator attention In very cold climates steam heatingshould be provided to prevent freezing in the cold water basin during shutdown periodsWater Side - The water circulation rate is maintained by operating one or more pumps of sufficient capacity to meet the systemdemand Flow-measuring facilities local temperature indicators and a high-temperature alarm are provided on the coolingtower water supply and return headers Hand-operated flow control valves are required for distributing the water equally to theindividual cells On counterflow towers provisions for measuring the flow to each cell (pitot tubes) are required They are notrequired on crossflow towers since the open distribution basin provides visual aid in balancing flows Shutoff valves should beprovided for servicing the flow control valves without shutting down the entire tower
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XXVII 22 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
STEP 5 - Provide chemical treatment facilities for the fresh water system
Alkalinity (pH) Control (if used)1 In cases where a controlled pH corrosionscale control program is being used the alkalinity (pH) of the recirculated cooling
water should be controlled in accordance with the program requirements Normally acid addition is required to accomplishthis This is due to free carbon dioxide being stripped from the cooling water in the tower to approximately 5 ppm For example if the alkalinity has to be controlled to between 25 and 50 ppm for a pH of 70 to 75 the volume of 98 sulfuricacid (66 degBeacute) required to treat the makeup water is calculated using the following equation
[Note 93 sulfuric acid should be used if ambient temperatures will be less than 35 degF (2 degC)]
GallonsHour 66 degBeacute H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate gpm) (326 x 10 -5)
For calculation in Metric limits
kgh of 100 (66 degBeacute) H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate Ls) (793 x 10 -3)
2 The metering pump should be designed for a maximum feed rate of two to three times the normal feed rate A glass woolfilter is required on the pump suction piping to minimize plugging Normally two - 100 capacity pumps are provided
Positive displacement metering pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumps with flow control areused for higher rates
3 Hydrochloric acid may also be used for alkalinity control The gallons per hour of 20 degBeacute hydrochloric acid required is36 times the amount of 66 degBeacute sulfuric acid In Metric units the weight of 32 (20 degBeacute) hydrochloric acid required is227 times the weight of 100 (66 degBeacute) sulfuric acid
4 Specifications for chemical feeders may be found in EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased)
5 A pH analyzer with high and low alarm is provided to control the amount of acid feed The acid should be added to thecooling tower makeup water (Figure 2 ) The lag time between the point of acid injection and the sample outlet to the pHcell should be less than two minutes for effective control
6 Spills or overflows from the sulfuric acid tank should be contained
Corrosion Inhibitors and Anti-Scalents1 Two chemical feeders are generally provided The following guides in sizing the equipment will allow the use of any of the
proprietary products from the vendors when final selection by the owner is made See Report No EE102E78 Cooling Tower Water Treatment Guidelines EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and Report No EE34E86 Environmentally - Acceptable Cooling Water Treatments for detailed discussion of acceptable commercial chemicals
bull If the water treatment program to be used is known design for the chemical rates specified by the vendor or COOLING WATER SYSTEMS SPECIALIST
bull Otherwise design for 100 wppm of each commercial liquid product in the recirculated cooling water The design feedrate should be based on the design total blowdown flow (both uncontrolled and controlled) since this is where theproduct is lost For 100 wppm concentration the chemical rates will be as follows
GallonsHour Chemical = Blowdown (gpm) x (0006)
LitersHour Chemical = Blowdown (Ls) x (036)
2 The design capacity of the metering pump should be two to three times this calculated number Normally 2 - 100capacity pumps are provided Positive displacement pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumpswith flow control are used for higher rates
3 Specifications for chemical feeders may be found in EMRErsquos Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased) Reference should also be made to the checklists containedin Report No EE92E94 Guidelines for Safety Evaluation of Chemical Injection Facilities
4 A final check should be made with the owner and the COOLING WATER SYSTEMS SPECIALISTS as to the treatmentprogram selected and if there is a need for an additional chemical feeder
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COOLING WATER SYSTEMS XXVII 23 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Biological ControlFacilities should be provided for feeding the biocide into the back of the cooling tower basin If chlorine is used it must bepresent in the cooling water as free residual to be effective However the following amounts of chlorine are first required toreact with the listed impurities when present in the cooling water before a free residual can be established
IMPURITIESAMOUNT OF CHLORINE REQUIRED (wppm)
WITH EACH WPPM OF IMPURITY
Ammonia (NH 3)
Hydrogen Sulfide (H 2S)
Sodium Sulfite (Na 2SO 3)
Ferrous Iron (Fe ++)
10
22
056
064
The design capacity of the chlorine feeder shall be based on achieving a 01 wppm free chlorine residual in the recirculatingwater flow or a minimum dosage of 7 wppm at maximum design makeup water flow rate and conditions (see EMRE Water and
Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service )Chlorine may be fed either as the commercial liquid sodium hypochlorite (124 wt Cl 2) or liquid chlorine (100 Cl 2) Liquidhypochlorite is being used increasingly because of the higher costs associated with precautions equipment and proceduresrequired to minimize the risks associated with handling gaseous chlorine The hypochlorite solution is fed using a meteringpump which should be sized to feed an equivalent of 15 ppm 100 Cl 2 (Refer to EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) ) In case of spills or an overflow from thestorage tank the sodium hypochlorite may be sent to the cooling tower basinGas chlorinators are covered by EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service The use of gas chlorination is being reduced but may still be appropriate in some locations subject tolocal management approval The gas from either commercial cylinders or an evaporator is dissolved in a water slip stream bymeans of an eductor This results in a chlorine solution strength of approximately 01 This solution should be evenlydistributed below the water surface by means of a header lateral pipe arrangement at the rear of the cooling tower basinopposite the cooling water pumpsSome additional guides on the use of gas chlorinators are given below It is also recommended that the safety guidelines
published by the Manufacturing Chemists Association and the Chlorine Institute be followedbull Dilution water for the chlorinator must not be less than 50 degF (10 degC) or ice formation may cause plugging problems
bull Continuous rates of chlorine gas withdrawal should not exceed 400 lb (180 kg)24 hours from each one ton cylinder atroom temperature [70 degF (21 degC)] This rate may be exceeded (up to 50) for periods not exceeding 2 hours Rule of thumb is not to design for more than 4 cylinders operating in parallel When feed rate exceeds 3000 lb (1360 kg) per daythe liquid chlorine should be fed to an evaporator upstream of the chlorinator
bull If the ambient temperature can be expected to be below 50 degF (10 degC) for extended periods a building should be providedfor the chlorinator and cylinders
bull Weighing scales should be provided to determine when shipping containers should be replaced and to verify chlorine feedrates
bull Analyzers to measure the free chlorine residual in the circulated cooling water should be provided
bull Chlorine leak detectors and a catch basin for major spills should be provided
Control SystemProvide automatic controllers to continuously monitor water condition and to add chemicals to the system (complete systemsmay be available through the water treatment program vendor as a lease or purchase)pH Control - This controller adds acid to correct water alkalinity a timer on the acid pump sounds an alarm if the pump doesnot shut off in a predetermined length of time Backup controls are required to prevent the cooling system from beingpermanently damaged in case the pH electrodes become fouledCorrosivity and Inhibitor Control - Systems are available which give a direct readout of corrosion rate in milsyear If thecorrosion rate exceeds a set point the controller shuts off the acid increases the inhibitor addition opens the blowdown valveand sounds an alarm Electrodes measure the completeness of the inhibitor film by the value of the potential differencebetween the electrodes The larger the difference the more incomplete the film and a potential reading exceeding a set levelactivates the inhibitor feed
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XXVII 24 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Free Residual Chlorine Control - Free residual chlorine should be controlled automatically for biocide treatment Severalgood analyzers are currently available These units can be incorporated into an automatic control system
Oil in Water DetectionMonitoring Corrosion and Deposit FormationThe following types of devices for measuring corrosion rates and deposit formation should be provided to allow the plant tomonitor the effectiveness of the cooling water treatment before scheduled turnaroundsCoupons - A corrosion coupon rack should be specified for the hot return water to the tower in accordance with IP 19-6-1 Corrosion Monitor - On-line corrosion analyzer if specified should monitor the hot return water to the tower in accordance withIP 19-6-1
On-Line Test Exchanger Although there are a variety of deposit monitors from the water treatment chemical vendors they do not necessarily representactual heat transfer conditions in process exchangers For this reason a test exchanger should be installed for evaluatingcorrosion and deposit formation Details are covered in Report No EE102E78 Cooling Tower Water Treatment Guidelines
Alternatively a critical process exchanger can be instrumented such that its heat transfer coefficient can be monitored
Instrumentation would include flows and inlet and outlet temperatures of process fluid and cooling waterSTEP 6 - Specify cooling tower location
Select a location for the cooling tower The following factors influence this selection
Location Factors1 The location should be within a reasonable distance of the units served and consistent with future refinery plans Piping
costs to and from battery limits can be a significant portion of the total cooling system cost and can be the location-determining factor particularly for salt water systems While piping costs can be easily determined other costs affectinglocation cannot be estimated readily Therefore judgment is required in selecting the location
2 Safety spacing requirements (see Section XV-G Equipment Spacing )3 Cooling tower contribution to plant boundary noise4 For salt water cooling towers wind direction and distance to other facilities is of major concern because of potential
corrosion problems
5 Factors affecting cooling tower performance ie orientation with respect to wind site congestion etc
Plot RequirementsIn estimating plot requirements (basin area) one should allow 5 to 6 ft of tower length for each 1000 gpm (2 m for each 60 Ls)of circulating capacity This figure is based on a typical counterflow design with a width of 50 ft (12 m) and does not includepump suction pit space Exact requirements can be obtained from cooling tower vendors after the tower capacity has beenestimated
Fogging and DriftTwo distinct problems are associated with the discharge air from a cooling tower (1) fogging which can be a nuisance or ahazard and (2) drift which is generally a nuisance because of increased corrosion of surrounding equipment Fogging fromcooling towers occurs during periods of combined high ambient humidity and low temperature as a result of mixing the warmmoist cooling tower exhaust air with the cooler ambient air The ambient air cannot absorb all moisture as vapor and thus tinyfog droplets are formed by condensation Fogging and resulting icing in cold climates has produced hazardous drivingconditions on highways up to 850 ft (250 m) awayProprietary designs are available to combat potential fogging problems Basically these designs incorporate heating of thedischarge air with hot return water precooling coils (similar to an air cooler) to eliminate any visible plume However thesedesigns are expensive and relatively unproven Therefore proper location based on reliable weather data is the preferredsolutionDrift from a cooling tower represents a more difficult problem since it does not depend on definable climatic conditions andoccurs for all wind directions The effects of drift on nearby equipment and vegetation are particularly severe and generally of concern only for salt water systems because of the high solids content of the drift (concentrated salt water) For salt water cooling systems electric lines substations and instrumentation should be located upwind in the direction of the predominantwind They should be provided with adequate shelter or materials when within 450 ft (140 m) of the cooling tower In additionspecifications for all equipment within 150 ft (45 m) of the tower should include surface treatment suitable for a marineenvironment
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COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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DESIGN PRACTICES December 2000
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DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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DESIGN PRACTICES December 2000
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TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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DESIGN PRACTICES December 2000
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FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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DESIGN PRACTICES December 2000
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FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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Section Page
XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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Section Page
COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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Section Page
XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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Section Page
XXVII 10 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
MATERIALS OF CONSTRUCTION
The EMRE Refinery Construction Materials Manual and IP 8-1-1 list acceptable materials for all components in each type of cooling water system (once-through and recirculating) For salt water systems emphasis must be placed on proper materialsselection throughout the system The external salt water environments in the vicinity of the tower itself and the intake stationsmust be taken into consideration Proper siting of the tower is extremely important to avoid salt water corrosion of adjacentunits or the need to use expensive materials to operate in such an environment
WATER TREATMENT FOR ONCE-THROUGH SYSTEMS
Once-through salt and fresh cooling water systems can develop a variety of problems such as fouling due to seaweed or other debris marine microorganisms such as muscles and shell fish and biological growth Corrosion and scaling problems areminimal in fresh water once-through systems because the water is not concentrated by evaporation In salt water systemscorrosion is minimized by proper materials selection and scaling problems generally do not occurProtection against fouling by seaweed or other debris is provided by careful selection of the water intake location and travelingscreens with washing facilities In some cases additional steps may be required These may include an additional secondaryset of traveling screens and possibly battery limit strainers on the cooling water supply lines to individual unitsProtection against fouling by marine microorganisms and biological growths is achieved primarily by injecting chlorinehypochlorite chlorinebromine or other similar chemicals which kill these growths Chlorine is commonly used and is injectedin sufficient quantities to produce a residual of 1 wppm in the effluent for one hour each day (shock chlorination) Continuouschlorination for 2 - 3 days per month at lower levels (03 wppm residual chlorine in the effluent) may also be used in addition toshock chlorinationThe large water volume discharged from once-through systems usually makes chemical treating for corrosion or deposit controltoo costly These problems are handled by proper materials selection and by keeping water velocities through exchanger tubesgreater than 3 fts (09 ms) Occasionally the water analysis of a fresh water source being considered for a once-throughsystem using carbon steel materials will indicate the need for some corrosion or deposit control In this case the cost of thethreshold treatment with the once-through system should be compared to the costs of a recirculated system before making afinal choice
WATER TREATMENT FOR SALT WATER COOLING TOWER SYSTEMS
A total dissolved salts (TDS) concentration limit of 55000 wppm for sea water in cooling tower systems requires high blowdownrates since the makeup sea water typically has from 35000 to 40000 wppm TDS Because of cost restraints the only water treatment normally specified for these systems is for biological control Chlorine is most commonly used either as directchlorine injection or hypochlorite produced in electrochlorinators Care must be taken in selecting the intake location to assurethat the suspended solids do not exceed 200 wppm in the recirculated water If it is not possible to find sea water with sufficientclarity clarification with polyelectrolytes should be evaluated Corrosion is one of the major problems with the use of sea water in cooling towers This requires proper materials selection
WATER TREATMENT FOR FRESH WATER COOLING TOWER SYSTEMS
Cooling tower water treatment is necessary to minimize or eliminate corrosion scale and biological fouling of process heattransfer surfaces caused by minerals and impurities in the water The difficulties caused by these impurities and the means of treatment are shown in Table 1 Water lost by evaporation results in an increase in total dissolved salts in the recirculated water To indicate the degree of concentration of impurities the term cycles of concentration is used
Corrosion is an electrochemical process that deteriorates metals exposed to water in the presence of corrosive agents such asacids oxygen or bacteria A common form of corrosion is pitting Severe corrosion can lead to equipment failure Corrosionis caused by many conditions including process leaks into the cooling water water flow velocity that is too low (causes depositsand fouling lending to corrosion) or too high (causes erosion and corrosion) low pH and high temperatures Corrosion is lesslikely in water with non-acidic pH values (greater than 7) although scaling is more likely in these pH rangesScaling is characterized by the formation of hard dense deposits on material surfaces These deposits impact heat transfer and can become a site for localized under-deposit corrosion Scaling is influenced by many factors including makeup water composition total dissolved solids in the recirculating water water temperature and pH and flow velocity Calcium carbonateis the main scaling constituent in all waters and it is the least soluble It forms from the hardness (calcium Ca ++ ) andbicarbonate (HCO 3
ndash ) alkalinity present in most waterFouling can be caused either by the settlement of suspended matter or by microbiological growth of algae bacterial slime or fungi Fouling will reduce heat transfer cause plugging and create corrosion sites
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Section Page
COOLING WATER SYSTEMS XXVII 11 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Treatment of the cooling water is broken down into two major categories external and internal External treatmentencompasses any treatment given to the cooling tower makeup water Internal treatment is the addition of chemicals directly to
the recirculated cooling water
EXTERNAL TREATMENT
External treatment to remove impurities in order to reduce the amount of blowdown should always be evaluated as to costeffectiveness The most frequently used techniques for treating makeup water are shown in Figure 11 Details of availabletreatment processes (see Table 1 for methods available) are covered in Section XXVI-A Boiler Feedwater Treatment For cooling tower systems that meet currently required drift limits in refineries and chemical plants uncontrolled cooling water losses (drift and miscellaneous process uses) normally amount to about 14 of the cooling water evaporation loss Under these conditions the maximum cycles of concentration achievable are 8 with no controlled blowdown Some cooling towersare operating at up to 10 cycles of concentration Additional controlled blowdown required to limit the cycles of concentrationper the water quality criteria is covered later
INTERNAL TREATMENT
The makeup water analyses amount of cooling water losses environmental restraints potential chemical hazards materials of construction and film temperatures of heat transfer surfaces determine the type of internal treatment required for cooling tower systems The internal water treatment program selected must address corrosion scale fouling biological growth andchemical attack of cooling tower componentsNumerous chemicals and treatment programs are continuously being developed for addressing the potential problems Theslightly acidic low pH programs of the past which used chromate based chemicals and usually required acid injection arebeing completely phased out because of environmental restrictions on chromate and the higher costs associated with meetingcurrent practices for handling hazardous chemicals such as acid Non-chromate treatments which use inhibitors such asphosphates zinc and organics along with higher pH have been developed to replace the earlier programs Although not aseffective as chromate these inhibitors operate at higher water pH which is a less corrosive environment The disadvantage of the higher pH is the greater tendency to form scale This tendency is controlled by the use of dispersants or scale inhibitorswhich are normally organic polymers or phosphenate A corrosion inhibitor for copper or brass is also normally required if these metals are present in the systemThe following paragraphs describe techniques and chemicals to control alkalinity (pH) corrosion scale sludge and biologicalgrowth The chemicals mentioned are normally purchased as pre-mixes from vendors and dosed in combination Mostcorrosion and scale control program chemicals are two drum formulations due to the incompatibility of some chemicals in theconcentrated state Therefore separate feed systems are generally provided for chemicals for control of corrosion and scale (2drums) and biological growth (if liquid) If a program includes fixed pH then an additional feed system is required to add acidWater treatment programs are site specific and changing The COOLING WATER SYSTEMS SPECIALISTS should beconsulted for recommendations on the internal water treatment programs when an analysis of the available water is known
Alkalinity (pH) Control
Cooling water treatment programs presently available fall into two basic categories those that do not control pH (floating pH or alkaline treatment programs without pH adjustment) and those that control pH within a specified range by adding acid Theprograms that do not control pH are gaining favor because of the additional cost equipment and precautions required tominimize the risk associated with handling acid and because of improvements in chemicals to control depositsHigh pH in the recirculating cooling water is a result of the alkalinity of the fresh water due to bicarbonate salts This combinedwith carbon dioxide results in the water having a pH range of 6 to 85 When this fresh makeup water is mixed with therecirculated cooling water and passed through the cooling tower the air flow strips out the carbon dioxide Bicarbonate ionsthen convert to carbonate ions raising the pH which can cause both calcium carbonate and calcium phosphate salts toprecipitate out of solution and form deposits Another debit in allowing the cooling water pH to increase is increased chlorineconsumption to control biological growthsIf a program using pH control is justified and chosen acid addition is used under control of a pH analyzer to controlrecirculated cooling water alkalinity and pH to the range required by the treatment program
Corrosion Control
To reduce corrosion to an acceptable level chemical corrosion inhibitors which form protective films on heat transfer surfacesare the most effective protection Inhibiting corrosion is accomplished by phosphates organics zinc nitrites and molybdatesalts Unfortunately the use of chromate which is a reliable corrosion inhibitor is prohibited by environmental constraintsNitrite is not practical in an open system due to atmospheric oxygen converting the nitrite to nitrate
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Section Page
XXVII 12 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Phosphate inhibitors (orthophosphate polyphosphate phosphonate) are the basic treatment for new towers in todaysenvironment In addition to the phosphate chemistry salts of zinc may be added to aid in inhibiting corrosion These ions
combine with the hydroxide ion forming thin protective films Zinc is used to provide a synergistic effect in combination with thephosphate salts for protecting carbon steel surfaces from corrosion Finally all-organic programs are offered by severalmanufacturers The use of these treatments has been made possible by the development of polymer dispersants which givecalcium salts a much higher solubility and reduce the tendency to scaleWhen a cooling water system contains copper alloys azole based and azole substitutes are required as copper corrosioninhibitors There are several effective inhibitors The most common is tolytriazole (TTA) due to its resistance to chlorineThe corrosion inhibitors most frequently used in ExxonMobil recirculating cooling water systems are shown below indescending order of preference provided their use meets all environmental requirements
bull Stabilized Phosphate (OrthoPolyphosphate) - Environmental restrictions on the use of zinc have resulted in ExxonMobilplants in the USA using a blend of orthophosphate and polyphosphate type corrosion inhibitors with good results Thistreatment forms a thin protective calcium phosphate film on heat transfer surfaces through the use of orthophosphateconcentrations as high as 12 - 17 wppm The use of calcium phosphate dispersants and Hydroxethylidene Diphosphonate(HEDP) stabilizes the treatment and prevents the formation of calcium phosphate scale Concentration of the dispersantsdepends upon the calcium concentration temperature and pH Stabilized Phosphate treatment programs can operate in apH range of 68 to 78 Alternatively an alkaline (floating pH) stabilized phosphate treatment program can be used Thisprogram uses little or no acid addition and is gaining in popularity although not well established yet It operates with pHvalues of 8 - 9 and uses higher quantities of dispersants and lower levels of phosphate A corrosion inhibitor for copper and brass is normally included for all stabilized phosphate programs Several vendors offer these programs Twoseparate chemical feeders are generally required
bull Zinc Alkaline Phosphate - Heritage Exxon plants in Europe where the use of zinc is not restricted have obtained goodcontrol of corrosion and scaling using zinc-polyphosphate treatment programs Low (65 - 72) and high (73 - 80) pHprograms have been used The high pH (zinc alkaline phosphate) programs are the lowest cost and are recommended
As with the Stabilized Phosphate treatment dispersants such as HEDP and polyacrylates are required to prevent theformation of zinc or calcium phosphate scale on heat transfer surfaces A corrosion inhibitor for copper or brass isnormally used with this program which is offered by several vendors
bull Organic Salts - All-organic (polymer) programs are offered by several manufacturers and have been used successfullywithin h Exxon In general these treatments are based on a combination of a phosphonate to control corrosion polymers(dispersants) to prevent scale deposition and a corrosion inhibitor for copper or brass This type of program is mosteffective in conditions of high hardness high alkalinity (M alkalinity gt300 wppm as CaCO 3) and high pH values (gt 85)However some manufacturers have reported success in softer water with alternative polymer component formulations Ingeneral an inorganic corrosion control program is preferred to the all-organic programs which are sensitive to alkalinityand more costly
Detailed control limits for effective use of each of the above inhibitors and materials selection are covered in Report NoEE34E86 Environmentally Acceptable Cooling Water Treatments Report No EE102E78 Cooling Tower Water Treatment Guidelines and in the EMRE Refinery Construction Materials Manual
Scale and Sludge Control
Scale and sludge deposits in cooling water systems cause loss of heat transfer and provide the conditions for pitting corrosionScale consists of inorganic salts which have precipitated from the cooling water forming a coating on heat transfer surfacesSludge consists of organic and inorganic suspended matter that settles in quiescent areas and in heat exchangers The key toprevention of both scale and sludge formation in a cooling system is keeping water velocities greater than 3 fts (09 ms) andthe use of chemical additives in combination with a blowdown rate that keeps the impurity concentration below the level whichcauses deposits (See Table 2 for limits on impurities) Scale control has become increasingly important in higher alkalinityldquofloating pHrdquo water treatment programsSalts most commonly found in scale deposits are calcium carbonate calcium sulfate tricalcium phosphate zinc phosphatemagnesium silicate and silica The control method for preventing scale from each of these salts is discussed below
bull Calcium Carbonate - Where justified using pHalkalinity control in the range of 70 to 75 will normally inhibit the formationof calcium carbonate scale if anti-scalent chemicals (phosphonates acrylates copolymers) are part of the chemicaltreatment (See Report No EE102E78 Cooling Tower Water Treatment Guidelines ) Higher pH water treatmentprograms require greater concentrations of these chemicals along with dispersants and flocculants to inhibit the formationof scale and precipitation of other salts
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Section Page
COOLING WATER SYSTEMS XXVII 13 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
bull Calcium Sulfate - When heat transfer skin temperatures are below 250 degF (120 degC) and the cooling water bulktemperatures are under 130 degF (54 degC) gypsum scale (CaSO 42H 2O) will form if the product of the calcium and sulfate ionsconcentrations each expressed as wppm CaCO 3 equivalent exceeds 500000 Gypsum scale is prevented bymaintaining bulk water temperature below the indicated limits and controlling calcium and sulfate ions through blowdownor through the use of newer dispersants on the market
bull Tri-Calcium and Zinc Phosphate - Almost all chemical additives for corrosion control contain some type of phosphatesalt These have a tendency to form the orthophosphate ion depending on time pH and temperature Cooling watersusing these corrosion inhibitors may have an orthophosphate concentration in excess of 5 wppm as PO 4
= To preventcalcium phosphate scale Table 2 shows a limit of 1000 wppm of calcium as CaCO 3 equivalent when using a phosphateinhibitor in combination with a chemical dispersant (See Report No EE102E78 Cooling Tower Water Treatment Guidelines for solubility of calcium phosphate) These inhibitors and dispersants generally fall into one of the followingcategories
ndash Sequestrants antiprecipitants and inhibitors such as hydroyethylidene diphosphonate (HEDP) phosphate esters andaminomethylene phosphonate (AMP) function by defyingdistorting the crystal structure of the scale formations toprevent nucleation and growth of the scale
ndash Dispersants based on either anionic or nonionic polymers such as polyacrylamide polyacrylate or polymaleic acidfunction to stabilize colloidal and scale-forming particles in suspension Anionic polymers prevent agglomeration of suspended particles by either charge repulsion or crystal distortion while nonionic polymers reduce the surfacetension of particles
bull Silica Deposits - Quantitative indices for determining limits for silica in combination with other ions are not available due tothe formation of complex polymers Instead the rule of thumb limit of keeping reactive silica concentrations in the coolingwater below 150 wppm as SiO 2 has been successful
Possible alternatives for controlling sludge are
bull Chemical Dispersants - Water treating chemical suppliers now offer many types of dispersants These are mostlypolyelectrolytes Claims for removal of slime silt and mud from cooling tower basins and exchangers are madeHowever satisfactory results using these chemicals as a coagulant aid require very accurate control of the amount fed tothe water and stable operation of the water system Also suspended solids should be kept below 200 wppm for particleslarger than 045 microns Dispersants are used in virtually all fresh water cooling tower systems despite their high cost
bull Sidestream Filtration - The use of a sand pressure filter is the most reliable method for sludge removal Operating andmaintenance costs are minimal for this equipment
bull Higher Blowdown Rates - This may be an economical alternative depending on the cost of makeup water Additionalchemical costs must be included in any evaluation of this method
bull Makeup Clarification - This alternative may require additional raw water treatment facilities to handle seasonal variationsin turbidity andor suspended solids This method is applicable only where suspended solids come from makeup water
Biological Control
Operating conditions in recirculated cooling water are ideal for the growth of biological matter Water temperatures from 70 to120 degF (20 to 50 degC) favorable water pH continuous supply of nutrients including carbon nitrogen and phosphorus inorganicsalts and sunlight all lend themselves to environmental conditions encouraging microbiological growth and plant lifeIf biological growths get out of control and form large sticky agglomerations some of the following operating problems willresult
ndash Fouling of heat transfer surfaces by bacterial slimes resulting in flow restrictions and high process temperatures ndash Reduced cooling tower efficiency resulting from algae fungi and bacterial slime growths in the water distribution basin
and fill area of the cooling tower ndash Excessive plugging of screens and filters ndash Corrosion ndash Wood destruction by fungi ndash Clogging of water distribution nozzles ndash Excessive sludge accumulation in the cooling tower basin
An important part of the water treatment program is to control biological activity by feeding a biocide chemical Oxidizingbiocides such as chlorine sodium hypochlorite chlorine dioxide or bromine-associated compounds are most commonly usedwith good results Plate counts should be maintained below 50000 countsml (bacterial)
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Section Page
XXVII 14 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Chlorine is the most effective biocide for cooling water because of its low cost extensive period of successful use ease of application and minimum environmental effects if applied under controlled conditions Either a liquid feed (sodium
hypochlorite) or a gas feed (100 chlorine) is added to the cooling tower basin A sufficient dosage is that which obtains a 01to 03 wppm continuous free residual in the hot return water to the cooling tower
Advantages to continuous chlorination over shock treatment based on ExxonMobil experience are ndash Minimizes chlorine degradation of polymers ndash Better reduction in total aerobic bacteria plate count resulting in cleaner heat transfer surfaces ndash Does not cause significant pH depressions ndash Better control results with less chlorine consumption
Details concerning chlorine addition as well as the other alternative biocides are given in Report No EE102E78 Cooling Tower Water Treatment Guidelines and in EMRE Engineering Water and Wastewater Design Guide TMEE 080 DG 11-3-1Gas Chlorinators for Water Treating Service and DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) Gaseous chlorine systems have become more costly in recent years because of the additional precautions equipment andprocedures required to minimize the risk associated with handling this gas Alternatives to gaseous chlorine are available butrequire more attention and have a higher operating cost Sodium hypochlorite (either purchased or generated) and sodiumbromide stabilized solid halide (chlorinebromine) products chlorine dioxide and ozone are all alternatives which may haveapplications Contact the COOLING WATER SYSTEMS SPECIALISTS for guidance in the use of these alternative treatmentschemes
BLOWDOWN
Blowdown control is required to maintain the proper cycles of concentration in the system The most reliable and simplest typeof control for blowdown is specific conductance measurement of the cooling water Electrical signals from the specificconductance measurements of some impurity in the cooling water are used to actuate an automatic blowdown valveImpurities normally used for blowdown control are dissolved solids chloride calcium magnesium and silica
CONTROLS AND MONITORS
Any chemical treatment used to control problems in a cooling water system must be carefully monitored to be completelyeffective
The installation of automatic controls should be considered for recirculated cooling water systems in all plants Designs shouldinclude pH conductivity corrosivity inhibitor and biocide control Automatic controllers are available which continuouslymonitor the condition of the water and automatically make chemical additions to keep the water within compositional limitsTest exchangers and corrosion coupons are also useful in monitoring the effectiveness of the water treatment program
An oil-in-water monitor should be considered to provide early detection of oil leaks into the cooling water This will enablecorrective action to be taken before extensive contamination of the cooling water circuit occurs This is especially important incooling towers with high efficiency cellular fill which can be difficult to clean if significantly fouledManual control systems may be used in locations that have adequate technical personnel to perform routine water analysesand to modify chemical treatment necessitated by sudden variations in water composition process contamination etc
WASTEWATER REUSE POSSIBILITIES
Recirculating cooling water systems normally have the highest plant raw water demand A potential alternative is to use treatedwastewater to supply part or all of the makeup water requirement The treatment required must be determined but will typicallyinclude primary oil removal (eg API Separator) secondary oil and suspended solids removal (eg filtration dissolved or induced air flotation) and soluble carbon removal utilizing biological treatmentThe reuse of treated wastewater as makeup currently exists at only a few locations If this alternative is being considered refer to Report No EE58E85 Reuse of Wastewater as Cooling Tower Makeup and Section XIX-B Water Reuse
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Section Page
COOLING WATER SYSTEMS XXVII 15 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS
STEP 1 - Select a location for the intake station and pumps
Factors which influence this selection are as followsbull Depth of water and tidal variations
bull Relative location of intake and outfall direction of current
bull Length and size of intake suction piping vs pump discharge piping
STEP 2 - Determine cooling water requirements
Determine the design cold water inlet temperature based on site data covering seasonal water temperature as a function of inlet location and depth (for existing locations refer to Table 2 in Section IX-B )Tabulate the cooling water requirements for each process unit and offsite consumer Apply the proper LGFs to all coolingloads depending on the stage of the project and whether the new units are exact duplicates of existing unitsIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The designer should investigate methods to increase effluent water temperature to the maximumrecommended limit for the particular type of water used One common method is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 3 - Determine the total required capacity for water intake facilities
Process Coolers - The total requirements determined in Step 2 including LGFsFirewater - This is often supplied from the intake bay of a once-through systemFuture Expansions - These should be considered in sizing intake facilitiesTotal Required Capacity - The sum of the above three items
STEP 4 - Provide water intake facilities
Intake Crib - The intake crib with its support structure is located at the extreme end of the water intake line (See Figure 4 )The intake is usually located 200 to 300 ft (60 to 90 m) offshore and at a sufficient depth below the lowest water level to
preclude vortex formation and subsequent pump cavitation during storm conditions Additionally the crib should have 3 to 5 ft(1 to 15 m) clearance above the sea floor to minimize sand and silt pickup A concrete velocity cap should be specified to cover the inlet crib This will minimize the incidence of fish and other foreignmatter entering the system A horizontal inlet flow pattern should be designed with a maximum entrance velocity of 1 fts(03 ms) Experience shows that vertical velocity flow confuses fish and they are drawn into the intake lineThe intake crib support structure protects the crib from adverse effects of storms and tidal motions as well as protecting it fromimpact of floating and submerged objects A self-cleaning screen can be specified to prevent entrance of large objects into thesystem at the cribIntake Line (Figure 5 ) - An intake line is required if the water depth along the shore does not permit water to flow directly intothe intake bay The intake line is usually buried in the sea floor and covered with a minimum required overburden Consultwith the COOLING WATER SYSTEMS SPECIALISTS for required coverage at a specific siteChlorine or sodium hypochlorite is injected at the intake crib to prevent marine growth in the lineNote that an intake crib and line will not be required if water of sufficient depth is available at the supply pump stationIntake Bay (Figure 6 ) - The intake bay is an enclosure which houses the pumps and screens Depending on a particular design a series of screens is used to ensure the supply of clean water Trash racks comprise the first element of the filter system These remove only very coarse material from the water such as bulk seaweed They can be supplied with atraversing raking mechanismLift screens provide secondary cleanup These screens can be lifted out of the intake bay for periodic cleaning Double liftscreens can be used if it is deemed necessary to have one in service at all timesFinal water filtering is usually done by traveling screens Details of the screen dimensions and the mounting are usuallydetermined during detailed engineering Screen net open area is typically approximately 50 of total screen area Thescreens should be sized for a normal water velocity of 12 fts (04 ms) through the open screen area Maximum velocityshould be limited to 24 fts (08 ms) with 50 of the screen area plugged This is the upper velocity limit for efficientscreening
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Section Page
XXVII 16 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
If high suspended solids greater than 200 mesh (eg sand) are expected to be a continuous problem ejectors may be installedupstream and downstream of the traveling screen to remove accumulated sand and solids from the intake bay in these areas
Traveling screens and ejectors are supplied with wash water from the raw water pumps or from separate wash water pumpsOnoff control using timers should allow optimum scheduling of the ejectors depending on the time of the year
STEP 5 - Specify water intake facilities
Water intake facilities should be designed in accordance with the following guidelines from the Hydraulic Institute Standards The water intake should be designed to provide an evenly distributed flow of water to the pump suction A properly designedintake reduces the possibility of vortex formation and results in better pump performance with less noise Geometry of theintake and the manner in which water is introduced into the intake are important parameters in providing an even distribution of water flow Complete analysis of intake structures is best accomplished by scale model testsThe pump manufacturer will generally render advice on specific problems while the intake design is still preliminary He shouldbe provided with the necessary intake layout drawings showing the physical limitations of the siteFigures 7A and 7B have been developed for single- and multiple-pump arrangements to show suggestions for basic sumpdimensions They are for pumps normally operating in the capacity range of approximately 3000 to 300000 gpm (200 to20000 Ls) Since these values are composite averages from a great many pump types and cover the entire range of specificspeeds they must not be thought of as absolute values but rather as basic guides subject to some possible variations
All the dimensions in Figures 7A and 7B are based on the rated capacity of the pump at the design head Any increase incapacity above these values should be momentary or very limited in time If operation at an increased capacity is to beundertaken for considerable periods of time the maximum capacity should be used for the design value in obtaining sumpdimensionsDimension C is an average value based on an analysis of many pumps Its final value should be specified by the pumpmanufacturerDimension B is a suggested maximum dimension which may be less depending on actual suction bell or bowl diameters in useby the pump manufacturer The edge of the bell should be close to the back wall of the sump When the position of the backwall is determined by the driving equipment or the discharge piping Dimension B may become excessive and a ldquofalserdquo backwall should be installedDimension S is a minimum value for the sump width for a single pump installation This dimension can be increased but if it isto be made smaller the manufacturer should be consulted or a sump model test should be run to determine its adequacy
Motor size may dictate minimum pump spacing be especially aware of this when specifying pumps with unusually high pumpheads eg greater than 80 psig (550 kPa)Dimension H is a minimum value based on the ldquonormal low water levelrdquo at the pump suction bell taking into considerationfriction losses through the inlet screen and approach channel This dimension can be considerably less momentarily or infrequently without excessive damage to the pump It should be remembered however that this does not representldquosubmergencerdquo Submergence is normally quoted as dimension ldquoHrdquo minus ldquoCrdquo This represents the physical height of water level above the bottom of the suction inletThe actual submergence of the pump is something less than this since the impeller eye is some distance above the bottom of the suction bell possibly as much as 3 to 4 ft (1 to 13 m) For the purposes of sump design in connection with this chart it isunderstood that the pump has been selected in accordance with specific speed charts The submergence referred to hereinhas to do only with vortexing and eddy formationsDimensions Y and A are recommended minimum values These dimensions can be as large as desired but should be limitedto the restrictions indicated on the curve If the design does not include a screen Dimension A should be considerably longerThe screen or gate widths should not be substantially less than S and heights should not be less than H If the main stream
velocity is more than 2 fts (06 ms) it may be necessary to construct straightening vanes in the approach channel increaseDimension A conduct a sump model test of the installation or work out some combination of these factorsDimension S becomes the width of an individual pump cell or the center-to-center distance of two pumps if no division walls areusedOn multiple pump installations the recommended dimensions in Figures 7A and 7B also apply as noted above and thefollowing additional determinants should be consideredFigure 8A - Low velocity and straight-line flow to all units simultaneously is the first recommended style of pit Velocities in thepump area should be approximately 1 fts (03 ms) Some pumps with velocities of 2 fts (06 ms) and higher have given goodresults This is particularly true where the design resulted from a model study Not recommended would be an abrupt changein size of inlet pipe to sump or inlet from one side introducing eddying
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Section Page
COOLING WATER SYSTEMS XXVII 17 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Figure 8B - A number of pumps in the same sump will operate best without separating walls unless all pumps are always inoperation at the same time in which case the use of separating walls may be beneficial If walls must be used for structural
purposes and pumps will operate intermittently one should leave flow space behind each wall from the pit floor up to at leastthe minimum water level and the wall should not extend upstream beyond the rim of the suction bell If walls are used increaseDimension ldquoSrdquo by the thickness of the wall for correct centerline spacing Round off the ends of the walls Not recommended isthe placement of a number of pumps around the edge of a sump with or without dividing wallsFigure 8C - Abrupt changes in size from inlet pipe or channel to pump bay are not desirable A relatively small pipe emptyinginto a large pump pit should connect to the pit with a gradually increasing taper section The angle should be as large aspossible preferably not less than 45 degrees With this arrangement pit velocities much less than one foot per second aredesirable Especially not recommended is a small pipe directly connected to a large pit with pumps close to the inlet Flow willhave excessive change of direction to get to most of the pumps Centering pumps in the pit leave large ldquovortex areasrdquo behindthe pumps which will cause operational troubleFigure 8D - If the pit velocity can be kept low enough [below 1 fts (03 ms)] an abrupt change from inlet pipe to pit can beaccommodated if the length equals or exceeds the values shown It is assumed that as ratio WP increases the inlet velocityat ldquoPrdquo will increase up to an allowed maximum of 8 fts (24 ms) at WP = 10 Pumps ldquoin linerdquo are not recommended unless theratio of pit to pump size is quite large and pumps are separated longitudinally by a generous margin A pit can generally be
constructed at much less cost by use of a recommended designFigure 8E - It is sometimes desirable to install pumps in tunnels or pipelines A drop pipe or false well to house the pump witha vaned inlet well facing upstream will be satisfactory in flows up to 8 fts (24 ms) Without the inlet well the pump section bellshould be positioned at least two pipe diameters vertically above the top of the tunnel not hung into the tunnel flow especiallywith tunnel velocities 2 fts (06 ms) or more There should be no signs of air along the top of the tunnel It may be necessaryto lower the scoop or to insist on a minimum water level in the vertical wellNote The foregoing statements apply to sumps for clear liquid
STEP 6 - Provide chemical treatment
Provide chemical treatment facilities for the once-through cooling water system Normally the only chemical treatment requiredin a once-through cooling system is for biological control Facilities for adding chlorine to once-through cooling water are thesame as those described for the recirculated cooling water system The design capacity of the chlorinator should be based onachieving a chlorine residual of 1 wppm in the effluent The chlorine required by the organics and other reducing agents in thewater must be satisfied before any residual chlorine will appear in the effluent The design capacity of the chlorinator should be
based on the total water chlorine demand plus 1 wppm in the effluent In the absence of water data the chlorination systemdesign capacity should be based on 10 wppm minimum instantaneous dosage for maximum cooling water flow For very largeonce-through cooling water systems it will be impractical to design for shock rates at this level In this case a continuousdosage of 1 - 2 wppm should be used (Refer to Step 5 of the DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS )
STEP 7 - Specify pumps and distribution system
Specify other major facilities in the once-through cooling water system The recommended materials for pumps valves andpiping are shown in the EMRErsquos Refinery Construction Materials Manual
Pumps1 Type - These pumps are usually vertical centrifugal pumps However horizontal centrifugal pumps have been used An
economic study andor consideration of existing facilities will determine which type should be specified In the event thathorizontal centrifugal pumps are specified facilities for pump priming will likely be required For large pumps in salt water service there is a very limited selection of pumps because of the special materials required Consult with the DISCIPLINESPECIALISTS early in the design effort to determine what specific pumps are currently available
2 Total Required Capacity - Firm capacity (excluding spares) of the supply pumps should equal the maximum cooling water requirements including LGF and RCF
3 NumberIndividual Pump Capacity - Normally one spare pump is provided for up to six operating pumps two spares for up to twelve operating pumps etc The most common arrangements for water supply pumps are
bull Two 100 capacity pumps
bull Three 50 capacity pumps
bull Four 33-13 capacity pumps
bull Five 25 capacity pumps
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Section Page
XXVII 18 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Maximum commercially available pump capacity ranges up to 300000 gpm (20000 Ls) at discharge pressures up to 100psi (700 kPa) Maximum capacity will be less for higher discharge pressures Other factors which may influence pump
selection arebull Selecting capacity to match existing pumps
bull Selecting capacity for future requirements
bull Selecting capacity corresponding to manufacturers available standardVendor information should be obtained on the availability size and cost of the pumps for several configurations Acomparison should be made of the investments and operating costs as well as other advantages and disadvantages toselect a pump configuration
4 Drivers - These pumps are usually motor-driven Because of the remote location of the intake facilities the steamdistribution costs are normally too high to justify the use of a steam turbine driver However use of a secondary selectivesubstation provides a reliable source of power to the pump drivers
5 Pump ∆∆∆∆P - Pump ∆P is based on supplying the maximum cooling water requirement at the battery limit design pressureestablished for the system The ∆P should include the total circulating system pressure drop plus any elevation differencebetween the low water level at the intake and the discharge point of contact with air
6 Pump Instrumentation - The remote location of the intake facilities makes remote control and monitoring of the water supply operation desirable Pump controls located in the control house provide for minimum manning Local controlbackup should be provided The Owners operating and manning philosophy determine the basis for instrumentation andcontrolThe spare pump is controlled to automatically start on low pressure in the supply header Concurrently alarms should beactuated in the control room to indicate the low-pressure situation and spare pump operation
Distribution Network1 Configuration - The piping network is a once-through system with a return line from each consumer or consumer group to
the cooling water effluent line2 Layout - The routing of the piping network should connect all cooling water consumers in the most economical manner
permitted by the plant layout Careful consideration should be given to the turnaround philosophy of the plant Thenetwork layout should preclude the necessity of having above-ground water lines that are in service running through anarea that is being turned around Underground piping or an extra lateral may be required to accomplish this objective
3 Valves and Blinds - Battery limit valves should be provided in the network to isolate individual process units from thesystem Valves should be provided to permit onstream removal and maintenance of spared equipment
4 Basket Strainers - Consideration should be given to the installation of basket strainers in the laterals serving each of theconsumer loops These provide added insurance against entrance of foreign material into the cooling equipment The useof intake screens with 025 in (63 mm) openings may eliminate the need for basket strainers in the laterals
Line Sizing1 Design Basis - The network is sized so that water is delivered to the most distant consumer at a pressure equal to the
required process design (battery limit) pressure2 Flow Rate - Each segment of the distribution network is sized for the simultaneous maximum flow in that segment for any
operating condition The main supply and discharge headers are sized for the simultaneous maximum flow of all segments(not necessarily the sum of the simultaneous maximum flows of the individual segments)
3 Pressure Drop
Line Sizing Basis - The distribution system should be sized according to the guidelines established in Section XIV The use of an ldquoold piperdquo friction factor of C = 90 - 100 (Hazen-Williams factors) in the main lines is recommended to assuregood operation in later years when the system experiences fouling For large diameter lines [greater than 48 in (1200mm)] maximum water velocity should be limited to 10 fts (3 ms) based on civilmechanical considerations (supports andanchoring required to handle potential pressure surges) For salt water systems utilizing cement-lined pipe consider thereduced inner diameter in the friction loss calculations A study may be necessary to determine the economic pump ∆P vsline diameters
4 Equivalent Length Factors - Equivalent length factors are used in pressure drop calculations for sizing the distributionsystem The piping length scaled from the plot plan is multiplied by an equivalent length factor to compensate for pipingelbows expansion loops and other piping variations
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Section Page
COOLING WATER SYSTEMS XXVII 19 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
The equivalent length factors are selected based on the expected variation from a straight piping run Typical equivalentlength factors suitable for planning purposes for different piping applications are as follows
TYPE OF PIPING EQUIVALENT LENGTH FACTORS
Long piping runs [gt 1000 ft (300 m)] 12 to 14
Onsite distribution piping 15 to 20
For design purposes the actual piping layout should be evaluated to ensure that the equivalent length factors areapplicable to the specific situation Particular attention is required for revamp work and saltwater systems utilizing cement-lined pipe
STEP 8 - Prepare a system flow plan similar to the one shown in Figure 1
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS
STEP 1 - Determine cooling water requirements
Tabulate the cooling water requirements and cooling water discharge temperature for each process unit and offsite consumer Apply the proper LGFs to all cooling loads depending on the stage of the project and whether the new units are exactduplicates of existing units Determine the total cooling water flow required including LGFs and the bulk temperature (T 1) of thecombined cooling water flow from all coolers to the cooling towerIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The onsite and offsite designers should investigate methods to increase effluent water temperature to themaximum recommended limit for the particular type of water used This will result in a lower cost cooling tower for a given heatrejection rate One common method of maximizing return water temperature is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 2 - Select cooling tower design parameters
Select the proper design conditions of ambient wet bulb temperature approach temperature range and flow rate for specifyingthe cooling tower Based on these conditions the cooling tower vendor will size the tower and provide the tower characteristicand performance curves which serve as the basis for the performance guarantee Required performance test and evaluationprocedures are described in IP 8-1-1 Economic studies may be required to select the ambient wet bulb temperature andapproach temperature for new locations or for expansion projects with special cooling requirements
STEP 3 - Prepare cooling tower specification sheet
Include the following information in the heat exchange section of the design specificationTower Flow Capacity - This was determined in Step 1Type of Tower - The tower usually specified is an induced draft multicell type suitable for fresh salt or brackish water depending on the water available Additional tower characteristics are usually developed and proposed by the tower vendor (eg crossflow versus counterflow type of tower fill etc) The vendor should be made aware of conditions such as thelikelihood of heavy oil leakage into the cooling water or high fouling potential which may influence the tower type or fill
selectionNumber of Cells - The minimum number of cells required (normally not less than 2) may be specified The maximum flow per cell is a function of the cooling tower design conditions selected in Step 2 Typical values range up to 10000 gpm (600 Ls) for crossflow designs and 8000 gpm (500 Ls) for counterflow designs Normally spare cells are not provided Maintenance iscarried out during cooler periods or load is shed
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XXVII 20 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Ambient Wet Bulb Temperature1 The ambient wet bulb temperature selected for specifying the cooling tower design is generally that temperature which is
equaled or exceeded 5 of the time during the four hottest months (5 of the hourly readings taken during the four hottestmonths will exceed this value) Specifying the 5 temperature should give satisfactory results since the time that the wetbulb temperature is exceeded is for short duration and the system inertia is usually sufficient to avoid problems The tower specification should make it clear that the wet bulb temperature specified is an ambient value as opposed to the enteringvalue which may be higher because of recirculation The tower manufacturer should adjust the ambient value upward totake into account his estimates of recirculation Currently cooling tower test codes provide procedures for measuringperformance in the case of either entering or ambient wet-bulb temperature specifications An increase of 2 degF (11 degC) isgenerally added to ambient wet bulb temperature to estimate entering wet bulb temperature for large cooling towers
2 If plant production will peak during the four summer months when the 5 ambient wet bulb temperature may be exceededconsideration should be given to specifying a higher design ambient wet bulb temperature (one that will be exceeded 212 of the time for example) However the incremental cost of the resulting closer approach and larger cooling tower must be justified by the value of the incremental products made
On-Tower Temperature (T1) - This temperature cannot be accurately determined until the design of all process coolers iscompleted and the volume of cooling water and outlet temperature are known for each (see Step 1) In the planning stage or early design stage of a project this temperature must be estimated and is usually set equal to the maximum design cooler outlet temperature shown under COOLING WATER REQUIREMENTS in the BASIC DESIGN CONSIDERATIONS portion of this sectionApproach Temperature - The approach temperature has the most pronounced effect on cooling tower size and cost For example reducing the approach from 10 to 5 degF (55 to 3 degC) can increase the cooling tower investment by 50 percent and thefan operating cost by 65 percent Thus various approach temperatures should be evaluated to determine the minimum cost of the total system including the process exchanger area Approaches used in the past designs have been from 5 to 12 degF (3 to7degC) Note that it is not customary in the cooling tower industry to guarantee approaches of less than 5 degF to (3 degC) to coolingtower design inlet wet bulb temperature A typical approach for a new grass roots cooling tower is 10 degF (6 degC)Off-Tower Temperature (T2) - This is the sum of the approach temperature and the ambient air wet bulb temperature For existing locations Table 2 in Section IX-B may be used
Range and Flow Rate1 The range for a given heat load is related to the circulating water rate as follows
Q = M C p ∆Tcong (500) (gpm) (range) Eq (1)
where Q = Heat load on the tower BtuhM = Water circulating rate lbhCp = Specific heat capacity of water at average temperature Btulb degF∆T = (T 1 ndash T 2) degF = (water cooling range)gpm = Circulation rate in US gallons per minute
For calculation in Metric units
Q = M C p ∆Tcong (1162 x 10 -3) (Ls) (range) Eq (1M)
where Q = Heat load on the tower kJh
M = Water circulating rate kghCp = Specific heat capacity of water at average temperature kJkg degC∆T = (T 1 ndash T 2) degC = (water cooling range)Ls = Circulation rate in liters per second
2 Generally lower tower investment fan horsepower and pump horsepower result from maximizing the range andminimizing the flow rate However based on fouling considerations of process coolers the maximum allowable hot water temperatures must be kept below recommended limits Also while increasing the range minimizes cooling tower investment and operating cost the cost of exchanger surface requirements is adversely affected by the LMTD and acompromise must be made The range is bounded by the maximum allowable hot water temperature and the Off-Tower temperature (T 2) For established locations refer to Section IX-B for design cold water temperatures to assist in selectingthe approach and range
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Section Page
COOLING WATER SYSTEMS XXVII 21 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Makeup Water Requirements - Calculate the various losses in the cooling water system to determine makeup requirementsThese losses include
1 Evaporation Loss (E) - Allow 1 of the circulation rate for each 10 degF (55 degC) temperature drop through the tower Notethat the actual evaporation loss will be somewhat less depending on ambient air temperature
( )sL555
TT)gpm(
1000TT
E 2121
minus=
minus= Eq (2)
2 Drift Loss (D) - IP 8-1-1 limits the maximum allowable drift loss to 0005 of the design water circulation rate for freshbrackish or salt water
( )sL1000050
)gpm(1000050
D
=
= Eq (3)
This low drift level contributes little to solids removal from the tower with the spray This allows higher cycles of concentration to be achieved in cooling towers which use clarified softened water for makeup and thus have low requiredblowdown rates Higher drift loss percentages in older or poorly maintained towers will limit the maximum cycles of
concentration achievable3 Miscellaneous Losses (O) - Miscellaneous losses are estimated to be one-seventh of the difference between evaporation
loss and drift loss
minus=
7DE
O Eq (4)
4 Blowdown Rate (B) ndash Guidelines for maximum concentrations of impurities allowed in recirculated fresh cooling water areshown in Table 2 For salt water the blowdown is based on keeping the total dissolved salts below 55000 wppm Obtainthe chemical analysis of the makeup water and divide each individual component concentration into its counterpart inTable 2 The result is the maximum allowable cycles of concentration for that constituent (a worksheet is provided inTable 4 ) The impurity with the lowest factor will be the control and will determine the required blowdown rate
)OD(1Cycles
EB +minus
minus= Eq (5)
The makeup water requirement is the sum of all losses shown above
MakeUp = E + D + O + B Eq (6)
STEP 4 - Specify the controls necessary for proper cooling tower operation
Air Side - Cooling towers are provided with motor-driven manually adjusted pitch multiple-bladed propeller fans Figures9A amp B may be used to estimate the normal operating horsepower requirements Since the fan motor and gear reducer arelocated at the top of the tower [generally 40 to 50 ft (12 to 15 m) above grade] the need for equipment to handle thesecomponents should be discussed with the affiliateControl of the air flow through the tower to maintain reasonably constant cold water temperatures can be accomplished byturning off individual fans in multiple-cell installations adjusting the blade pitch or using variable speed motors To turn off individual fans remote at grade start-stop buttons for the motors are usually specified
Whenever the ambient air temperature is 32 degF (0 degC) or below icing of the tower louvers and structural members in contact withthe incoming air will occur If the heat load on the tower is relatively free of seasonal variations two-speed fans should provideadequate ice control down to approximately 0 degF (-18 degC) ambient temperatures When sustained temperatures of 0 degF (-18 degC)and below occur two-speed fans capable of two-speed reverse operation with appropriate time delays for change of directionand speed should be specified to periodically eliminate ice which will build up on the louvers Other methods of winterizationare available but in general are more expensive and require greater operator attention In very cold climates steam heatingshould be provided to prevent freezing in the cold water basin during shutdown periodsWater Side - The water circulation rate is maintained by operating one or more pumps of sufficient capacity to meet the systemdemand Flow-measuring facilities local temperature indicators and a high-temperature alarm are provided on the coolingtower water supply and return headers Hand-operated flow control valves are required for distributing the water equally to theindividual cells On counterflow towers provisions for measuring the flow to each cell (pitot tubes) are required They are notrequired on crossflow towers since the open distribution basin provides visual aid in balancing flows Shutoff valves should beprovided for servicing the flow control valves without shutting down the entire tower
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XXVII 22 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
STEP 5 - Provide chemical treatment facilities for the fresh water system
Alkalinity (pH) Control (if used)1 In cases where a controlled pH corrosionscale control program is being used the alkalinity (pH) of the recirculated cooling
water should be controlled in accordance with the program requirements Normally acid addition is required to accomplishthis This is due to free carbon dioxide being stripped from the cooling water in the tower to approximately 5 ppm For example if the alkalinity has to be controlled to between 25 and 50 ppm for a pH of 70 to 75 the volume of 98 sulfuricacid (66 degBeacute) required to treat the makeup water is calculated using the following equation
[Note 93 sulfuric acid should be used if ambient temperatures will be less than 35 degF (2 degC)]
GallonsHour 66 degBeacute H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate gpm) (326 x 10 -5)
For calculation in Metric limits
kgh of 100 (66 degBeacute) H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate Ls) (793 x 10 -3)
2 The metering pump should be designed for a maximum feed rate of two to three times the normal feed rate A glass woolfilter is required on the pump suction piping to minimize plugging Normally two - 100 capacity pumps are provided
Positive displacement metering pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumps with flow control areused for higher rates
3 Hydrochloric acid may also be used for alkalinity control The gallons per hour of 20 degBeacute hydrochloric acid required is36 times the amount of 66 degBeacute sulfuric acid In Metric units the weight of 32 (20 degBeacute) hydrochloric acid required is227 times the weight of 100 (66 degBeacute) sulfuric acid
4 Specifications for chemical feeders may be found in EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased)
5 A pH analyzer with high and low alarm is provided to control the amount of acid feed The acid should be added to thecooling tower makeup water (Figure 2 ) The lag time between the point of acid injection and the sample outlet to the pHcell should be less than two minutes for effective control
6 Spills or overflows from the sulfuric acid tank should be contained
Corrosion Inhibitors and Anti-Scalents1 Two chemical feeders are generally provided The following guides in sizing the equipment will allow the use of any of the
proprietary products from the vendors when final selection by the owner is made See Report No EE102E78 Cooling Tower Water Treatment Guidelines EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and Report No EE34E86 Environmentally - Acceptable Cooling Water Treatments for detailed discussion of acceptable commercial chemicals
bull If the water treatment program to be used is known design for the chemical rates specified by the vendor or COOLING WATER SYSTEMS SPECIALIST
bull Otherwise design for 100 wppm of each commercial liquid product in the recirculated cooling water The design feedrate should be based on the design total blowdown flow (both uncontrolled and controlled) since this is where theproduct is lost For 100 wppm concentration the chemical rates will be as follows
GallonsHour Chemical = Blowdown (gpm) x (0006)
LitersHour Chemical = Blowdown (Ls) x (036)
2 The design capacity of the metering pump should be two to three times this calculated number Normally 2 - 100capacity pumps are provided Positive displacement pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumpswith flow control are used for higher rates
3 Specifications for chemical feeders may be found in EMRErsquos Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased) Reference should also be made to the checklists containedin Report No EE92E94 Guidelines for Safety Evaluation of Chemical Injection Facilities
4 A final check should be made with the owner and the COOLING WATER SYSTEMS SPECIALISTS as to the treatmentprogram selected and if there is a need for an additional chemical feeder
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 23 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Biological ControlFacilities should be provided for feeding the biocide into the back of the cooling tower basin If chlorine is used it must bepresent in the cooling water as free residual to be effective However the following amounts of chlorine are first required toreact with the listed impurities when present in the cooling water before a free residual can be established
IMPURITIESAMOUNT OF CHLORINE REQUIRED (wppm)
WITH EACH WPPM OF IMPURITY
Ammonia (NH 3)
Hydrogen Sulfide (H 2S)
Sodium Sulfite (Na 2SO 3)
Ferrous Iron (Fe ++)
10
22
056
064
The design capacity of the chlorine feeder shall be based on achieving a 01 wppm free chlorine residual in the recirculatingwater flow or a minimum dosage of 7 wppm at maximum design makeup water flow rate and conditions (see EMRE Water and
Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service )Chlorine may be fed either as the commercial liquid sodium hypochlorite (124 wt Cl 2) or liquid chlorine (100 Cl 2) Liquidhypochlorite is being used increasingly because of the higher costs associated with precautions equipment and proceduresrequired to minimize the risks associated with handling gaseous chlorine The hypochlorite solution is fed using a meteringpump which should be sized to feed an equivalent of 15 ppm 100 Cl 2 (Refer to EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) ) In case of spills or an overflow from thestorage tank the sodium hypochlorite may be sent to the cooling tower basinGas chlorinators are covered by EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service The use of gas chlorination is being reduced but may still be appropriate in some locations subject tolocal management approval The gas from either commercial cylinders or an evaporator is dissolved in a water slip stream bymeans of an eductor This results in a chlorine solution strength of approximately 01 This solution should be evenlydistributed below the water surface by means of a header lateral pipe arrangement at the rear of the cooling tower basinopposite the cooling water pumpsSome additional guides on the use of gas chlorinators are given below It is also recommended that the safety guidelines
published by the Manufacturing Chemists Association and the Chlorine Institute be followedbull Dilution water for the chlorinator must not be less than 50 degF (10 degC) or ice formation may cause plugging problems
bull Continuous rates of chlorine gas withdrawal should not exceed 400 lb (180 kg)24 hours from each one ton cylinder atroom temperature [70 degF (21 degC)] This rate may be exceeded (up to 50) for periods not exceeding 2 hours Rule of thumb is not to design for more than 4 cylinders operating in parallel When feed rate exceeds 3000 lb (1360 kg) per daythe liquid chlorine should be fed to an evaporator upstream of the chlorinator
bull If the ambient temperature can be expected to be below 50 degF (10 degC) for extended periods a building should be providedfor the chlorinator and cylinders
bull Weighing scales should be provided to determine when shipping containers should be replaced and to verify chlorine feedrates
bull Analyzers to measure the free chlorine residual in the circulated cooling water should be provided
bull Chlorine leak detectors and a catch basin for major spills should be provided
Control SystemProvide automatic controllers to continuously monitor water condition and to add chemicals to the system (complete systemsmay be available through the water treatment program vendor as a lease or purchase)pH Control - This controller adds acid to correct water alkalinity a timer on the acid pump sounds an alarm if the pump doesnot shut off in a predetermined length of time Backup controls are required to prevent the cooling system from beingpermanently damaged in case the pH electrodes become fouledCorrosivity and Inhibitor Control - Systems are available which give a direct readout of corrosion rate in milsyear If thecorrosion rate exceeds a set point the controller shuts off the acid increases the inhibitor addition opens the blowdown valveand sounds an alarm Electrodes measure the completeness of the inhibitor film by the value of the potential differencebetween the electrodes The larger the difference the more incomplete the film and a potential reading exceeding a set levelactivates the inhibitor feed
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XXVII 24 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Free Residual Chlorine Control - Free residual chlorine should be controlled automatically for biocide treatment Severalgood analyzers are currently available These units can be incorporated into an automatic control system
Oil in Water DetectionMonitoring Corrosion and Deposit FormationThe following types of devices for measuring corrosion rates and deposit formation should be provided to allow the plant tomonitor the effectiveness of the cooling water treatment before scheduled turnaroundsCoupons - A corrosion coupon rack should be specified for the hot return water to the tower in accordance with IP 19-6-1 Corrosion Monitor - On-line corrosion analyzer if specified should monitor the hot return water to the tower in accordance withIP 19-6-1
On-Line Test Exchanger Although there are a variety of deposit monitors from the water treatment chemical vendors they do not necessarily representactual heat transfer conditions in process exchangers For this reason a test exchanger should be installed for evaluatingcorrosion and deposit formation Details are covered in Report No EE102E78 Cooling Tower Water Treatment Guidelines
Alternatively a critical process exchanger can be instrumented such that its heat transfer coefficient can be monitored
Instrumentation would include flows and inlet and outlet temperatures of process fluid and cooling waterSTEP 6 - Specify cooling tower location
Select a location for the cooling tower The following factors influence this selection
Location Factors1 The location should be within a reasonable distance of the units served and consistent with future refinery plans Piping
costs to and from battery limits can be a significant portion of the total cooling system cost and can be the location-determining factor particularly for salt water systems While piping costs can be easily determined other costs affectinglocation cannot be estimated readily Therefore judgment is required in selecting the location
2 Safety spacing requirements (see Section XV-G Equipment Spacing )3 Cooling tower contribution to plant boundary noise4 For salt water cooling towers wind direction and distance to other facilities is of major concern because of potential
corrosion problems
5 Factors affecting cooling tower performance ie orientation with respect to wind site congestion etc
Plot RequirementsIn estimating plot requirements (basin area) one should allow 5 to 6 ft of tower length for each 1000 gpm (2 m for each 60 Ls)of circulating capacity This figure is based on a typical counterflow design with a width of 50 ft (12 m) and does not includepump suction pit space Exact requirements can be obtained from cooling tower vendors after the tower capacity has beenestimated
Fogging and DriftTwo distinct problems are associated with the discharge air from a cooling tower (1) fogging which can be a nuisance or ahazard and (2) drift which is generally a nuisance because of increased corrosion of surrounding equipment Fogging fromcooling towers occurs during periods of combined high ambient humidity and low temperature as a result of mixing the warmmoist cooling tower exhaust air with the cooler ambient air The ambient air cannot absorb all moisture as vapor and thus tinyfog droplets are formed by condensation Fogging and resulting icing in cold climates has produced hazardous drivingconditions on highways up to 850 ft (250 m) awayProprietary designs are available to combat potential fogging problems Basically these designs incorporate heating of thedischarge air with hot return water precooling coils (similar to an air cooler) to eliminate any visible plume However thesedesigns are expensive and relatively unproven Therefore proper location based on reliable weather data is the preferredsolutionDrift from a cooling tower represents a more difficult problem since it does not depend on definable climatic conditions andoccurs for all wind directions The effects of drift on nearby equipment and vegetation are particularly severe and generally of concern only for salt water systems because of the high solids content of the drift (concentrated salt water) For salt water cooling systems electric lines substations and instrumentation should be located upwind in the direction of the predominantwind They should be provided with adequate shelter or materials when within 450 ft (140 m) of the cooling tower In additionspecifications for all equipment within 150 ft (45 m) of the tower should include surface treatment suitable for a marineenvironment
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COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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XXVII 28 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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XXVII 30 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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COOLING WATER SYSTEMS XXVII 31 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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XXVII 32 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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COOLING WATER SYSTEMS XXVII 33 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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COOLING WATER SYSTEMS XXVII 35 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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Section Page
XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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COOLING WATER SYSTEMS XXVII 37 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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COOLING WATER SYSTEMS XXVII 11 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Treatment of the cooling water is broken down into two major categories external and internal External treatmentencompasses any treatment given to the cooling tower makeup water Internal treatment is the addition of chemicals directly to
the recirculated cooling water
EXTERNAL TREATMENT
External treatment to remove impurities in order to reduce the amount of blowdown should always be evaluated as to costeffectiveness The most frequently used techniques for treating makeup water are shown in Figure 11 Details of availabletreatment processes (see Table 1 for methods available) are covered in Section XXVI-A Boiler Feedwater Treatment For cooling tower systems that meet currently required drift limits in refineries and chemical plants uncontrolled cooling water losses (drift and miscellaneous process uses) normally amount to about 14 of the cooling water evaporation loss Under these conditions the maximum cycles of concentration achievable are 8 with no controlled blowdown Some cooling towersare operating at up to 10 cycles of concentration Additional controlled blowdown required to limit the cycles of concentrationper the water quality criteria is covered later
INTERNAL TREATMENT
The makeup water analyses amount of cooling water losses environmental restraints potential chemical hazards materials of construction and film temperatures of heat transfer surfaces determine the type of internal treatment required for cooling tower systems The internal water treatment program selected must address corrosion scale fouling biological growth andchemical attack of cooling tower componentsNumerous chemicals and treatment programs are continuously being developed for addressing the potential problems Theslightly acidic low pH programs of the past which used chromate based chemicals and usually required acid injection arebeing completely phased out because of environmental restrictions on chromate and the higher costs associated with meetingcurrent practices for handling hazardous chemicals such as acid Non-chromate treatments which use inhibitors such asphosphates zinc and organics along with higher pH have been developed to replace the earlier programs Although not aseffective as chromate these inhibitors operate at higher water pH which is a less corrosive environment The disadvantage of the higher pH is the greater tendency to form scale This tendency is controlled by the use of dispersants or scale inhibitorswhich are normally organic polymers or phosphenate A corrosion inhibitor for copper or brass is also normally required if these metals are present in the systemThe following paragraphs describe techniques and chemicals to control alkalinity (pH) corrosion scale sludge and biologicalgrowth The chemicals mentioned are normally purchased as pre-mixes from vendors and dosed in combination Mostcorrosion and scale control program chemicals are two drum formulations due to the incompatibility of some chemicals in theconcentrated state Therefore separate feed systems are generally provided for chemicals for control of corrosion and scale (2drums) and biological growth (if liquid) If a program includes fixed pH then an additional feed system is required to add acidWater treatment programs are site specific and changing The COOLING WATER SYSTEMS SPECIALISTS should beconsulted for recommendations on the internal water treatment programs when an analysis of the available water is known
Alkalinity (pH) Control
Cooling water treatment programs presently available fall into two basic categories those that do not control pH (floating pH or alkaline treatment programs without pH adjustment) and those that control pH within a specified range by adding acid Theprograms that do not control pH are gaining favor because of the additional cost equipment and precautions required tominimize the risk associated with handling acid and because of improvements in chemicals to control depositsHigh pH in the recirculating cooling water is a result of the alkalinity of the fresh water due to bicarbonate salts This combinedwith carbon dioxide results in the water having a pH range of 6 to 85 When this fresh makeup water is mixed with therecirculated cooling water and passed through the cooling tower the air flow strips out the carbon dioxide Bicarbonate ionsthen convert to carbonate ions raising the pH which can cause both calcium carbonate and calcium phosphate salts toprecipitate out of solution and form deposits Another debit in allowing the cooling water pH to increase is increased chlorineconsumption to control biological growthsIf a program using pH control is justified and chosen acid addition is used under control of a pH analyzer to controlrecirculated cooling water alkalinity and pH to the range required by the treatment program
Corrosion Control
To reduce corrosion to an acceptable level chemical corrosion inhibitors which form protective films on heat transfer surfacesare the most effective protection Inhibiting corrosion is accomplished by phosphates organics zinc nitrites and molybdatesalts Unfortunately the use of chromate which is a reliable corrosion inhibitor is prohibited by environmental constraintsNitrite is not practical in an open system due to atmospheric oxygen converting the nitrite to nitrate
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XXVII 12 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Phosphate inhibitors (orthophosphate polyphosphate phosphonate) are the basic treatment for new towers in todaysenvironment In addition to the phosphate chemistry salts of zinc may be added to aid in inhibiting corrosion These ions
combine with the hydroxide ion forming thin protective films Zinc is used to provide a synergistic effect in combination with thephosphate salts for protecting carbon steel surfaces from corrosion Finally all-organic programs are offered by severalmanufacturers The use of these treatments has been made possible by the development of polymer dispersants which givecalcium salts a much higher solubility and reduce the tendency to scaleWhen a cooling water system contains copper alloys azole based and azole substitutes are required as copper corrosioninhibitors There are several effective inhibitors The most common is tolytriazole (TTA) due to its resistance to chlorineThe corrosion inhibitors most frequently used in ExxonMobil recirculating cooling water systems are shown below indescending order of preference provided their use meets all environmental requirements
bull Stabilized Phosphate (OrthoPolyphosphate) - Environmental restrictions on the use of zinc have resulted in ExxonMobilplants in the USA using a blend of orthophosphate and polyphosphate type corrosion inhibitors with good results Thistreatment forms a thin protective calcium phosphate film on heat transfer surfaces through the use of orthophosphateconcentrations as high as 12 - 17 wppm The use of calcium phosphate dispersants and Hydroxethylidene Diphosphonate(HEDP) stabilizes the treatment and prevents the formation of calcium phosphate scale Concentration of the dispersantsdepends upon the calcium concentration temperature and pH Stabilized Phosphate treatment programs can operate in apH range of 68 to 78 Alternatively an alkaline (floating pH) stabilized phosphate treatment program can be used Thisprogram uses little or no acid addition and is gaining in popularity although not well established yet It operates with pHvalues of 8 - 9 and uses higher quantities of dispersants and lower levels of phosphate A corrosion inhibitor for copper and brass is normally included for all stabilized phosphate programs Several vendors offer these programs Twoseparate chemical feeders are generally required
bull Zinc Alkaline Phosphate - Heritage Exxon plants in Europe where the use of zinc is not restricted have obtained goodcontrol of corrosion and scaling using zinc-polyphosphate treatment programs Low (65 - 72) and high (73 - 80) pHprograms have been used The high pH (zinc alkaline phosphate) programs are the lowest cost and are recommended
As with the Stabilized Phosphate treatment dispersants such as HEDP and polyacrylates are required to prevent theformation of zinc or calcium phosphate scale on heat transfer surfaces A corrosion inhibitor for copper or brass isnormally used with this program which is offered by several vendors
bull Organic Salts - All-organic (polymer) programs are offered by several manufacturers and have been used successfullywithin h Exxon In general these treatments are based on a combination of a phosphonate to control corrosion polymers(dispersants) to prevent scale deposition and a corrosion inhibitor for copper or brass This type of program is mosteffective in conditions of high hardness high alkalinity (M alkalinity gt300 wppm as CaCO 3) and high pH values (gt 85)However some manufacturers have reported success in softer water with alternative polymer component formulations Ingeneral an inorganic corrosion control program is preferred to the all-organic programs which are sensitive to alkalinityand more costly
Detailed control limits for effective use of each of the above inhibitors and materials selection are covered in Report NoEE34E86 Environmentally Acceptable Cooling Water Treatments Report No EE102E78 Cooling Tower Water Treatment Guidelines and in the EMRE Refinery Construction Materials Manual
Scale and Sludge Control
Scale and sludge deposits in cooling water systems cause loss of heat transfer and provide the conditions for pitting corrosionScale consists of inorganic salts which have precipitated from the cooling water forming a coating on heat transfer surfacesSludge consists of organic and inorganic suspended matter that settles in quiescent areas and in heat exchangers The key toprevention of both scale and sludge formation in a cooling system is keeping water velocities greater than 3 fts (09 ms) andthe use of chemical additives in combination with a blowdown rate that keeps the impurity concentration below the level whichcauses deposits (See Table 2 for limits on impurities) Scale control has become increasingly important in higher alkalinityldquofloating pHrdquo water treatment programsSalts most commonly found in scale deposits are calcium carbonate calcium sulfate tricalcium phosphate zinc phosphatemagnesium silicate and silica The control method for preventing scale from each of these salts is discussed below
bull Calcium Carbonate - Where justified using pHalkalinity control in the range of 70 to 75 will normally inhibit the formationof calcium carbonate scale if anti-scalent chemicals (phosphonates acrylates copolymers) are part of the chemicaltreatment (See Report No EE102E78 Cooling Tower Water Treatment Guidelines ) Higher pH water treatmentprograms require greater concentrations of these chemicals along with dispersants and flocculants to inhibit the formationof scale and precipitation of other salts
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COOLING WATER SYSTEMS XXVII 13 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
bull Calcium Sulfate - When heat transfer skin temperatures are below 250 degF (120 degC) and the cooling water bulktemperatures are under 130 degF (54 degC) gypsum scale (CaSO 42H 2O) will form if the product of the calcium and sulfate ionsconcentrations each expressed as wppm CaCO 3 equivalent exceeds 500000 Gypsum scale is prevented bymaintaining bulk water temperature below the indicated limits and controlling calcium and sulfate ions through blowdownor through the use of newer dispersants on the market
bull Tri-Calcium and Zinc Phosphate - Almost all chemical additives for corrosion control contain some type of phosphatesalt These have a tendency to form the orthophosphate ion depending on time pH and temperature Cooling watersusing these corrosion inhibitors may have an orthophosphate concentration in excess of 5 wppm as PO 4
= To preventcalcium phosphate scale Table 2 shows a limit of 1000 wppm of calcium as CaCO 3 equivalent when using a phosphateinhibitor in combination with a chemical dispersant (See Report No EE102E78 Cooling Tower Water Treatment Guidelines for solubility of calcium phosphate) These inhibitors and dispersants generally fall into one of the followingcategories
ndash Sequestrants antiprecipitants and inhibitors such as hydroyethylidene diphosphonate (HEDP) phosphate esters andaminomethylene phosphonate (AMP) function by defyingdistorting the crystal structure of the scale formations toprevent nucleation and growth of the scale
ndash Dispersants based on either anionic or nonionic polymers such as polyacrylamide polyacrylate or polymaleic acidfunction to stabilize colloidal and scale-forming particles in suspension Anionic polymers prevent agglomeration of suspended particles by either charge repulsion or crystal distortion while nonionic polymers reduce the surfacetension of particles
bull Silica Deposits - Quantitative indices for determining limits for silica in combination with other ions are not available due tothe formation of complex polymers Instead the rule of thumb limit of keeping reactive silica concentrations in the coolingwater below 150 wppm as SiO 2 has been successful
Possible alternatives for controlling sludge are
bull Chemical Dispersants - Water treating chemical suppliers now offer many types of dispersants These are mostlypolyelectrolytes Claims for removal of slime silt and mud from cooling tower basins and exchangers are madeHowever satisfactory results using these chemicals as a coagulant aid require very accurate control of the amount fed tothe water and stable operation of the water system Also suspended solids should be kept below 200 wppm for particleslarger than 045 microns Dispersants are used in virtually all fresh water cooling tower systems despite their high cost
bull Sidestream Filtration - The use of a sand pressure filter is the most reliable method for sludge removal Operating andmaintenance costs are minimal for this equipment
bull Higher Blowdown Rates - This may be an economical alternative depending on the cost of makeup water Additionalchemical costs must be included in any evaluation of this method
bull Makeup Clarification - This alternative may require additional raw water treatment facilities to handle seasonal variationsin turbidity andor suspended solids This method is applicable only where suspended solids come from makeup water
Biological Control
Operating conditions in recirculated cooling water are ideal for the growth of biological matter Water temperatures from 70 to120 degF (20 to 50 degC) favorable water pH continuous supply of nutrients including carbon nitrogen and phosphorus inorganicsalts and sunlight all lend themselves to environmental conditions encouraging microbiological growth and plant lifeIf biological growths get out of control and form large sticky agglomerations some of the following operating problems willresult
ndash Fouling of heat transfer surfaces by bacterial slimes resulting in flow restrictions and high process temperatures ndash Reduced cooling tower efficiency resulting from algae fungi and bacterial slime growths in the water distribution basin
and fill area of the cooling tower ndash Excessive plugging of screens and filters ndash Corrosion ndash Wood destruction by fungi ndash Clogging of water distribution nozzles ndash Excessive sludge accumulation in the cooling tower basin
An important part of the water treatment program is to control biological activity by feeding a biocide chemical Oxidizingbiocides such as chlorine sodium hypochlorite chlorine dioxide or bromine-associated compounds are most commonly usedwith good results Plate counts should be maintained below 50000 countsml (bacterial)
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Section Page
XXVII 14 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Chlorine is the most effective biocide for cooling water because of its low cost extensive period of successful use ease of application and minimum environmental effects if applied under controlled conditions Either a liquid feed (sodium
hypochlorite) or a gas feed (100 chlorine) is added to the cooling tower basin A sufficient dosage is that which obtains a 01to 03 wppm continuous free residual in the hot return water to the cooling tower
Advantages to continuous chlorination over shock treatment based on ExxonMobil experience are ndash Minimizes chlorine degradation of polymers ndash Better reduction in total aerobic bacteria plate count resulting in cleaner heat transfer surfaces ndash Does not cause significant pH depressions ndash Better control results with less chlorine consumption
Details concerning chlorine addition as well as the other alternative biocides are given in Report No EE102E78 Cooling Tower Water Treatment Guidelines and in EMRE Engineering Water and Wastewater Design Guide TMEE 080 DG 11-3-1Gas Chlorinators for Water Treating Service and DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) Gaseous chlorine systems have become more costly in recent years because of the additional precautions equipment andprocedures required to minimize the risk associated with handling this gas Alternatives to gaseous chlorine are available butrequire more attention and have a higher operating cost Sodium hypochlorite (either purchased or generated) and sodiumbromide stabilized solid halide (chlorinebromine) products chlorine dioxide and ozone are all alternatives which may haveapplications Contact the COOLING WATER SYSTEMS SPECIALISTS for guidance in the use of these alternative treatmentschemes
BLOWDOWN
Blowdown control is required to maintain the proper cycles of concentration in the system The most reliable and simplest typeof control for blowdown is specific conductance measurement of the cooling water Electrical signals from the specificconductance measurements of some impurity in the cooling water are used to actuate an automatic blowdown valveImpurities normally used for blowdown control are dissolved solids chloride calcium magnesium and silica
CONTROLS AND MONITORS
Any chemical treatment used to control problems in a cooling water system must be carefully monitored to be completelyeffective
The installation of automatic controls should be considered for recirculated cooling water systems in all plants Designs shouldinclude pH conductivity corrosivity inhibitor and biocide control Automatic controllers are available which continuouslymonitor the condition of the water and automatically make chemical additions to keep the water within compositional limitsTest exchangers and corrosion coupons are also useful in monitoring the effectiveness of the water treatment program
An oil-in-water monitor should be considered to provide early detection of oil leaks into the cooling water This will enablecorrective action to be taken before extensive contamination of the cooling water circuit occurs This is especially important incooling towers with high efficiency cellular fill which can be difficult to clean if significantly fouledManual control systems may be used in locations that have adequate technical personnel to perform routine water analysesand to modify chemical treatment necessitated by sudden variations in water composition process contamination etc
WASTEWATER REUSE POSSIBILITIES
Recirculating cooling water systems normally have the highest plant raw water demand A potential alternative is to use treatedwastewater to supply part or all of the makeup water requirement The treatment required must be determined but will typicallyinclude primary oil removal (eg API Separator) secondary oil and suspended solids removal (eg filtration dissolved or induced air flotation) and soluble carbon removal utilizing biological treatmentThe reuse of treated wastewater as makeup currently exists at only a few locations If this alternative is being considered refer to Report No EE58E85 Reuse of Wastewater as Cooling Tower Makeup and Section XIX-B Water Reuse
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Section Page
COOLING WATER SYSTEMS XXVII 15 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS
STEP 1 - Select a location for the intake station and pumps
Factors which influence this selection are as followsbull Depth of water and tidal variations
bull Relative location of intake and outfall direction of current
bull Length and size of intake suction piping vs pump discharge piping
STEP 2 - Determine cooling water requirements
Determine the design cold water inlet temperature based on site data covering seasonal water temperature as a function of inlet location and depth (for existing locations refer to Table 2 in Section IX-B )Tabulate the cooling water requirements for each process unit and offsite consumer Apply the proper LGFs to all coolingloads depending on the stage of the project and whether the new units are exact duplicates of existing unitsIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The designer should investigate methods to increase effluent water temperature to the maximumrecommended limit for the particular type of water used One common method is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 3 - Determine the total required capacity for water intake facilities
Process Coolers - The total requirements determined in Step 2 including LGFsFirewater - This is often supplied from the intake bay of a once-through systemFuture Expansions - These should be considered in sizing intake facilitiesTotal Required Capacity - The sum of the above three items
STEP 4 - Provide water intake facilities
Intake Crib - The intake crib with its support structure is located at the extreme end of the water intake line (See Figure 4 )The intake is usually located 200 to 300 ft (60 to 90 m) offshore and at a sufficient depth below the lowest water level to
preclude vortex formation and subsequent pump cavitation during storm conditions Additionally the crib should have 3 to 5 ft(1 to 15 m) clearance above the sea floor to minimize sand and silt pickup A concrete velocity cap should be specified to cover the inlet crib This will minimize the incidence of fish and other foreignmatter entering the system A horizontal inlet flow pattern should be designed with a maximum entrance velocity of 1 fts(03 ms) Experience shows that vertical velocity flow confuses fish and they are drawn into the intake lineThe intake crib support structure protects the crib from adverse effects of storms and tidal motions as well as protecting it fromimpact of floating and submerged objects A self-cleaning screen can be specified to prevent entrance of large objects into thesystem at the cribIntake Line (Figure 5 ) - An intake line is required if the water depth along the shore does not permit water to flow directly intothe intake bay The intake line is usually buried in the sea floor and covered with a minimum required overburden Consultwith the COOLING WATER SYSTEMS SPECIALISTS for required coverage at a specific siteChlorine or sodium hypochlorite is injected at the intake crib to prevent marine growth in the lineNote that an intake crib and line will not be required if water of sufficient depth is available at the supply pump stationIntake Bay (Figure 6 ) - The intake bay is an enclosure which houses the pumps and screens Depending on a particular design a series of screens is used to ensure the supply of clean water Trash racks comprise the first element of the filter system These remove only very coarse material from the water such as bulk seaweed They can be supplied with atraversing raking mechanismLift screens provide secondary cleanup These screens can be lifted out of the intake bay for periodic cleaning Double liftscreens can be used if it is deemed necessary to have one in service at all timesFinal water filtering is usually done by traveling screens Details of the screen dimensions and the mounting are usuallydetermined during detailed engineering Screen net open area is typically approximately 50 of total screen area Thescreens should be sized for a normal water velocity of 12 fts (04 ms) through the open screen area Maximum velocityshould be limited to 24 fts (08 ms) with 50 of the screen area plugged This is the upper velocity limit for efficientscreening
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ExxonMobil Proprietary
Section Page
XXVII 16 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
If high suspended solids greater than 200 mesh (eg sand) are expected to be a continuous problem ejectors may be installedupstream and downstream of the traveling screen to remove accumulated sand and solids from the intake bay in these areas
Traveling screens and ejectors are supplied with wash water from the raw water pumps or from separate wash water pumpsOnoff control using timers should allow optimum scheduling of the ejectors depending on the time of the year
STEP 5 - Specify water intake facilities
Water intake facilities should be designed in accordance with the following guidelines from the Hydraulic Institute Standards The water intake should be designed to provide an evenly distributed flow of water to the pump suction A properly designedintake reduces the possibility of vortex formation and results in better pump performance with less noise Geometry of theintake and the manner in which water is introduced into the intake are important parameters in providing an even distribution of water flow Complete analysis of intake structures is best accomplished by scale model testsThe pump manufacturer will generally render advice on specific problems while the intake design is still preliminary He shouldbe provided with the necessary intake layout drawings showing the physical limitations of the siteFigures 7A and 7B have been developed for single- and multiple-pump arrangements to show suggestions for basic sumpdimensions They are for pumps normally operating in the capacity range of approximately 3000 to 300000 gpm (200 to20000 Ls) Since these values are composite averages from a great many pump types and cover the entire range of specificspeeds they must not be thought of as absolute values but rather as basic guides subject to some possible variations
All the dimensions in Figures 7A and 7B are based on the rated capacity of the pump at the design head Any increase incapacity above these values should be momentary or very limited in time If operation at an increased capacity is to beundertaken for considerable periods of time the maximum capacity should be used for the design value in obtaining sumpdimensionsDimension C is an average value based on an analysis of many pumps Its final value should be specified by the pumpmanufacturerDimension B is a suggested maximum dimension which may be less depending on actual suction bell or bowl diameters in useby the pump manufacturer The edge of the bell should be close to the back wall of the sump When the position of the backwall is determined by the driving equipment or the discharge piping Dimension B may become excessive and a ldquofalserdquo backwall should be installedDimension S is a minimum value for the sump width for a single pump installation This dimension can be increased but if it isto be made smaller the manufacturer should be consulted or a sump model test should be run to determine its adequacy
Motor size may dictate minimum pump spacing be especially aware of this when specifying pumps with unusually high pumpheads eg greater than 80 psig (550 kPa)Dimension H is a minimum value based on the ldquonormal low water levelrdquo at the pump suction bell taking into considerationfriction losses through the inlet screen and approach channel This dimension can be considerably less momentarily or infrequently without excessive damage to the pump It should be remembered however that this does not representldquosubmergencerdquo Submergence is normally quoted as dimension ldquoHrdquo minus ldquoCrdquo This represents the physical height of water level above the bottom of the suction inletThe actual submergence of the pump is something less than this since the impeller eye is some distance above the bottom of the suction bell possibly as much as 3 to 4 ft (1 to 13 m) For the purposes of sump design in connection with this chart it isunderstood that the pump has been selected in accordance with specific speed charts The submergence referred to hereinhas to do only with vortexing and eddy formationsDimensions Y and A are recommended minimum values These dimensions can be as large as desired but should be limitedto the restrictions indicated on the curve If the design does not include a screen Dimension A should be considerably longerThe screen or gate widths should not be substantially less than S and heights should not be less than H If the main stream
velocity is more than 2 fts (06 ms) it may be necessary to construct straightening vanes in the approach channel increaseDimension A conduct a sump model test of the installation or work out some combination of these factorsDimension S becomes the width of an individual pump cell or the center-to-center distance of two pumps if no division walls areusedOn multiple pump installations the recommended dimensions in Figures 7A and 7B also apply as noted above and thefollowing additional determinants should be consideredFigure 8A - Low velocity and straight-line flow to all units simultaneously is the first recommended style of pit Velocities in thepump area should be approximately 1 fts (03 ms) Some pumps with velocities of 2 fts (06 ms) and higher have given goodresults This is particularly true where the design resulted from a model study Not recommended would be an abrupt changein size of inlet pipe to sump or inlet from one side introducing eddying
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COOLING WATER SYSTEMS XXVII 17 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Figure 8B - A number of pumps in the same sump will operate best without separating walls unless all pumps are always inoperation at the same time in which case the use of separating walls may be beneficial If walls must be used for structural
purposes and pumps will operate intermittently one should leave flow space behind each wall from the pit floor up to at leastthe minimum water level and the wall should not extend upstream beyond the rim of the suction bell If walls are used increaseDimension ldquoSrdquo by the thickness of the wall for correct centerline spacing Round off the ends of the walls Not recommended isthe placement of a number of pumps around the edge of a sump with or without dividing wallsFigure 8C - Abrupt changes in size from inlet pipe or channel to pump bay are not desirable A relatively small pipe emptyinginto a large pump pit should connect to the pit with a gradually increasing taper section The angle should be as large aspossible preferably not less than 45 degrees With this arrangement pit velocities much less than one foot per second aredesirable Especially not recommended is a small pipe directly connected to a large pit with pumps close to the inlet Flow willhave excessive change of direction to get to most of the pumps Centering pumps in the pit leave large ldquovortex areasrdquo behindthe pumps which will cause operational troubleFigure 8D - If the pit velocity can be kept low enough [below 1 fts (03 ms)] an abrupt change from inlet pipe to pit can beaccommodated if the length equals or exceeds the values shown It is assumed that as ratio WP increases the inlet velocityat ldquoPrdquo will increase up to an allowed maximum of 8 fts (24 ms) at WP = 10 Pumps ldquoin linerdquo are not recommended unless theratio of pit to pump size is quite large and pumps are separated longitudinally by a generous margin A pit can generally be
constructed at much less cost by use of a recommended designFigure 8E - It is sometimes desirable to install pumps in tunnels or pipelines A drop pipe or false well to house the pump witha vaned inlet well facing upstream will be satisfactory in flows up to 8 fts (24 ms) Without the inlet well the pump section bellshould be positioned at least two pipe diameters vertically above the top of the tunnel not hung into the tunnel flow especiallywith tunnel velocities 2 fts (06 ms) or more There should be no signs of air along the top of the tunnel It may be necessaryto lower the scoop or to insist on a minimum water level in the vertical wellNote The foregoing statements apply to sumps for clear liquid
STEP 6 - Provide chemical treatment
Provide chemical treatment facilities for the once-through cooling water system Normally the only chemical treatment requiredin a once-through cooling system is for biological control Facilities for adding chlorine to once-through cooling water are thesame as those described for the recirculated cooling water system The design capacity of the chlorinator should be based onachieving a chlorine residual of 1 wppm in the effluent The chlorine required by the organics and other reducing agents in thewater must be satisfied before any residual chlorine will appear in the effluent The design capacity of the chlorinator should be
based on the total water chlorine demand plus 1 wppm in the effluent In the absence of water data the chlorination systemdesign capacity should be based on 10 wppm minimum instantaneous dosage for maximum cooling water flow For very largeonce-through cooling water systems it will be impractical to design for shock rates at this level In this case a continuousdosage of 1 - 2 wppm should be used (Refer to Step 5 of the DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS )
STEP 7 - Specify pumps and distribution system
Specify other major facilities in the once-through cooling water system The recommended materials for pumps valves andpiping are shown in the EMRErsquos Refinery Construction Materials Manual
Pumps1 Type - These pumps are usually vertical centrifugal pumps However horizontal centrifugal pumps have been used An
economic study andor consideration of existing facilities will determine which type should be specified In the event thathorizontal centrifugal pumps are specified facilities for pump priming will likely be required For large pumps in salt water service there is a very limited selection of pumps because of the special materials required Consult with the DISCIPLINESPECIALISTS early in the design effort to determine what specific pumps are currently available
2 Total Required Capacity - Firm capacity (excluding spares) of the supply pumps should equal the maximum cooling water requirements including LGF and RCF
3 NumberIndividual Pump Capacity - Normally one spare pump is provided for up to six operating pumps two spares for up to twelve operating pumps etc The most common arrangements for water supply pumps are
bull Two 100 capacity pumps
bull Three 50 capacity pumps
bull Four 33-13 capacity pumps
bull Five 25 capacity pumps
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Section Page
XXVII 18 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Maximum commercially available pump capacity ranges up to 300000 gpm (20000 Ls) at discharge pressures up to 100psi (700 kPa) Maximum capacity will be less for higher discharge pressures Other factors which may influence pump
selection arebull Selecting capacity to match existing pumps
bull Selecting capacity for future requirements
bull Selecting capacity corresponding to manufacturers available standardVendor information should be obtained on the availability size and cost of the pumps for several configurations Acomparison should be made of the investments and operating costs as well as other advantages and disadvantages toselect a pump configuration
4 Drivers - These pumps are usually motor-driven Because of the remote location of the intake facilities the steamdistribution costs are normally too high to justify the use of a steam turbine driver However use of a secondary selectivesubstation provides a reliable source of power to the pump drivers
5 Pump ∆∆∆∆P - Pump ∆P is based on supplying the maximum cooling water requirement at the battery limit design pressureestablished for the system The ∆P should include the total circulating system pressure drop plus any elevation differencebetween the low water level at the intake and the discharge point of contact with air
6 Pump Instrumentation - The remote location of the intake facilities makes remote control and monitoring of the water supply operation desirable Pump controls located in the control house provide for minimum manning Local controlbackup should be provided The Owners operating and manning philosophy determine the basis for instrumentation andcontrolThe spare pump is controlled to automatically start on low pressure in the supply header Concurrently alarms should beactuated in the control room to indicate the low-pressure situation and spare pump operation
Distribution Network1 Configuration - The piping network is a once-through system with a return line from each consumer or consumer group to
the cooling water effluent line2 Layout - The routing of the piping network should connect all cooling water consumers in the most economical manner
permitted by the plant layout Careful consideration should be given to the turnaround philosophy of the plant Thenetwork layout should preclude the necessity of having above-ground water lines that are in service running through anarea that is being turned around Underground piping or an extra lateral may be required to accomplish this objective
3 Valves and Blinds - Battery limit valves should be provided in the network to isolate individual process units from thesystem Valves should be provided to permit onstream removal and maintenance of spared equipment
4 Basket Strainers - Consideration should be given to the installation of basket strainers in the laterals serving each of theconsumer loops These provide added insurance against entrance of foreign material into the cooling equipment The useof intake screens with 025 in (63 mm) openings may eliminate the need for basket strainers in the laterals
Line Sizing1 Design Basis - The network is sized so that water is delivered to the most distant consumer at a pressure equal to the
required process design (battery limit) pressure2 Flow Rate - Each segment of the distribution network is sized for the simultaneous maximum flow in that segment for any
operating condition The main supply and discharge headers are sized for the simultaneous maximum flow of all segments(not necessarily the sum of the simultaneous maximum flows of the individual segments)
3 Pressure Drop
Line Sizing Basis - The distribution system should be sized according to the guidelines established in Section XIV The use of an ldquoold piperdquo friction factor of C = 90 - 100 (Hazen-Williams factors) in the main lines is recommended to assuregood operation in later years when the system experiences fouling For large diameter lines [greater than 48 in (1200mm)] maximum water velocity should be limited to 10 fts (3 ms) based on civilmechanical considerations (supports andanchoring required to handle potential pressure surges) For salt water systems utilizing cement-lined pipe consider thereduced inner diameter in the friction loss calculations A study may be necessary to determine the economic pump ∆P vsline diameters
4 Equivalent Length Factors - Equivalent length factors are used in pressure drop calculations for sizing the distributionsystem The piping length scaled from the plot plan is multiplied by an equivalent length factor to compensate for pipingelbows expansion loops and other piping variations
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Section Page
COOLING WATER SYSTEMS XXVII 19 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
The equivalent length factors are selected based on the expected variation from a straight piping run Typical equivalentlength factors suitable for planning purposes for different piping applications are as follows
TYPE OF PIPING EQUIVALENT LENGTH FACTORS
Long piping runs [gt 1000 ft (300 m)] 12 to 14
Onsite distribution piping 15 to 20
For design purposes the actual piping layout should be evaluated to ensure that the equivalent length factors areapplicable to the specific situation Particular attention is required for revamp work and saltwater systems utilizing cement-lined pipe
STEP 8 - Prepare a system flow plan similar to the one shown in Figure 1
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS
STEP 1 - Determine cooling water requirements
Tabulate the cooling water requirements and cooling water discharge temperature for each process unit and offsite consumer Apply the proper LGFs to all cooling loads depending on the stage of the project and whether the new units are exactduplicates of existing units Determine the total cooling water flow required including LGFs and the bulk temperature (T 1) of thecombined cooling water flow from all coolers to the cooling towerIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The onsite and offsite designers should investigate methods to increase effluent water temperature to themaximum recommended limit for the particular type of water used This will result in a lower cost cooling tower for a given heatrejection rate One common method of maximizing return water temperature is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 2 - Select cooling tower design parameters
Select the proper design conditions of ambient wet bulb temperature approach temperature range and flow rate for specifyingthe cooling tower Based on these conditions the cooling tower vendor will size the tower and provide the tower characteristicand performance curves which serve as the basis for the performance guarantee Required performance test and evaluationprocedures are described in IP 8-1-1 Economic studies may be required to select the ambient wet bulb temperature andapproach temperature for new locations or for expansion projects with special cooling requirements
STEP 3 - Prepare cooling tower specification sheet
Include the following information in the heat exchange section of the design specificationTower Flow Capacity - This was determined in Step 1Type of Tower - The tower usually specified is an induced draft multicell type suitable for fresh salt or brackish water depending on the water available Additional tower characteristics are usually developed and proposed by the tower vendor (eg crossflow versus counterflow type of tower fill etc) The vendor should be made aware of conditions such as thelikelihood of heavy oil leakage into the cooling water or high fouling potential which may influence the tower type or fill
selectionNumber of Cells - The minimum number of cells required (normally not less than 2) may be specified The maximum flow per cell is a function of the cooling tower design conditions selected in Step 2 Typical values range up to 10000 gpm (600 Ls) for crossflow designs and 8000 gpm (500 Ls) for counterflow designs Normally spare cells are not provided Maintenance iscarried out during cooler periods or load is shed
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XXVII 20 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Ambient Wet Bulb Temperature1 The ambient wet bulb temperature selected for specifying the cooling tower design is generally that temperature which is
equaled or exceeded 5 of the time during the four hottest months (5 of the hourly readings taken during the four hottestmonths will exceed this value) Specifying the 5 temperature should give satisfactory results since the time that the wetbulb temperature is exceeded is for short duration and the system inertia is usually sufficient to avoid problems The tower specification should make it clear that the wet bulb temperature specified is an ambient value as opposed to the enteringvalue which may be higher because of recirculation The tower manufacturer should adjust the ambient value upward totake into account his estimates of recirculation Currently cooling tower test codes provide procedures for measuringperformance in the case of either entering or ambient wet-bulb temperature specifications An increase of 2 degF (11 degC) isgenerally added to ambient wet bulb temperature to estimate entering wet bulb temperature for large cooling towers
2 If plant production will peak during the four summer months when the 5 ambient wet bulb temperature may be exceededconsideration should be given to specifying a higher design ambient wet bulb temperature (one that will be exceeded 212 of the time for example) However the incremental cost of the resulting closer approach and larger cooling tower must be justified by the value of the incremental products made
On-Tower Temperature (T1) - This temperature cannot be accurately determined until the design of all process coolers iscompleted and the volume of cooling water and outlet temperature are known for each (see Step 1) In the planning stage or early design stage of a project this temperature must be estimated and is usually set equal to the maximum design cooler outlet temperature shown under COOLING WATER REQUIREMENTS in the BASIC DESIGN CONSIDERATIONS portion of this sectionApproach Temperature - The approach temperature has the most pronounced effect on cooling tower size and cost For example reducing the approach from 10 to 5 degF (55 to 3 degC) can increase the cooling tower investment by 50 percent and thefan operating cost by 65 percent Thus various approach temperatures should be evaluated to determine the minimum cost of the total system including the process exchanger area Approaches used in the past designs have been from 5 to 12 degF (3 to7degC) Note that it is not customary in the cooling tower industry to guarantee approaches of less than 5 degF to (3 degC) to coolingtower design inlet wet bulb temperature A typical approach for a new grass roots cooling tower is 10 degF (6 degC)Off-Tower Temperature (T2) - This is the sum of the approach temperature and the ambient air wet bulb temperature For existing locations Table 2 in Section IX-B may be used
Range and Flow Rate1 The range for a given heat load is related to the circulating water rate as follows
Q = M C p ∆Tcong (500) (gpm) (range) Eq (1)
where Q = Heat load on the tower BtuhM = Water circulating rate lbhCp = Specific heat capacity of water at average temperature Btulb degF∆T = (T 1 ndash T 2) degF = (water cooling range)gpm = Circulation rate in US gallons per minute
For calculation in Metric units
Q = M C p ∆Tcong (1162 x 10 -3) (Ls) (range) Eq (1M)
where Q = Heat load on the tower kJh
M = Water circulating rate kghCp = Specific heat capacity of water at average temperature kJkg degC∆T = (T 1 ndash T 2) degC = (water cooling range)Ls = Circulation rate in liters per second
2 Generally lower tower investment fan horsepower and pump horsepower result from maximizing the range andminimizing the flow rate However based on fouling considerations of process coolers the maximum allowable hot water temperatures must be kept below recommended limits Also while increasing the range minimizes cooling tower investment and operating cost the cost of exchanger surface requirements is adversely affected by the LMTD and acompromise must be made The range is bounded by the maximum allowable hot water temperature and the Off-Tower temperature (T 2) For established locations refer to Section IX-B for design cold water temperatures to assist in selectingthe approach and range
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COOLING WATER SYSTEMS XXVII 21 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Makeup Water Requirements - Calculate the various losses in the cooling water system to determine makeup requirementsThese losses include
1 Evaporation Loss (E) - Allow 1 of the circulation rate for each 10 degF (55 degC) temperature drop through the tower Notethat the actual evaporation loss will be somewhat less depending on ambient air temperature
( )sL555
TT)gpm(
1000TT
E 2121
minus=
minus= Eq (2)
2 Drift Loss (D) - IP 8-1-1 limits the maximum allowable drift loss to 0005 of the design water circulation rate for freshbrackish or salt water
( )sL1000050
)gpm(1000050
D
=
= Eq (3)
This low drift level contributes little to solids removal from the tower with the spray This allows higher cycles of concentration to be achieved in cooling towers which use clarified softened water for makeup and thus have low requiredblowdown rates Higher drift loss percentages in older or poorly maintained towers will limit the maximum cycles of
concentration achievable3 Miscellaneous Losses (O) - Miscellaneous losses are estimated to be one-seventh of the difference between evaporation
loss and drift loss
minus=
7DE
O Eq (4)
4 Blowdown Rate (B) ndash Guidelines for maximum concentrations of impurities allowed in recirculated fresh cooling water areshown in Table 2 For salt water the blowdown is based on keeping the total dissolved salts below 55000 wppm Obtainthe chemical analysis of the makeup water and divide each individual component concentration into its counterpart inTable 2 The result is the maximum allowable cycles of concentration for that constituent (a worksheet is provided inTable 4 ) The impurity with the lowest factor will be the control and will determine the required blowdown rate
)OD(1Cycles
EB +minus
minus= Eq (5)
The makeup water requirement is the sum of all losses shown above
MakeUp = E + D + O + B Eq (6)
STEP 4 - Specify the controls necessary for proper cooling tower operation
Air Side - Cooling towers are provided with motor-driven manually adjusted pitch multiple-bladed propeller fans Figures9A amp B may be used to estimate the normal operating horsepower requirements Since the fan motor and gear reducer arelocated at the top of the tower [generally 40 to 50 ft (12 to 15 m) above grade] the need for equipment to handle thesecomponents should be discussed with the affiliateControl of the air flow through the tower to maintain reasonably constant cold water temperatures can be accomplished byturning off individual fans in multiple-cell installations adjusting the blade pitch or using variable speed motors To turn off individual fans remote at grade start-stop buttons for the motors are usually specified
Whenever the ambient air temperature is 32 degF (0 degC) or below icing of the tower louvers and structural members in contact withthe incoming air will occur If the heat load on the tower is relatively free of seasonal variations two-speed fans should provideadequate ice control down to approximately 0 degF (-18 degC) ambient temperatures When sustained temperatures of 0 degF (-18 degC)and below occur two-speed fans capable of two-speed reverse operation with appropriate time delays for change of directionand speed should be specified to periodically eliminate ice which will build up on the louvers Other methods of winterizationare available but in general are more expensive and require greater operator attention In very cold climates steam heatingshould be provided to prevent freezing in the cold water basin during shutdown periodsWater Side - The water circulation rate is maintained by operating one or more pumps of sufficient capacity to meet the systemdemand Flow-measuring facilities local temperature indicators and a high-temperature alarm are provided on the coolingtower water supply and return headers Hand-operated flow control valves are required for distributing the water equally to theindividual cells On counterflow towers provisions for measuring the flow to each cell (pitot tubes) are required They are notrequired on crossflow towers since the open distribution basin provides visual aid in balancing flows Shutoff valves should beprovided for servicing the flow control valves without shutting down the entire tower
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XXVII 22 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
STEP 5 - Provide chemical treatment facilities for the fresh water system
Alkalinity (pH) Control (if used)1 In cases where a controlled pH corrosionscale control program is being used the alkalinity (pH) of the recirculated cooling
water should be controlled in accordance with the program requirements Normally acid addition is required to accomplishthis This is due to free carbon dioxide being stripped from the cooling water in the tower to approximately 5 ppm For example if the alkalinity has to be controlled to between 25 and 50 ppm for a pH of 70 to 75 the volume of 98 sulfuricacid (66 degBeacute) required to treat the makeup water is calculated using the following equation
[Note 93 sulfuric acid should be used if ambient temperatures will be less than 35 degF (2 degC)]
GallonsHour 66 degBeacute H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate gpm) (326 x 10 -5)
For calculation in Metric limits
kgh of 100 (66 degBeacute) H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate Ls) (793 x 10 -3)
2 The metering pump should be designed for a maximum feed rate of two to three times the normal feed rate A glass woolfilter is required on the pump suction piping to minimize plugging Normally two - 100 capacity pumps are provided
Positive displacement metering pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumps with flow control areused for higher rates
3 Hydrochloric acid may also be used for alkalinity control The gallons per hour of 20 degBeacute hydrochloric acid required is36 times the amount of 66 degBeacute sulfuric acid In Metric units the weight of 32 (20 degBeacute) hydrochloric acid required is227 times the weight of 100 (66 degBeacute) sulfuric acid
4 Specifications for chemical feeders may be found in EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased)
5 A pH analyzer with high and low alarm is provided to control the amount of acid feed The acid should be added to thecooling tower makeup water (Figure 2 ) The lag time between the point of acid injection and the sample outlet to the pHcell should be less than two minutes for effective control
6 Spills or overflows from the sulfuric acid tank should be contained
Corrosion Inhibitors and Anti-Scalents1 Two chemical feeders are generally provided The following guides in sizing the equipment will allow the use of any of the
proprietary products from the vendors when final selection by the owner is made See Report No EE102E78 Cooling Tower Water Treatment Guidelines EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and Report No EE34E86 Environmentally - Acceptable Cooling Water Treatments for detailed discussion of acceptable commercial chemicals
bull If the water treatment program to be used is known design for the chemical rates specified by the vendor or COOLING WATER SYSTEMS SPECIALIST
bull Otherwise design for 100 wppm of each commercial liquid product in the recirculated cooling water The design feedrate should be based on the design total blowdown flow (both uncontrolled and controlled) since this is where theproduct is lost For 100 wppm concentration the chemical rates will be as follows
GallonsHour Chemical = Blowdown (gpm) x (0006)
LitersHour Chemical = Blowdown (Ls) x (036)
2 The design capacity of the metering pump should be two to three times this calculated number Normally 2 - 100capacity pumps are provided Positive displacement pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumpswith flow control are used for higher rates
3 Specifications for chemical feeders may be found in EMRErsquos Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased) Reference should also be made to the checklists containedin Report No EE92E94 Guidelines for Safety Evaluation of Chemical Injection Facilities
4 A final check should be made with the owner and the COOLING WATER SYSTEMS SPECIALISTS as to the treatmentprogram selected and if there is a need for an additional chemical feeder
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 23 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Biological ControlFacilities should be provided for feeding the biocide into the back of the cooling tower basin If chlorine is used it must bepresent in the cooling water as free residual to be effective However the following amounts of chlorine are first required toreact with the listed impurities when present in the cooling water before a free residual can be established
IMPURITIESAMOUNT OF CHLORINE REQUIRED (wppm)
WITH EACH WPPM OF IMPURITY
Ammonia (NH 3)
Hydrogen Sulfide (H 2S)
Sodium Sulfite (Na 2SO 3)
Ferrous Iron (Fe ++)
10
22
056
064
The design capacity of the chlorine feeder shall be based on achieving a 01 wppm free chlorine residual in the recirculatingwater flow or a minimum dosage of 7 wppm at maximum design makeup water flow rate and conditions (see EMRE Water and
Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service )Chlorine may be fed either as the commercial liquid sodium hypochlorite (124 wt Cl 2) or liquid chlorine (100 Cl 2) Liquidhypochlorite is being used increasingly because of the higher costs associated with precautions equipment and proceduresrequired to minimize the risks associated with handling gaseous chlorine The hypochlorite solution is fed using a meteringpump which should be sized to feed an equivalent of 15 ppm 100 Cl 2 (Refer to EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) ) In case of spills or an overflow from thestorage tank the sodium hypochlorite may be sent to the cooling tower basinGas chlorinators are covered by EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service The use of gas chlorination is being reduced but may still be appropriate in some locations subject tolocal management approval The gas from either commercial cylinders or an evaporator is dissolved in a water slip stream bymeans of an eductor This results in a chlorine solution strength of approximately 01 This solution should be evenlydistributed below the water surface by means of a header lateral pipe arrangement at the rear of the cooling tower basinopposite the cooling water pumpsSome additional guides on the use of gas chlorinators are given below It is also recommended that the safety guidelines
published by the Manufacturing Chemists Association and the Chlorine Institute be followedbull Dilution water for the chlorinator must not be less than 50 degF (10 degC) or ice formation may cause plugging problems
bull Continuous rates of chlorine gas withdrawal should not exceed 400 lb (180 kg)24 hours from each one ton cylinder atroom temperature [70 degF (21 degC)] This rate may be exceeded (up to 50) for periods not exceeding 2 hours Rule of thumb is not to design for more than 4 cylinders operating in parallel When feed rate exceeds 3000 lb (1360 kg) per daythe liquid chlorine should be fed to an evaporator upstream of the chlorinator
bull If the ambient temperature can be expected to be below 50 degF (10 degC) for extended periods a building should be providedfor the chlorinator and cylinders
bull Weighing scales should be provided to determine when shipping containers should be replaced and to verify chlorine feedrates
bull Analyzers to measure the free chlorine residual in the circulated cooling water should be provided
bull Chlorine leak detectors and a catch basin for major spills should be provided
Control SystemProvide automatic controllers to continuously monitor water condition and to add chemicals to the system (complete systemsmay be available through the water treatment program vendor as a lease or purchase)pH Control - This controller adds acid to correct water alkalinity a timer on the acid pump sounds an alarm if the pump doesnot shut off in a predetermined length of time Backup controls are required to prevent the cooling system from beingpermanently damaged in case the pH electrodes become fouledCorrosivity and Inhibitor Control - Systems are available which give a direct readout of corrosion rate in milsyear If thecorrosion rate exceeds a set point the controller shuts off the acid increases the inhibitor addition opens the blowdown valveand sounds an alarm Electrodes measure the completeness of the inhibitor film by the value of the potential differencebetween the electrodes The larger the difference the more incomplete the film and a potential reading exceeding a set levelactivates the inhibitor feed
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XXVII 24 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Free Residual Chlorine Control - Free residual chlorine should be controlled automatically for biocide treatment Severalgood analyzers are currently available These units can be incorporated into an automatic control system
Oil in Water DetectionMonitoring Corrosion and Deposit FormationThe following types of devices for measuring corrosion rates and deposit formation should be provided to allow the plant tomonitor the effectiveness of the cooling water treatment before scheduled turnaroundsCoupons - A corrosion coupon rack should be specified for the hot return water to the tower in accordance with IP 19-6-1 Corrosion Monitor - On-line corrosion analyzer if specified should monitor the hot return water to the tower in accordance withIP 19-6-1
On-Line Test Exchanger Although there are a variety of deposit monitors from the water treatment chemical vendors they do not necessarily representactual heat transfer conditions in process exchangers For this reason a test exchanger should be installed for evaluatingcorrosion and deposit formation Details are covered in Report No EE102E78 Cooling Tower Water Treatment Guidelines
Alternatively a critical process exchanger can be instrumented such that its heat transfer coefficient can be monitored
Instrumentation would include flows and inlet and outlet temperatures of process fluid and cooling waterSTEP 6 - Specify cooling tower location
Select a location for the cooling tower The following factors influence this selection
Location Factors1 The location should be within a reasonable distance of the units served and consistent with future refinery plans Piping
costs to and from battery limits can be a significant portion of the total cooling system cost and can be the location-determining factor particularly for salt water systems While piping costs can be easily determined other costs affectinglocation cannot be estimated readily Therefore judgment is required in selecting the location
2 Safety spacing requirements (see Section XV-G Equipment Spacing )3 Cooling tower contribution to plant boundary noise4 For salt water cooling towers wind direction and distance to other facilities is of major concern because of potential
corrosion problems
5 Factors affecting cooling tower performance ie orientation with respect to wind site congestion etc
Plot RequirementsIn estimating plot requirements (basin area) one should allow 5 to 6 ft of tower length for each 1000 gpm (2 m for each 60 Ls)of circulating capacity This figure is based on a typical counterflow design with a width of 50 ft (12 m) and does not includepump suction pit space Exact requirements can be obtained from cooling tower vendors after the tower capacity has beenestimated
Fogging and DriftTwo distinct problems are associated with the discharge air from a cooling tower (1) fogging which can be a nuisance or ahazard and (2) drift which is generally a nuisance because of increased corrosion of surrounding equipment Fogging fromcooling towers occurs during periods of combined high ambient humidity and low temperature as a result of mixing the warmmoist cooling tower exhaust air with the cooler ambient air The ambient air cannot absorb all moisture as vapor and thus tinyfog droplets are formed by condensation Fogging and resulting icing in cold climates has produced hazardous drivingconditions on highways up to 850 ft (250 m) awayProprietary designs are available to combat potential fogging problems Basically these designs incorporate heating of thedischarge air with hot return water precooling coils (similar to an air cooler) to eliminate any visible plume However thesedesigns are expensive and relatively unproven Therefore proper location based on reliable weather data is the preferredsolutionDrift from a cooling tower represents a more difficult problem since it does not depend on definable climatic conditions andoccurs for all wind directions The effects of drift on nearby equipment and vegetation are particularly severe and generally of concern only for salt water systems because of the high solids content of the drift (concentrated salt water) For salt water cooling systems electric lines substations and instrumentation should be located upwind in the direction of the predominantwind They should be provided with adequate shelter or materials when within 450 ft (140 m) of the cooling tower In additionspecifications for all equipment within 150 ft (45 m) of the tower should include surface treatment suitable for a marineenvironment
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COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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XXVII 28 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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XXVII 30 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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COOLING WATER SYSTEMS XXVII 31 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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XXVII 32 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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COOLING WATER SYSTEMS XXVII 33 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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Section Page
COOLING WATER SYSTEMS XXVII 35 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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COOLING WATER SYSTEMS XXVII 37 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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XXVII 12 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Phosphate inhibitors (orthophosphate polyphosphate phosphonate) are the basic treatment for new towers in todaysenvironment In addition to the phosphate chemistry salts of zinc may be added to aid in inhibiting corrosion These ions
combine with the hydroxide ion forming thin protective films Zinc is used to provide a synergistic effect in combination with thephosphate salts for protecting carbon steel surfaces from corrosion Finally all-organic programs are offered by severalmanufacturers The use of these treatments has been made possible by the development of polymer dispersants which givecalcium salts a much higher solubility and reduce the tendency to scaleWhen a cooling water system contains copper alloys azole based and azole substitutes are required as copper corrosioninhibitors There are several effective inhibitors The most common is tolytriazole (TTA) due to its resistance to chlorineThe corrosion inhibitors most frequently used in ExxonMobil recirculating cooling water systems are shown below indescending order of preference provided their use meets all environmental requirements
bull Stabilized Phosphate (OrthoPolyphosphate) - Environmental restrictions on the use of zinc have resulted in ExxonMobilplants in the USA using a blend of orthophosphate and polyphosphate type corrosion inhibitors with good results Thistreatment forms a thin protective calcium phosphate film on heat transfer surfaces through the use of orthophosphateconcentrations as high as 12 - 17 wppm The use of calcium phosphate dispersants and Hydroxethylidene Diphosphonate(HEDP) stabilizes the treatment and prevents the formation of calcium phosphate scale Concentration of the dispersantsdepends upon the calcium concentration temperature and pH Stabilized Phosphate treatment programs can operate in apH range of 68 to 78 Alternatively an alkaline (floating pH) stabilized phosphate treatment program can be used Thisprogram uses little or no acid addition and is gaining in popularity although not well established yet It operates with pHvalues of 8 - 9 and uses higher quantities of dispersants and lower levels of phosphate A corrosion inhibitor for copper and brass is normally included for all stabilized phosphate programs Several vendors offer these programs Twoseparate chemical feeders are generally required
bull Zinc Alkaline Phosphate - Heritage Exxon plants in Europe where the use of zinc is not restricted have obtained goodcontrol of corrosion and scaling using zinc-polyphosphate treatment programs Low (65 - 72) and high (73 - 80) pHprograms have been used The high pH (zinc alkaline phosphate) programs are the lowest cost and are recommended
As with the Stabilized Phosphate treatment dispersants such as HEDP and polyacrylates are required to prevent theformation of zinc or calcium phosphate scale on heat transfer surfaces A corrosion inhibitor for copper or brass isnormally used with this program which is offered by several vendors
bull Organic Salts - All-organic (polymer) programs are offered by several manufacturers and have been used successfullywithin h Exxon In general these treatments are based on a combination of a phosphonate to control corrosion polymers(dispersants) to prevent scale deposition and a corrosion inhibitor for copper or brass This type of program is mosteffective in conditions of high hardness high alkalinity (M alkalinity gt300 wppm as CaCO 3) and high pH values (gt 85)However some manufacturers have reported success in softer water with alternative polymer component formulations Ingeneral an inorganic corrosion control program is preferred to the all-organic programs which are sensitive to alkalinityand more costly
Detailed control limits for effective use of each of the above inhibitors and materials selection are covered in Report NoEE34E86 Environmentally Acceptable Cooling Water Treatments Report No EE102E78 Cooling Tower Water Treatment Guidelines and in the EMRE Refinery Construction Materials Manual
Scale and Sludge Control
Scale and sludge deposits in cooling water systems cause loss of heat transfer and provide the conditions for pitting corrosionScale consists of inorganic salts which have precipitated from the cooling water forming a coating on heat transfer surfacesSludge consists of organic and inorganic suspended matter that settles in quiescent areas and in heat exchangers The key toprevention of both scale and sludge formation in a cooling system is keeping water velocities greater than 3 fts (09 ms) andthe use of chemical additives in combination with a blowdown rate that keeps the impurity concentration below the level whichcauses deposits (See Table 2 for limits on impurities) Scale control has become increasingly important in higher alkalinityldquofloating pHrdquo water treatment programsSalts most commonly found in scale deposits are calcium carbonate calcium sulfate tricalcium phosphate zinc phosphatemagnesium silicate and silica The control method for preventing scale from each of these salts is discussed below
bull Calcium Carbonate - Where justified using pHalkalinity control in the range of 70 to 75 will normally inhibit the formationof calcium carbonate scale if anti-scalent chemicals (phosphonates acrylates copolymers) are part of the chemicaltreatment (See Report No EE102E78 Cooling Tower Water Treatment Guidelines ) Higher pH water treatmentprograms require greater concentrations of these chemicals along with dispersants and flocculants to inhibit the formationof scale and precipitation of other salts
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COOLING WATER SYSTEMS XXVII 13 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
bull Calcium Sulfate - When heat transfer skin temperatures are below 250 degF (120 degC) and the cooling water bulktemperatures are under 130 degF (54 degC) gypsum scale (CaSO 42H 2O) will form if the product of the calcium and sulfate ionsconcentrations each expressed as wppm CaCO 3 equivalent exceeds 500000 Gypsum scale is prevented bymaintaining bulk water temperature below the indicated limits and controlling calcium and sulfate ions through blowdownor through the use of newer dispersants on the market
bull Tri-Calcium and Zinc Phosphate - Almost all chemical additives for corrosion control contain some type of phosphatesalt These have a tendency to form the orthophosphate ion depending on time pH and temperature Cooling watersusing these corrosion inhibitors may have an orthophosphate concentration in excess of 5 wppm as PO 4
= To preventcalcium phosphate scale Table 2 shows a limit of 1000 wppm of calcium as CaCO 3 equivalent when using a phosphateinhibitor in combination with a chemical dispersant (See Report No EE102E78 Cooling Tower Water Treatment Guidelines for solubility of calcium phosphate) These inhibitors and dispersants generally fall into one of the followingcategories
ndash Sequestrants antiprecipitants and inhibitors such as hydroyethylidene diphosphonate (HEDP) phosphate esters andaminomethylene phosphonate (AMP) function by defyingdistorting the crystal structure of the scale formations toprevent nucleation and growth of the scale
ndash Dispersants based on either anionic or nonionic polymers such as polyacrylamide polyacrylate or polymaleic acidfunction to stabilize colloidal and scale-forming particles in suspension Anionic polymers prevent agglomeration of suspended particles by either charge repulsion or crystal distortion while nonionic polymers reduce the surfacetension of particles
bull Silica Deposits - Quantitative indices for determining limits for silica in combination with other ions are not available due tothe formation of complex polymers Instead the rule of thumb limit of keeping reactive silica concentrations in the coolingwater below 150 wppm as SiO 2 has been successful
Possible alternatives for controlling sludge are
bull Chemical Dispersants - Water treating chemical suppliers now offer many types of dispersants These are mostlypolyelectrolytes Claims for removal of slime silt and mud from cooling tower basins and exchangers are madeHowever satisfactory results using these chemicals as a coagulant aid require very accurate control of the amount fed tothe water and stable operation of the water system Also suspended solids should be kept below 200 wppm for particleslarger than 045 microns Dispersants are used in virtually all fresh water cooling tower systems despite their high cost
bull Sidestream Filtration - The use of a sand pressure filter is the most reliable method for sludge removal Operating andmaintenance costs are minimal for this equipment
bull Higher Blowdown Rates - This may be an economical alternative depending on the cost of makeup water Additionalchemical costs must be included in any evaluation of this method
bull Makeup Clarification - This alternative may require additional raw water treatment facilities to handle seasonal variationsin turbidity andor suspended solids This method is applicable only where suspended solids come from makeup water
Biological Control
Operating conditions in recirculated cooling water are ideal for the growth of biological matter Water temperatures from 70 to120 degF (20 to 50 degC) favorable water pH continuous supply of nutrients including carbon nitrogen and phosphorus inorganicsalts and sunlight all lend themselves to environmental conditions encouraging microbiological growth and plant lifeIf biological growths get out of control and form large sticky agglomerations some of the following operating problems willresult
ndash Fouling of heat transfer surfaces by bacterial slimes resulting in flow restrictions and high process temperatures ndash Reduced cooling tower efficiency resulting from algae fungi and bacterial slime growths in the water distribution basin
and fill area of the cooling tower ndash Excessive plugging of screens and filters ndash Corrosion ndash Wood destruction by fungi ndash Clogging of water distribution nozzles ndash Excessive sludge accumulation in the cooling tower basin
An important part of the water treatment program is to control biological activity by feeding a biocide chemical Oxidizingbiocides such as chlorine sodium hypochlorite chlorine dioxide or bromine-associated compounds are most commonly usedwith good results Plate counts should be maintained below 50000 countsml (bacterial)
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XXVII 14 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Chlorine is the most effective biocide for cooling water because of its low cost extensive period of successful use ease of application and minimum environmental effects if applied under controlled conditions Either a liquid feed (sodium
hypochlorite) or a gas feed (100 chlorine) is added to the cooling tower basin A sufficient dosage is that which obtains a 01to 03 wppm continuous free residual in the hot return water to the cooling tower
Advantages to continuous chlorination over shock treatment based on ExxonMobil experience are ndash Minimizes chlorine degradation of polymers ndash Better reduction in total aerobic bacteria plate count resulting in cleaner heat transfer surfaces ndash Does not cause significant pH depressions ndash Better control results with less chlorine consumption
Details concerning chlorine addition as well as the other alternative biocides are given in Report No EE102E78 Cooling Tower Water Treatment Guidelines and in EMRE Engineering Water and Wastewater Design Guide TMEE 080 DG 11-3-1Gas Chlorinators for Water Treating Service and DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) Gaseous chlorine systems have become more costly in recent years because of the additional precautions equipment andprocedures required to minimize the risk associated with handling this gas Alternatives to gaseous chlorine are available butrequire more attention and have a higher operating cost Sodium hypochlorite (either purchased or generated) and sodiumbromide stabilized solid halide (chlorinebromine) products chlorine dioxide and ozone are all alternatives which may haveapplications Contact the COOLING WATER SYSTEMS SPECIALISTS for guidance in the use of these alternative treatmentschemes
BLOWDOWN
Blowdown control is required to maintain the proper cycles of concentration in the system The most reliable and simplest typeof control for blowdown is specific conductance measurement of the cooling water Electrical signals from the specificconductance measurements of some impurity in the cooling water are used to actuate an automatic blowdown valveImpurities normally used for blowdown control are dissolved solids chloride calcium magnesium and silica
CONTROLS AND MONITORS
Any chemical treatment used to control problems in a cooling water system must be carefully monitored to be completelyeffective
The installation of automatic controls should be considered for recirculated cooling water systems in all plants Designs shouldinclude pH conductivity corrosivity inhibitor and biocide control Automatic controllers are available which continuouslymonitor the condition of the water and automatically make chemical additions to keep the water within compositional limitsTest exchangers and corrosion coupons are also useful in monitoring the effectiveness of the water treatment program
An oil-in-water monitor should be considered to provide early detection of oil leaks into the cooling water This will enablecorrective action to be taken before extensive contamination of the cooling water circuit occurs This is especially important incooling towers with high efficiency cellular fill which can be difficult to clean if significantly fouledManual control systems may be used in locations that have adequate technical personnel to perform routine water analysesand to modify chemical treatment necessitated by sudden variations in water composition process contamination etc
WASTEWATER REUSE POSSIBILITIES
Recirculating cooling water systems normally have the highest plant raw water demand A potential alternative is to use treatedwastewater to supply part or all of the makeup water requirement The treatment required must be determined but will typicallyinclude primary oil removal (eg API Separator) secondary oil and suspended solids removal (eg filtration dissolved or induced air flotation) and soluble carbon removal utilizing biological treatmentThe reuse of treated wastewater as makeup currently exists at only a few locations If this alternative is being considered refer to Report No EE58E85 Reuse of Wastewater as Cooling Tower Makeup and Section XIX-B Water Reuse
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COOLING WATER SYSTEMS XXVII 15 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS
STEP 1 - Select a location for the intake station and pumps
Factors which influence this selection are as followsbull Depth of water and tidal variations
bull Relative location of intake and outfall direction of current
bull Length and size of intake suction piping vs pump discharge piping
STEP 2 - Determine cooling water requirements
Determine the design cold water inlet temperature based on site data covering seasonal water temperature as a function of inlet location and depth (for existing locations refer to Table 2 in Section IX-B )Tabulate the cooling water requirements for each process unit and offsite consumer Apply the proper LGFs to all coolingloads depending on the stage of the project and whether the new units are exact duplicates of existing unitsIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The designer should investigate methods to increase effluent water temperature to the maximumrecommended limit for the particular type of water used One common method is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 3 - Determine the total required capacity for water intake facilities
Process Coolers - The total requirements determined in Step 2 including LGFsFirewater - This is often supplied from the intake bay of a once-through systemFuture Expansions - These should be considered in sizing intake facilitiesTotal Required Capacity - The sum of the above three items
STEP 4 - Provide water intake facilities
Intake Crib - The intake crib with its support structure is located at the extreme end of the water intake line (See Figure 4 )The intake is usually located 200 to 300 ft (60 to 90 m) offshore and at a sufficient depth below the lowest water level to
preclude vortex formation and subsequent pump cavitation during storm conditions Additionally the crib should have 3 to 5 ft(1 to 15 m) clearance above the sea floor to minimize sand and silt pickup A concrete velocity cap should be specified to cover the inlet crib This will minimize the incidence of fish and other foreignmatter entering the system A horizontal inlet flow pattern should be designed with a maximum entrance velocity of 1 fts(03 ms) Experience shows that vertical velocity flow confuses fish and they are drawn into the intake lineThe intake crib support structure protects the crib from adverse effects of storms and tidal motions as well as protecting it fromimpact of floating and submerged objects A self-cleaning screen can be specified to prevent entrance of large objects into thesystem at the cribIntake Line (Figure 5 ) - An intake line is required if the water depth along the shore does not permit water to flow directly intothe intake bay The intake line is usually buried in the sea floor and covered with a minimum required overburden Consultwith the COOLING WATER SYSTEMS SPECIALISTS for required coverage at a specific siteChlorine or sodium hypochlorite is injected at the intake crib to prevent marine growth in the lineNote that an intake crib and line will not be required if water of sufficient depth is available at the supply pump stationIntake Bay (Figure 6 ) - The intake bay is an enclosure which houses the pumps and screens Depending on a particular design a series of screens is used to ensure the supply of clean water Trash racks comprise the first element of the filter system These remove only very coarse material from the water such as bulk seaweed They can be supplied with atraversing raking mechanismLift screens provide secondary cleanup These screens can be lifted out of the intake bay for periodic cleaning Double liftscreens can be used if it is deemed necessary to have one in service at all timesFinal water filtering is usually done by traveling screens Details of the screen dimensions and the mounting are usuallydetermined during detailed engineering Screen net open area is typically approximately 50 of total screen area Thescreens should be sized for a normal water velocity of 12 fts (04 ms) through the open screen area Maximum velocityshould be limited to 24 fts (08 ms) with 50 of the screen area plugged This is the upper velocity limit for efficientscreening
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ExxonMobil Proprietary
Section Page
XXVII 16 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
If high suspended solids greater than 200 mesh (eg sand) are expected to be a continuous problem ejectors may be installedupstream and downstream of the traveling screen to remove accumulated sand and solids from the intake bay in these areas
Traveling screens and ejectors are supplied with wash water from the raw water pumps or from separate wash water pumpsOnoff control using timers should allow optimum scheduling of the ejectors depending on the time of the year
STEP 5 - Specify water intake facilities
Water intake facilities should be designed in accordance with the following guidelines from the Hydraulic Institute Standards The water intake should be designed to provide an evenly distributed flow of water to the pump suction A properly designedintake reduces the possibility of vortex formation and results in better pump performance with less noise Geometry of theintake and the manner in which water is introduced into the intake are important parameters in providing an even distribution of water flow Complete analysis of intake structures is best accomplished by scale model testsThe pump manufacturer will generally render advice on specific problems while the intake design is still preliminary He shouldbe provided with the necessary intake layout drawings showing the physical limitations of the siteFigures 7A and 7B have been developed for single- and multiple-pump arrangements to show suggestions for basic sumpdimensions They are for pumps normally operating in the capacity range of approximately 3000 to 300000 gpm (200 to20000 Ls) Since these values are composite averages from a great many pump types and cover the entire range of specificspeeds they must not be thought of as absolute values but rather as basic guides subject to some possible variations
All the dimensions in Figures 7A and 7B are based on the rated capacity of the pump at the design head Any increase incapacity above these values should be momentary or very limited in time If operation at an increased capacity is to beundertaken for considerable periods of time the maximum capacity should be used for the design value in obtaining sumpdimensionsDimension C is an average value based on an analysis of many pumps Its final value should be specified by the pumpmanufacturerDimension B is a suggested maximum dimension which may be less depending on actual suction bell or bowl diameters in useby the pump manufacturer The edge of the bell should be close to the back wall of the sump When the position of the backwall is determined by the driving equipment or the discharge piping Dimension B may become excessive and a ldquofalserdquo backwall should be installedDimension S is a minimum value for the sump width for a single pump installation This dimension can be increased but if it isto be made smaller the manufacturer should be consulted or a sump model test should be run to determine its adequacy
Motor size may dictate minimum pump spacing be especially aware of this when specifying pumps with unusually high pumpheads eg greater than 80 psig (550 kPa)Dimension H is a minimum value based on the ldquonormal low water levelrdquo at the pump suction bell taking into considerationfriction losses through the inlet screen and approach channel This dimension can be considerably less momentarily or infrequently without excessive damage to the pump It should be remembered however that this does not representldquosubmergencerdquo Submergence is normally quoted as dimension ldquoHrdquo minus ldquoCrdquo This represents the physical height of water level above the bottom of the suction inletThe actual submergence of the pump is something less than this since the impeller eye is some distance above the bottom of the suction bell possibly as much as 3 to 4 ft (1 to 13 m) For the purposes of sump design in connection with this chart it isunderstood that the pump has been selected in accordance with specific speed charts The submergence referred to hereinhas to do only with vortexing and eddy formationsDimensions Y and A are recommended minimum values These dimensions can be as large as desired but should be limitedto the restrictions indicated on the curve If the design does not include a screen Dimension A should be considerably longerThe screen or gate widths should not be substantially less than S and heights should not be less than H If the main stream
velocity is more than 2 fts (06 ms) it may be necessary to construct straightening vanes in the approach channel increaseDimension A conduct a sump model test of the installation or work out some combination of these factorsDimension S becomes the width of an individual pump cell or the center-to-center distance of two pumps if no division walls areusedOn multiple pump installations the recommended dimensions in Figures 7A and 7B also apply as noted above and thefollowing additional determinants should be consideredFigure 8A - Low velocity and straight-line flow to all units simultaneously is the first recommended style of pit Velocities in thepump area should be approximately 1 fts (03 ms) Some pumps with velocities of 2 fts (06 ms) and higher have given goodresults This is particularly true where the design resulted from a model study Not recommended would be an abrupt changein size of inlet pipe to sump or inlet from one side introducing eddying
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COOLING WATER SYSTEMS XXVII 17 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Figure 8B - A number of pumps in the same sump will operate best without separating walls unless all pumps are always inoperation at the same time in which case the use of separating walls may be beneficial If walls must be used for structural
purposes and pumps will operate intermittently one should leave flow space behind each wall from the pit floor up to at leastthe minimum water level and the wall should not extend upstream beyond the rim of the suction bell If walls are used increaseDimension ldquoSrdquo by the thickness of the wall for correct centerline spacing Round off the ends of the walls Not recommended isthe placement of a number of pumps around the edge of a sump with or without dividing wallsFigure 8C - Abrupt changes in size from inlet pipe or channel to pump bay are not desirable A relatively small pipe emptyinginto a large pump pit should connect to the pit with a gradually increasing taper section The angle should be as large aspossible preferably not less than 45 degrees With this arrangement pit velocities much less than one foot per second aredesirable Especially not recommended is a small pipe directly connected to a large pit with pumps close to the inlet Flow willhave excessive change of direction to get to most of the pumps Centering pumps in the pit leave large ldquovortex areasrdquo behindthe pumps which will cause operational troubleFigure 8D - If the pit velocity can be kept low enough [below 1 fts (03 ms)] an abrupt change from inlet pipe to pit can beaccommodated if the length equals or exceeds the values shown It is assumed that as ratio WP increases the inlet velocityat ldquoPrdquo will increase up to an allowed maximum of 8 fts (24 ms) at WP = 10 Pumps ldquoin linerdquo are not recommended unless theratio of pit to pump size is quite large and pumps are separated longitudinally by a generous margin A pit can generally be
constructed at much less cost by use of a recommended designFigure 8E - It is sometimes desirable to install pumps in tunnels or pipelines A drop pipe or false well to house the pump witha vaned inlet well facing upstream will be satisfactory in flows up to 8 fts (24 ms) Without the inlet well the pump section bellshould be positioned at least two pipe diameters vertically above the top of the tunnel not hung into the tunnel flow especiallywith tunnel velocities 2 fts (06 ms) or more There should be no signs of air along the top of the tunnel It may be necessaryto lower the scoop or to insist on a minimum water level in the vertical wellNote The foregoing statements apply to sumps for clear liquid
STEP 6 - Provide chemical treatment
Provide chemical treatment facilities for the once-through cooling water system Normally the only chemical treatment requiredin a once-through cooling system is for biological control Facilities for adding chlorine to once-through cooling water are thesame as those described for the recirculated cooling water system The design capacity of the chlorinator should be based onachieving a chlorine residual of 1 wppm in the effluent The chlorine required by the organics and other reducing agents in thewater must be satisfied before any residual chlorine will appear in the effluent The design capacity of the chlorinator should be
based on the total water chlorine demand plus 1 wppm in the effluent In the absence of water data the chlorination systemdesign capacity should be based on 10 wppm minimum instantaneous dosage for maximum cooling water flow For very largeonce-through cooling water systems it will be impractical to design for shock rates at this level In this case a continuousdosage of 1 - 2 wppm should be used (Refer to Step 5 of the DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS )
STEP 7 - Specify pumps and distribution system
Specify other major facilities in the once-through cooling water system The recommended materials for pumps valves andpiping are shown in the EMRErsquos Refinery Construction Materials Manual
Pumps1 Type - These pumps are usually vertical centrifugal pumps However horizontal centrifugal pumps have been used An
economic study andor consideration of existing facilities will determine which type should be specified In the event thathorizontal centrifugal pumps are specified facilities for pump priming will likely be required For large pumps in salt water service there is a very limited selection of pumps because of the special materials required Consult with the DISCIPLINESPECIALISTS early in the design effort to determine what specific pumps are currently available
2 Total Required Capacity - Firm capacity (excluding spares) of the supply pumps should equal the maximum cooling water requirements including LGF and RCF
3 NumberIndividual Pump Capacity - Normally one spare pump is provided for up to six operating pumps two spares for up to twelve operating pumps etc The most common arrangements for water supply pumps are
bull Two 100 capacity pumps
bull Three 50 capacity pumps
bull Four 33-13 capacity pumps
bull Five 25 capacity pumps
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Section Page
XXVII 18 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Maximum commercially available pump capacity ranges up to 300000 gpm (20000 Ls) at discharge pressures up to 100psi (700 kPa) Maximum capacity will be less for higher discharge pressures Other factors which may influence pump
selection arebull Selecting capacity to match existing pumps
bull Selecting capacity for future requirements
bull Selecting capacity corresponding to manufacturers available standardVendor information should be obtained on the availability size and cost of the pumps for several configurations Acomparison should be made of the investments and operating costs as well as other advantages and disadvantages toselect a pump configuration
4 Drivers - These pumps are usually motor-driven Because of the remote location of the intake facilities the steamdistribution costs are normally too high to justify the use of a steam turbine driver However use of a secondary selectivesubstation provides a reliable source of power to the pump drivers
5 Pump ∆∆∆∆P - Pump ∆P is based on supplying the maximum cooling water requirement at the battery limit design pressureestablished for the system The ∆P should include the total circulating system pressure drop plus any elevation differencebetween the low water level at the intake and the discharge point of contact with air
6 Pump Instrumentation - The remote location of the intake facilities makes remote control and monitoring of the water supply operation desirable Pump controls located in the control house provide for minimum manning Local controlbackup should be provided The Owners operating and manning philosophy determine the basis for instrumentation andcontrolThe spare pump is controlled to automatically start on low pressure in the supply header Concurrently alarms should beactuated in the control room to indicate the low-pressure situation and spare pump operation
Distribution Network1 Configuration - The piping network is a once-through system with a return line from each consumer or consumer group to
the cooling water effluent line2 Layout - The routing of the piping network should connect all cooling water consumers in the most economical manner
permitted by the plant layout Careful consideration should be given to the turnaround philosophy of the plant Thenetwork layout should preclude the necessity of having above-ground water lines that are in service running through anarea that is being turned around Underground piping or an extra lateral may be required to accomplish this objective
3 Valves and Blinds - Battery limit valves should be provided in the network to isolate individual process units from thesystem Valves should be provided to permit onstream removal and maintenance of spared equipment
4 Basket Strainers - Consideration should be given to the installation of basket strainers in the laterals serving each of theconsumer loops These provide added insurance against entrance of foreign material into the cooling equipment The useof intake screens with 025 in (63 mm) openings may eliminate the need for basket strainers in the laterals
Line Sizing1 Design Basis - The network is sized so that water is delivered to the most distant consumer at a pressure equal to the
required process design (battery limit) pressure2 Flow Rate - Each segment of the distribution network is sized for the simultaneous maximum flow in that segment for any
operating condition The main supply and discharge headers are sized for the simultaneous maximum flow of all segments(not necessarily the sum of the simultaneous maximum flows of the individual segments)
3 Pressure Drop
Line Sizing Basis - The distribution system should be sized according to the guidelines established in Section XIV The use of an ldquoold piperdquo friction factor of C = 90 - 100 (Hazen-Williams factors) in the main lines is recommended to assuregood operation in later years when the system experiences fouling For large diameter lines [greater than 48 in (1200mm)] maximum water velocity should be limited to 10 fts (3 ms) based on civilmechanical considerations (supports andanchoring required to handle potential pressure surges) For salt water systems utilizing cement-lined pipe consider thereduced inner diameter in the friction loss calculations A study may be necessary to determine the economic pump ∆P vsline diameters
4 Equivalent Length Factors - Equivalent length factors are used in pressure drop calculations for sizing the distributionsystem The piping length scaled from the plot plan is multiplied by an equivalent length factor to compensate for pipingelbows expansion loops and other piping variations
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Section Page
COOLING WATER SYSTEMS XXVII 19 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
The equivalent length factors are selected based on the expected variation from a straight piping run Typical equivalentlength factors suitable for planning purposes for different piping applications are as follows
TYPE OF PIPING EQUIVALENT LENGTH FACTORS
Long piping runs [gt 1000 ft (300 m)] 12 to 14
Onsite distribution piping 15 to 20
For design purposes the actual piping layout should be evaluated to ensure that the equivalent length factors areapplicable to the specific situation Particular attention is required for revamp work and saltwater systems utilizing cement-lined pipe
STEP 8 - Prepare a system flow plan similar to the one shown in Figure 1
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS
STEP 1 - Determine cooling water requirements
Tabulate the cooling water requirements and cooling water discharge temperature for each process unit and offsite consumer Apply the proper LGFs to all cooling loads depending on the stage of the project and whether the new units are exactduplicates of existing units Determine the total cooling water flow required including LGFs and the bulk temperature (T 1) of thecombined cooling water flow from all coolers to the cooling towerIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The onsite and offsite designers should investigate methods to increase effluent water temperature to themaximum recommended limit for the particular type of water used This will result in a lower cost cooling tower for a given heatrejection rate One common method of maximizing return water temperature is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 2 - Select cooling tower design parameters
Select the proper design conditions of ambient wet bulb temperature approach temperature range and flow rate for specifyingthe cooling tower Based on these conditions the cooling tower vendor will size the tower and provide the tower characteristicand performance curves which serve as the basis for the performance guarantee Required performance test and evaluationprocedures are described in IP 8-1-1 Economic studies may be required to select the ambient wet bulb temperature andapproach temperature for new locations or for expansion projects with special cooling requirements
STEP 3 - Prepare cooling tower specification sheet
Include the following information in the heat exchange section of the design specificationTower Flow Capacity - This was determined in Step 1Type of Tower - The tower usually specified is an induced draft multicell type suitable for fresh salt or brackish water depending on the water available Additional tower characteristics are usually developed and proposed by the tower vendor (eg crossflow versus counterflow type of tower fill etc) The vendor should be made aware of conditions such as thelikelihood of heavy oil leakage into the cooling water or high fouling potential which may influence the tower type or fill
selectionNumber of Cells - The minimum number of cells required (normally not less than 2) may be specified The maximum flow per cell is a function of the cooling tower design conditions selected in Step 2 Typical values range up to 10000 gpm (600 Ls) for crossflow designs and 8000 gpm (500 Ls) for counterflow designs Normally spare cells are not provided Maintenance iscarried out during cooler periods or load is shed
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XXVII 20 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Ambient Wet Bulb Temperature1 The ambient wet bulb temperature selected for specifying the cooling tower design is generally that temperature which is
equaled or exceeded 5 of the time during the four hottest months (5 of the hourly readings taken during the four hottestmonths will exceed this value) Specifying the 5 temperature should give satisfactory results since the time that the wetbulb temperature is exceeded is for short duration and the system inertia is usually sufficient to avoid problems The tower specification should make it clear that the wet bulb temperature specified is an ambient value as opposed to the enteringvalue which may be higher because of recirculation The tower manufacturer should adjust the ambient value upward totake into account his estimates of recirculation Currently cooling tower test codes provide procedures for measuringperformance in the case of either entering or ambient wet-bulb temperature specifications An increase of 2 degF (11 degC) isgenerally added to ambient wet bulb temperature to estimate entering wet bulb temperature for large cooling towers
2 If plant production will peak during the four summer months when the 5 ambient wet bulb temperature may be exceededconsideration should be given to specifying a higher design ambient wet bulb temperature (one that will be exceeded 212 of the time for example) However the incremental cost of the resulting closer approach and larger cooling tower must be justified by the value of the incremental products made
On-Tower Temperature (T1) - This temperature cannot be accurately determined until the design of all process coolers iscompleted and the volume of cooling water and outlet temperature are known for each (see Step 1) In the planning stage or early design stage of a project this temperature must be estimated and is usually set equal to the maximum design cooler outlet temperature shown under COOLING WATER REQUIREMENTS in the BASIC DESIGN CONSIDERATIONS portion of this sectionApproach Temperature - The approach temperature has the most pronounced effect on cooling tower size and cost For example reducing the approach from 10 to 5 degF (55 to 3 degC) can increase the cooling tower investment by 50 percent and thefan operating cost by 65 percent Thus various approach temperatures should be evaluated to determine the minimum cost of the total system including the process exchanger area Approaches used in the past designs have been from 5 to 12 degF (3 to7degC) Note that it is not customary in the cooling tower industry to guarantee approaches of less than 5 degF to (3 degC) to coolingtower design inlet wet bulb temperature A typical approach for a new grass roots cooling tower is 10 degF (6 degC)Off-Tower Temperature (T2) - This is the sum of the approach temperature and the ambient air wet bulb temperature For existing locations Table 2 in Section IX-B may be used
Range and Flow Rate1 The range for a given heat load is related to the circulating water rate as follows
Q = M C p ∆Tcong (500) (gpm) (range) Eq (1)
where Q = Heat load on the tower BtuhM = Water circulating rate lbhCp = Specific heat capacity of water at average temperature Btulb degF∆T = (T 1 ndash T 2) degF = (water cooling range)gpm = Circulation rate in US gallons per minute
For calculation in Metric units
Q = M C p ∆Tcong (1162 x 10 -3) (Ls) (range) Eq (1M)
where Q = Heat load on the tower kJh
M = Water circulating rate kghCp = Specific heat capacity of water at average temperature kJkg degC∆T = (T 1 ndash T 2) degC = (water cooling range)Ls = Circulation rate in liters per second
2 Generally lower tower investment fan horsepower and pump horsepower result from maximizing the range andminimizing the flow rate However based on fouling considerations of process coolers the maximum allowable hot water temperatures must be kept below recommended limits Also while increasing the range minimizes cooling tower investment and operating cost the cost of exchanger surface requirements is adversely affected by the LMTD and acompromise must be made The range is bounded by the maximum allowable hot water temperature and the Off-Tower temperature (T 2) For established locations refer to Section IX-B for design cold water temperatures to assist in selectingthe approach and range
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COOLING WATER SYSTEMS XXVII 21 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Makeup Water Requirements - Calculate the various losses in the cooling water system to determine makeup requirementsThese losses include
1 Evaporation Loss (E) - Allow 1 of the circulation rate for each 10 degF (55 degC) temperature drop through the tower Notethat the actual evaporation loss will be somewhat less depending on ambient air temperature
( )sL555
TT)gpm(
1000TT
E 2121
minus=
minus= Eq (2)
2 Drift Loss (D) - IP 8-1-1 limits the maximum allowable drift loss to 0005 of the design water circulation rate for freshbrackish or salt water
( )sL1000050
)gpm(1000050
D
=
= Eq (3)
This low drift level contributes little to solids removal from the tower with the spray This allows higher cycles of concentration to be achieved in cooling towers which use clarified softened water for makeup and thus have low requiredblowdown rates Higher drift loss percentages in older or poorly maintained towers will limit the maximum cycles of
concentration achievable3 Miscellaneous Losses (O) - Miscellaneous losses are estimated to be one-seventh of the difference between evaporation
loss and drift loss
minus=
7DE
O Eq (4)
4 Blowdown Rate (B) ndash Guidelines for maximum concentrations of impurities allowed in recirculated fresh cooling water areshown in Table 2 For salt water the blowdown is based on keeping the total dissolved salts below 55000 wppm Obtainthe chemical analysis of the makeup water and divide each individual component concentration into its counterpart inTable 2 The result is the maximum allowable cycles of concentration for that constituent (a worksheet is provided inTable 4 ) The impurity with the lowest factor will be the control and will determine the required blowdown rate
)OD(1Cycles
EB +minus
minus= Eq (5)
The makeup water requirement is the sum of all losses shown above
MakeUp = E + D + O + B Eq (6)
STEP 4 - Specify the controls necessary for proper cooling tower operation
Air Side - Cooling towers are provided with motor-driven manually adjusted pitch multiple-bladed propeller fans Figures9A amp B may be used to estimate the normal operating horsepower requirements Since the fan motor and gear reducer arelocated at the top of the tower [generally 40 to 50 ft (12 to 15 m) above grade] the need for equipment to handle thesecomponents should be discussed with the affiliateControl of the air flow through the tower to maintain reasonably constant cold water temperatures can be accomplished byturning off individual fans in multiple-cell installations adjusting the blade pitch or using variable speed motors To turn off individual fans remote at grade start-stop buttons for the motors are usually specified
Whenever the ambient air temperature is 32 degF (0 degC) or below icing of the tower louvers and structural members in contact withthe incoming air will occur If the heat load on the tower is relatively free of seasonal variations two-speed fans should provideadequate ice control down to approximately 0 degF (-18 degC) ambient temperatures When sustained temperatures of 0 degF (-18 degC)and below occur two-speed fans capable of two-speed reverse operation with appropriate time delays for change of directionand speed should be specified to periodically eliminate ice which will build up on the louvers Other methods of winterizationare available but in general are more expensive and require greater operator attention In very cold climates steam heatingshould be provided to prevent freezing in the cold water basin during shutdown periodsWater Side - The water circulation rate is maintained by operating one or more pumps of sufficient capacity to meet the systemdemand Flow-measuring facilities local temperature indicators and a high-temperature alarm are provided on the coolingtower water supply and return headers Hand-operated flow control valves are required for distributing the water equally to theindividual cells On counterflow towers provisions for measuring the flow to each cell (pitot tubes) are required They are notrequired on crossflow towers since the open distribution basin provides visual aid in balancing flows Shutoff valves should beprovided for servicing the flow control valves without shutting down the entire tower
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XXVII 22 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
STEP 5 - Provide chemical treatment facilities for the fresh water system
Alkalinity (pH) Control (if used)1 In cases where a controlled pH corrosionscale control program is being used the alkalinity (pH) of the recirculated cooling
water should be controlled in accordance with the program requirements Normally acid addition is required to accomplishthis This is due to free carbon dioxide being stripped from the cooling water in the tower to approximately 5 ppm For example if the alkalinity has to be controlled to between 25 and 50 ppm for a pH of 70 to 75 the volume of 98 sulfuricacid (66 degBeacute) required to treat the makeup water is calculated using the following equation
[Note 93 sulfuric acid should be used if ambient temperatures will be less than 35 degF (2 degC)]
GallonsHour 66 degBeacute H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate gpm) (326 x 10 -5)
For calculation in Metric limits
kgh of 100 (66 degBeacute) H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate Ls) (793 x 10 -3)
2 The metering pump should be designed for a maximum feed rate of two to three times the normal feed rate A glass woolfilter is required on the pump suction piping to minimize plugging Normally two - 100 capacity pumps are provided
Positive displacement metering pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumps with flow control areused for higher rates
3 Hydrochloric acid may also be used for alkalinity control The gallons per hour of 20 degBeacute hydrochloric acid required is36 times the amount of 66 degBeacute sulfuric acid In Metric units the weight of 32 (20 degBeacute) hydrochloric acid required is227 times the weight of 100 (66 degBeacute) sulfuric acid
4 Specifications for chemical feeders may be found in EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased)
5 A pH analyzer with high and low alarm is provided to control the amount of acid feed The acid should be added to thecooling tower makeup water (Figure 2 ) The lag time between the point of acid injection and the sample outlet to the pHcell should be less than two minutes for effective control
6 Spills or overflows from the sulfuric acid tank should be contained
Corrosion Inhibitors and Anti-Scalents1 Two chemical feeders are generally provided The following guides in sizing the equipment will allow the use of any of the
proprietary products from the vendors when final selection by the owner is made See Report No EE102E78 Cooling Tower Water Treatment Guidelines EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and Report No EE34E86 Environmentally - Acceptable Cooling Water Treatments for detailed discussion of acceptable commercial chemicals
bull If the water treatment program to be used is known design for the chemical rates specified by the vendor or COOLING WATER SYSTEMS SPECIALIST
bull Otherwise design for 100 wppm of each commercial liquid product in the recirculated cooling water The design feedrate should be based on the design total blowdown flow (both uncontrolled and controlled) since this is where theproduct is lost For 100 wppm concentration the chemical rates will be as follows
GallonsHour Chemical = Blowdown (gpm) x (0006)
LitersHour Chemical = Blowdown (Ls) x (036)
2 The design capacity of the metering pump should be two to three times this calculated number Normally 2 - 100capacity pumps are provided Positive displacement pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumpswith flow control are used for higher rates
3 Specifications for chemical feeders may be found in EMRErsquos Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased) Reference should also be made to the checklists containedin Report No EE92E94 Guidelines for Safety Evaluation of Chemical Injection Facilities
4 A final check should be made with the owner and the COOLING WATER SYSTEMS SPECIALISTS as to the treatmentprogram selected and if there is a need for an additional chemical feeder
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 23 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Biological ControlFacilities should be provided for feeding the biocide into the back of the cooling tower basin If chlorine is used it must bepresent in the cooling water as free residual to be effective However the following amounts of chlorine are first required toreact with the listed impurities when present in the cooling water before a free residual can be established
IMPURITIESAMOUNT OF CHLORINE REQUIRED (wppm)
WITH EACH WPPM OF IMPURITY
Ammonia (NH 3)
Hydrogen Sulfide (H 2S)
Sodium Sulfite (Na 2SO 3)
Ferrous Iron (Fe ++)
10
22
056
064
The design capacity of the chlorine feeder shall be based on achieving a 01 wppm free chlorine residual in the recirculatingwater flow or a minimum dosage of 7 wppm at maximum design makeup water flow rate and conditions (see EMRE Water and
Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service )Chlorine may be fed either as the commercial liquid sodium hypochlorite (124 wt Cl 2) or liquid chlorine (100 Cl 2) Liquidhypochlorite is being used increasingly because of the higher costs associated with precautions equipment and proceduresrequired to minimize the risks associated with handling gaseous chlorine The hypochlorite solution is fed using a meteringpump which should be sized to feed an equivalent of 15 ppm 100 Cl 2 (Refer to EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) ) In case of spills or an overflow from thestorage tank the sodium hypochlorite may be sent to the cooling tower basinGas chlorinators are covered by EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service The use of gas chlorination is being reduced but may still be appropriate in some locations subject tolocal management approval The gas from either commercial cylinders or an evaporator is dissolved in a water slip stream bymeans of an eductor This results in a chlorine solution strength of approximately 01 This solution should be evenlydistributed below the water surface by means of a header lateral pipe arrangement at the rear of the cooling tower basinopposite the cooling water pumpsSome additional guides on the use of gas chlorinators are given below It is also recommended that the safety guidelines
published by the Manufacturing Chemists Association and the Chlorine Institute be followedbull Dilution water for the chlorinator must not be less than 50 degF (10 degC) or ice formation may cause plugging problems
bull Continuous rates of chlorine gas withdrawal should not exceed 400 lb (180 kg)24 hours from each one ton cylinder atroom temperature [70 degF (21 degC)] This rate may be exceeded (up to 50) for periods not exceeding 2 hours Rule of thumb is not to design for more than 4 cylinders operating in parallel When feed rate exceeds 3000 lb (1360 kg) per daythe liquid chlorine should be fed to an evaporator upstream of the chlorinator
bull If the ambient temperature can be expected to be below 50 degF (10 degC) for extended periods a building should be providedfor the chlorinator and cylinders
bull Weighing scales should be provided to determine when shipping containers should be replaced and to verify chlorine feedrates
bull Analyzers to measure the free chlorine residual in the circulated cooling water should be provided
bull Chlorine leak detectors and a catch basin for major spills should be provided
Control SystemProvide automatic controllers to continuously monitor water condition and to add chemicals to the system (complete systemsmay be available through the water treatment program vendor as a lease or purchase)pH Control - This controller adds acid to correct water alkalinity a timer on the acid pump sounds an alarm if the pump doesnot shut off in a predetermined length of time Backup controls are required to prevent the cooling system from beingpermanently damaged in case the pH electrodes become fouledCorrosivity and Inhibitor Control - Systems are available which give a direct readout of corrosion rate in milsyear If thecorrosion rate exceeds a set point the controller shuts off the acid increases the inhibitor addition opens the blowdown valveand sounds an alarm Electrodes measure the completeness of the inhibitor film by the value of the potential differencebetween the electrodes The larger the difference the more incomplete the film and a potential reading exceeding a set levelactivates the inhibitor feed
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XXVII 24 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Free Residual Chlorine Control - Free residual chlorine should be controlled automatically for biocide treatment Severalgood analyzers are currently available These units can be incorporated into an automatic control system
Oil in Water DetectionMonitoring Corrosion and Deposit FormationThe following types of devices for measuring corrosion rates and deposit formation should be provided to allow the plant tomonitor the effectiveness of the cooling water treatment before scheduled turnaroundsCoupons - A corrosion coupon rack should be specified for the hot return water to the tower in accordance with IP 19-6-1 Corrosion Monitor - On-line corrosion analyzer if specified should monitor the hot return water to the tower in accordance withIP 19-6-1
On-Line Test Exchanger Although there are a variety of deposit monitors from the water treatment chemical vendors they do not necessarily representactual heat transfer conditions in process exchangers For this reason a test exchanger should be installed for evaluatingcorrosion and deposit formation Details are covered in Report No EE102E78 Cooling Tower Water Treatment Guidelines
Alternatively a critical process exchanger can be instrumented such that its heat transfer coefficient can be monitored
Instrumentation would include flows and inlet and outlet temperatures of process fluid and cooling waterSTEP 6 - Specify cooling tower location
Select a location for the cooling tower The following factors influence this selection
Location Factors1 The location should be within a reasonable distance of the units served and consistent with future refinery plans Piping
costs to and from battery limits can be a significant portion of the total cooling system cost and can be the location-determining factor particularly for salt water systems While piping costs can be easily determined other costs affectinglocation cannot be estimated readily Therefore judgment is required in selecting the location
2 Safety spacing requirements (see Section XV-G Equipment Spacing )3 Cooling tower contribution to plant boundary noise4 For salt water cooling towers wind direction and distance to other facilities is of major concern because of potential
corrosion problems
5 Factors affecting cooling tower performance ie orientation with respect to wind site congestion etc
Plot RequirementsIn estimating plot requirements (basin area) one should allow 5 to 6 ft of tower length for each 1000 gpm (2 m for each 60 Ls)of circulating capacity This figure is based on a typical counterflow design with a width of 50 ft (12 m) and does not includepump suction pit space Exact requirements can be obtained from cooling tower vendors after the tower capacity has beenestimated
Fogging and DriftTwo distinct problems are associated with the discharge air from a cooling tower (1) fogging which can be a nuisance or ahazard and (2) drift which is generally a nuisance because of increased corrosion of surrounding equipment Fogging fromcooling towers occurs during periods of combined high ambient humidity and low temperature as a result of mixing the warmmoist cooling tower exhaust air with the cooler ambient air The ambient air cannot absorb all moisture as vapor and thus tinyfog droplets are formed by condensation Fogging and resulting icing in cold climates has produced hazardous drivingconditions on highways up to 850 ft (250 m) awayProprietary designs are available to combat potential fogging problems Basically these designs incorporate heating of thedischarge air with hot return water precooling coils (similar to an air cooler) to eliminate any visible plume However thesedesigns are expensive and relatively unproven Therefore proper location based on reliable weather data is the preferredsolutionDrift from a cooling tower represents a more difficult problem since it does not depend on definable climatic conditions andoccurs for all wind directions The effects of drift on nearby equipment and vegetation are particularly severe and generally of concern only for salt water systems because of the high solids content of the drift (concentrated salt water) For salt water cooling systems electric lines substations and instrumentation should be located upwind in the direction of the predominantwind They should be provided with adequate shelter or materials when within 450 ft (140 m) of the cooling tower In additionspecifications for all equipment within 150 ft (45 m) of the tower should include surface treatment suitable for a marineenvironment
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COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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XXVII 28 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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XXVII 30 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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COOLING WATER SYSTEMS XXVII 31 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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XXVII 32 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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COOLING WATER SYSTEMS XXVII 33 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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COOLING WATER SYSTEMS XXVII 35 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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COOLING WATER SYSTEMS XXVII 37 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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COOLING WATER SYSTEMS XXVII 13 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
bull Calcium Sulfate - When heat transfer skin temperatures are below 250 degF (120 degC) and the cooling water bulktemperatures are under 130 degF (54 degC) gypsum scale (CaSO 42H 2O) will form if the product of the calcium and sulfate ionsconcentrations each expressed as wppm CaCO 3 equivalent exceeds 500000 Gypsum scale is prevented bymaintaining bulk water temperature below the indicated limits and controlling calcium and sulfate ions through blowdownor through the use of newer dispersants on the market
bull Tri-Calcium and Zinc Phosphate - Almost all chemical additives for corrosion control contain some type of phosphatesalt These have a tendency to form the orthophosphate ion depending on time pH and temperature Cooling watersusing these corrosion inhibitors may have an orthophosphate concentration in excess of 5 wppm as PO 4
= To preventcalcium phosphate scale Table 2 shows a limit of 1000 wppm of calcium as CaCO 3 equivalent when using a phosphateinhibitor in combination with a chemical dispersant (See Report No EE102E78 Cooling Tower Water Treatment Guidelines for solubility of calcium phosphate) These inhibitors and dispersants generally fall into one of the followingcategories
ndash Sequestrants antiprecipitants and inhibitors such as hydroyethylidene diphosphonate (HEDP) phosphate esters andaminomethylene phosphonate (AMP) function by defyingdistorting the crystal structure of the scale formations toprevent nucleation and growth of the scale
ndash Dispersants based on either anionic or nonionic polymers such as polyacrylamide polyacrylate or polymaleic acidfunction to stabilize colloidal and scale-forming particles in suspension Anionic polymers prevent agglomeration of suspended particles by either charge repulsion or crystal distortion while nonionic polymers reduce the surfacetension of particles
bull Silica Deposits - Quantitative indices for determining limits for silica in combination with other ions are not available due tothe formation of complex polymers Instead the rule of thumb limit of keeping reactive silica concentrations in the coolingwater below 150 wppm as SiO 2 has been successful
Possible alternatives for controlling sludge are
bull Chemical Dispersants - Water treating chemical suppliers now offer many types of dispersants These are mostlypolyelectrolytes Claims for removal of slime silt and mud from cooling tower basins and exchangers are madeHowever satisfactory results using these chemicals as a coagulant aid require very accurate control of the amount fed tothe water and stable operation of the water system Also suspended solids should be kept below 200 wppm for particleslarger than 045 microns Dispersants are used in virtually all fresh water cooling tower systems despite their high cost
bull Sidestream Filtration - The use of a sand pressure filter is the most reliable method for sludge removal Operating andmaintenance costs are minimal for this equipment
bull Higher Blowdown Rates - This may be an economical alternative depending on the cost of makeup water Additionalchemical costs must be included in any evaluation of this method
bull Makeup Clarification - This alternative may require additional raw water treatment facilities to handle seasonal variationsin turbidity andor suspended solids This method is applicable only where suspended solids come from makeup water
Biological Control
Operating conditions in recirculated cooling water are ideal for the growth of biological matter Water temperatures from 70 to120 degF (20 to 50 degC) favorable water pH continuous supply of nutrients including carbon nitrogen and phosphorus inorganicsalts and sunlight all lend themselves to environmental conditions encouraging microbiological growth and plant lifeIf biological growths get out of control and form large sticky agglomerations some of the following operating problems willresult
ndash Fouling of heat transfer surfaces by bacterial slimes resulting in flow restrictions and high process temperatures ndash Reduced cooling tower efficiency resulting from algae fungi and bacterial slime growths in the water distribution basin
and fill area of the cooling tower ndash Excessive plugging of screens and filters ndash Corrosion ndash Wood destruction by fungi ndash Clogging of water distribution nozzles ndash Excessive sludge accumulation in the cooling tower basin
An important part of the water treatment program is to control biological activity by feeding a biocide chemical Oxidizingbiocides such as chlorine sodium hypochlorite chlorine dioxide or bromine-associated compounds are most commonly usedwith good results Plate counts should be maintained below 50000 countsml (bacterial)
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XXVII 14 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Chlorine is the most effective biocide for cooling water because of its low cost extensive period of successful use ease of application and minimum environmental effects if applied under controlled conditions Either a liquid feed (sodium
hypochlorite) or a gas feed (100 chlorine) is added to the cooling tower basin A sufficient dosage is that which obtains a 01to 03 wppm continuous free residual in the hot return water to the cooling tower
Advantages to continuous chlorination over shock treatment based on ExxonMobil experience are ndash Minimizes chlorine degradation of polymers ndash Better reduction in total aerobic bacteria plate count resulting in cleaner heat transfer surfaces ndash Does not cause significant pH depressions ndash Better control results with less chlorine consumption
Details concerning chlorine addition as well as the other alternative biocides are given in Report No EE102E78 Cooling Tower Water Treatment Guidelines and in EMRE Engineering Water and Wastewater Design Guide TMEE 080 DG 11-3-1Gas Chlorinators for Water Treating Service and DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) Gaseous chlorine systems have become more costly in recent years because of the additional precautions equipment andprocedures required to minimize the risk associated with handling this gas Alternatives to gaseous chlorine are available butrequire more attention and have a higher operating cost Sodium hypochlorite (either purchased or generated) and sodiumbromide stabilized solid halide (chlorinebromine) products chlorine dioxide and ozone are all alternatives which may haveapplications Contact the COOLING WATER SYSTEMS SPECIALISTS for guidance in the use of these alternative treatmentschemes
BLOWDOWN
Blowdown control is required to maintain the proper cycles of concentration in the system The most reliable and simplest typeof control for blowdown is specific conductance measurement of the cooling water Electrical signals from the specificconductance measurements of some impurity in the cooling water are used to actuate an automatic blowdown valveImpurities normally used for blowdown control are dissolved solids chloride calcium magnesium and silica
CONTROLS AND MONITORS
Any chemical treatment used to control problems in a cooling water system must be carefully monitored to be completelyeffective
The installation of automatic controls should be considered for recirculated cooling water systems in all plants Designs shouldinclude pH conductivity corrosivity inhibitor and biocide control Automatic controllers are available which continuouslymonitor the condition of the water and automatically make chemical additions to keep the water within compositional limitsTest exchangers and corrosion coupons are also useful in monitoring the effectiveness of the water treatment program
An oil-in-water monitor should be considered to provide early detection of oil leaks into the cooling water This will enablecorrective action to be taken before extensive contamination of the cooling water circuit occurs This is especially important incooling towers with high efficiency cellular fill which can be difficult to clean if significantly fouledManual control systems may be used in locations that have adequate technical personnel to perform routine water analysesand to modify chemical treatment necessitated by sudden variations in water composition process contamination etc
WASTEWATER REUSE POSSIBILITIES
Recirculating cooling water systems normally have the highest plant raw water demand A potential alternative is to use treatedwastewater to supply part or all of the makeup water requirement The treatment required must be determined but will typicallyinclude primary oil removal (eg API Separator) secondary oil and suspended solids removal (eg filtration dissolved or induced air flotation) and soluble carbon removal utilizing biological treatmentThe reuse of treated wastewater as makeup currently exists at only a few locations If this alternative is being considered refer to Report No EE58E85 Reuse of Wastewater as Cooling Tower Makeup and Section XIX-B Water Reuse
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COOLING WATER SYSTEMS XXVII 15 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS
STEP 1 - Select a location for the intake station and pumps
Factors which influence this selection are as followsbull Depth of water and tidal variations
bull Relative location of intake and outfall direction of current
bull Length and size of intake suction piping vs pump discharge piping
STEP 2 - Determine cooling water requirements
Determine the design cold water inlet temperature based on site data covering seasonal water temperature as a function of inlet location and depth (for existing locations refer to Table 2 in Section IX-B )Tabulate the cooling water requirements for each process unit and offsite consumer Apply the proper LGFs to all coolingloads depending on the stage of the project and whether the new units are exact duplicates of existing unitsIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The designer should investigate methods to increase effluent water temperature to the maximumrecommended limit for the particular type of water used One common method is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 3 - Determine the total required capacity for water intake facilities
Process Coolers - The total requirements determined in Step 2 including LGFsFirewater - This is often supplied from the intake bay of a once-through systemFuture Expansions - These should be considered in sizing intake facilitiesTotal Required Capacity - The sum of the above three items
STEP 4 - Provide water intake facilities
Intake Crib - The intake crib with its support structure is located at the extreme end of the water intake line (See Figure 4 )The intake is usually located 200 to 300 ft (60 to 90 m) offshore and at a sufficient depth below the lowest water level to
preclude vortex formation and subsequent pump cavitation during storm conditions Additionally the crib should have 3 to 5 ft(1 to 15 m) clearance above the sea floor to minimize sand and silt pickup A concrete velocity cap should be specified to cover the inlet crib This will minimize the incidence of fish and other foreignmatter entering the system A horizontal inlet flow pattern should be designed with a maximum entrance velocity of 1 fts(03 ms) Experience shows that vertical velocity flow confuses fish and they are drawn into the intake lineThe intake crib support structure protects the crib from adverse effects of storms and tidal motions as well as protecting it fromimpact of floating and submerged objects A self-cleaning screen can be specified to prevent entrance of large objects into thesystem at the cribIntake Line (Figure 5 ) - An intake line is required if the water depth along the shore does not permit water to flow directly intothe intake bay The intake line is usually buried in the sea floor and covered with a minimum required overburden Consultwith the COOLING WATER SYSTEMS SPECIALISTS for required coverage at a specific siteChlorine or sodium hypochlorite is injected at the intake crib to prevent marine growth in the lineNote that an intake crib and line will not be required if water of sufficient depth is available at the supply pump stationIntake Bay (Figure 6 ) - The intake bay is an enclosure which houses the pumps and screens Depending on a particular design a series of screens is used to ensure the supply of clean water Trash racks comprise the first element of the filter system These remove only very coarse material from the water such as bulk seaweed They can be supplied with atraversing raking mechanismLift screens provide secondary cleanup These screens can be lifted out of the intake bay for periodic cleaning Double liftscreens can be used if it is deemed necessary to have one in service at all timesFinal water filtering is usually done by traveling screens Details of the screen dimensions and the mounting are usuallydetermined during detailed engineering Screen net open area is typically approximately 50 of total screen area Thescreens should be sized for a normal water velocity of 12 fts (04 ms) through the open screen area Maximum velocityshould be limited to 24 fts (08 ms) with 50 of the screen area plugged This is the upper velocity limit for efficientscreening
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XXVII 16 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
If high suspended solids greater than 200 mesh (eg sand) are expected to be a continuous problem ejectors may be installedupstream and downstream of the traveling screen to remove accumulated sand and solids from the intake bay in these areas
Traveling screens and ejectors are supplied with wash water from the raw water pumps or from separate wash water pumpsOnoff control using timers should allow optimum scheduling of the ejectors depending on the time of the year
STEP 5 - Specify water intake facilities
Water intake facilities should be designed in accordance with the following guidelines from the Hydraulic Institute Standards The water intake should be designed to provide an evenly distributed flow of water to the pump suction A properly designedintake reduces the possibility of vortex formation and results in better pump performance with less noise Geometry of theintake and the manner in which water is introduced into the intake are important parameters in providing an even distribution of water flow Complete analysis of intake structures is best accomplished by scale model testsThe pump manufacturer will generally render advice on specific problems while the intake design is still preliminary He shouldbe provided with the necessary intake layout drawings showing the physical limitations of the siteFigures 7A and 7B have been developed for single- and multiple-pump arrangements to show suggestions for basic sumpdimensions They are for pumps normally operating in the capacity range of approximately 3000 to 300000 gpm (200 to20000 Ls) Since these values are composite averages from a great many pump types and cover the entire range of specificspeeds they must not be thought of as absolute values but rather as basic guides subject to some possible variations
All the dimensions in Figures 7A and 7B are based on the rated capacity of the pump at the design head Any increase incapacity above these values should be momentary or very limited in time If operation at an increased capacity is to beundertaken for considerable periods of time the maximum capacity should be used for the design value in obtaining sumpdimensionsDimension C is an average value based on an analysis of many pumps Its final value should be specified by the pumpmanufacturerDimension B is a suggested maximum dimension which may be less depending on actual suction bell or bowl diameters in useby the pump manufacturer The edge of the bell should be close to the back wall of the sump When the position of the backwall is determined by the driving equipment or the discharge piping Dimension B may become excessive and a ldquofalserdquo backwall should be installedDimension S is a minimum value for the sump width for a single pump installation This dimension can be increased but if it isto be made smaller the manufacturer should be consulted or a sump model test should be run to determine its adequacy
Motor size may dictate minimum pump spacing be especially aware of this when specifying pumps with unusually high pumpheads eg greater than 80 psig (550 kPa)Dimension H is a minimum value based on the ldquonormal low water levelrdquo at the pump suction bell taking into considerationfriction losses through the inlet screen and approach channel This dimension can be considerably less momentarily or infrequently without excessive damage to the pump It should be remembered however that this does not representldquosubmergencerdquo Submergence is normally quoted as dimension ldquoHrdquo minus ldquoCrdquo This represents the physical height of water level above the bottom of the suction inletThe actual submergence of the pump is something less than this since the impeller eye is some distance above the bottom of the suction bell possibly as much as 3 to 4 ft (1 to 13 m) For the purposes of sump design in connection with this chart it isunderstood that the pump has been selected in accordance with specific speed charts The submergence referred to hereinhas to do only with vortexing and eddy formationsDimensions Y and A are recommended minimum values These dimensions can be as large as desired but should be limitedto the restrictions indicated on the curve If the design does not include a screen Dimension A should be considerably longerThe screen or gate widths should not be substantially less than S and heights should not be less than H If the main stream
velocity is more than 2 fts (06 ms) it may be necessary to construct straightening vanes in the approach channel increaseDimension A conduct a sump model test of the installation or work out some combination of these factorsDimension S becomes the width of an individual pump cell or the center-to-center distance of two pumps if no division walls areusedOn multiple pump installations the recommended dimensions in Figures 7A and 7B also apply as noted above and thefollowing additional determinants should be consideredFigure 8A - Low velocity and straight-line flow to all units simultaneously is the first recommended style of pit Velocities in thepump area should be approximately 1 fts (03 ms) Some pumps with velocities of 2 fts (06 ms) and higher have given goodresults This is particularly true where the design resulted from a model study Not recommended would be an abrupt changein size of inlet pipe to sump or inlet from one side introducing eddying
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Section Page
COOLING WATER SYSTEMS XXVII 17 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Figure 8B - A number of pumps in the same sump will operate best without separating walls unless all pumps are always inoperation at the same time in which case the use of separating walls may be beneficial If walls must be used for structural
purposes and pumps will operate intermittently one should leave flow space behind each wall from the pit floor up to at leastthe minimum water level and the wall should not extend upstream beyond the rim of the suction bell If walls are used increaseDimension ldquoSrdquo by the thickness of the wall for correct centerline spacing Round off the ends of the walls Not recommended isthe placement of a number of pumps around the edge of a sump with or without dividing wallsFigure 8C - Abrupt changes in size from inlet pipe or channel to pump bay are not desirable A relatively small pipe emptyinginto a large pump pit should connect to the pit with a gradually increasing taper section The angle should be as large aspossible preferably not less than 45 degrees With this arrangement pit velocities much less than one foot per second aredesirable Especially not recommended is a small pipe directly connected to a large pit with pumps close to the inlet Flow willhave excessive change of direction to get to most of the pumps Centering pumps in the pit leave large ldquovortex areasrdquo behindthe pumps which will cause operational troubleFigure 8D - If the pit velocity can be kept low enough [below 1 fts (03 ms)] an abrupt change from inlet pipe to pit can beaccommodated if the length equals or exceeds the values shown It is assumed that as ratio WP increases the inlet velocityat ldquoPrdquo will increase up to an allowed maximum of 8 fts (24 ms) at WP = 10 Pumps ldquoin linerdquo are not recommended unless theratio of pit to pump size is quite large and pumps are separated longitudinally by a generous margin A pit can generally be
constructed at much less cost by use of a recommended designFigure 8E - It is sometimes desirable to install pumps in tunnels or pipelines A drop pipe or false well to house the pump witha vaned inlet well facing upstream will be satisfactory in flows up to 8 fts (24 ms) Without the inlet well the pump section bellshould be positioned at least two pipe diameters vertically above the top of the tunnel not hung into the tunnel flow especiallywith tunnel velocities 2 fts (06 ms) or more There should be no signs of air along the top of the tunnel It may be necessaryto lower the scoop or to insist on a minimum water level in the vertical wellNote The foregoing statements apply to sumps for clear liquid
STEP 6 - Provide chemical treatment
Provide chemical treatment facilities for the once-through cooling water system Normally the only chemical treatment requiredin a once-through cooling system is for biological control Facilities for adding chlorine to once-through cooling water are thesame as those described for the recirculated cooling water system The design capacity of the chlorinator should be based onachieving a chlorine residual of 1 wppm in the effluent The chlorine required by the organics and other reducing agents in thewater must be satisfied before any residual chlorine will appear in the effluent The design capacity of the chlorinator should be
based on the total water chlorine demand plus 1 wppm in the effluent In the absence of water data the chlorination systemdesign capacity should be based on 10 wppm minimum instantaneous dosage for maximum cooling water flow For very largeonce-through cooling water systems it will be impractical to design for shock rates at this level In this case a continuousdosage of 1 - 2 wppm should be used (Refer to Step 5 of the DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS )
STEP 7 - Specify pumps and distribution system
Specify other major facilities in the once-through cooling water system The recommended materials for pumps valves andpiping are shown in the EMRErsquos Refinery Construction Materials Manual
Pumps1 Type - These pumps are usually vertical centrifugal pumps However horizontal centrifugal pumps have been used An
economic study andor consideration of existing facilities will determine which type should be specified In the event thathorizontal centrifugal pumps are specified facilities for pump priming will likely be required For large pumps in salt water service there is a very limited selection of pumps because of the special materials required Consult with the DISCIPLINESPECIALISTS early in the design effort to determine what specific pumps are currently available
2 Total Required Capacity - Firm capacity (excluding spares) of the supply pumps should equal the maximum cooling water requirements including LGF and RCF
3 NumberIndividual Pump Capacity - Normally one spare pump is provided for up to six operating pumps two spares for up to twelve operating pumps etc The most common arrangements for water supply pumps are
bull Two 100 capacity pumps
bull Three 50 capacity pumps
bull Four 33-13 capacity pumps
bull Five 25 capacity pumps
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Section Page
XXVII 18 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Maximum commercially available pump capacity ranges up to 300000 gpm (20000 Ls) at discharge pressures up to 100psi (700 kPa) Maximum capacity will be less for higher discharge pressures Other factors which may influence pump
selection arebull Selecting capacity to match existing pumps
bull Selecting capacity for future requirements
bull Selecting capacity corresponding to manufacturers available standardVendor information should be obtained on the availability size and cost of the pumps for several configurations Acomparison should be made of the investments and operating costs as well as other advantages and disadvantages toselect a pump configuration
4 Drivers - These pumps are usually motor-driven Because of the remote location of the intake facilities the steamdistribution costs are normally too high to justify the use of a steam turbine driver However use of a secondary selectivesubstation provides a reliable source of power to the pump drivers
5 Pump ∆∆∆∆P - Pump ∆P is based on supplying the maximum cooling water requirement at the battery limit design pressureestablished for the system The ∆P should include the total circulating system pressure drop plus any elevation differencebetween the low water level at the intake and the discharge point of contact with air
6 Pump Instrumentation - The remote location of the intake facilities makes remote control and monitoring of the water supply operation desirable Pump controls located in the control house provide for minimum manning Local controlbackup should be provided The Owners operating and manning philosophy determine the basis for instrumentation andcontrolThe spare pump is controlled to automatically start on low pressure in the supply header Concurrently alarms should beactuated in the control room to indicate the low-pressure situation and spare pump operation
Distribution Network1 Configuration - The piping network is a once-through system with a return line from each consumer or consumer group to
the cooling water effluent line2 Layout - The routing of the piping network should connect all cooling water consumers in the most economical manner
permitted by the plant layout Careful consideration should be given to the turnaround philosophy of the plant Thenetwork layout should preclude the necessity of having above-ground water lines that are in service running through anarea that is being turned around Underground piping or an extra lateral may be required to accomplish this objective
3 Valves and Blinds - Battery limit valves should be provided in the network to isolate individual process units from thesystem Valves should be provided to permit onstream removal and maintenance of spared equipment
4 Basket Strainers - Consideration should be given to the installation of basket strainers in the laterals serving each of theconsumer loops These provide added insurance against entrance of foreign material into the cooling equipment The useof intake screens with 025 in (63 mm) openings may eliminate the need for basket strainers in the laterals
Line Sizing1 Design Basis - The network is sized so that water is delivered to the most distant consumer at a pressure equal to the
required process design (battery limit) pressure2 Flow Rate - Each segment of the distribution network is sized for the simultaneous maximum flow in that segment for any
operating condition The main supply and discharge headers are sized for the simultaneous maximum flow of all segments(not necessarily the sum of the simultaneous maximum flows of the individual segments)
3 Pressure Drop
Line Sizing Basis - The distribution system should be sized according to the guidelines established in Section XIV The use of an ldquoold piperdquo friction factor of C = 90 - 100 (Hazen-Williams factors) in the main lines is recommended to assuregood operation in later years when the system experiences fouling For large diameter lines [greater than 48 in (1200mm)] maximum water velocity should be limited to 10 fts (3 ms) based on civilmechanical considerations (supports andanchoring required to handle potential pressure surges) For salt water systems utilizing cement-lined pipe consider thereduced inner diameter in the friction loss calculations A study may be necessary to determine the economic pump ∆P vsline diameters
4 Equivalent Length Factors - Equivalent length factors are used in pressure drop calculations for sizing the distributionsystem The piping length scaled from the plot plan is multiplied by an equivalent length factor to compensate for pipingelbows expansion loops and other piping variations
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Section Page
COOLING WATER SYSTEMS XXVII 19 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
The equivalent length factors are selected based on the expected variation from a straight piping run Typical equivalentlength factors suitable for planning purposes for different piping applications are as follows
TYPE OF PIPING EQUIVALENT LENGTH FACTORS
Long piping runs [gt 1000 ft (300 m)] 12 to 14
Onsite distribution piping 15 to 20
For design purposes the actual piping layout should be evaluated to ensure that the equivalent length factors areapplicable to the specific situation Particular attention is required for revamp work and saltwater systems utilizing cement-lined pipe
STEP 8 - Prepare a system flow plan similar to the one shown in Figure 1
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS
STEP 1 - Determine cooling water requirements
Tabulate the cooling water requirements and cooling water discharge temperature for each process unit and offsite consumer Apply the proper LGFs to all cooling loads depending on the stage of the project and whether the new units are exactduplicates of existing units Determine the total cooling water flow required including LGFs and the bulk temperature (T 1) of thecombined cooling water flow from all coolers to the cooling towerIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The onsite and offsite designers should investigate methods to increase effluent water temperature to themaximum recommended limit for the particular type of water used This will result in a lower cost cooling tower for a given heatrejection rate One common method of maximizing return water temperature is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 2 - Select cooling tower design parameters
Select the proper design conditions of ambient wet bulb temperature approach temperature range and flow rate for specifyingthe cooling tower Based on these conditions the cooling tower vendor will size the tower and provide the tower characteristicand performance curves which serve as the basis for the performance guarantee Required performance test and evaluationprocedures are described in IP 8-1-1 Economic studies may be required to select the ambient wet bulb temperature andapproach temperature for new locations or for expansion projects with special cooling requirements
STEP 3 - Prepare cooling tower specification sheet
Include the following information in the heat exchange section of the design specificationTower Flow Capacity - This was determined in Step 1Type of Tower - The tower usually specified is an induced draft multicell type suitable for fresh salt or brackish water depending on the water available Additional tower characteristics are usually developed and proposed by the tower vendor (eg crossflow versus counterflow type of tower fill etc) The vendor should be made aware of conditions such as thelikelihood of heavy oil leakage into the cooling water or high fouling potential which may influence the tower type or fill
selectionNumber of Cells - The minimum number of cells required (normally not less than 2) may be specified The maximum flow per cell is a function of the cooling tower design conditions selected in Step 2 Typical values range up to 10000 gpm (600 Ls) for crossflow designs and 8000 gpm (500 Ls) for counterflow designs Normally spare cells are not provided Maintenance iscarried out during cooler periods or load is shed
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XXVII 20 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Ambient Wet Bulb Temperature1 The ambient wet bulb temperature selected for specifying the cooling tower design is generally that temperature which is
equaled or exceeded 5 of the time during the four hottest months (5 of the hourly readings taken during the four hottestmonths will exceed this value) Specifying the 5 temperature should give satisfactory results since the time that the wetbulb temperature is exceeded is for short duration and the system inertia is usually sufficient to avoid problems The tower specification should make it clear that the wet bulb temperature specified is an ambient value as opposed to the enteringvalue which may be higher because of recirculation The tower manufacturer should adjust the ambient value upward totake into account his estimates of recirculation Currently cooling tower test codes provide procedures for measuringperformance in the case of either entering or ambient wet-bulb temperature specifications An increase of 2 degF (11 degC) isgenerally added to ambient wet bulb temperature to estimate entering wet bulb temperature for large cooling towers
2 If plant production will peak during the four summer months when the 5 ambient wet bulb temperature may be exceededconsideration should be given to specifying a higher design ambient wet bulb temperature (one that will be exceeded 212 of the time for example) However the incremental cost of the resulting closer approach and larger cooling tower must be justified by the value of the incremental products made
On-Tower Temperature (T1) - This temperature cannot be accurately determined until the design of all process coolers iscompleted and the volume of cooling water and outlet temperature are known for each (see Step 1) In the planning stage or early design stage of a project this temperature must be estimated and is usually set equal to the maximum design cooler outlet temperature shown under COOLING WATER REQUIREMENTS in the BASIC DESIGN CONSIDERATIONS portion of this sectionApproach Temperature - The approach temperature has the most pronounced effect on cooling tower size and cost For example reducing the approach from 10 to 5 degF (55 to 3 degC) can increase the cooling tower investment by 50 percent and thefan operating cost by 65 percent Thus various approach temperatures should be evaluated to determine the minimum cost of the total system including the process exchanger area Approaches used in the past designs have been from 5 to 12 degF (3 to7degC) Note that it is not customary in the cooling tower industry to guarantee approaches of less than 5 degF to (3 degC) to coolingtower design inlet wet bulb temperature A typical approach for a new grass roots cooling tower is 10 degF (6 degC)Off-Tower Temperature (T2) - This is the sum of the approach temperature and the ambient air wet bulb temperature For existing locations Table 2 in Section IX-B may be used
Range and Flow Rate1 The range for a given heat load is related to the circulating water rate as follows
Q = M C p ∆Tcong (500) (gpm) (range) Eq (1)
where Q = Heat load on the tower BtuhM = Water circulating rate lbhCp = Specific heat capacity of water at average temperature Btulb degF∆T = (T 1 ndash T 2) degF = (water cooling range)gpm = Circulation rate in US gallons per minute
For calculation in Metric units
Q = M C p ∆Tcong (1162 x 10 -3) (Ls) (range) Eq (1M)
where Q = Heat load on the tower kJh
M = Water circulating rate kghCp = Specific heat capacity of water at average temperature kJkg degC∆T = (T 1 ndash T 2) degC = (water cooling range)Ls = Circulation rate in liters per second
2 Generally lower tower investment fan horsepower and pump horsepower result from maximizing the range andminimizing the flow rate However based on fouling considerations of process coolers the maximum allowable hot water temperatures must be kept below recommended limits Also while increasing the range minimizes cooling tower investment and operating cost the cost of exchanger surface requirements is adversely affected by the LMTD and acompromise must be made The range is bounded by the maximum allowable hot water temperature and the Off-Tower temperature (T 2) For established locations refer to Section IX-B for design cold water temperatures to assist in selectingthe approach and range
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Section Page
COOLING WATER SYSTEMS XXVII 21 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Makeup Water Requirements - Calculate the various losses in the cooling water system to determine makeup requirementsThese losses include
1 Evaporation Loss (E) - Allow 1 of the circulation rate for each 10 degF (55 degC) temperature drop through the tower Notethat the actual evaporation loss will be somewhat less depending on ambient air temperature
( )sL555
TT)gpm(
1000TT
E 2121
minus=
minus= Eq (2)
2 Drift Loss (D) - IP 8-1-1 limits the maximum allowable drift loss to 0005 of the design water circulation rate for freshbrackish or salt water
( )sL1000050
)gpm(1000050
D
=
= Eq (3)
This low drift level contributes little to solids removal from the tower with the spray This allows higher cycles of concentration to be achieved in cooling towers which use clarified softened water for makeup and thus have low requiredblowdown rates Higher drift loss percentages in older or poorly maintained towers will limit the maximum cycles of
concentration achievable3 Miscellaneous Losses (O) - Miscellaneous losses are estimated to be one-seventh of the difference between evaporation
loss and drift loss
minus=
7DE
O Eq (4)
4 Blowdown Rate (B) ndash Guidelines for maximum concentrations of impurities allowed in recirculated fresh cooling water areshown in Table 2 For salt water the blowdown is based on keeping the total dissolved salts below 55000 wppm Obtainthe chemical analysis of the makeup water and divide each individual component concentration into its counterpart inTable 2 The result is the maximum allowable cycles of concentration for that constituent (a worksheet is provided inTable 4 ) The impurity with the lowest factor will be the control and will determine the required blowdown rate
)OD(1Cycles
EB +minus
minus= Eq (5)
The makeup water requirement is the sum of all losses shown above
MakeUp = E + D + O + B Eq (6)
STEP 4 - Specify the controls necessary for proper cooling tower operation
Air Side - Cooling towers are provided with motor-driven manually adjusted pitch multiple-bladed propeller fans Figures9A amp B may be used to estimate the normal operating horsepower requirements Since the fan motor and gear reducer arelocated at the top of the tower [generally 40 to 50 ft (12 to 15 m) above grade] the need for equipment to handle thesecomponents should be discussed with the affiliateControl of the air flow through the tower to maintain reasonably constant cold water temperatures can be accomplished byturning off individual fans in multiple-cell installations adjusting the blade pitch or using variable speed motors To turn off individual fans remote at grade start-stop buttons for the motors are usually specified
Whenever the ambient air temperature is 32 degF (0 degC) or below icing of the tower louvers and structural members in contact withthe incoming air will occur If the heat load on the tower is relatively free of seasonal variations two-speed fans should provideadequate ice control down to approximately 0 degF (-18 degC) ambient temperatures When sustained temperatures of 0 degF (-18 degC)and below occur two-speed fans capable of two-speed reverse operation with appropriate time delays for change of directionand speed should be specified to periodically eliminate ice which will build up on the louvers Other methods of winterizationare available but in general are more expensive and require greater operator attention In very cold climates steam heatingshould be provided to prevent freezing in the cold water basin during shutdown periodsWater Side - The water circulation rate is maintained by operating one or more pumps of sufficient capacity to meet the systemdemand Flow-measuring facilities local temperature indicators and a high-temperature alarm are provided on the coolingtower water supply and return headers Hand-operated flow control valves are required for distributing the water equally to theindividual cells On counterflow towers provisions for measuring the flow to each cell (pitot tubes) are required They are notrequired on crossflow towers since the open distribution basin provides visual aid in balancing flows Shutoff valves should beprovided for servicing the flow control valves without shutting down the entire tower
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XXVII 22 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
STEP 5 - Provide chemical treatment facilities for the fresh water system
Alkalinity (pH) Control (if used)1 In cases where a controlled pH corrosionscale control program is being used the alkalinity (pH) of the recirculated cooling
water should be controlled in accordance with the program requirements Normally acid addition is required to accomplishthis This is due to free carbon dioxide being stripped from the cooling water in the tower to approximately 5 ppm For example if the alkalinity has to be controlled to between 25 and 50 ppm for a pH of 70 to 75 the volume of 98 sulfuricacid (66 degBeacute) required to treat the makeup water is calculated using the following equation
[Note 93 sulfuric acid should be used if ambient temperatures will be less than 35 degF (2 degC)]
GallonsHour 66 degBeacute H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate gpm) (326 x 10 -5)
For calculation in Metric limits
kgh of 100 (66 degBeacute) H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate Ls) (793 x 10 -3)
2 The metering pump should be designed for a maximum feed rate of two to three times the normal feed rate A glass woolfilter is required on the pump suction piping to minimize plugging Normally two - 100 capacity pumps are provided
Positive displacement metering pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumps with flow control areused for higher rates
3 Hydrochloric acid may also be used for alkalinity control The gallons per hour of 20 degBeacute hydrochloric acid required is36 times the amount of 66 degBeacute sulfuric acid In Metric units the weight of 32 (20 degBeacute) hydrochloric acid required is227 times the weight of 100 (66 degBeacute) sulfuric acid
4 Specifications for chemical feeders may be found in EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased)
5 A pH analyzer with high and low alarm is provided to control the amount of acid feed The acid should be added to thecooling tower makeup water (Figure 2 ) The lag time between the point of acid injection and the sample outlet to the pHcell should be less than two minutes for effective control
6 Spills or overflows from the sulfuric acid tank should be contained
Corrosion Inhibitors and Anti-Scalents1 Two chemical feeders are generally provided The following guides in sizing the equipment will allow the use of any of the
proprietary products from the vendors when final selection by the owner is made See Report No EE102E78 Cooling Tower Water Treatment Guidelines EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and Report No EE34E86 Environmentally - Acceptable Cooling Water Treatments for detailed discussion of acceptable commercial chemicals
bull If the water treatment program to be used is known design for the chemical rates specified by the vendor or COOLING WATER SYSTEMS SPECIALIST
bull Otherwise design for 100 wppm of each commercial liquid product in the recirculated cooling water The design feedrate should be based on the design total blowdown flow (both uncontrolled and controlled) since this is where theproduct is lost For 100 wppm concentration the chemical rates will be as follows
GallonsHour Chemical = Blowdown (gpm) x (0006)
LitersHour Chemical = Blowdown (Ls) x (036)
2 The design capacity of the metering pump should be two to three times this calculated number Normally 2 - 100capacity pumps are provided Positive displacement pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumpswith flow control are used for higher rates
3 Specifications for chemical feeders may be found in EMRErsquos Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased) Reference should also be made to the checklists containedin Report No EE92E94 Guidelines for Safety Evaluation of Chemical Injection Facilities
4 A final check should be made with the owner and the COOLING WATER SYSTEMS SPECIALISTS as to the treatmentprogram selected and if there is a need for an additional chemical feeder
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 23 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Biological ControlFacilities should be provided for feeding the biocide into the back of the cooling tower basin If chlorine is used it must bepresent in the cooling water as free residual to be effective However the following amounts of chlorine are first required toreact with the listed impurities when present in the cooling water before a free residual can be established
IMPURITIESAMOUNT OF CHLORINE REQUIRED (wppm)
WITH EACH WPPM OF IMPURITY
Ammonia (NH 3)
Hydrogen Sulfide (H 2S)
Sodium Sulfite (Na 2SO 3)
Ferrous Iron (Fe ++)
10
22
056
064
The design capacity of the chlorine feeder shall be based on achieving a 01 wppm free chlorine residual in the recirculatingwater flow or a minimum dosage of 7 wppm at maximum design makeup water flow rate and conditions (see EMRE Water and
Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service )Chlorine may be fed either as the commercial liquid sodium hypochlorite (124 wt Cl 2) or liquid chlorine (100 Cl 2) Liquidhypochlorite is being used increasingly because of the higher costs associated with precautions equipment and proceduresrequired to minimize the risks associated with handling gaseous chlorine The hypochlorite solution is fed using a meteringpump which should be sized to feed an equivalent of 15 ppm 100 Cl 2 (Refer to EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) ) In case of spills or an overflow from thestorage tank the sodium hypochlorite may be sent to the cooling tower basinGas chlorinators are covered by EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service The use of gas chlorination is being reduced but may still be appropriate in some locations subject tolocal management approval The gas from either commercial cylinders or an evaporator is dissolved in a water slip stream bymeans of an eductor This results in a chlorine solution strength of approximately 01 This solution should be evenlydistributed below the water surface by means of a header lateral pipe arrangement at the rear of the cooling tower basinopposite the cooling water pumpsSome additional guides on the use of gas chlorinators are given below It is also recommended that the safety guidelines
published by the Manufacturing Chemists Association and the Chlorine Institute be followedbull Dilution water for the chlorinator must not be less than 50 degF (10 degC) or ice formation may cause plugging problems
bull Continuous rates of chlorine gas withdrawal should not exceed 400 lb (180 kg)24 hours from each one ton cylinder atroom temperature [70 degF (21 degC)] This rate may be exceeded (up to 50) for periods not exceeding 2 hours Rule of thumb is not to design for more than 4 cylinders operating in parallel When feed rate exceeds 3000 lb (1360 kg) per daythe liquid chlorine should be fed to an evaporator upstream of the chlorinator
bull If the ambient temperature can be expected to be below 50 degF (10 degC) for extended periods a building should be providedfor the chlorinator and cylinders
bull Weighing scales should be provided to determine when shipping containers should be replaced and to verify chlorine feedrates
bull Analyzers to measure the free chlorine residual in the circulated cooling water should be provided
bull Chlorine leak detectors and a catch basin for major spills should be provided
Control SystemProvide automatic controllers to continuously monitor water condition and to add chemicals to the system (complete systemsmay be available through the water treatment program vendor as a lease or purchase)pH Control - This controller adds acid to correct water alkalinity a timer on the acid pump sounds an alarm if the pump doesnot shut off in a predetermined length of time Backup controls are required to prevent the cooling system from beingpermanently damaged in case the pH electrodes become fouledCorrosivity and Inhibitor Control - Systems are available which give a direct readout of corrosion rate in milsyear If thecorrosion rate exceeds a set point the controller shuts off the acid increases the inhibitor addition opens the blowdown valveand sounds an alarm Electrodes measure the completeness of the inhibitor film by the value of the potential differencebetween the electrodes The larger the difference the more incomplete the film and a potential reading exceeding a set levelactivates the inhibitor feed
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Section Page
XXVII 24 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Free Residual Chlorine Control - Free residual chlorine should be controlled automatically for biocide treatment Severalgood analyzers are currently available These units can be incorporated into an automatic control system
Oil in Water DetectionMonitoring Corrosion and Deposit FormationThe following types of devices for measuring corrosion rates and deposit formation should be provided to allow the plant tomonitor the effectiveness of the cooling water treatment before scheduled turnaroundsCoupons - A corrosion coupon rack should be specified for the hot return water to the tower in accordance with IP 19-6-1 Corrosion Monitor - On-line corrosion analyzer if specified should monitor the hot return water to the tower in accordance withIP 19-6-1
On-Line Test Exchanger Although there are a variety of deposit monitors from the water treatment chemical vendors they do not necessarily representactual heat transfer conditions in process exchangers For this reason a test exchanger should be installed for evaluatingcorrosion and deposit formation Details are covered in Report No EE102E78 Cooling Tower Water Treatment Guidelines
Alternatively a critical process exchanger can be instrumented such that its heat transfer coefficient can be monitored
Instrumentation would include flows and inlet and outlet temperatures of process fluid and cooling waterSTEP 6 - Specify cooling tower location
Select a location for the cooling tower The following factors influence this selection
Location Factors1 The location should be within a reasonable distance of the units served and consistent with future refinery plans Piping
costs to and from battery limits can be a significant portion of the total cooling system cost and can be the location-determining factor particularly for salt water systems While piping costs can be easily determined other costs affectinglocation cannot be estimated readily Therefore judgment is required in selecting the location
2 Safety spacing requirements (see Section XV-G Equipment Spacing )3 Cooling tower contribution to plant boundary noise4 For salt water cooling towers wind direction and distance to other facilities is of major concern because of potential
corrosion problems
5 Factors affecting cooling tower performance ie orientation with respect to wind site congestion etc
Plot RequirementsIn estimating plot requirements (basin area) one should allow 5 to 6 ft of tower length for each 1000 gpm (2 m for each 60 Ls)of circulating capacity This figure is based on a typical counterflow design with a width of 50 ft (12 m) and does not includepump suction pit space Exact requirements can be obtained from cooling tower vendors after the tower capacity has beenestimated
Fogging and DriftTwo distinct problems are associated with the discharge air from a cooling tower (1) fogging which can be a nuisance or ahazard and (2) drift which is generally a nuisance because of increased corrosion of surrounding equipment Fogging fromcooling towers occurs during periods of combined high ambient humidity and low temperature as a result of mixing the warmmoist cooling tower exhaust air with the cooler ambient air The ambient air cannot absorb all moisture as vapor and thus tinyfog droplets are formed by condensation Fogging and resulting icing in cold climates has produced hazardous drivingconditions on highways up to 850 ft (250 m) awayProprietary designs are available to combat potential fogging problems Basically these designs incorporate heating of thedischarge air with hot return water precooling coils (similar to an air cooler) to eliminate any visible plume However thesedesigns are expensive and relatively unproven Therefore proper location based on reliable weather data is the preferredsolutionDrift from a cooling tower represents a more difficult problem since it does not depend on definable climatic conditions andoccurs for all wind directions The effects of drift on nearby equipment and vegetation are particularly severe and generally of concern only for salt water systems because of the high solids content of the drift (concentrated salt water) For salt water cooling systems electric lines substations and instrumentation should be located upwind in the direction of the predominantwind They should be provided with adequate shelter or materials when within 450 ft (140 m) of the cooling tower In additionspecifications for all equipment within 150 ft (45 m) of the tower should include surface treatment suitable for a marineenvironment
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Section Page
COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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XXVII 28 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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XXVII 30 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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COOLING WATER SYSTEMS XXVII 31 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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XXVII 32 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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COOLING WATER SYSTEMS XXVII 33 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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COOLING WATER SYSTEMS XXVII 35 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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Section Page
XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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COOLING WATER SYSTEMS XXVII 37 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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Section Page
XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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Section Page
COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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Section Page
XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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Section Page
COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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Section Page
COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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Section Page
XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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Section Page
COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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Section Page
XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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Section Page
COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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Section Page
XXVII 14 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
BASIC DESIGN CONSIDERATIONS (Cont)
Chlorine is the most effective biocide for cooling water because of its low cost extensive period of successful use ease of application and minimum environmental effects if applied under controlled conditions Either a liquid feed (sodium
hypochlorite) or a gas feed (100 chlorine) is added to the cooling tower basin A sufficient dosage is that which obtains a 01to 03 wppm continuous free residual in the hot return water to the cooling tower
Advantages to continuous chlorination over shock treatment based on ExxonMobil experience are ndash Minimizes chlorine degradation of polymers ndash Better reduction in total aerobic bacteria plate count resulting in cleaner heat transfer surfaces ndash Does not cause significant pH depressions ndash Better control results with less chlorine consumption
Details concerning chlorine addition as well as the other alternative biocides are given in Report No EE102E78 Cooling Tower Water Treatment Guidelines and in EMRE Engineering Water and Wastewater Design Guide TMEE 080 DG 11-3-1Gas Chlorinators for Water Treating Service and DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) Gaseous chlorine systems have become more costly in recent years because of the additional precautions equipment andprocedures required to minimize the risk associated with handling this gas Alternatives to gaseous chlorine are available butrequire more attention and have a higher operating cost Sodium hypochlorite (either purchased or generated) and sodiumbromide stabilized solid halide (chlorinebromine) products chlorine dioxide and ozone are all alternatives which may haveapplications Contact the COOLING WATER SYSTEMS SPECIALISTS for guidance in the use of these alternative treatmentschemes
BLOWDOWN
Blowdown control is required to maintain the proper cycles of concentration in the system The most reliable and simplest typeof control for blowdown is specific conductance measurement of the cooling water Electrical signals from the specificconductance measurements of some impurity in the cooling water are used to actuate an automatic blowdown valveImpurities normally used for blowdown control are dissolved solids chloride calcium magnesium and silica
CONTROLS AND MONITORS
Any chemical treatment used to control problems in a cooling water system must be carefully monitored to be completelyeffective
The installation of automatic controls should be considered for recirculated cooling water systems in all plants Designs shouldinclude pH conductivity corrosivity inhibitor and biocide control Automatic controllers are available which continuouslymonitor the condition of the water and automatically make chemical additions to keep the water within compositional limitsTest exchangers and corrosion coupons are also useful in monitoring the effectiveness of the water treatment program
An oil-in-water monitor should be considered to provide early detection of oil leaks into the cooling water This will enablecorrective action to be taken before extensive contamination of the cooling water circuit occurs This is especially important incooling towers with high efficiency cellular fill which can be difficult to clean if significantly fouledManual control systems may be used in locations that have adequate technical personnel to perform routine water analysesand to modify chemical treatment necessitated by sudden variations in water composition process contamination etc
WASTEWATER REUSE POSSIBILITIES
Recirculating cooling water systems normally have the highest plant raw water demand A potential alternative is to use treatedwastewater to supply part or all of the makeup water requirement The treatment required must be determined but will typicallyinclude primary oil removal (eg API Separator) secondary oil and suspended solids removal (eg filtration dissolved or induced air flotation) and soluble carbon removal utilizing biological treatmentThe reuse of treated wastewater as makeup currently exists at only a few locations If this alternative is being considered refer to Report No EE58E85 Reuse of Wastewater as Cooling Tower Makeup and Section XIX-B Water Reuse
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Section Page
COOLING WATER SYSTEMS XXVII 15 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS
STEP 1 - Select a location for the intake station and pumps
Factors which influence this selection are as followsbull Depth of water and tidal variations
bull Relative location of intake and outfall direction of current
bull Length and size of intake suction piping vs pump discharge piping
STEP 2 - Determine cooling water requirements
Determine the design cold water inlet temperature based on site data covering seasonal water temperature as a function of inlet location and depth (for existing locations refer to Table 2 in Section IX-B )Tabulate the cooling water requirements for each process unit and offsite consumer Apply the proper LGFs to all coolingloads depending on the stage of the project and whether the new units are exact duplicates of existing unitsIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The designer should investigate methods to increase effluent water temperature to the maximumrecommended limit for the particular type of water used One common method is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 3 - Determine the total required capacity for water intake facilities
Process Coolers - The total requirements determined in Step 2 including LGFsFirewater - This is often supplied from the intake bay of a once-through systemFuture Expansions - These should be considered in sizing intake facilitiesTotal Required Capacity - The sum of the above three items
STEP 4 - Provide water intake facilities
Intake Crib - The intake crib with its support structure is located at the extreme end of the water intake line (See Figure 4 )The intake is usually located 200 to 300 ft (60 to 90 m) offshore and at a sufficient depth below the lowest water level to
preclude vortex formation and subsequent pump cavitation during storm conditions Additionally the crib should have 3 to 5 ft(1 to 15 m) clearance above the sea floor to minimize sand and silt pickup A concrete velocity cap should be specified to cover the inlet crib This will minimize the incidence of fish and other foreignmatter entering the system A horizontal inlet flow pattern should be designed with a maximum entrance velocity of 1 fts(03 ms) Experience shows that vertical velocity flow confuses fish and they are drawn into the intake lineThe intake crib support structure protects the crib from adverse effects of storms and tidal motions as well as protecting it fromimpact of floating and submerged objects A self-cleaning screen can be specified to prevent entrance of large objects into thesystem at the cribIntake Line (Figure 5 ) - An intake line is required if the water depth along the shore does not permit water to flow directly intothe intake bay The intake line is usually buried in the sea floor and covered with a minimum required overburden Consultwith the COOLING WATER SYSTEMS SPECIALISTS for required coverage at a specific siteChlorine or sodium hypochlorite is injected at the intake crib to prevent marine growth in the lineNote that an intake crib and line will not be required if water of sufficient depth is available at the supply pump stationIntake Bay (Figure 6 ) - The intake bay is an enclosure which houses the pumps and screens Depending on a particular design a series of screens is used to ensure the supply of clean water Trash racks comprise the first element of the filter system These remove only very coarse material from the water such as bulk seaweed They can be supplied with atraversing raking mechanismLift screens provide secondary cleanup These screens can be lifted out of the intake bay for periodic cleaning Double liftscreens can be used if it is deemed necessary to have one in service at all timesFinal water filtering is usually done by traveling screens Details of the screen dimensions and the mounting are usuallydetermined during detailed engineering Screen net open area is typically approximately 50 of total screen area Thescreens should be sized for a normal water velocity of 12 fts (04 ms) through the open screen area Maximum velocityshould be limited to 24 fts (08 ms) with 50 of the screen area plugged This is the upper velocity limit for efficientscreening
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Section Page
XXVII 16 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
If high suspended solids greater than 200 mesh (eg sand) are expected to be a continuous problem ejectors may be installedupstream and downstream of the traveling screen to remove accumulated sand and solids from the intake bay in these areas
Traveling screens and ejectors are supplied with wash water from the raw water pumps or from separate wash water pumpsOnoff control using timers should allow optimum scheduling of the ejectors depending on the time of the year
STEP 5 - Specify water intake facilities
Water intake facilities should be designed in accordance with the following guidelines from the Hydraulic Institute Standards The water intake should be designed to provide an evenly distributed flow of water to the pump suction A properly designedintake reduces the possibility of vortex formation and results in better pump performance with less noise Geometry of theintake and the manner in which water is introduced into the intake are important parameters in providing an even distribution of water flow Complete analysis of intake structures is best accomplished by scale model testsThe pump manufacturer will generally render advice on specific problems while the intake design is still preliminary He shouldbe provided with the necessary intake layout drawings showing the physical limitations of the siteFigures 7A and 7B have been developed for single- and multiple-pump arrangements to show suggestions for basic sumpdimensions They are for pumps normally operating in the capacity range of approximately 3000 to 300000 gpm (200 to20000 Ls) Since these values are composite averages from a great many pump types and cover the entire range of specificspeeds they must not be thought of as absolute values but rather as basic guides subject to some possible variations
All the dimensions in Figures 7A and 7B are based on the rated capacity of the pump at the design head Any increase incapacity above these values should be momentary or very limited in time If operation at an increased capacity is to beundertaken for considerable periods of time the maximum capacity should be used for the design value in obtaining sumpdimensionsDimension C is an average value based on an analysis of many pumps Its final value should be specified by the pumpmanufacturerDimension B is a suggested maximum dimension which may be less depending on actual suction bell or bowl diameters in useby the pump manufacturer The edge of the bell should be close to the back wall of the sump When the position of the backwall is determined by the driving equipment or the discharge piping Dimension B may become excessive and a ldquofalserdquo backwall should be installedDimension S is a minimum value for the sump width for a single pump installation This dimension can be increased but if it isto be made smaller the manufacturer should be consulted or a sump model test should be run to determine its adequacy
Motor size may dictate minimum pump spacing be especially aware of this when specifying pumps with unusually high pumpheads eg greater than 80 psig (550 kPa)Dimension H is a minimum value based on the ldquonormal low water levelrdquo at the pump suction bell taking into considerationfriction losses through the inlet screen and approach channel This dimension can be considerably less momentarily or infrequently without excessive damage to the pump It should be remembered however that this does not representldquosubmergencerdquo Submergence is normally quoted as dimension ldquoHrdquo minus ldquoCrdquo This represents the physical height of water level above the bottom of the suction inletThe actual submergence of the pump is something less than this since the impeller eye is some distance above the bottom of the suction bell possibly as much as 3 to 4 ft (1 to 13 m) For the purposes of sump design in connection with this chart it isunderstood that the pump has been selected in accordance with specific speed charts The submergence referred to hereinhas to do only with vortexing and eddy formationsDimensions Y and A are recommended minimum values These dimensions can be as large as desired but should be limitedto the restrictions indicated on the curve If the design does not include a screen Dimension A should be considerably longerThe screen or gate widths should not be substantially less than S and heights should not be less than H If the main stream
velocity is more than 2 fts (06 ms) it may be necessary to construct straightening vanes in the approach channel increaseDimension A conduct a sump model test of the installation or work out some combination of these factorsDimension S becomes the width of an individual pump cell or the center-to-center distance of two pumps if no division walls areusedOn multiple pump installations the recommended dimensions in Figures 7A and 7B also apply as noted above and thefollowing additional determinants should be consideredFigure 8A - Low velocity and straight-line flow to all units simultaneously is the first recommended style of pit Velocities in thepump area should be approximately 1 fts (03 ms) Some pumps with velocities of 2 fts (06 ms) and higher have given goodresults This is particularly true where the design resulted from a model study Not recommended would be an abrupt changein size of inlet pipe to sump or inlet from one side introducing eddying
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Section Page
COOLING WATER SYSTEMS XXVII 17 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Figure 8B - A number of pumps in the same sump will operate best without separating walls unless all pumps are always inoperation at the same time in which case the use of separating walls may be beneficial If walls must be used for structural
purposes and pumps will operate intermittently one should leave flow space behind each wall from the pit floor up to at leastthe minimum water level and the wall should not extend upstream beyond the rim of the suction bell If walls are used increaseDimension ldquoSrdquo by the thickness of the wall for correct centerline spacing Round off the ends of the walls Not recommended isthe placement of a number of pumps around the edge of a sump with or without dividing wallsFigure 8C - Abrupt changes in size from inlet pipe or channel to pump bay are not desirable A relatively small pipe emptyinginto a large pump pit should connect to the pit with a gradually increasing taper section The angle should be as large aspossible preferably not less than 45 degrees With this arrangement pit velocities much less than one foot per second aredesirable Especially not recommended is a small pipe directly connected to a large pit with pumps close to the inlet Flow willhave excessive change of direction to get to most of the pumps Centering pumps in the pit leave large ldquovortex areasrdquo behindthe pumps which will cause operational troubleFigure 8D - If the pit velocity can be kept low enough [below 1 fts (03 ms)] an abrupt change from inlet pipe to pit can beaccommodated if the length equals or exceeds the values shown It is assumed that as ratio WP increases the inlet velocityat ldquoPrdquo will increase up to an allowed maximum of 8 fts (24 ms) at WP = 10 Pumps ldquoin linerdquo are not recommended unless theratio of pit to pump size is quite large and pumps are separated longitudinally by a generous margin A pit can generally be
constructed at much less cost by use of a recommended designFigure 8E - It is sometimes desirable to install pumps in tunnels or pipelines A drop pipe or false well to house the pump witha vaned inlet well facing upstream will be satisfactory in flows up to 8 fts (24 ms) Without the inlet well the pump section bellshould be positioned at least two pipe diameters vertically above the top of the tunnel not hung into the tunnel flow especiallywith tunnel velocities 2 fts (06 ms) or more There should be no signs of air along the top of the tunnel It may be necessaryto lower the scoop or to insist on a minimum water level in the vertical wellNote The foregoing statements apply to sumps for clear liquid
STEP 6 - Provide chemical treatment
Provide chemical treatment facilities for the once-through cooling water system Normally the only chemical treatment requiredin a once-through cooling system is for biological control Facilities for adding chlorine to once-through cooling water are thesame as those described for the recirculated cooling water system The design capacity of the chlorinator should be based onachieving a chlorine residual of 1 wppm in the effluent The chlorine required by the organics and other reducing agents in thewater must be satisfied before any residual chlorine will appear in the effluent The design capacity of the chlorinator should be
based on the total water chlorine demand plus 1 wppm in the effluent In the absence of water data the chlorination systemdesign capacity should be based on 10 wppm minimum instantaneous dosage for maximum cooling water flow For very largeonce-through cooling water systems it will be impractical to design for shock rates at this level In this case a continuousdosage of 1 - 2 wppm should be used (Refer to Step 5 of the DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS )
STEP 7 - Specify pumps and distribution system
Specify other major facilities in the once-through cooling water system The recommended materials for pumps valves andpiping are shown in the EMRErsquos Refinery Construction Materials Manual
Pumps1 Type - These pumps are usually vertical centrifugal pumps However horizontal centrifugal pumps have been used An
economic study andor consideration of existing facilities will determine which type should be specified In the event thathorizontal centrifugal pumps are specified facilities for pump priming will likely be required For large pumps in salt water service there is a very limited selection of pumps because of the special materials required Consult with the DISCIPLINESPECIALISTS early in the design effort to determine what specific pumps are currently available
2 Total Required Capacity - Firm capacity (excluding spares) of the supply pumps should equal the maximum cooling water requirements including LGF and RCF
3 NumberIndividual Pump Capacity - Normally one spare pump is provided for up to six operating pumps two spares for up to twelve operating pumps etc The most common arrangements for water supply pumps are
bull Two 100 capacity pumps
bull Three 50 capacity pumps
bull Four 33-13 capacity pumps
bull Five 25 capacity pumps
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ExxonMobil Proprietary
Section Page
XXVII 18 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Maximum commercially available pump capacity ranges up to 300000 gpm (20000 Ls) at discharge pressures up to 100psi (700 kPa) Maximum capacity will be less for higher discharge pressures Other factors which may influence pump
selection arebull Selecting capacity to match existing pumps
bull Selecting capacity for future requirements
bull Selecting capacity corresponding to manufacturers available standardVendor information should be obtained on the availability size and cost of the pumps for several configurations Acomparison should be made of the investments and operating costs as well as other advantages and disadvantages toselect a pump configuration
4 Drivers - These pumps are usually motor-driven Because of the remote location of the intake facilities the steamdistribution costs are normally too high to justify the use of a steam turbine driver However use of a secondary selectivesubstation provides a reliable source of power to the pump drivers
5 Pump ∆∆∆∆P - Pump ∆P is based on supplying the maximum cooling water requirement at the battery limit design pressureestablished for the system The ∆P should include the total circulating system pressure drop plus any elevation differencebetween the low water level at the intake and the discharge point of contact with air
6 Pump Instrumentation - The remote location of the intake facilities makes remote control and monitoring of the water supply operation desirable Pump controls located in the control house provide for minimum manning Local controlbackup should be provided The Owners operating and manning philosophy determine the basis for instrumentation andcontrolThe spare pump is controlled to automatically start on low pressure in the supply header Concurrently alarms should beactuated in the control room to indicate the low-pressure situation and spare pump operation
Distribution Network1 Configuration - The piping network is a once-through system with a return line from each consumer or consumer group to
the cooling water effluent line2 Layout - The routing of the piping network should connect all cooling water consumers in the most economical manner
permitted by the plant layout Careful consideration should be given to the turnaround philosophy of the plant Thenetwork layout should preclude the necessity of having above-ground water lines that are in service running through anarea that is being turned around Underground piping or an extra lateral may be required to accomplish this objective
3 Valves and Blinds - Battery limit valves should be provided in the network to isolate individual process units from thesystem Valves should be provided to permit onstream removal and maintenance of spared equipment
4 Basket Strainers - Consideration should be given to the installation of basket strainers in the laterals serving each of theconsumer loops These provide added insurance against entrance of foreign material into the cooling equipment The useof intake screens with 025 in (63 mm) openings may eliminate the need for basket strainers in the laterals
Line Sizing1 Design Basis - The network is sized so that water is delivered to the most distant consumer at a pressure equal to the
required process design (battery limit) pressure2 Flow Rate - Each segment of the distribution network is sized for the simultaneous maximum flow in that segment for any
operating condition The main supply and discharge headers are sized for the simultaneous maximum flow of all segments(not necessarily the sum of the simultaneous maximum flows of the individual segments)
3 Pressure Drop
Line Sizing Basis - The distribution system should be sized according to the guidelines established in Section XIV The use of an ldquoold piperdquo friction factor of C = 90 - 100 (Hazen-Williams factors) in the main lines is recommended to assuregood operation in later years when the system experiences fouling For large diameter lines [greater than 48 in (1200mm)] maximum water velocity should be limited to 10 fts (3 ms) based on civilmechanical considerations (supports andanchoring required to handle potential pressure surges) For salt water systems utilizing cement-lined pipe consider thereduced inner diameter in the friction loss calculations A study may be necessary to determine the economic pump ∆P vsline diameters
4 Equivalent Length Factors - Equivalent length factors are used in pressure drop calculations for sizing the distributionsystem The piping length scaled from the plot plan is multiplied by an equivalent length factor to compensate for pipingelbows expansion loops and other piping variations
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Section Page
COOLING WATER SYSTEMS XXVII 19 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
The equivalent length factors are selected based on the expected variation from a straight piping run Typical equivalentlength factors suitable for planning purposes for different piping applications are as follows
TYPE OF PIPING EQUIVALENT LENGTH FACTORS
Long piping runs [gt 1000 ft (300 m)] 12 to 14
Onsite distribution piping 15 to 20
For design purposes the actual piping layout should be evaluated to ensure that the equivalent length factors areapplicable to the specific situation Particular attention is required for revamp work and saltwater systems utilizing cement-lined pipe
STEP 8 - Prepare a system flow plan similar to the one shown in Figure 1
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS
STEP 1 - Determine cooling water requirements
Tabulate the cooling water requirements and cooling water discharge temperature for each process unit and offsite consumer Apply the proper LGFs to all cooling loads depending on the stage of the project and whether the new units are exactduplicates of existing units Determine the total cooling water flow required including LGFs and the bulk temperature (T 1) of thecombined cooling water flow from all coolers to the cooling towerIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The onsite and offsite designers should investigate methods to increase effluent water temperature to themaximum recommended limit for the particular type of water used This will result in a lower cost cooling tower for a given heatrejection rate One common method of maximizing return water temperature is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 2 - Select cooling tower design parameters
Select the proper design conditions of ambient wet bulb temperature approach temperature range and flow rate for specifyingthe cooling tower Based on these conditions the cooling tower vendor will size the tower and provide the tower characteristicand performance curves which serve as the basis for the performance guarantee Required performance test and evaluationprocedures are described in IP 8-1-1 Economic studies may be required to select the ambient wet bulb temperature andapproach temperature for new locations or for expansion projects with special cooling requirements
STEP 3 - Prepare cooling tower specification sheet
Include the following information in the heat exchange section of the design specificationTower Flow Capacity - This was determined in Step 1Type of Tower - The tower usually specified is an induced draft multicell type suitable for fresh salt or brackish water depending on the water available Additional tower characteristics are usually developed and proposed by the tower vendor (eg crossflow versus counterflow type of tower fill etc) The vendor should be made aware of conditions such as thelikelihood of heavy oil leakage into the cooling water or high fouling potential which may influence the tower type or fill
selectionNumber of Cells - The minimum number of cells required (normally not less than 2) may be specified The maximum flow per cell is a function of the cooling tower design conditions selected in Step 2 Typical values range up to 10000 gpm (600 Ls) for crossflow designs and 8000 gpm (500 Ls) for counterflow designs Normally spare cells are not provided Maintenance iscarried out during cooler periods or load is shed
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XXVII 20 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Ambient Wet Bulb Temperature1 The ambient wet bulb temperature selected for specifying the cooling tower design is generally that temperature which is
equaled or exceeded 5 of the time during the four hottest months (5 of the hourly readings taken during the four hottestmonths will exceed this value) Specifying the 5 temperature should give satisfactory results since the time that the wetbulb temperature is exceeded is for short duration and the system inertia is usually sufficient to avoid problems The tower specification should make it clear that the wet bulb temperature specified is an ambient value as opposed to the enteringvalue which may be higher because of recirculation The tower manufacturer should adjust the ambient value upward totake into account his estimates of recirculation Currently cooling tower test codes provide procedures for measuringperformance in the case of either entering or ambient wet-bulb temperature specifications An increase of 2 degF (11 degC) isgenerally added to ambient wet bulb temperature to estimate entering wet bulb temperature for large cooling towers
2 If plant production will peak during the four summer months when the 5 ambient wet bulb temperature may be exceededconsideration should be given to specifying a higher design ambient wet bulb temperature (one that will be exceeded 212 of the time for example) However the incremental cost of the resulting closer approach and larger cooling tower must be justified by the value of the incremental products made
On-Tower Temperature (T1) - This temperature cannot be accurately determined until the design of all process coolers iscompleted and the volume of cooling water and outlet temperature are known for each (see Step 1) In the planning stage or early design stage of a project this temperature must be estimated and is usually set equal to the maximum design cooler outlet temperature shown under COOLING WATER REQUIREMENTS in the BASIC DESIGN CONSIDERATIONS portion of this sectionApproach Temperature - The approach temperature has the most pronounced effect on cooling tower size and cost For example reducing the approach from 10 to 5 degF (55 to 3 degC) can increase the cooling tower investment by 50 percent and thefan operating cost by 65 percent Thus various approach temperatures should be evaluated to determine the minimum cost of the total system including the process exchanger area Approaches used in the past designs have been from 5 to 12 degF (3 to7degC) Note that it is not customary in the cooling tower industry to guarantee approaches of less than 5 degF to (3 degC) to coolingtower design inlet wet bulb temperature A typical approach for a new grass roots cooling tower is 10 degF (6 degC)Off-Tower Temperature (T2) - This is the sum of the approach temperature and the ambient air wet bulb temperature For existing locations Table 2 in Section IX-B may be used
Range and Flow Rate1 The range for a given heat load is related to the circulating water rate as follows
Q = M C p ∆Tcong (500) (gpm) (range) Eq (1)
where Q = Heat load on the tower BtuhM = Water circulating rate lbhCp = Specific heat capacity of water at average temperature Btulb degF∆T = (T 1 ndash T 2) degF = (water cooling range)gpm = Circulation rate in US gallons per minute
For calculation in Metric units
Q = M C p ∆Tcong (1162 x 10 -3) (Ls) (range) Eq (1M)
where Q = Heat load on the tower kJh
M = Water circulating rate kghCp = Specific heat capacity of water at average temperature kJkg degC∆T = (T 1 ndash T 2) degC = (water cooling range)Ls = Circulation rate in liters per second
2 Generally lower tower investment fan horsepower and pump horsepower result from maximizing the range andminimizing the flow rate However based on fouling considerations of process coolers the maximum allowable hot water temperatures must be kept below recommended limits Also while increasing the range minimizes cooling tower investment and operating cost the cost of exchanger surface requirements is adversely affected by the LMTD and acompromise must be made The range is bounded by the maximum allowable hot water temperature and the Off-Tower temperature (T 2) For established locations refer to Section IX-B for design cold water temperatures to assist in selectingthe approach and range
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Section Page
COOLING WATER SYSTEMS XXVII 21 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Makeup Water Requirements - Calculate the various losses in the cooling water system to determine makeup requirementsThese losses include
1 Evaporation Loss (E) - Allow 1 of the circulation rate for each 10 degF (55 degC) temperature drop through the tower Notethat the actual evaporation loss will be somewhat less depending on ambient air temperature
( )sL555
TT)gpm(
1000TT
E 2121
minus=
minus= Eq (2)
2 Drift Loss (D) - IP 8-1-1 limits the maximum allowable drift loss to 0005 of the design water circulation rate for freshbrackish or salt water
( )sL1000050
)gpm(1000050
D
=
= Eq (3)
This low drift level contributes little to solids removal from the tower with the spray This allows higher cycles of concentration to be achieved in cooling towers which use clarified softened water for makeup and thus have low requiredblowdown rates Higher drift loss percentages in older or poorly maintained towers will limit the maximum cycles of
concentration achievable3 Miscellaneous Losses (O) - Miscellaneous losses are estimated to be one-seventh of the difference between evaporation
loss and drift loss
minus=
7DE
O Eq (4)
4 Blowdown Rate (B) ndash Guidelines for maximum concentrations of impurities allowed in recirculated fresh cooling water areshown in Table 2 For salt water the blowdown is based on keeping the total dissolved salts below 55000 wppm Obtainthe chemical analysis of the makeup water and divide each individual component concentration into its counterpart inTable 2 The result is the maximum allowable cycles of concentration for that constituent (a worksheet is provided inTable 4 ) The impurity with the lowest factor will be the control and will determine the required blowdown rate
)OD(1Cycles
EB +minus
minus= Eq (5)
The makeup water requirement is the sum of all losses shown above
MakeUp = E + D + O + B Eq (6)
STEP 4 - Specify the controls necessary for proper cooling tower operation
Air Side - Cooling towers are provided with motor-driven manually adjusted pitch multiple-bladed propeller fans Figures9A amp B may be used to estimate the normal operating horsepower requirements Since the fan motor and gear reducer arelocated at the top of the tower [generally 40 to 50 ft (12 to 15 m) above grade] the need for equipment to handle thesecomponents should be discussed with the affiliateControl of the air flow through the tower to maintain reasonably constant cold water temperatures can be accomplished byturning off individual fans in multiple-cell installations adjusting the blade pitch or using variable speed motors To turn off individual fans remote at grade start-stop buttons for the motors are usually specified
Whenever the ambient air temperature is 32 degF (0 degC) or below icing of the tower louvers and structural members in contact withthe incoming air will occur If the heat load on the tower is relatively free of seasonal variations two-speed fans should provideadequate ice control down to approximately 0 degF (-18 degC) ambient temperatures When sustained temperatures of 0 degF (-18 degC)and below occur two-speed fans capable of two-speed reverse operation with appropriate time delays for change of directionand speed should be specified to periodically eliminate ice which will build up on the louvers Other methods of winterizationare available but in general are more expensive and require greater operator attention In very cold climates steam heatingshould be provided to prevent freezing in the cold water basin during shutdown periodsWater Side - The water circulation rate is maintained by operating one or more pumps of sufficient capacity to meet the systemdemand Flow-measuring facilities local temperature indicators and a high-temperature alarm are provided on the coolingtower water supply and return headers Hand-operated flow control valves are required for distributing the water equally to theindividual cells On counterflow towers provisions for measuring the flow to each cell (pitot tubes) are required They are notrequired on crossflow towers since the open distribution basin provides visual aid in balancing flows Shutoff valves should beprovided for servicing the flow control valves without shutting down the entire tower
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XXVII 22 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
STEP 5 - Provide chemical treatment facilities for the fresh water system
Alkalinity (pH) Control (if used)1 In cases where a controlled pH corrosionscale control program is being used the alkalinity (pH) of the recirculated cooling
water should be controlled in accordance with the program requirements Normally acid addition is required to accomplishthis This is due to free carbon dioxide being stripped from the cooling water in the tower to approximately 5 ppm For example if the alkalinity has to be controlled to between 25 and 50 ppm for a pH of 70 to 75 the volume of 98 sulfuricacid (66 degBeacute) required to treat the makeup water is calculated using the following equation
[Note 93 sulfuric acid should be used if ambient temperatures will be less than 35 degF (2 degC)]
GallonsHour 66 degBeacute H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate gpm) (326 x 10 -5)
For calculation in Metric limits
kgh of 100 (66 degBeacute) H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate Ls) (793 x 10 -3)
2 The metering pump should be designed for a maximum feed rate of two to three times the normal feed rate A glass woolfilter is required on the pump suction piping to minimize plugging Normally two - 100 capacity pumps are provided
Positive displacement metering pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumps with flow control areused for higher rates
3 Hydrochloric acid may also be used for alkalinity control The gallons per hour of 20 degBeacute hydrochloric acid required is36 times the amount of 66 degBeacute sulfuric acid In Metric units the weight of 32 (20 degBeacute) hydrochloric acid required is227 times the weight of 100 (66 degBeacute) sulfuric acid
4 Specifications for chemical feeders may be found in EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased)
5 A pH analyzer with high and low alarm is provided to control the amount of acid feed The acid should be added to thecooling tower makeup water (Figure 2 ) The lag time between the point of acid injection and the sample outlet to the pHcell should be less than two minutes for effective control
6 Spills or overflows from the sulfuric acid tank should be contained
Corrosion Inhibitors and Anti-Scalents1 Two chemical feeders are generally provided The following guides in sizing the equipment will allow the use of any of the
proprietary products from the vendors when final selection by the owner is made See Report No EE102E78 Cooling Tower Water Treatment Guidelines EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and Report No EE34E86 Environmentally - Acceptable Cooling Water Treatments for detailed discussion of acceptable commercial chemicals
bull If the water treatment program to be used is known design for the chemical rates specified by the vendor or COOLING WATER SYSTEMS SPECIALIST
bull Otherwise design for 100 wppm of each commercial liquid product in the recirculated cooling water The design feedrate should be based on the design total blowdown flow (both uncontrolled and controlled) since this is where theproduct is lost For 100 wppm concentration the chemical rates will be as follows
GallonsHour Chemical = Blowdown (gpm) x (0006)
LitersHour Chemical = Blowdown (Ls) x (036)
2 The design capacity of the metering pump should be two to three times this calculated number Normally 2 - 100capacity pumps are provided Positive displacement pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumpswith flow control are used for higher rates
3 Specifications for chemical feeders may be found in EMRErsquos Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased) Reference should also be made to the checklists containedin Report No EE92E94 Guidelines for Safety Evaluation of Chemical Injection Facilities
4 A final check should be made with the owner and the COOLING WATER SYSTEMS SPECIALISTS as to the treatmentprogram selected and if there is a need for an additional chemical feeder
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COOLING WATER SYSTEMS XXVII 23 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Biological ControlFacilities should be provided for feeding the biocide into the back of the cooling tower basin If chlorine is used it must bepresent in the cooling water as free residual to be effective However the following amounts of chlorine are first required toreact with the listed impurities when present in the cooling water before a free residual can be established
IMPURITIESAMOUNT OF CHLORINE REQUIRED (wppm)
WITH EACH WPPM OF IMPURITY
Ammonia (NH 3)
Hydrogen Sulfide (H 2S)
Sodium Sulfite (Na 2SO 3)
Ferrous Iron (Fe ++)
10
22
056
064
The design capacity of the chlorine feeder shall be based on achieving a 01 wppm free chlorine residual in the recirculatingwater flow or a minimum dosage of 7 wppm at maximum design makeup water flow rate and conditions (see EMRE Water and
Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service )Chlorine may be fed either as the commercial liquid sodium hypochlorite (124 wt Cl 2) or liquid chlorine (100 Cl 2) Liquidhypochlorite is being used increasingly because of the higher costs associated with precautions equipment and proceduresrequired to minimize the risks associated with handling gaseous chlorine The hypochlorite solution is fed using a meteringpump which should be sized to feed an equivalent of 15 ppm 100 Cl 2 (Refer to EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) ) In case of spills or an overflow from thestorage tank the sodium hypochlorite may be sent to the cooling tower basinGas chlorinators are covered by EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service The use of gas chlorination is being reduced but may still be appropriate in some locations subject tolocal management approval The gas from either commercial cylinders or an evaporator is dissolved in a water slip stream bymeans of an eductor This results in a chlorine solution strength of approximately 01 This solution should be evenlydistributed below the water surface by means of a header lateral pipe arrangement at the rear of the cooling tower basinopposite the cooling water pumpsSome additional guides on the use of gas chlorinators are given below It is also recommended that the safety guidelines
published by the Manufacturing Chemists Association and the Chlorine Institute be followedbull Dilution water for the chlorinator must not be less than 50 degF (10 degC) or ice formation may cause plugging problems
bull Continuous rates of chlorine gas withdrawal should not exceed 400 lb (180 kg)24 hours from each one ton cylinder atroom temperature [70 degF (21 degC)] This rate may be exceeded (up to 50) for periods not exceeding 2 hours Rule of thumb is not to design for more than 4 cylinders operating in parallel When feed rate exceeds 3000 lb (1360 kg) per daythe liquid chlorine should be fed to an evaporator upstream of the chlorinator
bull If the ambient temperature can be expected to be below 50 degF (10 degC) for extended periods a building should be providedfor the chlorinator and cylinders
bull Weighing scales should be provided to determine when shipping containers should be replaced and to verify chlorine feedrates
bull Analyzers to measure the free chlorine residual in the circulated cooling water should be provided
bull Chlorine leak detectors and a catch basin for major spills should be provided
Control SystemProvide automatic controllers to continuously monitor water condition and to add chemicals to the system (complete systemsmay be available through the water treatment program vendor as a lease or purchase)pH Control - This controller adds acid to correct water alkalinity a timer on the acid pump sounds an alarm if the pump doesnot shut off in a predetermined length of time Backup controls are required to prevent the cooling system from beingpermanently damaged in case the pH electrodes become fouledCorrosivity and Inhibitor Control - Systems are available which give a direct readout of corrosion rate in milsyear If thecorrosion rate exceeds a set point the controller shuts off the acid increases the inhibitor addition opens the blowdown valveand sounds an alarm Electrodes measure the completeness of the inhibitor film by the value of the potential differencebetween the electrodes The larger the difference the more incomplete the film and a potential reading exceeding a set levelactivates the inhibitor feed
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XXVII 24 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Free Residual Chlorine Control - Free residual chlorine should be controlled automatically for biocide treatment Severalgood analyzers are currently available These units can be incorporated into an automatic control system
Oil in Water DetectionMonitoring Corrosion and Deposit FormationThe following types of devices for measuring corrosion rates and deposit formation should be provided to allow the plant tomonitor the effectiveness of the cooling water treatment before scheduled turnaroundsCoupons - A corrosion coupon rack should be specified for the hot return water to the tower in accordance with IP 19-6-1 Corrosion Monitor - On-line corrosion analyzer if specified should monitor the hot return water to the tower in accordance withIP 19-6-1
On-Line Test Exchanger Although there are a variety of deposit monitors from the water treatment chemical vendors they do not necessarily representactual heat transfer conditions in process exchangers For this reason a test exchanger should be installed for evaluatingcorrosion and deposit formation Details are covered in Report No EE102E78 Cooling Tower Water Treatment Guidelines
Alternatively a critical process exchanger can be instrumented such that its heat transfer coefficient can be monitored
Instrumentation would include flows and inlet and outlet temperatures of process fluid and cooling waterSTEP 6 - Specify cooling tower location
Select a location for the cooling tower The following factors influence this selection
Location Factors1 The location should be within a reasonable distance of the units served and consistent with future refinery plans Piping
costs to and from battery limits can be a significant portion of the total cooling system cost and can be the location-determining factor particularly for salt water systems While piping costs can be easily determined other costs affectinglocation cannot be estimated readily Therefore judgment is required in selecting the location
2 Safety spacing requirements (see Section XV-G Equipment Spacing )3 Cooling tower contribution to plant boundary noise4 For salt water cooling towers wind direction and distance to other facilities is of major concern because of potential
corrosion problems
5 Factors affecting cooling tower performance ie orientation with respect to wind site congestion etc
Plot RequirementsIn estimating plot requirements (basin area) one should allow 5 to 6 ft of tower length for each 1000 gpm (2 m for each 60 Ls)of circulating capacity This figure is based on a typical counterflow design with a width of 50 ft (12 m) and does not includepump suction pit space Exact requirements can be obtained from cooling tower vendors after the tower capacity has beenestimated
Fogging and DriftTwo distinct problems are associated with the discharge air from a cooling tower (1) fogging which can be a nuisance or ahazard and (2) drift which is generally a nuisance because of increased corrosion of surrounding equipment Fogging fromcooling towers occurs during periods of combined high ambient humidity and low temperature as a result of mixing the warmmoist cooling tower exhaust air with the cooler ambient air The ambient air cannot absorb all moisture as vapor and thus tinyfog droplets are formed by condensation Fogging and resulting icing in cold climates has produced hazardous drivingconditions on highways up to 850 ft (250 m) awayProprietary designs are available to combat potential fogging problems Basically these designs incorporate heating of thedischarge air with hot return water precooling coils (similar to an air cooler) to eliminate any visible plume However thesedesigns are expensive and relatively unproven Therefore proper location based on reliable weather data is the preferredsolutionDrift from a cooling tower represents a more difficult problem since it does not depend on definable climatic conditions andoccurs for all wind directions The effects of drift on nearby equipment and vegetation are particularly severe and generally of concern only for salt water systems because of the high solids content of the drift (concentrated salt water) For salt water cooling systems electric lines substations and instrumentation should be located upwind in the direction of the predominantwind They should be provided with adequate shelter or materials when within 450 ft (140 m) of the cooling tower In additionspecifications for all equipment within 150 ft (45 m) of the tower should include surface treatment suitable for a marineenvironment
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COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
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DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
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COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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XXVII 30 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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DESIGN PRACTICES December 2000
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TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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DESIGN PRACTICES December 2000
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FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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DESIGN PRACTICES December 2000
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FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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Section Page
COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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Section Page
COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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Section Page
XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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COOLING WATER SYSTEMS XXVII 15 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS
STEP 1 - Select a location for the intake station and pumps
Factors which influence this selection are as followsbull Depth of water and tidal variations
bull Relative location of intake and outfall direction of current
bull Length and size of intake suction piping vs pump discharge piping
STEP 2 - Determine cooling water requirements
Determine the design cold water inlet temperature based on site data covering seasonal water temperature as a function of inlet location and depth (for existing locations refer to Table 2 in Section IX-B )Tabulate the cooling water requirements for each process unit and offsite consumer Apply the proper LGFs to all coolingloads depending on the stage of the project and whether the new units are exact duplicates of existing unitsIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The designer should investigate methods to increase effluent water temperature to the maximumrecommended limit for the particular type of water used One common method is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 3 - Determine the total required capacity for water intake facilities
Process Coolers - The total requirements determined in Step 2 including LGFsFirewater - This is often supplied from the intake bay of a once-through systemFuture Expansions - These should be considered in sizing intake facilitiesTotal Required Capacity - The sum of the above three items
STEP 4 - Provide water intake facilities
Intake Crib - The intake crib with its support structure is located at the extreme end of the water intake line (See Figure 4 )The intake is usually located 200 to 300 ft (60 to 90 m) offshore and at a sufficient depth below the lowest water level to
preclude vortex formation and subsequent pump cavitation during storm conditions Additionally the crib should have 3 to 5 ft(1 to 15 m) clearance above the sea floor to minimize sand and silt pickup A concrete velocity cap should be specified to cover the inlet crib This will minimize the incidence of fish and other foreignmatter entering the system A horizontal inlet flow pattern should be designed with a maximum entrance velocity of 1 fts(03 ms) Experience shows that vertical velocity flow confuses fish and they are drawn into the intake lineThe intake crib support structure protects the crib from adverse effects of storms and tidal motions as well as protecting it fromimpact of floating and submerged objects A self-cleaning screen can be specified to prevent entrance of large objects into thesystem at the cribIntake Line (Figure 5 ) - An intake line is required if the water depth along the shore does not permit water to flow directly intothe intake bay The intake line is usually buried in the sea floor and covered with a minimum required overburden Consultwith the COOLING WATER SYSTEMS SPECIALISTS for required coverage at a specific siteChlorine or sodium hypochlorite is injected at the intake crib to prevent marine growth in the lineNote that an intake crib and line will not be required if water of sufficient depth is available at the supply pump stationIntake Bay (Figure 6 ) - The intake bay is an enclosure which houses the pumps and screens Depending on a particular design a series of screens is used to ensure the supply of clean water Trash racks comprise the first element of the filter system These remove only very coarse material from the water such as bulk seaweed They can be supplied with atraversing raking mechanismLift screens provide secondary cleanup These screens can be lifted out of the intake bay for periodic cleaning Double liftscreens can be used if it is deemed necessary to have one in service at all timesFinal water filtering is usually done by traveling screens Details of the screen dimensions and the mounting are usuallydetermined during detailed engineering Screen net open area is typically approximately 50 of total screen area Thescreens should be sized for a normal water velocity of 12 fts (04 ms) through the open screen area Maximum velocityshould be limited to 24 fts (08 ms) with 50 of the screen area plugged This is the upper velocity limit for efficientscreening
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XXVII 16 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
If high suspended solids greater than 200 mesh (eg sand) are expected to be a continuous problem ejectors may be installedupstream and downstream of the traveling screen to remove accumulated sand and solids from the intake bay in these areas
Traveling screens and ejectors are supplied with wash water from the raw water pumps or from separate wash water pumpsOnoff control using timers should allow optimum scheduling of the ejectors depending on the time of the year
STEP 5 - Specify water intake facilities
Water intake facilities should be designed in accordance with the following guidelines from the Hydraulic Institute Standards The water intake should be designed to provide an evenly distributed flow of water to the pump suction A properly designedintake reduces the possibility of vortex formation and results in better pump performance with less noise Geometry of theintake and the manner in which water is introduced into the intake are important parameters in providing an even distribution of water flow Complete analysis of intake structures is best accomplished by scale model testsThe pump manufacturer will generally render advice on specific problems while the intake design is still preliminary He shouldbe provided with the necessary intake layout drawings showing the physical limitations of the siteFigures 7A and 7B have been developed for single- and multiple-pump arrangements to show suggestions for basic sumpdimensions They are for pumps normally operating in the capacity range of approximately 3000 to 300000 gpm (200 to20000 Ls) Since these values are composite averages from a great many pump types and cover the entire range of specificspeeds they must not be thought of as absolute values but rather as basic guides subject to some possible variations
All the dimensions in Figures 7A and 7B are based on the rated capacity of the pump at the design head Any increase incapacity above these values should be momentary or very limited in time If operation at an increased capacity is to beundertaken for considerable periods of time the maximum capacity should be used for the design value in obtaining sumpdimensionsDimension C is an average value based on an analysis of many pumps Its final value should be specified by the pumpmanufacturerDimension B is a suggested maximum dimension which may be less depending on actual suction bell or bowl diameters in useby the pump manufacturer The edge of the bell should be close to the back wall of the sump When the position of the backwall is determined by the driving equipment or the discharge piping Dimension B may become excessive and a ldquofalserdquo backwall should be installedDimension S is a minimum value for the sump width for a single pump installation This dimension can be increased but if it isto be made smaller the manufacturer should be consulted or a sump model test should be run to determine its adequacy
Motor size may dictate minimum pump spacing be especially aware of this when specifying pumps with unusually high pumpheads eg greater than 80 psig (550 kPa)Dimension H is a minimum value based on the ldquonormal low water levelrdquo at the pump suction bell taking into considerationfriction losses through the inlet screen and approach channel This dimension can be considerably less momentarily or infrequently without excessive damage to the pump It should be remembered however that this does not representldquosubmergencerdquo Submergence is normally quoted as dimension ldquoHrdquo minus ldquoCrdquo This represents the physical height of water level above the bottom of the suction inletThe actual submergence of the pump is something less than this since the impeller eye is some distance above the bottom of the suction bell possibly as much as 3 to 4 ft (1 to 13 m) For the purposes of sump design in connection with this chart it isunderstood that the pump has been selected in accordance with specific speed charts The submergence referred to hereinhas to do only with vortexing and eddy formationsDimensions Y and A are recommended minimum values These dimensions can be as large as desired but should be limitedto the restrictions indicated on the curve If the design does not include a screen Dimension A should be considerably longerThe screen or gate widths should not be substantially less than S and heights should not be less than H If the main stream
velocity is more than 2 fts (06 ms) it may be necessary to construct straightening vanes in the approach channel increaseDimension A conduct a sump model test of the installation or work out some combination of these factorsDimension S becomes the width of an individual pump cell or the center-to-center distance of two pumps if no division walls areusedOn multiple pump installations the recommended dimensions in Figures 7A and 7B also apply as noted above and thefollowing additional determinants should be consideredFigure 8A - Low velocity and straight-line flow to all units simultaneously is the first recommended style of pit Velocities in thepump area should be approximately 1 fts (03 ms) Some pumps with velocities of 2 fts (06 ms) and higher have given goodresults This is particularly true where the design resulted from a model study Not recommended would be an abrupt changein size of inlet pipe to sump or inlet from one side introducing eddying
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COOLING WATER SYSTEMS XXVII 17 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Figure 8B - A number of pumps in the same sump will operate best without separating walls unless all pumps are always inoperation at the same time in which case the use of separating walls may be beneficial If walls must be used for structural
purposes and pumps will operate intermittently one should leave flow space behind each wall from the pit floor up to at leastthe minimum water level and the wall should not extend upstream beyond the rim of the suction bell If walls are used increaseDimension ldquoSrdquo by the thickness of the wall for correct centerline spacing Round off the ends of the walls Not recommended isthe placement of a number of pumps around the edge of a sump with or without dividing wallsFigure 8C - Abrupt changes in size from inlet pipe or channel to pump bay are not desirable A relatively small pipe emptyinginto a large pump pit should connect to the pit with a gradually increasing taper section The angle should be as large aspossible preferably not less than 45 degrees With this arrangement pit velocities much less than one foot per second aredesirable Especially not recommended is a small pipe directly connected to a large pit with pumps close to the inlet Flow willhave excessive change of direction to get to most of the pumps Centering pumps in the pit leave large ldquovortex areasrdquo behindthe pumps which will cause operational troubleFigure 8D - If the pit velocity can be kept low enough [below 1 fts (03 ms)] an abrupt change from inlet pipe to pit can beaccommodated if the length equals or exceeds the values shown It is assumed that as ratio WP increases the inlet velocityat ldquoPrdquo will increase up to an allowed maximum of 8 fts (24 ms) at WP = 10 Pumps ldquoin linerdquo are not recommended unless theratio of pit to pump size is quite large and pumps are separated longitudinally by a generous margin A pit can generally be
constructed at much less cost by use of a recommended designFigure 8E - It is sometimes desirable to install pumps in tunnels or pipelines A drop pipe or false well to house the pump witha vaned inlet well facing upstream will be satisfactory in flows up to 8 fts (24 ms) Without the inlet well the pump section bellshould be positioned at least two pipe diameters vertically above the top of the tunnel not hung into the tunnel flow especiallywith tunnel velocities 2 fts (06 ms) or more There should be no signs of air along the top of the tunnel It may be necessaryto lower the scoop or to insist on a minimum water level in the vertical wellNote The foregoing statements apply to sumps for clear liquid
STEP 6 - Provide chemical treatment
Provide chemical treatment facilities for the once-through cooling water system Normally the only chemical treatment requiredin a once-through cooling system is for biological control Facilities for adding chlorine to once-through cooling water are thesame as those described for the recirculated cooling water system The design capacity of the chlorinator should be based onachieving a chlorine residual of 1 wppm in the effluent The chlorine required by the organics and other reducing agents in thewater must be satisfied before any residual chlorine will appear in the effluent The design capacity of the chlorinator should be
based on the total water chlorine demand plus 1 wppm in the effluent In the absence of water data the chlorination systemdesign capacity should be based on 10 wppm minimum instantaneous dosage for maximum cooling water flow For very largeonce-through cooling water systems it will be impractical to design for shock rates at this level In this case a continuousdosage of 1 - 2 wppm should be used (Refer to Step 5 of the DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS )
STEP 7 - Specify pumps and distribution system
Specify other major facilities in the once-through cooling water system The recommended materials for pumps valves andpiping are shown in the EMRErsquos Refinery Construction Materials Manual
Pumps1 Type - These pumps are usually vertical centrifugal pumps However horizontal centrifugal pumps have been used An
economic study andor consideration of existing facilities will determine which type should be specified In the event thathorizontal centrifugal pumps are specified facilities for pump priming will likely be required For large pumps in salt water service there is a very limited selection of pumps because of the special materials required Consult with the DISCIPLINESPECIALISTS early in the design effort to determine what specific pumps are currently available
2 Total Required Capacity - Firm capacity (excluding spares) of the supply pumps should equal the maximum cooling water requirements including LGF and RCF
3 NumberIndividual Pump Capacity - Normally one spare pump is provided for up to six operating pumps two spares for up to twelve operating pumps etc The most common arrangements for water supply pumps are
bull Two 100 capacity pumps
bull Three 50 capacity pumps
bull Four 33-13 capacity pumps
bull Five 25 capacity pumps
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Section Page
XXVII 18 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Maximum commercially available pump capacity ranges up to 300000 gpm (20000 Ls) at discharge pressures up to 100psi (700 kPa) Maximum capacity will be less for higher discharge pressures Other factors which may influence pump
selection arebull Selecting capacity to match existing pumps
bull Selecting capacity for future requirements
bull Selecting capacity corresponding to manufacturers available standardVendor information should be obtained on the availability size and cost of the pumps for several configurations Acomparison should be made of the investments and operating costs as well as other advantages and disadvantages toselect a pump configuration
4 Drivers - These pumps are usually motor-driven Because of the remote location of the intake facilities the steamdistribution costs are normally too high to justify the use of a steam turbine driver However use of a secondary selectivesubstation provides a reliable source of power to the pump drivers
5 Pump ∆∆∆∆P - Pump ∆P is based on supplying the maximum cooling water requirement at the battery limit design pressureestablished for the system The ∆P should include the total circulating system pressure drop plus any elevation differencebetween the low water level at the intake and the discharge point of contact with air
6 Pump Instrumentation - The remote location of the intake facilities makes remote control and monitoring of the water supply operation desirable Pump controls located in the control house provide for minimum manning Local controlbackup should be provided The Owners operating and manning philosophy determine the basis for instrumentation andcontrolThe spare pump is controlled to automatically start on low pressure in the supply header Concurrently alarms should beactuated in the control room to indicate the low-pressure situation and spare pump operation
Distribution Network1 Configuration - The piping network is a once-through system with a return line from each consumer or consumer group to
the cooling water effluent line2 Layout - The routing of the piping network should connect all cooling water consumers in the most economical manner
permitted by the plant layout Careful consideration should be given to the turnaround philosophy of the plant Thenetwork layout should preclude the necessity of having above-ground water lines that are in service running through anarea that is being turned around Underground piping or an extra lateral may be required to accomplish this objective
3 Valves and Blinds - Battery limit valves should be provided in the network to isolate individual process units from thesystem Valves should be provided to permit onstream removal and maintenance of spared equipment
4 Basket Strainers - Consideration should be given to the installation of basket strainers in the laterals serving each of theconsumer loops These provide added insurance against entrance of foreign material into the cooling equipment The useof intake screens with 025 in (63 mm) openings may eliminate the need for basket strainers in the laterals
Line Sizing1 Design Basis - The network is sized so that water is delivered to the most distant consumer at a pressure equal to the
required process design (battery limit) pressure2 Flow Rate - Each segment of the distribution network is sized for the simultaneous maximum flow in that segment for any
operating condition The main supply and discharge headers are sized for the simultaneous maximum flow of all segments(not necessarily the sum of the simultaneous maximum flows of the individual segments)
3 Pressure Drop
Line Sizing Basis - The distribution system should be sized according to the guidelines established in Section XIV The use of an ldquoold piperdquo friction factor of C = 90 - 100 (Hazen-Williams factors) in the main lines is recommended to assuregood operation in later years when the system experiences fouling For large diameter lines [greater than 48 in (1200mm)] maximum water velocity should be limited to 10 fts (3 ms) based on civilmechanical considerations (supports andanchoring required to handle potential pressure surges) For salt water systems utilizing cement-lined pipe consider thereduced inner diameter in the friction loss calculations A study may be necessary to determine the economic pump ∆P vsline diameters
4 Equivalent Length Factors - Equivalent length factors are used in pressure drop calculations for sizing the distributionsystem The piping length scaled from the plot plan is multiplied by an equivalent length factor to compensate for pipingelbows expansion loops and other piping variations
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Section Page
COOLING WATER SYSTEMS XXVII 19 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
The equivalent length factors are selected based on the expected variation from a straight piping run Typical equivalentlength factors suitable for planning purposes for different piping applications are as follows
TYPE OF PIPING EQUIVALENT LENGTH FACTORS
Long piping runs [gt 1000 ft (300 m)] 12 to 14
Onsite distribution piping 15 to 20
For design purposes the actual piping layout should be evaluated to ensure that the equivalent length factors areapplicable to the specific situation Particular attention is required for revamp work and saltwater systems utilizing cement-lined pipe
STEP 8 - Prepare a system flow plan similar to the one shown in Figure 1
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS
STEP 1 - Determine cooling water requirements
Tabulate the cooling water requirements and cooling water discharge temperature for each process unit and offsite consumer Apply the proper LGFs to all cooling loads depending on the stage of the project and whether the new units are exactduplicates of existing units Determine the total cooling water flow required including LGFs and the bulk temperature (T 1) of thecombined cooling water flow from all coolers to the cooling towerIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The onsite and offsite designers should investigate methods to increase effluent water temperature to themaximum recommended limit for the particular type of water used This will result in a lower cost cooling tower for a given heatrejection rate One common method of maximizing return water temperature is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 2 - Select cooling tower design parameters
Select the proper design conditions of ambient wet bulb temperature approach temperature range and flow rate for specifyingthe cooling tower Based on these conditions the cooling tower vendor will size the tower and provide the tower characteristicand performance curves which serve as the basis for the performance guarantee Required performance test and evaluationprocedures are described in IP 8-1-1 Economic studies may be required to select the ambient wet bulb temperature andapproach temperature for new locations or for expansion projects with special cooling requirements
STEP 3 - Prepare cooling tower specification sheet
Include the following information in the heat exchange section of the design specificationTower Flow Capacity - This was determined in Step 1Type of Tower - The tower usually specified is an induced draft multicell type suitable for fresh salt or brackish water depending on the water available Additional tower characteristics are usually developed and proposed by the tower vendor (eg crossflow versus counterflow type of tower fill etc) The vendor should be made aware of conditions such as thelikelihood of heavy oil leakage into the cooling water or high fouling potential which may influence the tower type or fill
selectionNumber of Cells - The minimum number of cells required (normally not less than 2) may be specified The maximum flow per cell is a function of the cooling tower design conditions selected in Step 2 Typical values range up to 10000 gpm (600 Ls) for crossflow designs and 8000 gpm (500 Ls) for counterflow designs Normally spare cells are not provided Maintenance iscarried out during cooler periods or load is shed
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Section Page
XXVII 20 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Ambient Wet Bulb Temperature1 The ambient wet bulb temperature selected for specifying the cooling tower design is generally that temperature which is
equaled or exceeded 5 of the time during the four hottest months (5 of the hourly readings taken during the four hottestmonths will exceed this value) Specifying the 5 temperature should give satisfactory results since the time that the wetbulb temperature is exceeded is for short duration and the system inertia is usually sufficient to avoid problems The tower specification should make it clear that the wet bulb temperature specified is an ambient value as opposed to the enteringvalue which may be higher because of recirculation The tower manufacturer should adjust the ambient value upward totake into account his estimates of recirculation Currently cooling tower test codes provide procedures for measuringperformance in the case of either entering or ambient wet-bulb temperature specifications An increase of 2 degF (11 degC) isgenerally added to ambient wet bulb temperature to estimate entering wet bulb temperature for large cooling towers
2 If plant production will peak during the four summer months when the 5 ambient wet bulb temperature may be exceededconsideration should be given to specifying a higher design ambient wet bulb temperature (one that will be exceeded 212 of the time for example) However the incremental cost of the resulting closer approach and larger cooling tower must be justified by the value of the incremental products made
On-Tower Temperature (T1) - This temperature cannot be accurately determined until the design of all process coolers iscompleted and the volume of cooling water and outlet temperature are known for each (see Step 1) In the planning stage or early design stage of a project this temperature must be estimated and is usually set equal to the maximum design cooler outlet temperature shown under COOLING WATER REQUIREMENTS in the BASIC DESIGN CONSIDERATIONS portion of this sectionApproach Temperature - The approach temperature has the most pronounced effect on cooling tower size and cost For example reducing the approach from 10 to 5 degF (55 to 3 degC) can increase the cooling tower investment by 50 percent and thefan operating cost by 65 percent Thus various approach temperatures should be evaluated to determine the minimum cost of the total system including the process exchanger area Approaches used in the past designs have been from 5 to 12 degF (3 to7degC) Note that it is not customary in the cooling tower industry to guarantee approaches of less than 5 degF to (3 degC) to coolingtower design inlet wet bulb temperature A typical approach for a new grass roots cooling tower is 10 degF (6 degC)Off-Tower Temperature (T2) - This is the sum of the approach temperature and the ambient air wet bulb temperature For existing locations Table 2 in Section IX-B may be used
Range and Flow Rate1 The range for a given heat load is related to the circulating water rate as follows
Q = M C p ∆Tcong (500) (gpm) (range) Eq (1)
where Q = Heat load on the tower BtuhM = Water circulating rate lbhCp = Specific heat capacity of water at average temperature Btulb degF∆T = (T 1 ndash T 2) degF = (water cooling range)gpm = Circulation rate in US gallons per minute
For calculation in Metric units
Q = M C p ∆Tcong (1162 x 10 -3) (Ls) (range) Eq (1M)
where Q = Heat load on the tower kJh
M = Water circulating rate kghCp = Specific heat capacity of water at average temperature kJkg degC∆T = (T 1 ndash T 2) degC = (water cooling range)Ls = Circulation rate in liters per second
2 Generally lower tower investment fan horsepower and pump horsepower result from maximizing the range andminimizing the flow rate However based on fouling considerations of process coolers the maximum allowable hot water temperatures must be kept below recommended limits Also while increasing the range minimizes cooling tower investment and operating cost the cost of exchanger surface requirements is adversely affected by the LMTD and acompromise must be made The range is bounded by the maximum allowable hot water temperature and the Off-Tower temperature (T 2) For established locations refer to Section IX-B for design cold water temperatures to assist in selectingthe approach and range
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Section Page
COOLING WATER SYSTEMS XXVII 21 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Makeup Water Requirements - Calculate the various losses in the cooling water system to determine makeup requirementsThese losses include
1 Evaporation Loss (E) - Allow 1 of the circulation rate for each 10 degF (55 degC) temperature drop through the tower Notethat the actual evaporation loss will be somewhat less depending on ambient air temperature
( )sL555
TT)gpm(
1000TT
E 2121
minus=
minus= Eq (2)
2 Drift Loss (D) - IP 8-1-1 limits the maximum allowable drift loss to 0005 of the design water circulation rate for freshbrackish or salt water
( )sL1000050
)gpm(1000050
D
=
= Eq (3)
This low drift level contributes little to solids removal from the tower with the spray This allows higher cycles of concentration to be achieved in cooling towers which use clarified softened water for makeup and thus have low requiredblowdown rates Higher drift loss percentages in older or poorly maintained towers will limit the maximum cycles of
concentration achievable3 Miscellaneous Losses (O) - Miscellaneous losses are estimated to be one-seventh of the difference between evaporation
loss and drift loss
minus=
7DE
O Eq (4)
4 Blowdown Rate (B) ndash Guidelines for maximum concentrations of impurities allowed in recirculated fresh cooling water areshown in Table 2 For salt water the blowdown is based on keeping the total dissolved salts below 55000 wppm Obtainthe chemical analysis of the makeup water and divide each individual component concentration into its counterpart inTable 2 The result is the maximum allowable cycles of concentration for that constituent (a worksheet is provided inTable 4 ) The impurity with the lowest factor will be the control and will determine the required blowdown rate
)OD(1Cycles
EB +minus
minus= Eq (5)
The makeup water requirement is the sum of all losses shown above
MakeUp = E + D + O + B Eq (6)
STEP 4 - Specify the controls necessary for proper cooling tower operation
Air Side - Cooling towers are provided with motor-driven manually adjusted pitch multiple-bladed propeller fans Figures9A amp B may be used to estimate the normal operating horsepower requirements Since the fan motor and gear reducer arelocated at the top of the tower [generally 40 to 50 ft (12 to 15 m) above grade] the need for equipment to handle thesecomponents should be discussed with the affiliateControl of the air flow through the tower to maintain reasonably constant cold water temperatures can be accomplished byturning off individual fans in multiple-cell installations adjusting the blade pitch or using variable speed motors To turn off individual fans remote at grade start-stop buttons for the motors are usually specified
Whenever the ambient air temperature is 32 degF (0 degC) or below icing of the tower louvers and structural members in contact withthe incoming air will occur If the heat load on the tower is relatively free of seasonal variations two-speed fans should provideadequate ice control down to approximately 0 degF (-18 degC) ambient temperatures When sustained temperatures of 0 degF (-18 degC)and below occur two-speed fans capable of two-speed reverse operation with appropriate time delays for change of directionand speed should be specified to periodically eliminate ice which will build up on the louvers Other methods of winterizationare available but in general are more expensive and require greater operator attention In very cold climates steam heatingshould be provided to prevent freezing in the cold water basin during shutdown periodsWater Side - The water circulation rate is maintained by operating one or more pumps of sufficient capacity to meet the systemdemand Flow-measuring facilities local temperature indicators and a high-temperature alarm are provided on the coolingtower water supply and return headers Hand-operated flow control valves are required for distributing the water equally to theindividual cells On counterflow towers provisions for measuring the flow to each cell (pitot tubes) are required They are notrequired on crossflow towers since the open distribution basin provides visual aid in balancing flows Shutoff valves should beprovided for servicing the flow control valves without shutting down the entire tower
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XXVII 22 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
STEP 5 - Provide chemical treatment facilities for the fresh water system
Alkalinity (pH) Control (if used)1 In cases where a controlled pH corrosionscale control program is being used the alkalinity (pH) of the recirculated cooling
water should be controlled in accordance with the program requirements Normally acid addition is required to accomplishthis This is due to free carbon dioxide being stripped from the cooling water in the tower to approximately 5 ppm For example if the alkalinity has to be controlled to between 25 and 50 ppm for a pH of 70 to 75 the volume of 98 sulfuricacid (66 degBeacute) required to treat the makeup water is calculated using the following equation
[Note 93 sulfuric acid should be used if ambient temperatures will be less than 35 degF (2 degC)]
GallonsHour 66 degBeacute H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate gpm) (326 x 10 -5)
For calculation in Metric limits
kgh of 100 (66 degBeacute) H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate Ls) (793 x 10 -3)
2 The metering pump should be designed for a maximum feed rate of two to three times the normal feed rate A glass woolfilter is required on the pump suction piping to minimize plugging Normally two - 100 capacity pumps are provided
Positive displacement metering pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumps with flow control areused for higher rates
3 Hydrochloric acid may also be used for alkalinity control The gallons per hour of 20 degBeacute hydrochloric acid required is36 times the amount of 66 degBeacute sulfuric acid In Metric units the weight of 32 (20 degBeacute) hydrochloric acid required is227 times the weight of 100 (66 degBeacute) sulfuric acid
4 Specifications for chemical feeders may be found in EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased)
5 A pH analyzer with high and low alarm is provided to control the amount of acid feed The acid should be added to thecooling tower makeup water (Figure 2 ) The lag time between the point of acid injection and the sample outlet to the pHcell should be less than two minutes for effective control
6 Spills or overflows from the sulfuric acid tank should be contained
Corrosion Inhibitors and Anti-Scalents1 Two chemical feeders are generally provided The following guides in sizing the equipment will allow the use of any of the
proprietary products from the vendors when final selection by the owner is made See Report No EE102E78 Cooling Tower Water Treatment Guidelines EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and Report No EE34E86 Environmentally - Acceptable Cooling Water Treatments for detailed discussion of acceptable commercial chemicals
bull If the water treatment program to be used is known design for the chemical rates specified by the vendor or COOLING WATER SYSTEMS SPECIALIST
bull Otherwise design for 100 wppm of each commercial liquid product in the recirculated cooling water The design feedrate should be based on the design total blowdown flow (both uncontrolled and controlled) since this is where theproduct is lost For 100 wppm concentration the chemical rates will be as follows
GallonsHour Chemical = Blowdown (gpm) x (0006)
LitersHour Chemical = Blowdown (Ls) x (036)
2 The design capacity of the metering pump should be two to three times this calculated number Normally 2 - 100capacity pumps are provided Positive displacement pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumpswith flow control are used for higher rates
3 Specifications for chemical feeders may be found in EMRErsquos Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased) Reference should also be made to the checklists containedin Report No EE92E94 Guidelines for Safety Evaluation of Chemical Injection Facilities
4 A final check should be made with the owner and the COOLING WATER SYSTEMS SPECIALISTS as to the treatmentprogram selected and if there is a need for an additional chemical feeder
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COOLING WATER SYSTEMS XXVII 23 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Biological ControlFacilities should be provided for feeding the biocide into the back of the cooling tower basin If chlorine is used it must bepresent in the cooling water as free residual to be effective However the following amounts of chlorine are first required toreact with the listed impurities when present in the cooling water before a free residual can be established
IMPURITIESAMOUNT OF CHLORINE REQUIRED (wppm)
WITH EACH WPPM OF IMPURITY
Ammonia (NH 3)
Hydrogen Sulfide (H 2S)
Sodium Sulfite (Na 2SO 3)
Ferrous Iron (Fe ++)
10
22
056
064
The design capacity of the chlorine feeder shall be based on achieving a 01 wppm free chlorine residual in the recirculatingwater flow or a minimum dosage of 7 wppm at maximum design makeup water flow rate and conditions (see EMRE Water and
Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service )Chlorine may be fed either as the commercial liquid sodium hypochlorite (124 wt Cl 2) or liquid chlorine (100 Cl 2) Liquidhypochlorite is being used increasingly because of the higher costs associated with precautions equipment and proceduresrequired to minimize the risks associated with handling gaseous chlorine The hypochlorite solution is fed using a meteringpump which should be sized to feed an equivalent of 15 ppm 100 Cl 2 (Refer to EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) ) In case of spills or an overflow from thestorage tank the sodium hypochlorite may be sent to the cooling tower basinGas chlorinators are covered by EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service The use of gas chlorination is being reduced but may still be appropriate in some locations subject tolocal management approval The gas from either commercial cylinders or an evaporator is dissolved in a water slip stream bymeans of an eductor This results in a chlorine solution strength of approximately 01 This solution should be evenlydistributed below the water surface by means of a header lateral pipe arrangement at the rear of the cooling tower basinopposite the cooling water pumpsSome additional guides on the use of gas chlorinators are given below It is also recommended that the safety guidelines
published by the Manufacturing Chemists Association and the Chlorine Institute be followedbull Dilution water for the chlorinator must not be less than 50 degF (10 degC) or ice formation may cause plugging problems
bull Continuous rates of chlorine gas withdrawal should not exceed 400 lb (180 kg)24 hours from each one ton cylinder atroom temperature [70 degF (21 degC)] This rate may be exceeded (up to 50) for periods not exceeding 2 hours Rule of thumb is not to design for more than 4 cylinders operating in parallel When feed rate exceeds 3000 lb (1360 kg) per daythe liquid chlorine should be fed to an evaporator upstream of the chlorinator
bull If the ambient temperature can be expected to be below 50 degF (10 degC) for extended periods a building should be providedfor the chlorinator and cylinders
bull Weighing scales should be provided to determine when shipping containers should be replaced and to verify chlorine feedrates
bull Analyzers to measure the free chlorine residual in the circulated cooling water should be provided
bull Chlorine leak detectors and a catch basin for major spills should be provided
Control SystemProvide automatic controllers to continuously monitor water condition and to add chemicals to the system (complete systemsmay be available through the water treatment program vendor as a lease or purchase)pH Control - This controller adds acid to correct water alkalinity a timer on the acid pump sounds an alarm if the pump doesnot shut off in a predetermined length of time Backup controls are required to prevent the cooling system from beingpermanently damaged in case the pH electrodes become fouledCorrosivity and Inhibitor Control - Systems are available which give a direct readout of corrosion rate in milsyear If thecorrosion rate exceeds a set point the controller shuts off the acid increases the inhibitor addition opens the blowdown valveand sounds an alarm Electrodes measure the completeness of the inhibitor film by the value of the potential differencebetween the electrodes The larger the difference the more incomplete the film and a potential reading exceeding a set levelactivates the inhibitor feed
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XXVII 24 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Free Residual Chlorine Control - Free residual chlorine should be controlled automatically for biocide treatment Severalgood analyzers are currently available These units can be incorporated into an automatic control system
Oil in Water DetectionMonitoring Corrosion and Deposit FormationThe following types of devices for measuring corrosion rates and deposit formation should be provided to allow the plant tomonitor the effectiveness of the cooling water treatment before scheduled turnaroundsCoupons - A corrosion coupon rack should be specified for the hot return water to the tower in accordance with IP 19-6-1 Corrosion Monitor - On-line corrosion analyzer if specified should monitor the hot return water to the tower in accordance withIP 19-6-1
On-Line Test Exchanger Although there are a variety of deposit monitors from the water treatment chemical vendors they do not necessarily representactual heat transfer conditions in process exchangers For this reason a test exchanger should be installed for evaluatingcorrosion and deposit formation Details are covered in Report No EE102E78 Cooling Tower Water Treatment Guidelines
Alternatively a critical process exchanger can be instrumented such that its heat transfer coefficient can be monitored
Instrumentation would include flows and inlet and outlet temperatures of process fluid and cooling waterSTEP 6 - Specify cooling tower location
Select a location for the cooling tower The following factors influence this selection
Location Factors1 The location should be within a reasonable distance of the units served and consistent with future refinery plans Piping
costs to and from battery limits can be a significant portion of the total cooling system cost and can be the location-determining factor particularly for salt water systems While piping costs can be easily determined other costs affectinglocation cannot be estimated readily Therefore judgment is required in selecting the location
2 Safety spacing requirements (see Section XV-G Equipment Spacing )3 Cooling tower contribution to plant boundary noise4 For salt water cooling towers wind direction and distance to other facilities is of major concern because of potential
corrosion problems
5 Factors affecting cooling tower performance ie orientation with respect to wind site congestion etc
Plot RequirementsIn estimating plot requirements (basin area) one should allow 5 to 6 ft of tower length for each 1000 gpm (2 m for each 60 Ls)of circulating capacity This figure is based on a typical counterflow design with a width of 50 ft (12 m) and does not includepump suction pit space Exact requirements can be obtained from cooling tower vendors after the tower capacity has beenestimated
Fogging and DriftTwo distinct problems are associated with the discharge air from a cooling tower (1) fogging which can be a nuisance or ahazard and (2) drift which is generally a nuisance because of increased corrosion of surrounding equipment Fogging fromcooling towers occurs during periods of combined high ambient humidity and low temperature as a result of mixing the warmmoist cooling tower exhaust air with the cooler ambient air The ambient air cannot absorb all moisture as vapor and thus tinyfog droplets are formed by condensation Fogging and resulting icing in cold climates has produced hazardous drivingconditions on highways up to 850 ft (250 m) awayProprietary designs are available to combat potential fogging problems Basically these designs incorporate heating of thedischarge air with hot return water precooling coils (similar to an air cooler) to eliminate any visible plume However thesedesigns are expensive and relatively unproven Therefore proper location based on reliable weather data is the preferredsolutionDrift from a cooling tower represents a more difficult problem since it does not depend on definable climatic conditions andoccurs for all wind directions The effects of drift on nearby equipment and vegetation are particularly severe and generally of concern only for salt water systems because of the high solids content of the drift (concentrated salt water) For salt water cooling systems electric lines substations and instrumentation should be located upwind in the direction of the predominantwind They should be provided with adequate shelter or materials when within 450 ft (140 m) of the cooling tower In additionspecifications for all equipment within 150 ft (45 m) of the tower should include surface treatment suitable for a marineenvironment
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COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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XXVII 28 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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XXVII 30 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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COOLING WATER SYSTEMS XXVII 31 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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XXVII 32 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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COOLING WATER SYSTEMS XXVII 33 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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COOLING WATER SYSTEMS XXVII 35 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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Section Page
XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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COOLING WATER SYSTEMS XXVII 37 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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Section Page
XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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Section Page
COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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Section Page
COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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Section Page
XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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XXVII 16 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
If high suspended solids greater than 200 mesh (eg sand) are expected to be a continuous problem ejectors may be installedupstream and downstream of the traveling screen to remove accumulated sand and solids from the intake bay in these areas
Traveling screens and ejectors are supplied with wash water from the raw water pumps or from separate wash water pumpsOnoff control using timers should allow optimum scheduling of the ejectors depending on the time of the year
STEP 5 - Specify water intake facilities
Water intake facilities should be designed in accordance with the following guidelines from the Hydraulic Institute Standards The water intake should be designed to provide an evenly distributed flow of water to the pump suction A properly designedintake reduces the possibility of vortex formation and results in better pump performance with less noise Geometry of theintake and the manner in which water is introduced into the intake are important parameters in providing an even distribution of water flow Complete analysis of intake structures is best accomplished by scale model testsThe pump manufacturer will generally render advice on specific problems while the intake design is still preliminary He shouldbe provided with the necessary intake layout drawings showing the physical limitations of the siteFigures 7A and 7B have been developed for single- and multiple-pump arrangements to show suggestions for basic sumpdimensions They are for pumps normally operating in the capacity range of approximately 3000 to 300000 gpm (200 to20000 Ls) Since these values are composite averages from a great many pump types and cover the entire range of specificspeeds they must not be thought of as absolute values but rather as basic guides subject to some possible variations
All the dimensions in Figures 7A and 7B are based on the rated capacity of the pump at the design head Any increase incapacity above these values should be momentary or very limited in time If operation at an increased capacity is to beundertaken for considerable periods of time the maximum capacity should be used for the design value in obtaining sumpdimensionsDimension C is an average value based on an analysis of many pumps Its final value should be specified by the pumpmanufacturerDimension B is a suggested maximum dimension which may be less depending on actual suction bell or bowl diameters in useby the pump manufacturer The edge of the bell should be close to the back wall of the sump When the position of the backwall is determined by the driving equipment or the discharge piping Dimension B may become excessive and a ldquofalserdquo backwall should be installedDimension S is a minimum value for the sump width for a single pump installation This dimension can be increased but if it isto be made smaller the manufacturer should be consulted or a sump model test should be run to determine its adequacy
Motor size may dictate minimum pump spacing be especially aware of this when specifying pumps with unusually high pumpheads eg greater than 80 psig (550 kPa)Dimension H is a minimum value based on the ldquonormal low water levelrdquo at the pump suction bell taking into considerationfriction losses through the inlet screen and approach channel This dimension can be considerably less momentarily or infrequently without excessive damage to the pump It should be remembered however that this does not representldquosubmergencerdquo Submergence is normally quoted as dimension ldquoHrdquo minus ldquoCrdquo This represents the physical height of water level above the bottom of the suction inletThe actual submergence of the pump is something less than this since the impeller eye is some distance above the bottom of the suction bell possibly as much as 3 to 4 ft (1 to 13 m) For the purposes of sump design in connection with this chart it isunderstood that the pump has been selected in accordance with specific speed charts The submergence referred to hereinhas to do only with vortexing and eddy formationsDimensions Y and A are recommended minimum values These dimensions can be as large as desired but should be limitedto the restrictions indicated on the curve If the design does not include a screen Dimension A should be considerably longerThe screen or gate widths should not be substantially less than S and heights should not be less than H If the main stream
velocity is more than 2 fts (06 ms) it may be necessary to construct straightening vanes in the approach channel increaseDimension A conduct a sump model test of the installation or work out some combination of these factorsDimension S becomes the width of an individual pump cell or the center-to-center distance of two pumps if no division walls areusedOn multiple pump installations the recommended dimensions in Figures 7A and 7B also apply as noted above and thefollowing additional determinants should be consideredFigure 8A - Low velocity and straight-line flow to all units simultaneously is the first recommended style of pit Velocities in thepump area should be approximately 1 fts (03 ms) Some pumps with velocities of 2 fts (06 ms) and higher have given goodresults This is particularly true where the design resulted from a model study Not recommended would be an abrupt changein size of inlet pipe to sump or inlet from one side introducing eddying
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COOLING WATER SYSTEMS XXVII 17 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Figure 8B - A number of pumps in the same sump will operate best without separating walls unless all pumps are always inoperation at the same time in which case the use of separating walls may be beneficial If walls must be used for structural
purposes and pumps will operate intermittently one should leave flow space behind each wall from the pit floor up to at leastthe minimum water level and the wall should not extend upstream beyond the rim of the suction bell If walls are used increaseDimension ldquoSrdquo by the thickness of the wall for correct centerline spacing Round off the ends of the walls Not recommended isthe placement of a number of pumps around the edge of a sump with or without dividing wallsFigure 8C - Abrupt changes in size from inlet pipe or channel to pump bay are not desirable A relatively small pipe emptyinginto a large pump pit should connect to the pit with a gradually increasing taper section The angle should be as large aspossible preferably not less than 45 degrees With this arrangement pit velocities much less than one foot per second aredesirable Especially not recommended is a small pipe directly connected to a large pit with pumps close to the inlet Flow willhave excessive change of direction to get to most of the pumps Centering pumps in the pit leave large ldquovortex areasrdquo behindthe pumps which will cause operational troubleFigure 8D - If the pit velocity can be kept low enough [below 1 fts (03 ms)] an abrupt change from inlet pipe to pit can beaccommodated if the length equals or exceeds the values shown It is assumed that as ratio WP increases the inlet velocityat ldquoPrdquo will increase up to an allowed maximum of 8 fts (24 ms) at WP = 10 Pumps ldquoin linerdquo are not recommended unless theratio of pit to pump size is quite large and pumps are separated longitudinally by a generous margin A pit can generally be
constructed at much less cost by use of a recommended designFigure 8E - It is sometimes desirable to install pumps in tunnels or pipelines A drop pipe or false well to house the pump witha vaned inlet well facing upstream will be satisfactory in flows up to 8 fts (24 ms) Without the inlet well the pump section bellshould be positioned at least two pipe diameters vertically above the top of the tunnel not hung into the tunnel flow especiallywith tunnel velocities 2 fts (06 ms) or more There should be no signs of air along the top of the tunnel It may be necessaryto lower the scoop or to insist on a minimum water level in the vertical wellNote The foregoing statements apply to sumps for clear liquid
STEP 6 - Provide chemical treatment
Provide chemical treatment facilities for the once-through cooling water system Normally the only chemical treatment requiredin a once-through cooling system is for biological control Facilities for adding chlorine to once-through cooling water are thesame as those described for the recirculated cooling water system The design capacity of the chlorinator should be based onachieving a chlorine residual of 1 wppm in the effluent The chlorine required by the organics and other reducing agents in thewater must be satisfied before any residual chlorine will appear in the effluent The design capacity of the chlorinator should be
based on the total water chlorine demand plus 1 wppm in the effluent In the absence of water data the chlorination systemdesign capacity should be based on 10 wppm minimum instantaneous dosage for maximum cooling water flow For very largeonce-through cooling water systems it will be impractical to design for shock rates at this level In this case a continuousdosage of 1 - 2 wppm should be used (Refer to Step 5 of the DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS )
STEP 7 - Specify pumps and distribution system
Specify other major facilities in the once-through cooling water system The recommended materials for pumps valves andpiping are shown in the EMRErsquos Refinery Construction Materials Manual
Pumps1 Type - These pumps are usually vertical centrifugal pumps However horizontal centrifugal pumps have been used An
economic study andor consideration of existing facilities will determine which type should be specified In the event thathorizontal centrifugal pumps are specified facilities for pump priming will likely be required For large pumps in salt water service there is a very limited selection of pumps because of the special materials required Consult with the DISCIPLINESPECIALISTS early in the design effort to determine what specific pumps are currently available
2 Total Required Capacity - Firm capacity (excluding spares) of the supply pumps should equal the maximum cooling water requirements including LGF and RCF
3 NumberIndividual Pump Capacity - Normally one spare pump is provided for up to six operating pumps two spares for up to twelve operating pumps etc The most common arrangements for water supply pumps are
bull Two 100 capacity pumps
bull Three 50 capacity pumps
bull Four 33-13 capacity pumps
bull Five 25 capacity pumps
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ExxonMobil Proprietary
Section Page
XXVII 18 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Maximum commercially available pump capacity ranges up to 300000 gpm (20000 Ls) at discharge pressures up to 100psi (700 kPa) Maximum capacity will be less for higher discharge pressures Other factors which may influence pump
selection arebull Selecting capacity to match existing pumps
bull Selecting capacity for future requirements
bull Selecting capacity corresponding to manufacturers available standardVendor information should be obtained on the availability size and cost of the pumps for several configurations Acomparison should be made of the investments and operating costs as well as other advantages and disadvantages toselect a pump configuration
4 Drivers - These pumps are usually motor-driven Because of the remote location of the intake facilities the steamdistribution costs are normally too high to justify the use of a steam turbine driver However use of a secondary selectivesubstation provides a reliable source of power to the pump drivers
5 Pump ∆∆∆∆P - Pump ∆P is based on supplying the maximum cooling water requirement at the battery limit design pressureestablished for the system The ∆P should include the total circulating system pressure drop plus any elevation differencebetween the low water level at the intake and the discharge point of contact with air
6 Pump Instrumentation - The remote location of the intake facilities makes remote control and monitoring of the water supply operation desirable Pump controls located in the control house provide for minimum manning Local controlbackup should be provided The Owners operating and manning philosophy determine the basis for instrumentation andcontrolThe spare pump is controlled to automatically start on low pressure in the supply header Concurrently alarms should beactuated in the control room to indicate the low-pressure situation and spare pump operation
Distribution Network1 Configuration - The piping network is a once-through system with a return line from each consumer or consumer group to
the cooling water effluent line2 Layout - The routing of the piping network should connect all cooling water consumers in the most economical manner
permitted by the plant layout Careful consideration should be given to the turnaround philosophy of the plant Thenetwork layout should preclude the necessity of having above-ground water lines that are in service running through anarea that is being turned around Underground piping or an extra lateral may be required to accomplish this objective
3 Valves and Blinds - Battery limit valves should be provided in the network to isolate individual process units from thesystem Valves should be provided to permit onstream removal and maintenance of spared equipment
4 Basket Strainers - Consideration should be given to the installation of basket strainers in the laterals serving each of theconsumer loops These provide added insurance against entrance of foreign material into the cooling equipment The useof intake screens with 025 in (63 mm) openings may eliminate the need for basket strainers in the laterals
Line Sizing1 Design Basis - The network is sized so that water is delivered to the most distant consumer at a pressure equal to the
required process design (battery limit) pressure2 Flow Rate - Each segment of the distribution network is sized for the simultaneous maximum flow in that segment for any
operating condition The main supply and discharge headers are sized for the simultaneous maximum flow of all segments(not necessarily the sum of the simultaneous maximum flows of the individual segments)
3 Pressure Drop
Line Sizing Basis - The distribution system should be sized according to the guidelines established in Section XIV The use of an ldquoold piperdquo friction factor of C = 90 - 100 (Hazen-Williams factors) in the main lines is recommended to assuregood operation in later years when the system experiences fouling For large diameter lines [greater than 48 in (1200mm)] maximum water velocity should be limited to 10 fts (3 ms) based on civilmechanical considerations (supports andanchoring required to handle potential pressure surges) For salt water systems utilizing cement-lined pipe consider thereduced inner diameter in the friction loss calculations A study may be necessary to determine the economic pump ∆P vsline diameters
4 Equivalent Length Factors - Equivalent length factors are used in pressure drop calculations for sizing the distributionsystem The piping length scaled from the plot plan is multiplied by an equivalent length factor to compensate for pipingelbows expansion loops and other piping variations
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Section Page
COOLING WATER SYSTEMS XXVII 19 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
The equivalent length factors are selected based on the expected variation from a straight piping run Typical equivalentlength factors suitable for planning purposes for different piping applications are as follows
TYPE OF PIPING EQUIVALENT LENGTH FACTORS
Long piping runs [gt 1000 ft (300 m)] 12 to 14
Onsite distribution piping 15 to 20
For design purposes the actual piping layout should be evaluated to ensure that the equivalent length factors areapplicable to the specific situation Particular attention is required for revamp work and saltwater systems utilizing cement-lined pipe
STEP 8 - Prepare a system flow plan similar to the one shown in Figure 1
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS
STEP 1 - Determine cooling water requirements
Tabulate the cooling water requirements and cooling water discharge temperature for each process unit and offsite consumer Apply the proper LGFs to all cooling loads depending on the stage of the project and whether the new units are exactduplicates of existing units Determine the total cooling water flow required including LGFs and the bulk temperature (T 1) of thecombined cooling water flow from all coolers to the cooling towerIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The onsite and offsite designers should investigate methods to increase effluent water temperature to themaximum recommended limit for the particular type of water used This will result in a lower cost cooling tower for a given heatrejection rate One common method of maximizing return water temperature is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 2 - Select cooling tower design parameters
Select the proper design conditions of ambient wet bulb temperature approach temperature range and flow rate for specifyingthe cooling tower Based on these conditions the cooling tower vendor will size the tower and provide the tower characteristicand performance curves which serve as the basis for the performance guarantee Required performance test and evaluationprocedures are described in IP 8-1-1 Economic studies may be required to select the ambient wet bulb temperature andapproach temperature for new locations or for expansion projects with special cooling requirements
STEP 3 - Prepare cooling tower specification sheet
Include the following information in the heat exchange section of the design specificationTower Flow Capacity - This was determined in Step 1Type of Tower - The tower usually specified is an induced draft multicell type suitable for fresh salt or brackish water depending on the water available Additional tower characteristics are usually developed and proposed by the tower vendor (eg crossflow versus counterflow type of tower fill etc) The vendor should be made aware of conditions such as thelikelihood of heavy oil leakage into the cooling water or high fouling potential which may influence the tower type or fill
selectionNumber of Cells - The minimum number of cells required (normally not less than 2) may be specified The maximum flow per cell is a function of the cooling tower design conditions selected in Step 2 Typical values range up to 10000 gpm (600 Ls) for crossflow designs and 8000 gpm (500 Ls) for counterflow designs Normally spare cells are not provided Maintenance iscarried out during cooler periods or load is shed
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Section Page
XXVII 20 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Ambient Wet Bulb Temperature1 The ambient wet bulb temperature selected for specifying the cooling tower design is generally that temperature which is
equaled or exceeded 5 of the time during the four hottest months (5 of the hourly readings taken during the four hottestmonths will exceed this value) Specifying the 5 temperature should give satisfactory results since the time that the wetbulb temperature is exceeded is for short duration and the system inertia is usually sufficient to avoid problems The tower specification should make it clear that the wet bulb temperature specified is an ambient value as opposed to the enteringvalue which may be higher because of recirculation The tower manufacturer should adjust the ambient value upward totake into account his estimates of recirculation Currently cooling tower test codes provide procedures for measuringperformance in the case of either entering or ambient wet-bulb temperature specifications An increase of 2 degF (11 degC) isgenerally added to ambient wet bulb temperature to estimate entering wet bulb temperature for large cooling towers
2 If plant production will peak during the four summer months when the 5 ambient wet bulb temperature may be exceededconsideration should be given to specifying a higher design ambient wet bulb temperature (one that will be exceeded 212 of the time for example) However the incremental cost of the resulting closer approach and larger cooling tower must be justified by the value of the incremental products made
On-Tower Temperature (T1) - This temperature cannot be accurately determined until the design of all process coolers iscompleted and the volume of cooling water and outlet temperature are known for each (see Step 1) In the planning stage or early design stage of a project this temperature must be estimated and is usually set equal to the maximum design cooler outlet temperature shown under COOLING WATER REQUIREMENTS in the BASIC DESIGN CONSIDERATIONS portion of this sectionApproach Temperature - The approach temperature has the most pronounced effect on cooling tower size and cost For example reducing the approach from 10 to 5 degF (55 to 3 degC) can increase the cooling tower investment by 50 percent and thefan operating cost by 65 percent Thus various approach temperatures should be evaluated to determine the minimum cost of the total system including the process exchanger area Approaches used in the past designs have been from 5 to 12 degF (3 to7degC) Note that it is not customary in the cooling tower industry to guarantee approaches of less than 5 degF to (3 degC) to coolingtower design inlet wet bulb temperature A typical approach for a new grass roots cooling tower is 10 degF (6 degC)Off-Tower Temperature (T2) - This is the sum of the approach temperature and the ambient air wet bulb temperature For existing locations Table 2 in Section IX-B may be used
Range and Flow Rate1 The range for a given heat load is related to the circulating water rate as follows
Q = M C p ∆Tcong (500) (gpm) (range) Eq (1)
where Q = Heat load on the tower BtuhM = Water circulating rate lbhCp = Specific heat capacity of water at average temperature Btulb degF∆T = (T 1 ndash T 2) degF = (water cooling range)gpm = Circulation rate in US gallons per minute
For calculation in Metric units
Q = M C p ∆Tcong (1162 x 10 -3) (Ls) (range) Eq (1M)
where Q = Heat load on the tower kJh
M = Water circulating rate kghCp = Specific heat capacity of water at average temperature kJkg degC∆T = (T 1 ndash T 2) degC = (water cooling range)Ls = Circulation rate in liters per second
2 Generally lower tower investment fan horsepower and pump horsepower result from maximizing the range andminimizing the flow rate However based on fouling considerations of process coolers the maximum allowable hot water temperatures must be kept below recommended limits Also while increasing the range minimizes cooling tower investment and operating cost the cost of exchanger surface requirements is adversely affected by the LMTD and acompromise must be made The range is bounded by the maximum allowable hot water temperature and the Off-Tower temperature (T 2) For established locations refer to Section IX-B for design cold water temperatures to assist in selectingthe approach and range
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Section Page
COOLING WATER SYSTEMS XXVII 21 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Makeup Water Requirements - Calculate the various losses in the cooling water system to determine makeup requirementsThese losses include
1 Evaporation Loss (E) - Allow 1 of the circulation rate for each 10 degF (55 degC) temperature drop through the tower Notethat the actual evaporation loss will be somewhat less depending on ambient air temperature
( )sL555
TT)gpm(
1000TT
E 2121
minus=
minus= Eq (2)
2 Drift Loss (D) - IP 8-1-1 limits the maximum allowable drift loss to 0005 of the design water circulation rate for freshbrackish or salt water
( )sL1000050
)gpm(1000050
D
=
= Eq (3)
This low drift level contributes little to solids removal from the tower with the spray This allows higher cycles of concentration to be achieved in cooling towers which use clarified softened water for makeup and thus have low requiredblowdown rates Higher drift loss percentages in older or poorly maintained towers will limit the maximum cycles of
concentration achievable3 Miscellaneous Losses (O) - Miscellaneous losses are estimated to be one-seventh of the difference between evaporation
loss and drift loss
minus=
7DE
O Eq (4)
4 Blowdown Rate (B) ndash Guidelines for maximum concentrations of impurities allowed in recirculated fresh cooling water areshown in Table 2 For salt water the blowdown is based on keeping the total dissolved salts below 55000 wppm Obtainthe chemical analysis of the makeup water and divide each individual component concentration into its counterpart inTable 2 The result is the maximum allowable cycles of concentration for that constituent (a worksheet is provided inTable 4 ) The impurity with the lowest factor will be the control and will determine the required blowdown rate
)OD(1Cycles
EB +minus
minus= Eq (5)
The makeup water requirement is the sum of all losses shown above
MakeUp = E + D + O + B Eq (6)
STEP 4 - Specify the controls necessary for proper cooling tower operation
Air Side - Cooling towers are provided with motor-driven manually adjusted pitch multiple-bladed propeller fans Figures9A amp B may be used to estimate the normal operating horsepower requirements Since the fan motor and gear reducer arelocated at the top of the tower [generally 40 to 50 ft (12 to 15 m) above grade] the need for equipment to handle thesecomponents should be discussed with the affiliateControl of the air flow through the tower to maintain reasonably constant cold water temperatures can be accomplished byturning off individual fans in multiple-cell installations adjusting the blade pitch or using variable speed motors To turn off individual fans remote at grade start-stop buttons for the motors are usually specified
Whenever the ambient air temperature is 32 degF (0 degC) or below icing of the tower louvers and structural members in contact withthe incoming air will occur If the heat load on the tower is relatively free of seasonal variations two-speed fans should provideadequate ice control down to approximately 0 degF (-18 degC) ambient temperatures When sustained temperatures of 0 degF (-18 degC)and below occur two-speed fans capable of two-speed reverse operation with appropriate time delays for change of directionand speed should be specified to periodically eliminate ice which will build up on the louvers Other methods of winterizationare available but in general are more expensive and require greater operator attention In very cold climates steam heatingshould be provided to prevent freezing in the cold water basin during shutdown periodsWater Side - The water circulation rate is maintained by operating one or more pumps of sufficient capacity to meet the systemdemand Flow-measuring facilities local temperature indicators and a high-temperature alarm are provided on the coolingtower water supply and return headers Hand-operated flow control valves are required for distributing the water equally to theindividual cells On counterflow towers provisions for measuring the flow to each cell (pitot tubes) are required They are notrequired on crossflow towers since the open distribution basin provides visual aid in balancing flows Shutoff valves should beprovided for servicing the flow control valves without shutting down the entire tower
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XXVII 22 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
STEP 5 - Provide chemical treatment facilities for the fresh water system
Alkalinity (pH) Control (if used)1 In cases where a controlled pH corrosionscale control program is being used the alkalinity (pH) of the recirculated cooling
water should be controlled in accordance with the program requirements Normally acid addition is required to accomplishthis This is due to free carbon dioxide being stripped from the cooling water in the tower to approximately 5 ppm For example if the alkalinity has to be controlled to between 25 and 50 ppm for a pH of 70 to 75 the volume of 98 sulfuricacid (66 degBeacute) required to treat the makeup water is calculated using the following equation
[Note 93 sulfuric acid should be used if ambient temperatures will be less than 35 degF (2 degC)]
GallonsHour 66 degBeacute H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate gpm) (326 x 10 -5)
For calculation in Metric limits
kgh of 100 (66 degBeacute) H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate Ls) (793 x 10 -3)
2 The metering pump should be designed for a maximum feed rate of two to three times the normal feed rate A glass woolfilter is required on the pump suction piping to minimize plugging Normally two - 100 capacity pumps are provided
Positive displacement metering pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumps with flow control areused for higher rates
3 Hydrochloric acid may also be used for alkalinity control The gallons per hour of 20 degBeacute hydrochloric acid required is36 times the amount of 66 degBeacute sulfuric acid In Metric units the weight of 32 (20 degBeacute) hydrochloric acid required is227 times the weight of 100 (66 degBeacute) sulfuric acid
4 Specifications for chemical feeders may be found in EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased)
5 A pH analyzer with high and low alarm is provided to control the amount of acid feed The acid should be added to thecooling tower makeup water (Figure 2 ) The lag time between the point of acid injection and the sample outlet to the pHcell should be less than two minutes for effective control
6 Spills or overflows from the sulfuric acid tank should be contained
Corrosion Inhibitors and Anti-Scalents1 Two chemical feeders are generally provided The following guides in sizing the equipment will allow the use of any of the
proprietary products from the vendors when final selection by the owner is made See Report No EE102E78 Cooling Tower Water Treatment Guidelines EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and Report No EE34E86 Environmentally - Acceptable Cooling Water Treatments for detailed discussion of acceptable commercial chemicals
bull If the water treatment program to be used is known design for the chemical rates specified by the vendor or COOLING WATER SYSTEMS SPECIALIST
bull Otherwise design for 100 wppm of each commercial liquid product in the recirculated cooling water The design feedrate should be based on the design total blowdown flow (both uncontrolled and controlled) since this is where theproduct is lost For 100 wppm concentration the chemical rates will be as follows
GallonsHour Chemical = Blowdown (gpm) x (0006)
LitersHour Chemical = Blowdown (Ls) x (036)
2 The design capacity of the metering pump should be two to three times this calculated number Normally 2 - 100capacity pumps are provided Positive displacement pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumpswith flow control are used for higher rates
3 Specifications for chemical feeders may be found in EMRErsquos Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased) Reference should also be made to the checklists containedin Report No EE92E94 Guidelines for Safety Evaluation of Chemical Injection Facilities
4 A final check should be made with the owner and the COOLING WATER SYSTEMS SPECIALISTS as to the treatmentprogram selected and if there is a need for an additional chemical feeder
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COOLING WATER SYSTEMS XXVII 23 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Biological ControlFacilities should be provided for feeding the biocide into the back of the cooling tower basin If chlorine is used it must bepresent in the cooling water as free residual to be effective However the following amounts of chlorine are first required toreact with the listed impurities when present in the cooling water before a free residual can be established
IMPURITIESAMOUNT OF CHLORINE REQUIRED (wppm)
WITH EACH WPPM OF IMPURITY
Ammonia (NH 3)
Hydrogen Sulfide (H 2S)
Sodium Sulfite (Na 2SO 3)
Ferrous Iron (Fe ++)
10
22
056
064
The design capacity of the chlorine feeder shall be based on achieving a 01 wppm free chlorine residual in the recirculatingwater flow or a minimum dosage of 7 wppm at maximum design makeup water flow rate and conditions (see EMRE Water and
Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service )Chlorine may be fed either as the commercial liquid sodium hypochlorite (124 wt Cl 2) or liquid chlorine (100 Cl 2) Liquidhypochlorite is being used increasingly because of the higher costs associated with precautions equipment and proceduresrequired to minimize the risks associated with handling gaseous chlorine The hypochlorite solution is fed using a meteringpump which should be sized to feed an equivalent of 15 ppm 100 Cl 2 (Refer to EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) ) In case of spills or an overflow from thestorage tank the sodium hypochlorite may be sent to the cooling tower basinGas chlorinators are covered by EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service The use of gas chlorination is being reduced but may still be appropriate in some locations subject tolocal management approval The gas from either commercial cylinders or an evaporator is dissolved in a water slip stream bymeans of an eductor This results in a chlorine solution strength of approximately 01 This solution should be evenlydistributed below the water surface by means of a header lateral pipe arrangement at the rear of the cooling tower basinopposite the cooling water pumpsSome additional guides on the use of gas chlorinators are given below It is also recommended that the safety guidelines
published by the Manufacturing Chemists Association and the Chlorine Institute be followedbull Dilution water for the chlorinator must not be less than 50 degF (10 degC) or ice formation may cause plugging problems
bull Continuous rates of chlorine gas withdrawal should not exceed 400 lb (180 kg)24 hours from each one ton cylinder atroom temperature [70 degF (21 degC)] This rate may be exceeded (up to 50) for periods not exceeding 2 hours Rule of thumb is not to design for more than 4 cylinders operating in parallel When feed rate exceeds 3000 lb (1360 kg) per daythe liquid chlorine should be fed to an evaporator upstream of the chlorinator
bull If the ambient temperature can be expected to be below 50 degF (10 degC) for extended periods a building should be providedfor the chlorinator and cylinders
bull Weighing scales should be provided to determine when shipping containers should be replaced and to verify chlorine feedrates
bull Analyzers to measure the free chlorine residual in the circulated cooling water should be provided
bull Chlorine leak detectors and a catch basin for major spills should be provided
Control SystemProvide automatic controllers to continuously monitor water condition and to add chemicals to the system (complete systemsmay be available through the water treatment program vendor as a lease or purchase)pH Control - This controller adds acid to correct water alkalinity a timer on the acid pump sounds an alarm if the pump doesnot shut off in a predetermined length of time Backup controls are required to prevent the cooling system from beingpermanently damaged in case the pH electrodes become fouledCorrosivity and Inhibitor Control - Systems are available which give a direct readout of corrosion rate in milsyear If thecorrosion rate exceeds a set point the controller shuts off the acid increases the inhibitor addition opens the blowdown valveand sounds an alarm Electrodes measure the completeness of the inhibitor film by the value of the potential differencebetween the electrodes The larger the difference the more incomplete the film and a potential reading exceeding a set levelactivates the inhibitor feed
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XXVII 24 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Free Residual Chlorine Control - Free residual chlorine should be controlled automatically for biocide treatment Severalgood analyzers are currently available These units can be incorporated into an automatic control system
Oil in Water DetectionMonitoring Corrosion and Deposit FormationThe following types of devices for measuring corrosion rates and deposit formation should be provided to allow the plant tomonitor the effectiveness of the cooling water treatment before scheduled turnaroundsCoupons - A corrosion coupon rack should be specified for the hot return water to the tower in accordance with IP 19-6-1 Corrosion Monitor - On-line corrosion analyzer if specified should monitor the hot return water to the tower in accordance withIP 19-6-1
On-Line Test Exchanger Although there are a variety of deposit monitors from the water treatment chemical vendors they do not necessarily representactual heat transfer conditions in process exchangers For this reason a test exchanger should be installed for evaluatingcorrosion and deposit formation Details are covered in Report No EE102E78 Cooling Tower Water Treatment Guidelines
Alternatively a critical process exchanger can be instrumented such that its heat transfer coefficient can be monitored
Instrumentation would include flows and inlet and outlet temperatures of process fluid and cooling waterSTEP 6 - Specify cooling tower location
Select a location for the cooling tower The following factors influence this selection
Location Factors1 The location should be within a reasonable distance of the units served and consistent with future refinery plans Piping
costs to and from battery limits can be a significant portion of the total cooling system cost and can be the location-determining factor particularly for salt water systems While piping costs can be easily determined other costs affectinglocation cannot be estimated readily Therefore judgment is required in selecting the location
2 Safety spacing requirements (see Section XV-G Equipment Spacing )3 Cooling tower contribution to plant boundary noise4 For salt water cooling towers wind direction and distance to other facilities is of major concern because of potential
corrosion problems
5 Factors affecting cooling tower performance ie orientation with respect to wind site congestion etc
Plot RequirementsIn estimating plot requirements (basin area) one should allow 5 to 6 ft of tower length for each 1000 gpm (2 m for each 60 Ls)of circulating capacity This figure is based on a typical counterflow design with a width of 50 ft (12 m) and does not includepump suction pit space Exact requirements can be obtained from cooling tower vendors after the tower capacity has beenestimated
Fogging and DriftTwo distinct problems are associated with the discharge air from a cooling tower (1) fogging which can be a nuisance or ahazard and (2) drift which is generally a nuisance because of increased corrosion of surrounding equipment Fogging fromcooling towers occurs during periods of combined high ambient humidity and low temperature as a result of mixing the warmmoist cooling tower exhaust air with the cooler ambient air The ambient air cannot absorb all moisture as vapor and thus tinyfog droplets are formed by condensation Fogging and resulting icing in cold climates has produced hazardous drivingconditions on highways up to 850 ft (250 m) awayProprietary designs are available to combat potential fogging problems Basically these designs incorporate heating of thedischarge air with hot return water precooling coils (similar to an air cooler) to eliminate any visible plume However thesedesigns are expensive and relatively unproven Therefore proper location based on reliable weather data is the preferredsolutionDrift from a cooling tower represents a more difficult problem since it does not depend on definable climatic conditions andoccurs for all wind directions The effects of drift on nearby equipment and vegetation are particularly severe and generally of concern only for salt water systems because of the high solids content of the drift (concentrated salt water) For salt water cooling systems electric lines substations and instrumentation should be located upwind in the direction of the predominantwind They should be provided with adequate shelter or materials when within 450 ft (140 m) of the cooling tower In additionspecifications for all equipment within 150 ft (45 m) of the tower should include surface treatment suitable for a marineenvironment
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Section Page
COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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XXVII 28 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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XXVII 30 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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COOLING WATER SYSTEMS XXVII 31 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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XXVII 32 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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COOLING WATER SYSTEMS XXVII 33 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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COOLING WATER SYSTEMS XXVII 35 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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COOLING WATER SYSTEMS XXVII 37 of 48
DESIGN PRACTICES December 2000
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FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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Section Page
COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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COOLING WATER SYSTEMS XXVII 17 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Figure 8B - A number of pumps in the same sump will operate best without separating walls unless all pumps are always inoperation at the same time in which case the use of separating walls may be beneficial If walls must be used for structural
purposes and pumps will operate intermittently one should leave flow space behind each wall from the pit floor up to at leastthe minimum water level and the wall should not extend upstream beyond the rim of the suction bell If walls are used increaseDimension ldquoSrdquo by the thickness of the wall for correct centerline spacing Round off the ends of the walls Not recommended isthe placement of a number of pumps around the edge of a sump with or without dividing wallsFigure 8C - Abrupt changes in size from inlet pipe or channel to pump bay are not desirable A relatively small pipe emptyinginto a large pump pit should connect to the pit with a gradually increasing taper section The angle should be as large aspossible preferably not less than 45 degrees With this arrangement pit velocities much less than one foot per second aredesirable Especially not recommended is a small pipe directly connected to a large pit with pumps close to the inlet Flow willhave excessive change of direction to get to most of the pumps Centering pumps in the pit leave large ldquovortex areasrdquo behindthe pumps which will cause operational troubleFigure 8D - If the pit velocity can be kept low enough [below 1 fts (03 ms)] an abrupt change from inlet pipe to pit can beaccommodated if the length equals or exceeds the values shown It is assumed that as ratio WP increases the inlet velocityat ldquoPrdquo will increase up to an allowed maximum of 8 fts (24 ms) at WP = 10 Pumps ldquoin linerdquo are not recommended unless theratio of pit to pump size is quite large and pumps are separated longitudinally by a generous margin A pit can generally be
constructed at much less cost by use of a recommended designFigure 8E - It is sometimes desirable to install pumps in tunnels or pipelines A drop pipe or false well to house the pump witha vaned inlet well facing upstream will be satisfactory in flows up to 8 fts (24 ms) Without the inlet well the pump section bellshould be positioned at least two pipe diameters vertically above the top of the tunnel not hung into the tunnel flow especiallywith tunnel velocities 2 fts (06 ms) or more There should be no signs of air along the top of the tunnel It may be necessaryto lower the scoop or to insist on a minimum water level in the vertical wellNote The foregoing statements apply to sumps for clear liquid
STEP 6 - Provide chemical treatment
Provide chemical treatment facilities for the once-through cooling water system Normally the only chemical treatment requiredin a once-through cooling system is for biological control Facilities for adding chlorine to once-through cooling water are thesame as those described for the recirculated cooling water system The design capacity of the chlorinator should be based onachieving a chlorine residual of 1 wppm in the effluent The chlorine required by the organics and other reducing agents in thewater must be satisfied before any residual chlorine will appear in the effluent The design capacity of the chlorinator should be
based on the total water chlorine demand plus 1 wppm in the effluent In the absence of water data the chlorination systemdesign capacity should be based on 10 wppm minimum instantaneous dosage for maximum cooling water flow For very largeonce-through cooling water systems it will be impractical to design for shock rates at this level In this case a continuousdosage of 1 - 2 wppm should be used (Refer to Step 5 of the DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS )
STEP 7 - Specify pumps and distribution system
Specify other major facilities in the once-through cooling water system The recommended materials for pumps valves andpiping are shown in the EMRErsquos Refinery Construction Materials Manual
Pumps1 Type - These pumps are usually vertical centrifugal pumps However horizontal centrifugal pumps have been used An
economic study andor consideration of existing facilities will determine which type should be specified In the event thathorizontal centrifugal pumps are specified facilities for pump priming will likely be required For large pumps in salt water service there is a very limited selection of pumps because of the special materials required Consult with the DISCIPLINESPECIALISTS early in the design effort to determine what specific pumps are currently available
2 Total Required Capacity - Firm capacity (excluding spares) of the supply pumps should equal the maximum cooling water requirements including LGF and RCF
3 NumberIndividual Pump Capacity - Normally one spare pump is provided for up to six operating pumps two spares for up to twelve operating pumps etc The most common arrangements for water supply pumps are
bull Two 100 capacity pumps
bull Three 50 capacity pumps
bull Four 33-13 capacity pumps
bull Five 25 capacity pumps
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XXVII 18 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Maximum commercially available pump capacity ranges up to 300000 gpm (20000 Ls) at discharge pressures up to 100psi (700 kPa) Maximum capacity will be less for higher discharge pressures Other factors which may influence pump
selection arebull Selecting capacity to match existing pumps
bull Selecting capacity for future requirements
bull Selecting capacity corresponding to manufacturers available standardVendor information should be obtained on the availability size and cost of the pumps for several configurations Acomparison should be made of the investments and operating costs as well as other advantages and disadvantages toselect a pump configuration
4 Drivers - These pumps are usually motor-driven Because of the remote location of the intake facilities the steamdistribution costs are normally too high to justify the use of a steam turbine driver However use of a secondary selectivesubstation provides a reliable source of power to the pump drivers
5 Pump ∆∆∆∆P - Pump ∆P is based on supplying the maximum cooling water requirement at the battery limit design pressureestablished for the system The ∆P should include the total circulating system pressure drop plus any elevation differencebetween the low water level at the intake and the discharge point of contact with air
6 Pump Instrumentation - The remote location of the intake facilities makes remote control and monitoring of the water supply operation desirable Pump controls located in the control house provide for minimum manning Local controlbackup should be provided The Owners operating and manning philosophy determine the basis for instrumentation andcontrolThe spare pump is controlled to automatically start on low pressure in the supply header Concurrently alarms should beactuated in the control room to indicate the low-pressure situation and spare pump operation
Distribution Network1 Configuration - The piping network is a once-through system with a return line from each consumer or consumer group to
the cooling water effluent line2 Layout - The routing of the piping network should connect all cooling water consumers in the most economical manner
permitted by the plant layout Careful consideration should be given to the turnaround philosophy of the plant Thenetwork layout should preclude the necessity of having above-ground water lines that are in service running through anarea that is being turned around Underground piping or an extra lateral may be required to accomplish this objective
3 Valves and Blinds - Battery limit valves should be provided in the network to isolate individual process units from thesystem Valves should be provided to permit onstream removal and maintenance of spared equipment
4 Basket Strainers - Consideration should be given to the installation of basket strainers in the laterals serving each of theconsumer loops These provide added insurance against entrance of foreign material into the cooling equipment The useof intake screens with 025 in (63 mm) openings may eliminate the need for basket strainers in the laterals
Line Sizing1 Design Basis - The network is sized so that water is delivered to the most distant consumer at a pressure equal to the
required process design (battery limit) pressure2 Flow Rate - Each segment of the distribution network is sized for the simultaneous maximum flow in that segment for any
operating condition The main supply and discharge headers are sized for the simultaneous maximum flow of all segments(not necessarily the sum of the simultaneous maximum flows of the individual segments)
3 Pressure Drop
Line Sizing Basis - The distribution system should be sized according to the guidelines established in Section XIV The use of an ldquoold piperdquo friction factor of C = 90 - 100 (Hazen-Williams factors) in the main lines is recommended to assuregood operation in later years when the system experiences fouling For large diameter lines [greater than 48 in (1200mm)] maximum water velocity should be limited to 10 fts (3 ms) based on civilmechanical considerations (supports andanchoring required to handle potential pressure surges) For salt water systems utilizing cement-lined pipe consider thereduced inner diameter in the friction loss calculations A study may be necessary to determine the economic pump ∆P vsline diameters
4 Equivalent Length Factors - Equivalent length factors are used in pressure drop calculations for sizing the distributionsystem The piping length scaled from the plot plan is multiplied by an equivalent length factor to compensate for pipingelbows expansion loops and other piping variations
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Section Page
COOLING WATER SYSTEMS XXVII 19 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
The equivalent length factors are selected based on the expected variation from a straight piping run Typical equivalentlength factors suitable for planning purposes for different piping applications are as follows
TYPE OF PIPING EQUIVALENT LENGTH FACTORS
Long piping runs [gt 1000 ft (300 m)] 12 to 14
Onsite distribution piping 15 to 20
For design purposes the actual piping layout should be evaluated to ensure that the equivalent length factors areapplicable to the specific situation Particular attention is required for revamp work and saltwater systems utilizing cement-lined pipe
STEP 8 - Prepare a system flow plan similar to the one shown in Figure 1
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS
STEP 1 - Determine cooling water requirements
Tabulate the cooling water requirements and cooling water discharge temperature for each process unit and offsite consumer Apply the proper LGFs to all cooling loads depending on the stage of the project and whether the new units are exactduplicates of existing units Determine the total cooling water flow required including LGFs and the bulk temperature (T 1) of thecombined cooling water flow from all coolers to the cooling towerIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The onsite and offsite designers should investigate methods to increase effluent water temperature to themaximum recommended limit for the particular type of water used This will result in a lower cost cooling tower for a given heatrejection rate One common method of maximizing return water temperature is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 2 - Select cooling tower design parameters
Select the proper design conditions of ambient wet bulb temperature approach temperature range and flow rate for specifyingthe cooling tower Based on these conditions the cooling tower vendor will size the tower and provide the tower characteristicand performance curves which serve as the basis for the performance guarantee Required performance test and evaluationprocedures are described in IP 8-1-1 Economic studies may be required to select the ambient wet bulb temperature andapproach temperature for new locations or for expansion projects with special cooling requirements
STEP 3 - Prepare cooling tower specification sheet
Include the following information in the heat exchange section of the design specificationTower Flow Capacity - This was determined in Step 1Type of Tower - The tower usually specified is an induced draft multicell type suitable for fresh salt or brackish water depending on the water available Additional tower characteristics are usually developed and proposed by the tower vendor (eg crossflow versus counterflow type of tower fill etc) The vendor should be made aware of conditions such as thelikelihood of heavy oil leakage into the cooling water or high fouling potential which may influence the tower type or fill
selectionNumber of Cells - The minimum number of cells required (normally not less than 2) may be specified The maximum flow per cell is a function of the cooling tower design conditions selected in Step 2 Typical values range up to 10000 gpm (600 Ls) for crossflow designs and 8000 gpm (500 Ls) for counterflow designs Normally spare cells are not provided Maintenance iscarried out during cooler periods or load is shed
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Section Page
XXVII 20 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Ambient Wet Bulb Temperature1 The ambient wet bulb temperature selected for specifying the cooling tower design is generally that temperature which is
equaled or exceeded 5 of the time during the four hottest months (5 of the hourly readings taken during the four hottestmonths will exceed this value) Specifying the 5 temperature should give satisfactory results since the time that the wetbulb temperature is exceeded is for short duration and the system inertia is usually sufficient to avoid problems The tower specification should make it clear that the wet bulb temperature specified is an ambient value as opposed to the enteringvalue which may be higher because of recirculation The tower manufacturer should adjust the ambient value upward totake into account his estimates of recirculation Currently cooling tower test codes provide procedures for measuringperformance in the case of either entering or ambient wet-bulb temperature specifications An increase of 2 degF (11 degC) isgenerally added to ambient wet bulb temperature to estimate entering wet bulb temperature for large cooling towers
2 If plant production will peak during the four summer months when the 5 ambient wet bulb temperature may be exceededconsideration should be given to specifying a higher design ambient wet bulb temperature (one that will be exceeded 212 of the time for example) However the incremental cost of the resulting closer approach and larger cooling tower must be justified by the value of the incremental products made
On-Tower Temperature (T1) - This temperature cannot be accurately determined until the design of all process coolers iscompleted and the volume of cooling water and outlet temperature are known for each (see Step 1) In the planning stage or early design stage of a project this temperature must be estimated and is usually set equal to the maximum design cooler outlet temperature shown under COOLING WATER REQUIREMENTS in the BASIC DESIGN CONSIDERATIONS portion of this sectionApproach Temperature - The approach temperature has the most pronounced effect on cooling tower size and cost For example reducing the approach from 10 to 5 degF (55 to 3 degC) can increase the cooling tower investment by 50 percent and thefan operating cost by 65 percent Thus various approach temperatures should be evaluated to determine the minimum cost of the total system including the process exchanger area Approaches used in the past designs have been from 5 to 12 degF (3 to7degC) Note that it is not customary in the cooling tower industry to guarantee approaches of less than 5 degF to (3 degC) to coolingtower design inlet wet bulb temperature A typical approach for a new grass roots cooling tower is 10 degF (6 degC)Off-Tower Temperature (T2) - This is the sum of the approach temperature and the ambient air wet bulb temperature For existing locations Table 2 in Section IX-B may be used
Range and Flow Rate1 The range for a given heat load is related to the circulating water rate as follows
Q = M C p ∆Tcong (500) (gpm) (range) Eq (1)
where Q = Heat load on the tower BtuhM = Water circulating rate lbhCp = Specific heat capacity of water at average temperature Btulb degF∆T = (T 1 ndash T 2) degF = (water cooling range)gpm = Circulation rate in US gallons per minute
For calculation in Metric units
Q = M C p ∆Tcong (1162 x 10 -3) (Ls) (range) Eq (1M)
where Q = Heat load on the tower kJh
M = Water circulating rate kghCp = Specific heat capacity of water at average temperature kJkg degC∆T = (T 1 ndash T 2) degC = (water cooling range)Ls = Circulation rate in liters per second
2 Generally lower tower investment fan horsepower and pump horsepower result from maximizing the range andminimizing the flow rate However based on fouling considerations of process coolers the maximum allowable hot water temperatures must be kept below recommended limits Also while increasing the range minimizes cooling tower investment and operating cost the cost of exchanger surface requirements is adversely affected by the LMTD and acompromise must be made The range is bounded by the maximum allowable hot water temperature and the Off-Tower temperature (T 2) For established locations refer to Section IX-B for design cold water temperatures to assist in selectingthe approach and range
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COOLING WATER SYSTEMS XXVII 21 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Makeup Water Requirements - Calculate the various losses in the cooling water system to determine makeup requirementsThese losses include
1 Evaporation Loss (E) - Allow 1 of the circulation rate for each 10 degF (55 degC) temperature drop through the tower Notethat the actual evaporation loss will be somewhat less depending on ambient air temperature
( )sL555
TT)gpm(
1000TT
E 2121
minus=
minus= Eq (2)
2 Drift Loss (D) - IP 8-1-1 limits the maximum allowable drift loss to 0005 of the design water circulation rate for freshbrackish or salt water
( )sL1000050
)gpm(1000050
D
=
= Eq (3)
This low drift level contributes little to solids removal from the tower with the spray This allows higher cycles of concentration to be achieved in cooling towers which use clarified softened water for makeup and thus have low requiredblowdown rates Higher drift loss percentages in older or poorly maintained towers will limit the maximum cycles of
concentration achievable3 Miscellaneous Losses (O) - Miscellaneous losses are estimated to be one-seventh of the difference between evaporation
loss and drift loss
minus=
7DE
O Eq (4)
4 Blowdown Rate (B) ndash Guidelines for maximum concentrations of impurities allowed in recirculated fresh cooling water areshown in Table 2 For salt water the blowdown is based on keeping the total dissolved salts below 55000 wppm Obtainthe chemical analysis of the makeup water and divide each individual component concentration into its counterpart inTable 2 The result is the maximum allowable cycles of concentration for that constituent (a worksheet is provided inTable 4 ) The impurity with the lowest factor will be the control and will determine the required blowdown rate
)OD(1Cycles
EB +minus
minus= Eq (5)
The makeup water requirement is the sum of all losses shown above
MakeUp = E + D + O + B Eq (6)
STEP 4 - Specify the controls necessary for proper cooling tower operation
Air Side - Cooling towers are provided with motor-driven manually adjusted pitch multiple-bladed propeller fans Figures9A amp B may be used to estimate the normal operating horsepower requirements Since the fan motor and gear reducer arelocated at the top of the tower [generally 40 to 50 ft (12 to 15 m) above grade] the need for equipment to handle thesecomponents should be discussed with the affiliateControl of the air flow through the tower to maintain reasonably constant cold water temperatures can be accomplished byturning off individual fans in multiple-cell installations adjusting the blade pitch or using variable speed motors To turn off individual fans remote at grade start-stop buttons for the motors are usually specified
Whenever the ambient air temperature is 32 degF (0 degC) or below icing of the tower louvers and structural members in contact withthe incoming air will occur If the heat load on the tower is relatively free of seasonal variations two-speed fans should provideadequate ice control down to approximately 0 degF (-18 degC) ambient temperatures When sustained temperatures of 0 degF (-18 degC)and below occur two-speed fans capable of two-speed reverse operation with appropriate time delays for change of directionand speed should be specified to periodically eliminate ice which will build up on the louvers Other methods of winterizationare available but in general are more expensive and require greater operator attention In very cold climates steam heatingshould be provided to prevent freezing in the cold water basin during shutdown periodsWater Side - The water circulation rate is maintained by operating one or more pumps of sufficient capacity to meet the systemdemand Flow-measuring facilities local temperature indicators and a high-temperature alarm are provided on the coolingtower water supply and return headers Hand-operated flow control valves are required for distributing the water equally to theindividual cells On counterflow towers provisions for measuring the flow to each cell (pitot tubes) are required They are notrequired on crossflow towers since the open distribution basin provides visual aid in balancing flows Shutoff valves should beprovided for servicing the flow control valves without shutting down the entire tower
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XXVII 22 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
STEP 5 - Provide chemical treatment facilities for the fresh water system
Alkalinity (pH) Control (if used)1 In cases where a controlled pH corrosionscale control program is being used the alkalinity (pH) of the recirculated cooling
water should be controlled in accordance with the program requirements Normally acid addition is required to accomplishthis This is due to free carbon dioxide being stripped from the cooling water in the tower to approximately 5 ppm For example if the alkalinity has to be controlled to between 25 and 50 ppm for a pH of 70 to 75 the volume of 98 sulfuricacid (66 degBeacute) required to treat the makeup water is calculated using the following equation
[Note 93 sulfuric acid should be used if ambient temperatures will be less than 35 degF (2 degC)]
GallonsHour 66 degBeacute H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate gpm) (326 x 10 -5)
For calculation in Metric limits
kgh of 100 (66 degBeacute) H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate Ls) (793 x 10 -3)
2 The metering pump should be designed for a maximum feed rate of two to three times the normal feed rate A glass woolfilter is required on the pump suction piping to minimize plugging Normally two - 100 capacity pumps are provided
Positive displacement metering pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumps with flow control areused for higher rates
3 Hydrochloric acid may also be used for alkalinity control The gallons per hour of 20 degBeacute hydrochloric acid required is36 times the amount of 66 degBeacute sulfuric acid In Metric units the weight of 32 (20 degBeacute) hydrochloric acid required is227 times the weight of 100 (66 degBeacute) sulfuric acid
4 Specifications for chemical feeders may be found in EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased)
5 A pH analyzer with high and low alarm is provided to control the amount of acid feed The acid should be added to thecooling tower makeup water (Figure 2 ) The lag time between the point of acid injection and the sample outlet to the pHcell should be less than two minutes for effective control
6 Spills or overflows from the sulfuric acid tank should be contained
Corrosion Inhibitors and Anti-Scalents1 Two chemical feeders are generally provided The following guides in sizing the equipment will allow the use of any of the
proprietary products from the vendors when final selection by the owner is made See Report No EE102E78 Cooling Tower Water Treatment Guidelines EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and Report No EE34E86 Environmentally - Acceptable Cooling Water Treatments for detailed discussion of acceptable commercial chemicals
bull If the water treatment program to be used is known design for the chemical rates specified by the vendor or COOLING WATER SYSTEMS SPECIALIST
bull Otherwise design for 100 wppm of each commercial liquid product in the recirculated cooling water The design feedrate should be based on the design total blowdown flow (both uncontrolled and controlled) since this is where theproduct is lost For 100 wppm concentration the chemical rates will be as follows
GallonsHour Chemical = Blowdown (gpm) x (0006)
LitersHour Chemical = Blowdown (Ls) x (036)
2 The design capacity of the metering pump should be two to three times this calculated number Normally 2 - 100capacity pumps are provided Positive displacement pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumpswith flow control are used for higher rates
3 Specifications for chemical feeders may be found in EMRErsquos Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased) Reference should also be made to the checklists containedin Report No EE92E94 Guidelines for Safety Evaluation of Chemical Injection Facilities
4 A final check should be made with the owner and the COOLING WATER SYSTEMS SPECIALISTS as to the treatmentprogram selected and if there is a need for an additional chemical feeder
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Section Page
COOLING WATER SYSTEMS XXVII 23 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Biological ControlFacilities should be provided for feeding the biocide into the back of the cooling tower basin If chlorine is used it must bepresent in the cooling water as free residual to be effective However the following amounts of chlorine are first required toreact with the listed impurities when present in the cooling water before a free residual can be established
IMPURITIESAMOUNT OF CHLORINE REQUIRED (wppm)
WITH EACH WPPM OF IMPURITY
Ammonia (NH 3)
Hydrogen Sulfide (H 2S)
Sodium Sulfite (Na 2SO 3)
Ferrous Iron (Fe ++)
10
22
056
064
The design capacity of the chlorine feeder shall be based on achieving a 01 wppm free chlorine residual in the recirculatingwater flow or a minimum dosage of 7 wppm at maximum design makeup water flow rate and conditions (see EMRE Water and
Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service )Chlorine may be fed either as the commercial liquid sodium hypochlorite (124 wt Cl 2) or liquid chlorine (100 Cl 2) Liquidhypochlorite is being used increasingly because of the higher costs associated with precautions equipment and proceduresrequired to minimize the risks associated with handling gaseous chlorine The hypochlorite solution is fed using a meteringpump which should be sized to feed an equivalent of 15 ppm 100 Cl 2 (Refer to EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) ) In case of spills or an overflow from thestorage tank the sodium hypochlorite may be sent to the cooling tower basinGas chlorinators are covered by EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service The use of gas chlorination is being reduced but may still be appropriate in some locations subject tolocal management approval The gas from either commercial cylinders or an evaporator is dissolved in a water slip stream bymeans of an eductor This results in a chlorine solution strength of approximately 01 This solution should be evenlydistributed below the water surface by means of a header lateral pipe arrangement at the rear of the cooling tower basinopposite the cooling water pumpsSome additional guides on the use of gas chlorinators are given below It is also recommended that the safety guidelines
published by the Manufacturing Chemists Association and the Chlorine Institute be followedbull Dilution water for the chlorinator must not be less than 50 degF (10 degC) or ice formation may cause plugging problems
bull Continuous rates of chlorine gas withdrawal should not exceed 400 lb (180 kg)24 hours from each one ton cylinder atroom temperature [70 degF (21 degC)] This rate may be exceeded (up to 50) for periods not exceeding 2 hours Rule of thumb is not to design for more than 4 cylinders operating in parallel When feed rate exceeds 3000 lb (1360 kg) per daythe liquid chlorine should be fed to an evaporator upstream of the chlorinator
bull If the ambient temperature can be expected to be below 50 degF (10 degC) for extended periods a building should be providedfor the chlorinator and cylinders
bull Weighing scales should be provided to determine when shipping containers should be replaced and to verify chlorine feedrates
bull Analyzers to measure the free chlorine residual in the circulated cooling water should be provided
bull Chlorine leak detectors and a catch basin for major spills should be provided
Control SystemProvide automatic controllers to continuously monitor water condition and to add chemicals to the system (complete systemsmay be available through the water treatment program vendor as a lease or purchase)pH Control - This controller adds acid to correct water alkalinity a timer on the acid pump sounds an alarm if the pump doesnot shut off in a predetermined length of time Backup controls are required to prevent the cooling system from beingpermanently damaged in case the pH electrodes become fouledCorrosivity and Inhibitor Control - Systems are available which give a direct readout of corrosion rate in milsyear If thecorrosion rate exceeds a set point the controller shuts off the acid increases the inhibitor addition opens the blowdown valveand sounds an alarm Electrodes measure the completeness of the inhibitor film by the value of the potential differencebetween the electrodes The larger the difference the more incomplete the film and a potential reading exceeding a set levelactivates the inhibitor feed
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XXVII 24 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Free Residual Chlorine Control - Free residual chlorine should be controlled automatically for biocide treatment Severalgood analyzers are currently available These units can be incorporated into an automatic control system
Oil in Water DetectionMonitoring Corrosion and Deposit FormationThe following types of devices for measuring corrosion rates and deposit formation should be provided to allow the plant tomonitor the effectiveness of the cooling water treatment before scheduled turnaroundsCoupons - A corrosion coupon rack should be specified for the hot return water to the tower in accordance with IP 19-6-1 Corrosion Monitor - On-line corrosion analyzer if specified should monitor the hot return water to the tower in accordance withIP 19-6-1
On-Line Test Exchanger Although there are a variety of deposit monitors from the water treatment chemical vendors they do not necessarily representactual heat transfer conditions in process exchangers For this reason a test exchanger should be installed for evaluatingcorrosion and deposit formation Details are covered in Report No EE102E78 Cooling Tower Water Treatment Guidelines
Alternatively a critical process exchanger can be instrumented such that its heat transfer coefficient can be monitored
Instrumentation would include flows and inlet and outlet temperatures of process fluid and cooling waterSTEP 6 - Specify cooling tower location
Select a location for the cooling tower The following factors influence this selection
Location Factors1 The location should be within a reasonable distance of the units served and consistent with future refinery plans Piping
costs to and from battery limits can be a significant portion of the total cooling system cost and can be the location-determining factor particularly for salt water systems While piping costs can be easily determined other costs affectinglocation cannot be estimated readily Therefore judgment is required in selecting the location
2 Safety spacing requirements (see Section XV-G Equipment Spacing )3 Cooling tower contribution to plant boundary noise4 For salt water cooling towers wind direction and distance to other facilities is of major concern because of potential
corrosion problems
5 Factors affecting cooling tower performance ie orientation with respect to wind site congestion etc
Plot RequirementsIn estimating plot requirements (basin area) one should allow 5 to 6 ft of tower length for each 1000 gpm (2 m for each 60 Ls)of circulating capacity This figure is based on a typical counterflow design with a width of 50 ft (12 m) and does not includepump suction pit space Exact requirements can be obtained from cooling tower vendors after the tower capacity has beenestimated
Fogging and DriftTwo distinct problems are associated with the discharge air from a cooling tower (1) fogging which can be a nuisance or ahazard and (2) drift which is generally a nuisance because of increased corrosion of surrounding equipment Fogging fromcooling towers occurs during periods of combined high ambient humidity and low temperature as a result of mixing the warmmoist cooling tower exhaust air with the cooler ambient air The ambient air cannot absorb all moisture as vapor and thus tinyfog droplets are formed by condensation Fogging and resulting icing in cold climates has produced hazardous drivingconditions on highways up to 850 ft (250 m) awayProprietary designs are available to combat potential fogging problems Basically these designs incorporate heating of thedischarge air with hot return water precooling coils (similar to an air cooler) to eliminate any visible plume However thesedesigns are expensive and relatively unproven Therefore proper location based on reliable weather data is the preferredsolutionDrift from a cooling tower represents a more difficult problem since it does not depend on definable climatic conditions andoccurs for all wind directions The effects of drift on nearby equipment and vegetation are particularly severe and generally of concern only for salt water systems because of the high solids content of the drift (concentrated salt water) For salt water cooling systems electric lines substations and instrumentation should be located upwind in the direction of the predominantwind They should be provided with adequate shelter or materials when within 450 ft (140 m) of the cooling tower In additionspecifications for all equipment within 150 ft (45 m) of the tower should include surface treatment suitable for a marineenvironment
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COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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XXVII 28 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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XXVII 30 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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COOLING WATER SYSTEMS XXVII 31 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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XXVII 32 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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COOLING WATER SYSTEMS XXVII 33 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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COOLING WATER SYSTEMS XXVII 35 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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COOLING WATER SYSTEMS XXVII 37 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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Section Page
XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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XXVII 18 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
Maximum commercially available pump capacity ranges up to 300000 gpm (20000 Ls) at discharge pressures up to 100psi (700 kPa) Maximum capacity will be less for higher discharge pressures Other factors which may influence pump
selection arebull Selecting capacity to match existing pumps
bull Selecting capacity for future requirements
bull Selecting capacity corresponding to manufacturers available standardVendor information should be obtained on the availability size and cost of the pumps for several configurations Acomparison should be made of the investments and operating costs as well as other advantages and disadvantages toselect a pump configuration
4 Drivers - These pumps are usually motor-driven Because of the remote location of the intake facilities the steamdistribution costs are normally too high to justify the use of a steam turbine driver However use of a secondary selectivesubstation provides a reliable source of power to the pump drivers
5 Pump ∆∆∆∆P - Pump ∆P is based on supplying the maximum cooling water requirement at the battery limit design pressureestablished for the system The ∆P should include the total circulating system pressure drop plus any elevation differencebetween the low water level at the intake and the discharge point of contact with air
6 Pump Instrumentation - The remote location of the intake facilities makes remote control and monitoring of the water supply operation desirable Pump controls located in the control house provide for minimum manning Local controlbackup should be provided The Owners operating and manning philosophy determine the basis for instrumentation andcontrolThe spare pump is controlled to automatically start on low pressure in the supply header Concurrently alarms should beactuated in the control room to indicate the low-pressure situation and spare pump operation
Distribution Network1 Configuration - The piping network is a once-through system with a return line from each consumer or consumer group to
the cooling water effluent line2 Layout - The routing of the piping network should connect all cooling water consumers in the most economical manner
permitted by the plant layout Careful consideration should be given to the turnaround philosophy of the plant Thenetwork layout should preclude the necessity of having above-ground water lines that are in service running through anarea that is being turned around Underground piping or an extra lateral may be required to accomplish this objective
3 Valves and Blinds - Battery limit valves should be provided in the network to isolate individual process units from thesystem Valves should be provided to permit onstream removal and maintenance of spared equipment
4 Basket Strainers - Consideration should be given to the installation of basket strainers in the laterals serving each of theconsumer loops These provide added insurance against entrance of foreign material into the cooling equipment The useof intake screens with 025 in (63 mm) openings may eliminate the need for basket strainers in the laterals
Line Sizing1 Design Basis - The network is sized so that water is delivered to the most distant consumer at a pressure equal to the
required process design (battery limit) pressure2 Flow Rate - Each segment of the distribution network is sized for the simultaneous maximum flow in that segment for any
operating condition The main supply and discharge headers are sized for the simultaneous maximum flow of all segments(not necessarily the sum of the simultaneous maximum flows of the individual segments)
3 Pressure Drop
Line Sizing Basis - The distribution system should be sized according to the guidelines established in Section XIV The use of an ldquoold piperdquo friction factor of C = 90 - 100 (Hazen-Williams factors) in the main lines is recommended to assuregood operation in later years when the system experiences fouling For large diameter lines [greater than 48 in (1200mm)] maximum water velocity should be limited to 10 fts (3 ms) based on civilmechanical considerations (supports andanchoring required to handle potential pressure surges) For salt water systems utilizing cement-lined pipe consider thereduced inner diameter in the friction loss calculations A study may be necessary to determine the economic pump ∆P vsline diameters
4 Equivalent Length Factors - Equivalent length factors are used in pressure drop calculations for sizing the distributionsystem The piping length scaled from the plot plan is multiplied by an equivalent length factor to compensate for pipingelbows expansion loops and other piping variations
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COOLING WATER SYSTEMS XXVII 19 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
The equivalent length factors are selected based on the expected variation from a straight piping run Typical equivalentlength factors suitable for planning purposes for different piping applications are as follows
TYPE OF PIPING EQUIVALENT LENGTH FACTORS
Long piping runs [gt 1000 ft (300 m)] 12 to 14
Onsite distribution piping 15 to 20
For design purposes the actual piping layout should be evaluated to ensure that the equivalent length factors areapplicable to the specific situation Particular attention is required for revamp work and saltwater systems utilizing cement-lined pipe
STEP 8 - Prepare a system flow plan similar to the one shown in Figure 1
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS
STEP 1 - Determine cooling water requirements
Tabulate the cooling water requirements and cooling water discharge temperature for each process unit and offsite consumer Apply the proper LGFs to all cooling loads depending on the stage of the project and whether the new units are exactduplicates of existing units Determine the total cooling water flow required including LGFs and the bulk temperature (T 1) of thecombined cooling water flow from all coolers to the cooling towerIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The onsite and offsite designers should investigate methods to increase effluent water temperature to themaximum recommended limit for the particular type of water used This will result in a lower cost cooling tower for a given heatrejection rate One common method of maximizing return water temperature is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 2 - Select cooling tower design parameters
Select the proper design conditions of ambient wet bulb temperature approach temperature range and flow rate for specifyingthe cooling tower Based on these conditions the cooling tower vendor will size the tower and provide the tower characteristicand performance curves which serve as the basis for the performance guarantee Required performance test and evaluationprocedures are described in IP 8-1-1 Economic studies may be required to select the ambient wet bulb temperature andapproach temperature for new locations or for expansion projects with special cooling requirements
STEP 3 - Prepare cooling tower specification sheet
Include the following information in the heat exchange section of the design specificationTower Flow Capacity - This was determined in Step 1Type of Tower - The tower usually specified is an induced draft multicell type suitable for fresh salt or brackish water depending on the water available Additional tower characteristics are usually developed and proposed by the tower vendor (eg crossflow versus counterflow type of tower fill etc) The vendor should be made aware of conditions such as thelikelihood of heavy oil leakage into the cooling water or high fouling potential which may influence the tower type or fill
selectionNumber of Cells - The minimum number of cells required (normally not less than 2) may be specified The maximum flow per cell is a function of the cooling tower design conditions selected in Step 2 Typical values range up to 10000 gpm (600 Ls) for crossflow designs and 8000 gpm (500 Ls) for counterflow designs Normally spare cells are not provided Maintenance iscarried out during cooler periods or load is shed
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XXVII 20 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Ambient Wet Bulb Temperature1 The ambient wet bulb temperature selected for specifying the cooling tower design is generally that temperature which is
equaled or exceeded 5 of the time during the four hottest months (5 of the hourly readings taken during the four hottestmonths will exceed this value) Specifying the 5 temperature should give satisfactory results since the time that the wetbulb temperature is exceeded is for short duration and the system inertia is usually sufficient to avoid problems The tower specification should make it clear that the wet bulb temperature specified is an ambient value as opposed to the enteringvalue which may be higher because of recirculation The tower manufacturer should adjust the ambient value upward totake into account his estimates of recirculation Currently cooling tower test codes provide procedures for measuringperformance in the case of either entering or ambient wet-bulb temperature specifications An increase of 2 degF (11 degC) isgenerally added to ambient wet bulb temperature to estimate entering wet bulb temperature for large cooling towers
2 If plant production will peak during the four summer months when the 5 ambient wet bulb temperature may be exceededconsideration should be given to specifying a higher design ambient wet bulb temperature (one that will be exceeded 212 of the time for example) However the incremental cost of the resulting closer approach and larger cooling tower must be justified by the value of the incremental products made
On-Tower Temperature (T1) - This temperature cannot be accurately determined until the design of all process coolers iscompleted and the volume of cooling water and outlet temperature are known for each (see Step 1) In the planning stage or early design stage of a project this temperature must be estimated and is usually set equal to the maximum design cooler outlet temperature shown under COOLING WATER REQUIREMENTS in the BASIC DESIGN CONSIDERATIONS portion of this sectionApproach Temperature - The approach temperature has the most pronounced effect on cooling tower size and cost For example reducing the approach from 10 to 5 degF (55 to 3 degC) can increase the cooling tower investment by 50 percent and thefan operating cost by 65 percent Thus various approach temperatures should be evaluated to determine the minimum cost of the total system including the process exchanger area Approaches used in the past designs have been from 5 to 12 degF (3 to7degC) Note that it is not customary in the cooling tower industry to guarantee approaches of less than 5 degF to (3 degC) to coolingtower design inlet wet bulb temperature A typical approach for a new grass roots cooling tower is 10 degF (6 degC)Off-Tower Temperature (T2) - This is the sum of the approach temperature and the ambient air wet bulb temperature For existing locations Table 2 in Section IX-B may be used
Range and Flow Rate1 The range for a given heat load is related to the circulating water rate as follows
Q = M C p ∆Tcong (500) (gpm) (range) Eq (1)
where Q = Heat load on the tower BtuhM = Water circulating rate lbhCp = Specific heat capacity of water at average temperature Btulb degF∆T = (T 1 ndash T 2) degF = (water cooling range)gpm = Circulation rate in US gallons per minute
For calculation in Metric units
Q = M C p ∆Tcong (1162 x 10 -3) (Ls) (range) Eq (1M)
where Q = Heat load on the tower kJh
M = Water circulating rate kghCp = Specific heat capacity of water at average temperature kJkg degC∆T = (T 1 ndash T 2) degC = (water cooling range)Ls = Circulation rate in liters per second
2 Generally lower tower investment fan horsepower and pump horsepower result from maximizing the range andminimizing the flow rate However based on fouling considerations of process coolers the maximum allowable hot water temperatures must be kept below recommended limits Also while increasing the range minimizes cooling tower investment and operating cost the cost of exchanger surface requirements is adversely affected by the LMTD and acompromise must be made The range is bounded by the maximum allowable hot water temperature and the Off-Tower temperature (T 2) For established locations refer to Section IX-B for design cold water temperatures to assist in selectingthe approach and range
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Section Page
COOLING WATER SYSTEMS XXVII 21 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Makeup Water Requirements - Calculate the various losses in the cooling water system to determine makeup requirementsThese losses include
1 Evaporation Loss (E) - Allow 1 of the circulation rate for each 10 degF (55 degC) temperature drop through the tower Notethat the actual evaporation loss will be somewhat less depending on ambient air temperature
( )sL555
TT)gpm(
1000TT
E 2121
minus=
minus= Eq (2)
2 Drift Loss (D) - IP 8-1-1 limits the maximum allowable drift loss to 0005 of the design water circulation rate for freshbrackish or salt water
( )sL1000050
)gpm(1000050
D
=
= Eq (3)
This low drift level contributes little to solids removal from the tower with the spray This allows higher cycles of concentration to be achieved in cooling towers which use clarified softened water for makeup and thus have low requiredblowdown rates Higher drift loss percentages in older or poorly maintained towers will limit the maximum cycles of
concentration achievable3 Miscellaneous Losses (O) - Miscellaneous losses are estimated to be one-seventh of the difference between evaporation
loss and drift loss
minus=
7DE
O Eq (4)
4 Blowdown Rate (B) ndash Guidelines for maximum concentrations of impurities allowed in recirculated fresh cooling water areshown in Table 2 For salt water the blowdown is based on keeping the total dissolved salts below 55000 wppm Obtainthe chemical analysis of the makeup water and divide each individual component concentration into its counterpart inTable 2 The result is the maximum allowable cycles of concentration for that constituent (a worksheet is provided inTable 4 ) The impurity with the lowest factor will be the control and will determine the required blowdown rate
)OD(1Cycles
EB +minus
minus= Eq (5)
The makeup water requirement is the sum of all losses shown above
MakeUp = E + D + O + B Eq (6)
STEP 4 - Specify the controls necessary for proper cooling tower operation
Air Side - Cooling towers are provided with motor-driven manually adjusted pitch multiple-bladed propeller fans Figures9A amp B may be used to estimate the normal operating horsepower requirements Since the fan motor and gear reducer arelocated at the top of the tower [generally 40 to 50 ft (12 to 15 m) above grade] the need for equipment to handle thesecomponents should be discussed with the affiliateControl of the air flow through the tower to maintain reasonably constant cold water temperatures can be accomplished byturning off individual fans in multiple-cell installations adjusting the blade pitch or using variable speed motors To turn off individual fans remote at grade start-stop buttons for the motors are usually specified
Whenever the ambient air temperature is 32 degF (0 degC) or below icing of the tower louvers and structural members in contact withthe incoming air will occur If the heat load on the tower is relatively free of seasonal variations two-speed fans should provideadequate ice control down to approximately 0 degF (-18 degC) ambient temperatures When sustained temperatures of 0 degF (-18 degC)and below occur two-speed fans capable of two-speed reverse operation with appropriate time delays for change of directionand speed should be specified to periodically eliminate ice which will build up on the louvers Other methods of winterizationare available but in general are more expensive and require greater operator attention In very cold climates steam heatingshould be provided to prevent freezing in the cold water basin during shutdown periodsWater Side - The water circulation rate is maintained by operating one or more pumps of sufficient capacity to meet the systemdemand Flow-measuring facilities local temperature indicators and a high-temperature alarm are provided on the coolingtower water supply and return headers Hand-operated flow control valves are required for distributing the water equally to theindividual cells On counterflow towers provisions for measuring the flow to each cell (pitot tubes) are required They are notrequired on crossflow towers since the open distribution basin provides visual aid in balancing flows Shutoff valves should beprovided for servicing the flow control valves without shutting down the entire tower
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XXVII 22 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
STEP 5 - Provide chemical treatment facilities for the fresh water system
Alkalinity (pH) Control (if used)1 In cases where a controlled pH corrosionscale control program is being used the alkalinity (pH) of the recirculated cooling
water should be controlled in accordance with the program requirements Normally acid addition is required to accomplishthis This is due to free carbon dioxide being stripped from the cooling water in the tower to approximately 5 ppm For example if the alkalinity has to be controlled to between 25 and 50 ppm for a pH of 70 to 75 the volume of 98 sulfuricacid (66 degBeacute) required to treat the makeup water is calculated using the following equation
[Note 93 sulfuric acid should be used if ambient temperatures will be less than 35 degF (2 degC)]
GallonsHour 66 degBeacute H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate gpm) (326 x 10 -5)
For calculation in Metric limits
kgh of 100 (66 degBeacute) H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate Ls) (793 x 10 -3)
2 The metering pump should be designed for a maximum feed rate of two to three times the normal feed rate A glass woolfilter is required on the pump suction piping to minimize plugging Normally two - 100 capacity pumps are provided
Positive displacement metering pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumps with flow control areused for higher rates
3 Hydrochloric acid may also be used for alkalinity control The gallons per hour of 20 degBeacute hydrochloric acid required is36 times the amount of 66 degBeacute sulfuric acid In Metric units the weight of 32 (20 degBeacute) hydrochloric acid required is227 times the weight of 100 (66 degBeacute) sulfuric acid
4 Specifications for chemical feeders may be found in EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased)
5 A pH analyzer with high and low alarm is provided to control the amount of acid feed The acid should be added to thecooling tower makeup water (Figure 2 ) The lag time between the point of acid injection and the sample outlet to the pHcell should be less than two minutes for effective control
6 Spills or overflows from the sulfuric acid tank should be contained
Corrosion Inhibitors and Anti-Scalents1 Two chemical feeders are generally provided The following guides in sizing the equipment will allow the use of any of the
proprietary products from the vendors when final selection by the owner is made See Report No EE102E78 Cooling Tower Water Treatment Guidelines EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and Report No EE34E86 Environmentally - Acceptable Cooling Water Treatments for detailed discussion of acceptable commercial chemicals
bull If the water treatment program to be used is known design for the chemical rates specified by the vendor or COOLING WATER SYSTEMS SPECIALIST
bull Otherwise design for 100 wppm of each commercial liquid product in the recirculated cooling water The design feedrate should be based on the design total blowdown flow (both uncontrolled and controlled) since this is where theproduct is lost For 100 wppm concentration the chemical rates will be as follows
GallonsHour Chemical = Blowdown (gpm) x (0006)
LitersHour Chemical = Blowdown (Ls) x (036)
2 The design capacity of the metering pump should be two to three times this calculated number Normally 2 - 100capacity pumps are provided Positive displacement pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumpswith flow control are used for higher rates
3 Specifications for chemical feeders may be found in EMRErsquos Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased) Reference should also be made to the checklists containedin Report No EE92E94 Guidelines for Safety Evaluation of Chemical Injection Facilities
4 A final check should be made with the owner and the COOLING WATER SYSTEMS SPECIALISTS as to the treatmentprogram selected and if there is a need for an additional chemical feeder
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COOLING WATER SYSTEMS XXVII 23 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Biological ControlFacilities should be provided for feeding the biocide into the back of the cooling tower basin If chlorine is used it must bepresent in the cooling water as free residual to be effective However the following amounts of chlorine are first required toreact with the listed impurities when present in the cooling water before a free residual can be established
IMPURITIESAMOUNT OF CHLORINE REQUIRED (wppm)
WITH EACH WPPM OF IMPURITY
Ammonia (NH 3)
Hydrogen Sulfide (H 2S)
Sodium Sulfite (Na 2SO 3)
Ferrous Iron (Fe ++)
10
22
056
064
The design capacity of the chlorine feeder shall be based on achieving a 01 wppm free chlorine residual in the recirculatingwater flow or a minimum dosage of 7 wppm at maximum design makeup water flow rate and conditions (see EMRE Water and
Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service )Chlorine may be fed either as the commercial liquid sodium hypochlorite (124 wt Cl 2) or liquid chlorine (100 Cl 2) Liquidhypochlorite is being used increasingly because of the higher costs associated with precautions equipment and proceduresrequired to minimize the risks associated with handling gaseous chlorine The hypochlorite solution is fed using a meteringpump which should be sized to feed an equivalent of 15 ppm 100 Cl 2 (Refer to EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) ) In case of spills or an overflow from thestorage tank the sodium hypochlorite may be sent to the cooling tower basinGas chlorinators are covered by EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service The use of gas chlorination is being reduced but may still be appropriate in some locations subject tolocal management approval The gas from either commercial cylinders or an evaporator is dissolved in a water slip stream bymeans of an eductor This results in a chlorine solution strength of approximately 01 This solution should be evenlydistributed below the water surface by means of a header lateral pipe arrangement at the rear of the cooling tower basinopposite the cooling water pumpsSome additional guides on the use of gas chlorinators are given below It is also recommended that the safety guidelines
published by the Manufacturing Chemists Association and the Chlorine Institute be followedbull Dilution water for the chlorinator must not be less than 50 degF (10 degC) or ice formation may cause plugging problems
bull Continuous rates of chlorine gas withdrawal should not exceed 400 lb (180 kg)24 hours from each one ton cylinder atroom temperature [70 degF (21 degC)] This rate may be exceeded (up to 50) for periods not exceeding 2 hours Rule of thumb is not to design for more than 4 cylinders operating in parallel When feed rate exceeds 3000 lb (1360 kg) per daythe liquid chlorine should be fed to an evaporator upstream of the chlorinator
bull If the ambient temperature can be expected to be below 50 degF (10 degC) for extended periods a building should be providedfor the chlorinator and cylinders
bull Weighing scales should be provided to determine when shipping containers should be replaced and to verify chlorine feedrates
bull Analyzers to measure the free chlorine residual in the circulated cooling water should be provided
bull Chlorine leak detectors and a catch basin for major spills should be provided
Control SystemProvide automatic controllers to continuously monitor water condition and to add chemicals to the system (complete systemsmay be available through the water treatment program vendor as a lease or purchase)pH Control - This controller adds acid to correct water alkalinity a timer on the acid pump sounds an alarm if the pump doesnot shut off in a predetermined length of time Backup controls are required to prevent the cooling system from beingpermanently damaged in case the pH electrodes become fouledCorrosivity and Inhibitor Control - Systems are available which give a direct readout of corrosion rate in milsyear If thecorrosion rate exceeds a set point the controller shuts off the acid increases the inhibitor addition opens the blowdown valveand sounds an alarm Electrodes measure the completeness of the inhibitor film by the value of the potential differencebetween the electrodes The larger the difference the more incomplete the film and a potential reading exceeding a set levelactivates the inhibitor feed
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XXVII 24 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Free Residual Chlorine Control - Free residual chlorine should be controlled automatically for biocide treatment Severalgood analyzers are currently available These units can be incorporated into an automatic control system
Oil in Water DetectionMonitoring Corrosion and Deposit FormationThe following types of devices for measuring corrosion rates and deposit formation should be provided to allow the plant tomonitor the effectiveness of the cooling water treatment before scheduled turnaroundsCoupons - A corrosion coupon rack should be specified for the hot return water to the tower in accordance with IP 19-6-1 Corrosion Monitor - On-line corrosion analyzer if specified should monitor the hot return water to the tower in accordance withIP 19-6-1
On-Line Test Exchanger Although there are a variety of deposit monitors from the water treatment chemical vendors they do not necessarily representactual heat transfer conditions in process exchangers For this reason a test exchanger should be installed for evaluatingcorrosion and deposit formation Details are covered in Report No EE102E78 Cooling Tower Water Treatment Guidelines
Alternatively a critical process exchanger can be instrumented such that its heat transfer coefficient can be monitored
Instrumentation would include flows and inlet and outlet temperatures of process fluid and cooling waterSTEP 6 - Specify cooling tower location
Select a location for the cooling tower The following factors influence this selection
Location Factors1 The location should be within a reasonable distance of the units served and consistent with future refinery plans Piping
costs to and from battery limits can be a significant portion of the total cooling system cost and can be the location-determining factor particularly for salt water systems While piping costs can be easily determined other costs affectinglocation cannot be estimated readily Therefore judgment is required in selecting the location
2 Safety spacing requirements (see Section XV-G Equipment Spacing )3 Cooling tower contribution to plant boundary noise4 For salt water cooling towers wind direction and distance to other facilities is of major concern because of potential
corrosion problems
5 Factors affecting cooling tower performance ie orientation with respect to wind site congestion etc
Plot RequirementsIn estimating plot requirements (basin area) one should allow 5 to 6 ft of tower length for each 1000 gpm (2 m for each 60 Ls)of circulating capacity This figure is based on a typical counterflow design with a width of 50 ft (12 m) and does not includepump suction pit space Exact requirements can be obtained from cooling tower vendors after the tower capacity has beenestimated
Fogging and DriftTwo distinct problems are associated with the discharge air from a cooling tower (1) fogging which can be a nuisance or ahazard and (2) drift which is generally a nuisance because of increased corrosion of surrounding equipment Fogging fromcooling towers occurs during periods of combined high ambient humidity and low temperature as a result of mixing the warmmoist cooling tower exhaust air with the cooler ambient air The ambient air cannot absorb all moisture as vapor and thus tinyfog droplets are formed by condensation Fogging and resulting icing in cold climates has produced hazardous drivingconditions on highways up to 850 ft (250 m) awayProprietary designs are available to combat potential fogging problems Basically these designs incorporate heating of thedischarge air with hot return water precooling coils (similar to an air cooler) to eliminate any visible plume However thesedesigns are expensive and relatively unproven Therefore proper location based on reliable weather data is the preferredsolutionDrift from a cooling tower represents a more difficult problem since it does not depend on definable climatic conditions andoccurs for all wind directions The effects of drift on nearby equipment and vegetation are particularly severe and generally of concern only for salt water systems because of the high solids content of the drift (concentrated salt water) For salt water cooling systems electric lines substations and instrumentation should be located upwind in the direction of the predominantwind They should be provided with adequate shelter or materials when within 450 ft (140 m) of the cooling tower In additionspecifications for all equipment within 150 ft (45 m) of the tower should include surface treatment suitable for a marineenvironment
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COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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XXVII 28 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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DESIGN PRACTICES December 2000
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TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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DESIGN PRACTICES December 2000
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TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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DESIGN PRACTICES December 2000
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FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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Section Page
COOLING WATER SYSTEMS XXVII 19 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR ONCE-THROUGH SYSTEMS (Cont)
The equivalent length factors are selected based on the expected variation from a straight piping run Typical equivalentlength factors suitable for planning purposes for different piping applications are as follows
TYPE OF PIPING EQUIVALENT LENGTH FACTORS
Long piping runs [gt 1000 ft (300 m)] 12 to 14
Onsite distribution piping 15 to 20
For design purposes the actual piping layout should be evaluated to ensure that the equivalent length factors areapplicable to the specific situation Particular attention is required for revamp work and saltwater systems utilizing cement-lined pipe
STEP 8 - Prepare a system flow plan similar to the one shown in Figure 1
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS
STEP 1 - Determine cooling water requirements
Tabulate the cooling water requirements and cooling water discharge temperature for each process unit and offsite consumer Apply the proper LGFs to all cooling loads depending on the stage of the project and whether the new units are exactduplicates of existing units Determine the total cooling water flow required including LGFs and the bulk temperature (T 1) of thecombined cooling water flow from all coolers to the cooling towerIt should be pointed out that process cooler outlet temperatures are highly dependent on the heat levels and volumes of theprocess streams The onsite and offsite designers should investigate methods to increase effluent water temperature to themaximum recommended limit for the particular type of water used This will result in a lower cost cooling tower for a given heatrejection rate One common method of maximizing return water temperature is to reuse water from a light ends cooler in aheavy fuel oil cooler This increases the effluent water temperature by 10 to 15 degF (5 to 8 degC) but the bulk temperature is stillwithin allowable limits The designer should determine whether all cooler outlet temperatures have been optimized beforeproceeding with the cooling water system design
STEP 2 - Select cooling tower design parameters
Select the proper design conditions of ambient wet bulb temperature approach temperature range and flow rate for specifyingthe cooling tower Based on these conditions the cooling tower vendor will size the tower and provide the tower characteristicand performance curves which serve as the basis for the performance guarantee Required performance test and evaluationprocedures are described in IP 8-1-1 Economic studies may be required to select the ambient wet bulb temperature andapproach temperature for new locations or for expansion projects with special cooling requirements
STEP 3 - Prepare cooling tower specification sheet
Include the following information in the heat exchange section of the design specificationTower Flow Capacity - This was determined in Step 1Type of Tower - The tower usually specified is an induced draft multicell type suitable for fresh salt or brackish water depending on the water available Additional tower characteristics are usually developed and proposed by the tower vendor (eg crossflow versus counterflow type of tower fill etc) The vendor should be made aware of conditions such as thelikelihood of heavy oil leakage into the cooling water or high fouling potential which may influence the tower type or fill
selectionNumber of Cells - The minimum number of cells required (normally not less than 2) may be specified The maximum flow per cell is a function of the cooling tower design conditions selected in Step 2 Typical values range up to 10000 gpm (600 Ls) for crossflow designs and 8000 gpm (500 Ls) for counterflow designs Normally spare cells are not provided Maintenance iscarried out during cooler periods or load is shed
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XXVII 20 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Ambient Wet Bulb Temperature1 The ambient wet bulb temperature selected for specifying the cooling tower design is generally that temperature which is
equaled or exceeded 5 of the time during the four hottest months (5 of the hourly readings taken during the four hottestmonths will exceed this value) Specifying the 5 temperature should give satisfactory results since the time that the wetbulb temperature is exceeded is for short duration and the system inertia is usually sufficient to avoid problems The tower specification should make it clear that the wet bulb temperature specified is an ambient value as opposed to the enteringvalue which may be higher because of recirculation The tower manufacturer should adjust the ambient value upward totake into account his estimates of recirculation Currently cooling tower test codes provide procedures for measuringperformance in the case of either entering or ambient wet-bulb temperature specifications An increase of 2 degF (11 degC) isgenerally added to ambient wet bulb temperature to estimate entering wet bulb temperature for large cooling towers
2 If plant production will peak during the four summer months when the 5 ambient wet bulb temperature may be exceededconsideration should be given to specifying a higher design ambient wet bulb temperature (one that will be exceeded 212 of the time for example) However the incremental cost of the resulting closer approach and larger cooling tower must be justified by the value of the incremental products made
On-Tower Temperature (T1) - This temperature cannot be accurately determined until the design of all process coolers iscompleted and the volume of cooling water and outlet temperature are known for each (see Step 1) In the planning stage or early design stage of a project this temperature must be estimated and is usually set equal to the maximum design cooler outlet temperature shown under COOLING WATER REQUIREMENTS in the BASIC DESIGN CONSIDERATIONS portion of this sectionApproach Temperature - The approach temperature has the most pronounced effect on cooling tower size and cost For example reducing the approach from 10 to 5 degF (55 to 3 degC) can increase the cooling tower investment by 50 percent and thefan operating cost by 65 percent Thus various approach temperatures should be evaluated to determine the minimum cost of the total system including the process exchanger area Approaches used in the past designs have been from 5 to 12 degF (3 to7degC) Note that it is not customary in the cooling tower industry to guarantee approaches of less than 5 degF to (3 degC) to coolingtower design inlet wet bulb temperature A typical approach for a new grass roots cooling tower is 10 degF (6 degC)Off-Tower Temperature (T2) - This is the sum of the approach temperature and the ambient air wet bulb temperature For existing locations Table 2 in Section IX-B may be used
Range and Flow Rate1 The range for a given heat load is related to the circulating water rate as follows
Q = M C p ∆Tcong (500) (gpm) (range) Eq (1)
where Q = Heat load on the tower BtuhM = Water circulating rate lbhCp = Specific heat capacity of water at average temperature Btulb degF∆T = (T 1 ndash T 2) degF = (water cooling range)gpm = Circulation rate in US gallons per minute
For calculation in Metric units
Q = M C p ∆Tcong (1162 x 10 -3) (Ls) (range) Eq (1M)
where Q = Heat load on the tower kJh
M = Water circulating rate kghCp = Specific heat capacity of water at average temperature kJkg degC∆T = (T 1 ndash T 2) degC = (water cooling range)Ls = Circulation rate in liters per second
2 Generally lower tower investment fan horsepower and pump horsepower result from maximizing the range andminimizing the flow rate However based on fouling considerations of process coolers the maximum allowable hot water temperatures must be kept below recommended limits Also while increasing the range minimizes cooling tower investment and operating cost the cost of exchanger surface requirements is adversely affected by the LMTD and acompromise must be made The range is bounded by the maximum allowable hot water temperature and the Off-Tower temperature (T 2) For established locations refer to Section IX-B for design cold water temperatures to assist in selectingthe approach and range
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COOLING WATER SYSTEMS XXVII 21 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Makeup Water Requirements - Calculate the various losses in the cooling water system to determine makeup requirementsThese losses include
1 Evaporation Loss (E) - Allow 1 of the circulation rate for each 10 degF (55 degC) temperature drop through the tower Notethat the actual evaporation loss will be somewhat less depending on ambient air temperature
( )sL555
TT)gpm(
1000TT
E 2121
minus=
minus= Eq (2)
2 Drift Loss (D) - IP 8-1-1 limits the maximum allowable drift loss to 0005 of the design water circulation rate for freshbrackish or salt water
( )sL1000050
)gpm(1000050
D
=
= Eq (3)
This low drift level contributes little to solids removal from the tower with the spray This allows higher cycles of concentration to be achieved in cooling towers which use clarified softened water for makeup and thus have low requiredblowdown rates Higher drift loss percentages in older or poorly maintained towers will limit the maximum cycles of
concentration achievable3 Miscellaneous Losses (O) - Miscellaneous losses are estimated to be one-seventh of the difference between evaporation
loss and drift loss
minus=
7DE
O Eq (4)
4 Blowdown Rate (B) ndash Guidelines for maximum concentrations of impurities allowed in recirculated fresh cooling water areshown in Table 2 For salt water the blowdown is based on keeping the total dissolved salts below 55000 wppm Obtainthe chemical analysis of the makeup water and divide each individual component concentration into its counterpart inTable 2 The result is the maximum allowable cycles of concentration for that constituent (a worksheet is provided inTable 4 ) The impurity with the lowest factor will be the control and will determine the required blowdown rate
)OD(1Cycles
EB +minus
minus= Eq (5)
The makeup water requirement is the sum of all losses shown above
MakeUp = E + D + O + B Eq (6)
STEP 4 - Specify the controls necessary for proper cooling tower operation
Air Side - Cooling towers are provided with motor-driven manually adjusted pitch multiple-bladed propeller fans Figures9A amp B may be used to estimate the normal operating horsepower requirements Since the fan motor and gear reducer arelocated at the top of the tower [generally 40 to 50 ft (12 to 15 m) above grade] the need for equipment to handle thesecomponents should be discussed with the affiliateControl of the air flow through the tower to maintain reasonably constant cold water temperatures can be accomplished byturning off individual fans in multiple-cell installations adjusting the blade pitch or using variable speed motors To turn off individual fans remote at grade start-stop buttons for the motors are usually specified
Whenever the ambient air temperature is 32 degF (0 degC) or below icing of the tower louvers and structural members in contact withthe incoming air will occur If the heat load on the tower is relatively free of seasonal variations two-speed fans should provideadequate ice control down to approximately 0 degF (-18 degC) ambient temperatures When sustained temperatures of 0 degF (-18 degC)and below occur two-speed fans capable of two-speed reverse operation with appropriate time delays for change of directionand speed should be specified to periodically eliminate ice which will build up on the louvers Other methods of winterizationare available but in general are more expensive and require greater operator attention In very cold climates steam heatingshould be provided to prevent freezing in the cold water basin during shutdown periodsWater Side - The water circulation rate is maintained by operating one or more pumps of sufficient capacity to meet the systemdemand Flow-measuring facilities local temperature indicators and a high-temperature alarm are provided on the coolingtower water supply and return headers Hand-operated flow control valves are required for distributing the water equally to theindividual cells On counterflow towers provisions for measuring the flow to each cell (pitot tubes) are required They are notrequired on crossflow towers since the open distribution basin provides visual aid in balancing flows Shutoff valves should beprovided for servicing the flow control valves without shutting down the entire tower
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XXVII 22 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
STEP 5 - Provide chemical treatment facilities for the fresh water system
Alkalinity (pH) Control (if used)1 In cases where a controlled pH corrosionscale control program is being used the alkalinity (pH) of the recirculated cooling
water should be controlled in accordance with the program requirements Normally acid addition is required to accomplishthis This is due to free carbon dioxide being stripped from the cooling water in the tower to approximately 5 ppm For example if the alkalinity has to be controlled to between 25 and 50 ppm for a pH of 70 to 75 the volume of 98 sulfuricacid (66 degBeacute) required to treat the makeup water is calculated using the following equation
[Note 93 sulfuric acid should be used if ambient temperatures will be less than 35 degF (2 degC)]
GallonsHour 66 degBeacute H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate gpm) (326 x 10 -5)
For calculation in Metric limits
kgh of 100 (66 degBeacute) H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate Ls) (793 x 10 -3)
2 The metering pump should be designed for a maximum feed rate of two to three times the normal feed rate A glass woolfilter is required on the pump suction piping to minimize plugging Normally two - 100 capacity pumps are provided
Positive displacement metering pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumps with flow control areused for higher rates
3 Hydrochloric acid may also be used for alkalinity control The gallons per hour of 20 degBeacute hydrochloric acid required is36 times the amount of 66 degBeacute sulfuric acid In Metric units the weight of 32 (20 degBeacute) hydrochloric acid required is227 times the weight of 100 (66 degBeacute) sulfuric acid
4 Specifications for chemical feeders may be found in EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased)
5 A pH analyzer with high and low alarm is provided to control the amount of acid feed The acid should be added to thecooling tower makeup water (Figure 2 ) The lag time between the point of acid injection and the sample outlet to the pHcell should be less than two minutes for effective control
6 Spills or overflows from the sulfuric acid tank should be contained
Corrosion Inhibitors and Anti-Scalents1 Two chemical feeders are generally provided The following guides in sizing the equipment will allow the use of any of the
proprietary products from the vendors when final selection by the owner is made See Report No EE102E78 Cooling Tower Water Treatment Guidelines EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and Report No EE34E86 Environmentally - Acceptable Cooling Water Treatments for detailed discussion of acceptable commercial chemicals
bull If the water treatment program to be used is known design for the chemical rates specified by the vendor or COOLING WATER SYSTEMS SPECIALIST
bull Otherwise design for 100 wppm of each commercial liquid product in the recirculated cooling water The design feedrate should be based on the design total blowdown flow (both uncontrolled and controlled) since this is where theproduct is lost For 100 wppm concentration the chemical rates will be as follows
GallonsHour Chemical = Blowdown (gpm) x (0006)
LitersHour Chemical = Blowdown (Ls) x (036)
2 The design capacity of the metering pump should be two to three times this calculated number Normally 2 - 100capacity pumps are provided Positive displacement pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumpswith flow control are used for higher rates
3 Specifications for chemical feeders may be found in EMRErsquos Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased) Reference should also be made to the checklists containedin Report No EE92E94 Guidelines for Safety Evaluation of Chemical Injection Facilities
4 A final check should be made with the owner and the COOLING WATER SYSTEMS SPECIALISTS as to the treatmentprogram selected and if there is a need for an additional chemical feeder
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COOLING WATER SYSTEMS XXVII 23 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Biological ControlFacilities should be provided for feeding the biocide into the back of the cooling tower basin If chlorine is used it must bepresent in the cooling water as free residual to be effective However the following amounts of chlorine are first required toreact with the listed impurities when present in the cooling water before a free residual can be established
IMPURITIESAMOUNT OF CHLORINE REQUIRED (wppm)
WITH EACH WPPM OF IMPURITY
Ammonia (NH 3)
Hydrogen Sulfide (H 2S)
Sodium Sulfite (Na 2SO 3)
Ferrous Iron (Fe ++)
10
22
056
064
The design capacity of the chlorine feeder shall be based on achieving a 01 wppm free chlorine residual in the recirculatingwater flow or a minimum dosage of 7 wppm at maximum design makeup water flow rate and conditions (see EMRE Water and
Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service )Chlorine may be fed either as the commercial liquid sodium hypochlorite (124 wt Cl 2) or liquid chlorine (100 Cl 2) Liquidhypochlorite is being used increasingly because of the higher costs associated with precautions equipment and proceduresrequired to minimize the risks associated with handling gaseous chlorine The hypochlorite solution is fed using a meteringpump which should be sized to feed an equivalent of 15 ppm 100 Cl 2 (Refer to EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) ) In case of spills or an overflow from thestorage tank the sodium hypochlorite may be sent to the cooling tower basinGas chlorinators are covered by EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service The use of gas chlorination is being reduced but may still be appropriate in some locations subject tolocal management approval The gas from either commercial cylinders or an evaporator is dissolved in a water slip stream bymeans of an eductor This results in a chlorine solution strength of approximately 01 This solution should be evenlydistributed below the water surface by means of a header lateral pipe arrangement at the rear of the cooling tower basinopposite the cooling water pumpsSome additional guides on the use of gas chlorinators are given below It is also recommended that the safety guidelines
published by the Manufacturing Chemists Association and the Chlorine Institute be followedbull Dilution water for the chlorinator must not be less than 50 degF (10 degC) or ice formation may cause plugging problems
bull Continuous rates of chlorine gas withdrawal should not exceed 400 lb (180 kg)24 hours from each one ton cylinder atroom temperature [70 degF (21 degC)] This rate may be exceeded (up to 50) for periods not exceeding 2 hours Rule of thumb is not to design for more than 4 cylinders operating in parallel When feed rate exceeds 3000 lb (1360 kg) per daythe liquid chlorine should be fed to an evaporator upstream of the chlorinator
bull If the ambient temperature can be expected to be below 50 degF (10 degC) for extended periods a building should be providedfor the chlorinator and cylinders
bull Weighing scales should be provided to determine when shipping containers should be replaced and to verify chlorine feedrates
bull Analyzers to measure the free chlorine residual in the circulated cooling water should be provided
bull Chlorine leak detectors and a catch basin for major spills should be provided
Control SystemProvide automatic controllers to continuously monitor water condition and to add chemicals to the system (complete systemsmay be available through the water treatment program vendor as a lease or purchase)pH Control - This controller adds acid to correct water alkalinity a timer on the acid pump sounds an alarm if the pump doesnot shut off in a predetermined length of time Backup controls are required to prevent the cooling system from beingpermanently damaged in case the pH electrodes become fouledCorrosivity and Inhibitor Control - Systems are available which give a direct readout of corrosion rate in milsyear If thecorrosion rate exceeds a set point the controller shuts off the acid increases the inhibitor addition opens the blowdown valveand sounds an alarm Electrodes measure the completeness of the inhibitor film by the value of the potential differencebetween the electrodes The larger the difference the more incomplete the film and a potential reading exceeding a set levelactivates the inhibitor feed
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XXVII 24 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Free Residual Chlorine Control - Free residual chlorine should be controlled automatically for biocide treatment Severalgood analyzers are currently available These units can be incorporated into an automatic control system
Oil in Water DetectionMonitoring Corrosion and Deposit FormationThe following types of devices for measuring corrosion rates and deposit formation should be provided to allow the plant tomonitor the effectiveness of the cooling water treatment before scheduled turnaroundsCoupons - A corrosion coupon rack should be specified for the hot return water to the tower in accordance with IP 19-6-1 Corrosion Monitor - On-line corrosion analyzer if specified should monitor the hot return water to the tower in accordance withIP 19-6-1
On-Line Test Exchanger Although there are a variety of deposit monitors from the water treatment chemical vendors they do not necessarily representactual heat transfer conditions in process exchangers For this reason a test exchanger should be installed for evaluatingcorrosion and deposit formation Details are covered in Report No EE102E78 Cooling Tower Water Treatment Guidelines
Alternatively a critical process exchanger can be instrumented such that its heat transfer coefficient can be monitored
Instrumentation would include flows and inlet and outlet temperatures of process fluid and cooling waterSTEP 6 - Specify cooling tower location
Select a location for the cooling tower The following factors influence this selection
Location Factors1 The location should be within a reasonable distance of the units served and consistent with future refinery plans Piping
costs to and from battery limits can be a significant portion of the total cooling system cost and can be the location-determining factor particularly for salt water systems While piping costs can be easily determined other costs affectinglocation cannot be estimated readily Therefore judgment is required in selecting the location
2 Safety spacing requirements (see Section XV-G Equipment Spacing )3 Cooling tower contribution to plant boundary noise4 For salt water cooling towers wind direction and distance to other facilities is of major concern because of potential
corrosion problems
5 Factors affecting cooling tower performance ie orientation with respect to wind site congestion etc
Plot RequirementsIn estimating plot requirements (basin area) one should allow 5 to 6 ft of tower length for each 1000 gpm (2 m for each 60 Ls)of circulating capacity This figure is based on a typical counterflow design with a width of 50 ft (12 m) and does not includepump suction pit space Exact requirements can be obtained from cooling tower vendors after the tower capacity has beenestimated
Fogging and DriftTwo distinct problems are associated with the discharge air from a cooling tower (1) fogging which can be a nuisance or ahazard and (2) drift which is generally a nuisance because of increased corrosion of surrounding equipment Fogging fromcooling towers occurs during periods of combined high ambient humidity and low temperature as a result of mixing the warmmoist cooling tower exhaust air with the cooler ambient air The ambient air cannot absorb all moisture as vapor and thus tinyfog droplets are formed by condensation Fogging and resulting icing in cold climates has produced hazardous drivingconditions on highways up to 850 ft (250 m) awayProprietary designs are available to combat potential fogging problems Basically these designs incorporate heating of thedischarge air with hot return water precooling coils (similar to an air cooler) to eliminate any visible plume However thesedesigns are expensive and relatively unproven Therefore proper location based on reliable weather data is the preferredsolutionDrift from a cooling tower represents a more difficult problem since it does not depend on definable climatic conditions andoccurs for all wind directions The effects of drift on nearby equipment and vegetation are particularly severe and generally of concern only for salt water systems because of the high solids content of the drift (concentrated salt water) For salt water cooling systems electric lines substations and instrumentation should be located upwind in the direction of the predominantwind They should be provided with adequate shelter or materials when within 450 ft (140 m) of the cooling tower In additionspecifications for all equipment within 150 ft (45 m) of the tower should include surface treatment suitable for a marineenvironment
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COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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DESIGN PRACTICES December 2000
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COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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DESIGN PRACTICES December 2000
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TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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DESIGN PRACTICES December 2000
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FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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DESIGN PRACTICES December 2000
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FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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Section Page
COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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XXVII 20 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Ambient Wet Bulb Temperature1 The ambient wet bulb temperature selected for specifying the cooling tower design is generally that temperature which is
equaled or exceeded 5 of the time during the four hottest months (5 of the hourly readings taken during the four hottestmonths will exceed this value) Specifying the 5 temperature should give satisfactory results since the time that the wetbulb temperature is exceeded is for short duration and the system inertia is usually sufficient to avoid problems The tower specification should make it clear that the wet bulb temperature specified is an ambient value as opposed to the enteringvalue which may be higher because of recirculation The tower manufacturer should adjust the ambient value upward totake into account his estimates of recirculation Currently cooling tower test codes provide procedures for measuringperformance in the case of either entering or ambient wet-bulb temperature specifications An increase of 2 degF (11 degC) isgenerally added to ambient wet bulb temperature to estimate entering wet bulb temperature for large cooling towers
2 If plant production will peak during the four summer months when the 5 ambient wet bulb temperature may be exceededconsideration should be given to specifying a higher design ambient wet bulb temperature (one that will be exceeded 212 of the time for example) However the incremental cost of the resulting closer approach and larger cooling tower must be justified by the value of the incremental products made
On-Tower Temperature (T1) - This temperature cannot be accurately determined until the design of all process coolers iscompleted and the volume of cooling water and outlet temperature are known for each (see Step 1) In the planning stage or early design stage of a project this temperature must be estimated and is usually set equal to the maximum design cooler outlet temperature shown under COOLING WATER REQUIREMENTS in the BASIC DESIGN CONSIDERATIONS portion of this sectionApproach Temperature - The approach temperature has the most pronounced effect on cooling tower size and cost For example reducing the approach from 10 to 5 degF (55 to 3 degC) can increase the cooling tower investment by 50 percent and thefan operating cost by 65 percent Thus various approach temperatures should be evaluated to determine the minimum cost of the total system including the process exchanger area Approaches used in the past designs have been from 5 to 12 degF (3 to7degC) Note that it is not customary in the cooling tower industry to guarantee approaches of less than 5 degF to (3 degC) to coolingtower design inlet wet bulb temperature A typical approach for a new grass roots cooling tower is 10 degF (6 degC)Off-Tower Temperature (T2) - This is the sum of the approach temperature and the ambient air wet bulb temperature For existing locations Table 2 in Section IX-B may be used
Range and Flow Rate1 The range for a given heat load is related to the circulating water rate as follows
Q = M C p ∆Tcong (500) (gpm) (range) Eq (1)
where Q = Heat load on the tower BtuhM = Water circulating rate lbhCp = Specific heat capacity of water at average temperature Btulb degF∆T = (T 1 ndash T 2) degF = (water cooling range)gpm = Circulation rate in US gallons per minute
For calculation in Metric units
Q = M C p ∆Tcong (1162 x 10 -3) (Ls) (range) Eq (1M)
where Q = Heat load on the tower kJh
M = Water circulating rate kghCp = Specific heat capacity of water at average temperature kJkg degC∆T = (T 1 ndash T 2) degC = (water cooling range)Ls = Circulation rate in liters per second
2 Generally lower tower investment fan horsepower and pump horsepower result from maximizing the range andminimizing the flow rate However based on fouling considerations of process coolers the maximum allowable hot water temperatures must be kept below recommended limits Also while increasing the range minimizes cooling tower investment and operating cost the cost of exchanger surface requirements is adversely affected by the LMTD and acompromise must be made The range is bounded by the maximum allowable hot water temperature and the Off-Tower temperature (T 2) For established locations refer to Section IX-B for design cold water temperatures to assist in selectingthe approach and range
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COOLING WATER SYSTEMS XXVII 21 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Makeup Water Requirements - Calculate the various losses in the cooling water system to determine makeup requirementsThese losses include
1 Evaporation Loss (E) - Allow 1 of the circulation rate for each 10 degF (55 degC) temperature drop through the tower Notethat the actual evaporation loss will be somewhat less depending on ambient air temperature
( )sL555
TT)gpm(
1000TT
E 2121
minus=
minus= Eq (2)
2 Drift Loss (D) - IP 8-1-1 limits the maximum allowable drift loss to 0005 of the design water circulation rate for freshbrackish or salt water
( )sL1000050
)gpm(1000050
D
=
= Eq (3)
This low drift level contributes little to solids removal from the tower with the spray This allows higher cycles of concentration to be achieved in cooling towers which use clarified softened water for makeup and thus have low requiredblowdown rates Higher drift loss percentages in older or poorly maintained towers will limit the maximum cycles of
concentration achievable3 Miscellaneous Losses (O) - Miscellaneous losses are estimated to be one-seventh of the difference between evaporation
loss and drift loss
minus=
7DE
O Eq (4)
4 Blowdown Rate (B) ndash Guidelines for maximum concentrations of impurities allowed in recirculated fresh cooling water areshown in Table 2 For salt water the blowdown is based on keeping the total dissolved salts below 55000 wppm Obtainthe chemical analysis of the makeup water and divide each individual component concentration into its counterpart inTable 2 The result is the maximum allowable cycles of concentration for that constituent (a worksheet is provided inTable 4 ) The impurity with the lowest factor will be the control and will determine the required blowdown rate
)OD(1Cycles
EB +minus
minus= Eq (5)
The makeup water requirement is the sum of all losses shown above
MakeUp = E + D + O + B Eq (6)
STEP 4 - Specify the controls necessary for proper cooling tower operation
Air Side - Cooling towers are provided with motor-driven manually adjusted pitch multiple-bladed propeller fans Figures9A amp B may be used to estimate the normal operating horsepower requirements Since the fan motor and gear reducer arelocated at the top of the tower [generally 40 to 50 ft (12 to 15 m) above grade] the need for equipment to handle thesecomponents should be discussed with the affiliateControl of the air flow through the tower to maintain reasonably constant cold water temperatures can be accomplished byturning off individual fans in multiple-cell installations adjusting the blade pitch or using variable speed motors To turn off individual fans remote at grade start-stop buttons for the motors are usually specified
Whenever the ambient air temperature is 32 degF (0 degC) or below icing of the tower louvers and structural members in contact withthe incoming air will occur If the heat load on the tower is relatively free of seasonal variations two-speed fans should provideadequate ice control down to approximately 0 degF (-18 degC) ambient temperatures When sustained temperatures of 0 degF (-18 degC)and below occur two-speed fans capable of two-speed reverse operation with appropriate time delays for change of directionand speed should be specified to periodically eliminate ice which will build up on the louvers Other methods of winterizationare available but in general are more expensive and require greater operator attention In very cold climates steam heatingshould be provided to prevent freezing in the cold water basin during shutdown periodsWater Side - The water circulation rate is maintained by operating one or more pumps of sufficient capacity to meet the systemdemand Flow-measuring facilities local temperature indicators and a high-temperature alarm are provided on the coolingtower water supply and return headers Hand-operated flow control valves are required for distributing the water equally to theindividual cells On counterflow towers provisions for measuring the flow to each cell (pitot tubes) are required They are notrequired on crossflow towers since the open distribution basin provides visual aid in balancing flows Shutoff valves should beprovided for servicing the flow control valves without shutting down the entire tower
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XXVII 22 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
STEP 5 - Provide chemical treatment facilities for the fresh water system
Alkalinity (pH) Control (if used)1 In cases where a controlled pH corrosionscale control program is being used the alkalinity (pH) of the recirculated cooling
water should be controlled in accordance with the program requirements Normally acid addition is required to accomplishthis This is due to free carbon dioxide being stripped from the cooling water in the tower to approximately 5 ppm For example if the alkalinity has to be controlled to between 25 and 50 ppm for a pH of 70 to 75 the volume of 98 sulfuricacid (66 degBeacute) required to treat the makeup water is calculated using the following equation
[Note 93 sulfuric acid should be used if ambient temperatures will be less than 35 degF (2 degC)]
GallonsHour 66 degBeacute H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate gpm) (326 x 10 -5)
For calculation in Metric limits
kgh of 100 (66 degBeacute) H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate Ls) (793 x 10 -3)
2 The metering pump should be designed for a maximum feed rate of two to three times the normal feed rate A glass woolfilter is required on the pump suction piping to minimize plugging Normally two - 100 capacity pumps are provided
Positive displacement metering pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumps with flow control areused for higher rates
3 Hydrochloric acid may also be used for alkalinity control The gallons per hour of 20 degBeacute hydrochloric acid required is36 times the amount of 66 degBeacute sulfuric acid In Metric units the weight of 32 (20 degBeacute) hydrochloric acid required is227 times the weight of 100 (66 degBeacute) sulfuric acid
4 Specifications for chemical feeders may be found in EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased)
5 A pH analyzer with high and low alarm is provided to control the amount of acid feed The acid should be added to thecooling tower makeup water (Figure 2 ) The lag time between the point of acid injection and the sample outlet to the pHcell should be less than two minutes for effective control
6 Spills or overflows from the sulfuric acid tank should be contained
Corrosion Inhibitors and Anti-Scalents1 Two chemical feeders are generally provided The following guides in sizing the equipment will allow the use of any of the
proprietary products from the vendors when final selection by the owner is made See Report No EE102E78 Cooling Tower Water Treatment Guidelines EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and Report No EE34E86 Environmentally - Acceptable Cooling Water Treatments for detailed discussion of acceptable commercial chemicals
bull If the water treatment program to be used is known design for the chemical rates specified by the vendor or COOLING WATER SYSTEMS SPECIALIST
bull Otherwise design for 100 wppm of each commercial liquid product in the recirculated cooling water The design feedrate should be based on the design total blowdown flow (both uncontrolled and controlled) since this is where theproduct is lost For 100 wppm concentration the chemical rates will be as follows
GallonsHour Chemical = Blowdown (gpm) x (0006)
LitersHour Chemical = Blowdown (Ls) x (036)
2 The design capacity of the metering pump should be two to three times this calculated number Normally 2 - 100capacity pumps are provided Positive displacement pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumpswith flow control are used for higher rates
3 Specifications for chemical feeders may be found in EMRErsquos Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased) Reference should also be made to the checklists containedin Report No EE92E94 Guidelines for Safety Evaluation of Chemical Injection Facilities
4 A final check should be made with the owner and the COOLING WATER SYSTEMS SPECIALISTS as to the treatmentprogram selected and if there is a need for an additional chemical feeder
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COOLING WATER SYSTEMS XXVII 23 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Biological ControlFacilities should be provided for feeding the biocide into the back of the cooling tower basin If chlorine is used it must bepresent in the cooling water as free residual to be effective However the following amounts of chlorine are first required toreact with the listed impurities when present in the cooling water before a free residual can be established
IMPURITIESAMOUNT OF CHLORINE REQUIRED (wppm)
WITH EACH WPPM OF IMPURITY
Ammonia (NH 3)
Hydrogen Sulfide (H 2S)
Sodium Sulfite (Na 2SO 3)
Ferrous Iron (Fe ++)
10
22
056
064
The design capacity of the chlorine feeder shall be based on achieving a 01 wppm free chlorine residual in the recirculatingwater flow or a minimum dosage of 7 wppm at maximum design makeup water flow rate and conditions (see EMRE Water and
Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service )Chlorine may be fed either as the commercial liquid sodium hypochlorite (124 wt Cl 2) or liquid chlorine (100 Cl 2) Liquidhypochlorite is being used increasingly because of the higher costs associated with precautions equipment and proceduresrequired to minimize the risks associated with handling gaseous chlorine The hypochlorite solution is fed using a meteringpump which should be sized to feed an equivalent of 15 ppm 100 Cl 2 (Refer to EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) ) In case of spills or an overflow from thestorage tank the sodium hypochlorite may be sent to the cooling tower basinGas chlorinators are covered by EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service The use of gas chlorination is being reduced but may still be appropriate in some locations subject tolocal management approval The gas from either commercial cylinders or an evaporator is dissolved in a water slip stream bymeans of an eductor This results in a chlorine solution strength of approximately 01 This solution should be evenlydistributed below the water surface by means of a header lateral pipe arrangement at the rear of the cooling tower basinopposite the cooling water pumpsSome additional guides on the use of gas chlorinators are given below It is also recommended that the safety guidelines
published by the Manufacturing Chemists Association and the Chlorine Institute be followedbull Dilution water for the chlorinator must not be less than 50 degF (10 degC) or ice formation may cause plugging problems
bull Continuous rates of chlorine gas withdrawal should not exceed 400 lb (180 kg)24 hours from each one ton cylinder atroom temperature [70 degF (21 degC)] This rate may be exceeded (up to 50) for periods not exceeding 2 hours Rule of thumb is not to design for more than 4 cylinders operating in parallel When feed rate exceeds 3000 lb (1360 kg) per daythe liquid chlorine should be fed to an evaporator upstream of the chlorinator
bull If the ambient temperature can be expected to be below 50 degF (10 degC) for extended periods a building should be providedfor the chlorinator and cylinders
bull Weighing scales should be provided to determine when shipping containers should be replaced and to verify chlorine feedrates
bull Analyzers to measure the free chlorine residual in the circulated cooling water should be provided
bull Chlorine leak detectors and a catch basin for major spills should be provided
Control SystemProvide automatic controllers to continuously monitor water condition and to add chemicals to the system (complete systemsmay be available through the water treatment program vendor as a lease or purchase)pH Control - This controller adds acid to correct water alkalinity a timer on the acid pump sounds an alarm if the pump doesnot shut off in a predetermined length of time Backup controls are required to prevent the cooling system from beingpermanently damaged in case the pH electrodes become fouledCorrosivity and Inhibitor Control - Systems are available which give a direct readout of corrosion rate in milsyear If thecorrosion rate exceeds a set point the controller shuts off the acid increases the inhibitor addition opens the blowdown valveand sounds an alarm Electrodes measure the completeness of the inhibitor film by the value of the potential differencebetween the electrodes The larger the difference the more incomplete the film and a potential reading exceeding a set levelactivates the inhibitor feed
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XXVII 24 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Free Residual Chlorine Control - Free residual chlorine should be controlled automatically for biocide treatment Severalgood analyzers are currently available These units can be incorporated into an automatic control system
Oil in Water DetectionMonitoring Corrosion and Deposit FormationThe following types of devices for measuring corrosion rates and deposit formation should be provided to allow the plant tomonitor the effectiveness of the cooling water treatment before scheduled turnaroundsCoupons - A corrosion coupon rack should be specified for the hot return water to the tower in accordance with IP 19-6-1 Corrosion Monitor - On-line corrosion analyzer if specified should monitor the hot return water to the tower in accordance withIP 19-6-1
On-Line Test Exchanger Although there are a variety of deposit monitors from the water treatment chemical vendors they do not necessarily representactual heat transfer conditions in process exchangers For this reason a test exchanger should be installed for evaluatingcorrosion and deposit formation Details are covered in Report No EE102E78 Cooling Tower Water Treatment Guidelines
Alternatively a critical process exchanger can be instrumented such that its heat transfer coefficient can be monitored
Instrumentation would include flows and inlet and outlet temperatures of process fluid and cooling waterSTEP 6 - Specify cooling tower location
Select a location for the cooling tower The following factors influence this selection
Location Factors1 The location should be within a reasonable distance of the units served and consistent with future refinery plans Piping
costs to and from battery limits can be a significant portion of the total cooling system cost and can be the location-determining factor particularly for salt water systems While piping costs can be easily determined other costs affectinglocation cannot be estimated readily Therefore judgment is required in selecting the location
2 Safety spacing requirements (see Section XV-G Equipment Spacing )3 Cooling tower contribution to plant boundary noise4 For salt water cooling towers wind direction and distance to other facilities is of major concern because of potential
corrosion problems
5 Factors affecting cooling tower performance ie orientation with respect to wind site congestion etc
Plot RequirementsIn estimating plot requirements (basin area) one should allow 5 to 6 ft of tower length for each 1000 gpm (2 m for each 60 Ls)of circulating capacity This figure is based on a typical counterflow design with a width of 50 ft (12 m) and does not includepump suction pit space Exact requirements can be obtained from cooling tower vendors after the tower capacity has beenestimated
Fogging and DriftTwo distinct problems are associated with the discharge air from a cooling tower (1) fogging which can be a nuisance or ahazard and (2) drift which is generally a nuisance because of increased corrosion of surrounding equipment Fogging fromcooling towers occurs during periods of combined high ambient humidity and low temperature as a result of mixing the warmmoist cooling tower exhaust air with the cooler ambient air The ambient air cannot absorb all moisture as vapor and thus tinyfog droplets are formed by condensation Fogging and resulting icing in cold climates has produced hazardous drivingconditions on highways up to 850 ft (250 m) awayProprietary designs are available to combat potential fogging problems Basically these designs incorporate heating of thedischarge air with hot return water precooling coils (similar to an air cooler) to eliminate any visible plume However thesedesigns are expensive and relatively unproven Therefore proper location based on reliable weather data is the preferredsolutionDrift from a cooling tower represents a more difficult problem since it does not depend on definable climatic conditions andoccurs for all wind directions The effects of drift on nearby equipment and vegetation are particularly severe and generally of concern only for salt water systems because of the high solids content of the drift (concentrated salt water) For salt water cooling systems electric lines substations and instrumentation should be located upwind in the direction of the predominantwind They should be provided with adequate shelter or materials when within 450 ft (140 m) of the cooling tower In additionspecifications for all equipment within 150 ft (45 m) of the tower should include surface treatment suitable for a marineenvironment
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COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
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COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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DESIGN PRACTICES December 2000
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TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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DESIGN PRACTICES December 2000
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FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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Section Page
XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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Section Page
COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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Section Page
XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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COOLING WATER SYSTEMS XXVII 21 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Makeup Water Requirements - Calculate the various losses in the cooling water system to determine makeup requirementsThese losses include
1 Evaporation Loss (E) - Allow 1 of the circulation rate for each 10 degF (55 degC) temperature drop through the tower Notethat the actual evaporation loss will be somewhat less depending on ambient air temperature
( )sL555
TT)gpm(
1000TT
E 2121
minus=
minus= Eq (2)
2 Drift Loss (D) - IP 8-1-1 limits the maximum allowable drift loss to 0005 of the design water circulation rate for freshbrackish or salt water
( )sL1000050
)gpm(1000050
D
=
= Eq (3)
This low drift level contributes little to solids removal from the tower with the spray This allows higher cycles of concentration to be achieved in cooling towers which use clarified softened water for makeup and thus have low requiredblowdown rates Higher drift loss percentages in older or poorly maintained towers will limit the maximum cycles of
concentration achievable3 Miscellaneous Losses (O) - Miscellaneous losses are estimated to be one-seventh of the difference between evaporation
loss and drift loss
minus=
7DE
O Eq (4)
4 Blowdown Rate (B) ndash Guidelines for maximum concentrations of impurities allowed in recirculated fresh cooling water areshown in Table 2 For salt water the blowdown is based on keeping the total dissolved salts below 55000 wppm Obtainthe chemical analysis of the makeup water and divide each individual component concentration into its counterpart inTable 2 The result is the maximum allowable cycles of concentration for that constituent (a worksheet is provided inTable 4 ) The impurity with the lowest factor will be the control and will determine the required blowdown rate
)OD(1Cycles
EB +minus
minus= Eq (5)
The makeup water requirement is the sum of all losses shown above
MakeUp = E + D + O + B Eq (6)
STEP 4 - Specify the controls necessary for proper cooling tower operation
Air Side - Cooling towers are provided with motor-driven manually adjusted pitch multiple-bladed propeller fans Figures9A amp B may be used to estimate the normal operating horsepower requirements Since the fan motor and gear reducer arelocated at the top of the tower [generally 40 to 50 ft (12 to 15 m) above grade] the need for equipment to handle thesecomponents should be discussed with the affiliateControl of the air flow through the tower to maintain reasonably constant cold water temperatures can be accomplished byturning off individual fans in multiple-cell installations adjusting the blade pitch or using variable speed motors To turn off individual fans remote at grade start-stop buttons for the motors are usually specified
Whenever the ambient air temperature is 32 degF (0 degC) or below icing of the tower louvers and structural members in contact withthe incoming air will occur If the heat load on the tower is relatively free of seasonal variations two-speed fans should provideadequate ice control down to approximately 0 degF (-18 degC) ambient temperatures When sustained temperatures of 0 degF (-18 degC)and below occur two-speed fans capable of two-speed reverse operation with appropriate time delays for change of directionand speed should be specified to periodically eliminate ice which will build up on the louvers Other methods of winterizationare available but in general are more expensive and require greater operator attention In very cold climates steam heatingshould be provided to prevent freezing in the cold water basin during shutdown periodsWater Side - The water circulation rate is maintained by operating one or more pumps of sufficient capacity to meet the systemdemand Flow-measuring facilities local temperature indicators and a high-temperature alarm are provided on the coolingtower water supply and return headers Hand-operated flow control valves are required for distributing the water equally to theindividual cells On counterflow towers provisions for measuring the flow to each cell (pitot tubes) are required They are notrequired on crossflow towers since the open distribution basin provides visual aid in balancing flows Shutoff valves should beprovided for servicing the flow control valves without shutting down the entire tower
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XXVII 22 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
STEP 5 - Provide chemical treatment facilities for the fresh water system
Alkalinity (pH) Control (if used)1 In cases where a controlled pH corrosionscale control program is being used the alkalinity (pH) of the recirculated cooling
water should be controlled in accordance with the program requirements Normally acid addition is required to accomplishthis This is due to free carbon dioxide being stripped from the cooling water in the tower to approximately 5 ppm For example if the alkalinity has to be controlled to between 25 and 50 ppm for a pH of 70 to 75 the volume of 98 sulfuricacid (66 degBeacute) required to treat the makeup water is calculated using the following equation
[Note 93 sulfuric acid should be used if ambient temperatures will be less than 35 degF (2 degC)]
GallonsHour 66 degBeacute H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate gpm) (326 x 10 -5)
For calculation in Metric limits
kgh of 100 (66 degBeacute) H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate Ls) (793 x 10 -3)
2 The metering pump should be designed for a maximum feed rate of two to three times the normal feed rate A glass woolfilter is required on the pump suction piping to minimize plugging Normally two - 100 capacity pumps are provided
Positive displacement metering pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumps with flow control areused for higher rates
3 Hydrochloric acid may also be used for alkalinity control The gallons per hour of 20 degBeacute hydrochloric acid required is36 times the amount of 66 degBeacute sulfuric acid In Metric units the weight of 32 (20 degBeacute) hydrochloric acid required is227 times the weight of 100 (66 degBeacute) sulfuric acid
4 Specifications for chemical feeders may be found in EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased)
5 A pH analyzer with high and low alarm is provided to control the amount of acid feed The acid should be added to thecooling tower makeup water (Figure 2 ) The lag time between the point of acid injection and the sample outlet to the pHcell should be less than two minutes for effective control
6 Spills or overflows from the sulfuric acid tank should be contained
Corrosion Inhibitors and Anti-Scalents1 Two chemical feeders are generally provided The following guides in sizing the equipment will allow the use of any of the
proprietary products from the vendors when final selection by the owner is made See Report No EE102E78 Cooling Tower Water Treatment Guidelines EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and Report No EE34E86 Environmentally - Acceptable Cooling Water Treatments for detailed discussion of acceptable commercial chemicals
bull If the water treatment program to be used is known design for the chemical rates specified by the vendor or COOLING WATER SYSTEMS SPECIALIST
bull Otherwise design for 100 wppm of each commercial liquid product in the recirculated cooling water The design feedrate should be based on the design total blowdown flow (both uncontrolled and controlled) since this is where theproduct is lost For 100 wppm concentration the chemical rates will be as follows
GallonsHour Chemical = Blowdown (gpm) x (0006)
LitersHour Chemical = Blowdown (Ls) x (036)
2 The design capacity of the metering pump should be two to three times this calculated number Normally 2 - 100capacity pumps are provided Positive displacement pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumpswith flow control are used for higher rates
3 Specifications for chemical feeders may be found in EMRErsquos Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased) Reference should also be made to the checklists containedin Report No EE92E94 Guidelines for Safety Evaluation of Chemical Injection Facilities
4 A final check should be made with the owner and the COOLING WATER SYSTEMS SPECIALISTS as to the treatmentprogram selected and if there is a need for an additional chemical feeder
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COOLING WATER SYSTEMS XXVII 23 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Biological ControlFacilities should be provided for feeding the biocide into the back of the cooling tower basin If chlorine is used it must bepresent in the cooling water as free residual to be effective However the following amounts of chlorine are first required toreact with the listed impurities when present in the cooling water before a free residual can be established
IMPURITIESAMOUNT OF CHLORINE REQUIRED (wppm)
WITH EACH WPPM OF IMPURITY
Ammonia (NH 3)
Hydrogen Sulfide (H 2S)
Sodium Sulfite (Na 2SO 3)
Ferrous Iron (Fe ++)
10
22
056
064
The design capacity of the chlorine feeder shall be based on achieving a 01 wppm free chlorine residual in the recirculatingwater flow or a minimum dosage of 7 wppm at maximum design makeup water flow rate and conditions (see EMRE Water and
Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service )Chlorine may be fed either as the commercial liquid sodium hypochlorite (124 wt Cl 2) or liquid chlorine (100 Cl 2) Liquidhypochlorite is being used increasingly because of the higher costs associated with precautions equipment and proceduresrequired to minimize the risks associated with handling gaseous chlorine The hypochlorite solution is fed using a meteringpump which should be sized to feed an equivalent of 15 ppm 100 Cl 2 (Refer to EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) ) In case of spills or an overflow from thestorage tank the sodium hypochlorite may be sent to the cooling tower basinGas chlorinators are covered by EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service The use of gas chlorination is being reduced but may still be appropriate in some locations subject tolocal management approval The gas from either commercial cylinders or an evaporator is dissolved in a water slip stream bymeans of an eductor This results in a chlorine solution strength of approximately 01 This solution should be evenlydistributed below the water surface by means of a header lateral pipe arrangement at the rear of the cooling tower basinopposite the cooling water pumpsSome additional guides on the use of gas chlorinators are given below It is also recommended that the safety guidelines
published by the Manufacturing Chemists Association and the Chlorine Institute be followedbull Dilution water for the chlorinator must not be less than 50 degF (10 degC) or ice formation may cause plugging problems
bull Continuous rates of chlorine gas withdrawal should not exceed 400 lb (180 kg)24 hours from each one ton cylinder atroom temperature [70 degF (21 degC)] This rate may be exceeded (up to 50) for periods not exceeding 2 hours Rule of thumb is not to design for more than 4 cylinders operating in parallel When feed rate exceeds 3000 lb (1360 kg) per daythe liquid chlorine should be fed to an evaporator upstream of the chlorinator
bull If the ambient temperature can be expected to be below 50 degF (10 degC) for extended periods a building should be providedfor the chlorinator and cylinders
bull Weighing scales should be provided to determine when shipping containers should be replaced and to verify chlorine feedrates
bull Analyzers to measure the free chlorine residual in the circulated cooling water should be provided
bull Chlorine leak detectors and a catch basin for major spills should be provided
Control SystemProvide automatic controllers to continuously monitor water condition and to add chemicals to the system (complete systemsmay be available through the water treatment program vendor as a lease or purchase)pH Control - This controller adds acid to correct water alkalinity a timer on the acid pump sounds an alarm if the pump doesnot shut off in a predetermined length of time Backup controls are required to prevent the cooling system from beingpermanently damaged in case the pH electrodes become fouledCorrosivity and Inhibitor Control - Systems are available which give a direct readout of corrosion rate in milsyear If thecorrosion rate exceeds a set point the controller shuts off the acid increases the inhibitor addition opens the blowdown valveand sounds an alarm Electrodes measure the completeness of the inhibitor film by the value of the potential differencebetween the electrodes The larger the difference the more incomplete the film and a potential reading exceeding a set levelactivates the inhibitor feed
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XXVII 24 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Free Residual Chlorine Control - Free residual chlorine should be controlled automatically for biocide treatment Severalgood analyzers are currently available These units can be incorporated into an automatic control system
Oil in Water DetectionMonitoring Corrosion and Deposit FormationThe following types of devices for measuring corrosion rates and deposit formation should be provided to allow the plant tomonitor the effectiveness of the cooling water treatment before scheduled turnaroundsCoupons - A corrosion coupon rack should be specified for the hot return water to the tower in accordance with IP 19-6-1 Corrosion Monitor - On-line corrosion analyzer if specified should monitor the hot return water to the tower in accordance withIP 19-6-1
On-Line Test Exchanger Although there are a variety of deposit monitors from the water treatment chemical vendors they do not necessarily representactual heat transfer conditions in process exchangers For this reason a test exchanger should be installed for evaluatingcorrosion and deposit formation Details are covered in Report No EE102E78 Cooling Tower Water Treatment Guidelines
Alternatively a critical process exchanger can be instrumented such that its heat transfer coefficient can be monitored
Instrumentation would include flows and inlet and outlet temperatures of process fluid and cooling waterSTEP 6 - Specify cooling tower location
Select a location for the cooling tower The following factors influence this selection
Location Factors1 The location should be within a reasonable distance of the units served and consistent with future refinery plans Piping
costs to and from battery limits can be a significant portion of the total cooling system cost and can be the location-determining factor particularly for salt water systems While piping costs can be easily determined other costs affectinglocation cannot be estimated readily Therefore judgment is required in selecting the location
2 Safety spacing requirements (see Section XV-G Equipment Spacing )3 Cooling tower contribution to plant boundary noise4 For salt water cooling towers wind direction and distance to other facilities is of major concern because of potential
corrosion problems
5 Factors affecting cooling tower performance ie orientation with respect to wind site congestion etc
Plot RequirementsIn estimating plot requirements (basin area) one should allow 5 to 6 ft of tower length for each 1000 gpm (2 m for each 60 Ls)of circulating capacity This figure is based on a typical counterflow design with a width of 50 ft (12 m) and does not includepump suction pit space Exact requirements can be obtained from cooling tower vendors after the tower capacity has beenestimated
Fogging and DriftTwo distinct problems are associated with the discharge air from a cooling tower (1) fogging which can be a nuisance or ahazard and (2) drift which is generally a nuisance because of increased corrosion of surrounding equipment Fogging fromcooling towers occurs during periods of combined high ambient humidity and low temperature as a result of mixing the warmmoist cooling tower exhaust air with the cooler ambient air The ambient air cannot absorb all moisture as vapor and thus tinyfog droplets are formed by condensation Fogging and resulting icing in cold climates has produced hazardous drivingconditions on highways up to 850 ft (250 m) awayProprietary designs are available to combat potential fogging problems Basically these designs incorporate heating of thedischarge air with hot return water precooling coils (similar to an air cooler) to eliminate any visible plume However thesedesigns are expensive and relatively unproven Therefore proper location based on reliable weather data is the preferredsolutionDrift from a cooling tower represents a more difficult problem since it does not depend on definable climatic conditions andoccurs for all wind directions The effects of drift on nearby equipment and vegetation are particularly severe and generally of concern only for salt water systems because of the high solids content of the drift (concentrated salt water) For salt water cooling systems electric lines substations and instrumentation should be located upwind in the direction of the predominantwind They should be provided with adequate shelter or materials when within 450 ft (140 m) of the cooling tower In additionspecifications for all equipment within 150 ft (45 m) of the tower should include surface treatment suitable for a marineenvironment
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COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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XXVII 28 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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XXVII 30 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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XXVII 32 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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COOLING WATER SYSTEMS XXVII 35 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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Section Page
XXVII 22 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
STEP 5 - Provide chemical treatment facilities for the fresh water system
Alkalinity (pH) Control (if used)1 In cases where a controlled pH corrosionscale control program is being used the alkalinity (pH) of the recirculated cooling
water should be controlled in accordance with the program requirements Normally acid addition is required to accomplishthis This is due to free carbon dioxide being stripped from the cooling water in the tower to approximately 5 ppm For example if the alkalinity has to be controlled to between 25 and 50 ppm for a pH of 70 to 75 the volume of 98 sulfuricacid (66 degBeacute) required to treat the makeup water is calculated using the following equation
[Note 93 sulfuric acid should be used if ambient temperatures will be less than 35 degF (2 degC)]
GallonsHour 66 degBeacute H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate gpm) (326 x 10 -5)
For calculation in Metric limits
kgh of 100 (66 degBeacute) H 2SO 4 = (Alkalinity in Makeup Water as wppm CaCO 3) (Makeup Rate Ls) (793 x 10 -3)
2 The metering pump should be designed for a maximum feed rate of two to three times the normal feed rate A glass woolfilter is required on the pump suction piping to minimize plugging Normally two - 100 capacity pumps are provided
Positive displacement metering pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumps with flow control areused for higher rates
3 Hydrochloric acid may also be used for alkalinity control The gallons per hour of 20 degBeacute hydrochloric acid required is36 times the amount of 66 degBeacute sulfuric acid In Metric units the weight of 32 (20 degBeacute) hydrochloric acid required is227 times the weight of 100 (66 degBeacute) sulfuric acid
4 Specifications for chemical feeders may be found in EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased)
5 A pH analyzer with high and low alarm is provided to control the amount of acid feed The acid should be added to thecooling tower makeup water (Figure 2 ) The lag time between the point of acid injection and the sample outlet to the pHcell should be less than two minutes for effective control
6 Spills or overflows from the sulfuric acid tank should be contained
Corrosion Inhibitors and Anti-Scalents1 Two chemical feeders are generally provided The following guides in sizing the equipment will allow the use of any of the
proprietary products from the vendors when final selection by the owner is made See Report No EE102E78 Cooling Tower Water Treatment Guidelines EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and Report No EE34E86 Environmentally - Acceptable Cooling Water Treatments for detailed discussion of acceptable commercial chemicals
bull If the water treatment program to be used is known design for the chemical rates specified by the vendor or COOLING WATER SYSTEMS SPECIALIST
bull Otherwise design for 100 wppm of each commercial liquid product in the recirculated cooling water The design feedrate should be based on the design total blowdown flow (both uncontrolled and controlled) since this is where theproduct is lost For 100 wppm concentration the chemical rates will be as follows
GallonsHour Chemical = Blowdown (gpm) x (0006)
LitersHour Chemical = Blowdown (Ls) x (036)
2 The design capacity of the metering pump should be two to three times this calculated number Normally 2 - 100capacity pumps are provided Positive displacement pumps are used for rates up to 100 gph (01 Ls) Centrifugal pumpswith flow control are used for higher rates
3 Specifications for chemical feeders may be found in EMRErsquos Water and Wastewater Design Guide TMEE 080 DG 11-6-2Chemical Feeders for Cooling Towers (Leased or Purchased) Reference should also be made to the checklists containedin Report No EE92E94 Guidelines for Safety Evaluation of Chemical Injection Facilities
4 A final check should be made with the owner and the COOLING WATER SYSTEMS SPECIALISTS as to the treatmentprogram selected and if there is a need for an additional chemical feeder
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COOLING WATER SYSTEMS XXVII 23 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Biological ControlFacilities should be provided for feeding the biocide into the back of the cooling tower basin If chlorine is used it must bepresent in the cooling water as free residual to be effective However the following amounts of chlorine are first required toreact with the listed impurities when present in the cooling water before a free residual can be established
IMPURITIESAMOUNT OF CHLORINE REQUIRED (wppm)
WITH EACH WPPM OF IMPURITY
Ammonia (NH 3)
Hydrogen Sulfide (H 2S)
Sodium Sulfite (Na 2SO 3)
Ferrous Iron (Fe ++)
10
22
056
064
The design capacity of the chlorine feeder shall be based on achieving a 01 wppm free chlorine residual in the recirculatingwater flow or a minimum dosage of 7 wppm at maximum design makeup water flow rate and conditions (see EMRE Water and
Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service )Chlorine may be fed either as the commercial liquid sodium hypochlorite (124 wt Cl 2) or liquid chlorine (100 Cl 2) Liquidhypochlorite is being used increasingly because of the higher costs associated with precautions equipment and proceduresrequired to minimize the risks associated with handling gaseous chlorine The hypochlorite solution is fed using a meteringpump which should be sized to feed an equivalent of 15 ppm 100 Cl 2 (Refer to EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) ) In case of spills or an overflow from thestorage tank the sodium hypochlorite may be sent to the cooling tower basinGas chlorinators are covered by EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service The use of gas chlorination is being reduced but may still be appropriate in some locations subject tolocal management approval The gas from either commercial cylinders or an evaporator is dissolved in a water slip stream bymeans of an eductor This results in a chlorine solution strength of approximately 01 This solution should be evenlydistributed below the water surface by means of a header lateral pipe arrangement at the rear of the cooling tower basinopposite the cooling water pumpsSome additional guides on the use of gas chlorinators are given below It is also recommended that the safety guidelines
published by the Manufacturing Chemists Association and the Chlorine Institute be followedbull Dilution water for the chlorinator must not be less than 50 degF (10 degC) or ice formation may cause plugging problems
bull Continuous rates of chlorine gas withdrawal should not exceed 400 lb (180 kg)24 hours from each one ton cylinder atroom temperature [70 degF (21 degC)] This rate may be exceeded (up to 50) for periods not exceeding 2 hours Rule of thumb is not to design for more than 4 cylinders operating in parallel When feed rate exceeds 3000 lb (1360 kg) per daythe liquid chlorine should be fed to an evaporator upstream of the chlorinator
bull If the ambient temperature can be expected to be below 50 degF (10 degC) for extended periods a building should be providedfor the chlorinator and cylinders
bull Weighing scales should be provided to determine when shipping containers should be replaced and to verify chlorine feedrates
bull Analyzers to measure the free chlorine residual in the circulated cooling water should be provided
bull Chlorine leak detectors and a catch basin for major spills should be provided
Control SystemProvide automatic controllers to continuously monitor water condition and to add chemicals to the system (complete systemsmay be available through the water treatment program vendor as a lease or purchase)pH Control - This controller adds acid to correct water alkalinity a timer on the acid pump sounds an alarm if the pump doesnot shut off in a predetermined length of time Backup controls are required to prevent the cooling system from beingpermanently damaged in case the pH electrodes become fouledCorrosivity and Inhibitor Control - Systems are available which give a direct readout of corrosion rate in milsyear If thecorrosion rate exceeds a set point the controller shuts off the acid increases the inhibitor addition opens the blowdown valveand sounds an alarm Electrodes measure the completeness of the inhibitor film by the value of the potential differencebetween the electrodes The larger the difference the more incomplete the film and a potential reading exceeding a set levelactivates the inhibitor feed
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XXVII 24 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Free Residual Chlorine Control - Free residual chlorine should be controlled automatically for biocide treatment Severalgood analyzers are currently available These units can be incorporated into an automatic control system
Oil in Water DetectionMonitoring Corrosion and Deposit FormationThe following types of devices for measuring corrosion rates and deposit formation should be provided to allow the plant tomonitor the effectiveness of the cooling water treatment before scheduled turnaroundsCoupons - A corrosion coupon rack should be specified for the hot return water to the tower in accordance with IP 19-6-1 Corrosion Monitor - On-line corrosion analyzer if specified should monitor the hot return water to the tower in accordance withIP 19-6-1
On-Line Test Exchanger Although there are a variety of deposit monitors from the water treatment chemical vendors they do not necessarily representactual heat transfer conditions in process exchangers For this reason a test exchanger should be installed for evaluatingcorrosion and deposit formation Details are covered in Report No EE102E78 Cooling Tower Water Treatment Guidelines
Alternatively a critical process exchanger can be instrumented such that its heat transfer coefficient can be monitored
Instrumentation would include flows and inlet and outlet temperatures of process fluid and cooling waterSTEP 6 - Specify cooling tower location
Select a location for the cooling tower The following factors influence this selection
Location Factors1 The location should be within a reasonable distance of the units served and consistent with future refinery plans Piping
costs to and from battery limits can be a significant portion of the total cooling system cost and can be the location-determining factor particularly for salt water systems While piping costs can be easily determined other costs affectinglocation cannot be estimated readily Therefore judgment is required in selecting the location
2 Safety spacing requirements (see Section XV-G Equipment Spacing )3 Cooling tower contribution to plant boundary noise4 For salt water cooling towers wind direction and distance to other facilities is of major concern because of potential
corrosion problems
5 Factors affecting cooling tower performance ie orientation with respect to wind site congestion etc
Plot RequirementsIn estimating plot requirements (basin area) one should allow 5 to 6 ft of tower length for each 1000 gpm (2 m for each 60 Ls)of circulating capacity This figure is based on a typical counterflow design with a width of 50 ft (12 m) and does not includepump suction pit space Exact requirements can be obtained from cooling tower vendors after the tower capacity has beenestimated
Fogging and DriftTwo distinct problems are associated with the discharge air from a cooling tower (1) fogging which can be a nuisance or ahazard and (2) drift which is generally a nuisance because of increased corrosion of surrounding equipment Fogging fromcooling towers occurs during periods of combined high ambient humidity and low temperature as a result of mixing the warmmoist cooling tower exhaust air with the cooler ambient air The ambient air cannot absorb all moisture as vapor and thus tinyfog droplets are formed by condensation Fogging and resulting icing in cold climates has produced hazardous drivingconditions on highways up to 850 ft (250 m) awayProprietary designs are available to combat potential fogging problems Basically these designs incorporate heating of thedischarge air with hot return water precooling coils (similar to an air cooler) to eliminate any visible plume However thesedesigns are expensive and relatively unproven Therefore proper location based on reliable weather data is the preferredsolutionDrift from a cooling tower represents a more difficult problem since it does not depend on definable climatic conditions andoccurs for all wind directions The effects of drift on nearby equipment and vegetation are particularly severe and generally of concern only for salt water systems because of the high solids content of the drift (concentrated salt water) For salt water cooling systems electric lines substations and instrumentation should be located upwind in the direction of the predominantwind They should be provided with adequate shelter or materials when within 450 ft (140 m) of the cooling tower In additionspecifications for all equipment within 150 ft (45 m) of the tower should include surface treatment suitable for a marineenvironment
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COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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XXVII 28 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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XXVII 30 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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COOLING WATER SYSTEMS XXVII 31 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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XXVII 32 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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COOLING WATER SYSTEMS XXVII 33 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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COOLING WATER SYSTEMS XXVII 35 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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COOLING WATER SYSTEMS XXVII 37 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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COOLING WATER SYSTEMS XXVII 23 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Biological ControlFacilities should be provided for feeding the biocide into the back of the cooling tower basin If chlorine is used it must bepresent in the cooling water as free residual to be effective However the following amounts of chlorine are first required toreact with the listed impurities when present in the cooling water before a free residual can be established
IMPURITIESAMOUNT OF CHLORINE REQUIRED (wppm)
WITH EACH WPPM OF IMPURITY
Ammonia (NH 3)
Hydrogen Sulfide (H 2S)
Sodium Sulfite (Na 2SO 3)
Ferrous Iron (Fe ++)
10
22
056
064
The design capacity of the chlorine feeder shall be based on achieving a 01 wppm free chlorine residual in the recirculatingwater flow or a minimum dosage of 7 wppm at maximum design makeup water flow rate and conditions (see EMRE Water and
Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service )Chlorine may be fed either as the commercial liquid sodium hypochlorite (124 wt Cl 2) or liquid chlorine (100 Cl 2) Liquidhypochlorite is being used increasingly because of the higher costs associated with precautions equipment and proceduresrequired to minimize the risks associated with handling gaseous chlorine The hypochlorite solution is fed using a meteringpump which should be sized to feed an equivalent of 15 ppm 100 Cl 2 (Refer to EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) ) In case of spills or an overflow from thestorage tank the sodium hypochlorite may be sent to the cooling tower basinGas chlorinators are covered by EMRE Water and Wastewater Design Guide TMEE 080 DG 11-3-1 Gas Chlorinators for Water Treating Service The use of gas chlorination is being reduced but may still be appropriate in some locations subject tolocal management approval The gas from either commercial cylinders or an evaporator is dissolved in a water slip stream bymeans of an eductor This results in a chlorine solution strength of approximately 01 This solution should be evenlydistributed below the water surface by means of a header lateral pipe arrangement at the rear of the cooling tower basinopposite the cooling water pumpsSome additional guides on the use of gas chlorinators are given below It is also recommended that the safety guidelines
published by the Manufacturing Chemists Association and the Chlorine Institute be followedbull Dilution water for the chlorinator must not be less than 50 degF (10 degC) or ice formation may cause plugging problems
bull Continuous rates of chlorine gas withdrawal should not exceed 400 lb (180 kg)24 hours from each one ton cylinder atroom temperature [70 degF (21 degC)] This rate may be exceeded (up to 50) for periods not exceeding 2 hours Rule of thumb is not to design for more than 4 cylinders operating in parallel When feed rate exceeds 3000 lb (1360 kg) per daythe liquid chlorine should be fed to an evaporator upstream of the chlorinator
bull If the ambient temperature can be expected to be below 50 degF (10 degC) for extended periods a building should be providedfor the chlorinator and cylinders
bull Weighing scales should be provided to determine when shipping containers should be replaced and to verify chlorine feedrates
bull Analyzers to measure the free chlorine residual in the circulated cooling water should be provided
bull Chlorine leak detectors and a catch basin for major spills should be provided
Control SystemProvide automatic controllers to continuously monitor water condition and to add chemicals to the system (complete systemsmay be available through the water treatment program vendor as a lease or purchase)pH Control - This controller adds acid to correct water alkalinity a timer on the acid pump sounds an alarm if the pump doesnot shut off in a predetermined length of time Backup controls are required to prevent the cooling system from beingpermanently damaged in case the pH electrodes become fouledCorrosivity and Inhibitor Control - Systems are available which give a direct readout of corrosion rate in milsyear If thecorrosion rate exceeds a set point the controller shuts off the acid increases the inhibitor addition opens the blowdown valveand sounds an alarm Electrodes measure the completeness of the inhibitor film by the value of the potential differencebetween the electrodes The larger the difference the more incomplete the film and a potential reading exceeding a set levelactivates the inhibitor feed
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XXVII 24 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Free Residual Chlorine Control - Free residual chlorine should be controlled automatically for biocide treatment Severalgood analyzers are currently available These units can be incorporated into an automatic control system
Oil in Water DetectionMonitoring Corrosion and Deposit FormationThe following types of devices for measuring corrosion rates and deposit formation should be provided to allow the plant tomonitor the effectiveness of the cooling water treatment before scheduled turnaroundsCoupons - A corrosion coupon rack should be specified for the hot return water to the tower in accordance with IP 19-6-1 Corrosion Monitor - On-line corrosion analyzer if specified should monitor the hot return water to the tower in accordance withIP 19-6-1
On-Line Test Exchanger Although there are a variety of deposit monitors from the water treatment chemical vendors they do not necessarily representactual heat transfer conditions in process exchangers For this reason a test exchanger should be installed for evaluatingcorrosion and deposit formation Details are covered in Report No EE102E78 Cooling Tower Water Treatment Guidelines
Alternatively a critical process exchanger can be instrumented such that its heat transfer coefficient can be monitored
Instrumentation would include flows and inlet and outlet temperatures of process fluid and cooling waterSTEP 6 - Specify cooling tower location
Select a location for the cooling tower The following factors influence this selection
Location Factors1 The location should be within a reasonable distance of the units served and consistent with future refinery plans Piping
costs to and from battery limits can be a significant portion of the total cooling system cost and can be the location-determining factor particularly for salt water systems While piping costs can be easily determined other costs affectinglocation cannot be estimated readily Therefore judgment is required in selecting the location
2 Safety spacing requirements (see Section XV-G Equipment Spacing )3 Cooling tower contribution to plant boundary noise4 For salt water cooling towers wind direction and distance to other facilities is of major concern because of potential
corrosion problems
5 Factors affecting cooling tower performance ie orientation with respect to wind site congestion etc
Plot RequirementsIn estimating plot requirements (basin area) one should allow 5 to 6 ft of tower length for each 1000 gpm (2 m for each 60 Ls)of circulating capacity This figure is based on a typical counterflow design with a width of 50 ft (12 m) and does not includepump suction pit space Exact requirements can be obtained from cooling tower vendors after the tower capacity has beenestimated
Fogging and DriftTwo distinct problems are associated with the discharge air from a cooling tower (1) fogging which can be a nuisance or ahazard and (2) drift which is generally a nuisance because of increased corrosion of surrounding equipment Fogging fromcooling towers occurs during periods of combined high ambient humidity and low temperature as a result of mixing the warmmoist cooling tower exhaust air with the cooler ambient air The ambient air cannot absorb all moisture as vapor and thus tinyfog droplets are formed by condensation Fogging and resulting icing in cold climates has produced hazardous drivingconditions on highways up to 850 ft (250 m) awayProprietary designs are available to combat potential fogging problems Basically these designs incorporate heating of thedischarge air with hot return water precooling coils (similar to an air cooler) to eliminate any visible plume However thesedesigns are expensive and relatively unproven Therefore proper location based on reliable weather data is the preferredsolutionDrift from a cooling tower represents a more difficult problem since it does not depend on definable climatic conditions andoccurs for all wind directions The effects of drift on nearby equipment and vegetation are particularly severe and generally of concern only for salt water systems because of the high solids content of the drift (concentrated salt water) For salt water cooling systems electric lines substations and instrumentation should be located upwind in the direction of the predominantwind They should be provided with adequate shelter or materials when within 450 ft (140 m) of the cooling tower In additionspecifications for all equipment within 150 ft (45 m) of the tower should include surface treatment suitable for a marineenvironment
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COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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XXVII 28 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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XXVII 30 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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COOLING WATER SYSTEMS XXVII 31 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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XXVII 32 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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COOLING WATER SYSTEMS XXVII 33 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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COOLING WATER SYSTEMS XXVII 35 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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COOLING WATER SYSTEMS XXVII 37 of 48
DESIGN PRACTICES December 2000
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FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
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FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
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FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
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FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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XXVII 24 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Free Residual Chlorine Control - Free residual chlorine should be controlled automatically for biocide treatment Severalgood analyzers are currently available These units can be incorporated into an automatic control system
Oil in Water DetectionMonitoring Corrosion and Deposit FormationThe following types of devices for measuring corrosion rates and deposit formation should be provided to allow the plant tomonitor the effectiveness of the cooling water treatment before scheduled turnaroundsCoupons - A corrosion coupon rack should be specified for the hot return water to the tower in accordance with IP 19-6-1 Corrosion Monitor - On-line corrosion analyzer if specified should monitor the hot return water to the tower in accordance withIP 19-6-1
On-Line Test Exchanger Although there are a variety of deposit monitors from the water treatment chemical vendors they do not necessarily representactual heat transfer conditions in process exchangers For this reason a test exchanger should be installed for evaluatingcorrosion and deposit formation Details are covered in Report No EE102E78 Cooling Tower Water Treatment Guidelines
Alternatively a critical process exchanger can be instrumented such that its heat transfer coefficient can be monitored
Instrumentation would include flows and inlet and outlet temperatures of process fluid and cooling waterSTEP 6 - Specify cooling tower location
Select a location for the cooling tower The following factors influence this selection
Location Factors1 The location should be within a reasonable distance of the units served and consistent with future refinery plans Piping
costs to and from battery limits can be a significant portion of the total cooling system cost and can be the location-determining factor particularly for salt water systems While piping costs can be easily determined other costs affectinglocation cannot be estimated readily Therefore judgment is required in selecting the location
2 Safety spacing requirements (see Section XV-G Equipment Spacing )3 Cooling tower contribution to plant boundary noise4 For salt water cooling towers wind direction and distance to other facilities is of major concern because of potential
corrosion problems
5 Factors affecting cooling tower performance ie orientation with respect to wind site congestion etc
Plot RequirementsIn estimating plot requirements (basin area) one should allow 5 to 6 ft of tower length for each 1000 gpm (2 m for each 60 Ls)of circulating capacity This figure is based on a typical counterflow design with a width of 50 ft (12 m) and does not includepump suction pit space Exact requirements can be obtained from cooling tower vendors after the tower capacity has beenestimated
Fogging and DriftTwo distinct problems are associated with the discharge air from a cooling tower (1) fogging which can be a nuisance or ahazard and (2) drift which is generally a nuisance because of increased corrosion of surrounding equipment Fogging fromcooling towers occurs during periods of combined high ambient humidity and low temperature as a result of mixing the warmmoist cooling tower exhaust air with the cooler ambient air The ambient air cannot absorb all moisture as vapor and thus tinyfog droplets are formed by condensation Fogging and resulting icing in cold climates has produced hazardous drivingconditions on highways up to 850 ft (250 m) awayProprietary designs are available to combat potential fogging problems Basically these designs incorporate heating of thedischarge air with hot return water precooling coils (similar to an air cooler) to eliminate any visible plume However thesedesigns are expensive and relatively unproven Therefore proper location based on reliable weather data is the preferredsolutionDrift from a cooling tower represents a more difficult problem since it does not depend on definable climatic conditions andoccurs for all wind directions The effects of drift on nearby equipment and vegetation are particularly severe and generally of concern only for salt water systems because of the high solids content of the drift (concentrated salt water) For salt water cooling systems electric lines substations and instrumentation should be located upwind in the direction of the predominantwind They should be provided with adequate shelter or materials when within 450 ft (140 m) of the cooling tower In additionspecifications for all equipment within 150 ft (45 m) of the tower should include surface treatment suitable for a marineenvironment
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COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
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DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
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DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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XXVII 28 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
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COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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XXVII 30 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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XXVII 32 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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COOLING WATER SYSTEMS XXVII 35 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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COOLING WATER SYSTEMS XXVII 37 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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COOLING WATER SYSTEMS XXVII 25 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
Recirculation and InterferenceThe tower location should provide for an orientation relative to the wind direction during peak wet bulb temperature periods thatwill result in a minimum recirculation of the tower on itself and a minimum interference between the new tower and other towersin the area
A tower placed with the wind broadside experiences recirculation on the middle cells on the lee side of the tower with the endcells relatively unaffected Towers with the wind in a longitudinal direction experience recirculation on all cells downwind of thefirst few cellsRecirculation on a given tower depending on its length can be minimized by proper orientation Cooling towers less than 250ft (75 m) long should be aligned with the prevailing summer wind Towers with lengths between 250 and 350 ft (75 and 100 m)should be placed perpendicular to the prevailing wind If the estimated tower length exceeds 350 ft (100 m) it should be splitinto multiple units The problem then becomes similar to placing a new tower in the vicinity of existing towers and shouldachieve minimum interference between units Spacing recommendations are shown in Figure 10
STEP 7 - Provide facilities for chemical preparation handling and distribution
See ERampE Report No EE92E94 or EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feedersfor Cooling Towers (Leased or Purchased) for additional design practices for chemical injection systems
Also refer to Table 3 for Method of Chemical ReceiptBulk Liquid - This is the preferred method of receipt because the chemical can be distributed as is with no preparationrequiredIn some areas bulk liquid chemicals will be available in returnable liquid containers Where this is the case the followingshould be provided1 Storage area for a reserve supply of full containers and for holdup of empty containers prior to return The amount of
holdup will be dependent on the location of the supplier delivery frequency and regularity of deliveries Sufficient holdupshould be provided to reduce the chance of running out of chemical to essentially zero The storage area may have to besheltered and heated depending on the location and the chemical properties
2 Provide equipment to offload and handle the full and empty containers if this equipment is not available in the plant3 Provide an access road for truck deliveries4 Provide a safety shower and eye wash near the chemical handling area
In other cases bulk liquids may be available as truck delivered parcels In this case the following should be provided1 Provide a minimum storage quantity equal to the lesser of 30 days supply or the chemical shelf life in a minimum of two
tanks Heating may be required depending on the properties of the chemical2 Provide a compressed air supply at 30 psig (210 kPa) for unloading trucks andor rail cars3 Provide an access road for tank truck deliveries4 Provide hose connections to storage tanks5 Provide a gauge glass or local indicator for tank level plus remote level indication6 Provide an overflow line on tanks with a connection to the chemical sewer if available7 Provide a dessicator in the vent line of the sulfuric acid tank8 Provide a safety shower and eye wash near all chemical storage tanksLiquid Gas - Refer to Report No EE102E78 Cooling Tower Water Treatment Guidelines for information on chlorine handlingand chlorinator operation A copy of the safety guidelines published by the Manufacturing Chemists Association should be
available for each plant using a gas chlorinatorDrums or Bags - Chemical handling facilities should be designed for operation by one man only A fork lift should be availablefor moving chemical containers1 Outside Storage requires the following
a Some protection for chemicals and operators
b Cold treated soft water [less than 100 degF (38 degC)] for dilution of chemicals Treated soft water should be piped into thearea where chemicals are mixed
c Drum pump (with spare) to transfer liquid chemical into the day tankd Wet chemical feeder (see EMRE Water and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for
Cooling Towers (Leased or Purchased) )
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
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DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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XXVII 28 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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XXVII 30 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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DESIGN PRACTICES December 2000
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TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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DESIGN PRACTICES December 2000
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FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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DESIGN PRACTICES December 2000
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FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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DESIGN PRACTICES December 2000
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FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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DESIGN PRACTICES December 2000
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FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
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FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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XXVII 26 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
2 Inside Storage (Drums or Bags) - When the ambient temperature is below 55 degF (13 degC) for more than 48 consecutivehours a heated and lighted building is required The building should contain the following
a Monorail or hoist to move chemicalsb Drum pump (with spare) to transfer liquid chemical into the day tankc Operators testing lab for monitoring chemical feedd Day tanks metering pumps and 30-day storage for all chemicalse Special wetting equipment for polyphosphatef Eductor or feed screw for dry chemicals which are packaged in fiber drums
Dry Materials (In bags)1 For quantities of less than 1000 lbday (450 kgday) use an eductor or feed screw2 For quantities greater than 1000 lbday (450 kgday) provide a variable speed conveyor automatic bag breaker and
automatic pneumatic loading into silos Each silo must be provided with high and low level indicators isolation valves andchutes for transferring chemical by gravity to the dry chemical feeder located under the silo Electric rappers andcompressed air should be provided to keep the dry chemical flowing freely through the chutes
3 For solid Bromine andor Chlorine special feed systems are required depending on the chemical used Refer to EMREWater and Wastewater Design Guide TMEE 080 DG 11-6-2 Chemical Feeders for Cooling Towers (Leased or Purchased) and EMRE specialists
STEP 8 - Provide instrumentation to automatically control cooling tower blowdown
Use specific conductance measurement of dissolved salts in the water to operate the automatic blowdown control valveLocate the automatic control valve at some point in the distribution system so that the blowdown will go directly into thewastewater sewer Note the control valve and piping should be sized based on the maximum system blowdown neglectingdrift and miscellaneous losses
Any recirculated water that must be discarded after process use due to a contamination (eg barometric condensers invacuum pipestills) may be counted as tower blowdown A small blowdown should always be taken from the tower basin toallow hydrocarbons that may accumulate on the surface to be removed However the blowdown weir should be sized for themaximum system blowdown
STEP 9 - Provide sidestream filtration if local data indicates need for this
Provide sidestream filtration if it is anticipated that sludge will accumulate in the cooling water system during certain seasons of the year Use a sand pressure filter for sludge removal The amount of recycled water to be filtered is calculated from thefollowing formula
])sL(gpmRateBlowdown[1200
Water CoolinginSolidsSuspendedppm)sL(gpmRateFilter
minus=
Backwash water from the filters can either be from another source or if cooling water is used this water should be consideredas part of the tower blowdown In either case backwash effluent water must be sent to the waste treatment plant
STEP 10 - Specify pumps and distribution system
Size recirculation pumps and distribution piping using the same design considerations and procedures as those presented for the once-through system Steam turbine drivers are normally used along with motors as the cooling tower location is usuallyless remote than once-through system intake stations
Recirculating pump ∆P should include the total circulating system pressure drop (including the frictional pressure drop in thecooling tower pipe) plus the discharge pressure required at the cooling tower inlet to its distribution system plus the elevationdifference between the centerline of the inlet to the tower distribution system and the minimum water level in the basin
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
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DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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XXVII 28 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
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COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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XXVII 30 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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XXVII 32 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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COOLING WATER SYSTEMS XXVII 33 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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COOLING WATER SYSTEMS XXVII 35 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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DESIGN PRACTICES December 2000
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FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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DESIGN PRACTICES December 2000
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FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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DESIGN PRACTICES December 2000
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FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
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FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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DESIGN PRACTICES December 2000
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FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
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FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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COOLING WATER SYSTEMS XXVII 27 of 48
DESIGN PRACTICES December 2000
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DESIGN PROCEDURE FOR RECIRCULATING SYSTEMS (Cont)
For counterflow cooling towers an open disengaging stack shall be provided on each vertical return riser at the cooling tower toprotect the t ower from damage resulting from large gas releases associated with the failure of high pressure gas coolers
Figure 12 sh ows the conceptual arrangement of this open stack which is an open vertical extension of the riser pipe to eachcell The height of the extension should equal the pressure drop through the cooling tower inlet distribution plus an allowanceof 6 ft (2 m) to allow for higher-than-normal return flows and distributor pressure drops A small [1-12 in (38 mm)] diameter connection should be provided at the expected water height in the stack to the tower basin to withdraw small amounts of hydrocarbons which may accumulate on the surface The discharge point to the basin should be visible from outside the tower with the tower in service A hydrocarbon detector shall be provided at the top of one riser (preferably the first one in thedirection of flow) in a tower with multiple risers The risers (cooling water return line) should be grounded for lightningprotection in accordance with IP 16-4-1 For recirculating water systems a makeup water intake facility pump and lines to the cooling tower are required Theseshould also be sized using the same design considerations and procedures as those presented for the once-through systemIn this case however the makeup water rate is used as the design basis Note that mechanical treatment for biological controlis required at the intake facility in addition to those facilities required at the cooling towerProvide a bypass line between the supply and return headers to (1) limit velocities in exchangers and (2) keep the coolingtower wet to minimize fire hazards during startup and turnarounds
STEP 11 - Prepare a system flow plan similar to the one shown in Figure 2
REVAMP AND EXPANSION PROJECTS
The majority of recent projects have involved plant revamp and expansion As a result the offsite effort has been concentratedon reuse of existing facilities as opposed to grass roots projects There are a number of special items which should beconsidered with respect to the cooling water systemBefore starting any revamp or expansion work on a cooling tower it is essential to determine the actual capacity of the tower and compare this to the original design capacity This can be accomplished by running a performance test Typically somedropoff in capacity can be expected Common causes of reduced capacity are damaged fill cooling water pump and pumpimpeller erosion increased drift losses due to the poor condition of the drift eliminators fan blade pitch not set properly andother general deterioration These maintenance items should be corrected before determining further steps to be taken for increased tower capacityThere is a strong incentive to upgrade an existing cooling tower rather than add new cells or if plot space does not permit toprovide a new tower to meet future cooling water requirements Cooling tower performance can be increased by revampingwith improved components A number of these components are described below
bull Changeout to high-performance PVC cellular film-type fill if the cooling tower is equipped with splash type fill This has thepotential for the highest level of improvement in counter flow cooling towers where the air and water flows are parallel (inopposite directions) In crossflow towers where the air flows across the fill perpendicular to the direction of water flowcellular fill is less applicable A small section of cellular fill may be added at the top of the tower under the distributionbasin to improve water distribution to the splash fill below Some vendors also have developed schemes which useinclined cellular fill but this may make maintenance more difficult Replacing wood splash fill with fiberglass grid splash fillin crossflow towers will also improve performance somewhat Changing to cellular fill may not be appropriate for servicescooling mainly heavy fuels where a leak would cause fill plugging or for services with a high fouling potential It may alsonot be appropriate for revamps which replace only a portion of the splash fill with film fill where the film fill will be difficult toget to after the revamp
bull Change wood to cellular plastic high-efficiency drift eliminators
bull Replace old in-tower distribution system with low pressure PVC piping in conjunction with large square spray nozzlesbull Change the pitch of the fan blades to the maximum angle consistent with the motor horsepower (or replace motors) and
within the limits of drift-eliminator efficiency
bull Other potential fan modifications include use of automatic variable-pitch fans eased inlet fan cylinders elevated fancylinders to reduce the recirculation effect and use of variable-speed drivers
Whenever increasing the air flow the increased noise potential must be investigatedIncreased cooling tower capacity can be in the form of increased water flow at the same range increased range at the sameflow or a combination of these Increased flow is required if new coolers are added to the system Increased range is requiredif the cooling water return temperature (T1) is increased or the required cold water temperature (T2) and approach aredecreased
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XXVII 28 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
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COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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XXVII 30 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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DESIGN PRACTICES December 2000
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TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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DESIGN PRACTICES December 2000
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FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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COOLING WATER SYSTEMS XXVII 37 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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XXVII 28 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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REVAMP AND EXPANSION PROJECTS (Cont)
In the investigation of the cooling water distribution system it is necessary to develop the current piping layout and the flowsand pressure drops through each branch As flow meters are not commonly installed to provide the flow rates the use of an
ultrasonic flow meter has proved useful in developing this data Water rates can also be estimated by heat balance if theprocess fluid flow and ∆T and the cooling water ∆T are known In existing plants it is common to find cooling water distributionsystems which are not operating according to the original designHigher-than-design flow rates frequently exist This is usually the result of lower-than-design pressure drops in individualcoolers and uncontrolled flow through them (overcooling or over-recirculation) Conservatively designed cooling water linesand supply pumps may contribute to this The result is that although the total heat rejected to the cooling water is per thedesign this heat is being rejected at a higher flow and a lower water return temperature (lower ∆T) The cooling tower like anyheat exchanger is not able to reject the design heat rate at a lower-than-design temperature difference with the ambient wetbulb temperature This results in higher cold water temperatures (T2) which may cause cooling problems on days with highambient wet bulb temperatures The cooling water distribution system must be balanced by trimming the flow to individualcoolers to match the design process fluid outlet temperature and cooling water velocity Note that cooling water velocity in thecoolers must be maintained above the recommended minimum value (see Section IX Heat Exchange Equipment ) Coolingwater flow through a cooler may be trimmed by pinching back on a valve in the cooling water line Section XIV-A recommendsthe use of a throttling valve on one side (inlet or outlet) of all coolers or condensers
The following is a simplified plan for balancing cooling water exchanger water usage This procedure can be used in one areaof the plant or the entire plant
bull The following data should be collected before altering the cooling water flow rates
+ Cooling water bulk supply temperature and return temperature
+ List of operating recirculation pumps (if cooling tower used) or supply pumps (once-through system) and dischargepressure
+ Cooling water flow rates to each area (wherever flow meters are available) Note that portable ultrasonic flow metersare available for measuring liquid in a line
+ Cooling water temperature to and from each exchanger
bull Make Initial Adjustments and readjust as required
While the cooling water supply temperature is likely to be different from the design temperature the ∆T across eachexchanger should not vary much from design Comparing the design ∆T with the measured ∆T should indicate which
exchangers are receiving more water than design For these exchangers the cooling water throttling valve should bepinched in until the cooling water outlet temperature approaches the design value The cooled process streamtemperature should be checked to assure that it is adequately cooled (at or below design cooled temperature) When theexchangers have been adjusted and the system has stabilized a repeat round of data should be taken Comparison of this data with the design data will determine if further adjustments are required
bull Make Permanent ChangesWhen a satisfactory balance of the cooling water system has been achieved the cooling water flow rate to each exchanger can be calculated by balancing heat on the process and cooling water sides Restriction orifice plates of the appropriatesize can then be installed as necessary to keep the system balanced A control valve in a header may be appropriatewhere excess pressure results in much higher than design cooling water flows to a major branch of the system whilestarving other sections
Flow velocities less than the recommended minimum of 3 fts (09 ms) may also be encountered in branches of an existingdistribution system This may be the result of changing process requirements projects to reduce energy consumption whichhave eliminated coolers or winter operations If low velocities are found it may be necessary to make some hardwarechanges (such as adding a controlled cooling water exchanger bypass) in specific branches to ensure that adequate velocitiesare maintained at all times
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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XXVII 30 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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COOLING WATER SYSTEMS XXVII 35 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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Section Page
XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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COOLING WATER SYSTEMS XXVII 37 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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COOLING WATER SYSTEMS XXVII 29 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
COMPUTER PROGRAMS
Software for designing and evaluating cooling water systems is constantly being introduced and revised For currentinformation regarding the availability and use of cooling water system software contact the OffsitesUtilities Engineering
Section in EMRE Software presently available covers component design (eg cooling tower parameters pumps fans)cooling water distribution system analysis and cooling system chemical analysisThird party software for use on a PC is available for network analysis The programs titled Inplant and Micro Hardy Cross havethe capabilities to perform network analysis and to determine pressure drops and hydraulic profiles taking into account linediameters elevation changes pump head curves control valves and process pressure drops
A program titled Environmental Simulation Program (ESP) evaluates cooling tower chemistry It is capable of predicting water chemistry parameters such as pH scaling index and gas evolution based on water analysis
NOMENCLATURE
A = Sump dimension in (mm) (Figures 7A and 7B )B = Blowdown rate gpm (Ls) sump dimension in (mm) (Figures 7A and 7B )C = Sump dimension in (mm) (Figures 7A and 7B )D = Drift loss gpm (Ls) diameter of sump in (mm) (Figure 8 )E = Evaporation loss gpm (Ls)H = Sump dimension in (mm) (Figures 7A and 7B )L = Dimensions in multiple pump pits (Figure 8 )M = Water recirculation rate lbh (kgh)O = Miscellaneous losses gpm (Ls)P = Dimensions in multiple pump pits (Figure 8 )Q = Cooling tower duty (heat load) Btuh (kJh)S = Sump dimension in (mm) (Figures 7A and 7B )
T1 = Temperature of water entering the cooling tower degF ( degC)
T2 = Temperature of water leaving the cooling tower degF ( degC)
∆T = Temperature drop across the cooling tower degF ( degC)V = Inlet velocity fts (ms)W = Dimensions in multiple pump pits (Figure 8 )Y = Sump dimensions in (mm) (Figures 7A and 7B )
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XXVII 30 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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COOLING WATER SYSTEMS XXVII 31 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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XXVII 32 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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COOLING WATER SYSTEMS XXVII 33 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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COOLING WATER SYSTEMS XXVII 35 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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Section Page
XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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COOLING WATER SYSTEMS XXVII 37 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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XXVII 30 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 1IMPURITIES FOUND IN COOLING WATER
CONSTITUENT CHEMICAL COMPOSITION DIFFICULTIES CAUSED MEANS OF TREATMENTHardness Calcium and
Magnesium saltsexpressed as CaCO 3
Forms scale deposit on heattransfer surfaces
Lime andor Zeolitesoftened makeup water pHcontrol blowdown of recycled water
Alkalinity Bicarbonate saltsexpressed as CaCO 3
Forms calcium carbonate scalesattacks wood materials
Dealkalized makeup waterpH control dispersantsblowdown of recycled water
Sulfate Sulfate ions SO 4 ndash Reacts with calcium in waterforming calcium sulfate depositson condensers and coolers
Remove calcium ions bylime andor Zeolitesoftening blowdown for dissolved solids control
Chlorides Chloride ions Cl ndash Adds to dissolved solids content
and increases corrosion potentialof cooling water
Blowdown for dissolved
solids control
Silica Reactive SiO 2Silica
Reacts with calcium magnesiumiron in water forming silicatedeposits
Hot lime softeningblowdown for concentration control
Oil HHydrocarbons - C -
H
Forms sludge accumulations andbiological slimes films surfacesinhibiting heat transfer
Add dispersant with highblowdown correct oil leaksin process equipment
Ammonia Ammonium ion NH 4+ Corrosion of copper and zincalloys forms complex ion withzinc component in corrosioninhibitors rendering themineffective Excessiveconsumption of chlorine
Blowdown correct processleaks chlorination
Dissolved Solids High concentrations of dissolved
solids can cause corrosion andincrease the potential for salts toprecipitate out of solution andform scale deposits on heattransfer surfaces
Lime softening blowdown to
control concentrationdispersants
SuspendedSolids(UndissolvedMatter)
Settle out in low velocity areascausing plugging deposits in heatexchange equipment andenhancing biological growth andfouling
Pretreatment of makeupwater (coagulationsettling filtration)sidestream filtration of recirculating watercareful location of coolingtower
Oxygen andCarbonDioxide
O2 CO 2 General corrosion and local pittingof pumps piping heat exchanger tubes ie all metal surfaces
Add corrosion inhibitorsand control pH
Algae BacteriaFungi etc
Organic growths and slimedeposits
Add chemical biocidesrepair leaks into system
Acid Gases H 2S SO 2 Corrosion of carbon steel andcopper alloys
Chlorination inhibitorsrepair leaks into system
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COOLING WATER SYSTEMS XXVII 31 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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XXVII 32 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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COOLING WATER SYSTEMS XXVII 33 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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COOLING WATER SYSTEMS XXVII 35 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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Section Page
XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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COOLING WATER SYSTEMS XXVII 37 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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COOLING WATER SYSTEMS XXVII 31 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 2ALLOWABLE CONCENTRATION OF IMPURITIES IN RECIRCULATED FRESH COOLING WATER
IMPURITIES MAXIMUM CONCENTRATIONS
Total Dissolved Salts 3000 wppm ldquoas is
Total Suspended Solids 200 wppm gt 045 microns
Total Hydrocarbons 10 wppm as carbon
Biological Oxygen Demand (5 day test) 60 wppm as O 2
MagnesiumSilicate (Mg ++ x SiO 2) lt 30000 pH lt8
lt 12000 pH = 8-9
lt 8000 pH gt 9
CalciumMagnesiumSilicate (Ca ++ x Mg ++ x SiO 2) lt 1000000 pH = 85
SPECIFIC IONS
Alkalinity 50 - 500 wppm as CaCO 3 Equivalent
Aluminum lt 1 wppm as Al
Ammonia lt 10 wppm as ammonia
Calcium 1000 wppm as CaCO 3 equivalent
Magnesium 250 wppm as Mg ++ pH = 8
Copper lt 02 wppm as Cu
Chlorides lt 1000 wppm as Cl ndash
Iron lt 5 wppm as Fe
Silica lt 150 wppm as SiO 2
Sulfate as SO 4 ndash
++CaaswppmCalcium
000500
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XXVII 32 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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COOLING WATER SYSTEMS XXVII 35 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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COOLING WATER SYSTEMS XXVII 37 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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Section Page
XXVII 32 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 3METHOD OF CHEMICAL RECEIPT
BULK LIQUID (2)DRY MATERIAL
BAGS (1)
TANK TRUCKRAIL CAR OR
PIPED FROM PLANT 55-gal DRUMS
LIQUID GAS(ONE-TON
CYLINDERS)
SodiumHexametaphosphate
Sodium Hydroxide Sodium Hydroxide Sodium Hypo-Chlorite
Chlorine
Sulfuric Acid mdash mdashZinc Metaphosphate Cooling Tower FormulatedCorrosionInhibitors
Hydrochloric Acid Biocidesmdash
Phosphonates Sulfuric AcidHydrochloric Acid
Chlorine Gas Cooling Tower FormulatedCorrosion
Inhibitors
mdash
Zinc Organic SodiumHypoChlorite mdash
Cooling Tower FormulatedScale Inhibitors
mdash
Biocides
bull Brominatedchlorinatedhydantoin
bull Chlorinated isocyanuricacid salts
Cooling Tower Formulated ScaleInhibitors
mdash mdash mdash
Notes
(1) Cooling tower chemicals are rarely purchased in the dry form
(2) Bulk liquid chemicals may also be available in returnable containers in some locations
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Section Page
COOLING WATER SYSTEMS XXVII 33 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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Section Page
XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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COOLING WATER SYSTEMS XXVII 35 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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Section Page
XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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COOLING WATER SYSTEMS XXVII 37 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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Section Page
XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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Section Page
COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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Section Page
XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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COOLING WATER SYSTEMS XXVII 33 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
TABLE 4WATER TREATMENT ANALYSIS WORKSHEET
CONSTITUENT CATIONS wppm AS MAKEUP WATER RECIRCULATEDCOOLING WATER
CATIONS
Calcium (Ca ++ )
Magnesium (Mg ++ )
Sodium (Na +)
Ammonium (NH 4+)
CaCO 3
TOTAL CATIONS
ANIONS
Bicarbonate (HCO 3 ndash )
Carbonate (CO 3 ndash )
Hydroxide (OH ndash )Sulfate (SO 4 ndash )
Chloride (Cl ndash )
Nitrate (NO 3 ndash )
CaCO 3
TOTAL ANIONS
OTHER
Total Hardness
Methyl Orange Alkalinity ldquoM
ldquoPrdquo Alkalinity
Carbon Dioxide Free
Silica
TurbidityBOD (O 2 consumption)
pH
Phosphates
Hydrocarbons
Iron Total
Aluminum Total
Copper Total
CaCO 3
CO 2SiO 2
NTUO2mdash
PO 4 ndash
C
Fe
Al
Cu
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XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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COOLING WATER SYSTEMS XXVII 35 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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COOLING WATER SYSTEMS XXVII 37 of 48
DESIGN PRACTICES December 2000
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FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
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FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
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FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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DESIGN PRACTICES December 2000
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FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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XXVII 34 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 1TYPICAL ONCE-THROUGH COOLING WATER SYSTEM
DP27f01
PdIC
PdIC
PdIC
PdIC
Cooling Water Pumps
INTAKE BAYFrom ScreenWash Pumps
Water Inlet
T r a s h
R a c k
T r a s h
R a c k
S c r e e n
S c r e e n
S c r e e n
S c r e e n
C h l o r i n e I n j e c t i o n From Screen
Wash Pumps
PdIC PdHA
ToStart Screen Motor
Ejectors
Chlorinators
ChlorineVaporizer
ChlorineCylinders
Screen Wash Pumps
T o T r a v e l i n g S c r e e n s
To Wastewater Sewer Outfall
Condensers
FE
FE
FE
FE
ProcessUnit
ProcessUnit
Power Plant
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DESIGN PRACTICES December 2000
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FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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DESIGN PRACTICES December 2000
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FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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DESIGN PRACTICES December 2000
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FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
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FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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DESIGN PRACTICES December 2000
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FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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DESIGN PRACTICES December 2000
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FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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COOLING WATER SYSTEMS XXVII 35 of 48
DESIGN PRACTICES December 2000
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FIGURE 2TYPICAL RECIRCULATED COOLING WATER SYSTEM
Notes (1) Piping for concentrated acid may be carbon steel The acid injection tube should be polypropylene (2) Rubber or polypropylene lining is required about 2 feet upstream and 4 feet downstream of injection point
T r a s h
R a c k s a n d
S c r e e n s
S U C T I O N B A Y
S i x - C e l l C o o l i n g
T o w e r
F I
T H A
T L A T I L P
F R
F I L P
A I C C L 2
C h l o r i n a t o r
C h l o r i n e
C y l i n d e r
S i d e s t r e a m
F i l t e r
F l o w V a l v e
F E
P r o c e s s
U n i t
P o w e r
P l a n t
P r o c e s s
U n i t
F E F E
C o o l i n g
W a t e r R e t u r n
L P
F I
F R
I T b a s i n o p p o s i t e
c i r c u l a t i n g
p u m p s
C h l o r i n e
i s
d i s t r i b u t e d
a l o n g
b a c k o f
O i l
S k i m m
i n g
O v e r -
f l o w
W e i r
F i l t e r
F O ( s )
B l o w d o w n
t o
W a s t e w a t e r
S e w e r
A u t o m a t i c
C o n t r o l l e r
p h R e c o r d e r
o r I n d i c a t o r
C o n d u c t i v i t y
R e c o r I n d
C o r r o s i v i t y
R e c o r I n d
R e c o r I n d
I n h i b i t o r
P L C I
M
M
P L ( C I ) A
S e l e c t o r
S w
i t c h
T
C o o l i n g
W a t e r
R e c i r c u
l a t i o n
P u m p s
M H S
S t e a m
F O ( s )
A u x i l i a r y
P u m p
H a n d
S w
i t c h
I n h i b i t o r I n j e c t i o n
P u m p s
S e d i m e n t
P o t
S p a r e
L I
L L A
M
I n h i b i t o r
S t o r a g e ( amp M i x i n g )
T a n k O v e r f l o w
H i n g e d
C o v e r
W a t e r
S V S V
G L
D e t a i l A
N o t e s
1 2
M a k e u p
W a t e r
F r o m R a w
W a t e r S o u r c e A
u x i l i a r y P u m p
H a n d S w
i t c h
S u p p l y
C o n n e c t i o n
A c i d
S t o r a g e
T a n k
M M
S V
H S
S V
L H A
L L A
L I
F I L L A
S p a r e
A c i d
I n j e c t i o n
P u m p s
L I C
S e e
D e t a i l A
D P 2 7 f 0 2
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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COOLING WATER SYSTEMS XXVII 37 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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XXVII 36 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 3COOLING TOWER TYPES
Distribution PipeMist
Eliminator
Fan Fan
Induced-Draft Counterflow Induced-Draft Crossflow
Air Air
Air Air
MistEliminator
Water Deck
Water Deck
Mist Eliminator
Forced-Draft
Air
Air
Mist Eliminator
Air
HyperbolicCrossflow
Water Water Air Air
Hyperbolic Counterflow
Mist Eliminator
Distribution
DP27f03
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COOLING WATER SYSTEMS XXVII 37 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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COOLING WATER SYSTEMS XXVII 37 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 4SUPPORT STRUCTURE FOR WATER INTAKE CRIB
SupportMembers
Batter Piles
Intake Pipe
Collar Weldedto Intake Pipe
SECTION A-A
Elevation
LLW
Batter
A A
Details of IntakePipe and Crib Shown
Pipe to be Driven
ELEVATION
Cut Off for ClarityDetails Shown in Elevation
Intake Pipe
Batter Piles
PLAN VIEW
120deg
120deg
DP27f04
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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Section Page
COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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XXVII 38 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 5WATER INTAKE PIPE
MinimumDistance to
PreventVortex Formation
Intake Crib
Transition
Support Collar
Chlorine PipeOpen End
1 ft (03 m) Min
1 ft (03 m)Min
3 to 5 ft (1 to 15 m)Clearance AboveSea Floor
Existing Ground Elevation(Slope Exaggerated)
Required Overburden
Water Intake PipeChlorine Injection Pipe
DP27f05
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COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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Section Page
COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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Section Page
XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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Section Page
XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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Section Page
COOLING WATER SYSTEMS XXVII 39 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 6WATER INTAKE BAY
DieselDrive
Firewater Pumps
Cooling Water Makeup Pumps
MMM
SECTION A-A
A Thickness of Wallsnot Shown in Plan
View for Sake of Clarity
FW
FW
CW
CW
B
Traveling ScreenWash Trough
Sloped as Shown
Handrails
Chlorine Injection
Trash Rack Lift Screens
Screen Lift Mechanism
Drain
Future
Access
Pipeway
Pipeway
A
Firewater Mains
Firewater PressurizingConnection
B
PLAN VIEW
To Chlorinator To TravelingScreen Wash
F W F W C W
Handrail
Trash Rack
Lift ScreenFrame
TravelingScreen
M
FlowLLW
5 ft(15 m)
MinLevel of Topof Inlet Pipe
SECTION B-B
Change in Directionof Sidewall Minimum Distance = 5x (Largest
Pump Suction Bell Diameter)
Extreme High Water During Storms
Existing GroundElevation
Level of Top of Pipe
LLW
RequiredOverburden
DP27f06
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Section Page
XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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Section Page
COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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XXVII 40 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7ASUMP DIMENSIONS VS FLOW RATE
CUSTOMARY UNITS
987654329876543298765
4
3
5
6
789
10 4
2
3
4
5
6
789
10 5
2
3
10 10 2 10 3
Recommended Sump Dimensions Inches
DP27f07a
F l o w R a t e p e r P u m p g p m
H Y A S B C
( V = 2 f t s )
H Y
H Y
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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Section Page
COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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Section Page
XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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COOLING WATER SYSTEMS XXVII 41 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 7BSUMP DIMENSIONS VS FLOW RATE
METRIC UNITS
2987654329876543210 410 310 2
10 2
2
3
45
6789
10 3
2
3
4
5
6
789
10 4
2
Recommended Sump Dimensions mm
F l o w R a t e p e r P u m p
L s
YH
A
Y H S B C
H
Y
( V = 0 6 m
s )
DP27f07b
Figures apply to sumps for clear liquid Letter parameters refer to dimensions on Figure 7C
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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Section Page
COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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Section Page
XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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Section Page
COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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Section Page
XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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ExxonMobil Proprietary
Section Page
COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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Section Page
XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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Section Page
COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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Section Page
XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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COOLING WATER SYSTEMS XXVII 43 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 8CONFIGURATIONS FOR MULTIPLE-PUMP PITS
D D
D
D D
D
CLCL Add wall thickness
to - spacingRound off ends of walls Gap at rear
of wall asymp D3
A
B
C
D
E
D
RECOMMENDED
V lt 1 fts (03 ms)S
V X S = 15 to 2D
V
916 D
α
Minimum α = 45degPreferred α = 75deg
V
P
W
L
Baffles grating or strainer should be introducedacross inlet channel at beginning of maximum width section
WP 10 15 20 40 100LV [fts(ms)]
3D13
6D26
7D412
10D618
15D824
FMin2F
V lt 8 fts (24 ms)
Note The dimension D is generally the diameter of the suction bell measured at the inlet This dimension may vary depending on the pump design Refer to the pump manufacturer for specific dimensions
NOT RECOMMENDED
Alternate Inlets
D
V
A V gt 2 fts (06 ms)if A lt 8D
A
V A2
V
W
S L
W gt 5D or V lt 02 fts (006 ms)and L = Same as in chartat left and S gt 4D
V = 2 fts(06 ms)
DP27f08
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
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FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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XXVII 44 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9AFAN HORSEPOWER REQUIREMENTS
CUSTOMARY UNITS
5550454035302520151050
Cooling Range degF
242220181614121008060402Cooling Tower Size Factor
6 0
7 0 7 2
7 4 7 6
7 8 8 0
8 2
A m b i e n t
W e t B
u l b
T e m p e r
a t u r e deg F
DP27f09a
A p p r o a
c h t o W
e t B u l b
T e m p e r
a t u r e
deg C
5
6
7
8
1 0 1 1
1 2
1 5
2 0
Total Fan BHP1000 gpm = (Size Factor) (14)Example (follow arrows on graph) For 30degFCooling Range 82degF Approach to Wet BulbTemperature and 805degF Ambient Wet BulbTemperature read Size Factor = 11Then BHP1000 gpm = (11) (14) = 154
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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COOLING WATER SYSTEMS XXVII 45 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 9BFAN HORSEPOWER REQUIREMENTS
METRIC UNITS
302520151050Cooling Range degC
2 7 deg
4 deg
6 deg
7 deg
8 deg
1 0 deg
1 1 deg
5 deg A p p
r o a c h t
o W e t B
u l b
T e m p e r
a t u r e deg
C
Total Fan kW for 100 Ls = (Size Factor) (166) Example (follow arrows on graph) For 165degC Cooling Range 45degC Approach to Wet Bulb Temperature and 27degC Ambient Wet Bulb Temperature read Size Factor = 11 Then kW100 Ls = 11 x 166 = 183
A m b i e n t
W e t B
u l b
T e m p e
r a t u r e
deg C
2 7 8 2 7
2 6 2 5 2 4
2 3 2 2
2 1
2 0
1 5 5
DP27f09b
02 04 06 08 10 12 14 16 18 20 22 24Cooling Tower Size Factor
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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Section Page
COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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ExxonMobil Proprietary
Section Page
XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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XXVII 46 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 10COOLING TOWER ORIENTATION
A Cooling Tower Length lt 250 ft (75 m)
B Cooling Tower Length 250 to 350 ft (75 to 100 m)
C Multiple Tower Systems
MinimumDistanceEqual toTower Length
MinimumDistanceEqual toTower Length
DP27f10Note Indicates prevailing wind direction during peak wet bulb temperature periods
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Section Page
COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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Section Page
XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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Section Page
COOLING WATER SYSTEMS XXVII 47 of 48
DESIGN PRACTICES December 2000
ExxonMobil Research and Engineering Company ndash Fairfax VA
FIGURE 11WATER TREATMENT FLOW PLAN FOR
RECIRCULATING COOLING WATER SYSTEMS
IMPORTANTPlease read this table before referring to flow plan
Raw Water Reused Water
Cold Lime Softener and Clarifier
Hot Lime Softener and Clarifier
10 ppm 1
35 ppm 2
70 ppm 3
40 ppm 4
1 ppm 2
1 ppm 3
2 ppm 4
1 ppm 1
Filters
1 10 ppm
2 25 ppm
3 5 ppm
5 1 ppm
4 30 ppm
2 1 ppm
3 1 ppm
SodiumZeoliteSoftener
Recycled Cooling Water
EvaporationLoss
DriftLoss
SidestreamFilters
CoolingTower
Process
Losses
BlowdownControl
Scale InhibitorsDispersants
Scale Inhibitors
Dispersants
BiologicalControl
Alkalinity control(Low pH Programs)
HydrogenSodiumZeolite Softeners
DP27f11
The table below explains the code used in the flow planNumbers in circles indicate the impurity reduced by the
treatment equipment The numbers in ppm indicate theamount of impurities remaining after treatment
1 Suspended solids mud silt clay
2 Calcium Hardness (as CaCO 3)
3 Magnesium Hardness (as CaCO 3)
4 Alkalinity (as CaCO 3)
5 Silica (as SiO 2)
Impurity in Water Number Code
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ExxonMobil Proprietary
Section Page
XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes
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ExxonMobil Proprietary
Section Page
XXVII 48 of 48 COOLING WATER SYSTEMSDecember 2000 DESIGN PRACTICES
FIGURE 12COOLING TOWER DISENGAGING STACK
A
SUCTION BAY
Cooling Tower Return Line
Overflow Line A
PLAN VIEW
(empty = 1 12)Overflow Line
N o t e s
1 2
Open Riser
Head equal to pressuredrop through distribution
Distribution
SuctionBay
Over flow Cooling Tower Basin
VIEW A-A
Cooling Water Return Line
Ground for Lightning Protection
Notes