TET-DG-5001- Basic Design Criteria v1.1

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Design GuidelinesBASIC DESIGN CRITERIA

TET/SS/5501

Date: 14/05/2015Issue: 1.1

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Electronic Document, only the original archived in Quality Department is signed / Printed copies uncontrolled

Revision Date Details

1.0 22/04/12 First issue

1.1 14/05/2015 - Page 3 section 1.1 last paragraph’’ Future population andGrowth rates’’ updated after publication of Population ProjectionSultanate of Oman (2015-2040) by National centre for Statisticsand Information (NCSI)

- Section 1.3 LPCD table updated.

Contents

Introduction ................................................................................................ ............................................................. 2

1. WATER DEMAND FORECAST ................................................................................................................ 3

1.1 Population forecast ................................................................................................................................... 3

1.2 Categories of Water Demand ................................................................................................................... 4

1.3 Domestic Consumption / Pipe Supply...................................................................................................... 4 1.4 Domestic Consumption / Tankers Supply ................................................................................................ 5

1.5 Non Domestic Consumption .................................................................................................................... 5

1.6 Leakage and other technical losses........................................................................................................... 6 1.7 Water Used by PAEW in its Normal Operations (after leakage) ............................................................ 6

1.8 Fire Fighting .................................................................................................... .......................................... 6

1.9 Commercial Losses .................................................................................... ............................................... 7

1.10 Peak Factors .............................................................................................................................................. 8 2. DISTRIBUTION SYSTEMS .......................................................................................................... .............. 9

2.1

Future Water demand – Design horizon and phasing of Assets .............................................................. 9 2.1.1 General ................................................................................................................... ................................... 9

2.1.2 Water Demand .......................................................................................................................................... 9

2.1.3 An `` All-In`` figure for preliminary estimating ...................................................................................... 9 2.2 Pressure in the network ............................................................................................................................. 9

3. TRANSMISSION AND TRUNK MAIN DESIGN ................................................................................... 10

3.1 Headroom ................................................................................................................................................ 10 3.2 Route and elevation ................................................................................................................................ 10

3.3 Desired pressure .............................................................................................. ........................................ 11

3.4 Velocity ............................................................................................................................................. ...... 11 3.5 Surge ....................................................................................................................................................... 11 3.6 Valves ............................................................................... ....................................................................... 11

3.7 Air valves ................................................................................................................................................ 12

3.8 Pipe materials ...................................................................................... .................................................... 12

3.9

Thrust blocks ............................................................................................. .............................................. 13

3.10 Energy considerations ................................................................................ ............................................. 13 4. WATER QUALITY .......................................................................................................... .......................... 13

5. CONTROL PHILOSOPHY ...................................................................................... .................................. 14 6. SECURITY OF SUPPLY – RISK MANAGEMENT ............................................................................... 15


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INTRODUCTION

This document has been prepared as a Guide to PAEW engineers and their consultants for theplanning of water transmission and distribution systems.

It is not intended as a complete Manual for pipe design, but is intended to:

1. Set out the PAEW policy for pipeline design;2. Inform upon standard pipeline design criteria;3. Provide basic pipeline requirements to ensure the safe operation of the pipeline, and4. Act as an aid memoire where specialist design skills are required e.g. surge.

In the event of any doubt concerning the use of this Guide, the advice of the General Manager:Planning and Asset Management Department should be sought.

Wherever possible, the water demand of an area should be based upon researched datarelating to the specific area under study. Only where such data is not available, should the datain this Guide be used.

The Guide provides the minimum requirements/standards to be met and the dataprovided is to be used as the minimum acceptable.

System InputVolume

(Corrected for

known errors)

WATER

DEMAND

Authorised

Consumption

Billed

Authorised

Consumption

Billed Metered Consumption

(including water exported)Revenue Water

Billed Unmetered Consumption

Unbilled

Authorised

Consumption

Unbilled Metered Consumption

Non Revenue

Water

Unbilled UnmeteredConsumption

Water Losses

Unaccounted for

Water

Apparent Losses

(Commercial

losses)

Unauthorised Consumption

Customer Metering Inaccuracies

Real Losses

(technical losses)

Leakage on Transmission and /

or Distribution Mains

Leakage and Overflows at

Utility’s Storage Tanks

Leakage on Service Connections

up to point of CustomerMetering

Table 1: IWA Standard International Water Balance and Terminology


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1. WATER DEMAND FORECAST

1.1 Population forecast

Population

Future water domestic demand depends upon the future population and future per capitaconsumption.Population in an area can be determined in number of ways:

1. As provided by a developer in his request for a water supply to be made available;2. From the planning proposals of the Omani Planning Authorities;3. Existing population + growth rate;

4. Calculated (N×P×O) from the number of plots (N), properties per plot (P) andoccupancy rates (O), and

5. Using a population/acre figure

Domestic Occupancy

Domestic occupancy rates are difficult to ascertain due to lack of appropriate data.Computations are complicated by the large number of empty houses1, holiday homes and forother reasons.From the 2010 Population Census and the 2010 water demand study, the following have beendeduced for 2010 and can be used for future developments unless more explicit data isavailable.

Parameter Governorate As assessed

for 2010

Domestic occupancy Muscat 5.1

Batinah 6.0

Buraymi 4.6

Dakhliyah 6.0

Dhahirah 6.1

Sharqiyah 4.7

Wusta 7.6

Musandam 3.7

1 According to the 2010 Population Census of the total 551,058 housing units only 396,421 were occupied

2 From Population Projection Sultanate of Oman (2015‐2040) issued by NCSI in September 2014.


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Future population and Growth rates

Unless more specific information is available, for the foreseeable future the following growthrates can be assumed2 and applied. The following table contains the average populationgrowth rate deducted from the Population Projection Sultanate of Oman (2015-2040) issued bythe National Centre for Statistics and Information (NCSI) published in September 2014.

2015/20 2020/25 2025/30 2030/35 2035/40

Omani 13.9% 12.1% 10.0% 8.8% 8.4%

Non-Omani 3.6% 1.8% -0.3% -1.6% -2.2%

For local evolutions and detailed estimates data published by NCSI can be referred. NCSI isupdating this information regularly, for detailed information the designer should refer to the latestdata published.

1.2 Categories of Water Demand

For designing and planning a water main, the most important factor is to understand the waterrequirement of the area to be supplied and the various categories of demand.

There is a distinct difference between water demand, which is the quantity of water requiredwithin an area to satisfy all the water needs of the area, and water consumption, which is theactual quantity of water drawn by customers through their service pipes.

In some cases due to inadequacies in the water supply system, customers demand may not bemet. In which case, their consumption is referred to as “restrained demand”. However watersystems shall be designed for unrestrained water demand.

For water demand forecast the following categories can be used.

Domestic consumption by piped supply;

Domestic consumption by tanker supply;

Non domestic consumption;

Unaccounted for water including:o Leakage and other technical losses;o Water used by PAEW in its normal operations;o Fire fighting usageo Commercial losses

1.3 Domestic Consumption / Pipe Supply

The required water demand of customers to be met by the PAEW for their domestic use, i.e.within their residences, varies depending on several considerations such as socio-economic,cultural and climatic.

Water consumption is expressed as “per capita consumption”, and is the average quantity ofwater used per person per day and is generally termed as “lpcd”.

Wherever data is available, domestic per capita demand should be based upon theconsumption at the time of the design, increased appropriately over the project design horizon,which is usually 25yrs.


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The following data should be used as guidance.

Parameter Governorate

2010

from

Water

Balance

2015 2020 2025 2030 2035 2040

Domestic

consumption as

drawn at tap in

litres/head/day

Muscat175 180 185 190 195 200 200

Batinah North 167 175 180 185 190 195 195

Batinah South 156 165 170 175 180 185 185

Buraymi 180 180 180 180 185 190 190

Dakhliyah 150 158 165 170 175 180 180

Dhahirah 148 154 160 165 170 175 175

Sharqiyah (North and

South)134 152 160 165 170 175 175

Wusta 130 135 140 145 155 160 160

Musandam 191 191 191 191 191 191 191

These are indicative figures only that apply across a Region. It can be expected that there willbe variations within a Region.

1.4 Domestic Consumption / Tankers Supply

Where water is being distributed by the tankers, the consumption of 22 gallons /capita/day is tobe used.

1.5 Non Domestic Consumption

Where there is not expected to be any specific non-domestic high-usage, there are differentmethods to calculate the water consumption, such as:

Commercial user can be converted into the equivalent number of domestic user,

Number of employees at the commercial site divided by the population density perhouse in the region to get the equivalent number of properties,

Hotels, clinics and hospitals can be converted into domestic properties by consideringthe number of employees so that in turn the usage can be calculated.

Generally the domestic usage uses the greatest volume of water per square meter of any othertype of user.

The following table shows some values of non-domestic consumption for special uses:

Category Average per Capita consumption

Public office 60 l / c/day (6 days per week)

Hospital 200 – 500 l / bed / daySchool 20 l / pupil / day (5 days per week)

Hotel 200 – 500 l / person/day

Restaurant 100 l / person/day

Cattle breeding 6 -10 l/ animal/ day

Gardens / plantations 3 -10 l / m / day


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A general “rule of thumb” is to allow 20% of the domestic consumption for non-domestic usage(including government).

Non Domestic Usage – Industrial and High Usage Concerns

Where there are known to be industrial premises that will have a high water consumption or

other developments such as tourist complexes, shopping malls and industrial parks, it isessential that the anticipated water demand is discussed with the appropriate industrialists,developers and planning authorities.

Beware that there is a tendency for planners to over-estimate water needs in order to ensurethe adequacy of the water supply to a development.

1.6 Leakage and other technical losses

Water is lost from the PAEW’s system through a number of ways:

1. Leakage – slow continual loss of water through small holes in the mains, poor joints andthe like;

2. Bursts, where the volume lost can be large but are generally of short duration;3. Water lost when a mains system has to be drained down in order to repair a leak or

burst, including water used to flush the main and take water quality samples after theburst;

4. Water lost from reservoirs and other storage facilities due to non-closing inlet valves,wash-out valves not fully closed, incorrectly set water level recorders when a reservoir isfilled by pumps controlled by the reservoir level and from leakage through the structure,and

5. Water lost from leaking pump glands, inadequately maintained sluice valves, hydrantsetc.

For a new distribution system that has been correctly designed, installed and commissioned,losses should be negligible within the early years of the pipe system. In order to allow for some

leakage, especially during the later life of the system, an allowance needs to be made.

This will be the higher of the following two calculations:

15% of total domestic and non-domestic consumption;

10 m3/d/Km of network in dense urban or 5 m3/d/Km in other areas

1.7 Water Used by PAEW in its Normal Operations (after leakage)

The PAEW will use water during its normal day-to-day operations for such activities as mainscleansing and running hydrants for water quality sample analysis.

Normally, the water used is minimal and can be considered as included within the technicallosses. If there is known to be specific problems in the area, for example with debris settling inthe mains due to the condition of the mains or as sand “carried over” from the treatment/sourceworks, an allowance of 2% of customer demand can be assumed.

1.8 Fire Fighting

Oman’s statutory requirements should be ascertained, and met.

In Oman the Directorate General of Civil Defence set the requirements for fire fighting.


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The design of new distribution network should incorporate fire hydrants in accordance with theDirectorate General of Civil Defence r equirements. Each project must get their approval for thelocation of fire hydrants.

The additional ground service reservoir capacity for fire fighting is as follows:

Population (Capita) Capacity (m )

Less than 5,000 50

Less than 10,000 100







When designing the flow capacity of a distribution system, the following fire flows shall beincorporated, depending on the population of the area served.

Population

(Capita)

Fire flow

(m3 /min)

Less than 5,000 1

Less than 10,000 2

Less than 20,000 4

Less than 40,000 6Less than 60,000 8

Less than 80,000 9

Less than100,000 10

1.9 Commercial Losses

Commercial losses are generally included at design stage within domestic and non-domesticconsumptions.

Meter and other income determination errors

In addition to the recorded flow of water to a customer, there might be an additional flow notrecorded by the meter due to wear of the meter; the meter being incorrectly installed or, inextreme, the meter has become blocked or has stopped recording for some other reason.

A key consideration can be the type of meter installed being one that is not capable of recordingvery low flows that can occur when a customer’s inlet valve to a cistern is not closing off.


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It should be noted that the meter error is included within the above per capita consumptionrates. Thus, meter error normally affects the income received by the PAEW and the historicalrecords of consumption in a given supply area, not the quantity of water that it has to supply.

Unknown Connections and Theft

These are obviously not known as they would be stopped, disconnected or converted into alegitimate connection. As such no specific allowance need to be made, unless it is known thatthe mains are to be laid in an area where illegal connections may be made.

1.10 Peak Factors

Water demand of customers can vary by the hour, by the day and seasonally.

The total quantity of water supplied or drawn for 365 days, divided by 365 is the average dailydemand. This is the normal reference flow.

Domestic per capita figures quoted relate to the annual average demand.

At various times of the year and as a result of seasonal needs, people will use more water insome days. A similar higher demand can occur at weekends when customers are at home. Thisis known as the peak day demand. The peak can vary from one area to another. Past recordsof the study area, or a similar one, should be consulted to determine the historic peak day factorto be used in the design.

People draw water according to the needs. The PAEW has to meet the instantaneous waterdemand when many people wish to draw water at the same time e.g. in the morning beforegoing to work. This is known as the peak hour demand. The peak hour demand can beextremely variable, depending as it does on the actions of people in an area. Within apredominantly residential area where all the customers leave at around the same time each dayto go to work, take children to school, the peak hour factor can be as much as 5 times the

annual average. The peak hour demand can be significantly reduced where water goes first intoa storage tank, and people draw from the storage. The flow to a property is then limited by thecapacity of the service pipe; not the quantity of water used instantaneously.

The amount by which a peak demand exceeds the annual average demand is known as thepeak day or peak hour factor. The peak factors do not apply to all categories of demand. Forexample, leakage will not vary. Industrial demand is usually more consistent – indeed thePAEW can ensure a consistent draw-off by requiring the industrial premise to have storage andby limiting the size of the service pipe to the premise.

Where there is no historic data available, the following can be used as a guide:

Peak day flow = 1.3 × annual average demand (based on the average day demand in the peakweek) excluding leakage or 1.24 times water in to supply i.e. inclusive of leakage

Peak hour flow = 2.5 × peak day flow.


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2. DISTRIBUTION SYSTEMS

2.1 Future Water demand – Design horizon and phasing of Assets

2.1.1 General

Any design must take into account the likely growth in demand in the areas that will be fed fromthe new main(s).

Whilst it is normally not desirable to lay a second pipe in the future to allow for future growth,there can sometimes be an advantage. For example, if the development is to be staged it mightbe cost beneficial, if space permits, to lay one main for the immediate development and tofollow this with a duplication when subsequent development takes place. An alternative wouldbe to convert a gravity main to a pumped main to “force through” more water.Such phasing is only likely to be cost effective for large diameter mains and when there will be asignificant time difference between the development phases. The justification can only be aftera cost benefit analysis is made.

A more likely, and advantageous, scenario would be to lay one main along one route and thenduplicate the main along a second, separate route.

In some instances, short sections of main across roads have been installed and left blanked offallowing an additional main to be laid in the future picking up these blanked off mains. Thisallows an extra parallel main to be laid at minimal cost as the difficult sections have beeninstalled at the time of the original main being laid.

The design horizon for PAEW projects is 25 years. However for consistency issues, horizoncurrently used is year 2040.

2.1.2 Water Demand

See above.

2.1.3 An `` All-In`` figure for preliminary estimating

Surprisingly and consistently, a “rule of thumb” all-in peak day design flow that can be used forpreliminary estimating within an area, is found to be between 300 and 350litres/head/day.

2.2 Pressure in the network

In a distribution system water should be supplied with adequate pressure and flow. However,pressure is lost by the action of friction at the pipe wall and in pipe line components such as

valves. The amount of pressure loss is also dependant on the water demand from customers,pipe material, length, gradient and diameter.

To deliver sufficient quantities of water the pressure head in the network should, whereverpossible, be at least 1.5 bar (15 m.w.c., worst point peak day, peak hour) in all parts of thenetwork, including the remotest and highest points. The maximum pressure should not exceed6 bar (60 m.w.c.).

In case of fire fighting flows, pressure in the network shall be maintained as minimum positivevalue i.e. negative pressure should not be developed in the network.


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3. TRANSMISSION AND TRUNK MAIN DESIGN

3.1 Headroom

Not used in the design of distribution mains, but used for source/treatment works, pumping

stations and transmission mains, is the concept of “headroom”.

Following a major incident, the PAEW’s reservoirs and mains will be depleted. The quantity ofwater to be supplied by the PAEW in these conditions must be such as to not only meetdemand, but also to refill the reservoirs and re-charge the mains.

Customers may have been without water for a considerable time. They will want to use waterfor a backlog of purposes, and their storage will need to be re-filled.

In order to provide for this addition demand, a treatment works etc. is designed to meet theaverage daily flow not over 24 hours, but over 21 hours i.e. the works are designed to supply24/21 (1.14) times the annual daily flow or, put another way, with 15% extra capacity than wouldotherwise be provided. This ratio can be refined for large systems where several sources of

water or pumping stations may be used concurrently to supply the demand.

3.2 Route and elevation

In consideration of the route of a pipeline, a number of factors must be taken into account:1. The hydraulic gradient along the pipeline;2. Where there is space to lay a main and to subsequently maintain the pipeline;3. Ground conditions;4. Obstructions are other difficulties to be overcome both in the laying and continued

operation of the main, such as wadis;5. Major road crossings;6. Environmentally sensitive areas;7. Safety of the PAEW staff in gaining access to the pipeline, and

8. The proximity of areas that could be serviced from the main.9. Other existing or planned utilities (sewer, oil/gas pipelines, cables) should be takeninto consideration;

10. Roads easements standards should be considered.

Sometimes it will be necessary to consider alternative routes and also intermediate boosters tore-lift the pressure.

Sometimes it is necessary to tunnel under obstacles with a relatively short section of smallerdiameter main. Directional boring of large diameter pipes can be a major cost and so reducingthe size of the main at such crossing points is often desirable. Often for security of supply suchcrossings are duplicated. For example a 400mm main could cross a major road with 2 parallel

300mm diameter mains. If this action is taken, the twin bores should be suitably distanced apart

so that an incident to the first bore does not affect the second. The water carrier pipe should belaid within a sleeve to enable a failed pipe to be readily removed and replaced. For the samereason, two adjustable couplings with a connecting piece should be provided.

All crossings of major obstacles should be guarded by in-line valves either side of the obstacle.

All branches off a transmission and trunk mains should be metered, as required by the PAEWNon-Revenue Water Manager, within the Planning and Asset Management Department.


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No service connection should be taken directly off a transmission or trunk main. A fault with theconnection may require the main to be shut down, with possible consequences on maintainingsupplies. If a connection is required, it should be taken from a tapping drilled into a blank flangeon a branch tee, “guarded” by a valve.

3.3 Desired pressure

The pressure within a pipeline is determined by the pressure at the start of the pipeline and atthe terminal point. Pressure at various points along the pipeline is shown on the hydraulicgradient that must be prepared for all transmission or trunk mains during preliminary design atthe latest.

The hydraulic gradient can be used to determine the pressure rating of the pipe to be used.It is important to ensure that:

1. Pipeline profile should go above the gradient line at any point;2. The pipeline does not empty into a receiving reservoir due to the hydraulic

conditions at the end of the pipeline;3. At no place must the pressure within the pipeline exceed the manufacturer’s

pressure rating for the pipe.

To avoid the above happening, a break pressure tank may be required, or an elaborate controlsystem installed at the terminal reservoir.

If branches are to be taken off the main under design, the adequacy of the pressure at the take-off must be determined and, if necessary, a booster pump(s) or a pressure reducing valveinstalled.

Normally the aim of the design is to transfer a desired flow down the main whilst achieving aminimum pressure at the far end. This minimum pressure could be the top water level of aservice reservoir or water tower. The pressure could be a service level required to feedproperties or the desired suction pressure of a transfer pump at the end of the main being

designed.

3.4 Velocity

Normal design velocity is 1 m/s. This figure is the point at which particles in the main will bepicked up. This helps to ensure that the main is self-cleaning and reduces long term build upinside the main. A peak velocity as high as 2.5 m/s is possible but is seldom considered due to the high stressesthis can cause and high pressure loss than can be generated over short distances. A peakvelocity of 2 m/s is more common but will depend on pipe diameter (more acceptable for largepipes than small diameters).

3.5 Surge

Surge is a specialist subject for which advice should be taken.

All transmission and trunk mains, pumped or gravity, should be evaluated for the risk ofexcessive surge pressures developing. If required, anti-surge measures should be adopted andnecessary protection provided to the pipeline.

3.6 Valves


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Valves should be installed so that, where possible, all parts of the network can be controlledwithout adversely affecting another part. As a minimum valve shall be installed:

On all branch connections,

On all branches from feeder mains,

Between feeder pipes and hydrants;

Not more than 2 valves at a tee,

Preferably at a uniform distance from pipe intersections; Not more than 3 valves at a cross,

Washouts at all valley points.

The frequency of in-line isolation valves should be determined for each pipeline and shall belocated strategically. A maximum spacing of line valves for transmission main should notexceed 5km and 2 km for primary trunk mains and maximum 1 km for distribution lines.

The following types of valves are used in a water supply system:

Isolating valves (Gate valves, butterfly valves);

Non-return valves;

Air release valves;

Washout valves;

Pressure regulating valves;

Flow control valves.

Gate valves with flange joints shall be used on pipelines ≤ DN 400. For pipelines > DN 400butterfly valves shall be used.

All valves must be located with due concern for the safety of the PAEW staff/contractor duringtheir operation.

Valves can be buried or installed in a chamber. The criteria for chambers are given in theDesign Guidelines for Transmission and distribution pipelines TET/DG/5005.

The chambers should be constructed to take the end thrust of the pipe against the closed valve.

Some in-line valves may be electrically operated and some provided with a by-pass(systematically above DN 400).

3.7 Air valves

In order to prevent an air lock in the pipeline, an adequate number of suitably designed airvalves need to be incorporated into the pipeline design.

See TET/SS/5008 for technical specifications of air valves.

Air valve design is a special subject but generally air valves require to be located:

1. At high points on a pipeline;2. Where the gradient of a pipeline significantly increases or decreases;3. On pumping mains close to the outlet from a pumping station;4. Between inline valves to allow for mains to be drained and re-charged.

3.8 Pipe materials


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Consideration needs to be given to the pipe material.

PAEW recommend the following:

Distribution pipeso HDPE up to 300 mm diameter (OD 110mm, 180mm and 225mm shall be

used);

o Ductile Iron above 300 mm diameter;o And DI for special locations like road/wadi crossings.

Transmission Lineso Ductile Iron up to 600 mm diameter;o Above 600 mm dia comparison should be made with MS pipes (generally

epoxy coated in and out or 3 layers PPE);o In special cases like high pressure, small diameter pipes MS pipes may also

be used.

Pre-stressed Concrete, GRE and GRP are not in use yet.

3.9 Thrust blocks

Where a pipeline contains flexible joints, concrete thrust blocks must be designed andconstructed at all changes in pipe direction horizontal and vertical. The thrust block must becapable of resisting the pipe test pressure; not the operating pressure which will be lower.

3.10 Energy considerations

Sometimes along long sections of trunk main it is necessary to re-lift the pressure.

For a pumping main, there is normally a choice between a large pipe diameter with acorresponding low pumping head – a high CAPEX/low OPEX option - or a smaller pipe

diameter with a larger pumping requirement – a low CAPEX/high OPEX option.

In order to evaluate the options and determine the most suitable, a cost benefit analysis isrequired considering both OPEX and CAPEX, over the whole life of the assets. The cost of thebooster maintenance, replacement and running costs should be considered.

Having a transfer booster mid-point along a transmission main can have a dramatic impact onpressures along the length of the main. It is possible to use mains with lower pressure ratingand background leakage may also be reduced. Additionally future mains breaks should bereduced as a result of operating at a lower average operating pressure along the length of themain.

4. WATER QUALITY

Consideration to water age should be made at design stage. Increased residence may lead thedeterioration in water quality e.g. reduce chlorine content, taste ,odour and microbiologicalgrowth.

Hydraulic modelling should be used to assess age and potential water quality impact.

Designing pipeline`s future growth should consider range of flows (retention time) over the lifetime of the pipe e.g. initially residence time may be significant due to low demand.


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5. CONTROL PHILOSOPHY

There are numerous methods of controlling a transmission main:

Simple transmission main to a demand area

In this instance the demand from the customers determines how much water will flow down themain. There is little control that can be used on such a system other than pressure control.

The main will have been sized to meet peak demand usage however at night demand is muchlower. Therefore the pressure loss across the main will be lower and the pressure on the mainand within the demand area will increase.

It is possible to install a pressure reducing valve at a point along the new main that will reducepressures at night to minimize leakage and burst rate.

In most PAEW systems, transmission main do not feed directly distribution zones.

Transmission main into storage

Within PAEW the water is generally not directly sent to distribution mains in demand areas.Generally all the water reaches the storage service reservoir and is distributed by gravity.

In this case the normal method of control is to install a control valve on the inlet to the reservoir.This can be controlled in three ways:

1. A valve – When water level in the reservoir falls below certain level the valve opensautomatically and allowing unrestricted flow into the reservoir. If the level exceeds anupper control point the inlet valve shuts. This can be a Floating valve. However thissystem is not desired as it often generate overflow if the valve is not closing properly. An

electrical valve controlled by the level in the reservoir is therefore preferred.

2. Flow Control – A modulating control valve on the inlet to the reservoir operates such thata set flow is allowed into this reservoir. This control can be adjusted remotely from anoperational control room.

3. Pressure Sustaining Valve – A modulating control valve on the inlet to the reservoir thatmaintains a set pressure upstream on the transmission main. This form of control ismost common where there is a high point on the transmission main that preventsunrestricted flow from going into the reservoir. A PSV will ensure that positive pressurealways exists on the transmission main.

Transmission main to the suction of a booster

This can occur if there is a high point at the end of the transmission route which requires abooster to lift the water or if the transmission main is of such a long length that a simpletransmission system with realistic mains diameters is not possible.

It is possible that an economic assessment of mains capacity vs. boosting energy indicates thatthis is the preferred solution.

The size of the transmission main will influence the suction and delivery at the transfer booster.


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It is possible to combine all these elements and controls to ensure that it is possible to transfer agiven amount of water along any feasible construction route.

6. SECURITY OF SUPPLY – RISK MANAGEMENT

See also the Emergency Response Manual.

Wherever possible, risk should be designed out of the water supply system.

Reservoir provides adequate security of supply to a network allowing enough time for atransmission repair to take place, or a treatment works failure rectified. All supply areas shouldhave a minimum of 48 hours storage located after the transmission system and as close aspossible to the area of supply.

Whenever possible and providing a significant risk reduction effect compared with additionalcosts the design of the system will take into account the following considerations:

Alternative supply routes;

Limiting repair time through the use of standard pipe diameters and adequate provision

of inline valves and corresponding fittings (washout and air valves); Parallel pipes or ring main arrangements.

Documents

TET-DG-5001- Basic Design Criteria v1.1