Relief Valve-Eng Handbook

5/25/2018 Relief Valve-Eng Handbook..

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Technical Document No. TP-V3Effective: May 19

Crosby

Pressure Relief ValveEngineering Handbook

Crosby Valve Inc.

An FMC Corporation subsidi

Table of Contents


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Notes:


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*United States Customary System

Warning: The information contained in this handbook is for informational purposes only.

See also Crosby's computer sizing program, CROSBY-SIZE. The actual selection of valvesand valve products is dependent upon numerous factors and should be made only after

consultation with applicable Crosby personnel. Crosby assumes no responsibility for the

actual selection of such products and hereby expressly disclaims liability for any and all

claims and damages which may result from the use or application of this information or from

any consultation with Crosby personnel.

CROSBY

Pressure Relief Valve

ENGINEERING HANDBOOK

CONTENTS

Chapter 1 Introduction to Crosby Engineering Handbook

Chapter 2 Fundamentals of Pressure Relief Valve Design

Chapter 3 Terminology

Chapter 4 Codes and Standards - Summary

Chapter 5 Valve Sizing and Selection - U.S.C.S.* Units

Chapter 6 Valve Sizing and Selection - Metric Units

Chapter 7 Engineering Support Information

Appendix ASME Section VIII, Division 1, 1992 Edition Exerpts

OtherInformation Ordering Information

Pressure Relief Valve Specification Sheet

click on chapter for quick access


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The CrosbyPressure Relief Valve Engineering Hand-book contains important technical information relatingto pressure relief valves.

The primary purpose of a pressure relief valve is protec-tion of life and property by venting fluid from anoverpressurized vessel. Information contained in this

handbook applies to the overpressure protection ofpressure vessels, lines and systems.

Reference is made to the ASME Boiler and Pressure

Vessel Code, Section VIII, Pressure Vessels. The

information in this handbook is NOT to be used for

the application of overpressure protection to power

boilers and nuclear power plant components which

are addressed in the ASME Boiler and Pressure

Vessel Code, Section I, Power Boilers, and Section

III, Nuclear Power Plant Components, respectively.

Proper sizing, selection, manufacture, assembly, test,installation and maintenance of a pressure relief valveare all critical to obtaining maximum protection.

This handbook has been designed to provide a serviceto Crosbys customers by presenting reference data andtechnical recommendations based on our many years ofexperience in sizing, selecting, testing, installing andoperating pressure relief valves. Sufficient data issupplied so to properly size and select Crosby pressurerelief valves for specific applications. Information cov-ering terminology, standards, codes, basic design, siz-ing and selection information, including examples, are

presented in an easy to use format.Some of the material in this handbook is reprinted orexcerpted from publications developed by associationsor committees in which Crosby has participated. Theinformation contained in the manual is offered as aguide. Those who use the information are reminded ofthe limitations of such a publication and that there is nosubstitute for qualified engineering analysis.

Crosby pressure relief valves are manufactured in cordance with a controlled Quality Assurance Progrwhich meets or exceeds ASME Code Quality ConProgram requirements. Capacities are certified byNational Board of Boiler and Pressure Vessel Insptors. These features are assured by the presence oASME Code Symbol Stamp and the letters NB on ea

valve nameplate. Crosby's valves are designed, mafactured and tested in accordance with a quality magement system approved to the International Stdard Organization's ISO 9000 Quality Standard Serequirements. With proper sizing and selection, user can thus be assured that Crosby products arethe highest quality and technical standards in the woof pressure relief technology.

When in doubt as to the proper application of aparticular data, the user is advised to contact the neest Crosby Regional Office or Representative. Crohas a large staff of highly trained people strategic

located throughout the world who should be contacwhen a question arises. Refer to Crosby's WorldwDirectory for an up-to-date contact listing.

Crosby's Computer Aided ValveSizing Program - "CROSBY-SIZE"

Crosby has designed a computer sizing prograCROSBY-SIZE, to provide maximum service to our ctomers by presenting recommendations based Crosby's many years of experience. Use of this prograllows an accurate determination of such parametersorifice size, maximum flow and predicted sound leve

The program is a powerful tool, yet easy to use. Its mafeatures include quick and accurate calculations, uselected units, selection of valve size and style, vadata storage, printed reports, specification sheets adimensional drawings.

CrosbyEngineering Handbook

Technical Publication No. TP-V300

Chapter I

Introduction

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HOME


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Crosby Engineering HandbookChapter 1

Introduction

Program control via pop-up windows, function keys,extensive on-line help facilities, easy to read formattedscreens, immediate flagging of errors, easy editing ofdisplayed inputs and other features combine to makethe program easy to understand and operate.

It is assumed that the user ofCROSBY-SIZEhas a basicunderstanding of relief valve sizing calculations. Theuser is responsible for correct determination of serviceconditions and the suitability of this program for aspecific application.

CROSBY-SIZE and Crosby's Engineering Handbookare useful tools in sizing pressure relief valves. Shouldadditional clarification be required, contact Crosby.


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IntroductionA pressure relief valve is a safety device designed toprotect a pressurized vessel or system during an over-pressure event. An overpressure event refers to anycondition which would cause pressure in a vessel orsystem to increase beyond the specified design pres-sure or maximum allowable working pressure (MAWP).

Since pressure relief valves are safety devices, there aremany Codes and Standards written to control theirdesign and application. The purpose of this discussion isto familiarize you with the various parameters involved inthe design of a pressure relief valve and provide a briefintroduction to some of the Codes and Standards whichgovern the design and use of pressure relief valves.Excerpts of various applicable Codes and Standards areincluded in other sections of this handbook.

Many electronic, pneumatic and hydraulic systems existtoday to control fluid system variables, such as pressure,

temperature and flow. Each of these systems requiresa power source of some type, such as electricity orcompressed air in order to operate. A pressure reliefvalve must be capable of operating at all times, espe-cially during a period of power failure when systemcontrols are nonfunctional. The sole source of power forthe pressure relief valve, therefore, is the process fluid.

Once a condition occurs that causes the pressure in asystem or vessel to increase to a dangerous level, thepressure relief valve may be the only device remaining toprevent a catastrophic failure. Since reliability is directlyrelated to the complexity of the device, it is important thatthe design of the pressure relief valve be as simple as

possible.

The pressure relief valve must open at a predeterminedset pressure, flow a rated capacity at a specified over-pressure, and close when the system pressure hasreturned to a safe level. Pressure relief valves must bedesigned with materials compatible with many processfluids from simple air and water to the most corrosive



Chapter 2

Design Fundamentals

Crosby Style JOS Spring LoadedPressure Relief Valve

Figure F2-1

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media. They must also be designed to operate consistently smooth and stable manner on a varietfluids and fluid phases. These design parameters to the wide array of Crosby products available in market today and provide the challenge for future puct development.

Spring Loaded DesignThe basic spring loaded pressure relief valve has bdeveloped to meet the need for a simple, reliable, sysactuated device to provide overpressure protection. ure F2-1 shows the construction of a spring loapressure relief valve. The valve consists of a valve or nozzle mounted on the pressurized system, a held against the nozzle to prevent flow under norsystem operating conditions, a spring to hold the closed, and a body/bonnet to contain the operaelements. The spring load is adjustable to vary pressure at which the valve will open.

HOM


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Crosby Engineering Handbook

2 - 2

Chapter 2Design

Fundamentals

The design of the control or huddling chamber invoa series of design tradeoffs. If the design maximizeeffort then blowdown will be long. If the design objecis to minimize blowdown, then the lift effort willdiminished. Many pressure relief valves are, therefequipped with a nozzle ring which can be adjustevary the geometry of the control chamber to meeparticular system operating requirement (Figures Fand F2-3).

Liquid Trim Designs

For liquid applications, Crosby offers a unique, patenliquid trim design designated as Style JLT-JOS or JJBS. See Figure F2-4 showing liquid trim availabmetal or soft seated valves. These designs provstable non-chattering valve performance and hcapacity at 10% overpressure.

Figure F2-2 is a simple sketch showing the disc held inthe closed position by the spring. When system pressurereaches the desired opening pressure, the force ofpressure acting over Area A1equals the force of thespring, and the disc will lift and allow fluid to flow out

through the valve. When pressure in the system returnsto a safe level, the valve will return to the closed position.

When a pressure relief valve begins to lift, the springforce increases. Thus system pressure must increase iflift is to continue. For this reason pressure relief valvesare allowed an overpressure allowance to reach full lift.This allowable overpressure is generally 10% for valveson unfired systems. This margin is relatively small andsome means must be provided to assist in the lift effort.

Trim Areas DiagramFigure F2-2

Most pressure relief valves, therefore, have a secondarycontrol chamber or huddling chamber to enhance lift. Atypical configuration is shown in Figure F2-3. As the discbegins to lift, fluid enters the control chamber exposinga larger area A

2of the disc (Figure F2-2) to system

pressure. This causes an incremental change in forcewhich overcompensates for the increase in spring forceand causes the valve to open at a rapid rate. At the sametime, the direction of the fluid flow is reversed and themomentum effect resulting from the change in flowdirection further enhances lift. These effects combine toallow the valve to achieve maximum lift and maximumflow within the allowable overpressure limits. Because ofthe larger disc area A

2

(Figure F2-2) exposed to systempressure after the valve achieves lift, the valve will notclose until system pressure has been reduced to somelevel below the set pressure. The design of the controlchamber determines where the closing point will occur.

The difference between the set pressure and the closingpoint pressure is called blowdown and is usually ex-pressed as a percentage of set pressure.

Crosby Style JOS Pressure Relief Valve TrimFigure F2-3

Metal Seat O-Ring Soft SeaCrosby Styles JLT-JOS and JLT-JBS

Figure F2-4


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Chapter 2Design

Fundamentals

Materials of ConstructionCompatibility with the process fluid is achieved by care-ful selection of materials of construction. Materials mustbe chosen with sufficient strength to withstand the pres-sure and temperature of the system fluid. Materials mustalso resist chemical attack by the process fluid and thelocal environment to ensure valve function is not im-paired over long periods of exposure. Bearing proper-ties are carefully evaluated for parts with guiding sur-faces. The ability to achieve a fine finish on the seatingsurfaces of the disc and nozzle is required for tight shutoff. Rates of expansion caused by temperature ofmating parts is another design factor.

Back Pressure ConsiderationsPressure relief valves on clean non-toxic, non-corrosivesystems may be vented directly to atmosphere. Pres-sure relief valves on corrosive, toxic or valuable recover-able fluids are vented into closed systems. Valves thatvent to the atmosphere, either directly or through short

vent stacks, are not subjected to elevated back pressureconditions. For valves installed in a closed system, orwhen a long vent pipe is used, there is a possibility ofdeveloping high back pressure. The back pressure on apressure relief valve must always be evaluated and itseffect on valve performance and relieving capacity mustbe considered.

A review of the force balance on the disc (Figure F2-2 onpage 2-2) shows that the force of fluid pressure acting onthe inlet side of the disc will be balanced by the force ofthe spring plus whatever pressure exists on the outletside of the valve. If pressure in the valve outlet varieswhile the valve is closed, the valve set pressure will

change. If back pressure varies while the valve is openand flowing, valve lift and flow rate through the valve canbe affected. Care must be taken in the design andapplication of pressure relief valves to compensate forthese variations.

Conventional ValvesBack pressure which may occur in the downstreamsystem while the valve is closed is called superimposedback pressure. This back pressure may be a result of thevalve outlet being connected to a normally pressurizedsystem or may be caused by other pressure relief valvesventing into a common header. Compensation for su-perimposed back pressure which is constant may be

provided by reducing the spring force. Under this condi-tion the force of the spring plus back pressure acting onthe disc would equal the force of the inlet set pressureacting to open the disc. It must be remembered, how-ever, that the value of the set pressure will vary directlywith any change in back pressure.

Balanced Bellows Valves and Balanced Piston ValvesWhen superimposed back pressure is variable, a bal-

anced bellows or balanced piston design is recmended. Typical balanced bellows and piston svalves are shown in Figure F2-5. The bellows or piis designed with an effective pressure area equal toseat area of the disc. The bonnet is vented to ensurethe pressure area of the bellows or piston will alwayexposed to atmospheric pressure and to provide a tale sign should the bellows or piston begin to leVariations in back pressure, therefore, will have no efon set pressure. Back pressure may, however, afflow.

Back pressure, which may occur after the valve is oand flowing, is called dynamic or built up back pressThis type of back pressure is caused by fluid flowing fthe pressure relief valve through the downstream pipsystem. Built up back pressure will not affect the vopening pressure, but may have an effect on valvand flow. On applications of 10% overpressure, anced bellows or balanced piston designs are recmended when built-up back pressure is expecteexceed 10% of the cold differential test pressure (CDT

In addition to offsetting the effects of variable bpressure, the bellows or piston acts to seal process from escaping to atmosphere and isolates the sprbonnet and guiding surfaces from contacting the cess fluid. This is especially important for corroservices.

Balanced Pressure Relief ValvesFigure F2-5

Nozzle TypeThe inlet construction of pressure relief valves is eithfull nozzle as used in Styles JOS, JBS and JLT, Se800/900 OMNI-TRIMand Series BP, or semi nozzl

Crosby Style JBS Crosby Series


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Chapter 2Design

Fundamentals

used in Styles JPV/JPVM. In a full nozzle valve, only thenozzle and disc are exposed to the fluid media when thevalve is closed. In a semi nozzle valve, the nozzle, disc,and part of the valve body are exposed to the inlet fluidwhen the valve is closed.

Seat LeakageAnother important consideration in the design of a pres-sure relief valve is the ability to maintain tight shut off.Pressure relief valves are required to remain on systemsfor long periods of time under widely varying conditionsof pressure and temperature. Seat leakage will result incontinuous loss of system fluid and may cause progres-sive damage to the valve seating surfaces. Extremeleakage could result in premature opening of the valve.Allowable seat leakage limits for pressure relief valvesare many orders of magnitude more stringent thanrequired for other types of valves.

These extremes of tightness are achieved by closecontrol of part alignment, optically flat seating surfaces,and careful selection of materials for each application. Adiligent maintenance schedule must be carried out in thefield to maintain the leak tight integrity of the valve,particularly on a system where the pressure relief valveis cycled often. For additional tightness, where systemconditions permit, soft seat or elastomer seat construc-tion may be employed (see Figure F2-6). Most manu-facturers recommend that system operating pressuresnot exceed 90% of set pressure to achieve and maintainproper seat tightness integrity.

Metal Seat O-Ring Soft SeatCrosby Styles JOS and JBS

Figure F2-6

Screwed Connection ValvesFor applications requiring smaller sizes (0.074 to 0.503sq. in. orifices), maximum versatility and premium per-formance, Crosby offers Series 800 AdjustableBlowdown, Series 900 Fixed Blowdown OMNI-TRIM

and Series BP (Balanced Piston) pressure relief valves.See Figure F2-7 for these screwed connection valves

which also can be furnished with welding end or flanconnections. See Figure F2-5 for Series BP valve.

Series 900 pressure relief valve trim is unique wisingle trim configuration used to provide smooth sta

operation on gas, vapor, liquid and steam applicatio

Adjustable Blowdown Fixed BlowdownCrosby Series 800 Crosby Series 900

(Compressible Fluids Only)

Figure F2-7

Pilot Operated DesignsA second type of pressure relief valve which ofadvantages in selected applications is the pilot operapressure relief valve. Crosby Snap Acting Style JP

shown in Figure F2-8.

Crosby Snap Acting Style JPVPilot Operated Pressure Relief Valve

Figure F2-8


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Chapter 2Design

Fundamentals

Pilot operated pressure relief valves consist of a mainvalve with piston or diaphragm operated disc and a pilot.Under normal operating conditions the pilot allows sys-tem pressure into the piston chamber. Since the pistonarea is greater than the disc seat area, the disc is held

closed. When the set pressure is reached, the pilotactuates to shut off system fluid to the piston chamberand simultaneously vents the piston chamber. Thiscauses the disc to open.

The pilot operated pressure relief valve has severaladvantages. As the system pressure increases, theforce holding the disc in the closed position increases.This allows the system operating pressure to be in-creased to values within 5% of set pressure withoutdanger of increased seat leakage in the main valve.Pilots are generally designed with a separate control forset pressure and blowdown. Valves can be set to openfully at the set pressure and close with a very shortblowdown. Modulating pilot valve designs, as shown inFigure F2-9, control the main valve such that minoroverpressure conditions are controlled without fully open-ing the main valve. This limits fluid loss and systemshock. Another advantage of pilot operated pressurerelief valves is the reduced cost of larger valve sizes. Thelarge spring and associated envelope is replaced by asmall pilot, thus reducing the mass and cost of the valve.

Pilot operated pressure relief valves are supplied withfilters to protect against foreign matter and are generallyrecommended for relatively clean service.

Codes, Standards and RecommendedPracticesMany Codes and Standards are published throughthe world which address the design and applicatiopressure relief valves. The most widely used and rec

nized of these is the ASME Boiler and Pressure VesCode, commonly called the ASME Code.

Most Codes and Standards are voluntary, which methat they are available for use by manufacturers users and may be written into purchasing and consttion specifications. The ASME Code is unique inUnited States and Canada, having been adopted bymajority of state and provincial legislatures and mdated by law.

The ASME Code provides rules for the design construction of pressure vessels. Various sections oCode cover fired vessels, nuclear vessels, unfired v

sels and additional subjects, such as welding nondestructive examination. Vessels manufactureaccordance with the ASME Code are required to hoverpressure protection. The type and design of alable overpressure protection devices is spelled oudetail in the Code.

Certain sizes and types of vessels are specificallycluded from the scope of the ASME Code. For examvessels with operating pressure not exceeding 15 pare excluded from the scope of Section VIII.

A manufacturer, in order to comply with ASME Crequirements, must first prepare a Quality AssuraProgram and submit to periodic on-site inspectionsASME. Completion of this task qualifies the manuturer and allows him to apply an ASME Code stamapproved products. Each product, however, musthrough a specific qualification process.

The product inspection agency for ASME is the NatioBoard of Boiler and Pressure Vessel Inspectors comonly referred to as The National Board. Beforpressure relief valve can be sold with an ASME Cstamp, a group of valves, generally a quantity of nmust be subjected to a flow test conducted in acdance with rules in the ASME Code. From this testin

flow coefficient is determined and submitted to National Board. Once the results of the tests are proved, the flow coefficient is published by the NatioBoard to be used for valve sizing. Thereafter, a samof valves must be submitted to the National Board operiodic basis for flow verification. Any major changethe valve design require that the certification bepeated. All testing is conducted in laboratories whichcertified and inspected by the National Board.

Crosby Modulating Style JPVMPilot Operated Pressure Relief Valve

Figure F2-9


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Chapter 2Design

Fundamentals

A more difficult task is determining the required relievcapacity. The pressure relief valve must relieve a scient amount of fluid to ensure that pressure in the vesor system never exceeds the specified overpressThis means that all possible sources and causes

overpressure must be evaluated. Some examples cobe failure of a stop valve to close, control system failfire, pump failure, uncontrolled chemical reaction, veisolation, and many more. The worst case combinatiothese factors is used to determine the required capac

Total rated relieving capacity of the selected valvevalves if multiple valves are used) must be greater tthe required capacity determined from the worst csystem failure analysis.

SummaryThe purpose of this discussion has been to provideintroduction to some of the considerations emplo

when designing pressure relief valves and to the Coand Standards employed in this industry to maintahigh level of product quality and reliability. More speinformation may be found by referencing the ASCode, various published Standards, and by consuliterature published by the pressure relief valve mafacturers.

It is important to remember that a pressure revalve is a safety device employed to protect prsure vessels or systems from catastrophic failuWith this in mind, the application of pressure revalves should be assigned only to fully train

personnel and be in strict compliance with ruprovided by the governing Codes and Standard

The ASME requirement for capacity certification onceapplied to valves on compressible fluid service only. InJanuary 1985, the ASME rules were expanded to includevalves for liquid service at 10% overpressure, as well asgas, steam and vapor services.

The ASME Code also provides specific rules governingthe application of overpressure protection, determina-tion of and allowable tolerance on set pressure, allow-able overpressure, required blowdown, application ofmultiple valves, sizing for fire, requirements for materialsof construction, and rules for installation.

The most widely used pressure relief valve voluntarystandards in the United States are published by theAmerican Petroleum Institute (API). These Standardsprovide recommended practices for pressure relief valveconstruction, sizing, installation and maintenance. TheAPI, more than any other body, has worked to standard-

ize the ratings and sizes of pressure relief valves, includ-ing pressure/temperature limits and center-to-face di-mensions.

API developed a series of inlet, orifice, outlet combina-tions for various flanged valve pressure classes whichare utilized throughout the petroleum and hydrocarbonprocessing industry. These standard sizes are charac-terized by a series of fourteen standard letter orificesranging from D through T. Each letter refers to a specificeffective orifice area. As an example, the effective areaof a J orifice valve is 1.287 square inches. This orificearea is used in standard API formulations to calculatevalve flow rate. The manufacturer is not required to

produce a valve with a bore area equal to the effectivearea. Rather, he is obliged to produce a valve which willhave a flow rate equal to or greater than that determinedby the API formulation.

Many other Standards are published which deal with theapplication and design of pressure relief valves particu-lar to a specific industry. Additional Codes and Stan-dards are written by various bodies throughout theworld.

Sizing Pressure Relief ValvesThe first step in applying overpressure protection to a

vessel or system is to determine the set pressure, backpressure, allowable overpressure, and required relievingcapacity. Set pressure and allowable overpressure canbe determined by reference to the operating pressuresof the system and the Code under which the system orvessel will be built and inspected.


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Chapter 3

Terminology

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This chapter contains common and standardized termi-nology related to pressure relief devices and is in accor-dance with, and adapted from, ANSI/ASME PerformanceTest Code PTC-25.3-1988, Appendix I and other ac-cepted practices.

Terminology for Pressure Relief Devices

A. General

A.1 Pressure Relief DevicesA pressure relief device is a device designed to preventinternal fluid pressure from rising above a predeterminedmaximum pressure in a pressure vessel exposed toemergency or abnormal conditions.

A.2 Flow Capacity TestingTesting of a pressure relief device to determine itsoperating characteristics including measured relievingcapacity.

A.3 In-Service Testing

Testing of a pressure relief device while protecting thesystem on which it is installed to determine some or allof its operating characteristics using system pressuresolely or in conjunction with an auxiliary lift device orother pressure source.

A.4 Bench TestingTesting of a pressure relief device on a pressurizedsystem to determine set pressure and seat tightness.

B. Types of Devices

B.1 Reclosing Pressure Relief Devices(a) Pressure Relief Valve. A pressure relief valve isa spring loaded pressure relief device which is de-

signed to open to relieve excess pressure and toreclose and prevent the further flow of fluid after normalconditions have been restored. It is characterized byrapid opening pop action or by opening generallyproportional to the increase in pressure over the open-ing pressure. It may be used for either compressible orincompressible fluids, depending on design, adjust-ment, or application.

(b) Safety Valve. A safety valve is a pressure revalve actuated by inlet static pressure and characized by rapid opening or pop action. (It is normused for steam and air services.)

(1) Low-Lift Safety Valve. A low-lift safety valva safety valve in which the disc lifts automatic

such that the actual discharge area is determinethe position of the disc.

(2) Full-Lift Safety Valve. A full-lift safety valva safety valve in which the disc lifts automaticsuch that the actual discharge area is not demined by the position of the disc.

(c) Relief Valve. A relief valve is a pressure rdevice actuated by inlet static pressure havingradual lift generally proportional to the increaspressure over opening pressure. It may be proviwith an enclosed spring housing suitable for clodischarge system application and is primarily used

liquid service.

(d) Safety Relief Valve. A safety relief valve pressure relief valve characterized by rapid openor pop action, or by opening in proportion to increase in pressure over the opening pressdepending on the application and may be used eifor liquid or compressible fluid.

(1) Conventional Safety Relief Valve. A convtional safety relief valve is a pressure relief vawhich has its spring housing vented to the dischaside of the valve. The operational characteris(opening pressure, closing pressure, and reliev

capacity) are directly affected by changes of back pressure on the valve.

(2) Balanced Safety Relief Valve.A balanced sarelief valve is a pressure relief valve which incorates means of minimizing the effect of back presson the operational characteristics (opening pressclosing pressure, and relieving capacity).

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Chapter 3

Terminology

(e) Pilot-Operated Pressure Relief Valve. A pilot-operated pressure relief valve is a pressure relief valvein which the major relieving device is combined withand is controlled by a self-actuated auxiliary pressurerelief valve.

(f) Power-Actuated Pressure Relief Valve. A power-actuated pressure relief valve is a pressure reliefvalve in which the major relieving device is combinedwith and controlled by a device requiring an externalsource of energy.

(g) Temperature-Actuated Pressure Relief Valve. Atemperature-actuated pressure relief valve is a pres-sure relief valve which may be actuated by external orinternal temperature or by pressure on the inlet side.

(h) Vacuum Relief Valve. A vacuum relief valve is apressure relief device designed to admit fluid to pre-vent an excessive internal vacuum; it is designed to

reclose and prevent further flow of fluid after normalconditions have been restored.

B.2 Non-Reclosing Pressure Relief Devices. A non-reclosing pressure relief device is a pressure reliefdevice designed to remain open after operation. Amanual resetting means may be provided.

(a) Rupture Disc Device. A rupture disc device is anon-reclosing pressure relief device actuated by inletstatic pressure and designed to function by the burst-ing of a pressure containing disc.

(b) Breaking Pin Device. A breaking pin device is anon-reclosing pressure relief device actuated by inlet

static pressure and designed to function by the break-age of a load-carrying section of a pin which supportsa pressure containing member.

C. Parts of Pressure Relief Devices

approach channel - the passage through which thefluid must pass to reach the operating parts of a pres-sure relief device

breaking pin - the load-carrying element of a breakingpin device

breaking pin housing -the structure which enclosesthe breaking pin mechanism

discharge channel - the passage through which thefluid must pass between the operating parts of a pres-sure relief device and its outlet

disc - the pressure containing movable element of apressure relief valve which effects closure

huddling chamber - the annular pressure chamberlocated beyond the valve seat for the purpose of gener-ating a popping characteristic

lifting device- a device for manually opening a psure relief valve by the application of external forcelessen the spring loading which holds the valve clo

lifting lever - seelifting device

nozzle -a pressure containing element which contutes the inlet flow passage and includes the fiportion of the seat closure

pilot valve -an auxiliary valve which actuates a mrelieving device (Crosby sometimes calls pilot actua

pressure containing member (of a pressure redevice) - a part which is in actual contact with pressure media in the protected vessel

pressure retaining member (of a pressure redevice) - a part which is stressed due to its functioholding one or more pressure containing memberposition

rupture disc- the pressure containing and presssensitive element of a rupture disc device

rupture disc holder -the structure which encloses clamps the rupture disc in position

seat - the pressure containing contact betweenfixed and moving portions of the pressure containelements of a valve

vacuum support -an auxiliary element of a rupture device designed to prevent rupture or deformation ofdisc due to vacuum or back pressure

D. Pressure Relief Valve DimensionalCharacteristics

actual discharge area- the measured minimum area which determines the flow through a valve.

bore area - the minimum cross-sectional flow area nozzle

bore diameter -the minimum diameter of a nozzle

curtain area - the area of the cylindrical or condischarge opening between the seating surfaces ated by the lift of the disc above the seat

developed lift -the actual travel of the disc from cloposition to the position reached when the valve iflow-rating pressure

discharge area - see actual discharge area

effective discharge area - a nominal or computed aof flow through a pressure relief valve, differing fromactual discharge area, for use in recognized flow forlas to determine the capacity of a pressure relief va


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Chapter 3Terminology

inlet size - the nominal pipe size of the inlet of apressure relief valve, unless otherwise designated

lift - the actual travel of the disc away from closedposition when a valve is relieving

nozzle area, nozzle throat area- see bore area

nozzle diameter - see bore diameter

orifice area - see effective discharge area

outlet size - the nominal pipe size of the outlet of apressure relief valve, unless otherwise designated

rated lift -the design lift at which a valve attains its ratedrelieving capacity

seat angle - the angle between the axis of a valve andthe seating surface. A flat-seated valve has a seat angleof 90 degrees.

seat area - the area determined by the seat diameter

seat diameter - the smallest diameter of contact be-tween the fixed and moving portions of the pressurecontaining elements of a valve

seat flow area- see curtain area

throat area - seebore area

throat diameter- seebore diameter

E. Operational Characteristics of PressureRelief Devices

back pressure -the static pressure existing at the outletof a pressure relief device due to pressure in thedischarge system

blowdown - the difference between actual poppingpressure of a pressure relief valve and actual reseatingpressure expressed as a percentage of set pressure orin pressure units

blowdown pressure - the value of decreasing inletstatic pressure at which no further discharge is detectedat the outlet of a pressure relief valve after the valve hasbeen subjected to a pressure equal to or above thepopping pressure

breaking pressure -the value of inlet static pressure atwhich a breaking pin or shear pin device functions

built-up back pressure - pressure existing at the outletof a pressure relief device caused by the flow throughthat particular device into a discharge system

burst pressure- the value of inlet static pressure atwhich a rupture disc device functions

chatter -abnormal rapid reciprocating motion of movable parts of a pressure relief valve in which the dcontacts the seat

closing pressure -the value of decreasing inlet stpressure at which the valve disc reestablishes conwith the seat or at which lift becomes zero

coefficient of discharge- the ratio of the measurelieving capacity to the theoretical relieving capac

cold differential test pressure -the inlet static psure at which a pressure relief valve is adjusted to oon the test stand. This test pressure includes cortions for service conditions of superimposed back psure and/or temperature.

constant back pressure -a superimposed back psure which is constant with time

cracking pressure -see opening pressure

flow capacity -see measured relieving capacity

flow-rating pressure -the inlet static pressure at whthe relieving capacity of a pressure relief devicemeasured

flutter - abnormal, rapid reciprocating motion of movable parts of a pressure relief valve in which the ddoes not contact the seat

leak pressure - seestart-to-leak pressure

leak test pressure -the specified inlet static press

at which a quantitative seat leakage test is performeaccordance with a standard procedure

marked breaking pressure - the value of pressmarked on a breaking pin device or its nameplate

marked burst pressure -the value of pressure maron the rupture disc device or its nameplate or on theof the rupture disc and indicates the burst pressurthe coincident disc temperature

marked pressure - the value or values of pressmarked on a pressure relief device

marked relieving capacity -see rated relieving capa

measured relieving capacity -the relieving capacita pressure relief device measured at the flow-rapressure, expressed in gravimetric or volumetric u

opening pressure- the value of increasing inlet stpressure of a pressure relief valve at which there measurable lift, or at which the discharge becomcontinuous as determined by seeing, feeling, or hea


15/93


3 - 4

Chapter 3

Terminology

start-to-discharge pressure- see opening press

start-to-leak pressure - the value of increasing static pressure at which the first bubble occurs whepressure relief valve is tested by means of air undespecified water seal on the outlet

superimposed back pressure - the static pressexisting at the outlet of a pressure relief device attime the device is required to operate. It is the resupressure in the discharge system from other sourc

test pressure -seerelieving pressure

theoretical relieving capacity- the computed capaexpressed in gravimetric or volumetric units of a thretically perfect nozzle having a minimum cross-stional flow area equal to the actual discharge area pressure relief valve or relief area of a non-reclospressure relief device

vapor-tight pressure- seeresealing pressure

variable back pressure -a superimposed back psure that will vary with time

warn- see simmer

CEN Definitions

accumulation- a pressure increase over the set psure of a pressure relief valve, usually expressed apercentage of the set pressure.

pilot-operated safety valve - safety valve, the option of which is initiated and controlled by the f

discharged from a pilot valve which is itself a dirloaded safety valve.

supplementary loaded safety valve - safety vawhich has, until the pressure at the inlet to the savalve reaches the set pressure, an additional fowhich increases the sealing force. This additional fo(supplementary load), which may be provided by meof an extraneous power source, is reliably releawhen the pressure at the inlet of the safety vareaches the set pressure. The amount of supplemtary loading is so arranged that if such supplementanot released, the safety valve attains its certified charge capacity at a pressure not greater than 1

above the allowable pressure.

overpressure -a pressure increase over the set pres-sure of a pressure relief valve, usually expressed as apercentage of set pressure

popping pressure -the value of increasing inlet staticpressure at which the disc moves in the opening direc-

tion at a faster rate as compared with correspondingmovement at higher or lower pressures. It applies onlyto safety or safety relief valves on compressible-fluidservice.

primary pressure - the pressure at the inlet in a safety,safety relief, or relief valve

rated relieving capacity- that portion of the measuredrelieving capacity permitted by the applicable code orregulation to be used as a basis for the application of apressure relief device

reference conditions -those conditions of a test me-dium which are specified by either an applicable stan-dard or an agreement between the parties to the test,which may be used for uniform reporting of measuredflow test results

relieving pressure - set pressure plus overpressure

resealing pressure -the value of decreasing inlet staticpressure at which no further leakage is detected afterclosing. The method of detection may be a specifiedwater seal on the outlet or other means appropriate forthis application.

reseating pressure -see closing pressure

seal-off pressure- seeresealing pressure

secondary pressure - the pressure existing in thepassage between the actual discharge area and thevalve outlet in a safety, safety relief, or relief valve

set pressure -the value of increasing inlet static pres-sure at which a pressure relief valve displays one of theoperational characteristics as defined under openingpressure, popping pressure, or start-to-leak pressure

simmer -the audible or visible escape of fluid betweenthe seat and disc at an inlet static pressure below thepopping pressure and at no measurable capacity. Itapplies to safety or safety relief valves on compressible-

fluid service.specified burst pressure (of a rupture disc device)-the value of increasing inlet static pressure, at a speci-fied temperature, at which a rupture disc is designed tofunction


16/93



Chapter 4

Codes and Standards

American Petroleum Institute (API)

ANSI/API Recommended Practice 520 Part I, Sizingand Selection. This API design manual is widely usedfor sizing of relief valves on both liquid and gas filledvessels: (a) liquid vessels - paragraphs 5 and 6, and (b)gas filled vessels - Appendix D-3. This RP covers onlyvessels above 15 psig.

ANSI/API Recommended Practice 520 Part II, In-stallation. This includes: (a) recommended piping prac-tices, (b) calculation formula for reactive force on valve(2.4), and (c) precautions on preinstallation handling anddirt.

ANSI/API Recommended Practice 521, Guide forPressure Relief and Depressuring Systems. An ex-cellent document on everything from causes of overpres-sure through flare stacks.

ANSI/API Recommended Practice 526, FlangedSteel Relief Valves. Gives industry standards as todimensions, pressure-temperature ratings, maximum setpressures, body materials.

ANSI/API Recommended Practice 527, Seat Tight-ness of Pressure Relief Valves. Permissible leakagerate of conventional and bellows valves and testingprocedure.

API Guide for Inspection of Refinery Equipment,Chapter XVI Pressure Relieving Devices. Gives: (a)guide for inspection and record keeping, and (b) fre-quency of inspection, Paragraph 1602.03.

American Society of Mechanical Engineers(ASME)

ASME B31.1. Power Piping - Code 1995 Edition

Reference sections:

Chapter II, Part 3, Paragraph 107.8 Safety and reliefvalves including general information, safety and relief

4 - 1

valves on boiler external piping, safety relief valvesnon boiler external piping,and non mandatory appeces on valve installations.

Chapter II, Part 6, Paragraph 122.6 - Pressure Relief Pip

American National Standards Institute(AN

ASME/ANSI B16.5. Pipe flanges and flanged tings.This standard provides allowable materials, psure temperature limits and flange dimensions for stdard ANSI flanges.

ASME/ANSI B16.34. Valves - Flanged, Threaand Welding End.Standard covers pressure, tempeture ratings, dimensions, tolerances, materials, nonstructive examination requirements, testing and markfor cast, forged and manufactured flanged, threaded welding end valves. (End connection dimensions tolerances are applicable only.)

ANSI B31.8. Gas Transmission and DistributSystems. Portions of this large document pertainpressure relief and its limitations.

Manufacturers Standardizations SocieStandard Practices (MSS-SP)

SP-25. (Not applicable to pressure relief valvStandard marking system for valves, fittings, flanges unions. Refer to UG-129 of ASME Section VIIImarking information for pressure relief valves.

SP-55.Quality standards for steel castings for valvflanges and fittings and other piping components.

SP-61. (Not applicable to pressure relief valvPressure testing of steel valves (refer to API Recomended Practice 527 for commercial seat tightntests).

Other Standards to be considered:

See pages 4-2 and 4-3.

HOME


17/93


Chapter 4Codes and

Standards

4 - 2

Codes and StandardsRegulatory Body Codes and Standards

Allami Energerhkai esEnergiabiztonsagtechnikai Felugyelet (AEEF)(State Authority for Energy, Management and Safety)

Budapest VIIIKoztarsasag ter 7, Hungary

American National Standards Institute1430 BroadwayNew York, NY 10018

American Petroleum Institute2101 L Street NorthwestWashington, DC 20037

The American Society of Mechanical EngineersUnited Engineering Center345 East 47th StreetNew York, NY 10017

Association Francaise de NormalisationTour Europe

Cedex 7F-92049 Paris La Defence, France

Australian Standards AssociationNo. 1 The Crescent HomebushNew South Wales 2140, Australia

British Standards Institute389 Chiswick High RoadLondon W4 4AL, England

Canadian Standards Association178 Rexdale BoulevardToronto, Ontario M9W 1R3

Chlorine Institute Inc.2001 L Street, NWWashington, DC 20036

CC NASTHOLShenogina Street123007 Moscow, Russia

Safety Valves 22/1969/VI.12 (mod) 29/1960/VI.7 (orig)

B16.34 Steel Valves, Flanged and Buttwelded EndsB16.5 Steel Pipe Flanges and Flanged FittingsB31.1 Power PipingB31.3 Chemical Plant and Petroleum Refinery PipingB31.4 Liquid Petroleum Transportation Piping SystemsB95.1 Terminology for Pressure Relief DevicesANSI/ASME PTC 25.3 Performance Test Code, Safety

and Relief Valves

API RP 510 Pressure Vessel Inspection CodeAPI RP 520 Recommended Practice for the Design and

Installation of Pressure Relieving Systems inRefineries: Part 1 - Design; Part II - Installation

API RP 521 Guide for Pressure Relief and DepressuringSystems

API Standard 526 Flanged Steel Safety Relief ValvesAPI Standard 527 Commercial Seat Tightness of SafetyRelief Valves with Metal to Metal Seats

API Standard 2000 Venting Atmospheric and LowPressure StorageTanks

API Guide for Inspection of Refinery EquipmentChapter XVI - Pressure Relieving Devices

Boiler and Pressure Vessel CodeSection I - Power BoilersSection II - MaterialsSection IV - Heating BoilersSection VII - Care of Power BoilersSection VIII - Pressure VesselsSection IX - Welding and Brazing Qualifications

NFE 29-410 to 420

AS1271 Safety Valves, Other Valves, Liquid Level Gagesand Other Fittings for Boilers and UnfiredPressure Vessels 1990 Edition

AS1210 Unfired Pressure Vessels (EAA UnfiredPressure Vessel Code) 1989 Edition

AS1200 Pressure Equipment 1994 Edition

BS6759 Parts 1, 2 and 3 Safety Valves

CSA Z299.2.85 (R1991) - Quality Assurance Program -Category 1



Pamphlet 39 Type 1-1/2" JQPamphlet 41 Type 4" JQ

GOST R Certification System


18/93


4 - 3

Chapter 4Codes and

Standards

Codes and Standards(Cont.)Codes and StandardsRegulatory Body

DIN 50049 Materials Testing Certificates

CEN Standards for Safety ValvesPressure Equipment Directive

HEI Standards for Closed Feedwater Heaters

ISO-9000 Quality SystemISO-4126 Safety Valves - General Requirements

Romanian Pressure Vessel Standard

JIS B8210 Spring Loaded Safety Valvesfor Steam Boilers and Pressure Vessels.

SP-6 Finishes of Contact Faces of Connecting EndFlanges

SP-9 MSS Spot Facing StandardSP-55 Quality Standard for Steel Castings

Stoomwezen Specification A1301

NACE MR0175

NB-25 National Board Inspectors CodeNB-65 National Board Authorization to Repair ASME

and National Board Stamped Safety Valvesand Relief Valves

NFPA 30 Flammable and Combustible Liquids Code

Specifications 602 - Safety Valves for Boilers andPressure Vessels

TBK General Rules for Pressure Vessels

TRD 421 AD-Merkblatt A2

Deutsche Institut Fur NormungBurggrafenstrasse 6D-10787 Berlin, Germany

Comite Europeen de Normalisation(Europeon Committee for Standardisation)rue de Stassart 36B-1050 Brussels, Belgium

Heat Exchange Institute, Inc.1300 Sumner AvenueCleveland, OH 44115

International Organisation for StandardisationCase Postale 56CH-1211Geneve 20, Switzerland

I.S.C.I.R. CentralBucurestiFrumoasa nr. 26, Romania

Japanese Industrial Standard CommitteeJapanese Standards Association1-24, Akasaka 4-chome, Minato-kuTokyo 107 Japan

Manufacturers' Standardization Society of the Valveand Fitting Industry1815 North Fort Myer DriveArlington, VA 22209

Ministerie Van Sociale Zaken En WerkgelegenheidDirectoraat Generaal Van De ArbeidDienst Voor Het Stoomwezen2517 KL Gravenhage - Eisenhowerlaan 102 Holland

National Association of Corrosion EngineersP.O. Box 1499Houston, TX 77001

National Board of Boiler and PressureVessel inspectors1055 Crupper AvenueColumbus, OH 43229

National Fire Protection AssociationBatterymarch ParkQuincy, MA 02269

Schweizerisher Verein furDruckbehalteruberwachung (SVDB)Postfach 358030 Zurich, Switzerland

Den Norske Trykkbeholderkomite (TBK)Norsk Verkstedsindustris StandardiseringssentralOscarsgate 20, Oslo, Norway

Verband der TechnischenUberwachungs-Vereine e. V (TUV)Kurfurstenstrafe 564300 Essen 1, Germany


19/93



Chapter 5

Valve Sizing and SelectionU.S.C.S. Units (United States Customary System)

NOTE: Crosby offers a com-

puter program, CROSBY-SIZE,

for sizing pressure relief valves.

See page 1-1 for additional in-

formation or contact your local

Crosby Representative.

5 - 1

IntroductionThis section of the Crosby Pressure Relief Valve Engi-neering Handbook is designed to assist the user in thesizing and selection of pressure relief valves whensystem parameters are expressed in U.S.C.S. units.Please refer to Chapter 6 for sizing using metric unitformulations.

The basic formulae and capacity correction factorscontained in this handbook have been developed atCrosby and by others within the industry and reflectcurrent state-of-the-art pressure relief valve sizing tech-nology. Typical valve sizing examples have been in-cluded to assist in understanding how specific formulaeare applied. Useful technical data is included for easyreference.

This handbook is limited to spring loaded and pilot

operated pressure relief valves. Formulations in thischapter are in U.S.C.S. Units and are consistent with therequirements of ASME Section VIII and API Recom-mended Practice 520.

Sizing formulae in this handbook are used to calculate therequired effective area for a pressure relief valve that willflow the required volume of system fluid at anticipatedrelieving conditions. The appropriate valve size and stylemay then be selected having a nominal effective areaequal to or greater than the calculated required effectivearea. Effective areas for Crosby pressure relief valvesare shown on pages 7-30 and 7-31 along with a crossreference to the applicable product catalogs, styles or

series. Crosby uses "effective" areas in these formulaeconsistent with API RP520.

Crosby pressure relief valves are manufactured andtested in accordance with requirements of the ASMEBoiler and Pressure Vessel Code. Relieving capacitieshave been certified, as required, by The National Boardof Boiler and Pressure Vessel Inspectors.

Pressure relief valves must be selected by those whave complete knowledge of the pressure relievrequirements of the system to be protected and environmental conditions particular to that installat

Selection should not be based on arbitrarily assumconditions or incomplete information. Valve selecand sizing is the responsibility of the system enginand the user of the equipment to be protected.

HOME


20/93


5 - 2

Chapter 5Valve Sizing

and SelectionU.S.C.S. Units

REQUIRED SIZING DATAThe following is a suggested list of service conditions which must be provided in order to properly size and selea pressure relief valve.

1.Fluid Properties:

a. Fluid and State

b. Molecular Weight

c. Viscosity

d. Specific Gravity

Liquid (referred to water)

Gas (referred to air)e. Ratio of Specific Heats (k)

f. Compressibility Factor (Z)

2.Operating Conditions:

a. Operating Pressure (psig maximum)

b. Operating Temperature (F maximum)

c. Max. Allowable Working Pressure (psig)

3. Relieving Conditions:a. Required Relieving Capacity

Gas or Vapor (lb/hr)

Gas or Vapor (scfm)

Liquid (gpm)

b. Set Pressure (psig)

c. Allowable Overpressure %

d. Superimposed Back Pressure (psig)(specify constant or variable)

e. Built-Up Back Pressure (psig)

f. Relieving Temperature (F)


21/93


5 - 3



EXAMPLE #1Atmospheric Back Pressure

Fluid: Natural GasRequired Capacity: 5900 lb/hrSet Pressure 210 psigOverpressure: 10%Back Pressure: AtmosphericInlet Relieving Temperature: 120FMolecular Weight: 19.0

A =W TZ

C K P1Kb MWhere:

A = Minimum required effective discharge area,square inches

W = 5900 lb/hrT = 120F + 460 = 580RZ = Compressibility Factor, use Z = 1.0

P1

= Absolute relieving pressure 210 + 21 + 14.7 =245.7 psia

C = 344 (Table T7-7 on page 7-26)K = 0.975

Kb

= Capacity correction factor due to back pressure.Use K

b= 1.0 for atmospheric back pressure.

M = 19.0 (Table T7-7 on page 7-26)

A =5900 580 (1)

= 0.396 sq.in.(344) (0.975) (245.7) (1) 19

A "G" orifice valve with an effective area of 0.503 squainches is the smallest standard size valve that will flow trequired relieving capacity. From Crosby Catalog No.3select a 1-1/2G2-1/2 Style JOS-15 with Type J caStandard materials of construction are satisfactory for tapplication (natural gas).

EXAMPLE #2Superimposed Constant Back Pressure

In the preceding example, any change in service contions would necessitate recalculation of the required orifarea. For example, rather than atmospheric back prsure, consider that there is a superimposed constant bapressure of 195 psig.

Since the superimposed back pressure is constantconventional valve may be used.

To find the value of the capacity correction factor Kb, u

Table T7-1 on page 7-3.

Pb = Back Pressure PercentageP1

=Back Pressure (psia)

X 100Relieving Pressure (psia)

(195 psig + 14.7 psi)X 100 = 85.3%

(210 psig + 21 psig + 14.7 psi)

The following formula is used for sizing valves for gases and

vapor (except steam) when required flow is expressed as amass flow rate, pounds per hour. Correction factors areincluded to account for the effects of back pressure, com-pressibility and subcritical flow conditions. For steam appli-cation use the formula on page 5-6.

A =W TZ

C K P1K

b M

Where:A = Minimum required effective discharge area,

square inches.C = Coefficient determined from an expression of the

ratio of specific heats of the gas or vapor atstandard conditions (see Table T7-7 on page 7-26),or if ratio of specific heats value is known, seepage 7-9. Use C = 315 if value is unknown.

K = Effective coefficient of discharge, K = 0.975

Kb = Capacity correction factor due to back pressuFor standard valves with superimposed (con-stant) back pressure exceeding critical see TaT7-1 on page 7-3. For bellows or Series BPvalves with superimposed or variable backpressure see Figure F7-2 on page 7-5. For poperated valves see discussion on page 7-4.

M = Molecular weight of the gas or vapor obtainedfrom standard tables or Table T7-7 on page 726.

P1

= Relieving pressure, pounds per square inchabsolute. This is the set pressure (psig) + ovpressure (psi) + atmospheric pressure (psia).

T = Absolute temperature of the fluid at the valveinlet, degrees Rankine (F + 460).W = Required relieving capacity, pounds per hourZ = Compressibility factor (see Figure F7-1 on pa

7-2). Use Z = 1.0 if value is unknown.

Gas and Vapor Sizing10% Overpressure (lb/hr)


22/93


5 - 4



A =500 530(1)

= 0.163 sq(356) (0.975) (37.7) (1) 28.97

From Catalog No. 902, select a 1" x 1-1/2" Crosby Se

900 valve with a No.7, 0.196 sq.in. orifice, Type D liflever and standard materials. Therefore, Model Numb972103M-D.

EXAMPLE #4Variable Superimposed Back Pressure

When a pressure relief valve is exposed to a variaback pressure the set pressure of the valve mayeffected unless either a balanced bellows or seriesstyle valve is selected.

Fluid: Air (UV Stamp Required)

Required Capacity: 280 lb/hrInlet Relieving Temp.: 140 deg FSet Pressure: 58 psigBack Pressure: 0-20.3 psigOverpressure: 10%A BP-Omni threaded valve is preferred for this applicat

A =W TZ

C K P1K

b M

Where:W = 280 lb/hrT = 140F + 460 = 600RZ = Compressibility Factor = 1.0

P1 = Absolute relieving pressure = 58 + 5.8 + 1478.5 psiaC = 356 from Table T7-7 on page 7-26.K = 0.975

Kb = Capacity correction factor from Table F7-2BP Omni on page 7-5 = 0.650.

M = 28.97 from Table T7-7 on page 7-26.

A =280 600(1)

(356) (0.975) (78.5) (0.65) 28.97

= 0.072 sq.in.

From Catalog No. 905, select a 3/4" x 1" Series BP

a 0.074 sq. in. orifice, type D lifting lever and standmaterial. Therefore the Model No. is BP51701M-D.

Gas and Vapor Sizing10% Overpressure (Cont.)

Interpolating from Table T7-1 on page 7-3, Kb= 0.76

A = W TZ = 5900 580 (1)C K P

1K

b M (344) (0.975) (245.7)(.76) 19

= 0.520 sq. in.

A Crosby "H" orifice valve with an effective area of 0.785square inches is the smallest standard valve orifice thatwill flow the required relieving capacity. Since the backpressure is constant a conventional Style JOS valve canbe used. From Crosby Catalog No.310, select a 1-1/2H3Style JOS-15 with Type J cap. For the production testthis valve would be adjusted to open at 15 psig. This iscalled the cold differential test pressure (CDTP) and is

equal to the set pressure minus superimposed constantback pressure. The opening pressure under serviceconditions, however, would equal the sum of the colddifferential test pressure plus the superimposed constantback pressure (210 psig = 15 psig + 195 psig). Theproper valve spring for this particular application wouldbe the spring specified for a CDTP of 15 psig.

EXAMPLE #3Set Pressure Below 30 psig

When a pressure relief valve is to be used with a setpressure below 30 psig, the ASME Boiler and PressureVessel Code, Section VIII, specifies a maximum allow-

able overpressure of 3 psi.

Fluid: Air (UV Stamp Required)Required Capacity: 500 lb/hrInlet Relieving Temp.: 70FSet Pressure: 20 psigOverpressure: 3 psi

A =W TZ

C K P1K

b M

Where:W = 500 lb/hrT = 70F + 460 = 530R

Z = Compressibility Factor, use Z = 1.0P

1= Absolute relieving pressure = 20 psig + 3 psi +

14.7 psia = 37.7 psiaC = 356 from Table T7-7 on page 7-26.K = 0.975

Kb

= Capacity correction factor due to back pressure.Use K

b= 1.0 for atmospheric back pressure.

M = 28.97 from Table T7-7 on page 7-26.


23/93


5 - 5



A=12000 660(.968)(1)

= 4.282 sq1.175(341) (0.975) (201.7) (0.899)

Standard ValveAn "N" orifice valve with an effective area of 4.34 squinches is the smallest standard size valve that will flow

required relieving capacity. From Crosby Catalog No.select a 4N6 JBS-15 with a Type L cap. Standard mateof construction are satisfactory for this application (Etene).

Pilot ValveNote that Crosby Style JPV Pilot Operated Valve malso be selected for this application. Since pilot oated valve performance is unaffected by back psure,* the flow correction factor Kb is not applicaexcept when subcritical flow is encountered. Thus inexample above, the Kbcorrection factor (0.899) shonot be applied if a pilot operated valve is to be selec

A = 12000 660(.968) (1) = 3.849 sq.in1.175 (341) (0.975) (201.7)

From Crosby Catalog No. 318, select a 4N6 JPV-15.

* For Style JPVM, up to 70% back pressure is permisswith exhaust connected to outlet of main valve. Ab70% the exhaust should vent to a suitable low presslocation.

The following formula is used for sizing valves for gases

and vapor (except steam) when required flow is ex-pressed as a volumetric flow rate, scfm. Correctionfactors are included to account for the effects ofbackpressure, compressibility and subcritical flow.

A =SCFM TGZ

1.175 C K P1Kb


square inches.C = Coefficient determined from an expression

of the ratio of specific heats of the gas or

vapor at standard conditions (see Table T7-7on page 7-26) or if ratio of specific heatsvalue is known, see page 7-9.Use C = 315 if value is unknown.

K = Effective coefficient of discharge, K = 0.975

G = Specific gravity of the gas or vapor.

Kb = Capacity correction factor due to backpressure. For standard valves with superposed constant back pressure exceedingcritical see Table T7-1 on page 7-3. For lows or Series BP valves with superimposor variable back pressure see Figure F7on page 7-5. For pilot valves see discusson page 7-4.

P1 = Relieving pressure, pounds per square incabsolute. This is the set pressure (psig) +overpressure (psi) + atmospheric pressure(psia).

T = Absolute temperature of the fluid at the va

inlet, degrees Rankine (

F + 460).SCFM = Required relieving capacity, standard cubfeet per minute (scfm).

Z = Compressibility factor (see Figure F7-1 onpage 7-2). Use Z = 1.0 if value is unknow

Gas and Vapor Sizing10% Overpressure (scfm)

EXAMPLE #1Built-up Variable Back Pressure

Fluid: Ethylene GasRequired Capacity: 12,000 scfmSet Pressure: 170 psigOverpressure: 10%Back Pressure: 0-75 psig

Inlet Relieving Temp.: 200FSpecific Gravity: 0.968Special Requirement: Bolted cap requested

A = SCFM TGZ

1.175 C K P1 Kb


square inchesSCFM = 12,000 standard cubic feet per minute

T = 200F + 460 = 660RG = 0.968 relative to airZ = Compressibility factor, use Z = 1.0

P1 = Absolute relieving pressure 170 psig + 17 psi+14.7 psia = 201.7 psia

C = 341 (from Table T7-7 on page 7-26.)K = 0.975

Kb = Capacity correction factor for bellows stylevalves from Figure F7-2 on page 7-5.

Back Pressure X 100 = 75 X 100 = 44.1%, Kb= 0.899

Set Pressure 170


24/93


5 - 6



The following formula is used for sizing valves for steam

service at 10% overpressure. This formula is based on theempirical Napier formula for steam flow. Correction factorsare included to account for the effects of superheat, backpressure and subcritical flow. An additional correction factor K

n

is required by ASME when relieving pressure (P1) is above

1500 psia.

A =W

51.5 K P1KshKnKbWhere:

A = Minimum required effective discharge area,square inches

W = Required relieving capacity, pounds per hourK = Effective coefficient of discharge, K = 0.975

An "N" orifice valve with an effective area of 4.34 squinches is the smallest standard size valve that will flowrequired relieving capacity. From Crosby Catalog No.3select a 4N6 JOS-46 valve with a Type C lifting lever alloy steel spring. Standard materials of constructionsatisfactory for this superheated steam application.

EXAMPLE #3Saturated Steam at a Relieving Pressure Greatethan 1500 psig

Required Capacity: 88,000 lb/hr saturated stea

Set Pressure: 2750 psigOverpressure: 10%Back Pressure: AtmosphericSpecial Requirement: Open BonnetRelieving Pressure: P1 =2750 psig + 275 psi

14.7 psi = 3039.7 psiaFrom Figure F7-4

on page 7-6: Capacity Correction FactoK

n= 1.155

A =W

51.5 K P1K

shK

nK

b

A =88,000

51.5 (0.975) (3039.7) (1) (1.155) (1)

A = 0.499 sq. in.

A "G" orifice valve with an effective area of 0.503 squinches is the smallest standard size valve that wil l flowrequired relieving capacity. From Crosby Catalog No.3select a 2G3 JOS-76 valve with a Type C lifting lever alloy steel spring. Standard materials of constructionsatisfactory for this saturated steam application.

EXAMPLE #1Saturated Steam (lb/hr)

Required Capacity: 21,500 lb/hr saturated steamSet Pressure: 225 psigOverpressure: 10%Relieving Pressure: P

1= 225 psig + 22.5 psi +14.7 psi

= 262.2 psiaBack Pressure: Atmospheric

A =W

51.5 K P1 KshKn Kb

A =21,500

= 1.633 sq.in.(51.5) (0.975) (262.2) (1) (1) (1)

A "K" orifice valve with an effective area of 1.838 squareinches is the smallest standard size valve that will flow therequired capacity. From Crosby Catalog No.310, selecta 3K4 JOS-15 valve with a Type C lifting lever. Standardmaterials of construction are satisfactory for this satu-rated steam application.

EXAMPLE #2Superheated Steam (lb/hr)

Required Capacity: 108,500 lb/hr superheated steam

Relieving Temp.: 750FSet Pressure: 532 psigRelieving Pressure: P1=532 psig +53.2 psi +14.7 psi

= 599.9 psiaBack Pressure: AtmosphericFrom page 7-8: Capacity Correction Factor,

Ksh

= 0.844

A =108,500

= 4.268 sq.in.(51.5) (0.975) (599.9) (.844) (1) (1)

P1

= Relieving pressure, pounds per square inch

absolute. This is the set pressure (psig) +ovepressure (psi) + atmospheric pressure (psia).

Ksh

= Capacity correction factor due to the degree osuperheat in the steam. For saturated steamK

sh= 1.00. See Table T7-2 on page 7-8 for o

values.Kn = Capacity correction factor for dry saturated steam

at set pressures above 1500 psia and up to 3psia. See Figure F7-4 on page 7-6.

Kb

= Capacity correction factor due to back pressuFor conventional valves with superimposed(constant) back pressure exceeding critical seTable T7-1 on page 7-3. For bellows valves wsuperimposed or variable back pressure see

Figure F7-2 on page 7-5. For pilot valves, sediscussion on page 7-4.

Steam Sizing10% Overpressure (lb/hr)


25/93


5 - 7



EXAMPLE #2Liquid, gpm

Fluid: Castor OilRelieving Cap: 100 gpmSet Pressure: 210 psigOverpressure: 10%Back Pressure: 35 psig (constant)Relieving Temperature: 60FSpecific Gravity: 0.96

A =GPM G

28.14 Kw K

v P


square inchesGPM = 100 gallons per minute

G = 0.96K

w= 1.0 (Page 7-5)

Kv

= 1.0 for non-viscous fluidP = 210 psig + 21 psi - 35 psig = 196 psi

A =100 0.96

= 0.249 sq.in28.14 (1)(1) 196

A number "8" orifice with an effective area of 0.307 sis the smallest Series 900 OMNI-TRIM valve that will the required relieving capacity. Since the back presis constant a conventional Style JOS or Series 900 vcan be used. Therefore, from Crosby Catalog No. 9select a 981105M-A.

EXAMPLE #1Liquid, gpm

Fluid: Sodium TrisulfateRelieving Capacity: 125 gpmSet Pressure: 100 psigOverpressure: 10%Back Pressure: 0-30 psig (built-up)Relieving Temperature: 60FSpecific Gravity: 1.23

A = GPM G28.14 Kv Kw P



G = 1.23Kw = .866 (Figure F7-3 on page 7-5)Kv = 1.0 for non-viscous fluidP = 100 psig + 10 psi - 30 psig = 80 psi

A =125 1.23

= 0.636 sq. in.28.14(1)(.866) 80

An "H" orifice valve with an effective area of 0.785 squareinches is the smallest standard size valve that will flow therequired relieving capacity. Since the built-up back pres-sure exceeds 10% a bellows style valve, Style JBS, isrequired. From Crosby Catalog No. 310, standard materi-als were selected. Therefore, Model Number is 1-1/2H3Style JLT-JBS-15 valve with a Type J cap.

Liquid SizingSpring Loaded Valves

Styles JLT-JOS, JLT-JBS, Series 900 and Series BP

Note: See page 7-25 for information on two phase f

The following formula has been developed for valve

Styles JLT-JOS, JLT-JBS, Series 900 and Series BPpressure relief valves using valve capacities certified bythe National Board of Boiler and Pressure Vessel Inspec-tors in accordance with the rules of the ASME Boiler andPressure Vessel Code, Section VIII. This formula appliesto, and is to be used exclusively for, sizing Crosby StylesJLT, Series 900 and Series BP pressure relief valves forliquid service applications. Valve sizing using this formula-tion is not permitted for overpressures less than 10%.

A =GPM G

28.14 KwK

vP

Where:

A = Minimum required effective discharge area,square inches.G = Specific gravity of the liquid at flowing conditio

GPM = Required relieving capacity, U.S. gallons pminute at flowing temperature.

P = Differential pressure (psi). This is the setpressure (psig) + overpressure (psi) - backpressure (psig). Pressures expressed, psi.

Kv

= Flow correction factor due to viscosity of thfluid at flowing conditions (see page 7-7).

Kw

= Capacity correction factor due to backpressure on bellows or Series BP valves onliquid service. Refer to Figure F7-3 on page


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Crosby Style JPVM Pilot Operated Pressure Relief Valves

may be used on liquid service. The coefficient of dis-charge for these valves has been certified at 10% over-pressure in accordance with the rules of the ASME Boilerand Pressure Vessel Code, Section VIII. Capacities arecertified by the National Board of Boiler and PressureVessel Inspectors. The following formula is to be usedexclusively for Crosby Style JPVM valve.

Note: A Style JPVM on liquid service provides 30%greater capacity than spring loaded type valves withliquid trim. This can permit use of a much smallervalve than would otherwise be required.

A = GPM G36.81 (K

v) P

Where:

A = Minimum required effective discharge areasquare inches.

G = Specific gravity of the liquid at flowing condtions.

GPM = Required relieving capacity, U.S. gallons peminute at flowing temperature

P = Differential pressure (psi). This is the setpressure (psig) + overpressure (psi) - backpressure (psig).

Kv = Flow correction factor due to viscosity of thfluid at flowing conditions (see page 7-7).Note: For optimum operation, fluid viscositshould be no greater than 300 SSU, and in

case Kv= 1.0 may be used.Note: See page 7-25 for information on two phaseflow.

Liquid SizingPilot Operated Valves

Style JPVM

A "G" orifice valve with an effective area of 0.503 squinches is the smallest standard size valve that will flowrequired relieving capacity. From Crosby Catalog 318, standard materials were selected. Therefore, MoNumber is 1-1/2G3 Style JPVM-15.

EXAMPLE #1Liquid, GPMCrosby Style JPVM Valve

Fluid: Sodium Trisulfate

Relieving Cap: 125 GPMSet Pressure: 100 psigOverpressure: 10%Back Pressure: 0-30 psig (built-up)Relieving Temperature: 60FSpecific Gravity: 1.23

A =GPM G36.81 (K

v) P



G = 1.23Kv

= 1.0 for non-viscous fluidP = 100 psig + 10 psi - 30 psig = 80 psi

A =125 1.23 = 0.421 sq.in.36.81 (1.0) 80


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Multiple Valve Sizing

When multiple pressure relief valves are used, one valveshall be set at or below the Maximum Allowable Working

Pressure, MAWP, and the remaining valve(s) may be setup to 5% over the MAWP. When sizing for multiple valveapplications, the total required area is calculated on anoverpressure of 16% or 4 psi, whichever is greater.

When exposure to fire is a consideration, please reence liquid relief valve sizing under fire conditions (

page 7-17).

Example #2

Fluid: AirRequired Capacity: 150000 lb/hrSet Pressure: 200 psigOverpressure: 16%Back Pressure: AtmosphericInlet Relieving Temperature: 150F

A = W TZC K P1Kb M


square inchesW = 150000 lb/hrT = 150 + 460 = 610RZ = Compressibility factor, use Z = 1.0

P1

= Absolute relieving pressure 200 + 32 + 14.7246.7 psia

C = 356 (Table T7-7 on page 7-26)K = 0.975

Kb = Capacity correction factor due to back pres-sure. For standard valves with superimpos(constant) back pressure exceeding critical seTable T7-1 on page 7-3. For bellows valvewith superimposed variable back pressure sFigure F7-2 on page 7-5. Use Kb= 1.0 foratmospheric back pressure.

M = 28.97 (Table T7-7 on page 7-26)

A =(150000) (610)(1)

(356)(0.975)(246.7)(1) 28.97

A = 8.038 sq. in.

Total required orifice area is 8.038 square inches. Vaselected are "N" orifice with an effective area of 4.square inches each. Total area of two "N" orifice vaequals 8.680 square inches. On multiple valve apptions, only one valve needs to be set at or below MAWPadditional valves may be set up to and including 105%MAWP. From Crosby Catalog No. 310, standard matewere selected. Therefore, Model Number is 4N6 JOS-1

Example #1

Reference Example #1, page 5-3, except that this is amultiple valve application:

MAWP: 210 psigFluid: Natural GasRequired Capacity: 5900 lb/hrSet Pressure: 210 psigOverpressure: 16%Back Pressure: AtmosphericInlet Relieving Temperature: 120FMolecular Weight: 19.0

A =W TZC K P

1K

bM


square inchesW = 5900 lb/hrT = 120 + 460 = 580R

Z = Compressibility factor, use Z = 1.0P1 = (210)(1.16) + 14.7 = 258.3 psiaC = 344 (Table T7-7 on page 7-26)K = 0.975

Kb

= Capacity correction factor due to back pres-sure. Use Kb= 1.0 for atmospheric backpressure.

M = 19.0 (Table T7-7 on page 7-26)

A =(5900) (580)(1)

(344)(0.975)(258.3)(1) 19.0

A = 0.376 sq. in.

Therefore, two "E" orifice valves with a total area of .392square inches are selected to meet the required flow for thismultiple valve application: one valve set at MAWP equals210 psig, and one set at 105% of MAWP equals 220.5 psig.The effective area of each "E" orifice valve is .196 squareinches. From Crosby Catalog No. 310, standard materialswere selected. Therefore, Model Number is 1E2 JOS-15-J.


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Fluid: Natural GasRequired Capacity: 7400 lb/hrSet Pressure: 210 psigOverpressure: 10%Back Pressure: AtmosphericInlet Relieving Temperature: 120FMolecular Weight: 19.0

A =W TZC K P

1K

b M


square inchesW = 7400 lb/hrT = 120F + 460 = 580RZ = Compressibility factor, use Z = 1.0

P1

= (210)(1.10) + 14.7 = 245.7 psiaC = 344 (Table T7-7, page 7-26)

K = 0.975Kb

= Capacity correction factor due to backpressure. Use K

b= 1.0 for atmospheric back

pressureM =19.0 (Table T7-7, page 7-26)

spectors in the Pressure Relief Device Certificatio

publication, NB-18. This publication lists the combintion capacity factors to be used with specific ruptudevice and relief valve by manufacturer rupture devicvalve models.

When a combination capacity factor that has been detmined by test for the specific rupture disc and relief vacombination is not available, a combination capacfactor of 0.9 may be used.

Combination Devices

The rated relieving capacity of a pressure relief valve in

combination with a rupture disc is equal to the capacityof the pressure relief valve multiplied by a combinationcapacity factor to account for any flow losses attributedto the rupture disc.

Combination capacity factors that have been deter-mined by test and are acceptable to use are compiled byThe National Board of Boiler and Pressure Vessel In-

Example #1Gas/Vapor Mass Flow (lb/hr)

(See page 5-3)

A =(7400) (580)(1)

(344)(0.975)(245.7)(1) 19.0

A = 0.496 sq. in.

A standard application would require a G orifice SJOS valve with an effective orifice area of 0.503 squinches. However, this application requires a rupture dSince a specific rupture disc has not been specifiedrupture disc combination factor of 0.9 could be used. minimum required effective discharge area mustscaled up by dividing it by the combination factor.

Required Area = (A) / (Fcomb

)

= (.496) / (0.9)

= 0.551 sq. in.

Therefore, this application with rupture disc requan H orifice Style JOS valve of standard materials an effective area of 0.785 square inches, an increasone valve size. However, in this example, if usinspecific rupture disc having a combination factor (Fwhen used with Crosby valves that is 0.986 or highe

larger valve size may not be necessary. (See TNational Board of Boiler and Pressure Vessel InspecNB-18, "Pressure Relief Device Certifications" - SecIV.)


29/93

NOTE: Crosby offers a com-

puter program,CROSBY-SIZE,

for sizing pressure relief valves.

See page 1-1 for additional in-

formation or contact your local

Crosby Representative.

6 - 1



Chapter 6

Valve Sizing and SelectionMetric Units

IntroductionThis section is provided to assist in calculating the re-quired effective area of a pressure relief valve that willflow the required volume of system fluid at anticipated

relieving conditions when system parameters are ex-pressed in metric units. The appropriate valve size andstyle may then be selected having a nominal effectivearea equal to or greater than the calculated requiredeffective area. Detailed explanations and illustrative ex-amples for sizing using U.S.C.S. Units may also be foundin Chapter 5.

Effective areas for Crosby pressure relief valves are shownon pages 7-30 and 7-31 along with a cross reference to theapplicable product catalogs, styles or series. Crosby uses"effective" areas in these formulae consistent with APIRP520.

The basic formulae and capacity correction factors con-tained in this handbook have been developed at Crosbyand by others within the industry and reflect current state-of-the-art pressure relief valve sizing technology. Typicalvalve sizing examples have been included to assist inunderstanding how specific formulae are applied. Usefultechnical data is included for easy reference.

Crosby pressure relief valves are manufactured and testedin accordance with requirements of the ASME Boiler andPressure Vessel Code. Relieving capacities have beencertified, as required, by The National Board of Boiler andPressure Vessel Inspectors.

Pressure relief valves must be selected by those who havecomplete knowledge of the pressure relieving require-ments of the system to be protected and the environmen-tal conditions particular to that installation. Selection

should not be based on arbitrarily assumed conditinor incomplete information. Valve selection and sizis the responsibility of the system engineer and the uof the equipment to be protected.

HOME


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and SelectionMetric Units

REQUIRED SIZING DATAThe following is a suggested list of service conditions which must be provided in order to properly size and sela pressure relief valve.

1.Fluid Properties:

a. Fluid and State

b. Molecular Weight

c. Viscosity

d. Specific Gravity

Liquid (referred to water)

Gas (referred to air)

e. Ratio of Specific Heats (k)

f. Compressibility Factor (Z)

2.Operating Conditions:

a. Operating Pressure (kPag maximum)

b. Operating Temperature (C maximum)

c. Max. Allowable Working Pressure (kPag)

3.Relieving Conditions:

a. Required Relieving Capacity

Gas or Vapor (kg/hr)

Gas or Vapor (Sm3/min)

Liquid (liter/minute)

b. Set Pressure (kPag)

c. Allowable Overpressure %

d. Superimposed Back Pressure (kPag)

(specify constant or variable)

e. Built-Up Back Pressure (kPag)

f. Relieving Temperature (C)


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The following formula is used for sizing valves for gases andvapor (except steam) when required flow is expressed as a

mass flow rate, kilograms per hour. Correction factors areincluded to account for the effects of back pressure, com-pressibility and subcritical flow conditions. For steam appli-cations use the formula on page 6-6.

A = 13160W TZ

C K P1K

b MWhere:

A = Minimum required effective discharge area,square millimeters.

C = Coefficient determined from an expression of theratio of specific heats of the gas or vapor atstandard conditions (seeTable T7-7 on page 7-26).

Use C = 315 if value is unkown.K = Effective coefficient of discharge. K = 0.975

A "G" orifice valve with an effective area of 325 squmillimeters is the smallest standard size valve thatflow the required relieving capacity. From CroCatalog No. 310, select a 1-1/2 G 2-1/2 Style JOS-15 Type J cap. Standard materials of construction satisfactory for this application (natural gas).

EXAMPLE #2Superimposed Constant Back Pressure

In the preceding example, any change in service cotions would necessitate recalculation of the requorifice area. For example, rather than atmospheric bpressure, consider that there is a superimposed consback pressure of 1345 kPag.

Since the superimposed back pressure is constanconventional valve may be used.

To find the value of the capacity correction factor Kb,

Table T7-1 on page 7-3.

Pb =Back Pressure PercentageP1

=Back Pressure (kPag)

x 100Relieving Pressure (kPag)

(1345 kPag + 101 kPa)x 100 = 85.

(1450 kPag + 145 kPag + 101 kPa)

EXAMPLE #1Atmospheric Back Pressure

Fluid: Natural GasRequired Capacity: 2675 kg/hrSet Pressure: 1450 kPagOverpressure: 10%Back Pressure: AtmosphericInlet Relieving Temperature: 50C

Molecular Weight: 19.0

A =13160 W TZ

C KP

1K

b M


square millimetersW = 2675 kg/hrT = 50 + 273 = 323KZ = Compressibility Factor, use Z = 1.0P1 = Absolute relieving pressure 1450 + 145 + 101

= 1696 kPaaC = 344 (Table T7-7 on page 7-26)

K = 0.975K

b= Capacity correction factor due to back pres-

sure. Use Kb= 1.0 for atmospheric backpressure

M = 19 (Table T7-7 on page 7-26)

A =13160 (2675) (323)(1.0)

= 255 sq.mm(344) (0.975) (1696) (1.0) 19

Gas and Vapor Sizing10% Overpressure (kg/hr)

Kb

= Capacity correction factor due to back pressFor standard valves with superimposed (constant) back pressure exceeding critical seeTable T7-1 on page 7-3. For bellows or SeriBP valves with superimposed or variable bacpressure see Figure F7-2 on page 7-5. For poperated valves see discussion on page 7-4

M = Molecular weight of the gas or vapor obtainefrom standard tables or Table T7-7 on page 7-2

P1

= Relieving pressure, kiloPascals absolute. Ththe set pressure (kPa) + overpressure (kPa) atmospheric pressure (kPaa).

T = Absolute temperature of the fluid at the valveinlet, degrees Kelvin (C + 273).

W = Required relieving capacity, kilograms per ho

Z = Compressibility factor (see Figure F7-1 on pa7-2). Use Z = 1.0 if value is unknown.


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Interpolating from Table T7-1 on page 7-3, Kb= 0.76

A = 13160 WTZ =13160 (2675) 323 (1)

C KP1Kb M 344 (0.975) (1696) (0.76) 19

= 335 sq.mm

A Crosby "H" orifice valve with an effective area of 506square millimeters is the smallest standard valve orificethat will flow the required relieving capacity. Since theback pressure is constant a conventional Style JOS valvecan be used. From Crosby Catalog No. 310, selecta 1-1/2 H 3 Style JOS-15 with Type J cap. For the

Documents

Relief Valve-Eng Handbook