Sizing And Selecting Pressure Relief Valves

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramco’s employees.Any material contained in this document which is not already in the publicdomain may not be copied, reproduced, sold, given, or disclosed to thirdparties, or otherwise used in whole, or in part, without the written permissionof the Vice President, Engineering Services, Saudi Aramco.

Chapter : Instrumentations For additional information on this subject, contactFile Reference: PCI11003 D.W. Buerkel on 874-7339

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Sizing And Selecting Pressure Relief Valves

Engineering Encyclopedia Instrumentations


Saudi Aramco DeskTop Standards

Content Page

INTRODUCTION................................................................................................................ 1

SPECIFICATIONS REQUIRED FOR SIZING PRESSURE RELIEF VALVES.................. 2

Process Flow Diagrams ............................................................................................. 4

Piping & Instrument Diagrams................................................................................... 6

Instrument Specification Sheets ................................................................................. 6

Basis of Selection .....................................................................................................11

Conditions Requiring Overpressure Protection..............................................11

Contingencies That Cause Overpressure........................................................13

Operational Requirements for Overpressure Protection .................................14

Effective-Area Concept.................................................................................14

Methods for Determining Relieving Pressure ............................................................14

Operating Contingencies ...............................................................................15

Fire Contingencies ........................................................................................17

Steam Service ...............................................................................................18

CALCULATING THE SIZE OF A PRESSURE RELIEF VALVE - HANDCALCULATOR METHOD.................................................................................................19

Basis for Calculating Valve Size ...............................................................................19

Service Conditions........................................................................................20

Flow Rate .....................................................................................................20

Effective Discharge Area ..............................................................................21

Sizing Equations For Specific Applications...............................................................22

Sizing for Gas and Vapor Relief....................................................................22

Sizing for Steam Relief .................................................................................24

Sizing for Liquid Relief .................................................................................25

Sizing for Two-Phase Liquid-Vapor Relief....................................................26

SELECTING A PRESSURE RELIEF VALVE - MANUFACTURER’S CATALOGMETHOD ...........................................................................................................................28

Sources of Required Data.........................................................................................28

Instrument Specification Sheets.....................................................................28




Manufacturers’ Catalogs ...............................................................................28

Administrative Requirements ....................................................................................31

SIZING AND SELECTING A PRESSURE RELIEF VALVE - COMPUTERIZEDSIZING METHOD..............................................................................................................32

Program Applications ...............................................................................................32

RELIEF VALVE AUTHORIZATION PROCESS...............................................................37

Relief Valve Authorization, Form 3099A..................................................................40

Form 8020-611 ........................................................................................................40

Relief Valve Test Report, Form 3750 .......................................................................42

WORK AID 1: RESOURCES FOR DETERMINING RELIEF VALVESPECIFICATIONS .............................................................................................................44

Work Aid 1A: Procedure for Determining Applicable Contingencies for PZVSizing and Selection .................................................................................................44

Work Aid 1B: Procedure for Determining Relieving Pressure of PZVs .....................45

WORK AID 2: RESOURCES USED TO CALCULATE THE SIZE OF A RELIEFVALVE-HAND CALCULATOR METHOD.......................................................................47

Work Aid 2A: Formulas and Procedures to Size PZVs for Gas and Vapor................47

Work Aid 2B: Formulas and Procedure to Size PZVs for Steam Relief .....................51

Work Aid 2C: Formulas and Procedure to Size PZVs for Liquid Relief.....................52

Work Aid 2D: Procedure to Size PZVs for Two-Phase Liquid/Vapor Relief .............54

WORK AID 3: RESOURCES USED TO SELECT A RELIEF VALVE -MANUFACTURER’S CATALOG......................................................................................56

WORK AID 4: RESOURCES USED TO SIZE AND SELECT A PRESSURE RELIEFVALVE - COMPUTERIZED SIZING METHOD...............................................................57

GLOSSARY........................................................................................................................61

ADDENDUM......................................................................................................................62

Addendum 1: Crosby Engineering Handbook ...........................................................63

Addendum 2: Properties of Gases.............................................................................64

Addendum 3: SAES-J-600, Pressure Relief Devices, ................................................65

ADDENDUM 4: Crosby Catalogs............................................................................66




Table of Figures Page

Figure 1. Information Sources for PZV Sizing and Selection..................................... 3

Figure 2. Process Flow Diagram of HPPT ................................................................ 5

Figure 3.Typical P & ID ............................................................................................ 6

Figure 4. ISS Form No. 8020-611-ENG, Sheet 1...................................................... 8

Figure 5. ISS Form No. 8020-611-ENG, Sheet 2...................................................... 9

Figure 6. ISS Form No. 8020-611-ENG, Sheet 3.....................................................10

Figure 7. Basis of Relief Capacities Under Selected Conditions1..............................12

Figure 8. Example Determination of Relieving Pressure for a Single ValveInstallation (Operating Contingencies)1 ....................................................15

Figure 9. Example Determination of Relieving Pressure for a Multiple ValveInstallation (Operating Contingencies)1 ....................................................16

Figure 10. Example Determination of Relieving Pressure for a Single ValveInstallation (Fire Contingencies)1..............................................................17

Figure 11. Example Determination of Relieving Pressure for a Multiple ValveInstallation (Fire Contingencies) 1.............................................................17

Figure 12. Inputs and Considerations for PZV Size Calculation ...............................19

Figure 13. PZV-100 Thermal Relief Valve...............................................................29

Figure 14. CROSBY-SIZE Report Sheet for PZV-200 ............................................36

Figure 15. Chart I - Projects Authorization Procedure for RV Installation andChanges....................................................................................................37

Figure 16. Chart II - Operations Facilities Authorization Procedure for ReliefValve Installation and Changes .................................................................39

Figure 17. Relief Valve Authorization, Form 3099A ................................................41

Figure 18. Relief Valve Test Report, Form 3750......................................................43

Figure 19. Set Pressure and Accumulation Limits for Pressure Relief Valves2..........46

Figure 20. Company Database Administration Menu................................................58



Saudi Aramco DeskTop Standards 1

INTRODUCTION

The process of specifying a pressure relief valve involves three main phases:

1. Determining the specifications for the PZV

2. Calculating the flow area of the PZV orifice (effective area)

3. Selecting and verifying the size of the PZV

The first phase involves both the collection of data from existing sources and the determination ofthe design basis of the PZV. The data that must be collected to size and select pressure reliefvalves are specified in API RP-520. The collected data is then recorded on Saudi Aramco’sInstrument Specification Sheet (ISS) Form 8020-611 ENG. The design basis includes theevaluation of all potential causes of overpressure and the calculation of the PZV’s relievingpressure.

In the second phase, a preliminary valve size is calculated based on the particular valvespecifications that were determined in the first phase. For the required relieving conditions, thearea of the PZV orifice is calculated so that it provides the required flow rate and volume ofdischarge. This calculated area is called "effective area" because it is based on values that areassumed, or conceptually assigned from design requirements, rather than values that are actuallymeasured. This module will demonstrate two methods to calculate the effective area—using ahand calculator and using a manufacturer’s computer program.

In the third phase, the calculated "effective area" and the service conditions that are specified onthe ISS are used to select a valve from either a manufacturer's catalog or a manufacturer’scomputer program.

This module also describes the relief valve authorization process. As code certified valves, theevents in the service history of each PZV require authorization by responsible engineers.Signatures are recorded for the approvals required for the origination, installation, maintenanceand removal of each PZV.




SPECIFICATIONS REQUIRED FOR SIZING PRESSURE RELIEF VALVES

Figure 1 shows the relationship between the information sources for the data that must bespecified on the Instrument Specification Sheet and the proces of sizing a PZV.

The first step in establishing the size of a PZV is to determine the conditions for whichoverpressure protection may be required. In order to determine the overpressure conditions, onemust obtain process and equipment information about the physical system that requiresoverpressure protection. Then, each of the potential causes of overpressure must be evaluatedboth in terms of the pressures that may be generated and the rates at which fluids must berelieved. API RP-520, Part I, Section 4.1, "Determination of Relief Requirements," lists thefollowing information that is needed for calculating relieving rates:

• Process flow diagram (PFD)

• Piping & Instrument Diagram (P & ID)

• Design Basis (or Basis of Selection)

All of these items are described in more detail on the following pages. API RP-520 also statesthat the material balance is needed; however, it will be described later. In addition to theinformation above, the following items should be obtained.

• Equipment Specifications - Originating engineers involved in the sizing and selectionof PZVs must review specifications of the equipment to be protected in order toconfirm MAWP, and establish the set pressure for its PZV.

• Construction Layout Drawings - The location of each major piece of processequipment is shown on construction layout drawings. Often layout drawings areused as background drawings for pipe routing drawing. In turn pipe routingdrawings often include vent relief header routing information. These and otherengineering drawing are used to evaluate the physical relationship between theprotected equipment and associated equipment and piping.

• Fluid Properties Data - Fluid properties data are taken from standard engineeringreferences. 'Flow of Fluids Through Valves, Fittings, and Pipe', Technical Paper No.410 (Crane) is an excellent standard engineering reference.




Obtain Required Infomation

PFD P&ID Equipment Specification Sheets Layout Drawings Material Balance Design Basis Vendor Data Fluid Properties Data

Record data on appropriate

ISS Form

Determine Basis for Relief

Calculate Effective Area

Use ISS Form for Bid Solicitation

Figure 1. Information Sources for PZV Sizing and Selection




Process Flow Diagrams

Process conceptual designs are depicted on PFDs (Figure 2). PFDs also include the material andheat balance information that is related to each step in the process. After a PFD is completed, theminimum pressure and temperature design ratings for equipment and piping are determined.Engineering designers use these design ratings for the detail design of equipment and pipe. Afterengineering and design approval, final design pressure and temperature (P&T) ratings areassigned. Design P&T ratings are classified as "Maximum Authorized Working Pressure"(MAWP) and "Maximum Authorized Working Temperature" (MAWT) for equipment and pipe.These ratings are used when equipment is purchased.

Data obtained from the PFD (and P & ID) drawings should not be used blindly. Pressure andtemperature gauge readings may be reported on the wrong process line. They can be located onthe right line, but in a different place than the drawing shows. This does not mean that acquiringaccurate data is a hopeless task, but a certain degree of caution is required in using data forcalculations. The data should stand the test of reasonableness. Are the temperature, pressure,and flow in the expected range? Is the pressure higher at the pump and continuously decreasingas the stream progresses through the plant? Does the temperature rise as expected after passingthrough a heat exchanger?




Figure 2. Process Flow Diagram of HPPT




Piping & Instrument Diagrams

Piping and Instrument Diagrams (P & IDs) contain some of the data that are required for sizingand selecting PZVs. Figure 3 shows a typical P & ID for a High Pressure Production Trap. Notethat the PZV instrument tag number (PZV 132) is printed on the P & ID. The PZV set pressures,and sometimes the maximum operating pressure (MAWP), are also shown on P & IDs.

Figure 3.Typical P & ID

The data from P & IDs are not sufficient for final PZV sizing and purchase specification becauseP & ID revisions lag behind equipment specification changes. Final PZV set pressure and sizingdeterminations must come from MAWPs that are included in either “As-Built” or “Approved forConstruction” equipment specifications. Data that are taken from P & IDs for PZV sizing shouldonly be used for estimating sizes in Rev.A of the ISS.

After a PZV is sized and selected the Set Pressure and size of the PZV are printed next to theInstrument Society of America (ISA) symbol (balloon), which identifies the PZV on the P & ID.

Instrument Specification Sheets

Instrument Specification Sheets (ISSs) are used to record detailed engineering information onto astandard form, which is suitable for the sizing and selection of PZVs. Saudi Aramco EngineeringStandard J-007, Instrumentation Forms, lists three ISS forms for PZV specification. ISS formnumber 8020-611-ENG is used for specifying spring loaded, screwed or flanged, PZVs in Englishunits. ISS 8020-611M-ENG is used to specify PZVs in metric units, and ISS 8020-612-ENG isused to specify pilot operated PZVs. Only ISS 8020-611-ENG will be described in this module.

ISS form 8020-611-ENG (Figures 4 - 6) consists of three sheets that have a total of 98 lines forrecording data. There is a sidebar space on each sheet for recording revisions, authorizationsignatures, and for identifying the data lines on each sheet. Each sheet contains an itemidentification area along the bottom. Sheet 2 (Figure 5) contains data and a space for a simplifiedrelief valve sketch or other information that is not identified by a line number. Sheet 3 (Figure 6)contains data and space for calculating the orifice area.




Four PZVs can be specified on one ISS form. Each PZV has a data entry column on the form;however, multiple PZVs that are specified on one ISS should be related by model and/or processapplication. For example, a spare PZV can share an ISS with the main PZV, or with PZVs onduplicate process equipment (e.g., two identical heat exchangers), or with several PZVs in amultiple relief valve application.

Initially, the ISS form contains generic information that is presented for competitive bidsolicitation. At this point, the form is referred to as Revision A (Rev.A). ISS Rev.A is completedafter the basis for relief has been selected and after an effective sizing area has been calculated.Rev. A is based on API sizing estimates unless a PZV vendor is specified at the time of initial areasizing. All ISS revisions after vendor selection must be based on the manufacturer's data for theselected PZV.




Figure 4. ISS Form No. 8020-611-ENG, Sheet 1




Basis of Selection

Conditions Requiring Overpressure Protection

All causes of overpressure, or contingencies, must be evaluated for each PZV installation in termsof the pressures generated and the rates at which fluids must be relieved. Causes of overpressurein process equipment can range from a single event to a complex combination of events. Figure 7below is a table that summarizes some, but not all, operating conditions that lead to the indicatedrelief capacities.

Item No. Condition Pressure Relief Device(Liquid Relief)

Pressure Relief Device(Vapor Relief)*

1 Closed outlets on vessels Maximum liquidpump-in rate

Total incoming steam and vapor plus thatgenerated therein at relieving conditions

2 Cooling water failure tocondenser

— Total vapor to condenser at relievingconditions

3 Top-tower reflux failure — Total incoming steam and vapor plus thatgenerated therein at relieving conditions lessvapor condensed by sidestream reflux

4 Sidestream reflux failure — Difference between vapor entering andleaving section at relieving conditions

5 Lean oil failure to absorber — None, normally

6 Accumulation ofnoncondensables

— Same effect in towers as found for Item 2; inother vessels, same effect as found for Item 1

7 Entrance of highly volatilematerial

Water in hot oil

Light hydrocarbons in hot oil

— For towers , usually not predictable

8 Overfilling storage or surgevessel

Maximum liquidPump--in rate

—

9 Failure of automaticcontrols

— Must be analyzed on a case-by-case basis

10 Abnormal heat or vaporinput

— Estimated maximum vapor generationincluding noncondensables from overheating

11 Split exchanger tube — Steam or vapor entering from twice the cross-sectional area of one tube; also same effectsfound in Item 7 for exchangers

12 Internal Explosions — Not controlled by conventional relief devicesbut by avoidance of circumstances




13 Chemical reaction — Estimated vapor generation from both normaland uncontrolled conditions

14 Hydraulic expansion

Cold fluid shut in

Line outside process area

shut in

See C.2

See C.2

—

—

15 Exterior fire — Estimate by the method given in D.5

16 Power failure (steam, electric, or other)

Fractionators

Reactors

Air-cooled exchangers

Surge vessels

—

—

—

—

Maximum liquid

inlet rate

Study the installation to determine the effectof power failure; size relief valve for the worstcondition that can occur

All pumps could be down, with the result thatreflux and cooling water would fail

Consider failure of agitation or stirring,quench or retarding steam; size valves forvapor generation from a runaway reaction

Fans would fail; size valves for the differencebetween normal and emergency duty

—

* Considerations may be given to the suppression of vapor production as the result of the device’s relievingpressure being above operating pressure, assuming constant heat input. (Procedures for sizing pressure reliefdevices are presented in Section 4 of API-RP-520.)

Figure 7. Basis of Relief Capacities Under Selected Conditions1




Contingencies That Cause Overpressure

Fire Contingency - Any pressure vessel in a plant that processes flammable liquids may beexposed to fire at some point in its life, even if the vessel does not contain flammable liquid. If anopen, free-burning fire occurs, vessels and other equipment that are exposed to the flame willabsorb heat by radiation or by direct contact with the flame or hot gases. Escaping flammableliquids may be carried away For this reason, a PZV should be provided for pressure vessels torelieve overpressure that is caused by fire. (API RP-2001 describes ways to limit heat input froma fire using surface drainage and firewater application.)

Fire contingency applies to any liquid-filled process equipment (within a fire zone) that has awetted surface. A wetted surface is any surface that is both in contact with the process liquid andcan be exposed to fire. An uninsulated, wetted surface of a vessel will absorb radiation as sensibleheat. As the temperatures of the vessel and the liquid rise, the temperatures will essentiallybecome equal. At the boiling point of the liquid, the radiation will be absorbed as latent heat andthe resulting vapor generation will cause the pressure to rise to the set pressure of the PZV. Aslong as the vapor that is generated is less than the flow capacity of the PZV, the valve willintermittently open and close to protect the vessel. If the rate of vapor generation is greater thanthe rated capacity of the PZV, the pressure will increase beyond the permissible accumulation andcreate an unsafe situation.

A PZV may not protect a pressure vessel that contains only vapor because the vessel’s walltemperature can rise very rapidly and lead to vessel failure. For this reason, a vessel that containsonly vapor should be protected by reducing its pressure to atmospheric and by limiting the heatinput from a fire.

Equipment that does not have a reasonable quantity of wetted surface cannot be protected by aPZV against a fire contingency. Prime movers (pumps, compressors, etc.) and low-volumeequipment (pipes, tubes, etc.) are typical examples of equipment that cannot be protected againsta fire contingency by a PZV. In addition, jacketed vessels are also exempt from fire contingenciesunless the jacket contains a liquid, or the unjacketed area exposed to fire has sufficient wettedsurface to protect the vessel from heat damage. SADP-Section XII, Section 5.3.1 lists exceptionsto fire risk contingencies in paragraphs (a), (b), and (d). Paragraph (d). This reference should bereviewed when determining fire contingencies.

Blocked Discharge Contingency (BD) results from overpressure caused by cessation of fluidtransport out of process equipment. Some possible causes of BD can be determined from P &IDs. For example, when maintenance, process control, and check valves are present, BD must beconsidered. Other conditions that can cause BD can only be determined from equipmentspecification data, and/or chemical properties. If a valid BD contingency is discovered, 10%overpressure is allowed. Relieving pressure is 1.1 times set pressure converted to appropriatepressure units.




Finally, BD alone does not cause overpressure. Evaluation of the energy source (mechanical,thermal, chemical, etc.) under BD conditions determines if overpressure is possible. Thisinformation is only available from engineering specification data and/or chemical properties.Once the energy source is known (e.g., maximum discharge pressure of a prime-mover), the MOPat the equipment must be determined. If the MOP is higher than MAWP, a valid BD contingencyexists, and PZV area calculations must be completed.

Other Contingencies, which are not related to fire or BD, can cause overpressure of processequipment. Thermal relief and run-away chemical reactions are the most common examples.Thermal relief is a special case of BD where a liquid that is trapped in a piece of equipmentexpands as a result heat transfer from an external source (except fire). The thermal reliefcontingency must be considered as a “basis of selection” if the hydraulic pressure due toexpansion of the liquid exceeds the MAWP of the equipment. Thus, the surface area and heattransfer rate are required to solve thermal relief contingencies.

No API sizing procedure can protect against some run-away chemical reactions. The designengineer must recognize these cases and review engineering literature for possible solutions forprotecting against these contingencies.

Operational Requirements for Overpressure Protection

To meet the requirements of the ASME Code, accumulated pressure must be limited to 110percent of MAWP for a single-valve installation sized for operating (non-fire) contingencies.

For multiple PZV applications, ASME code requirements limit accumulated pressure to 116percent of MAWP in vessels that are protected by for a multiple-valves sized for operating (non-fire) contingencies.

Effective-Area Concept

If protection against more than one contingent event can be provided by a PZV, an “effectivearea” calculation must be made for each particular contingency. The largest effective areacalculation from all of the contingencies is selected as the “Basis for Relief,” which is used tospecify the PZV. The effective-area concept allows area sizing by calculation, which isindependent of the manufacturer.

Methods for Determining Relieving Pressure

Relieving pressure for PZVs in liquid service is defined in Section 4.2.1 of API RP-520 as the setpressure plus the allowable overpressure. The allowable overpressure can vary depending onthe contingency and on whether the installation involves a single PZV or multiple PZVs (notspare PZVs). The units of measurement for relieving pressures depend on process fluidproperties as reflected in the area sizing equation. Because liquids are generally considered to benon-compressible, gauge pressures (psig) are generally used. For compressible fluids,atmospheric pressures (psia, psia = psig + 14.7) are generally used.




The three methods for determining the relieving pressure of PZVs are categorized as follows:

• Operating contingencies

• Fire contingencies

• Steam service

Operating Contingencies

Single-Valve Installation - For operating (nonfire) contingencies, Section XIII of the ASME Coderequires that the accumulated pressure shall be limited to 110 percent of the MAWP. The setpressure of the valve shall not exceed the MAWP. Figure 8 shows an example of thedetermination of relieving pressure for a single valve.

Characteristic Value

Valve Set Pressure Less Than MAWP

Protected Vessel MAWP, psig 100

Maximum accumulated pressure 110

Valve set pressure, psig 90

Allowable overpressure, psi 20

Relieving Pressure, psia 124.7

Valve Set Pressure Equal to MAWP






Figure 8. Example Determination of Relieving Pressurefor a Single Valve Installation (Operating Contingencies)1

Multiple-Valve Installation - For operating (nonfire) contingencies, Section VIII of the ASMECode requires that the accumulated pressure shall be limited to 116 percent of the MAWP. Theset pressure of the first valve shall not exceed the MAWP. The set pressure of the additionalvalves shall not exceed 105 of the MAWP. Figure 9 shows an example of the determination ofrelieving pressure for a multiple valve installation.




The staging of the valves, helps to preventing chattering, since not all relieving cases are atmaximum flow. (Only one valve is required). ASME Section VIII allows these 105% valves tohave 116% accumulation. The reason for this is that these same valves only reach full lift at 10%overpressure. Thus 10% of 105% set pressure = 1.1 * 105 = 115.5 or round up to 116.

Characteristic Value

First Valve

(Valve Set Pressure Equal to MAWP)






Additional Valve

(Valve Set Pressure Equal to 105 Percent of MAWP)






Figure 9. Example Determination of Relieving Pressurefor a Multiple Valve Installation (Operating Contingencies)1

Supplemental-Valve Installations provide protection against additional hazards from fire orother sources of heat. They are used only in addition to valves that are sized for operating(nonfire) contingencies. The set pressure for a supplemental valve for fire is limited to 110percent of the MAWP.

Such supplemental pressure relieving devices shall be capable of preventing the pressure formrising more than 21% above the maximum allowable working pressure.

Again we see that the supplemental valves like any other Section VIII valve, reaches full lift at10% overpressure. Hence, with a setpressure allowable of 110% we have 110% * 1.1 = 121%accumulation.




Fire Contingencies

For fire contingencies, Section VIII of the ASME Code requires that the accumulated pressureshall be limited to 121 percent of the MAWP. This requirement applies to single-, multiple-, andsupplemental-valve installationFigure 10 shows an example of the determination of relievingpressure for a single valve installation.

Characteristic ValueValve Set Pressure Less Than MAWP

Protected Vessel MAWP, psig 100Maximum accumulated pressure 121Valve set pressure, psig 90Allowable overpressure, psi 31Relieving Pressure, psia 135.7

Valve Set Pressure Equal to MAWPProtected Vessel MAWP, psig 100Maximum accumulated pressure 121Valve set pressure, psig 100Allowable overpressure, psi 21Relieving Pressure, psia 135.7

Figure 10. Example Determination of Relieving Pressurefor a Single Valve Installation (Fire Contingencies)1

Figure 11 shows an example of the determination of relieving pressure for a multiple valveinstallation.

Characteristic ValueFirst Valve

(Valve Set Pressure Equal to 100 Percent of MAWP)Protected Vessel MAWP, psig 100Maximum accumulated pressure 121Valve set pressure, psig 100Allowable overpressure, psi 21Relieving Pressure, psia 135.7

Additional Valve(Valve Set Pressure Equal to 105 Percent of MAWP)

Protected Vessel MAWP, psig 100Maximum accumulated pressure 121Valve set pressure, psig 105Allowable overpressure, psi 16Relieving Pressure, psia 135.7

Figure 11. Example Determination of Relieving Pressurefor a Multiple Valve Installation (Fire Contingencies) 1




Steam Service

Protection against overpressure by steam is treated as any other ASME Section VIII material forAPI PZV sizing except that area sizing equations contain a superheat correction factor KSH, and aNapier KN correction factor. Generally contingencies regarding steam relief outside of boilerhouses are caused by pressure regulator failures in tropical climates.

API/ASME area sizing equations use saturated steam as a basis for the flow coefficients used inthe equations. If process fluid steam is saturated the value of KSH is one (KSH = 1) otherwise thevalue is taken from the API RP-520 Table 10-Superheat Correction Factors KSH.

The Napier factor KN corrects for changes in steam properties at high pressures above 1515 psiawhere KN = (0.1906P1 - 1000)/(0.2292P1 - 1061) and P1 = psia Relieving Pressure (Set Pressure+ Overpressure + Atmospheric Pressure).




CALCULATING THE SIZE OF A PRESSURE RELIEF VALVE - HANDCALCULATOR METHOD

After the specifications are determined and recorded on the ISS, the next activity is to calculate apreliminary valve size. This preliminary valve size is called the effective discharge area. Figure12 shows the inputs that are used to determine the basis for calculating the size of a PZV.

Blocked Flow Contingency

Fire Contingency

Thermal Contingency

Worst Case Area Calculations

Other Contingency

ASME SEC VII Standards

API RP-520 Standards

SAES-J-600 Standards

SADP 600 Standards

Figure 12. Inputs and Considerations for PZV Size Calculation

Basis for Calculating Valve Size

The basis for calculating a valve size follows calculations of valid contingencies. The contingencythat requires the largest effective area dictates the size of the PZV. The basis for calculating valvesize can be divided into three areas:

• Service conditions

• Flow rate

• Effective nozzle area




Service Conditions

The ISS Service Conditions Section includes details about the process fluid and the process itself.Fluid properties such as molecular weight, compressibility, and specific heat, and processrequirements such as temperature and pressure are used to determine the basis for relief and tocalculate the effective area of the valve. As discussed below, data listed in the ServiceConditions section of the ISS is used to select the appropriate sizing equation. Also, correctionfactors are used when the ideal or assumed conditions associated with each of the sizing equationsare different from the actual service conditions. Backpressure is an example of a servicecondition variable that requires the use of correction factors.

Flow Rate

The effective area sizing equations included in Work Aid 2 show that the required effectivedischarge area, variable A, equals a flow rate (variable W, V, or Q), which is modified by othercoefficients in the equation. The modifiers and coefficients are derived from published chemicalproperties.

BD Contingencies - In BD (Blocked Discharge) contingencies, the flow rate is determined by theprime-mover, which has a capacity to generate a flow rate in the protected device at some MOP.The flow rate is taken from process information in the equipment specifications and it issubstituted into the appropriate API area sizing equation.

Thermal Relief Contingencies - In thermal relief contingencies, the flow rate is derived fromchemical properties data that relate temperature to liquid expansion. The design engineer mustdetermine the surface area of the blocked pipe and calculate the liquid expansion using the SaudiAramco solar heat rate of 950 W/sq m (300 BTU/hr-ft2).The thermal expansion of trapped fluids can be approximated by using the following formula:

gpm BH500GC=

Where:

gpm = flow rate at the flowing temperature, in U.S. gallons per minute

B = cubical expansion coefficient per °F for the liquid at the expected temperature

H = total heat transfer rate in BTU per hour (see formula below)

G = specific gravity referred to water (1.0 at 60°F. Liquid compressibility is usuallyignored.)

The following formula can be used to calculate the total heat transfer rate for Saudi Aramcoapplications.

H = Hs As




Where:

H = total heat transfer rate in BTU/hr-ft2

Hs = solar heat rate in BTU/hr (300 BTU/hr from SAES L-043, Section 4.2.2)

As = heat transfer surface area in ft2

Fire Contingencies - Flow rate in fire contingencies is the vapor generated by the energyreleased into the process liquid by the fire. Flow rates are likewise derived from heat transferrates and chemical properties as shown in the following equations.

W = Q/HvapWhere:

W = mass flow in pounds per hour

Q = total heat absorption in BTU per hour

Hvap = latent heat of vaporization in BTU per pound

The total heat absorption, Q, depends on the vessel insulation and whether prompt fire fightingefforts and adequate drainage exist. When prompt firefighting efforts and adequate drainageexist,

Q = 21,000F (Awet)0.82

When prompt firefighting efforts and adequate drainage do not exist,Q = 34,500F (Awet)0.82

Where:

Q = total heat absorption to the wetted surface in BTU per hour

F = environmental factor (page 7-18 of Addendum 1)

Awet = total wetted surface in square feet (page 7-19 of Addendum 1)

Because this module is an introductory review of relief valve sizing, the flow that is generated in acontainer by an external fire is considered to be a stable saturated vapor.

Effective Discharge Area

API RP-520 defines the effective discharge area, or equivalent flow area, as a nominal orcomputed area of a pressure relief valve used in recognized flow formulas to determine the size ofthe valve. The effective discharge area is generally less than the actual discharge area. Aneffective discharge area is calculated in order to specify the actual orifice area that is required fora PZV.

The effective discharge area for a PZV is calculated in using the API critical flow equation. Thefirst area solution is often a trial estimation for sizing. The back pressure factor, Kb, is assumedto be 1 unless reliable engineering data indicates that back pressure will exist in the system. Forexample, if a vent header is known to have a constant pressure of 15 psig, then a superimposedback pressure of 15 psig would be used to determine Kb.




Sizing Equations For Specific Applications

The terms that are used in engineering equations are not standardized throughout the industry.API uses P1 to designate gage pressure (psig) in liquid equations and absolute pressure (psia) ingas and vapor equations. P1 is always the upstream relieving pressure into the PZV in APIequations. Different variable designations often share the same letter symbol in API equations.For example, A is the effective discharge area in the area sizing equation and it is the total wettedsurface in the heat transfer rate equation.

ISS 8020-611 ENG defines terms and units on each printed line. In some cases two options areprinted (e.g., Item 37, MW or SP GR @ FLOWING TEMP). In these cases the design engineershould circle the option that applies to the value that is entered in the data block.

Sizing for Gas and Vapor Relief

The sizing equations for gas and vapor are divided into two main categories depending onwhether the flow is critical or subcritical. Critical and subcritical flow are best described using anexample of the expansion of a compressible gas across a nozzle. If the downstream pressuredecreases, the velocity and specific volume of the gas will increase until the mass flow rate of thegas reaches a limiting velocity. This limiting velocity is equal to the velocity of sound in theflowing gas in the throat of the nozzle. The mass flow rate that corresponds to the limitingvelocity is called the critical flow rate.

The pressure in the throat at sonic velocity is called the critical flow pressure, Pcf. Under criticalflow conditions, the pressure in the throat cannot fall below the critical flow pressure even if thedownstream pressure is much lower. At critical flow, the expansion that occurs as the pressuredecreases from the throat pressure to the downstream pressure is irreversible. The energy isdissipated in turbulence into the surrounding fluid.

The ratio of Pcf to the inlet pressure, P1, is called the critical pressure ratio. The critical flowpressure ratio may be estimated using the following equation.

PP

2k 1

cf

1

k/(k 1)= +

+

Where:

Pcf = critical flow throat pressure in psia

P1 = upstream relieving pressure in psia

k = ratio of specific heats for any ideal gas

If the downstream pressure is less than or equal to Pcf, critical flow will occur. If the downstreampressure is greater than Pcf, subcritical flow will occur. The sizing equations for these twoinstances are described below.




Sizing for Critical Flow - When sizing gas or vapor PZVs, one must calculate the critical backpressure to determine whether the flow is critical or subcritical. The critical back pressure iscalculated by multiplying the relieving pressure by the critical flow pressure ratio for theparticular gas, which can be found on page 27 of Addendum 2. If pressure downstream of thethroat (use total back pressure until a PZV is selected) is less than the critical back pressure, theequation below may be used to calculate the effective discharge area, A, for the PZV. Themanufacturer’s PZV with an effective discharge area equal to or greater than the calculated valueof A is chosen for the application.

A WCKPK

TZM1 b

=

Where:

A = effective discharge area of the PZV expressed in sq. in.

W = required flow rate through the valve expressed in lb/hr (mass flow units)

K = effective coefficient of discharge = 0.975, or certified manufacturer’s value.

Kb = capacity correction factor due to back pressure, Bellows PZVs only. UseAddendum 1 or manufacturer’s values.

M = molecular weight of hydrocarbon gas or vapor

P1 = upstream relieving pressure in psia, (Set Pressure psig x % Overpressure) +14.7 = P1 psia)

C = specific heats ratio coefficient. Use page 7-9 of Addendum 1 to derive Cbased on the ratio of the specific heats, k.

T = relieving temperature °R (°Rankine = °F + 460°)

Z = compressibility factor at inlet conditions. Use Pr and Tr to derive Z fromAddendum 4. Z = 1 is a conservative value.

For an example of relief valve sizing for critical flow, refer to pages 7-22 and 7-23 of the CrosbyEngineering Handbook in Addendum 1.

Sizing for Subcritical Flow - If the back pressure is greater than the critical back pressure, eitherof the equations below may be used to calculate the effective discharge area, A, for the PZV.

A W735F K

ZTMP (P P )2 1 1 2

= −

A V4645.2F K

ZTMP (P P )2 1 1 2

= −

A V863.63F K

ZTGP (P P )2 1 1 2

= −




Where:



F2 = coefficient of subcritical flow =

( )kk 1

r1 r

1 r2/k

(k 1)/k

−

−−

−

k = specific heat ratio k = Cp / Cv.

r = ratio of back pressure to upstream relieving pressure, P2/P1.

V = required flow rate through the valve expressed in scfm (standard cubic feet perminute at 14.7 psia and 60°F)

K = effective coefficient of discharge = 0.975, or certified manufacturer’s value.


G = specific gravity relative to Air @ STP, (G = M / 28.97)

P1 = upstream relieving pressure in psia, ((Set Pressure psig x % Overpressure) +14.7 = P1 psia)

P2 = back pressure in psia



Z = compressibility factor at relieving inlet conditions. Use Pr and Tr to derive Zfrom Addendum 4. Z = 1 is a conservative value.

Bellows PZVs operating in the subcritical flow regime are sized using the critical flow equationexcept that the manufacturers K factor must be used.

Saudi Aramco design engineers shall not size spring loaded PZVs under API subcritical flowconditions outlined in API RP-520, Section 4.3.3. SAES-J-600 section 5.2.1, expressly forbidsthe use of conventional PZVs when total back pressure on the valve exceeds 10% of set pressure.

Sizing for Steam Relief

PZVs in steam service may be sized using the following equation:

A W51.5P KK K1 N SH

=




Where:

A = effective discharge area expressed in sq. in.

W = required flow rate through the valve expressed in lbs/hr (mass flow units)

K = effective coefficient of discharge = 0.975, or certified manufacturers value.

P1 = upstream relieving pressure in psia, ((Set Pressure psig x %Overpressure) +14.7 = P1 psia)

KSH = superheat correction factor. KSH = 1 for saturated steam. Otherwise useAddendum 7.

KN = correction factor for Napier equation. KN = 1 where P1 ≤ 1515 psia. Where P1

> 1515 psia and ≤ 3215 psia, use Page 7-6 of Addendum 1 or KN = (0.1906P1 -1000) / (0.2292P1 - 1061)

Section 4.4, 'Sizing for Steam Relief', in API RP-520 outlines requirements, and includesexamples for sizing PZV's in steam service under ASME Section VIII. Reliable steam tables canbe found in Crane TP-410. There are two steam tables in Crane, 'Properties of Saturated Steamand Saturated Water' and 'Properties of Superheated Steam.' Superheated steam has differentvolume/mass, temperature, and pressure properties than saturated steam; therefore, ASME/APIrelief valves for steam service require a superheat correction factor KSH in the effective areasizing equation. Likewise, steam at pressures above 1515 psia require a correction factor KN.

Sizing for Liquid Relief

PZVs for liquid relief may be sized by using the following equation:

AQ

38K K KG

P Pd W V 1 2

=−

Where:

A - Effective discharge area expressed in sq. in.

Q - Required flow rate through the valve expressed in gpm, (volumetric flow units)

Kd - Effective coefficient of discharge = 0.650, or certified manufacturers value.

KW - Capacity correction factor due to back pressure, Bellows PZVs only. (Pages 7-3 to 7-5 of Addendum 1 or manufacturer’s value.)

Kv - Capacity correction factor due to viscosity. (Page 7-7 of Addendum 1 ormanufacturer’s value.)

G = Specific gravity (water = 1 @ 60°F)

P1 = Upstream relieving pressure in psig. This is the set pressure plus allowableoverpressure. (Note this Value is in gauge units not absolute).

P2 = Total back pressure in psig.




The PZV should first be sized for nonviscous-type application to obtain a preliminary dischargearea, A. From the manufacturer’s standard orifice sizes, the next orifice size that is larger than Ais used to determine the Reynold’s number using the following equation.

RQ(2800G)

m A=

Where:

R = Reynold’s number

Q = flow rate at the flowing temperature, in U.S. gallons per minute

G = specific gravity (water = 1 @ 60°F)

m = absolute viscosity at the flowing temperature, in centipoises

A = effective discharge area in square inches

After the Reynold’s number is determined, the factor viscosity correction factor, KV, is used tocorrect the preliminary discharge area as shown in the following equation.

AAK

RV

=

Where:

A = area corrected for viscosity

AR = required area without viscosity correction

Kv = viscosity correction factor

Kv can be obtained from the graph on page 7-7 of Addendum 1. If the size of the corrected areaexceeds the chosen standard orifice area, the calculations above are repeated using the next largerstandard orifice area.

Sizing for Two-Phase Liquid-Vapor Relief

API RP-520, section 4.7, page 37, 'Sizing for Two-Phase Liquid/Vapor Relief', presentsmethodology for sizing PZVs which relieve process fluids that are partially liquid and partiallygas. API methodology recommends the following:

1. Determine the quantity of liquid and calculate an effective area based on thatdetermination.

2. Determine the quantity of vapor and calculate an effective area based on thatdetermination.

3. Add the two areas together and choose a PZV with an orifice greater than the sumof liquid and vapor areas.

4. Use the same pressure values, relieving pressure and back pressure in bothcalculations.




5. Select either liquid or gas trim. Rule of Thumb. If more thane 50% of the flow (on amass basis) is liquid, then use liquid trim.

Relief valve problems involving two phase flow conditions beyond a 3/4D1 size PZV used forthermal relief should only be resolved by senior engineers specializing in relief valve technology. Itis further recommended that a test stand or bench scale test apparatus be used to test the selectedvalve. Oversized PZVs tend to chatter, and often leak after a release. And most calculations ofPZVs for two phase flow conditions will be oversize effective relief areas.

The test apparatus must have close temperature control as well as pressure control. Furthermoreunder flashing conditions both wet-bulb and dry temperatures must be measured. Data on thevalve performance should be measured and recorded over a wide 'true temperature' range atconstant relieving pressure. Finally back pressure should be varied over a reasonable range basedon installed valve conditions.

Information about and from the test apparatus must have the approval of the valve manufacture.Capacity certification testing for ASME stamps does not involve two phase fluids except steam.

In the cases where the process fluid is a mass produced refrigerant, the manufacturer of therefrigerant will usually be able to recommend a suitable relief device for their product. They canalso direct Saudi Aramco design engineers to client engineers who have process experience withthe product. Complex relief valve sizing and selection problems must be reviewed by seniorengineers who have experience with the process involving protection.




SELECTING A PRESSURE RELIEF VALVE - MANUFACTURER’S CATALOGMETHOD

There are two parts to the selection of relief valves. The first part is an engineering decisionregarding the PZV type and features. The second part is the selection of a manufactured reliefvalve that meets the requirements determined. Both parts are interdependent because designengineers must specify features that are available in manufactured relief valves.

This section will describe the sources of required data on which the design engineer will base hisdecisions, and the administrative (documentation) requirements for relief valve selection.

Sources of Required Data

Instrument Specification Sheets

The ISS 8020-611 ENG specifies relief valves that meet the requirements of Saudi Aramco.These requirements include specifications from ASME and API standards and recommendedpractices. Rev.A also includes specifications based on engineering judgment. For example,although balanced bellows and soft seats are not required by Saudi Aramco standards, they maybe specified on the ISS because they are considered to be cost effective by the design engineerand the engineering manager.

After approval, ISS 8020-611 ENG will specify a PZV that is suitable for a vendor quotation.Saudi Aramco engineers may allow vendors to recommend products that conform to the ISS.The design engineer must then review and approve the vendor’s selection. Final revision of theISS represents agreement between Saudi Aramco engineering and the relief valve manufacturerthat the valve delivered to Saudi Aramco will meet the requirements of the ISS and theASME/API certification.

Manufacturers’ Catalogs

After a vendor selects and quotes a PZV, catalogs and other publications published by themanufacturer are required to verify that the quoted PZV meets the requirements of the ISS. Insome cases, where non-standard options are required, factory correspondence is required to verifythe quotation.

Manufacturers' catalogs contain both certified and non-certified valve capacities. Catalogs also listmaterials of construction for each class of relief valve, and they include instructions for relating amodel, or style, number to the features that are available in a particular class, or series, of reliefvalve.




Valve capacity tables are always included for Air, Water, and Steam in catalogs for ASMESection I or VIII certified relief valves. Capacity tables relate certified capacities to set pressurevalues at a fixed allowable overpressure (usually 10%). Capacity units are usually gpm for water,lb/hr for steam, and scfh for air. Some catalogs include effective discharge coefficients, K values.

Addendum 4 contains two Crosby relief valve catalogs. Crosby Style JOS, JBS and JLT andCrosby Series 800 Adjustable Blowdown and Series 900 OMNI-TRIM® Pressure Relief Valves,Catalog No. 902 (Cat. 902) contains information and certified capacities for Crosby's small size,conventional, and thermal relief PZVs that conform to the requirements of the PZV.

As an example, assume a simple case of a single contingency thermal relief valve, PZV-100,which is located on a length of pipe between two block valves (see P & ID in Figure 13).

Figure 13. PZV-100 Thermal Relief Valve

Assume that the specifications from the ISS are as follows:

• calculated flow rate, Q = 0.0254 gpm

• effective discharge are, A = 9.29 x 10-5 sq. in.

• conventional relief valve

• bronze or brass construction materials required (6.2.2, SAES-J-600)

• blowdown adjustment is not required

• non-sour service

• special accessories are not required

• lifting lever is not required

• test gag is not required

Assume that one of the approved vendors (found in SAES-J-002) quotes a Crosby relief valvewith a style number of 951501MA; however, the valve is not bronze or brass becausemanufacturers no longer use these materials.




The engineer should revise the ISS and obtain approval for a carbon steel valve with stainlesssteel trim because only the stainless steel will be in contact with the process fluid duringdischarge.

Page 3 of Cat. 902 (Addendum 4) compares the Series 800 pressure relief valves with the Series900. Observe that Series 900 is designed for Thermal Relief and it does not have adjustableblowdown. The PZV does not require a blowdown adjustment reseating rate after discharge, soSeries 900 is selected.

The procedure for selecting a style number for Crosby Series 900 PZVs is illustrated on page 7 ofthe catalog. From the style designation information on page 7, number 951501MA is chosen.The meaning of the style designation is as follows:

1st digit - 9 for Series 900

2nd digit - 5 for 0.074 sq. in. Effective Orifice Area

3rd digit - 1 for 1500 psig Maximum set Pressure

4th digit - 5 Kalrez Seat Material (Inert Fluorocarbon) soft seat (p. 11)

5th digit - 0 Standard Materials (p. 9)

6th digit - 1 Connection Size 3/4 in. x 1 in. NPS (p. 16, P&T ratings)

7th digit - M Connection Type Male NPT inlet and Female NPT discharge

8th digit - A Standard Screwed Cap (p. 10)

Materials of construction for Series 900 relief valves are listed on page 9 of Cat. 902. CrosbySeries 900 relief valves have carbon steel cylinders (Fluid Cavity & Bonnet) and 316 stainlesssteel (316 SS) Base (Body). Disk and seat assemblies are stainless steel, and a Kalrezfluorocarbon soft seat is specified. Soft seat details are shown on page 11 of Cat. 902.

Connections are 3/4" MNPT (0.75 SC-NPTM, SAEP-1131) inlet and 1" FNPT (1 SC-NPTF,SAEP-1131) outlet. A type 'A' carbon steel cap completes the selected style.

Cap details are shown on page 10 of Cat. 902.

Note also that the data for the PZV, records that certified bronze/brass relief valve production bySaudi Aramco approved vendors has been discontinued. The Crosby Series 900 Style 951501MAhas all inert materials, stainless steel and Kalrez, in contact with the process fluid (process water)under normal conditions.




Certified water capacities for Crosby Series 900 relief valves are listed on page 22 of Cat. 902. At120 psig set pressure the selected valve (0.074 sq. in effective area) will discharge 23 gpm. Thesame valve is certified for 543 lb/hr steam capacity. Both capacities exceed the required flowrated calculated for the PZV.

The P & ID can be revised to show: 3/4 x 1 Set @ 120 psig next to PZV-100. Rev. C of the ISScan be prepared to show Crosby on line 4, and 951501MA on line 5. Lines 10, 13, 15, 49, 60,61, 76, and 82 should also be changed to include vendor data.

Administrative Requirements

Saudi Aramco standard 34-SAMSS-611, 'Safety Relief Valves Flanged Conventional AndBalanced Types', Issue Date 1 DEC 96, section 1.2, requires ISS form 8020-611 ENG to beincluded with "Buyer's Quotation Request or Purchase Order." Official Saudi Aramcoadministrative requirements related to quotations and purchasing beyond this statement and theacceptable vendor list in SAES-J002 are beyond the scope of this module.

The ISS should be revised after the selection of the Crosby 3/4" x 1" 951501MA relief valve. ISSform 8020-611 ENG is a generic specification designed to solicit competitive bids fromacceptable manufacturers. After bid selection the ISS form is revised with the manufacturer’smodel number and other special information related to the selected manufacturers valve. Therevised (Rev.B or higher) ISS form 8020-611-1-ENG then becomes part of the purchase order.

After purchasing the PZV, Saudi Aramco form 3099A 'Relief Valve Authorization' must becompleted. Form 3099A is central to the disposition of a PZV throughout its useful life, even ifthe valve is 'Mothballed'. Saudi Aramco standards SAEP-318 and SAEP-1131 are devotedexclusively to instructions and procedures related to form 3099A and relief valve authorization.

Saudi Aramco form 3750 'Pressure Relieving Device Maintenance Report' is the on-site version of8020-611 ENG. It contains all information related to the testing, and inspection of a PZV inmuch the same way as 8020-611 ENG contains all of the information related to sizing, andselection of a PZV. Instructions and procedures routine testing and inspection of relief valves iscontained in SAEP-319. Instructions for filling out form 3750 are outlined in Saudi Aramcostandard SAEP-1133.

Both Forms 3099A and 3750 as well as 8020-611 ENG are part of the Saudi Aramco companywide computer system.




SIZING AND SELECTING A PRESSURE RELIEF VALVE - COMPUTERIZED SIZINGMETHOD

Crosby-Size is the relief valve sizing computer software provided by Crosby Valve and GageCompany. The computer application’s features include:

• accurate calculations

• user-selected units

• selection of valve size and style

• valve data storage

• printed reports

• specification sheets

Crosby-Size is a menu-driven application. Prior to using this application, the user must have abasic understanding of the relief valve sizing calculations that were described in WorkAid 2. The main menu of Version 2.1 of the software contains the following options:

1. Section I Steam Valve Sizing

2. Section I Steam Valve Flow Calculation

3. Section VIII Gas/Steam/Liquid Sizing

4. Section VIII Valve Flow Calculation

5. API RP-520 Fire Sizing

6. Miscellaneous Reports and Utilities Menu

7. CROSBY-SIZE Program Configuration Menu

The first five options are used to size and select PZVs, and they relate to ASME Section I andVIII, and API RP-520. The sixth option, “Miscellaneous Reports/Utilities Menu,” is used to savesizing and selection data and to print reports of the selected PZV. The last option, “CROSBY-SIZE Program Configuration Menu,” is for program administration, which is used to organizefiles, select a printer, select program variables, and 'toggle' certain program features. Each Optionin the main menu is described in a section of the software manual, which is indexed byidentification tabs bearing each option name. Work Aid 3 contains the procedures for using theapplication.

Program Applications

The 'Valve Sizing Options:' screen provides nine sizing options based on a combination of thestate of the process fluid and the type of PZV that will be used. In some cases, Crosby PZVclasses are part of the sizing and selection.




For sizing and selection of PZVs for liquids, the available option selection presupposes that theuser is familiar with Crosby relief valves. If a wrong class of valve is chosen during the initialentry from the selection screen, the user can return to the selection screen and try a differentchoice. Most of the data entered into the program is retained until the user returns to the 'MainMenu'.

The discussion of CROSBY-SIZE will now follow program selections that verify the sizing andselection example on pages 7-22 and 7-23 of the Crosby Engineering Handbook in Addendum 1.The fluid and vessel data are as follows:

Fluid Data

Fluid: Benzene

Required Capacity: 5000 lb/hr

Set Pressure: 200 psig

Back Pressure: Atmospheric

Inlet Relieving Temperature: 100°F

Molecular Weight: 78.11

Latent Heat: 172 BTU/lb

Specific Heat Ratio (k): 1.12 (p. 7-26 of Addendum 1)

Vessel Data

Diameter (D): 15 ft.

Length (L): 30 ft.

Elevation (H): 15 ft.

Max. Fluid Level (F): 147 inches (12.25 ft.)

Type: Cylindrical with spherical ends

Prompt fire-fighting efforts and adequate drainage exist.

Placement: Horizontal

Insulation: None

Assume the ISS states the following:

Tag No.: PZV-200

Valve conn.: 300 lb. x 150 lb.

At the Main Menu of the CROSBY-SIZE program, the API RP520 Fire Sizing option is selectedand the following data are enter at the prompts.




Prompt EnterSelect Tank Type: Sphere or Cylinder(S/C)?

C

Cylinder Type: Horizontal or Vertical(H/V)?

H

Type of Ends: Flat or Spherical (F/S)? S

Enter wetted area manually? N

[D] Diameter, in Ft 15

[L] End-to-End Length. in Ft 30

[H] Height off Ground, in Ft 15

[F] Fluid Level, in Ft 12.25

[T] Latent Heat, in Btu/lb 172

Adequate Drainage & Fire FightingEquipment Provided (Y/N)?

Y

Insulation Types: Options: Bare Vessel

Accept These Options (Y/N/Quit) ? Y

Req. Cap. Other Than Fire Exp.: 0

Set Pressure: 200

Types Of Back pressure: Superimposed Constant

Back Pressure: Superimposed Constant: 0

Overpressure: 21

Specific Heat Ratio: 1.12

Molecular Weight: 78.11

Temperature: 100

Compressibility Factor: 1.0


Valve Types: Options: JOS

Do you require all Stainless Steel Valve(Y/N) ?

N

Selected Valves Include: JOS-25,300 x 150

Cap Type Selections: Options: ScrewedCap

[C]onditions, [S]ave, [R]eport. [V]alveType, or [Q]uit?

R




CROSBY-SIZE Sizing Reports: ReportOptions:

CrosbyReportSheet w/Formulas

Print to Screen/Printer (S/P) P

Customer Name: SAUDIARAMCO

Customer Reference: EXAMPLE

Comments [ENTER]

[C]onditions, [S]ave, [R]eport. [V]alveType, or [Q]uit?

Q

[ESC] to Quit ESC

The program generates the report shown in Figure 14.

CROSBY-SIZE ASME Section VIII Date: Sept. 21, 96Pressure Relief Valve Report Sheet (Note 1)

Customer: SAUDI ARAMCO Prepared By:Reference: EXAMPLE Saudi AramcoQuote/Tag: EXAMPLE/

Gas & Vapor Mass Flow Spring Loaded Pressure Relief Valve Sizing

Service ConditionsRequired Capacity······················32330.8 lb/hrSet Pressure··························200.0 psigOverpressure ·························21 %Temperature··························100 °FBackpressure - Constant ·················0.0 psigSpecific Heat Ratio ·····················1.12

Calculated ValuesRequired Orifice Area ···················1.052 sq inEffective Orifice Area····················1.287 sq inCalculated Flow ·······················39568.1 lb/hr

Calculation Formula (USCS) (Note 1)A = --------------------------------------Pi * C * K * h-bm




Required Orifice Area (A) ·················1.052 sq inRequired Capacity (W) ···················32330.8 lb /hrSet Pressure (P) ·······················200.0 psigTemperature (T)························100 °FRelieving Pressure (P1) ··················256.7 psiaMolecular Weight (M) ····················78.11Back Pressure Correction (Kb) ·············1.000Effective Coefficient Of Discharge (K) ········0.975Specific Heat Coefficient (C) ···············329Compressibility Factor (Z) ················1.000Valve Style····························JOS-25-AValve Size (Inlet-Orifice-Outlet) ·············2 J 3Valve Connection (inlet-Outlet) ·············300 x 150

(Note 1)API effective orifice area and effective coefficient of discharge area used in the flow and sizingcalculation shown on this report.

CROSBY-SIZE Section VIII Report Sheet, Page #2 Date: Sept. 21, 96

Customer: SAUDI ARAMCO Prepared By:Reference: EXAMPLE Saudi Aramco1-508-384-3i2i

Fire Sizing Vessel Dimensions

Tank Type···································CylinderOrientation···································Horiz.Type of Ends·································SphericalDiameter····································15.00 ft.End-To-End Length····························30.00 ft.Evaluation···································15.00 ft..Fluid Level ··································12.25. ft.Latent Heat of Vaporization ······················172.00 BTU/lbAdequate Fire Fighting & Drainage·················YesInsulation Type ·······························Bare VesselInsulation Conduction Factor ·····················1.0000Wetted Area ·································901.13 sq ft.Required Flow Due To Fire Capacity ···············32330.8 lb/hr

Figure 14. CROSBY-SIZE Report Sheet for PZV-200




RELIEF VALVE AUTHORIZATION PROCESSThe authorization process is the final phase in the sizing and selection of a PZV. Theauthorization process that is followed depends on the responsible organization, Projects orOperations. The authorization process for Projects is shown graphically in Chart I of SAEP-318,(Figure 15). Chart I has an authorization, administration path diagram for new PZVs providedwith projects. These procedures are governed by Saudi Aramco standards SAEP-318 SAEP-319.

Figure 15. Chart I - Projects Authorization Procedure forRV Installation and Changes




The authorization process for Projects is further explained below.

1. The Originating Engineer, or Originator, completes lines 1-4 of Form 3099A inaccordance with the requirements of Saudi Aramco specification SAEP-1131,Section 2. ISS Form 8020-611-ENG is attached to form 3099A until the original3099A/ISS 8020-611ENG is filed by the Relief Valve Administrator.

Form 3099A, is forwarded to the Senior Project Engineer, or SuperintendentOperations Engineering for approval and authorization signature.

2. A signed copy of Form 3099A is returned to the Originating Engineer.

3. The Originating Engineer submits 3099A to the Projects Inspection Supervisor orOperations Foreman for disposition.

4. The Project Inspection Supervisor reviews and signs off Form 3099A using ISS8020-611ENG to verify the data on Form 3099A for newly constructed facilities.

5. The Originator, or someone under his supervision, enters the data into the reliefvalve database. The original 3099A is distributed to the Relief Valve Administratorfor approval signature.

6. The Relief Valve Administrator assigns a relief valve number to the PZV, thenapproves, and signs 3099A. The original 3099A and a copy of 3750 is returned tothe Originating Engineer. The original 3750 and a copy of 3099A is distributed tothe Computer Operations Department. The Computer Operations Departmententers data from Forms 3099A and 3750 into the Saudi Aramco computer, signs theforms, and returns 3099A

7. After receipt of an approved copy of 3099A, the Originating Engineer completeswork order Form 981-1, and distributes a copy to the Maintenance Relief ValveTest Unit, or Contractor Shops Division.

8. The Maintenance Relief Valve Test Unit, or the Contractor Shops Division, willrecord the test data on form 3750, and mark the PZV in accordance with SAEP-1131. Form 3750 for the PZV is distributed to the Relief Valve Administrator.

9. The Projects Inspection Supervisor, or Operations Foreman checks and approvesfinal installation of the PZV. He then signs Form 3099A, and distributes the form tothe Supervisor of the Commissioning Unit.

10. The Supervisor of the Commissioning Unit also inspects the installation of the PZVto insure the relief valve is ready for commissioning of the process equipment. TheSupervisor of the Commissioning Unit then signs Form 3099A, and submits it to theRelief Valve Administrator.

11. The Relief Valve Coordinator checks and files original Form 3099A. He thendistributes copies of Forms 3099A and 3750 to the Computer OperationsDepartment, and a copy of 3099A to the Supervisor of Operations EngineeringInspection.




The authorization process for Operations is shown in Chart II of (Figure 16).

Figure 16. Chart II - Operations Facilities Authorization Procedure for Relief Valve Installation and Changes




Relief Valve Authorization, Form 3099A

The Relief Valve Authorization Form 3099A consists of two major sections and an area forremarks (Figure 17). The top section is divided into data areas and letter/number blocks. Theareas are identified by line numbers and column numbers. Specific details for the entry ofinformation in each area are outlined in SAEP-1131, Section 2.

Authorization regulations, distribution requirements, and disposition rules regarding Form 3099Aare outlined in SAEP-318.

Form 8020-611

The Form 8020-611 (ISS) is used to record detailed engineering information, which is suitable forthe sizing and selection of PZVs. Saudi Aramco Engineering Standard J-007, InstrumentationForms, lists three ISS forms for PZV specification. ISS form number 8020-611-ENG is used forspecifying spring loaded, screwed or flanged, PZVs in English units. ISS 8020-611M-ENG isused to specify PZVs in metric units, and ISS 8020-612-ENG is used to specify pilot operatedPZVs.

Unlike 3099A and 3750, an ISS for a PZV is not a perpetual document. The final revision of anISS for a PZV is attached to the initial 3099A. The distribution and disposition of 3099A/8020-611-ENG was presented above.

Requirements regarding ISS 8020-611-ENG are contained in Saudi Aramco standards SAES-J-600, and 34-SAMSS-611.




Figure 17. Relief Valve Authorization, Form 3099A




Relief Valve Test Report, Form 3750Form 3750, Relief Valve Test Report is part of the PZV authorization process. Each time a workorder Form 981-1 is issued for a PZV, a Form 3750 must be completed.

The Form 3750 is organized by areas, which are identified by line numbers and column numbers.Specific details for information entry in each area are outlined in SAEP-1133, Section 3. Asample Form 3750 is shown in Figure 18. Data entry into the form is associated with the trouble-shooting discussions in Module 5. SAEP-1133 should be referenced in conjunction with variousPZV T&I operations. Note that the inspection period required in 3099A line 2, columns 74-75should originate from Form 3750 after initial testing of the PZV.




Figure 18. Relief Valve Test Report, Form 3750




WORK AID 1: RESOURCES FOR DETERMINING RELIEF VALVESPECIFICATIONS

This Work Aid provides the procedures for determining the Basis of Selection and the relievingpressure of PZVs in accordance with SAES-J-600. These procedures are taken from API RP-520and Section VIII of the ASME code.

Work Aid 1A: Procedure for Determining Applicable Contingencies for PZV Sizingand Selection

This procedure can be used to determine all applicable contingency events. This procedurecannot be used to calculate the effective area for each applicable contingency.

1. Determine whether blocked discharge contingency is applicable.

A. On the P & ID, follow each process feed line connected to the protected device(whose PZV is to be sized) back to a likely pressure source upstream. Thepressure source will usually be a prime mover (i.e., turbine, compressor, pump,etc.), but could be any process equipment which can generate pressure in the lineunder study.

B. Follow each process discharge line that is connected to the protected devicedownstream as far as the first device that is protected by a PZV. Note all devicesthat can block the discharge in that line. The last blocking device (usually a valve)in the line upstream of the device protected by the PZV will be the final device onthat discharge line that can block the flow out of the protected device under study.

2. Determine whether fire contingency is applicable.

A. If the information is not provided, determine whether the equipment is in a firezone (wholly or partly below 25 ft. above a fire-bearing surface) using a plot plan.

Yes, the equipment is in a fire zone. Go to step 2B.

No, the equipment is not in a fire zone. Fire contingency is not applicable in thiscase.*

B. Does the equipment contain liquid and does it have a sufficient amount of wettedsurface?

Yes. Go to step 2C.

No. Fire contingency is not applicable in this case.*

C. Is the equipment jacketed?

Yes. Fire contingency is not applicable in this case.*

No. Fire contingency is applicable in this case.*




* If in doubt, select fire as a contingency or compare the fire contingency area toother contingencies. Seek the guidance of a senior engineer or engineeringmanager.

3. Determine whether thermal relief contingency is applicable.

A. Can the liquid-filled equipment be sealed closed?

Yes. Go to step 3B.

No. Thermal relief contingency is not applicable in this case.

B. Can continued heat input be avoided?

Yes. Thermal relief contingency is not applicable in this case.

No. Thermal relief contingency is applicable in this case.

Work Aid 1B: Procedure for Determining Relieving Pressure of PZVsThe following procedure and the table in Figure 23 can be used to determine the maximumaccumulation and set pressures of PZVs for single- and multiple-valve installations inaccordance with Section VIII of the ASME Code.

Procedure

1. Obtain the maximum allowable working pressure (MAWP) of the equipment or vesselfrom “As Built” or “Approved for Construction” equipment specifications.

2. Calculate the maximum accumulated pressure by multiplying the MAWP by the percentMaximum Accumulated Pressure from Figure 23.

3. Calculate the relief valve set pressure by multiplying the MAWP by the percent SetPressure from Figure 23.

4. Calculate the relieving pressure by adding the set pressure and the percent overpressureplus 14.7.




Single-Valve Installations Multiple-Valve Installations

Maximum Maximum

Set Accumulated Set Accumulated

Pressure Pressure Pressure Pressure

Contingency (percent) (percent) (percent) (percent)

Nonfire only

First Valve 100 110 100 116

Additional valve(s) — — 105 116

Fire only

First valve 100 121 100 121

Additional valve(s) — — 105 121

Supplemental valve — — 110 121

Note: All values are percentages of the maximum allowable working pressure.

Figure 19. Set Pressure and Accumulation Limits for Pressure Relief Valves2




WORK AID 2: RESOURCES USED TO CALCULATE THE SIZE OF A RELIEFVALVE-HAND CALCULATOR METHOD

This Work Aid provides the formulas and procedures to calculate the size of a pressure reliefvalve. These procedures assume that the basis of relief has been selected.

Work Aid 2A: Formulas and Procedures to Size PZVs for Gas and VaporFormulas

Equation 1: Flow Rate for Thermal Contingencies

gpm BH500GC=

Where:

gpm = flow rate at the flowing temperature, in U.S. gallons per minute

B = cubical expansion coefficient per °F for the liquid at the expectedtemperature

H = total heat transfer rate in BTU per hour (see formula below)

G = specific gravity referred to water (1.0 at 60°F. Liquid compressibility isusually ignored.)

Equation 2: Total Heat Absorption for Fire Contingencies

When prompt firefighting efforts and adequate drainage exist,

Q = 21,000F (Awet)0.82

When prompt firefighting efforts and adequate drainage do not exist,

Q = 34,500F (Awet)0.82

Where:


F = environmental factor (page 7-18 of Addendum 1) or (Table D-3 API520 PT1) or (Table D-3 API 520 PT1)

Awet = total wetted surface in square feet (Page 7-19 of Addendum 1)

Equation 3: Flow Rate for Fire Contingencies

W = Q/Hvap

Where:

W = mass flow in pounds per hour





Hvap = latent heat of vaporization in BTU per pound

Equation 4: Effective Discharge Area for Critical Flow

A WCKP K

TZM1 b

=

Where:


W = required flow rate through the valve expressed in lb/hr (mass flowunits)

C = specific heats ratio coefficient. Table 8 and Table 9 of API 520 PT1(Addendum 1)

K = effective coefficient of discharge = 0.975, or certified manufacturer’svalue.

P1 = upstream relieving pressure in psia, ((Set Pressure psig x%Overpressure) + 14.7 = P1 psia)

Kb = capacity correction factor due to back pressure, Bellows PZVs only.Use Addendum 1 or manufacturer’s value.


Z = compressibility factor at inlet conditions. Use Addendum 4.


Equation 5: Effective Discharge Area for Subcritical Flow

A W735F K

ZTMP (P P )2 d 1 1 2

= −

A V4645.2F K

ZTMP (P P )2 d 1 1 2

= −

A V863.63F K

ZTGP (P P )2 d 1 1 2

= −

Where:



F2 = coefficient of subcritical flow ( )= −

−−

−kk 1 r 1 r

1 r2/k (k 1)/k

k = specific heat ratio k = Cp / Cv.




r = ratio of back pressure to upstream relieving pressure, P2/P1.

V = required flow rate through the valve expressed in scfm (standard cubic feetper minute at 14.7 psia and 60°F)

Kd = effective coefficient of discharge = 0.975, or certified manufacturer’s value.


G = specific gravity relative to Air @ STP, (G = M / 28.97)

P1 = upstream relieving pressure in psia, (Set Pressure psig + Overpressure psig+ 14.7 = P1 psia)

P2 = back pressure in psia



Z = compressibility factor at relieving inlet conditions. Use Pr and Tr to deriveZ from Addendum 4. Z = 1 is a conservative value.

Equation 6: Critical Pressure Ratio

PP

2k 1

cf

1

k/(k 1)= +

−

Where:

Pcf = critical flow throat pressure in psia


k = ratio of specific heats for any ideal gas

Procedure for Determining Effective Discharge Area for Vaporizing Liquids

1.0 Obtain the following information:

• molecular weight of hydrocarbon vapor, M, from the ISS

• relieving temperature, T, from the ISS

• relieving pressure, P1 (the set pressure and the percent overpressure plus 14.7)

2.0 Determine the total wetted surface area.

2.1 Calculate, Awet, the total wetted surface area using Page 7-19 of Addendum 1. Ifthe vessel dimensions are not known, obtain them from the vessel design sheet.To calculate the end-to-end length of an elliptical vessel, add the tangent-to-tangent length to 2 times the tangent-to-end length.




3.0 Determine the rate of vapor or gas vaporized from the liquid.

3.1 Calculate, Q, the total heat absorption to the wetted surface in BTU per hourusing Equation 2.

3.2 Calculate, W, the mass flow rate in pounds per hour using Equation 3.

4.0 Calculate the effective discharge area.

4.1 If the specific heats ratio coefficient is not known, find the value of k on page 7-9of Addendum 1. Then find the value of C based on k by using the equation onpage 7-9 of Addendum 1.

4.2 If the effective coefficient of discharge, Kd, is not known, use 0.975.

4.3 Determine the back pressure correction factor, Kb, using Addendum 1, or use themanufacturer’s value.

4.4 If the compressibility factor, Z, is not known, use Addendum 4 to determine Z.

4.5 Substitute the values for W, M, C, K, P1, Kb, T, and Z into Equation 4 and solvefor A.

Procedure for Determining Effective Discharge Area for Subcritical Flow


• required hydrocarbon flow, W, in pounds per hour

• molecular weight of hydrocarbon vapor, M

• relieving temperature, T

• relieving pressure, P1, the set pressure and the percent overpressure plus 14.7.

2.0 If the compressibility factor, Z, is not known, use Addendum 4 to determine Z.

3.0 Determine whether flow is subcritical by calculating the critical back pressure.

3.1 Find the Critical Flow Pressure Ratio for the gas in Table 8, page 27 of Addendum2.

3.2 Multiply the relieving pressure, P1, by the Critical Flow Pressure Ratio in 3.1 tocalculate the critical back pressure.

3.3 If the back pressure is greater than the critical back pressure, the PZV can be sizedusing Equation 5 (subcritical flow).

4.0 Calculate the total back pressure.

4.1 Total Back Pressure - Superimposed Back Pressure + Built-up Backpressure.

5.0 Determine the coefficient of subcritical flow, F2.

5.1 Find the value of k on page 7-26 (Table T7-7) of Addendum 1, the CrosbyEngineering Handbook or other handbook.




5.2 Calculate, r, the ratio of the back pressure to the upstream pressure by dividing P2

(from 4.1) by the relieving pressure P1 (from 2.0).

5.3 Use the F2 equation under Equation 5 with the values for r and k to determine F2.

6.0 Substitute the values for W, F2, K, Z, T, M, P1, P2, V, and G into the Effective DischargeArea for Subcritical Flow Equation and solve for A. (Equation 5)

7.0 Select the manufacturer’s standard orifice size that is equal to or greater than the orificesize calculated for A.

Work Aid 2B: Formulas and Procedure to Size PZVs for Steam Relief

This Work Aid contains the formulas and procedure to size PZVs for steam service.

Formulas

Equation 7. Effective Discharge Area for Steam Relief

A W51.5P K K K1 d N S H

=

Where:

A = effective discharge area expressed in sq. in.

W = required flow rate through the valve expressed in lbs/hr

Kd = effective coefficient of discharge = 0.975, or certified manufacturers value.


KSH = superheat correction factor.

KN = correction factor for Napier equation.

Procedure


• flow rate in lbs/hr

• set pressure in psig

• percent accumulation

• superheat correction factor, KSH (KSH = 1 for saturated steam. Otherwise useAddendum 7.)

2.0 Calculate the upstream relieving pressure, P1, in psia.

2.1 Multiply the set press by the percent accumulation to obtain the allowableoverpressure.




2.2 Add the allowable overpressure from 2.1 to the set pressure plus 14.7 to obtain theupstream relieving pressure.

3.0 Determine the correction factor for Napier equation, KN.

3.1 If P1 ≤ 1515 psia, KN = 1. If P1 > 1515 psia and ≤ 3215 psia, use Page7-6 of Addendum 1 or substitute P1 into the equation below and solve for KN.

KN = (0.1906P1 - 1000) / (0.2292P1 - 1061)

4.0 Substitute W, K (0.975), P1, KSH, and KN into the effective area discharge equation andsolve for A.

5.0 Select the manufacturer’s standard orifice size that is equal to or greater than the orificesize calculated for A.

Work Aid 2C: Formulas and Procedure to Size PZVs for Liquid Relief

This Work Aid contains the formulas and procedure to size PZVs that require capacitycertification in accordance with Section VIII, Division I, of the ASME Code. The procedureincludes determining the coefficient of discharge at 10% overpressure.

Formulas

Equation 8. Total Heat Transfer Rate

H = Hs As

Where:

H = total heat transfer rate in BTU/hr-ft2

Hs = solar heat rate in BTU/hr (300 BTU/hr from SAES L-043, Section 4.2.2)

As = heat transfer surface area in ft2

Equation 9. Liquid Expansion Rate

gpm BH500GC=

Where:

gpm = flow rate in gallons per minute

B = cubical expansion coefficient per °F

H = total heat transfer rate in BTU/hr

G = specific gravity

C = specific heat of trapped fluid in BTU/lb. °F




Equation 10. API Liquid PZV Sizing Equation

A Q38K K K

GP Pd W V 1 2

= −

Where:

A = effective discharge area in in2

Q = flow rate in gpm

Kd = effective discharge coefficient (Use Kd = .650 if unknown)

Kv = viscosity correction factor

Kw = back pressure correction factor

G = specific gravity (water = 1 @ 60 F)

P1 = upstream relieving pressure in psig

P2 = total back pressure in psig

Equation 11. Reynold’s Number

RQ(2800G)

A=

µ

Where:

R = Reynold’s number

Q = flow rate at the flowing temperature, in U.S. gallons per minute

G = specific gravity (water = 1 @ 60 F)

m = absolute viscosity at the flowing temperature, in centipoise

A = effective discharge area in square inches

Procedure

1.0 Calculate the total heat transfer rate, H.

1.1 Obtain the following information

• surface area of the equipment in ft2, As.

• Saudi Aramco solar heat rate (300 BTU/hr-ft2), Hs.

1.2 Multiply the heat transfer surface area of the equipment by the Saudi Aramco solarheat rate to calculate the total heat transfer rate.

2.0 Calculate the expansion rate of the trapped fluid, gpm.


• cubical expansion coefficient per °F, B.




• total heat transfer rate in BTU/hr (H from 1.0 above)

• specific gravity, G (water = 1 @ 60°F & 1 atm)

• specific heat of the trapped fluid in BTU/lb. °F, C.

2.2 Substitute the values obtained in 2.1 into the Liquid Expansion Rate equation tocalculate the expansion rate of the trapped fluid.

3.0 Determine the Reynold’s number and the capacity correction factor for viscosity, Kv.


• discharge temperature (°F) and pressure (psig)

• flow rate of the fluid in gallons per minute (gpm from 2.0 above)

• specific gravity, G, of the liquid at the flowing temperature

• absolute viscosity, m, (Figure ?? at discharge temperature)

• area, A, from manufacturer’s standard orifice sizes

3.2 Substitute the values obtained in 3.1 into the Reynold’s Number equation andcalculate the Reynold’s number.

3.3 Use the Reynold’s number that was calculated in 3.2 to determine the viscositycorrection factor, Kv, in Figure ?.

3.4 Divide the area of the manufacturer’s standard orifice sizes, A, by the viscositycorrection factor, Kv, from 3.3 to obtain the orifice area corrected for viscosity.

3.5 If the corrected area from 3.4 exceeds the manufacturer’s standard orifice size,repeat the steps starting with 3.0 and substitute the manufacturer’s next largeststandard orifice size for A. If the corrected area from 3.4 is less than or equal tothe manufacturer’s standard orifice size, select that standard orifice size.

Work Aid 2D: Procedure to Size PZVs for Two-Phase Liquid/Vapor Relief

This Work Aid contains the formulas and procedure to size PZVs for two-phase liquid/vaporrelief. This procedure was taken directly from API RP-520, Section 4.7.

Procedure

1.0 Determine the amount of liquid that flashes by an isenthalpic (adiabatic) expansion fromthe relieving condition either to the critical downstream pressure for the flashed vapor orto the back pressure, whichever is greater.

2.0 Calculate individually the orifice area required to pass the flashed vapor component, usingthe critical or subcritical equation in Work Aid 2A as appropriate, according to service,type of valve, and whether the back pressure is greater or less than the criticaldownstream pressure.




3.0 Calculate individually the orifice area required to pass the unflashed liquid componentusing the effective discharge equation in Work Aid 2C. The pressure drop (P1 - P2) is theinlet relieving pressure minus the back pressure.

4.0 Add the individual areas calculated for the vapor and liquid components to obtain the totalorifice area, A, that is required.

5.0 Select a pressure relief valve that has an effective discharge area equal to or greater thanthe total calculated orifice area. The designer should recheck the back pressure that willexist for the specific relief valve selected, with its particular discharge installation, byexamining the vapor generation downstream of the pressure relief valve nozzle. Whereappropriate, corrections can be applied to the individual orifice areas previouslycalculated. Furthermore, selecting a balanced pressure relief valve is often desirable tominimize the effect of flashed vapor on the valve capacity.




WORK AID 3: RESOURCES USED TO SELECT A RELIEF VALVE -MANUFACTURER’S CATALOG

This Work Aid contains a procedure for selecting a PZV from a relief valve manufacturer’scatalog. A completed ISS and the manufacturer’s quotation and catalog are needed for thisprocedure.

Procedure

1.0 Obtain the style designation for the PZV from the manufacturer’s quotation.

2.0 Using the PZV style designation and the manufacturer’s catalog, determine the following:

• Valve size

• Style

• Pressure and temperature ratings for the connections

• Materials of construction

• Type of cap and lever

3.0 Compare the PZV specifications on the ISS with the valve characteristics from themanufacturer’s catalog. If the answer to any of the questions below is no and the optionoffered by the manufacturer is not acceptable, select a different PZV that meets thespecifications on the ISS.

3.1 Do the inlet and outlet sizes match the sizes specified in the BODY section of theISS?

3.2 Do the materials of construction in the catalog meet the requirements in theBODY and TRIM MATERIALS sections of the ISS?

3.3 Is the PZV orifice area equal to, or greater than, the size requirement in theORIFICE AREA section of the ISS?

3.4 Does the valve include other specifications from the ISS such as a lifting lever,balanced bellows, etc.




WORK AID 4: RESOURCES USED TO SIZE AND SELECT A PRESSURERELIEF VALVE - COMPUTERIZED SIZING METHOD

This Work Aid contains the procedure for using the CROSBY-SIZE application (Version 2.1).This procedure assumes that the user has a basic knowledge of personal computers and that theproper printer was selected during the software installation process. A completed ISS isrequired to perform this procedure.

Procedure

1.0 To access Crosby-Size enter the “csize” subdirectory and type "crosby" then press the[ENTER] key. After the start-up screen appears, press the any key to make the MainMenu appear.

2.0 If you are instructed that your computer has been configured and that you can begin thesizing procedure, go to Step 8.0. Otherwise, at the Main Menu, select the last option,CROSBY-SIZE Program Configuration Menu, and press [ENTER]. The CROSBY-SIZEConfiguration menu will replace the Main Menu.

• Option one, 'Company Database Administration,' is used to create report headerswhich will identify the author of the reports generated by the software.

• Option two, 'Printer Maintenance Menu,' is used to identify the printer that will beused with the software.

• Options three through five, 'Program Options Settings,' 'Computation UnitSettings,' and 'Atmospheric Pressure Settings' are important for PZV sizing andselection.

• The last option, 'Reindex Files,' is used to save the selected configurations fordatabase presentations.

3.0 Select the 'Company Database Administration' option and press [ENTER]. Read the'Company Administration Menu' section in the manual pages 12-14 for instructions onediting the information on this menu. Make changes to the data entry areas so that themenu appears as shown in Figure 24.




Figure 20. Company Database Administration Menu

3.1 When you have finished editing the data entry areas, review the information on thescreen, and correct any errors. Press [ESCAPE] to return to the Main Menu. Anychanges will be saved by the program.

4.0 At the Main Menu, select the 'Program Options Settings' option and press [ENTER].Read the 'Program Options Settings' section of the manual on pages 15 & 16.

4.1 Select 'Reactive Force' and toggle the block from "OFF" to "ON" by pressing[ENTER]. With ‘Reactive Force’ "ON", the program will calculate the reactiveforce at PZV discharge.

4.2 Toggle 'Noise' and 'Weights Printing' from "OFF" to "ON". With ‘Noise' "ON",the program will calculate PZV discharge dBa (Sound pressure level in decibels onthe 'a' scale) at a distance requested by the design engineer. With 'WeightsPrinting' "ON", the program will list the weight of the selected PZV.

4.3 Leave 'CxK Manual Factor' "OFF". The program will require the data entry of k(Specific Heats Ratio Cp/Cv) and will calculate C (Specific Heats RatioCoefficient).

4.4 Leave 'Rupture Disk' "OFF" because no rupture disk is used with PZVs.




4.5 Press [ESCAPE] to save the entries and return to the 'Program ConfigurationMenu'. Entries that have been changed in the 'Program Option Settings' screen arenot saved permanently by the program. When the user 'quits' CROSBY-SIZEthese values return to the default "OFF", positions.

5.0 Highlight 'Computation Unit Settings' and press [ENTER]. Read the 'Computation UnitSettings' section of the manual pages 17-19.

5.1 Select 'Date Format' and press [ENTER]. The program provides a selection optionscreen 'Date Format'. Select 'USA' and press [ENTER].

5.2 Review the other settings in the menu; however, do not change any setting. Press[ESCAPE] to return to the 'Program Configuration Menu'.

6.0 Highlight the 'Atmospheric Pressure Settings' option and review this section of the manualpages 20 & 21. Observe the selection option screen, and then press [ESCAPE] to returnto the 'Program Configuration Menu'. (Any accidental change will be ignored by theprogram.)

7.0 Crosby-Size program configuration is complete press [ESCAPE] and an inquiry screenwill prompt with a question "Save Settings Permanently". Note that the default "Y", press[ENTER] to save changes. (Note: Typing "N" will return all selected options and data tothe Crosby-Size default values.)

8.0 Record the data from Exercise 1 into the table below. Other data have been provided inthe table for you. At the 'Main Menu' screen, select the 'API RP520 Fire Sizing' option.Enter the appropriate data when you receive the following prompts.

Prompt Enter

Select Tank Type: Sphere or Cylinder (S/C)? C

Cylinder Type: Horizontal or Vertical (H/V)? H

Type of Ends: Flat or Spherical (F/S)? S

Enter wetted area manually? N

[D] Diameter, in Ft

[L] End-to-End Length. in Ft

[H] Height off Ground, in Ft

[F] Fluid Level, in Ft

[T] Latent Heat, in Btu/lb

Adequate Drainage & Fire Fighting Equipment Provided (Y/N)? Y

Insulation Types: Options: Bare Vessel





Req. Cap. Other Than Fire Exp.: 0

Set Pressure:

Types Of Back Pressure: SuperimposedConstant

Back Pressure: Superimposed Constant:

Overpressure:

Specific Heat Ratio:

Molecular Weight:

Temperature:

Compressibility Factor:


Valve Types: Options: JOS

Do you require all Stainless Steel Valve (Y/N) ? N

Selected Valves Include:

Cap Type Selections: Options: Screwed Cap

[C]onditions, [S]ave, [R]eport. [V]alve Type, or [Q]uit? R

CROSBY-SIZE Sizing Reports: Report Options: Crosby Report Sheetw/ Formulas

Print to Screen/Printer (S/P) P

Customer Name: SAUDI ARAMCO

Customer Reference: PZV-131

Comments [ENTER]

[C]onditions, [S]ave, [R]eport. [V]alve Type, or [Q]uit? Q

[ESC] to Quit ESC




GLOSSARYallowable overpressure The pressure increase over the set pressure of the relieving

device, expressed in pressure units or as a percent. It is thesame as accumulation when the relieving device is set at themaximum allowable working pressure of the vessel.

critical flow rate The flow rate that corresponds to the limiting velocity.

critical flow pressure ratio The absolute pressure ratio of the pressure in the throat ofa PZV at sonic velocity (Pcf) to the inlet pressure (P1).

critical flow rate The flow rate that corresponds to the limiting velocity.

effective discharge area A nominal or computed area of a pressure relief valve usedin recognized flow formulas to determine the size of thevalve. It will b less than the actual discharge area.

limiting velocity The maximum mass flow rate through the PZV nozzle,which is equal to the velocity of sound in the flowing mediaat that location.

originator An engineer who initiates a RV installation or change thatrequires authorization approvals by providing the designand specification information. He can be an engineer forany discipline (i.e.; Operations Engineering, MaintenanceEngineer, Inspection Engineer, Projects Engineer, PlantEngineer, etc.)

relieving pressure The total of the set pressure plus overpressure plusatmospheric pressure. For gases and vapors, the relievingpressure is expressed in absolute units (psia). For liquids,the relieving pressure is expressed in gauge units (psig).

set pressure The Inlet gauge pressure at which the pressure relief valveis set to open under service conditions.

wetted surface Any surface that is both in contact with the process liquidand can be exposed to fire.




ADDENDUM1. Crosby Engineering Handbook, Technical Publication No. TP-V300

2. API RP-520

3. SAES-J-600, Pressure Relief Devices, Chapter 7.0

4. Crosby Catalogs




Addendum 1:Crosby Engineering Handbook




Addendum 2:Properties of Gases




Addendum 3:SAES-J-600, Pressure Relief Devices,




ADDENDUM 4: Crosby Catalogs

Sizing And Selecting Pressure Relief Valves

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Transcript of Sizing And Selecting Pressure Relief Valves