119473517 Sizing and Selecting Pressure Relief Valves

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Sizing And Selecting Pressure Relief Valves

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Instrumentations File Reference: PCI11003

For additional information on this subject, contact D.W. Buerkel on 874-7339

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Instrumentations Sizing And Selecting Pressure Relief Valves

Content

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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 - HAND CALCULATOR 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 CATALOG METHOD ...........................................................................................................................28 Sources of Required Data.........................................................................................28 Instrument Specification Sheets.....................................................................28 Saudi Aramco DeskTop Standards

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Manufacturers’ Catalogs ...............................................................................28 Administrative Requirements ....................................................................................31 SIZING AND SELECTING A PRESSURE RELIEF VALVE - COMPUTERIZED SIZING 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 VALVE SPECIFICATIONS .............................................................................................................44 Work Aid 1A: Procedure for Determining Applicable Contingencies for PZV Sizing 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 RELIEF VALVE-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 RELIEF VALVE - 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 Saudi Aramco DeskTop Standards

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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 Valve Installation (Operating Contingencies)1 ....................................................15 Figure 9. Example Determination of Relieving Pressure for a Multiple Valve Installation (Operating Contingencies)1 ....................................................16 Figure 10. Example Determination of Relieving Pressure for a Single Valve Installation (Fire Contingencies)1..............................................................17 Figure 11. Example Determination of Relieving Pressure for a Multiple Valve Installation (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 and Changes....................................................................................................37 Figure 16. Chart II - Operations Facilities Authorization Procedure for Relief Valve 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

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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 of the design basis of the PZV. The data that must be collected to size and select pressure relief valves are specified in API RP-520. The collected data is then recorded on Saudi Aramco’s Instrument Specification Sheet (ISS) Form 8020-611 ENG. The design basis includes the evaluation of all potential causes of overpressure and the calculation of the PZV’s relieving pressure. In the second phase, a preliminary valve size is calculated based on the particular valve specifications that were determined in the first phase. For the required relieving conditions, the area of the PZV orifice is calculated so that it provides the required flow rate and volume of discharge. This calculated area is called "effective area" because it is based on values that are assumed, or conceptually assigned from design requirements, rather than values that are actually measured. This module will demonstrate two methods to calculate the effective area—using a hand calculator and using a manufacturer’s computer program. In the third phase, the calculated "effective area" and the service conditions that are specified on the ISS are used to select a valve from either a manufacturer's catalog or a manufacturer’s computer program. This module also describes the relief valve authorization process. As code certified valves, the events in the service history of each PZV require authorization by responsible engineers. Signatures are recorded for the approvals required for the origination, installation, maintenance and removal of each PZV.

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SPECIFICATIONS REQUIRED FOR SIZING PRESSURE RELIEF VALVES Figure 1 shows the relationship between the information sources for the data that must be specified 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 which overpressure protection may be required. In order to determine the overpressure conditions, one must obtain process and equipment information about the physical system that requires overpressure protection. Then, each of the potential causes of overpressure must be evaluated both in terms of the pressures that may be generated and the rates at which fluids must be relieved. API RP-520, Part I, Section 4.1, "Determination of Relief Requirements," lists the following 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 states that the material balance is needed; however, it will be described later. In addition to the information above, the following items should be obtained. •

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

•

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

•

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

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PFD P&ID Equipment Specification Sheets Layout Drawings Material Balance

Obtain Required Infomation

Determine Basis for Relief

Design Basis Vendor Data

Calculate Effective Area

Fluid Properties Data Record data on appropriate ISS Form

Use ISS Form for Bid Solicitation

Figure 1. Information Sources for PZV Sizing and Selection

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Process Flow Diagrams Process conceptual designs are depicted on PFDs (Figure 2). PFDs also include the material and heat balance information that is related to each step in the process. After a PFD is completed, the minimum 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. After engineering and design approval, final design pressure and temperature (P&T) ratings are assigned. 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 and temperature gauge readings may be reported on the wrong process line. They can be located on the right line, but in a different place than the drawing shows. This does not mean that acquiring accurate data is a hopeless task, but a certain degree of caution is required in using data for calculations. 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 decreasing as the stream progresses through the plant? Does the temperature rise as expected after passing through a heat exchanger?

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Figure 2. Process Flow Diagram of HPPT Saudi Aramco DeskTop Standards

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Piping & Instrument Diagrams Piping and Instrument Diagrams (P & IDs) contain some of the data that are required for sizing and selecting PZVs. Figure 3 shows a typical P & ID for a High Pressure Production Trap. Note that 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 because P & ID revisions lag behind equipment specification changes. Final PZV set pressure and sizing determinations must come from MAWPs that are included in either “As-Built” or “Approved for Construction” equipment specifications. Data that are taken from P & IDs for PZV sizing should only 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 the Instrument 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 a standard form, which is suitable for the sizing and selection of PZVs. Saudi Aramco Engineering Standard J-007, Instrumentation Forms, lists three ISS forms for PZV specification. ISS form number 8020-611-ENG is used for specifying spring loaded, screwed or flanged, PZVs in English units. ISS 8020-611M-ENG is used to specify PZVs in metric units, and ISS 8020-612-ENG is used 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 for recording data. There is a sidebar space on each sheet for recording revisions, authorization signatures, and for identifying the data lines on each sheet. Each sheet contains an item identification area along the bottom. Sheet 2 (Figure 5) contains data and a space for a simplified relief 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.

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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 process application. For example, a spare PZV can share an ISS with the main PZV, or with PZVs on duplicate process equipment (e.g., two identical heat exchangers), or with several PZVs in a multiple relief valve application. Initially, the ISS form contains generic information that is presented for competitive bid solicitation. At this point, the form is referred to as Revision A (Rev.A). ISS Rev.A is completed after 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 area sizing. All ISS revisions after vendor selection must be based on the manufacturer's data for the selected PZV.

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Figure 4. ISS Form No. 8020-611-ENG, Sheet 1 Saudi Aramco DeskTop Standards

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Figure 5. ISS Form No. 8020-611-ENG, Sheet 2 Saudi Aramco DeskTop Standards

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Figure 6. ISS Form No. 8020-611-ENG, Sheet 3 Saudi Aramco DeskTop Standards

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Basis of Selection Conditions Requiring Overpressure Protection All causes of overpressure, or contingencies, must be evaluated for each PZV installation in terms of the pressures generated and the rates at which fluids must be relieved. Causes of overpressure in process equipment can range from a single event to a complex combination of events. Figure 7 below is a table that summarizes some, but not all, operating conditions that lead to the indicated relief capacities. Item No.

Condition

Pressure Relief Device (Liquid Relief)

Pressure Relief Device (Vapor Relief)*

1

Closed outlets on vessels

Maximum liquid pump-in rate

Total incoming steam and vapor plus that generated therein at relieving conditions

2

Cooling water failure to condenser

—

Total vapor to condenser at relieving conditions

3

Top-tower reflux failure

—

Total incoming steam and vapor plus that generated therein at relieving conditions less vapor condensed by sidestream reflux

4

Sidestream reflux failure

—

Difference between vapor entering and leaving section at relieving conditions

5

Lean oil failure to absorber

—

None, normally

6

Accumulation of noncondensables

—

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

7

Entrance of highly volatile material

—

For towers , usually not predictable

Water in hot oil Light hydrocarbons in hot oil 8

Overfilling storage or surge vessel

9

Failure of automatic controls

—

Must be analyzed on a case-by-case basis

10

Abnormal heat or vapor input

—

Estimated maximum vapor generation including noncondensables from overheating

11

Split exchanger tube

—

Steam or vapor entering from twice the crosssectional area of one tube; also same effects found in Item 7 for exchangers

12

Internal Explosions

—

Not controlled by conventional relief devices but by avoidance of circumstances

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Maximum liquid Pump--in rate

—

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13

Chemical reaction

14

Hydraulic expansion Cold fluid shut in

—

Estimated vapor generation from both normal and uncontrolled conditions

See C.2

—

See C.2

—

Line outside process area shut in 15

Exterior fire

—

Estimate by the method given in D.5

16

Power failure (steam, electric, or other)

—

Study the installation to determine the effect of power failure; size relief valve for the worst condition that can occur

Fractionators

Reactors

—

—

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

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

Fans would fail; size valves for the difference between normal and emergency duty Air-cooled exchangers

— —

Surge vessels

Maximum liquid inlet rate

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

Figure 7. Basis of Relief Capacities Under Selected Conditions1

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Contingencies That Cause Overpressure Fire Contingency - Any pressure vessel in a plant that processes flammable liquids may be exposed to fire at some point in its life, even if the vessel does not contain flammable liquid. If an open, free-burning fire occurs, vessels and other equipment that are exposed to the flame will absorb heat by radiation or by direct contact with the flame or hot gases. Escaping flammable liquids may be carried away For this reason, a PZV should be provided for pressure vessels to relieve overpressure that is caused by fire. (API RP-2001 describes ways to limit heat input from a fire using surface drainage and firewater application.) Fire contingency applies to any liquid-filled process equipment (within a fire zone) that has a wetted surface. A wetted surface is any surface that is both in contact with the process liquid and can be exposed to fire. An uninsulated, wetted surface of a vessel will absorb radiation as sensible heat. As the temperatures of the vessel and the liquid rise, the temperatures will essentially become equal. At the boiling point of the liquid, the radiation will be absorbed as latent heat and the resulting vapor generation will cause the pressure to rise to the set pressure of the PZV. As long as the vapor that is generated is less than the flow capacity of the PZV, the valve will intermittently open and close to protect the vessel. If the rate of vapor generation is greater than the rated capacity of the PZV, the pressure will increase beyond the permissible accumulation and create an unsafe situation. A PZV may not protect a pressure vessel that contains only vapor because the vessel’s wall temperature can rise very rapidly and lead to vessel failure. For this reason, a vessel that contains only vapor should be protected by reducing its pressure to atmospheric and by limiting the heat input from a fire. Equipment that does not have a reasonable quantity of wetted surface cannot be protected by a PZV against a fire contingency. Prime movers (pumps, compressors, etc.) and low-volume equipment (pipes, tubes, etc.) are typical examples of equipment that cannot be protected against a fire contingency by a PZV. In addition, jacketed vessels are also exempt from fire contingencies unless the jacket contains a liquid, or the unjacketed area exposed to fire has sufficient wetted surface to protect the vessel from heat damage. SADP-Section XII, Section 5.3.1 lists exceptions to fire risk contingencies in paragraphs (a), (b), and (d). Paragraph (d). This reference should be reviewed when determining fire contingencies. Blocked Discharge Contingency (BD) results from overpressure caused by cessation of fluid transport 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 be considered. Other conditions that can cause BD can only be determined from equipment specification 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 appropriate pressure units.

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Finally, BD alone does not cause overpressure. Evaluation of the energy source (mechanical, thermal, chemical, etc.) under BD conditions determines if overpressure is possible. This information 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 MOP at the equipment must be determined. If the MOP is higher than MAWP, a valid BD contingency exists, and PZV area calculations must be completed. Other Contingencies, which are not related to fire or BD, can cause overpressure of process equipment. 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 equipment expands as a result heat transfer from an external source (except fire). The thermal relief contingency must be considered as a “basis of selection” if the hydraulic pressure due to expansion of the liquid exceeds the MAWP of the equipment. Thus, the surface area and heat transfer rate are required to solve thermal relief contingencies. No API sizing procedure can protect against some run-away chemical reactions. The design engineer must recognize these cases and review engineering literature for possible solutions for protecting against these contingencies. Operational Requirements for Overpressure Protection To meet the requirements of the ASME Code, accumulated pressure must be limited to 110 percent of MAWP for a single-valve installation sized for operating (non-fire) contingencies. For multiple PZV applications, ASME code requirements limit accumulated pressure to 116 percent of MAWP in vessels that are protected by for a multiple-valves sized for operating (nonfire) contingencies. Effective-Area Concept If protection against more than one contingent event can be provided by a PZV, an “effective area” calculation must be made for each particular contingency. The largest effective area calculation from all of the contingencies is selected as the “Basis for Relief,” which is used to specify the PZV. The effective-area concept allows area sizing by calculation, which is independent 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 set pressure plus the allowable overpressure. The allowable overpressure can vary depending on the contingency and on whether the installation involves a single PZV or multiple PZVs (not spare PZVs). The units of measurement for relieving pressures depend on process fluid properties as reflected in the area sizing equation. Because liquids are generally considered to be non-compressible, gauge pressures (psig) are generally used. For compressible fluids, atmospheric pressures (psia, psia = psig + 14.7) are generally used. Saudi Aramco DeskTop Standards

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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 Code requires that the accumulated pressure shall be limited to 110 percent of the MAWP. The set pressure of the valve shall not exceed the MAWP. Figure 8 shows an example of the determination 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 Protected Vessel MAWP, psig

100

Maximum accumulated pressure

110

Valve set pressure, psig

100

Allowable overpressure, psi Relieving Pressure, psia

10 124.7

Figure 8. Example Determination of Relieving Pressure for a Single Valve Installation (Operating Contingencies)1 Multiple-Valve Installation - For operating (nonfire) contingencies, Section VIII of the ASME Code requires that the accumulated pressure shall be limited to 116 percent of the MAWP. The set pressure of the first valve shall not exceed the MAWP. The set pressure of the additional valves shall not exceed 105 of the MAWP. Figure 9 shows an example of the determination of relieving pressure for a multiple valve installation.

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The staging of the valves, helps to preventing chattering, since not all relieving cases are at maximum flow. (Only one valve is required). ASME Section VIII allows these 105% valves to have 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) Protected Vessel MAWP, psig

100

Maximum accumulated pressure

116

Valve set pressure, psig

100

Allowable overpressure, psi Relieving Pressure, psia

16 130.7

Additional Valve (Valve Set Pressure Equal to 105 Percent of MAWP) Protected Vessel MAWP, psig

100

Maximum accumulated pressure

116

Valve set pressure, psig

105

Allowable overpressure, psi Relieving Pressure, psia

11 130.7

Figure 9. Example Determination of Relieving Pressure for a Multiple Valve Installation (Operating Contingencies)1 Supplemental-Valve Installations provide protection against additional hazards from fire or other 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 110 percent of the MAWP. Such supplemental pressure relieving devices shall be capable of preventing the pressure form rising 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 at 10% overpressure. Hence, with a setpressure allowable of 110% we have 110% * 1.1 = 121% accumulation.

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Fire Contingencies For fire contingencies, Section VIII of the ASME Code requires that the accumulated pressure shall be limited to 121 percent of the MAWP. This requirement applies to single-, multiple-, and supplemental-valve installationFigure 10 shows an example of the determination of relieving pressure for a single valve installation. Characteristic Valve Set Pressure Less Than MAWP Protected Vessel MAWP, psig Maximum accumulated pressure Valve set pressure, psig Allowable overpressure, psi Relieving Pressure, psia Valve Set Pressure Equal to MAWP Protected Vessel MAWP, psig Maximum accumulated pressure Valve set pressure, psig Allowable overpressure, psi Relieving Pressure, psia

Value 100 121 90 31 135.7 100 121 100 21 135.7

Figure 10. Example Determination of Relieving Pressure for a Single Valve Installation (Fire Contingencies)1 Figure 11 shows an example of the determination of relieving pressure for a multiple valve installation. Characteristic Value First Valve (Valve Set Pressure Equal to 100 Percent of MAWP) Protected Vessel MAWP, psig 100 Maximum accumulated pressure 121 Valve set pressure, psig 100 Allowable overpressure, psi 21 Relieving Pressure, psia 135.7 Additional Valve (Valve Set Pressure Equal to 105 Percent of MAWP) Protected Vessel MAWP, psig 100 Maximum accumulated pressure 121 Valve set pressure, psig 105 Allowable overpressure, psi 16 Relieving Pressure, psia 135.7 Figure 11. Example Determination of Relieving Pressure for a Multiple Valve Installation (Fire Contingencies) 1 Saudi Aramco DeskTop Standards

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Steam Service Protection against overpressure by steam is treated as any other ASME Section VIII material for API PZV sizing except that area sizing equations contain a superheat correction factor KSH, and a Napier KN correction factor. Generally contingencies regarding steam relief outside of boiler houses 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 in the equations. If process fluid steam is saturated the value of KSH is one (KSH = 1) otherwise the value 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 psia where KN = (0.1906P1 - 1000)/(0.2292P1 - 1061) and P1 = psia Relieving Pressure (Set Pressure + Overpressure + Atmospheric Pressure).

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CALCULATING THE SIZE OF A PRESSURE RELIEF VALVE - HAND CALCULATOR METHOD After the specifications are determined and recorded on the ISS, the next activity is to calculate a preliminary valve size. This preliminary valve size is called the effective discharge area. Figure 12 shows the inputs that are used to determine the basis for calculating the size of a PZV. Fire Contingency

Blocked Flow Contingency

Thermal Contingency

Other Contingency

Worst Case Area Calculations

SAES-J-600 Standards

API RP-520 Standards

SADP 600 Standards

ASME SEC VII 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 contingency that requires the largest effective area dictates the size of the PZV. The basis for calculating valve size can be divided into three areas: • Service conditions •

Flow rate

•

Effective nozzle area

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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 process requirements such as temperature and pressure are used to determine the basis for relief and to calculate the effective area of the valve. As discussed below, data listed in the Service Conditions section of the ISS is used to select the appropriate sizing equation. Also, correction factors are used when the ideal or assumed conditions associated with each of the sizing equations are different from the actual service conditions. Backpressure is an example of a service condition variable that requires the use of correction factors. Flow Rate The effective area sizing equations included in Work Aid 2 show that the required effective discharge area, variable A, equals a flow rate (variable W, V, or Q), which is modified by other coefficients in the equation. The modifiers and coefficients are derived from published chemical properties. BD Contingencies - In BD (Blocked Discharge) contingencies, the flow rate is determined by the prime-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 is substituted into the appropriate API area sizing equation. Thermal Relief Contingencies - In thermal relief contingencies, the flow rate is derived from

chemical properties data that relate temperature to liquid expansion. The design engineer must determine the surface area of the blocked pipe and calculate the liquid expansion using the Saudi Aramco 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: BH gpm = 500GC 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 usually ignored.) The following formula can be used to calculate the total heat transfer rate for Saudi Aramco applications. H = Hs As

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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) = heat transfer surface area in ft2

As Fire Contingencies - Flow rate in fire contingencies is the vapor generated by the energy released into the process liquid by the fire. Flow rates are likewise derived from heat transfer rates and chemical properties as shown in the following equations. W = Q/Hvap Where: 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 fighting efforts and adequate drainage exist. 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: 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 a container 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 or computed area of a pressure relief valve used in recognized flow formulas to determine the size of the valve. The effective discharge area is generally less than the actual discharge area. An effective discharge area is calculated in order to specify the actual orifice area that is required for a PZV. The effective discharge area for a PZV is calculated in using the API critical flow equation. The first area solution is often a trial estimation for sizing. The back pressure factor, Kb, is assumed to be 1 unless reliable engineering data indicates that back pressure will exist in the system. For example, if a vent header is known to have a constant pressure of 15 psig, then a superimposed back pressure of 15 psig would be used to determine Kb. Saudi Aramco DeskTop Standards

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Instrumentations Sizing And Selecting Pressure Relief Valves

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) in gas and vapor equations. P1 is always the upstream relieving pressure into the PZV in API equations. 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 wetted surface in the heat transfer rate equation. ISS 8020-611 ENG defines terms and units on each printed line. In some cases two options are printed (e.g., Item 37, MW or SP GR @ FLOWING TEMP). In these cases the design engineer should 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 on whether the flow is critical or subcritical. Critical and subcritical flow are best described using an example of the expansion of a compressible gas across a nozzle. If the downstream pressure decreases, the velocity and specific volume of the gas will increase until the mass flow rate of the gas reaches a limiting velocity. This limiting velocity is equal to the velocity of sound in the flowing gas in the throat of the nozzle. The mass flow rate that corresponds to the limiting velocity is called the critical flow rate. The pressure in the throat at sonic velocity is called the critical flow pressure, Pcf. Under critical flow conditions, the pressure in the throat cannot fall below the critical flow pressure even if the downstream pressure is much lower. At critical flow, the expansion that occurs as the pressure decreases from the throat pressure to the downstream pressure is irreversible. The energy is dissipated in turbulence into the surrounding fluid. The ratio of Pcf to the inlet pressure, P1, is called the critical pressure ratio. The critical flow pressure ratio may be estimated using the following equation. Pcf  2  k/(k +1) = P1  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 downstream pressure is greater than Pcf, subcritical flow will occur. The sizing equations for these two instances are described below.

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Instrumentations Sizing And Selecting Pressure Relief Valves

Sizing for Critical Flow - When sizing gas or vapor PZVs, one must calculate the critical back pressure to determine whether the flow is critical or subcritical. The critical back pressure is calculated by multiplying the relieving pressure by the critical flow pressure ratio for the particular gas, which can be found on page 27 of Addendum 2. If pressure downstream of the throat (use total back pressure until a PZV is selected) is less than the critical back pressure, the equation below may be used to calculate the effective discharge area, A, for the PZV. The manufacturer’s PZV with an effective discharge area equal to or greater than the calculated value of A is chosen for the application. W TZ A= CKP1K b M 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. Use Addendum 1 or manufacturer’s values.

M

=

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 C based on the ratio of the specific heats, k.

T

=

Z

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

molecular weight of hydrocarbon gas or vapor

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

For an example of relief valve sizing for critical flow, refer to pages 7-22 and 7-23 of the Crosby Engineering Handbook in Addendum 1. Sizing for Subcritical Flow - If the back pressure is greater than the critical back pressure, either of the equations below may be used to calculate the effective discharge area, A, for the PZV.

Saudi Aramco DeskTop Standards

A=

W ZT 735F2K MP1(P1 − P2 )

A=

V ZTM 4645.2F2K P1(P1 − P2 )

A=

V ZTG 863.63F2K P1(P1 − P2 )

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Instrumentations Sizing And Selecting Pressure Relief Valves

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)

F2

=

coefficient of subcritical flow = (k − 1)/ k   k  2/k 1 − r ( ) r      k − 1  1− r 

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 per minute at 14.7 psia and 60°F)

K

=

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

M

=

molecular weight of hydrocarbon gas or vapor

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

=

C

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

T

=

Z

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

back pressure in psia

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

Bellows PZVs operating in the subcritical flow regime are sized using the critical flow equation except that the manufacturers K factor must be used. Saudi Aramco design engineers shall not size spring loaded PZVs under API subcritical flow conditions outlined in API RP-520, Section 4.3.3. SAES-J-600 section 5.2.1, expressly forbids the 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: W A= 51.5P1KKNK SH

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Instrumentations Sizing And Selecting Pressure Relief Valves

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. Addendum 7.

KSH = 1 for saturated steam. Otherwise use

= 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 includes examples for sizing PZV's in steam service under ASME Section VIII. Reliable steam tables can be found in Crane TP-410. There are two steam tables in Crane, 'Properties of Saturated Steam and Saturated Water' and 'Properties of Superheated Steam.' Superheated steam has different volume/mass, temperature, and pressure properties than saturated steam; therefore, ASME/API relief valves for steam service require a superheat correction factor KSH in the effective area sizing equation. Likewise, steam at pressures above 1515 psia require a correction factor KN. KN

Sizing for Liquid Relief PZVs for liquid relief may be sized by using the following equation: Q G A= 38KdK WK V P1 − P2 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 73 to 7-5 of Addendum 1 or manufacturer’s value.) Kv

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

G

=

P1

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

P2

=

Specific gravity (water = 1 @ 60°F)

Total back pressure in psig.

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Instrumentations Sizing And Selecting Pressure Relief Valves

The PZV should first be sized for nonviscous-type application to obtain a preliminary discharge area, A. From the manufacturer’s standard orifice sizes, the next orifice size that is larger than A is used to determine the Reynold’s number using the following equation. Q(2800G) R= 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 to correct the preliminary discharge area as shown in the following equation. A A= R KV 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 area exceeds the chosen standard orifice area, the calculations above are repeated using the next larger standard orifice area. Sizing for Two-Phase Liquid-Vapor Relief API RP-520, section 4.7, page 37, 'Sizing for Two-Phase Liquid/Vapor Relief', presents methodology for sizing PZVs which relieve process fluids that are partially liquid and partially gas. API methodology recommends the following: 1. Determine the quantity of liquid and calculate an effective area based on that determination. 2.

Determine the quantity of vapor and calculate an effective area based on that determination.

3.

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

4.

Use the same pressure values, relieving pressure and back pressure in both calculations.

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Instrumentations Sizing And Selecting Pressure Relief Valves

5.

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

Relief valve problems involving two phase flow conditions beyond a 3/4D1 size PZV used for thermal relief should only be resolved by senior engineers specializing in relief valve technology. It is further recommended that a test stand or bench scale test apparatus be used to test the selected valve. Oversized PZVs tend to chatter, and often leak after a release. And most calculations of PZVs for two phase flow conditions will be oversize effective relief areas. The test apparatus must have close temperature control as well as pressure control. Furthermore under flashing conditions both wet-bulb and dry temperatures must be measured. Data on the valve performance should be measured and recorded over a wide 'true temperature' range at constant relieving pressure. Finally back pressure should be varied over a reasonable range based on 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 the refrigerant will usually be able to recommend a suitable relief device for their product. They can also direct Saudi Aramco design engineers to client engineers who have process experience with the product. Complex relief valve sizing and selection problems must be reviewed by senior engineers who have experience with the process involving protection.

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Instrumentations Sizing And Selecting Pressure Relief Valves

SELECTING A PRESSURE RELIEF VALVE - MANUFACTURER’S CATALOG METHOD There are two parts to the selection of relief valves. The first part is an engineering decision regarding the PZV type and features. The second part is the selection of a manufactured relief valve that meets the requirements determined. Both parts are interdependent because design engineers 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 his decisions, 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 recommended practices. 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 may be specified on the ISS because they are considered to be cost effective by the design engineer and 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 the ISS represents agreement between Saudi Aramco engineering and the relief valve manufacturer that the valve delivered to Saudi Aramco will meet the requirements of the ISS and the ASME/API certification. Manufacturers’ Catalogs After a vendor selects and quotes a PZV, catalogs and other publications published by the manufacturer are required to verify that the quoted PZV meets the requirements of the ISS. In some cases, where non-standard options are required, factory correspondence is required to verify the quotation. Manufacturers' catalogs contain both certified and non-certified valve capacities. Catalogs also list materials of construction for each class of relief valve, and they include instructions for relating a model, or style, number to the features that are available in a particular class, or series, of relief valve.

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Instrumentations Sizing And Selecting Pressure Relief Valves

Valve capacity tables are always included for Air, Water, and Steam in catalogs for ASME Section I or VIII certified relief valves. Capacity tables relate certified capacities to set pressure values 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 and Crosby 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 valve with a style number of 951501MA; however, the valve is not bronze or brass because manufacturers no longer use these materials.

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Instrumentations Sizing And Selecting Pressure Relief Valves

The engineer should revise the ISS and obtain approval for a carbon steel valve with stainless steel trim because only the stainless steel will be in contact with the process fluid during discharge. Page 3 of Cat. 902 (Addendum 4) compares the Series 800 pressure relief valves with the Series 900. Observe that Series 900 is designed for Thermal Relief and it does not have adjustable blowdown. The PZV does not require a blowdown adjustment reseating rate after discharge, so Series 900 is selected. The procedure for selecting a style number for Crosby Series 900 PZVs is illustrated on page 7 of the 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. Crosby Series 900 relief valves have carbon steel cylinders (Fluid Cavity & Bonnet) and 316 stainless steel (316 SS) Base (Body). Disk and seat assemblies are stainless steel, and a Kalrez fluorocarbon 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 by Saudi Aramco approved vendors has been discontinued. The Crosby Series 900 Style 951501MA has all inert materials, stainless steel and Kalrez, in contact with the process fluid (process water) under normal conditions.

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Instrumentations Sizing And Selecting Pressure Relief Valves

Certified water capacities for Crosby Series 900 relief valves are listed on page 22 of Cat. 902. At 120 psig set pressure the selected valve (0.074 sq. in effective area) will discharge 23 gpm. The same valve is certified for 543 lb/hr steam capacity. Both capacities exceed the required flow rated 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 ISS can 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 And Balanced Types', Issue Date 1 DEC 96, section 1.2, requires ISS form 8020-611 ENG to be included with "Buyer's Quotation Request or Purchase Order." Official Saudi Aramco administrative requirements related to quotations and purchasing beyond this statement and the acceptable 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. ISS form 8020-611 ENG is a generic specification designed to solicit competitive bids from acceptable manufacturers. After bid selection the ISS form is revised with the manufacturer’s model number and other special information related to the selected manufacturers valve. The revised (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 be completed. Form 3099A is central to the disposition of a PZV throughout its useful life, even if the valve is 'Mothballed'. Saudi Aramco standards SAEP-318 and SAEP-1131 are devoted exclusively 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 of 8020-611 ENG. It contains all information related to the testing, and inspection of a PZV in much the same way as 8020-611 ENG contains all of the information related to sizing, and selection of a PZV. Instructions and procedures routine testing and inspection of relief valves is contained in SAEP-319. Instructions for filling out form 3750 are outlined in Saudi Aramco standard SAEP-1133. Both Forms 3099A and 3750 as well as 8020-611 ENG are part of the Saudi Aramco company wide computer system.

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Instrumentations Sizing And Selecting Pressure Relief Valves

SIZING AND SELECTING A PRESSURE RELIEF VALVE - COMPUTERIZED SIZING METHOD Crosby-Size is the relief valve sizing computer software provided by Crosby Valve and Gage Company. 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 a basic understanding of the relief valve sizing calculations that were described in Work Aid 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 and VIII, and API RP-520. The sixth option, “Miscellaneous Reports/Utilities Menu,” is used to save sizing and selection data and to print reports of the selected PZV. The last option, “CROSBYSIZE Program Configuration Menu,” is for program administration, which is used to organize files, select a printer, select program variables, and 'toggle' certain program features. Each Option in the main menu is described in a section of the software manual, which is indexed by identification tabs bearing each option name. Work Aid 3 contains the procedures for using the application. Program Applications The 'Valve Sizing Options:' screen provides nine sizing options based on a combination of the state of the process fluid and the type of PZV that will be used. In some cases, Crosby PZV classes are part of the sizing and selection.

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For sizing and selection of PZVs for liquids, the available option selection presupposes that the user is familiar with Crosby relief valves. If a wrong class of valve is chosen during the initial entry from the selection screen, the user can return to the selection screen and try a different choice. Most of the data entered into the program is retained until the user returns to the 'Main Menu'. The discussion of CROSBY-SIZE will now follow program selections that verify the sizing and selection 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 selected and the following data are enter at the prompts.

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Prompt Select Tank Type: Sphere or Cylinder (S/C)? Cylinder Type: Horizontal or Vertical (H/V)? Type of Ends: Flat or Spherical (F/S)? Enter wetted area manually? [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)? Insulation Types: Options: Accept These Options (Y/N/Quit) ? Req. Cap. Other Than Fire Exp.: Set Pressure: Types Of Back pressure: Back Pressure: Superimposed Constant: Overpressure: Specific Heat Ratio: Molecular Weight: Temperature: Compressibility Factor: Accept These Options (Y/N/Quit) ? Valve Types: Options: Do you require all Stainless Steel Valve (Y/N) ? Selected Valves Include: Cap Type Selections: Options: [C]onditions, [S]ave, [R]eport. [V]alve Type, or [Q]uit?

Saudi Aramco DeskTop Standards

Enter C H S N 15 30 15 12.25 172 Y Bare Vessel Y 0 200 Superimpos ed Constant 0 21 1.12 78.11 100 1.0 Y JOS N JOS-25, 300 x 150 Screwed Cap R

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Instrumentations Sizing And Selecting Pressure Relief Valves

CROSBY-SIZE Sizing Reports: Report Options:

Print to Screen/Printer (S/P) Customer Name: Customer Reference: Comments [C]onditions, [S]ave, [R]eport. [V]alve Type, or [Q]uit? [ESC] to Quit

Crosby Report Sheet w/ Formulas P SAUDI ARAMCO EXAMPLE [ENTER] Q ESC

The program generates the report shown in Figure 14. CROSBY-SIZE ASME Section VIII Pressure Relief Valve Report Sheet (Note 1) Customer: SAUDI ARAMCO Reference: EXAMPLE Quote/Tag: EXAMPLE/

Date: Sept. 21, 96

Prepared By: Saudi Aramco

Gas & Vapor Mass Flow Spring Loaded Pressure Relief Valve Sizing Service Conditions Required Capacity······················32330.8 lb/hr Set Pressure··························200.0 psig Overpressure ·························21 % Temperature··························100 °F Backpressure - Constant ·················0.0 psig Specific Heat Ratio ·····················1.12 Calculated Values Required Orifice Area ···················1.052 sq in Effective Orifice Area····················1.287 sq in Calculated Flow ·······················39568.1 lb/hr Calculation Formula (USCS) (Note 1) A = -------------------------------------Pi * C * K * h-bm

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Instrumentations Sizing And Selecting Pressure Relief Valves

Required Orifice Area (A) ·················1.052 sq in Required Capacity (W) ···················32330.8 lb /hr Set Pressure (P) ·······················200.0 psig Temperature (T)························100 °F Relieving Pressure (P1) ··················256.7 psia Molecular Weight (M) ····················78.11 Back Pressure Correction (Kb) ·············1.000 Effective Coefficient Of Discharge (K) ········0.975 Specific Heat Coefficient (C) ···············329 Compressibility Factor (Z) ················1.000 Valve Style····························JOS-25-A Valve Size (Inlet-Orifice-Outlet) ·············2 J 3 Valve Connection (inlet-Outlet) ·············300 x 150 (Note 1) API effective orifice area and effective coefficient of discharge area used in the flow and sizing calculation shown on this report. CROSBY-SIZE Section VIII Report Sheet, Page #2

Date: Sept. 21, 96

Customer: SAUDI ARAMCO Reference: EXAMPLE 1-508-384-3i2i

Prepared By: Saudi Aramco

Fire Sizing Vessel Dimensions Tank Type···································Cylinder Orientation···································Horiz. Type of Ends·································Spherical Diameter····································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/lb Adequate Fire Fighting & Drainage·················Yes Insulation Type ·······························Bare Vessel Insulation Conduction Factor ·····················1.0000 Wetted Area ·································901.13 sq ft. Required Flow Due To Fire Capacity ···············32330.8 lb/hr Figure 14. CROSBY-SIZE Report Sheet for PZV-200 Saudi Aramco DeskTop Standards

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RELIEF VALVE AUTHORIZATION PROCESS The authorization process is the final phase in the sizing and selection of a PZV. The authorization process that is followed depends on the responsible organization, Projects or Operations. 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 provided with projects. These procedures are governed by Saudi Aramco standards SAEP-318 SAEP-319.

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

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The authorization process for Projects is further explained below. 1. The Originating Engineer, or Originator, completes lines 1-4 of Form 3099A in accordance with the requirements of Saudi Aramco specification SAEP-1131, Section 2. ISS Form 8020-611-ENG is attached to form 3099A until the original 3099A/ISS 8020-611ENG is filed by the Relief Valve Administrator. Form 3099A, is forwarded to the Senior Project Engineer, or Superintendent Operations 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 or Operations Foreman for disposition.

4.

The Project Inspection Supervisor reviews and signs off Form 3099A using ISS 8020-611ENG to verify the data on Form 3099A for newly constructed facilities.

5.

The Originator, or someone under his supervision, enters the data into the relief valve database. The original 3099A is distributed to the Relief Valve Administrator for approval signature.

6.

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

7.

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

8.

The Maintenance Relief Valve Test Unit, or the Contractor Shops Division, will record the test data on form 3750, and mark the PZV in accordance with SAEP1131. Form 3750 for the PZV is distributed to the Relief Valve Administrator.

9.

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

10.

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

11.

The Relief Valve Coordinator checks and files original Form 3099A. He then distributes copies of Forms 3099A and 3750 to the Computer Operations Department, and a copy of 3099A to the Supervisor of Operations Engineering Inspection.

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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

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Relief Valve Authorization, Form 3099A The Relief Valve Authorization Form 3099A consists of two major sections and an area for remarks (Figure 17). The top section is divided into data areas and letter/number blocks. The areas are identified by line numbers and column numbers. Specific details for the entry of information in each area are outlined in SAEP-1131, Section 2. Authorization regulations, distribution requirements, and disposition rules regarding Form 3099A are outlined in SAEP-318. Form 8020-611 The Form 8020-611 (ISS) is used to record detailed engineering information, which is suitable for the sizing and selection of PZVs. Saudi Aramco Engineering Standard J-007, Instrumentation Forms, lists three ISS forms for PZV specification. ISS form number 8020-611-ENG is used for specifying spring loaded, screwed or flanged, PZVs in English units. ISS 8020-611M-ENG is used to specify PZVs in metric units, and ISS 8020-612-ENG is used to specify pilot operated PZVs. Unlike 3099A and 3750, an ISS for a PZV is not a perpetual document. The final revision of an ISS for a PZV is attached to the initial 3099A. The distribution and disposition of 3099A/8020611-ENG was presented above. Requirements regarding ISS 8020-611-ENG are contained in Saudi Aramco standards SAES-J600, and 34-SAMSS-611.

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Figure 17. Relief Valve Authorization, Form 3099A

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Relief Valve Test Report, Form 3750 Form 3750, Relief Valve Test Report is part of the PZV authorization process. Each time a work order 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. A sample Form 3750 is shown in Figure 18. Data entry into the form is associated with the troubleshooting discussions in Module 5. SAEP-1133 should be referenced in conjunction with various PZV T&I operations. Note that the inspection period required in 3099A line 2, columns 74-75 should originate from Form 3750 after initial testing of the PZV.

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Figure 18. Relief Valve Test Report, Form 3750

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WORK AID 1:

RESOURCES FOR DETERMINING RELIEF VALVE SPECIFICATIONS

This Work Aid provides the procedures for determining the Basis of Selection and the relieving pressure of PZVs in accordance with SAES-J-600. These procedures are taken from API RP-520 and Section VIII of the ASME code. Work Aid 1A:

Procedure for Determining Applicable Contingencies for PZV Sizing and Selection

This procedure can be used to determine all applicable contingency events. cannot be used to calculate the effective area for each applicable contingency. 1.

2.

This procedure

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. The pressure 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 line under study.

B.

Follow each process discharge line that is connected to the protected device downstream as far as the first device that is protected by a PZV. Note all devices that 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 on that discharge line that can block the flow out of the protected device under study.

Determine whether fire contingency is applicable. A.

If the information is not provided, determine whether the equipment is in a fire zone (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 this case.*

B.

Does the equipment contain liquid and does it have a sufficient amount of wetted surface? 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.*

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*

3.

If in doubt, select fire as a contingency or compare the fire contingency area to other contingencies. Seek the guidance of a senior engineer or engineering manager.

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 PZVs The following procedure and the table in Figure 23 can be used to determine the maximum accumulation and set pressures of PZVs for single- and multiple-valve installations in accordance with Section VIII of the ASME Code. Procedure 1.

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

2.

Calculate the maximum accumulated pressure by multiplying the MAWP by the percent Maximum Accumulated Pressure from Figure 23.

3.

Calculate the relief valve set pressure by multiplying the MAWP by the percent Set Pressure from Figure 23.

4.

Calculate the relieving pressure by adding the set pressure and the percent overpressure plus 14.7.

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Single-Valve Installations

Multiple-Valve Installations

Maximum

Maximum

Set

Accumulated

Set

Accumulated

Pressure

Pressure

Pressure

Pressure

(percent)

(percent)

(percent)

(percent)

First Valve

100

110

100

116

Additional valve(s)

—

—

105

116

First valve

100

121

100

121

Additional valve(s)

—

—

105

121

Supplemental valve

—

—

110

121

Contingency Nonfire only

Fire only

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

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

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WORK AID 2:

RESOURCES USED TO CALCULATE THE SIZE OF A RELIEF VALVE-HAND CALCULATOR METHOD

This Work Aid provides the formulas and procedures to calculate the size of a pressure relief valve. These procedures assume that the basis of relief has been selected. Work Aid 2A: Formulas

Formulas and Procedures to Size PZVs for Gas and Vapor

Equation 1: Flow Rate for Thermal Contingencies gpm =

BH 500GC

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 usually 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: Q

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

F

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

Q

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

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Hvap

= latent heat of vaporization in BTU per pound

Equation 4: Effective Discharge Area for Critical Flow A=

W CKP1K b

TZ M

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)

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’s value.

P1

= upstream relieving pressure in %Overpressure) + 14.7 = P1 psia)

Kb

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

T

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

Z

= compressibility factor at inlet conditions. Use Addendum 4.

M

= molecular weight of hydrocarbon gas or vapor

psia,

((Set

Pressure

psig

x

Equation 5: Effective Discharge Area for Subcritical Flow A=

W ZT 735F2K d MP1(P1 − P2 )

A=

V ZTM 4645.2F2K d P1(P1 − P2 )

A=

V ZTG 863.63F2K d P1(P1 − P2 )

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) F2

(k −1)/k   = coefficient of subcritical flow =  k  ( r) 2/k  1− r  k −1  1− r 

k

= specific heat ratio k = Cp / Cv.

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r

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

V

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

Kd = effective coefficient of discharge = 0.975, or certified manufacturer’s value. M = molecular weight of hydrocarbon gas or vapor 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

C

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

T

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

Z

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

Equation 6: Critical Pressure Ratio Pcf  2  k/(k −1) = P1  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

Procedure for Determining Effective Discharge Area for Vaporizing Liquids 1.0

2.0

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)

Determine the total wetted surface area. 2.1

Calculate, Awet, the total wetted surface area using Page 7-19 of Addendum 1. If the 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-totangent length to 2 times the tangent-to-end length.

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3.0

4.0

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 hour using Equation 2.

3.2

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

Calculate the effective discharge area. 4.1

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

Procedure for Determining Effective Discharge Area for Subcritical Flow 1.0

Obtain the following information: •

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.

4.0

3.1

Find the Critical Flow Pressure Ratio for the gas in Table 8, page 27 of Addendum 2.

3.2

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

3.3

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

Calculate the total back pressure. 4.1

5.0

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

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 Crosby Engineering Handbook or other handbook.

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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 Discharge Area 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 orifice size 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 =

W 5 1 . 5 P1K d K N K 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. P1

= upstream relieving pressure in psia

KSH = superheat correction factor. KN = correction factor for Napier equation. Procedure 1.0

2.0

Obtain the following information: •

flow rate in lbs/hr

•

set pressure in psig

•

percent accumulation

•

superheat correction factor, KSH (KSH = 1 for saturated steam. Otherwise use Addendum 7.)

Calculate the upstream relieving pressure, P1, in psia. 2.1

Multiply the set press by the percent accumulation to obtain the allowable overpressure.

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2.2 3.0

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

Determine the correction factor for Napier equation, KN. 3.1

If P1 ≤ 1515 psia, KN = 1. If P1 > 1515 psia and ≤ 3215 psia, use Page 7-6 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 and solve for A.

5.0

Select the manufacturer’s standard orifice size that is equal to or greater than the orifice size 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 capacity certification in accordance with Section VIII, Division I, of the ASME Code. The procedure includes 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 =

BH 500GC

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

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Equation 10. API Liquid PZV Sizing Equation A=

Q G 38K dK W K V P1 − P2

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 R=

Q(2800G) µ 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 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 2.0

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

Calculate the expansion rate of the trapped fluid, gpm. 2.1

Obtain the following information: • cubical expansion coefficient per °F, B.

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• 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 3.0

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

Determine the Reynold’s number and the capacity correction factor for viscosity, Kv. 3.1

Obtain the following information: • 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 and calculate the Reynold’s number.

3.3

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

3.4

Divide the area of the manufacturer’s standard orifice sizes, A, by the viscosity correction 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 largest standard orifice size for A. If the corrected area from 3.4 is less than or equal to the 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/vapor relief. 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 from the relieving condition either to the critical downstream pressure for the flashed vapor or to the back pressure, whichever is greater.

2.0

Calculate individually the orifice area required to pass the flashed vapor component, using the 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 critical downstream pressure.

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3.0

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

4.0

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

5.0

Select a pressure relief valve that has an effective discharge area equal to or greater than the total calculated orifice area. The designer should recheck the back pressure that will exist for the specific relief valve selected, with its particular discharge installation, by examining the vapor generation downstream of the pressure relief valve nozzle. Where appropriate, corrections can be applied to the individual orifice areas previously calculated. Furthermore, selecting a balanced pressure relief valve is often desirable to minimize the effect of flashed vapor on the valve capacity.

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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’s catalog. A completed ISS and the manufacturer’s quotation and catalog are needed for this procedure. 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:

3.0

•

Valve size

•

Style

•

Pressure and temperature ratings for the connections

•

Materials of construction

•

Type of cap and lever

Compare the PZV specifications on the ISS with the valve characteristics from the manufacturer’s catalog. If the answer to any of the questions below is no and the option offered by the manufacturer is not acceptable, select a different PZV that meets the specifications on the ISS. 3.1

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

3.2

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

3.3

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

3.4

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

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WORK AID 4:

RESOURCES USED TO SIZE AND SELECT A PRESSURE RELIEF 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 the proper printer was selected during the software installation process. A completed ISS is required 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 Main Menu appear.

2.0

If you are instructed that your computer has been configured and that you can begin the sizing 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-SIZE Configuration menu will replace the Main Menu.

3.0

•

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

•

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

•

Options three through five, 'Program Options Settings,' 'Computation Unit Settings,' and 'Atmospheric Pressure Settings' are important for PZV sizing and selection.

•

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

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

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Figure 20. Company Database Administration Menu 3.1

4.0

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

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 reactive force 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 on the 'a' scale) at a distance requested by the design engineer. With 'Weights Printing' "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 Ratio Coefficient).

4.4

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

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4.5

5.0

Press [ESCAPE] to save the entries and return to the 'Program Configuration Menu'. Entries that have been changed in the 'Program Option Settings' screen are not saved permanently by the program. When the user 'quits' CROSBY-SIZE these values return to the default "OFF", positions.

Highlight 'Computation Unit Settings' and press [ENTER]. Read the 'Computation Unit Settings' section of the manual pages 17-19. 5.1

Select 'Date Format' and press [ENTER]. The program provides a selection option screen '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 manual pages 20 & 21. Observe the selection option screen, and then press [ESCAPE] to return to the 'Program Configuration Menu'. (Any accidental change will be ignored by the program.)

7.0

Crosby-Size program configuration is complete press [ESCAPE] and an inquiry screen will 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 to the Crosby-Size default values.)

8.0

Record the data from Exercise 1 into the table below. Other data have been provided in the 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

Accept These Options (Y/N/Quit) ?

Y

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Req. Cap. Other Than Fire Exp.:

0

Set Pressure: Types Of Back Pressure:

Superimposed Constant

Back Pressure: Superimposed Constant: Overpressure: Specific Heat Ratio: Molecular Weight: Temperature: Compressibility Factor: Accept These Options (Y/N/Quit) ?

Y

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 Sheet w/ 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

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GLOSSARY allowable overpressure

critical flow rate

The pressure increase over the set pressure of the relieving device, expressed in pressure units or as a percent. It is the same as accumulation when the relieving device is set at the maximum allowable working pressure of the vessel. The flow rate that corresponds to the limiting velocity.

critical flow pressure ratio The absolute pressure ratio of the pressure in the throat of a 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 used in recognized flow formulas to determine the size of the valve. 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 media at that location.

originator

An engineer who initiates a RV installation or change that requires authorization approvals by providing the design and specification information. He can be an engineer for any discipline (i.e.; Operations Engineering, Maintenance Engineer, Inspection Engineer, Projects Engineer, Plant Engineer, etc.)

relieving pressure

The total of the set pressure plus overpressure plus atmospheric pressure. For gases and vapors, the relieving pressure 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 valve is set to open under service conditions.

wetted surface

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

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ADDENDUM 1. 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

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Addendum 1: Crosby Engineering Handbook

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Addendum 2: Properties of Gases

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

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ADDENDUM 4:

Crosby Catalogs

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