Jdm1374c0002 Design Criteria for Safety Valve

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This document is of a confidential nature and is the property of JGC CORPORATION, TOKYO, JAPAN, and shall not

be traced, photographed, Photostatted or reproduced in any manner, nor used for any purpose whatsoever except by

written permission of JGC CORPORATION.

REV. DATE PAGE DESCRIPTION PREPD CHKD APPD

29-Sep.-00 ALL First Issue Y.Kubo K.Yokobayashi A.Iwasaki

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JDM - 1374 - C 0002

JGC DESIGN MANUAL

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DESIGN CRITERIA

FOR

SAFETY RELIEF

VALVES

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C O N T E N T S

PAGE

1. SCOPE 3

2. GENERAL 3

3. TYPE SELECTION 3

4. DESIGN CRITERIA FOR EACH TYPE OF INSTRUMENT 4

5. NOTES 7

6. ABBREVIATIONS 9

TABLES

1 Orifice areas and Designations (API-526) 10

2 Manufacture reference data for Orifice areas and Designations for large size pilot operated

safety relief valve

10

3 Blowdown of the pressure relief valves can be assumed as follows: 10

4 Maximum Working Pressure for Bellows 11

APPENDIX

A Sizing (8)

B Process Conditions (2)

C Definitions (5)

D Safety Relief Valve Type (2)

FLOW CHART FOR TYPE SELECTION (2)

( ) : Number of sheets

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1. SCOPE

This manual covers the design criteria for safety relief valves (*).

(*) Pressure relief valve is the proper term to represent all valve types of pressure relief devices, such as safety valve,

relief valve, safety relief valve, pilot operated safety relief valve, etc. Refer to APPENDIX-3 for the definitions of the

types. This manual covers all these valves.

2. GENERAL

2.1 Most design criteria may be specified in the General Specification or the Detailed Engineering Design Data (herein

after called GS for both) for the project. Therefore this manual is applicable for the items for which no design criteria

is covered in the GS.

Note that this practice is not applicable for package units unless the GS calls for uniformity throughout the project.

2.2 The following documents shall be referred to in the priority order shown below:

1) JOB Instruction

2) GS (Detailed Engineering Design Data or General Specification for instrumentation for the project)

3) Other Project Specifications applicable to the design basis for the instruments, such as

Material Selection Basis for Instruments / Equipment

4) This Design Criteria, JDM-1374-C-0002

5) Preparation Guidance & Sample Specifications for pressure measurement, such as:

JDM-1374-G-0001, Preparation Guidance & Sample Specifications for Safety Relief Valves

6) Standard Practice of Process Group

JDM-1370-C-0012, Safety Relief Devices

Note that the materials specified above shall be of equivalent or higher grade than those specified in the piping material

specification.

2.3 Definitions

Definitions used in this criteria and also important terms to understand the literatures about the pressure relief valves

are given in the APPENDIX-3.

3 TYPE SELECTION

3.1 General

1) Pressure relief valve types are shown on the P&ID. However, in most cases, especially in early stage of the project,

the types are selected by a process engineer without the detailed process data. Accordingly, it is important to verify

whether the type shown on P&ID are properly selected in accordance with the given process data, The GS and the

criteria described in this Section. When any discrepancies on the type selection are found, recommendations shall be

informed to the JGC instrument key person in order to reflect those on the P&ID. When a suitable type cannot be

found, the JGC instrument key person shall be consulted.

Regarding design criteria and typical specification for each type of valve mentioned in this Section 3, Section 4 shall

be referred to.

2) For slurry and especially corrosive services, rupture discs may be used. In such case, Materials except nozzle may be

relaxed for corrosiveness. Required orifice area shall be larger than that for the case without rupture discs.

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3) For type selection, refer to the attached FLOW CHART FOR TYPE SELLECTION. When the flow charts for type

selection show no suitable instrument or when the selected type is not the first choice, the JGC key person for the

project shall be consulted.

4. DESIGN CRITERIA FOR EACH TYPE OF INSTRUMENTS

4.1 General

1) Process conditions

Following process data shall be given by process engineering group. However, it is recommended to verify the

given data referring the APPENDIX-B, PROCESS CONDITIONS, as some data, especially Required

capacity, Set pressure and Accumulation effect the valve design, such as valve size and type.

In cases where the multiple-valve installations are applied, the set pressures and accumulations may different

from those for the single-valve installation. Refer to the APPENDIX-B.

a) Accumulation

In cases where the valve shall conform to the applicable standard or code, a accumulation shall be determined by

that applicable. Refer to APPENDIX-B.

b) Required capacity

Required capacities shall be determined from the various relieving cases (failure cases), such as, fire,

cooling water failure, instrument air failure, electric power interruption, thermal expansion, etc. The

maximum capacity among those cases shall normally be given as the required capacity.

Since the safety valves tend to chatter at low discharge flow (less than 25 30 % of the rated capacity), care

shall be taken for the other cases than the maximum capacity case. Refer to Section 5.1.

c) Design temperature/ Design pressure

The design temperature/ Design pressure shall be transcribed from the line index, as a rule.

In some cases, the designing temperature may be different from the design temperature for the line on which the

valve is installed. Refer to 5.5.

The design pressure is used to determine the hydrostatic test pressure of the valve.

d) Operating pressure and set pressure

The design pressure (*1) shall normally be used as the set pressure of the valve for single-valve installation.

The differential between the operating pressure and the set pressure is the important factor to select the valve type

(i.e. safety relief valve, safety valve, relief valve and pilot operated valve). Namely, this differential pressure (*2)

shall be smaller than the blowdown pressure, which varys depending on the valves type and their manufacturer.

Accordingly, it shall be confirmed with the process engineer that all design cases including start-up, emergency

conditions, SOR (start of run), EOR (end of run), etc. are covered.

(*1) ASME Sect.VIII refers to the maximum allowable working pressure (MAWP) of the vessel to be protected,

which is normally lower than the design pressure, as the set pressure. However, it is recommended to used the

design pressure as MAWP since the valve shall protect the system which contains not only vessel but also

other piping system of which the maximum pressure is normally not available.

The APPENDIX-B shall be also referred to.

(*2) Refer to the APPENDIX-B for the differential pressure.

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f) Relieving temperature

The valve inlet temperature when the inlet pressure reaches relieving pressure (=set pressure + accumulation)

shall be indicated.

g) Viscosity (CP)

Where the valve is used in viscous service (higher than 10 cP), viscosity at relieving condition shall be indicated

because viscosity-compensation is needed. Note that Reynolds number at orifice is higher than 100,000, vsicosity

correction is required, i.e. required orifice area become larger.

h) Back pressure

The maximum variable and the constant back pressure shall be specified. The maximum variable is the sum of the

variable superimposed back pressure plus the built-up back pressure. The constant back pressure is the constant the

superimposed back pressure.

The superimposed back pressure is the static back pressure that exists at the outlet of a pressure relief device at the

time the device is required to operate. It is result of pressure in the discharge system coming from other sources and

may be variable or constant. The built-up back pressure is the increase in pressure in the discharge header that

develops as a result of flow after the pressure relief devices or devices open. And the built-up back pressure is

caused by the particular device (the valve concerned) or others, if any, which simultaneously discharge into the

disposal system (the discharge header). Namely total back pressure when the valve concerned opens is the

superimposed and the built-up back pressure.

Where the constant are expected, those pressures shall be used to determine the spring set pressure, i.e. the spring

set pressure is the set pressure minus constant back pressure.

i) Failure case

This is used to check the accumulation.

j) Discharge to

The process on the outlet side of the valve shall be indicated. This is used to check back pressure.

2) Valve sizing and size selection

A valve shall be sized in the following steps:

a) Calculation of the required effective area

The required effective area shall be calculated using the process data in 1) in accordance with the applicable

standard and code for each item. In cases where no applicable standard and code for the valve, the API 520

formulas shall be applied. The sizing formulas are shown in the APPENDIX-A. Note that the formulas in

APPENDIX-A are basically for the cases without rupture disc. In case where the pressure relief valves are used is

used in series with rupture disc, use different Kd.

b) Size selection

Valve size shall be selected so that the effective orifice area is equal to or greater than the required orifice area.

The effective orifice areas shall be referred to manufacturers data. Typical effective orifice areas are shown in

Table-1.

In cases where the maximum area of the conventional type (*) is not enough for the required area, the process

engineer shall be informed and consulted. There are following two options, which effect the process design, such

as the set pressure, accumulation.

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- Application of pilot operated valve, which has a larger capacity than a conventional type.

(Refer to Table-2 for the available orifice areas)

- Multiple-valve installation

(*) According to API 526.

Up to 20BB224 (orifice area of 1,194 cm2) is available, but only from the limited manufacturer and only for

the limited rating. Accordingly, T shall be considered as the maximum orifice area.

3) Material

The materials for all wetted parts shall be specified in accordance with relevant specifications mentioned in

paragraph 2.2. Materials for services not specified in the relevant specifications shall be consulted with JGC key

person for the project.

The materials underlined below are generally used unless otherwise specified by the relevant specifications.

Body : CS, 304SS, 316SS

Nozzle & Disk : 304 SS, 316 SS

Spindle : Manufacturers standard

Spring : Manufacturers standard

Bellows : 316LSS

In cases where rupture disc is installed in series with the valve, consult with JGC about relaxation of resistant for

fluid corrosivity.

4) Connection rating

The rating and connection type shall be specified in accordance with the piping material specification.

In case when flange material is different from piping material, it is shall be confirmed that PT rating of the flange

material is equal or higher than that of piping material at design pressure and all temperature range up to design

temperature. This is because even higher-grade materials (e.g. 316SS) may have lower PT rating than that of

piping materials (e.g. CS) for the class at certain temperature and pressure. Refer to ANSI B16.5. In case the PT

of the selected material is lower than the PT of the piping material, the higher rating of the flange shall be

applied. Those item(s) and changed connection rating shall be informed to JGC key person and relevant groups,

such as process engineering group and piping design group.

5) disk and seat

a) Solid particles between seat and disc

Use a special knife-edge seat arrangement, or a resilient O-ring seat.

b) Nature of process fluid

For light, hard-to-hold fluid such as hydrogen will leak through metal-to-metal seats, use a resilient O-ring seat.

O-ring seats are advisable for corrosive or toxic service too. Specify greater tightness when ordering the valve.

4.2 Conventional Type

1) Construction

a) Bonnet

Closed type shall be selected for the services where contain a fluid which shall not be released to atmosphere, such

as flammable, toxic, corrosive fluid.

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Open type, in general, is used for air and steam service.

A Lifting lever is to open the valve manually. The necessity of lifting levers shall be specified in accordance with

the applicable standard/regulation, P&ID and GS. In general, those are provided for air and steam service. In case

where ASME is applicable for the services, lifting levers shall be specified for air and steam services.

c) Cap

Cap is to protect the set screw. The provision of caps shall be specified in accordance with GS. As a rule, caps

shall be provided.

d) Test gag

Test gag (rod) is device which is installed on top of bonnet and avoids lifting up of disk locking spindle even

during overpressure, such as during hydrostatic test. The provision of caps shall be specified in accordance with

GS. As a rule, caps shall be provided.

6) Form 839 shall be used to specify safety relief valves.

4.3 Pilot operated safety relief valves

1) For dirty service, it is recommended to provide with filter in pilot supply line.


4.4 Thermal relief valves

1) Reference data for control valves, such as rating, maximum working temperature, rangeability and seat leakage from

some manufacturers catalogues are shown in Table-1.


5. NOTES

The following items are mostly the nature of basic engineering for which the process engineer shall be responsible.

However, these items have been experienced in the past projects. Accordingly, the instrument engineer shall also pay

attentions on these items in order to avoid serious mistakes in the project.

1) Oversizing

If the flow rate is less than 25 - 30 o/o of the rated capacity of the valve, the valve may tend to chatter due to lack of

lifting force. Multiple-valve installation may be considered.

2) Too small pressure differential

If the pressure differential between the set pressure and the operating pressure is too small normal leakage flow will

be sufficient to pop the valve open prematurely. Refer to the term, simmer in the APPENDIX-B.

3) Blowdown pressure

A blowdown pressure shall be higher than the operating pressure. Otherwise, the valves will never reclose after the

opening. Especially for liquid service, a short-term pressure rise (surging) due to quick process valve operation may

open the relief valve. Since the relief valve has normally a large blow down, care shall be taken to determine the set

pressure. Typical blowdown pressures are shown in Table-3.

3) Thermal relief valve installation

In cases where thermal relief valve discharges are connected serially to the upstream block (see below), the followings

shall be taken in account of:

a) Thermal relief valve discharge

In off-site where intermittent operation is normal operation mode, in order to avoid product losses and

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environmental hazards, thermal relief valve discharges are normally introduced to the closed system. (the most

case, to the next upstream block).

b) Set pressure

The back pressure of the RV3 is a sum of the set pressures of the valves in the system (in the case below, RV1 and

RV2). Accordingly, the sum of all set pressure (in series) of those valves shall be the design pressure minus the

maximum tank head. In the other hand, if the set pressure of RV5 is lower than the maximum tank head (TA), the

unloading liquid may leak to TB.

5) Design temperature for relief valves

For the services where the liquids have a higher vapor pressure than atmosphere at ambient temperature and are

discharged to atmospheric pressure system, the valve temperatures will decrease to saturate temperature of the liquid

at atmosphere due to adiabatic expansion. Design temperature shall be that temperature (=design temperature of

discharge line) and the design pressure shall be the design pressure of valve inlet line. Accordingly materials selection

shall be also based on that temperature. For example, the relief vales on ambient temperature propane line shall be

design for 45.

R VR V R V

TA

TB

1

2 3RV4

5RV

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6. ABBREVIATIONS

Metric English

A = required effective discharge area of the valve cm2 in2

C = coefficient determined from "k" at standard conditions - -

F2 = coefficient of subcritical flow - -

G = liquid specific gravity at the flowing condition - -

Kb = capacity correction factor due to back pressure - -

Kd (K) = effective coefficient of discharge - -

Kn = correction factor for Napier equation - -

Kp = correction factor due to overpressure - -

KSH = superheat steam correction factor - -

KV = correction factor due to viscosity - -

KW = correction factor due to back pressure - -

k = ratio of specific heats Cp/Cv - -

M = molecular weight of the gas or vapor - -

Pl = upstream relieving pressure

= Ps x (1+accumulation) + atmospheric pressure

For non-certified valves for liquid service, use 1.25 Ps

kg/cm2A psia

P2 = back pressure kg/cm2A psia

Pcf = critical flow throat pressure kg/cm2A psia

ps = set pressure kg/cm2G psig

pl = upstream relieving pressure kg/cm2G psig

p2 = total back pressure kg/cm2G psig

Pb = total back pressure kg/cm2G psig

Q = required flow rate through the valve m3/h SCFM

r = pressure ratio, P2 / Pl - -

T = relieving temperature of the inlet gas or vapor oK oR

V = required flow rate through the valve NM3/h SCFM

W = required flow rate through the valve kg/h Lb/h

Z = compressibility factor at inlet conditions - -

= liquid viscosity at flowing condition cP cP

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Table-1 Orifice areas and Designations (API-526)

Orifice

Designation

Orifice Area in

cm2 (in2)

Nozzle Sizes

(Inlet-Orifice-Outlet)

D 0.710 (0.110) 1D1 11/2D2, 11/2D2

1/2

E 1.265 (0.196) E2 11/2E2, 11/2E2

1/2

F 1.981 (0.307) 11/2F2 11/2F2

1/2 -

G 3.245 (0.503) 2G3 11/2G21/2 -

H 5.065 (0.785) 2H3 11/2H3 -

J 8.303 (1.287) 2J3 3J4, 21/2J4

K 11.858 (1.838) 3K4 3K6 -

L 18.406 (2.853) 3L4 4L6 -

M 23.266 (3.60) 4M6 - -

N 28.000 (4.34) 4N6 - -

P 41.161 (6.38) 4P6 - -

Q 71.290 (11.05) 6Q8 - -

R 103.226 (16.0) 6R8 6R10 -

T 167.742 (26.0) 8T10 8T10 -

Table-2 Manufacture reference data for Orifice areas and Designations

for large size pilot operated safety relief valve

Orifice

Designation

Orifice Area in

cm2 (in2)

Nozzle Sizes

(Inlet-Orifice-Outlet)

V 285.02 (44.18) 10V14

W 410.43 (63.62) 12W16

Y 558.64 (86.59) 14Y18

Z 615.31 (95.38) 16Z20

Z2 729.65 (113.1) 16Z220

A 923.47 (143.1) 18A24

B 1140.0(176.7) 20B24

B2 1464.3 (227.0) 20B224

Table 3 Blowdown of the pressure relief valves can be assumed as follows:

Valve type Blowdown

Relief valves 20 % of set pressure

Safety valves 5 % of set pressure with adjusting ring, 7 % without adjusting ring

Safety relief valves 5 % of set pressure with adjusting ring, 7 % without adjusting ring

Pilot-operated relief valves 3 % of set pressure

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Table-4 Maximum Working Pressure for Bellows

Orifice Designation Max. Work. Press.

(Kg/cm2G)

D - J 15

K 10 to 15

L 7 to 12

M P 5 to 10

Q 4 to 8

R 4 to 7

T 2 to 4

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APPENDIX-A CALCULATION FORMULAS FOR THE REQUIRED EFFECTIVE ORIFICE AREA

This APPENDIX shows mainly the formulas for the cases without rupture disc. In case where the pressure relief

valves are used is used in series with rupture disc, use different Kd. Details shall be consult with JGC.

Abbreviations shall be referred to paragraph 6.

1. ASTM Sect. I (Power Boiler)

A = S19.045.51 1 shKKP

W For steam

Where: P1 = (1.03x ps) + Atmospheric pressure

2. ASTM Sect. VIII (Pressure Vessel)

A = S1KP

W

5.51For Steam

A = S2M

TZ

CKP

W For Gas or Vapor

Where: P = (1.10x ps) + Atmospheric pressure

3. API RP 520

All formulas are for the cases without rupture disk.

For Steam

A = S1shnbd KKKKP

W

15.51

For Gas or Vapor (Critical flow*)

A = S2M

TZ

KPCK

W

bd 1

For Gas or Vapor (Sub-critical flow*)

A = S2)(735 2112 PPMP

ZT

KF

W

d

For Liquid for certified valves

A = S32138 PP

G

KKK

W

VWd

For Two-phases and flashing flow Note below

Where: P1 = Ps x (1+accumulation) + atmospheric pressure

For non-certified valves for liquid service, use 1.25 Ps

S1= Conversion factor, 1.0 for the English unit, and 1.0 for the metric unit



(*) If the pressure downstream of the throat (practically, use P2) is less than or equal to the critical

pressure, Pcf, then critical flow occur, and the formula for the critical flow shall be applied. The

critical gas flow pressure ratio should be estimated using the following ideal gas relationship.)1/(

1 1

2

kkcf

kP

P

Refer to the API 520 table-7 for typical pressure ratio, Pcf/P1. When Pcf is unknown, 0.5P1 shall

be used.

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Note : API 620 Appendix-D shows the calculation method. However, it should be aware that there are

currently no pressure relief devices with certified capacities for two-phases flow since there are

no testing methods for certification. It is recommended to have a manufacturer size. For

preliminary sizing, add the required an area for a liquid and an area for a gas and/or flashed

vapor. The flow rates (liquid, and gas and/or vapor) to be used for the calculations shall be

those at valve outlet (i.e. after flashing). For saturated water, refer to ASME Sect.VIII,

Appendix 11.

To determine the saturated water capacity of a valve rated under UG-131 (certification of

capacity of pressure relief valve), above, refer to Fig. Fig.-A1.

4. Coefficient of discharge, K

The coefficients shall be referred to manufacturers data. For preliminary calculation (for inquiry to bidder), use

the following reference data from manufacturers catalogue:

Table-A1

Non-certified Motoyama Fukui

ASME Sect. I (*1) : - 0.905 0.975

ASME Sect. VIII (*1) : - 0.862 0.869, 0.878

API 520 (Gas/Vapor) : 0.975 - -

API 520 (Liquid ) : 0.62 - -

(*1) Certified

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Fig-A1 (API 520, Fig 32) Curve for Evaluating Coefficient C in the Flow Equation

From the Specific Heat Ratio, Assuming Ideal Gas Behavior

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Fig-A2 (API 520, Fig 35) Constant Back Pressure Sizing Factor, Kb, for Conventional Safety Relief Valves

Fig-A3 (API 520, Fig 30) Constant Back Pressure Sizing Factor, Kb, for Balanced-Bellows Pressure Relief Valves

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Table-A2 (API 520, Table 7) Property of Gases

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Table-A3 (API 520, Table 9) Superheat Steam Correction Factor, KSH

Fig-A4 (API 520, Fig 34) Values of F2 for Subcritical Flow

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Fig-A5 (API 520, Fig 31) Capacity Correction factor, KW, due to Back Pressure

on Balanced-Bellows Pressure Relief Valves in Liquid Service

Fig-A6 (API 520, Fig 37) Capacity Correction factor, KP, due to Over Pressure

For Relief and Safety Relief Valves in Liquid Service

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Fig-A7 (API 520, Fig 36) Capacity Correction factor, KV, due to Viscosity

Fig-A7 (API 520, Fig 36) Capacity Correction factor, KV, due to Viscosity

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APPENDIX-B PROCESS CONDITIONS

1. Differential pressure between the set pressure and the operation pressure

1) The minimum pressure differentials, between the set pressure of the valve and the operation pressure of the vessel are

recommended by ASME Sect. VIII as shown below:

Table-B1 Recommended minimum pressure differentials (ASME VIII, Appendix MM-11)

Set Pressure (ps) Recomd minimum

pressure differential

Set pressure tolerance

(UG-126(d),134(d)(1))

Differential to the leak test

pressure

ps 70 psi (4.9 Kg/cm2) 5 psi (0.35 Kg/cm2) 2 psi (0.14 Kg/cm2) 10 % of ps or 5 psi (0.35

Kg/cm2), whichever larger

ps 1000 psi (70 Kg/cm2) 10 % of ps 3 % of ps 10 % of ps

ps > 1000 psi (70 Kg/cm2) 7 % of ps 3 % of ps 5 % of ps

2) To minimize leakage, the pressure differentials listed above Table-XX can be increased for metal-seated valves if the

fluid is toxic, corrosive, cryogenic, or exceptionally valuable; or if the system pressure fluctuates, as in the discharge

line of reciprocating pumps and compressors.

2. Set pressure and Accumulation

1) ASME Sect. I

3 % of set pressure for steam

2) ASME Sect. VIII and API 520 (sect. 3.5)

Table-B2 Summary of the set pressure and accumulation (API 520 3.5)

Single valve installations Multiple valve installations

Set pressure

(%)

Max. accum.

Press. (%)

Set pressure

(%)

Max. accum.

Press. (%)

Non-fire case

First valve 100 110 (*1) 100 116 (*2)

Additional valve(s) 105 116 (*2)

Fire only 100 121

First valve 100 121

Additional valve(s) 105 121

Supplemental valve (*3) 110 121

ASME Sect. VIII Div. 1 UG-134 UG-125 UG-134 UG-125

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

(* 1) 10 % or 3 psi (0.21 kg/cm2), whichever is greater.

(*2) 16 % or 4 psi (0.28 kg/cm2), whichever is greater.

(*3) A supplemental-valve installation provides relieving capacity for an additional hazard created by exposure to

fire or other unexpected sources of external heat. Supplemental valves are used only in addition to valves sized

for operating (non-fire) contingencies.

3) In the case of ASME-application liquid service valves (that is, for protection of a liquid-full vessel), maximum

accumulated pressure is limited to 110 % of the maximum allowable working pressure for operating contingencies.

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In the case of non-ASME-application liquid service valves (that is, for protection of piping without vessels included),

25 % overpressure is generally specified in accordance with Japanese code, High Pressure Gas Control Law (HPGCL).

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APPENDIX-C DIFINITIONS

5.1 Pressure Relief Devices

Pressure relief device is actuated by inlet static pressure and designed to open during an emergency or abnormal

conditions to prevent a rise of internal fluid pressure in excess of a specified value. The device also may be designed to

prevent excessive internal vacuum. A brief description of the several types of pressure relief devices is given in

Appendix-A .

5.2 Types of devices

1) Pressure Relief Valve is a spring loaded pressure relief device which is designed to open to relieve excess pressure

and to reclose and prevent the further flow of fluid after normal conditions have been restored. It is characterized by

rapid opening pop action or by opening generally proportional to the increase in pressure over the opening pressure. It

may be used for either compressible or incompressible fluids, depending on design, adjustment, or application.

2) Safety Valve is a pressure relief valve actuated by inlet static pressure and characterized by rapid opening or pop

action. (It is normally used for steam and air services.)

(1) Low-Lift Safety Valve is a safety valve in which the disc lifts automatically such that the actual discharge

area is determined by the position of the disc.

(2) Full-Lift Safety Valve is a safety valve in which the disc lifts automatically such that the actual discharge area is

not determined by the position of the disc.

3) Relief Valve is a pressure relief device actuated by inlet static pressure having a gradual lift generally proportional to

the increase in pressure over opening pressure. It may be provided with an enclosed spring housing suitable for closed

discharge system application and is primarily used for liquid service.

4) Safety Relief Valve is a pressure relief valve characterized by rapid opening or pop action, or by opening in

proportion to the increase in pressure over the opening pressure, de-pending on the application and may be used either

for liquid or compressible fluid.

(1) Conventional Safety Relief Valve is a pressure relief valve which has its spring housing vented to the discharge

side of the valve. The operational characteristics (opening pressure, closing pressure, and relieving capacity) are

directly affected by changes of the back pressure on the valve.

(2) Balanced Safety Relief Valve is a pressure relief valve which incorporates means of minimizing the effect of

back pressure on the operational characteristics (opening pressure , closing pressure, and relieving capacity).

5) Pilot-Operated Pressure Relief Valve is a pressure relief valve in which the major relieving device is combined with

and is controlled by a self-actuated auxiliary pressure relief valve .

6) Power-Actuated Pressure Relief Valve is a pressure relief valve in which the major relieving device is combined with

and controlled by a device requiring an external source of energy.

7) Vacuum Relief Valve is a pressure relief device designed to admit fluid to prevent an excessive internal vacuum; it is

designed to reclose and prevent further flow of fluid after normal conditions have been restored.

5.3 Dimensional Characteristics of Pressure Relief Devices

Actual discharge area is the measured minimum net area that determines the flow through the valve.

Curtain area is the area of the cylindrical or conical discharge opening between the seating surfaces above the nozzle

seat created by the lift of the disc.

Effective discharge area, or equivalent flow area, is a nominal or computed area of a pressure relief valve used in

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recognized flow formulas to determine the size of the valve. It will be less than the actual discharge area.

Nozzle area is the cross-sectional flow area of a nozzle at the minimum diameter.

Huddling chamber is an annular pressure chamber located beyond the pressure relief valve seat to open the valve

rapidly.

Inlet size is the nominal pipe size of the valve at the inlet connection, unless otherwise designated.

Outlet size is the nominal pipe size of the valve at the discharge connection, unless otherwise designated.

Lift is the actual travel of the disc from the closed position to the open position when a valve is relieving.

5.3 Operational Characteristics

For reference, pressure level Relationships for Safety Relief valves are attached in the Appendix. Refer to Fig.-C1.

1) System Pressure

Maximum operating pressure is the expected maximum pressure during operation of the system upstream of the

pressure relief valve.

Maximum allowable working pressure (MAWP) is the maximum gauge pressure permissible at the top of a

completed vessel in its operating position for a designated temperature. The pressure is based on calculations for

each element in a vessel using nominal thickness, exclusive of additional metal thickness allowed for corrosion

and loading other than pressure. The maximum allowable working pressure is the basis for the pressure setting of

the pressure relief devices that protect the vessel.

Design gauge pressure refers to at least the most severe conditions of coincident temperature and pressure

expected during operation. This pressure may be used in place of the maximum allowable working pressure in

all cases where the MAWP has not been established. The design pressure is equal to or less than the MAWP.

Accumulation is the pressure increase over the maximum allowable working pressure of the vessel during

discharge through the pressure relief device, expressed in pressure units or as a percent. Maximum allowable

accumulations are established by applicable codes for operating and fire contingencies.

Overpressure is the pressure increase over the set pressure of the pressure relief 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.

Rated relieving capacity is the portion of the measured relieving capacity permitted by the applicable code

regulation to be used as a basis for the application of a pressure relief device.

Stamped capacity is the rated relieving capacity that appears on the device nameplate. The stamped capacity is

based on the set pressure plus the allowable overpressure for compressible fluids and the differential pressure for

incompressible fluids

The stamped capacity shall not exceed 90 %of the average capacity of the valves tested. [ASME VIII Div, l

UG-131 (d)(1)]

2) Device Pressure

Set pressure is the inlet gauge pressure at which the pressure relief device is set to open under service conditions.

Pressure differential is the difference between the set pressure of the pressure relief device and the operating

pressure of the protected vessel.

Cold differential test pressure is the pressure at which the pressure relief valve is adjusted to open on the test stand.

The cold differential test pressure includes corrections for the service conditions of back pressure or temperature

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or both.

Back pressure is the pressure that exists at the outlet of a pressure relief devices as a result of the pressure in the

discharge system. It is the sum of the superimposed and built-up back pressure.

Superimposed back pressure is the static back pressure that exists at the outlet of a pressure relief device at the

time the device is required to operate It is result of pressure in the discharge system coming from other sources

and may be constant or variable.

Built-up back-pressure is the built-up back pressure is the increase in pressure in the discharge header that

develops as a result of flow after the pressure relief device or devices open. The built-up back pressure is caused

by flow from the particular device and others, if any, which simultaneously discharge into the disposal system.

This type of back pressure is variable.

The built-up back pressure shall be less than the allowable back pressure, 10 % of the set pressure for

conventional type or 50 % of the set pressure for balanced-bellows type.

Constant Back pressure is the static back pressure that exists under normal operation where no relief device is

operated; i.e. constant back pressure of the superimposed back pressure. The constant back pressure is used to

determine the spring set pressure of conventional type pressure relief valve as a difference between the set

pressure and the constant back pressure. Therefore, the opening pressure will vary depend on the built-up back

pressure.

The opening pressure of balanced-bellows type is the set pressure of the valve and is independent of any back

pressure.

Blowdown is the difference between the set pressure and the closing pressure of a pressure relief valve, expressed

as a percent of the set pressure or in pressure units.

Opening pressure is the value of increasing inlet static pressure at which there is a measurable lift of the disc or at

which discharge of the fluid becomes continuous.

Closing pressure is the value of decreasing inlet static pressure at which the valve disc reestablishes contact with

the seat or at which lift becomes zero.

Simmer is the audible or visible escape of compressible fluid between the seat and disc at an inlet static pressure

below the set pressure and at no measurable capacity.

Leak-test pressure is the specified inlet static pressure at which a seat leak test is performed.

Relieving conditions is the term used to indicate the inlet pressure and temperature on a pressure relief device at a

specific overpressure. The relieving pressure is equal to the valve set pressure plus the overpressure.

The temperature of the flowing fluid at relieving conditions may be higher or lower than the operating

temperature

3) Abnormal Conditions

Chatter refers to the motion that causes the disc to contact the seat and damage the valve and associated piping-

Chattering may result in lowered capacity and damage to the seating surfaces.

Flutter refers to the abnormally rapid reciprocating motion of the movable parts of a pressure relief valve in which

the disc does not contact the seat.

5.5 Disposal Systems

Atmospheric discharge is the release of vapors and gases from pressure relief or depressuring devices to the

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atmosphere.

Flare system is a means for safe disposal of waste gasses by closed pipeline and combustion system. With an

elevated flare the combustion is carried out at the top of a pipe or stack where the burner and igniter are located.

A ground flare is similarly equipped except that combustion is carried out at or near ground level.

A burn pit differs from a flare in that it is normally designed to handle both liquids and vapors.

Vent stack is the elevated vertical termination of a disposal system which discharges vapors into the atmosphere

without combustion or chemical conversion of the relieved fluid.

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Fig.-C1 Pressure level Relationships for Safety Relief valves

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APPENDIXD TYPE OF PRESSRE RELIEF VALVES

Type General Advantages Disadvantages Application

Relief valvesSee Fig Al-l

Plain disc forsimplest form.Flow deflectoravailable forhighspecific flow.

Available in smallsizes.Low cost.Reseat after relieving.Suitable for toxicmaterials.

Not good for polymers.Limited to about 150 kg/cm2

Inlet.Not available with bellows.Not suitable for service onsteam boilers.

Pump discharge.Thermal relief of lines andheat exchangers.Water heaters.

Safety valvesSee Fig Al-2

Normally openbonnet to protectthe spring fromexcessive heat.Bonnet is notfluid tight.

Open bonnet isolatesspring from processtemperature.Reseat after relieving.

For steam service only.Not available with balancedbellows; however does notnormally discharge intoclosed systems.

ASME power boilers andgeneral steam services.

Conventionalsafety reliefvalvesSee Fig Al-3

Bonnet is fluidtight and isvented toValve outlet.

Fluid tight.Available with soft-sealingMaterials.

Back pressure more than 10% of set pressure affectsvalve set pressure andperformance.

General use for processindustry ASME unfiredpressure vessels,particularly clean high-pressure gas service.

Balancedsafetyrelief valvesSeeFig Al-4

Bonnet is fluidtight. Bonnetarea and bellowsarea are ventedto atmosphere.Two principaltypes:piston, bellows.

Fluid tight.Can be used for highback pressure(up to 50 % of setpressure).Available withunbalanced bellowsfor corrosionisolation.Available with soft-sealing materials.

Bonnet vent to be piped tothe safe location.

General use for processindustry ASME unfiredpressure vessels,particularly clean low-pressure gas service.

Pilot-operatedsafety reliefvalvesSee Fig Al-5

Consists of twovalves.Pilot valvecontrolsthe main valve.Two principaltypes:piston,diaphragm.

Can withstand highinlet pressure.Can be designed torelieve nearoperating pressure (upto 90% ofset pressure).Can be set atblowdown as short as2%Can be remotelyoperated formanual depressuring.

Not suitable for temperaturesabove 180.Not recommended for dirtyservices, slurries, polymers.

ASMIE unfired pressurevessels, particularly cleanhigh-pressure gas serviceGas pipeline andcompressors.

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FLOW CHART FOR TYPE SELECTION

C onventional, standard bonnet

Therm alrelief

C onventionalstandardbonnet

M ulti-installationconventional

standard bonnet

Pilot operat.standardbonnet

C onventional,bellows

Therm al relief

Safety relief valve

Back press.10%

Blow dow n5%

C orrosive

Largecapacity

Low setpress.

Blow dow n5%

C onventional,bellow sM ulti-installation

*1

*2

*5

*3

*4

Low setpress.

Y

N

Y

N

*2

*6

*7

Y

N

O rificeT

Pilot valveaccepted

Pilot valveaccepted

*5

*4

*6

*7

Pilot operat.bellow s

Y

N Y

N Y

N

Y

Y

Y

Y

Y

N

N

N

N

N

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Attachment A

*1 Whether the purpose for the valve is thermal relief.

*2 Whether process fluid is such corrosive that the spring, bonnet and guiding surface shall avoid to be exposed by the

process fluid.

*3 Whether the build-up back pressure of the valve (with 10 % allowable overpressure) exceeds 10 % of the set pressure

(for safety sake, consider the set pressure means the spring set pressure). For the valves having the higher allowable

overpressure than 10 %, the build-up back pressure can be allowed up to the maximum allowable overpressure. This is

because the higher built-up back pressure mat cause chatter or flatter. Refer to the API 52, paragraph 3.3.3.1.3.

*4 Whether the set pressure is higher than 10 %. Most conventional safety relief valves have the set pressure lower than 1.0

Kg/cm2.

*5 Whether the allowable blowdown pressure is lower than 5 %. The 5 % of blow down is the minimum blowdown

available by the conventional type. Refer to Table 3.

*6

*7

Whether the required orifice area is larger than T (size designation). Refer to paragraph 4.1.2).

Whether a use of the pilot type pressure relief valve for mild services is accepted by client.

Documents

Jdm1374c0002 Design Criteria for Safety Valve