Pressure relief valve
Engineering handbook
Anderson Greenwood, Crosby and Varec products | Technical publication No. TP-V300

Pressure relief valve engineering handbook
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Copyright © 2012 Emerson. All rights reserved. No part of this publication may be reproduced or distributedin any form or by any means, or stored in a database or retrieval system, without written permission. Emerson
provides the information herein in good faith but makes no representation as to its comprehensiveness or
accuracy. Individuals using this information in this publication must exercise their independent judgment inevaluating product selection and determining product appropriateness for their particular purpose and system
requirements. Emerson makes no representations or warranties, either express or implied, including without
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set forth herein or the product(s) to which the information refers. Accordingly, Emerson will not be responsiblefor damages (of any kind or nature, including incidental, direct, indirect, or consequential damages) resulting
from the use of or reliance upon this information. Emerson reserves the right to change product designs andspecifications without notice. All registered trademarks are the property of their respective owners. Printed in
the USA.

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Pressure relief valve engineering handbook
Contents
Table of Contents
Chapter 1 - Introduction������������������������������������������������������������������������������������������������������������������������1.1
Chapter 2 - Terminology�����������������������������������������������������������������������������������������������������������������������2.1
	I.	General���������������������������������������������������������������������������������������������������������������������������������������2.1
	II. Types of Devices������������������������������������������������������������������������������������������������������������������������2.1
	III.	Parts of Pressure Relief Devices���������������������������������������������������������������������������������������������2.1
	IV.	Dimensional Characteristics - Pressure Relief Valves����������������������������������������������������������2.2
V.	Operational Characteristics - Pressure Relief Devices����������������������������������������������������������2.3
VI. System Characteristics�������������������������������������������������������������������������������������������������������������2.4
Chapter 3 - Codes and Standards��������������������������������������������������������������������������������������������������������3.1
	I.	Introduction�������������������������������������������������������������������������������������������������������������������������������3.3
	II.	American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code������3.3
	III.	International Organization for Standardization (ISO)������������������������������������������������������������3.18
	IV.	European Union Directives�����������������������������������������������������������������������������������������������������3.22
V.	American Petroleum Institute (API)���������������������������������������������������������������������������������������3.23
VI.	National Fire Protection Agency (NFPA)�������������������������������������������������������������������������������3.26
VII.	National Board of Boiler and Pressure Vessel Inspectors��������������������������������������������������3.27
Chapter 4 - Design Fundamentals�������������������������������������������������������������������������������������������������������4.1
	I.	Introduction�������������������������������������������������������������������������������������������������������������������������������4.3
	II.	Direct Spring Operated Pressure Relief Valves����������������������������������������������������������������������4.3
	III.	Pilot Operated Pressure Relief Valves�����������������������������������������������������������������������������������4.15
	IV. 	Advantages and Limitations of Valve Types���������������������������������������������������������������������������4.27
Chapter 5 - Valve Sizing and Selection (USCS Units)��������������������������������������������������������������������������5.1
	I.	Introduction�������������������������������������������������������������������������������������������������������������������������������5.3
	II.	Gas/Vapor Sizing - Sonic Flow�������������������������������������������������������������������������������������������������5.4
	III.	Gas/Vapor Sizing - Subsonic Flow�������������������������������������������������������������������������������������������5.5
	IV. Steam Sizing������������������������������������������������������������������������������������������������������������������������������5.5
V.	Liquid Sizing����������������������������������������������������������������������������������������������������������������������������5.11
VI.	Fire Sizing��������������������������������������������������������������������������������������������������������������������������������5.11
VII. Two-Phase Flow Sizing�����������������������������������������������������������������������������������������������������������5.18
VIII.	Noise Level Calculations��������������������������������������������������������������������������������������������������������5.25
	IX.	Reaction Forces����������������������������������������������������������������������������������������������������������������������5.26
Chapter 6 - Valve Sizing and Selection (Metric Units)�������������������������������������������������������������������������6.1
	I.	Introduction�������������������������������������������������������������������������������������������������������������������������������6.3
	II.	Gas/Vapor Sizing - Sonic Flow�������������������������������������������������������������������������������������������������6.4
	III.	Gas/Vapor Sizing - Subsonic Flow�������������������������������������������������������������������������������������������6.5
	IV. Steam Sizing������������������������������������������������������������������������������������������������������������������������������6.6
V.	Liquid Sizing����������������������������������������������������������������������������������������������������������������������������6.11
VI.	Fire Sizing��������������������������������������������������������������������������������������������������������������������������������6.13
VII. Two-Phase Flow Sizing�����������������������������������������������������������������������������������������������������������6.15
VIII.	Noise Level Calculations��������������������������������������������������������������������������������������������������������6.23
	IX.	Reaction Forces����������������������������������������������������������������������������������������������������������������������6.24

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Pressure relief valve engineering handbook
Contents
Table of Contents (continued)
Chapter 7 - Engineering Support Information (USCS Units)��������������������������������������������������������������7.1
	I.	Compressibility Factor��������������������������������������������������������������������������������������������������������������7.3
	II.	Capacity Correction Factor for Back Pressure�����������������������������������������������������������������������7.4
	III.	Capacity Correction Factor for High Pressure Steam����������������������������������������������������������7.31
	IV.	Capacity Correction Factor for Viscosity�������������������������������������������������������������������������������7.31
V.	Capacity Correction Factor for Superheat����������������������������������������������������������������������������7.33
VI.	Ratio of Specific Heats and Coefficient C������������������������������������������������������������������������������7.35
VII. Typical Fluid Properties����������������������������������������������������������������������������������������������������������7.36
VIII. Saturated Steam Pressure Table�������������������������������������������������������������������������������������������7.40
	IX.	Orifice Area and Coefficient of Discharge for Anderson Greenwood and Crosby
Pressure Relief Valves������������������������������������������������������������������������������������������������������������7.41
X.	Equivalents and Conversion Factors�������������������������������������������������������������������������������������7.48
XI.	Capacity Correction Factor for Rupture Disc/Pressure Relief Valve Combination������������7.54
Chapter 8 - Engineering Support Information (Metric Units)�������������������������������������������������������������8.1
	I.	Compressibility Factor��������������������������������������������������������������������������������������������������������������8.3
	II.	Capacity Correction Factor for Back Pressure�����������������������������������������������������������������������8.4
	III.	Capacity Correction Factor for High Pressure Steam����������������������������������������������������������8.31
	IV.	Capacity Correction Factor for Viscosity�������������������������������������������������������������������������������8.31
V.	Capacity Correction Factor for Superheat����������������������������������������������������������������������������8.33
VI.	Ratio of Specific Heats and Coefficient C������������������������������������������������������������������������������8.35
VII. Typical Fluid Properties����������������������������������������������������������������������������������������������������������8.36
VIII. Saturated Steam Pressure Table�������������������������������������������������������������������������������������������8.40
	IX.	Orifice Area and Coefficient of Discharge for Anderson Greenwood and Crosby
Pressure Relief Valves������������������������������������������������������������������������������������������������������������8.41
X.	Equivalents and Conversion Factors�������������������������������������������������������������������������������������8.48
XI.	Capacity Correction Factor for Rupture Disc/Pressure Relief Valve Combination������������8.54

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Pressure relief valve engineering handbook
Chapter 1 - Introduction
The primary purpose of a pressure or vacuum relief valve is to protect
life and property by venting process fluid from an overpressurized vesselor adding fluid (such as air) to prevent formation of a vacuum strong
enough to cause a storage tank to collapse.
Proper sizing, selection, manufacture, assembly, testing, installation,and maintenance of a pressure relief valve are all critical for optimal
protection of the vessel or system.
Please note that the brand names of pressure relief devices covered(Anderson Greenwood, Crosby, Whessoe and Varec) are of Emerson
manufacture. A specific valve brand is selected, according to pressure
range, temperature range, valve size, industry application and otherapplicable factors.
This manual has been designed to provide a service to Emerson
customers by presenting reference data and technical recommendations
based on over 125 years of pioneering research, development, design,
manufacture and application of pressure relief valves. Sufficient data
is supplied so that an individual will be able to use this manual as aneffective aid to properly size and select Emerson-manufactured pressure
relief devices for specific applications. Information covering terminology,
standards, codes, basic design, sizing and selection are presented in aneasy to use format.
The information contained in this manual is offered as a guide. The
actual selection of valves and valve products is dependent on numerous
factors and should be made only after consultation with qualified
Emerson personnel. Those who utilize this information are reminded ofthe limitations of such publications and that there is no substitute for
qualified engineering analysis.
Emerson pressure relief devices are manufactured in accordancewith a controlled quality assurance program which meets or exceeds
ASME Code quality control requirements. Capacities of valves with setpressures of 15 psig (1.03 barg], or higher, are certified by the National
Board of Boiler and Pressure Vessel Inspectors. These attributes are
assured by the presence of an ASME Code Symbol Stamp and the letters
NB on each pressure relief valve nameplate. Lower set pressures are
not addressed by either the National Board or ASME; however, capacities
at lower set pressures have been verified by actual testing at Emerson’s
extensive flow lab facilities. Emerson’s range of pressure relief valves
are designed, manufactured, and tested in strict accordance with aquality management system approved to the International Standard
Organization’s ISO 9000 quality standard requirements. With proper
sizing and selection, the user can thus be assured that Emerson’s
products are of the highest quality and technical standards in the worldof pressure relief technology.
When in doubt as to the proper application of any particular data, theuser is advised to contact the nearest Emerson sales office or sales
representative. Emerson has a large staff of highly trained personnel
strategically located throughout the world, who are available for yourconsultation.
Emerson has designed and has available to customers a computer sizing
program for pressure relief valves, PRV2SIZE (Pressure Relief Valve andVent Sizing Software). The use of this comprehensive program allows
an accurate and documented determination of such parameters aspressure relief valve orifice area and maximum available flow.

This sizing program is a powerful tool, yet easy to use. Its many
features include quick and accurate calculations, user-selected units ofmeasurement, selection of pressure relief valve size and style, valve data
storage, printed reports, valve specification sheets and outline drawings.
Program control via pop-up windows, function keys, extensive on-line
help facilities, easy-to-read formatted screens, flagging of errors, andeasy editing of displayed inputs make the program easy to understandand operate.
It is assumed that the program user has a general understandingof pressure relief valve sizing calculations. The program user must
remember they are responsible for the correct determination ofservice conditions and the various data necessary for input to the sizing
program.
For download instructions for the latest PRV2SIZE please contact yoursales representative or factory.
The information in this manual is not to be used for ASME Section
III nuclear applications. If you need assistance with pressure relief
valves for ASME Section III service, please contact our nuclear
industry experts at 508-384-3121.

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Pressure relief valve engineering handbook
Chapter 2 - Terminology
This chapter contains common and standardized terminology related to pressure relief devices used throughout this handbook and is in accordance
with, and adopted from, ANSI/ASME Performance Test Code PTC-25-2008 and other widely accepted practices.
I. General
Bench Testing
Testing of a pressure relief device on a test stand using an externalpressure source with or without an auxiliary lift device to determinesome or all of its operating characteristics.
Flow Capacity Testing
Testing of a pressure relief device to determine its operating
characteristics including measured relieving capacity.
In-Place Testing
Testing of a pressure relief device installed on but not protecting asystem, using an external pressure source, with or without an auxiliary
lift device to determine some or all of its operating characteristics.
In-Service Testing
Testing of a pressure relief device installed on and protecting asystem using system pressure or an external pressure source, with orwithout an auxiliary lift device to determine some or all of its operating
characteristics.
Pressure Relief Device
A device designed to prevent pressure or vacuum from exceeding apredetermined value in a pressure vessel by the transfer of fluid during
emergency or abnormal conditions.
II. Types of Devices
Pressure Relief Valve (PRV)
A pressure relief device designed to actuate on inlet static pressure
and to reclose after normal conditions have been restored. It may be
one of the following types and have one or more of the following design
features.
A.	Restricted lift PRV: a pressure relief valve in which the actual
discharge area is determined by the position of the disc.
B.	Full lift PRV: a pressure relief valve in which the actual discharge
area is not determined by the position of the disc.
C.	Reduced bore PRV: a pressure relief valve in which the flow path area
below the seat is less than the flow area at the inlet to the valve.
D.	Full bore PRV: a pressure relief valve in which the bore area is equal
to the flow area at the inlet to the valve and there are no protrusions
in the bore.
E.	Direct spring loaded PRV: a pressure relief valve in which the disc isheld closed by a spring.
F.	Pilot operated PRV: a pressure relief valve in which a piston ordiaphragm is held closed by system pressure and the holding
pressure is controlled by a pilot valve actuated by system pressure.
G.	Conventional direct spring loaded PRV: a direct spring loadedpressure relief valve whose operational characteristics are directly
affected by changes in the back pressure.

H.	Balanced direct spring loaded PRV: a direct spring loaded pressure
relief valve which incorporates means of minimizing the effect ofback pressure on the operational characteristics (opening pressure,closing pressure, and relieving capacity).
I.	Internal spring PRV: a direct spring loaded pressure relief valve
whose spring and all or part of the operating mechanism is exposedto the system pressure when the valve is in the closed position.
J. Temperature and pressure relief valve: a pressure relief valve that
may be actuated by pressure at the valve inlet or by temperature atthe valve inlet.
K.	Power actuated PRV: a pressure relief valve actuated by an externally
powered control device.
Safety Valve
A pressure relief valve characterized by rapid opening or closing andnormally used to relieve compressible fluids.
Relief Valve
A pressure relief valve characterized by gradual opening or closing
generally proportional to the increase or decrease in pressure. It is
normally used for incompressible fluids.
Safety Relief Valve
A pressure relief valve characterized by rapid opening or closing or by
gradual opening or closing, generally proportional to the increase or
decrease in pressure. It can be used for compressible or incompressible
fluids.
III. Parts of Pressure Relief Devices
Adjusting Ring: a ring assembled to the nozzle and/or guide of a direct
spring valve used to control the opening characteristics and/or the
reseat pressure.
Adjustment Screw: a screw used to adjust the set pressure or the reseatpressure of a reclosing pressure relief device.
Backflow Preventer: a part or a feature of a pilot operated pressure
relief valve used to prevent the valve from opening and flowing
backwards when the pressure at the valve outlet is greater than thepressure at the valve inlet.

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Pressure relief valve engineering handbook
Chapter 2 - Terminology
Bellows: a flexible component of a balanced direct spring valve used
to prevent changes in set pressure when the valve is subjected to asuperimposed back pressure, or to prevent corrosion between the discholder and guide.
Blowdown Ring: see adjusting ring.
Body: a pressure retaining or containing member of a pressure
relief device that supports the parts of the valve assembly and has
provisions(s) for connecting to the primary and/or secondary pressuresource(s).
Bonnet: a component of a direct spring valve or of a pilot in a pilot
operated valve that supports the spring. It may or may not be pressure
containing.
Cap: a component used to restrict access and/or protect the adjustment
screw in a reclosing pressure relief device. It may or may not be apressure containing part.
Diaphragm: a flexible metallic, plastic, or elastomer member of a
reclosing pressure relief device used to sense pressure or provide
opening or closing force.
Disc: a moveable component of a pressure relief device that contains theprimary pressure when it rests against the nozzle.
Disc Holder: a moveable component in a pressure relief device that
contains the disc.
Dome: the volume of the side of the unbalanced moving member
opposite the nozzle in the main relieving valve of a pilot operatedpressure relief device.
Field Test: a device for in-service or bench testing of a pilot operatedpressure relief device to measure the set pressure.
Gag: a device used on reclosing pressure relief devices to prevent thevalve from opening.
Guide: a component in a direct spring or pilot operated pressure relief
device used to control the lateral movement of the disc or disc holder.
Huddling Chamber: the annular pressure chamber between the nozzle
exit and the disc or disc holder that produces the lifting force to obtain
lift.
Lift Lever: a device to apply an external force to the stem of a pressure
relief valve to manually operate the valve at some pressure below the set
pressure.
Main Relieving Valve: that part of a pilot operated pressure relief device
through which the rated flow occurs during relief.
Nozzle: a primary pressure containing component in a pressure relief
valve that forms a part or all of the inlet flow passage.
Pilot: the pressure or vacuum sensing component of a pilot operatedpressure relief valve that controls the opening and closing of the main
relieving valve.
Piston: the moving element in the main relieving valve of a pilot
operated, piston type pressure relief valve which contains the seat that
forms the primary pressure containment zone when in contact with thenozzle.
Pressure Containing Member: a component which is exposed to andcontains pressure.
Pressure Retaining Member: a component which holds one or morepressure containing members together but is not exposed to thepressure.

Seat: the pressure sealing surfaces of the fixed and moving pressure
containing components.
Spindle: a part whose axial orientation is parallel to the travel of the disc.
It may be used in one or more of the following functions:a. assist in alignment,b. guide disc travel, andc. transfer of internal or external forces to the seats.
Spring: the element in a pressure relief valve that provides the force tokeep the disc on the nozzle.
Spring Step: a load transferring component in a pressure relief valvethat supports the spring.
Spring Washer: see spring step.
Spring Button: see spring step.
Stem: see spindle.
Yoke: a pressure retaining component in a pressure relief device that
supports the spring in a pressure relief valve but does not enclose thespring from the surrounding ambient environment.
IV. Dimensional Characteristics - Pressure Relief Valves
Actual Discharge Area: the measured minimum net area which
determines the flow through a valve.
Actual Orifice Area: see actual discharge area.
Bore Area: the minimum cross-sectional flow area of a nozzle.
Bore Diameter: the minimum diameter of a nozzle.
Curtain Area: the area of the cylindrical or conical discharge opening
between the seating surfaces created by the lift of the disc above theseat.
Developed Lift: the actual travel of the disc from closed position to theposition reached when the valve is at flow rating pressure.
Discharge Area: see actual discharge area.
Effective Discharge Area: a nominal or computed area of flow througha pressure relief valve used with an effective discharge coefficient tocalculate minimum required relieving capacity.

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Pressure relief valve engineering handbook
Chapter 2 - Terminology
Effective Orifice Area: see effective discharge area.
Inlet Size: the nominal pipe size of the inlet of a pressure relief valve,
unless otherwise designated.
Lift: the actual travel of the disc away from closed position when a valve
is relieving.
Nozzle Area, Nozzle Throat Area: see bore area.
Nozzle Diameter: see bore diameter.
Outlet Size: the nominal pipe size of the outlet of a pressure relief valve,
unless otherwise designated.
Rated Lift: the design lift at which a valve attains its rated relieving
capacity.
Seat Angle: the angle between the axis of a valve and the seating
surface. A flat-seated valve has a seat angle of 90 degrees.
Seat Area: the area determined by the seat diameter.
Seat Diameter: the smallest diameter of contact between the fixed andmoving portions of the pressure containing elements of a valve.
Seat Flow Area: see curtain area.
Throat Area: see bore area.
Throat Diameter: see bore diameter.
V. Operational Characteristics of Pressure Relief Devices
Back Pressure: the static pressure existing at the outlet of a pressure
relief device due to pressure in the discharge system. It is the sum of
superimposed and built-up back pressure.
Blowdown: the difference between actual set pressure of a pressure
relief valve and actual reseating pressure, expressed as a percentage ofset pressure or in pressure units.
Blowdown Pressure: the value of decreasing inlet static pressure atwhich no further discharge is detected at the outlet of a pressure relief
valve after the valve has been subjected to a pressure equal to or above
the set pressure.
Built-Up Back Pressure: pressure existing at the outlet of a pressure
relief device caused by the flow through that particular device into adischarge system.
Chatter: abnormal, rapid reciprocating motion of the moveable parts of apressure relief valve in which the disc contacts the seat.
Closing Pressure: the value of decreasing inlet static pressure atwhich the valve disc re-establishes contact with the seat or at which lift
becomes zero.
Coefficient of Discharge: the ratio of the measured relieving capacity to
the theoretical relieving capacity.
Cold Differential Test Pressure: the inlet static pressure at which apressure relief valve is adjusted to open on the test stand. This test
pressure includes corrections for service conditions of superimposedback pressure and/or temperature. Abbreviated as CDTP and stamped
on the nameplate of a pressure relief valve.

Constant Back Pressure: a superimposed back pressure which isconstant with time.
Cracking Pressure: see opening pressure.
Dynamic Blowdown: the difference between the set pressure andclosing pressure of a pressure relief valve when it is overpressured to
the flow rating pressure.
Effective Coefficient of Discharge: a nominal value used with theeffective discharge area to calculate the minimum required relieving
capacity of a pressure relief valve.
Flow Capacity: see measured relieving capacity.
Flow Rating Pressure: the inlet stagnation pressure at which therelieving capacity of a pressure relief device is measured.
Flutter: abnormal, rapid reciprocating motion of the movable parts of apressure relief valve in which the disc does not contact the seat.
Leak Pressure: see start-to-leak pressure.
Leak Test Pressure: the specified inlet static pressure at which aquantitative seat leakage test is performed in accordance with astandard procedure.
Marked Set Pressure: the value or values of pressure marked on apressure relief device.
Marked Relieving Capacity: see rated relieving capacity.
Measured Relieving Capacity: the relieving capacity of a pressure relief
device measured at the flow rating pressure, expressed in gravimetric orvolumetric units.
Opening Pressure: the value of increasing static pressure of a pressure
relief valve at which there is a measurable lift, or at which the discharge
becomes continuous as determined by seeing, feeling, or hearing.
Overpressure: a pressure increase over the set pressure of a pressure
relief valve, usually expressed as a percentage of set pressure.
Popping Pressure: the value on increasing inlet static pressure at which
the disc moves in the opening direction at a faster rate as compared withcorresponding movement at higher or lower pressures.
Primary Pressure: the pressure at the inlet in a pressure relief device.
Rated Coefficient of Discharge: the coefficient of discharge determinedin accordance with the applicable code or regulation and is used with theactual discharge area to calculate the rated flow capacity of a pressure
relief valve.
Rated Relieving Capacity: that portion of the measured relieving
capacity permitted by the applicable code or regulation to be used as abasis for the application of a pressure relief device.

2.3

Pressure relief valve engineering handbook
Chapter 2 - Terminology
Reference Conditions: those conditions of a test medium which are
specified by either an applicable standard or an agreement between theparties to the test, which may be used for uniform reporting of measured
flow test results.
Relieving Conditions: the inlet pressure and temperature on a pressure
relief device during an overpressure condition. The relieving pressure isequal to the valve set pressure plus the overpressure. (The temperatureof the flowing fluid at relieving conditions may be higher or lower thanthe operating temperature.)
Relieving Pressure: set pressure plus overpressure.
Resealing Pressure: the value of decreasing inlet static pressure
at which no further leakage is detected after closing. The method of
detection may be a specified water seal on the outlet or other means
appropriate for this application.
Reseating Pressure: see closing pressure.
Seal-Off Pressure: see resealing pressure.
Secondary Pressure: the pressure existing in the passage between theactual discharge area and the valve outlet in a safety, safety relief, orrelief valve.
Set Pressure: the value of increasing inlet static pressure at which apressure relief device displays one of the operational characteristics
as defined under opening pressure, popping pressure or start-to-leakpressure. (The applicable operating characteristic for a specific device
design is specified by the device manufacturer.)
Simmer: the audible or visible escape of fluid between the seat anddisc at an inlet static pressure below the popping pressure and at
no measurable capacity. It applies to safety or safety relief valves oncompressible fluid service.
Start-to-Discharge Pressure: see opening pressure.
Start-to-Leak Pressure: the value of increasing inlet static pressure atwhich the first bubble occurs when a pressure relief valve is tested bymeans of air under a specified water seal on the outlet.
Static Blowdown: the difference between the set pressure and theclosing pressure of a pressure relief valve when it is not overpressured
to the flow rating pressure.
Superimposed Back Pressure: the static pressure existing at the outletof a pressure relief device at the time the device is required to operate. It
is the result of pressure in the discharge system from other sources andmay be constant or variable.
Test Pressure: see relieving pressure.
Theoretical Relieving Capacity: the computed capacity expressed ingravimetric or volumetric units of a theoretically perfect nozzle having aminimum cross-sectional flow area equal to the actual discharge area ofa pressure relief valve or net flow area of a non-reclosing pressure relief
device.
Vapor-Tight Pressure: see resealing pressure.
Variable Back Pressure: a superimposed back pressure that will varywith time.
Warn: see simmer.

VI. System Characteristics
Accumulation: is the pressure increase over the maximum allowable
working pressure (MAWP) of the process vessel or storage tank allowed
during discharge through the pressure relief device. It is expressedin pressure units or as a percentage of MAWP or design pressure.
Maximum allowable accumulations are typically established byapplicable codes for operating and fire overpressure contingencies.
Design Pressure: is the pressure of the vessel along with the designtemperature that is used to determine the minimum permissible
thickness or physical characteristic of each vessel component as
determined by the vessel design rules. The design pressure is selectedby the user to provide a suitable margin above the most severe pressure
expected during normal operation at a coincident temperature. It is thepressure specified on the purchase order. This pressure may be used inplace of the maximum allowable working pressure (MAWP) in all cases
where the MAWP has not been established. The design pressure is equal
to or less than the MAWP.
Maximum Allowable Working Pressure: is the maximum gaugepressure permissible at the top of a completed process vessel or storagetank in its normal operating position at the designated coincident
temperature specified for that pressure. The pressure is the least of
the values for the internal or external pressure as determined by thevessel design rules for each element of the vessel using actual nominal
thickness, exclusive of additional metal thickness allowed for corrosion
and loadings other than pressure. The maximum allowable working
pressure (MAWP) is the basis for the pressure setting of the pressure
relief devices that protect the vessel. The MAWP is normally greater thanthe design pressure but must be equal to the design pressure when thedesign rules are used only to calculate the minimum thickness for eachelement and calculations are not made to determine the value of the
MAWP.
Maximum Operating Pressure: is the maximum pressure expectedduring normal system operation.

2.4

Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
The following data is included in this chapter
I.	Introduction�������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������3.3
II.	American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code�����������������������������������������������������������������������������������������������3.3
Section I - Rules for Construction of Power Boilers��������������������������������������������������������������������������������������������������������������������������������������������������������3.3
Section VIII - Rules for Construction of Pressure Vessels����������������������������������������������������������������������������������������������������������������������������������������������3.9
	PTC 25 - Performance Test Code�����������������������������������������������������������������������������������������������������������������������������������������������������������������������������������3.18
	B16.34 - Valves - Flanged, Threaded and Welded Ends������������������������������������������������������������������������������������������������������������������������������������������������3.18
	B16.5 - Pipe Flanges and Flanges Fittings���������������������������������������������������������������������������������������������������������������������������������������������������������������������3.18
III.	International Organization for Standardization (ISO)�����������������������������������������������������������������������������������������������������������������������������������������������������3.18
	ISO 4126 - Safety Devices for Protection Against Excessive Pressure�������������������������������������������������������������������������������������������������������������������������3.18
	ISO 23251 - Petroleum and Natural Gas Industries - Pressure Relieving and Depressurizing Systems�����������������������������������������������������������������3.21
	ISO 28300 - Petroleum and Natural Gas Industries - Venting of Atmospheric and Low Pressure Storage Tanks��������������������������������������������������3.21
IV.	European Union Directives����������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������3.22
	Pressure Equipment Directive (PED) 97/23/EC�������������������������������������������������������������������������������������������������������������������������������������������������������������3.22
	ATEX Directive 94/9/EC����������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������3.23
V.	American Petroleum Institute (API)��������������������������������������������������������������������������������������������������������������������������������������������������������������������������������3.23
	API Standard/Recommended Practice 520 - Sizing, Selection and Installation of Pressure Relieving Devices in Refineries��������������������������������3.23
	API Standard 521 - Guide to Pressure Relieving and Depressuring Systems�������������������������������������������������������������������������������������������������������������3.23
	API Standard 526 - Flanged Steel Pressure Relief Valves��������������������������������������������������������������������������������������������������������������������������������������������3.24
	API Standard 527 - Seat Tightness of Pressure Relief Valves��������������������������������������������������������������������������������������������������������������������������������������3.24
	API Standard 2000 - Venting Atmospheric and Low Pressure Storage Tanks������������������������������������������������������������������������������������������������������������3.24
	API Recommended Practice 576 - Inspection of Pressure Relief Devices������������������������������������������������������������������������������������������������������������������3.25
	API Standard 620 - Design and Construction of Large, Welded, Low Pressure Storage Tanks��������������������������������������������������������������������������������3.25
	API Standard 625 - Tank Systems for Refrigerated Liquid Gas Storage����������������������������������������������������������������������������������������������������������������������3.25
	API Standard 650 - Welded Steel Tanks for Oil Storage������������������������������������������������������������������������������������������������������������������������������������������������3.26
VI.	National Fire Protection Agency (NFPA)������������������������������������������������������������������������������������������������������������������������������������������������������������������������3.26
	NFPA 30 - Flammable and Combustible Liquids Code�������������������������������������������������������������������������������������������������������������������������������������������������3.26
	NFPA 58 - Liquefied Petroleum Gas Code���������������������������������������������������������������������������������������������������������������������������������������������������������������������3.26
	NFPA 59A - Standard for the Production, Storage and Handling of Liquefied Natural Gas (LNG)����������������������������������������������������������������������������3.27
VII.	National Board of Boiler and Pressure Vessel Inspectors��������������������������������������������������������������������������������������������������������������������������������������������3.27
	National Board Inspection Code (NBIC) 23..3.27	NB18 Pressure Relief Device Certifications�����������������������������������������������������������������������������������3.28

3.1

Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
The following Figures are included in this chapter
Figure 3-1
Typical Section I Single PRV Installation�����������������������������������������������������������������������������������������������������������������������������������������������������������3.4
Figure 3-2
Typical Section I Multiple PRV Installation��������������������������������������������������������������������������������������������������������������������������������������������������������3.5
Figure 3-3	Direct Spring Operated PRV with Lift Lever������������������������������������������������������������������������������������������������������������������������������������������������������3.7
Figure 3-4	Pilot Operated PRV Field Test Assembly�����������������������������������������������������������������������������������������������������������������������������������������������������������3.7
Figure 3-5
Safety Selector Valve������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������3.8
Figure 3-6	Recommended ASME Section I Piping Arrangement��������������������������������������������������������������������������������������������������������������������������������������3.8
Figure 3-7
Typical Section VIII Single Device Installation (Non-Fire) - Set at the MAWP of the Vessel������������������������������������������������������������������������3.10
Figure 3-8
Typical Section VIII Single Device Installation (Non-Fire) - Set below the MAWP of the Vessel�����������������������������������������������������������������3.11
Figure 3-9
Typical Section VIII Single Device Installation (Fire) - Set at the MAWP of the Vessel���������������������������������������������������������������������������������3.12
Figure 3-10 Typical Section VIII Multiple Valve (Non-Fire Case) Installation��������������������������������������������������������������������������������������������������������������������3.13
Figure 3-11 Typical Section VIII Multiple Valve (Fire Case) Installation�����������������������������������������������������������������������������������������������������������������������������3.14
Figure 3-12 Typical ASME Section VIII Nameplate��������������������������������������������������������������������������������������������������������������������������������������������������������������3.18
Figure 3-13	Isolation Valve Requirements��������������������������������������������������������������������������������������������������������������������������������������������������������������������������3.20
Figure 3-14	PRV Discharge Piping Example�����������������������������������������������������������������������������������������������������������������������������������������������������������������������3.21
Figure 3-15	API 527 Leak Test for Gas Service�������������������������������������������������������������������������������������������������������������������������������������������������������������������3.24
The following Tables are included in this chapter
Table 3-1
Section I Set Pressure Tolerances��������������������������������������������������������������������������������������������������������������������������������������������������������������������3.7
Table 3-2	ASME Section VIII Set Pressure Tolerance�����������������������������������������������������������������������������������������������������������������������������������������������������3.16
Table 3-3	Design Basis for Sizing Downstream Piping���������������������������������������������������������������������������������������������������������������������������������������������������3.21
Table 3-4	API 527 Leakage Rate Acceptance for Metal Seated PRV (Gas Service)�������������������������������������������������������������������������������������������������������3.25

3.2

Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
I. Introduction
This section will provide highlights (please note this is not a complete
review) of several commonly used global codes, standards andrecommended practices that may be referenced when selecting
pressure relief valves. The documents that are listed in this handbook
are subject to revision and the user should be aware that the following
information may not reflect the most current editions.
II. American Society of Mechanical Engineers (ASME) Boiler
and Pressure Vessel Code
There is information contained within various sections in the Code thatprovide rules for design, fabrication, testing, materials and certificationof appurtenances, such as pressure relief valves that are used in the new
construction of a boiler or pressure vessel. The scope of this handbook
will limit this discussion to the Section I and Section VIII portion of theCode. The text is based upon the 2013 revision of the Code.
Section I - Rules for Construction of Power Boilers
Scope
The general requirements found in part PG of the Section I Codeprovides rules that are applicable to the construction of new boilers thatgenerate steam at a pressure equal to or more than 15 psig (1.03 barg).
In addition, these rules will apply to the construction of new hot water
boilers that operate above 160 psig (11.0 barg) and/or when the operating
temperature exceeds 250°F (120°C). For boilers that operate outside ofthese parameters, the user may wish to review Section IV of the Codethat deals with rules for heating boilers.
Acceptable Valve Designs
ASME Section I traditionally allowed only the use of direct acting springloaded pressure relief valves, but the use of self-actuated pilot operatedpressure relief valves is now allowed. The use of power-actuatedpressure relief valves can be used in some circumstances for a forcedflow steam generator. No other types of pressure relief valves or nonclosing devices such as rupture disks can be used for this section of the
Code.
Allowable Vessel Accumulation
One requirement in Section I is that the maximum accumulation allowed
during an overpressure event must be limited to 3% when one pressure
relief valve is used to provide protection. There are specific rules listed
in Section I that will oftentimes require the use of two or more pressure
relief valves to provide protection. More details on these multiple valve
installation requirements are found in Chapter 5 (USCS units) or Chapter
6 (Metric units) that deal with sizing and selection. When multiple PRVs
are used, the allowable accumulation for a fired vessel can be 6%.
For a single PRV installation, the Code will allow the highest set
pressure to be equal to maximum allowable working pressure (MAWP).

Therefore, the design of this valve must allow adequate lift to obtain theneeded capacity within 3% overpressure. Chapter 4 of the handbook will
discuss how the design of a Section I valve provides this needed lift withminimal overpressure. Although most users desire this highest possible
set pressure (equal to MAWP) to avoid unwanted cycles, the Code does
allow this PRV to be set below the MAWP.
For a multiple PRV installation, the Code will allow for a staggeredor variable set pressure regime for the valves. This helps to avoid
interaction between the safety valves during their open and closing cycle.
As noted above, the accumulation rule allows for 6% rise in pressure
above the MAWP. One of the multiple valves, sometimes called theprimary pressure relief valve, must still be set no higher than the MAWP
but the additional or supplemental pressure relief valve can be set up toa maximum of 3% above the MAWP. In this case, the same valve design
criteria, obtaining the needed valve lift with 3% overpressure, is stillrequired. The Code requires that the overall range of set pressures fora multiple valve installation not exceed 10% of the highest set pressure
PRV. Figures 3-1 and 3-2 help to illustrate the single and multiple valve
installation.
Pressure Relief Valve Certification Requirements
The ASME organization itself does not do the actual inspection andacceptance of a pressure relief valve design to meet the requirementsof the Code. Traditionally, it has been the National Board of Boiler andPressure Vessel Inspectors (National Board) that has been designated by
the ASME to perform this duty.
One test that is performed is to demonstrate that an individual valve will
provide the capacity of steam that is expected when the valve is called
upon to relieve. For each combination of valve and orifice size, valve
design and set pressure, there are to be three valves tested to measure
their capacity. These capacity certification tests are done with saturated
steam at a flowing pressure using the greater of 3% or 2 psi (0.138 bar)
overpressure. The requirement is that the measured capacity from anyof the three valves must fall within a plus or minus 5% of the averagecapacity of the three valve test. If one valve were to fail to meet thiscriteria, then rules in the Code allow for two more valves to be tested.
Now, all four valves must fall within a plus or minus 5% of the averagecapacity of all four valves now tested. If either of the two additional valvesfail to meet this range, then valve certification is denied.
When the valve capacity certification is approved, this individual valve will
be given a rated capacity that is 90% of the average capacity found during
the testing. It is this rated capacity that is used to size and select valves
per the ASME Section I procedures in Chapters 5 and 6.

3.3

Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
Figure 3-1
Typical Section I Single PRV Installation
PRV Specifications

Vessel Pressure %Vessel Specifications
Maximum
Accumulation

103

Accumulation(3%)

Overpressure(3%)

Set Pressure

100

MAWP

Blowdown(4%)
Simmer Pressure

98

Reseat Pressure

96

Leak Test Pressure

93

90

Possible
OperatingPressure

3.4

Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
Figure 3-2
Typical Section I Multiple PRV Installation
Primary PRV Specifications

Supplemental PRV Specifications

Vessel Pressure %106

Vessel Specifications

Maximum
Accumulation

Supplemental PRV
Overpressure (3%)
Supplemental PRV
Set Pressure
Primary PRV
Overpressure (3%)

Accumulation (6%)

Supplemental PRV
Blowdown (4%)
Simmer Pressure

Primary PRV
Set Pressure
Primary PRV
Blowdown (4%)

103

101
100

Reseat Pressure

SimmerPressure

MAWP

99
98

Leak Test Pressure

97

ReseatPressure

96

Leak Test
Pressure

93

90

Possible
OperatingPressure

3.5

Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
This three valve test is normally used for a very narrow, oftentimes nonstandard, application. Please note that the set pressure cannot vary inorder to provide a code stamp for the safety valve. If a safety valve will
be used in multiple applications that have different set pressures, thenanother capacity certification test procedure can be used. A ratio of themeasured capacity over the flowing pressure (using an overpressure of
3% or 2 psi (0.138 bar), whichever is greater) is established with testing
four valves of the same connection and orifice size. These four valves
are tested at different set pressures that would be representative of
their expected application. This ratio is plotted to give a slope that will
determine the straight line relationship between the capacity and theflowing pressure of the valve during relief.
All four valves tested must fall within plus or minus 5% of the averagestraight line slope. If one valve were to fall outside of this plus or minus
5% range, then two additional valves can be tested. No more than four
additional valves can be tested or the certification will be denied.
When the valve capacity certification is approved then the rated slope,used to size and select valves, is limited to 90% of the average slope
measured during testing.
A third, and frequently used, capacity certification test is available when
the design of a safety valve encompasses many different sizes and set
pressure requirements. One requirement for grouping different size
safety valves as one specific design family is that the ratio of the valve
bore diameter to the valve inlet diameter must not exceed the range of
0.15 to 0.75 when the nozzle of the valve controls the capacity. If the lift ofthe valve trim parts controls the capacity, then the lift to nozzle diameter(L/D) of the safety valves in the design family must be the same.
Once the design family is determined, then three valve sizes from thefamily and three valves for each size, for a total of nine valves, are tested
to measure their capacity with steam. Once again, these flow tests aredone with 3% or 2 psi (0.138 bar), whichever is greater. These measured
values are compared to the expected theoretical capacity delivered
through an ideal nozzle or flow area where there are no losses to reduce
flow. A coefficient of discharge (Kd) is denoted for each of the nine tests
as follows:
Kd =Actual Flow
Theoretical Flow

Similar to the other two capacity tests above, each of the nine values of
Kd must fall within plus or minus 5% of the average of the nine tests. If
one valve falls outside of this range then two more valves may be tested,
up to a limit of four total additional valves. When excluding the replaced
valves, the Kd of all valves tested must fall in the plus or minus 5% of theoverall average or the certification is denied.
If the capacity certification test is successful, then the rated coefficient ofdischarge (K) is established for the valve design family. The K is equal to90% of the Kd value.

In addition to establishing the rated capacities, the certification
testing will also require that the blowdown of any Section I valve bedemonstrated not to exceed 4% when the certification set pressure is
above 100 psig (6.90 barg) or not to exceed 4 psi (0.276 bar) when thecertification set pressure is below 100 psig (6.90 barg).
If a pressure relief valve is to be used to protect an economizer (seeFigure 5-2 or 6-1) then this device must be capacity certified on water
as well as saturated steam. The same set pressure tolerances andmaximum blowdown criteria that is required for steam as the test
media is also required for water as the test media.
The Code requires that the manufacturer demonstrate that eachindividual pressure relief valve or valve design family tested per theabove requirements also provide similar operational performance whenbuilt on the production line. Therefore, every six years, two production
valves are chosen for each individual valve or valve design family for set
pressure, capacity, and blowdown testing. As with the initial certification
testing an ASME designated third party, such as the National Board, ispresent to witness these production valve tests.
Pressure Relief Valve Design Criteria
Each production PRV must have its set pressure demonstrated withthe valve being tested on steam. When the testing equipment andvalve configuration will allow, this set pressure test is done by the
manufacturer prior to shipping. If the set pressure requirement is higheror the test drum volume requirement is larger than the capabilities that
reside at the manufacturing facility, then the valve can be sent to the site,
mounted on the boiler and tested. This in situ testing is rarely performed
today due to safety concerns and possible damage to the safety valve andother equipment. The Code recognizes these concerns and will allow
the manufacturer to use two alternative methods to demonstrate the set
pressure on steam.
When there is limited capacity on the test stand, the rapid opening of asteam safety valve will deplete the force holding the seat in lift during
testing. This can damage the seating surfaces during the reclosureof the valve. Therefore, one alternative method is to limit the lift ofthe safety valve seat when tested. This can be done by externally
blocking the movement of the valve trim parts, such as the spindle
assembly shown in Figure 3-3, that move upward when the safety valve
opens. If this restricted lift test is performed, the manufacturer mustmechanically confirm the actual required lift is met.

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Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
When the required set pressure exceeds the manufacturer’s test boiler
capabilities, another acceptable alternate test method is to use what
is called a lift assist device. These devices attach to the same spindle
assembly discussed above. The safety valve is subjected to the steampressure from the test boiler. Since the test boiler pressure is limited,
the lift assist device must have the ability to add upward lifting force,
typically via some hydraulically powered system, to overcome thespring compression. The lift assist device has instrumentation that canmeasure the upward force being applied. Using the safety valve seatdimensions and the operating pressure from the test boiler, the set
pressure can be determined with minimal lift of the seat. As with therestricted lift test above, the manufacturer must mechanically confirm
the actual required lift is met.
A recent change in the Section I Code does not require a demonstrated
test of the valve blowdown for production safety valves. For example, thetypical blowdown setting for a production Section I PRV is 4% for valves
set above 375 psig (25.9 barg) and the valve adjustments are to be set per
manufacturer’s instructions to reflect this blowdown.
Since the test stand accumulators are of limited volume in a valve
manufacturing environment, there is no requirement to measure thecapacity of a production safety valve. The initial certification and renewal
testing of valve capacities are discussed above.
A seat leakage test is required at the maximum expected operatingpressure, or at a pressure not exceeding the reseat pressure of thevalve. The requirement is that there is to be no visible leakage.
Each production PRV will have its pressure containing components
either hydrostatically tested at 1.5 times the design of the part orpneumatically tested at 1.25 times the design of the part. This proof test
is now required even for non-cast pressure containing parts such as bar
stock or forgings where the test pressures could exceed 50% of their
allowable stress. A pressure containing part made in a cast or welded
form will always be proof tested no matter what its allowable stress
may be.

Figure 3-3
Direct Spring Operated PRV with Lift Lever

Lift Lever

Spindle Assembly

Figure 3-4
Pilot Operated PRV Field Test Assembly

Active Process

Table 3-1 - Section I Set Pressure Tolerances
Set Pressure, psig (barg)Less than or equal to 70 (4.82)
More than 70 (4.82) and equal to or less than 300 (20.7)
More than 300 (2.07) and equal to or less than 1000 (70.0)
More than 1000 (70.0)

Tolerance (plus or minus) from the set pressure
2 psi (0.137 bar)
3% of the set pressure
10 psi (0.690 bar)
1% of the set pressure
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Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
A Section I PRV with an inlet that is equal to or greater than 3” (80 mm)
in size must have a flanged or welded inlet connection. Any PRV withan inlet less than 3” (80 mm) can have a screwed, flanged or welded
connection.
All pressure relief valves must have a device to check if the trim parts
are free to move when the valve is exposed to a minimum of 75% of
its set pressure. This device is normally a lift lever (see Figure 3-3) fora direct spring loaded or pilot operated valve. A pilot operated valve
may also use what is called a field test connection, where an externalpressure can be supplied to function the valve (see Figure 3-4).
Pressure Relief Valve Installation
There are specific maximum lengths of inlet piping specified by ASME
Section I that mandate a close coupling of the safety valve to the vessel.
The inlet and outlet piping shall have at least the area of the respective
valve inlet or outlet area. If there are multiple valves mounted on one
connection, then this connection must have an area at least as large as
the two safety valves inlet connection areas in total. These installation
requirements are extremely important for these safety valves that havevery minimal blowdown settings. There will be more on this topic in
Chapter 4.
There can be no intervening isolation valve between the vessel and thesafety valve. There also cannot be any isolation valve downstream of thesafety valve.
An exception to the mandate of no isolation valves for the inlet
connection of a Section I safety valve lies in what is called an ASME CodeCase. These code cases are not a part of the main body of the document
as they are a vehicle to respond to inquiries asking for clarifications oralternatives to the rules. These code cases may be published as often
as four times a year and their implementation is immediate when thereis latitude that has been granted to modify a requirement. In someinstances, a code case will become a part of the Code in some futurerevision.
Code Case 2254 allows the use of diverter, or changeover valves,
when the steam drum has a MAWP of 800 psig (55.2 barg) or less.
The Anderson Greenwood Safety Selector Valve (see Figure 3-5) is adiverter valve that will meet the requirements laid out in the code case.
These requirements include that the diverter valve never be in a position
where both safety valves could be blocked at the same time, there must
be a positive indication of the active safety valve, vent valves to safely
bleed pressure for a newly isolated safety valve are to be provided,
and that a minimum flow coefficient (Cv) is met. With any code case,the device, in this instance the diverter valve, must be marked with theCode Case 2254 on the nameplate.

Figure 3-5
Safety Selector Valve
PRV
Connection

Flow

Bleed Port forStandby PRD

Integral Cast Inlet Flange(ANSI Raised Face)

Process Connection

Figure 3-6
Recommended ASME Section I Piping Arrangement
Fixed support anchored tobuilding structure

Discharge
Pipe

Drip Pan
Drain

NOTE
Allow sufficient space toprevent bottoming or sidebinding of the drip pan on
the discharge pipe undermaximum conditions of
expansion.

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Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
The discharge piping is also required to be short and straight as possible
and also designed to reduce stress on the safety valve body. It is notuncommon to find the outlet piping causing distortion of the valve bodywhich in turn causes the seat and nozzle to not properly align, therefore
causing leakage. The discharge piping should also be designed toeliminate condensation and water to gather in the discharge of the safetyvalve. Figure 3-6 illustrates an ideal installation with a short discharge
angled tailpipe that is inserted into, but not attached to, an externally
supported pipe riser.
Assemblers
There is wording in the Code that defines a manufacturer as the entitythat is responsible for meeting the design criteria to produce the valve
components that can be put together to build a valve that has beencertified by the testing requirements listed above. This approval by the
ASME designee to produce valves with a Code stamp symbol is specificto the manufacturer’s physical location.
To best serve the user community, the Code allows the manufacturer
to designate other locations that will inventory valve components
to efficiently build and test pressure relief valves that mirror those
produced at the manufacturer’s location. These organizations are called“assemblers,” and are allowed to assemble, adjust, test and code stamp
certified designs. They are required to use OEM parts to assemble
valves, and can only purchase these parts direct from the manufacturer
or another certified assembler. The assembler is required to use thesame assembly and test procedures as the manufacturer and is notallowed to machine or fabricate parts. An assembler may be owned by
the manufacturer, or be a separate entity.
As with the manufacturer’s location, an assembler has their quality
system reviewed and approved by an ASME designated third party,
such as the National Board. The assembler most likely will not be able
to produce all of the valves that are certified by the manufacturer per
the Code and they must define in detail what valve designs they canassemble and what, if any limitations, there may be in the actions taken
to configure these valve designs to meet the customer requirements.
As with the manufacturer, the Code requires that the assembler
demonstrate that each individual pressure relief valve or valve design
family where they are approved, be tested. Therefore, every six years,
two assembler built valves are chosen for each individual valve or
valve design family and are sent in for set pressure, capacity, and valve
stability testing. As with the manufacturer production valve testing, an
ASME designated third party, such as the National Board, is present towitness these production valve tests.
This assembler program is strictly to be used to provide new, notrepaired, pressure relief valves.

Nameplates
All pressure relief valves built in accordance with ASME Section I are
required to have specific information contained on a nameplate thatis attached to the valve. The manufacturer’s name along with theassembler’s name, if applicable, is to be shown. The rated capacity
is to be shown in superheated steam for reheaters and superheaters(see Figures 5-2 or 6-1), water and saturated steam for economizers,
and saturated steam for other Section I locations. Recall that this rated
capacity is 90% of that measured during certification testing at a flowingpressure at 3% overpressure or 2 psi (0.138 bar) whichever is greater.
The valve model number, set pressure and inlet size are also required
fields for the nameplate.
You can identify a pressure relief valve that has been certified to ASME
Section I by locating a “V” marked on the nameplate.
In addition to this nameplate identification, the PRV is required to haveall parts used in the adjustment of the set pressure and blowdown tobe sealed by the manufacturer or assembler. This seal will carry theidentification of which authorized facility built and tested the PRV.
Section VIII - Rules for Construction of Pressure Vessels
Scope
Division I of ASME Section VIII will provide rules for the new constructionof vessels which contain pressure that is supplied via an external sourceor pressure generated by heat input or a combination of both. Since thedesigns of these vessels can be numerous, it may be easier to provide
examples of what type of pressure containers might not be considered
an ASME Section VIII vessel. Some common examples can include thefollowing:• Vessels having an inside diameter or cross section diagonal not
exceeding 6” (152 mm) of any length at any design pressure.
• Vessels having a design pressure below 15 psig (1.03 barg).
• Fired tubular heaters.
• Components, such as pump casings or compressor cylinders, of amoving mechanical piece of equipment that are a part of the device
and designed to meet the working conditions of the device.
• Piping systems that are required to transport gases or liquids between
areas.
The reader should note that there may be local or country statutes that
determine whether or not a certain vessel is to conform to the rules of
ASME Section VIII.
The requirements for ASME Section VIII are less stringent than thosein Section I. It is permissible to use a PRV certified for Section I in any
Section VIII application provided than the design will meet all of therequirements of the application.

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Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
Figure 3-7
Typical Section VIII Single Device Installation (Non-Fire) - Set at the MAWP of the Vessel
Vessel Pressure %PRV Specifications

Vessel Specifications

Maximum
Accumulation

110

Accumulation(10%)

Overpressure(10%)

Set Pressure

100

Simmer Pressure

98

MAWP

Blowdown (8%)

Reseat Pressure

92

Leak Test Pressure

92

84

Possible
OperatingPressure

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Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
Figure 3-8
Typical Section VIII Single Device Installation (Non-Fire) - Set below the MAWP of the Vessel
PRV Specifications

Vessel Pressure %Vessel Specifications

110

Maximum
Accumulation

Overpressure(14%)

Accumulation
(10%)

100

Set Pressure

96

Simmer Pressure

94

MAWP

Blowdown(8%)
Reseat Pressure

88

Leak Test Pressure

86
84

Possible
OperatingPressure

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Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
Figure 3-9
Typical Section VIII Single Device Installation (Fire) - Set at the MAWP of the Vessel
PRV Specifications

Vessel Pressure %121

Overpressure(21%)

Vessel Specifications

Maximum
Accumulation

Accumulation(21%)

Set Pressure

100

Simmer Pressure

98

MAWP

Blowdown(8%)
Reseat Pressure

92

Leak Test Pressure

92

84

Possible
OperatingPressure

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Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
Figure 3-10
Typical Section VIII Multiple Valve (Non-Fire Case) Installation
Primary PRV Specifications

Supplemental PRV Specifications

Vessel Pressure %Vessel Specifications

116

Maximum
Accumulation

Supplemental PRV
Overpressure (10%)
Primary PRV
Overpressure (16%)

Primary PRV
Set Pressure
Simmer Pressure

Primary PRV
Blowdown (8%)

Accumulation (16%)
Supplemental PRV
Set Pressure

105

Simmer Pressure

103

Supplemental PRV
Blowdown (8%)

100

MAWP

98

Reseat Pressure

97

Leak Test Pressure

95

Reseat Pressure

92

Leak Test
Pressure

90

80

Possible
OperatingPressure

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Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
Figure 3-11
Typical Section VIII Multiple Valve (Non-Fire Case) Installation
Primary PRV Specifications

Supplemental PRV Specifications

Vessel Pressure %Vessel Specifications

121

Maximum
Accumulation

Supplemental PRV
Overpressure (10%)

Primary PRV
Overpressure (21%)

Supplemental PRV
Set Pressure

110

Simmer Pressure

108

Accumulation (21%)

Supplemental PRV
Blowdown (8%)

Primary PRV
Set Pressure
Simmer Pressure

Reseat Pressure

102

Leak Test
Pressure

100

MAWP

98

Primary PRV
Blowdown (8%)
Reseat Pressure

92

Leak Test
Pressure

90

80

Possible
OperatingPressure

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Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
Acceptable Designs
As with ASME Section I, reclosing direct acting spring loaded andreclosing self-actuated pilot operated pressure relief valves can be used
for Section VIII vessel protection. Unlike Section I, this part of the Codeallows the use of non-reclosing devices such as rupture disks, nonclosing direct acting spring loaded valves, and pin devices where the pinholds the pressure containing component closed. A combination of a
non-reclosing device mounted in series with a reclosing device can also
be an acceptable relieving system. There is also a choice to use simple
openings that flow or vent away excessive pressure.
Allowable Vessel Accumulation
There are different levels of accumulation that are permissible fora Section VIII vessel. When the source of overpressure is not being
generated by an external fire and there is one pressure relieving device
to be used, the vessel is allowed to experience an accumulation inpressure, during an upset condition, up to 10% over the maximum
allowable working pressure (MAWP). Most users desire the highest
possible set pressure to avoid unwanted PRV cycles. When a single
pressure relieving device is used, the maximum set or burst pressure
allowed is equal to the MAWP. In this case, the value of the vessel
accumulation and the device’s overpressure are the same (seeFigure 3-7). Therefore, the design of a pressure relief valve must allow
adequate lift to obtain the needed capacity within 10% overpressure.
Chapter 4 of the handbook will discuss how the design of a Section VIII
valve provides this needed lift with minimal overpressure.
The Code does allow this pressure relief device to be set below the
MAWP. When the device is set to open below the MAWP, it may be sizedusing the overpressure (the difference between the set or burst pressure
and the maximum allowable accumulation) as shown in Figure 3-8.
When a pressure vessel can experience an external fire that would
cause an overpressure condition, the Code allows for a maximum
accumulation of 21%. The rule is the same as the non-fire condition, inthat the maximum set or burst pressure for a single device installation
cannot be higher than the MAWP of the vessel. If a pressure relief valve
is selected, it typically will have the same operational characteristics as
the one selected for a non-fire relieving case. An overpressure of 21%
can be used to size this valve. See Figure 3-9.
There is no mandate in Section VIII that requires the use of multiple
relieving devices. However, in some applications it may be that therequired capacity to be relieved is too much for a single relieving device.
If more than one device is needed, the accumulation, for a non-firegenerated overpressure scenario, is to not exceed 16% above the MAWP.
This additional accumulation will allow for the multiple pressure relief
valves to be set at different pressures. As mentioned previously, thisstaggered set point regime will help to avoid interaction between themultiple PRVs. Similar to Section I, the rules are that a primary PRV
can be set no higher than the MAWP of the vessel. Any additional or
supplemental PRV can be set above the MAWP, but at a level no higher
than 5% above the MAWP. These multi-device rules in Section VIII willoftentimes allow for the operating pressure to remain at the samelevel as they would be with a single valve installation. Figure 3-10 will
illustrate this multiple PRV scenario. There is no requirement thatmultiple valves be of the same size, although this is often found to be
the case in order to best utilize the inventory of spare parts.

When multiple PRVs are required when the relieving case contingency
is heat input from an external source, such as a fire, the primary valve
can again be set no higher than the MAWP. Any supplemental valve canbe set to open at a pressure 10% above the MAWP. The overall vessel
accumulation that is allowed by the Code is now 21%. Please note that
if there are any non-fire case contingencies that are to be handled withthese multiple valves, any supplemental valve set above 105% of the
MAWP cannot be counted in the available relieving capacity. Figure 3-11
provides an example of multiple PRVs for fire cases.
Pressure Relief Valve Certification Requirements
As we learned in the Section I certification discussion, there are capacity
certifications required by the Code for specific valve designs or families.
These capacity tests are performed on saturated steam, air or anothertype of gas such as nitrogen for safety and safety relief valve designs
used for compressible fluids. If the design is to be used in steam and inany other non-steam vapor/ gas, then at least one capacity test must
be done with steam with the remainder of the tests to be performedon the non-steam vapor or gas. Any relief or safety relief valve used
for incompressible media must be capacity certified on water. If thesafety relief valve is to have certification on both compressible andincompressible media, then individual capacity tests with gas and withliquid are required.
The steam, gas, or liquid capacity tests are performed with 10% or 3 psi(0.207 bar) overpressure in most instances. Using this flowing pressure
criteria, the same three capacity tests outlined above for Section I can beincorporated.
• Specific valve design, size and set pressure testing (3 valves minimum).
• Specific valve design and size using the slope method (4 valvesminimum).
• Valve design family using the coefficient of discharge method (9 valvesminimum).
The same requirement to meet no more than a plus or minus 5%
variance in every capacity test is mandated in Section VIII. Once thespecific valve design or family testing meets this requirement, then the
rated capacity is taken as 90% of the values measured in the capacity
testing. It is this rated capacity that is used to size and select valves per
the ASME Section VIII procedures in Chapters 5 and 6.

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Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
Since non-reclosing devices such as rupture disks are allowed for use
in Section VIII, there may be occasion to use these devices upstream ofa pressure relief valve. This pairing of two pressure relieving devices
may be necessary to protect the valve from the process conditions andis discussed more thoroughly in Chapter 4. The rupture disk will add
additional resistance to flow through the PRD installation and there arecapacity testing methods in Section VIII to determine what is denotedas the combination factor used in sizing the devices. The non-reclosing
device and the PRV combination factor are based upon the capacity
testing of specific device designs. The capacity test of the two devices
in series will be done at the minimum set or burst pressure of the nonreclosing device. The combination factor is the capacity measured using
the two devices in series divided by the capacity measured in testing only
the PRV. Two more additional combination tests are then performed
and each capacity measured must fall within plus or minus 10% of theaverage capacity of the three tests used to establish the combination
factor. Additional tests can be performed to use larger non-reclosing
devices than the one initially tested and for establishing a combination
factor for higher set or burst pressures. If there are no combination
factors available via actual testing, then the PRV rated capacity is to be
reduced by 10% when any non-reclosing device is installed upstream ofthe valve.
If a non-reclosing device is used on the downstream connection of a
PRV, there is no combination flow testing required, nor is there any
required reduction in the PRD installation rated capacity.
In addition to establishing the rated capacities, the capacity certification
test procedure will also require that the PRV blowdown be recorded. As
you will learn in Chapter 4, there are designs of safety valves (used forcompressible fluids) that have fixed blowdowns and there are designs
that have adjustments to alter the blowdown. If the safety valve design
has a fixed blowdown, the reseat pressure is simply denoted after
testing. If the safety valve design has an adjustable blowdown, thenthe reseat pressure cannot be any more than 5% or 3 psi (0.207 bar],whichever is greater, below the set pressure. All relief or safety relief
valve designs for liquid service have fixed blowdowns and as such, thereseat pressures are simply recorded during these water capacity tests.
The Code requires that the manufacturer demonstrate that eachindividual pressure relief valve or valve design family tested per theabove requirements also provide similar operational performance whenbuilt on the production line. Therefore, every six years, two production
valves are chosen for each individual valve or valve design family forset pressure (see below for requirements), capacity, and blowdown(if applicable) testing. As with the initial certification testing an ASME
designated third party, such as the National Board, is present to witnessthese production valve tests.

Every valve built and assembled for production will be tested using oneof the three media listed above to meet this set pressure tolerance. The
capacity that is listed on the nameplate will indicate whether steam,gas, or water was used for this test. Actual service conditions, suchas a higher than ambient operating or relieving temperature and backpressure on some types of valve designs, may require an adjustment in
the test bench setting. In Chapter 4, you will read about closed springbonnet designs that can confine the high temperature and lower thespring rate. This may require the test bench setting to be higher so
the PRV will open at the right pressure in service. You will also learn
about constant superimposed back pressure and how this downstreampressure can cause some valve designs to open at a higher in situ
pressure. The test bench setting in this case may need to be lowered tocompensate. This production shop test bench pressure is called the cold
differential test pressure (CDTP) of the PRV.
There is no requirement within ASME Section VIII to test the blowdown
for a production PRV. With the Section VIII safety valves that have an
adjustable blowdown as described in Chapter 4, the typical reseatpressure will be 7% to 10% below the set pressure. The size and set
pressure of the production safety valve will determine the size of theaccumulation vessel needed to obtain the lift to check the blowdown.
Many manufacturing and assembly facilities do not have the large
vessels needed to perform this test. Therefore, many production safetyvalves have their blowdown adjustments set empirically based uponlaboratory type testing of the valve design. These same assembly shop
limitations are one reason there is no requirement for a production valve
to undergo a capacity test.
After the production PRV has undergone the set pressure testing, thetightness of the seat of the valve is examined. The seat tightness criteria
found in Section VIII is API Standard 527, which is discussed below, themanufacturer’s published specification, or a mutually agreed criteria
between the customer and manufacturer. You should note that the
manufacturer’s published specification may or may not meet the API 527
requirements.

Pressure Relief Valve Design Criteria
The set pressure tolerance for Section VIII pressure relief valves forsteam, gas or liquid service is as follows:
Table 3-2 - ASME Section VIII Set Pressure Tolerance
Set Pressure
psig (barg)Less than or equal to 70 (4.82)
More than 70 (4.82)

Tolerance (plus or minus)
from the set pressure
2 psi (0.137 bar)
3% of the set pressure

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Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
The proof tests of the pressure containing components are the sameas outlined above in Section I, where each production PRV will have thecomponents either hydrostatically tested at 1.5 times the design of thepart or pneumatically tested at 1.25 times the design of the part. This
proof test is now required even for non-cast pressure containing parts
such as bar stock or forgings where the test pressures could exceed
50% of their allowable stress. A pressure containing part made in a castor welded form will always be proof tested no matter what its allowablestress may be.
There is no restriction on the type of inlet or outlet connection that canbe provided. A Section VIII PRV may have threaded, flanged, socket
welded, hubbed, union, tube fitting or other type of connection. The only
size limitation is that the inlet of any liquid relief valve be a minimum of½” (DN 15). Any threaded PRV must have flats to allow a wrench to be
used to install the valve without damage or altering the set pressure.
Any pressure valves that are used in Section VIII applications, where theservice is air, steam or water (when the water temperature is greaterthan 140°F or 60°C when the PRV is closed) must have a device tocheck if the trim parts are free to move when the valve is exposed toa minimum of 75% of its set pressure. This device is a lift lever (seeFigure 3-3) for a direct spring loaded valve. A pilot operated valve may
also use this lift lever accessory but these designs can also incorporate
what is called a field test connection, where an external pressure can besupplied to function the valve (see Figure 3-4).
This lift lever or field test requirement can be removed via the use of acode case. Code Case 2203 will allow the end user to install a valve inthese three services without the lifting device provided the following is
met:• The user has a documented procedure and implementation programfor periodic removal of the PRVs for inspection, testing and repair asnecessary.
• The user obtains permission for this deletion from any possible
jurisdictional authorities.

ASME Section VIII allows the use of inlet block valves (See Figure 3-13) toisolate the PRV, provided there is a management system where closing
any particular individual or series of block valves does not reduce theavailable relieving capacity provided from other on-line pressure relief
devices. The non-mandatory appendix M will allow block valves, both
upstream and downstream of the PRV, that could provide complete orpartial isolation of the required relieving capacity. The purpose of theseblock valves is solely for the inspection, testing, and repair for the PRV.
In this appendix there are specific requirements to be followed:• Management procedures are in place to ensure there is no
unauthorized PRV isolation.
• Block valves are provided with a mechanical locking feature.
• Procedures are in place when the block valve has isolated the PRVthat will provide overpressure protection. This may simply be having
personnel monitor the vessel pressure at all time with the ability toprevent a continual pressure rise or be able to vent and reduce anypressure in the vessel.
• The block valve can only be closed for the time duration of theinspection, test or repair of the PRV.
• The block valve must be of a full area design so the capacity throughthe PRV is minimally affected.
It is recommended to install PRVs in an orientation where the moving
parts are vertical, primarily so there is no added frictional force that
could alter the opening pressure.
When a non-reclosing device such as a rupture disk is installed
upstream of a PRV, the Code requires that there be a method to monitor
the pressure in the space between the two devices. If pressure were to
be trapped between the disk and valve, the disk may not operate properly
to provide a full opening for the PRV to deliver the required capacity. A
bleed ring is oftentimes used with a pressure indicating device installedin the vent connection to monitor this space.

Pressure Relief Valve Installation
Unlike Section I, there are no limits to the maximum length of the inlet
piping that can be used to connect the PRV to the vessel. The area of
the inlet piping shall at least be equal to that area of the valve inlet. The
same area requirement is true for the outlet piping which must meet or
exceed the valve outlet size. If there are multiple valves mounted on one
connection, then this connection must have an area at least as large as
the multiple valve inlet area in total.
The longer the inlet piping from the vessel to the PRV, more resistanceto flow will occur due to non-recoverable pressure losses that areprimarily caused by friction. Since the Code allows unlimited inlet piping
lengths, fittings, and transitions, there is a statement in Section VIII
that will tell the designer to check this pressure drop so that it does not
reduce the available PRV capacity below the required amount. The Codealso points out that the actual functionality of a PRV can be affected bythese inlet line pressure losses. There is no limitation in the main bodyof ASME Section VIII regarding the magnitude of these non-recoverable
losses. However, a non-mandatory Appendix M of Section VIII will statea limit of 3% of the set pressure for these losses. These piping loss
calculations are to be done using the rated capacity of the PRV.
The same design cautions, without the 3% limitation provided for inlet
piping, are also denoted for the outlet piping from a PRV. In Chapter 4,
we will go into more details surrounding the proper design of inlet andoutlet piping for various types of PRVs.

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Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
When a rupture disk is used downstream of a PRV, the space between
the devices must be vented. These rupture disks are usually set to burst
close to atmospheric pressure. The PRV in this type of installation needs
what is called a balanced design (see Chapter 4) so that if any pressure
were to gather between the devices, it would not affect the set pressure
or lift characteristics of the PRV.

ASME B16.5 (2013) - Pipe Flanges and Flanged Fittings
This standard provides allowable materials, pressure/ temperature
limits and flange dimensions for standard flange classes. Most flanged
ended pressure relief valves will conform to these requirements but itshould be noted that there may be other valve components outside of theflanges that determine the overall design pressure for the PRV.

Assemblers
The information presented above for Section I will also apply to
Section VIII assembler program.

III. International Organization for Standardization (ISO)Nameplates
All pressure relief valves built in accordance with ASME Section VIII
are required to have specific information contained on a nameplatethat is attached to the valve. The manufacturer’s name along with theassembler’s name, if applicable, is to be shown. The rated capacity is
to be shown at an overpressure of 10% or 3 psi (0.207 bar] of the set
pressure. The unit of capacity will be reflected in mass flow rate ofsteam, or the volume flow rate of air or water depending upon the media
used to calibrate the set pressure of the PRV. Recall that this rated
capacity is 90% of that measured during certification testing. The valve
model number, set pressure and inlet size are also required fields forthe nameplate.
For pilot operated valves, this ASME nameplate is to be affixed to themain valve since this portion of the valve determines the capacity.
You can identify a pressure relief valve that has been certified to ASME
Section VIII by locating a “UV” marked on the nameplate.
In addition to this nameplate identification, the PRV is required to haveall parts used in the adjustment of the set pressure and blowdown, if
applicable, to be sealed by the manufacturer or assembler. This seal
will carry the identification of which authorized facility built and tested
the PRV.
ASME PTC 25 (2014) - Pressure Relief Devices Performance Test Code
When performing testing to certify a device to the ASME Code, theprocedures outlined in this document should be followed. This
performance code is presented in three parts with a general section thatprovides a reference for pressure relief device terminology in part I. The
second part provides requirements for the flow capacity testing process.
Finally the third part lays out acceptable techniques for observing theproper set pressure and lift of the valve’s seat. This last section is
also a reference used in the writing of acceptable procedures for the
production bench testing or in situ setting of the pressure relief valves.

ISO 4126 - Safety Devices for Protection Against Excessive Pressure
To begin with some background, as part of the standardization process
within CEN (Comité Européen de Normalisation), work started back inthe early 1990’s on preparing product standards to replace those as thenused by the various European national bodies.
Working group 10 of the CEN Technical Committee 69 (Industrial
Valves) was formed with a view to prepare new EU (or EN) standards
for safety devices for protection against excessive pressure. After many
years of work within both CEN and ISO (International Organization forStandardization) with joint voting through what is called the Vienna
Agreement, the following cooperative EN and ISO standards wereprepared. In this chapter, we will refer to these standards as ISO
documents to reflect their global reach.
• ISO 4126-1 - Safety Valves (Spring Loaded)• ISO 4126-2 - Bursting Disc Safety Devices• ISO 4126-3 - Safety Valves and Bursting Disc Safety Devices in
Combination• ISO 4126-4 - Pilot Operated Safety Valves• ISO 4126-5 - Controlled Safety Pressure Relief Systems (CSPRS)• ISO 4126-6 - Application, Selection and Installation of Bursting Disc
Safety Devices• ISO 4126-7 - Common Data
• ISO 4126-9 - Application and Installation of Safety Devices ExcludingStand-Alone Bursting Discs• ISO 4126-10 - Sizing of Safety Valves for Gas/Liquid Two-Phase Flow

ASME B16.34 (2009) - Valves - Flanged, Threaded and Welding End
This standard covers pressure, temperature ratings, dimensions,tolerances, materials, non-destructive examination requirements,and marking for cast and forged valves. This standard is not directly
applicable to pressure relief valves but it is often used by manufacturers
as good engineering practice.

Figure 3-12
Typical ASME Section VIII Nameplate

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Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
The above standards, in their entirety, provide related requirements
and recommendations for overpressure protection devices that arefound in the ASME Boiler and Pressure Vessel Code and API Standards
and Recommended Practices. Currently, there is work underway for
ISO 4126-11 that will provide standards for the performance testing ofvarious overpressure protection devices.
The intent in this handbook is to draw your attention to some of therequirements found in the ISO standards that may differ from theprevious ASME Code discussion. This dialog is not meant to be acomplete comparison but a highlight of these items. We will be using
the third edition of 4126-1 (July 15, 2013), the second edition of 4126-4(July 15, 2013), the second edition of 4126-5 (July 12, 2013) and thefirst edition of 4126-9 (Apr 15, 2008) as references for the information
presented.
Scope
The scope of the ISO 4126 standards begin at a set pressure of 1.45 psig
(0.1 barg]. This is significantly lower than the scope of ASME Code. Thereis no distinction in the ISO product standards, such as ISO 4126-1, thatchange the performance criteria for safety valves used on fired vessels(ASME Section I) versus unfired vessels (ASME Section VIII).
These standards are centered on pressure relief device products and do
not address the equipment they are protecting. Therefore, one shouldreference the applicable design standard for the vessel, pipe, or otherpressure containing component to determine requirements such as
the allowable accumulation.
Allowable Vessel Accumulation
The requirement in ISO 4126-9 is that the maximum vessel accumulation
is to be defined by the applicable local regulation or directive. If
there is a need for multiple pressure relief devices, perhaps due tocapacity requirements, then one device must be set at no higher thanthe maximum allowable pressure set forth by the local regulations.
Any additional devices can be set up to a maximum of 5% above thismaximum allowable pressure. If local regulations allow, such as afire contingency, these set pressures may be higher than 105% of the
maximum allowable set pressure.

The ISO standard requires that flow testing be demonstrated whena valve is designed to operate when the total back pressure is morethan 25% of the set pressure. These tests establish a curve of theflow coefficient of the valve versus the back pressure ratio. This leads
to some differences in the way to approach sizing of PRVs underback pressure conditions between API and ISO standards. In API, allcorrections (Kb factor) to the flow due to the back pressure may be fromboth mechanical and fluid flow effects (see Chapter 7 or 8). In ISO, theback pressure correction factor (Kb) includes only the fluid flow effect,
and the flow coefficient (Kd) includes the correction for the mechanicaleffect that is discussed in Chapter 4. The sizing procedures in Chapter 5
and 6 will use the API approach for K, Kd, and Kb factors. However, muchof the Kb data is derived and based upon ISO back pressure testing so
the output from the API approach will be the same as the result from theISO procedures.
There are PRV/non-reclosing combination tests described in ISO 4126-3.
There are also functional test requirements where the PRVs must meetthe set pressure criteria listed below. In addition, every direct acting or
pilot operated valve design must demonstrate a blowdown on steam or
gas to fall within 2% to 15% or 4.35 psi (0.3 bar), whichever is greater.
There is no distinction between valves with a fixed blowdown or thosehaving an adjustable blowdown. For liquid service, the blowdown must
fall between 2.5% to 20% or 8.70 psi (0.6 bar), whichever is greater.
In either media, if the PRV is a modulating type design, the minimum
blowdown can be less. You may recall that ASME has no minimum or
maximum blowdown requirement for fixed blowdown valve designs.
There is no requirement in ISO 4126 for any follow-up or renewal
capacity testing as there is required for ASME.

Acceptable Valve Designs
Depending upon the application, there is the possibility of having morechoices in the selection of a pressure relieving device in these ISO
standards versus the ASME Code. The requirements for the use of thedirect acting and pilot operated PRV designs are outlined in 4126-1 and4126-4 respectively. For example 4126-1 discusses the use of external
power sources that provide additional seat loading to that provided by the
spring, and points the user to ISO 4126-5 for the details on using a device
called a controlled safety pressure relief system.
Pressure Relief Valve Certification Requirements
There are capacity certification tests that are similar to ASME on steam,gas and water if applicable. The coefficient of discharge method isused to compare tested flows at 10% or 1.45 psi (0.1 bar) overpressure,whichever is greater, to that of an ideal nozzle. There are multiple tests,
anywhere from 4 tests to 9 tests as a minimum, and the ratio fromeach test must fall within plus or minus 5% of the average. These tests
can be performed for a specific valve size or valve design family. Oncea coefficient of discharge is established by test, the rated coefficient isreduced by 10% as it is with ASME.

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Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
Pressure Relief Valve Design Criteria
The set pressure tolerance varies little from the ASME Section VIII
requirements. For the direct acting and pilot operated PRVs, the range isplus or minus 3% or 2.18 psi (0.15 bar), whichever is greater. There areno significant design criteria differences with the ASME Code.
There is no restriction on the type of end connections that can be
specified.
The seat tightness criteria is to be agreed upon between the user andmanufacturer.
ISO 4126 requires a proof test that is similar to ASME. No matter what
the design of the primary pressure containing parts, this portion of
the valve is to be hydrostatically (or pneumatically) tested to 1.5 times
its design pressure with duration times that lengthen as the size anddesign pressures increase. One other difference with ASME is that thesecondary pressure containing zone on the discharge side of the valve
is also proof tested to 1.5 times the manufacturer’s stated maximum
back pressure to which the valve is designed. ASME requires a 30 psig
(2.06 barg) test pressure for valves discharging into a closed header
system, which is normally less than the ISO test requirement.
There is no mandate in the ISO documents for valves to have lifting
devices, as shown in Figure 3-3, for any service conditions.

This inlet loss is to be calculated using the rated capacity of the valve.
There is also a requirement that the difference between the valve
blowdown and the inlet loss be at least 2%. An exception to these inlet
loss limits is allowed using remote sensed pilot operated PRVs. This typeof installation is discussed in Chapter 4.
Nameplates
There are some additional items of information in the ISO standards
versus ASME with regards to identification on the valve nameplates. For
example, a direct acting safety valve needs to be marked “ISO 4126-1”
and a pilot operated valve marked “4126-4”. The derated nozzle
coefficient, designated with the applicable fluid (gas, steam or liquid),
is to be shown on the nameplate, as well as the actual flow area andminimum lift.

Pressure Relief Valve Installation
In ISO 4126-9 there are no limits to the maximum length of the inlet
piping that can be used to connect the PRV to the vessel. The area of
the inlet piping shall at least be equal to that area of the valve inlet. The
same area requirement is true for the outlet piping which must equal or
exceed the valve outlet.
The allowance for the use of an isolation valve for a pressure relief
device has a significant difference to those requirements in ASME
Section VIII Appendix M. The source of the pressure for the vessel being
protected must itself be blocked. See Figure 3-13 to help illustrate thispoint. It should be noted that the vessel may still have an overpressure
contingency due to external fire or thermal relief.
As discussed above, the longer the inlet piping from the vessel to the
PRV, the more resistance to flow will occur due to non-recoverablepressure losses that are primarily caused by friction. If there is no
local regulation specification being used, ISO 4126-9 will require thatthe non-recoverable inlet line loss have a maximum value of 3% of theset pressure or 1/3 of the valve’s blowdown, whichever may be less.

Figure 3-13
Isolation Valve Requirements

PRV
PRV

PRV

PRV

Isolation Valve

Isolation Valve

Isolation Valve

VesselPressure
SourceIsolation Valve

VesselPressure
SourceISO 4126-9
VesselASME Section VIII Appendix M

Vessel3.20

Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
ISO 23251 (2006) - Petroleum and Natural Gas Industries - Pressurerelieving and Depressuring Systems
This document provides the process engineer with guidance on how toanalyze potential sources of overpressure and to determine the required
relieving loads necessary to mitigate the potentially unsafe scenario.
There is no minimum pressure in the scope, but most of the information
presented will deal with process equipment that have design pressuresequal to or above 15 psig (1.03 barg).
The document will refer to ISO 4126 and API Recommended Practice
520 part I for the sizing of the pressure relief device orifice. These sizing
procedures will be discussed in Chapters 5 and 6.
The standard will also provide information on determining the required
specifications for the fluid disposal systems downstream of the pressure
relieving device. For example, the design basis for determining therelieving loads into this downstream piping are listed in Table 3-3. The
definition of a lateral pipe is that section where a single pressure relief
device is attached, as shown in Figure 3-14. It should be noted that if
the required relieving rate is used for the pressure drop calculation andthe requirements should change, then the lateral piping pressure dropshould be recalculated.
This document will also describe guidelines used to estimate the noiseproduced by an open PRV vent to atmosphere via a vent stack. Thismethodology is found in Chapters 5 and 6.

Figure 3-14
PRV Discharge Piping Example

PRV

PRV

To Flare and/or Burner
Stack to Atmos

LateralHeaders

Closed Discharge
Header System

LateralHeaders
LateralHeaders

PRV

LateralHeaders
r
de
ea
H
ain
M

Protected Systems

ISO 28300 (2008) - Petroleum and Natural Gas Industries - Venting ofAtmospheric and Low Pressure Storage Tanks
The scope of this standard is the overpressure and vacuum protection of
fixed roof storage tanks that have a design from full vacuum to 15 psig
(1.03 barg). This document is very complete in that it encompasses
the process of examining what can cause the tank design pressure or
vacuum to be exceeded, much like the ISO 23251 standard, all the way
to methods of certifying and testing relief devices. There are techniques
described in the standard to provide required relieving rates forthese pressure and vacuum contingencies and sizing procedures arepresented to select the required flow orifices of the relieving devices.
The types of devices discussed in the standard are simple open pipe
vents, direct acting weight loaded and spring loaded vent valves, andpilot operated pressure relief valves. In addition, there is guidance on
the proper installation of these devices.

Table 3-3 - Design Basis for Sizing Downstream Piping
Pressure Relief Device
Direct Acting Spring Loaded PRV
Pop Action POPRV
Modulating POPRV
Non-reclosing Device

Lateral Header
PRV Rated Capacity
PRV Rated Capacity
Required Relieving Rate
Required Relieving Rate

Main Header
Required Relieving Rate
Required Relieving Rate
Required Relieving Rate
Required Relieving Rate
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Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
IV. European Union Directives
Pressure Equipment Directive (PED) 97/23/EC (May 1997)
Pressure equipment, which includes pressure relief valves, installed
within any country of the European Union (EU) since May 28, 2002, mustcomply with the Pressure Equipment Directive (PED). Please note that
there may also be countries, such as Norway, Switzerland, or Turkey,
that are not in the EU, which may require compliance with the PED.
The PED applies to any pressure equipment and assembly of pressure
equipment with a maximum allowable pressure above (and excluding)
7.25 psig (0.5 barg). However the following applications are excluded
from the scope of the PED:
• Pipelines for the conveyance of any fluid to or from an installation
starting from and including the last isolation device located within
the installation.
• “Simple pressure vessels” covered by what is known as the EU
Directive 87/404/EEC. These are basically welded vessels intended
to contain air or nitrogen at a gauge pressure greater than 7.25 psig
(0.5 barg), not intended for exposure to flame, and having certain
characteristics by which the vessel manufacturer is able to certify it asa “Simple Pressure Vessel”.
• Items specifically designed for nuclear use, the failure of which may
cause an emission of radioactivity.
• Petroleum production well head control equipment including thechristmas tree and underground storage facilities.
• Exhausts and inlet silencers.
• Ships, rockets, aircraft and mobile offshore units.
Any other equipment with a maximum allowable pressure higher than
7.25 psig (0.5 barg) falls within the scope of the PED, including the“safety devices,” such as the pressure relief valves and rupture disks,
protecting this equipment. The PED applies to both power boilers andprocess pressure and storage vessels.
The certification of equipment in compliance with the PED is throughwhat are called notified bodies, which are approved to carry out
these certifications by the European authorities. There are different
certification processes available, depending on the type of productand its potential applications. The choice for the type of certification
processes lies with the manufacturer, but the level of certification will
depend on the intended use of the equipment. Most of the pressure
safety devices will have to be certified for the highest level, level IV,
except for pressure safety devices that are designed solely for one typeof specific equipment, and this equipment itself is in a lower category.
Equipment that is certified in compliance with the PED will have to bear
the “CE” (Conformite Europeene or European Conformity) mark on its
nameplate. However, the CE mark implies also that the equipment isin compliance with any EU directive that may apply to this equipment(like for example the directive on explosive atmosphere to be discussed
below). Therefore the CE mark ensures to the user that the equipment
complies with any of the applicable EU directives.
It is illegal to affix the CE mark on a product that is outside of the scope(such as a vacuum breather vent) of the PED or any other directive forwhich the CE marking would show compliance. However, it is possible toaffix the CE mark on a product destined to a country outside of the EU, aslong as the product itself is within the scope of the PED.
There are some noticeable differences with other codes shown in thishandbook. These include the following.

• PED applies to both fired and unfired vessels.
• There is no imposition on the minimum quantity of pressure safety
devices that protect a specific type of equipment.
• There is only one vessel accumulation allowed: 10%, with no fixedminimum (i.e. a storage vessel with a maximum allowable pressure of
9 psig (0.62 barg) will have an accumulation of 0.9 psig (62.0 mbarg)).
This applies to all cases, including when several pressure safety
devices protect the same equipment, but it does not apply to the “firecase.” For “fire case” relieving scenarios, the accumulation selectedby the equipment designer has to be proven safe for example (there
is no loss of containment). Therefore, the “proven safe” level may belower, higher or equal to 21% that is often used in ASME applications.
PED does not address sizing of the pressure relief valve, nor any sortof capacity certification.
• The scope of the PED is for new construction of equipment. This meansthat repairs are not within the scope of the PED, provided these repairs
do not significantly change the characteristics of the product.
• All pressure containing parts have to be pressure tested at 1.43 times
the maximum allowable pressure or 1.25 the stress at pressure andtemperature, whichever is higher. For pressure relief valves, thismeans that the outlet zone, outside of the primary pressure containing
area of the valve, needs also to be pressure tested.
Parts 1, 2, 4 and 5 of the ISO 4126 standards discussed previously areharmonized to the PED. In the EU, these standards are referred to as
EN 4126. This means that they include requirements which addresssome of the mandatory Essential Safety Requirements (ESR) of the PED.
In each of the EN version of these standards, there is an informative
annex, Annex ZA, that shows the relationship between clauses of the
EN standard and the Essential Safety Requirements of the PED. Thisis a necessary part of the EN 4126 version but is not required within
the ISO 4126 version as the PED is not mandatory outside of the EU.
Following these harmonized standards for the design, construction andtesting of the pressure relief valve will give presumption of conformity
to the PED (to the extent of the scope of the Annex ZA of the standard)
but it is not mandatory to follow the EN 4126 standards to comply with
the PED. The EN 4126 standards give one way, amongst many, to comply
with some of the requirements of the PED. As long as the standards orcodes used, such as ASME Section VIII, meet all the requirements of the
PED to the satisfaction of the notified body, then valves can be suppliedwith a CE mark.

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Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
ATEX Directive 94/9/EC (March 1994, Updated October 2013)
Since July 1, 2003, the 94/9/EC directive, also known as ATEX 100a, ismandatory for all equipment and protective systems intended for use inpotentially explosive atmospheres in the EU. It covers not only electricalequipment but also non-electrical devices such as valves.
Like the PED, ATEX 94/9/EC is a “product oriented” directive andmust be used in conjunction with the ATEX “user” Directive 1999/92/
EC. This directive helps the user to identify the zones of his facilities inaccordance with their risks of having a potentially explosive atmosphere:• Zone 0 = explosive atmosphere is continuously, or frequently present.
• Zone 1 = explosive atmosphere is likely to occur during normal
operation, occasionally.
• Zone 2 = explosive atmosphere is unlikely to occur, and if it does it will
be only for a short period.
• Zones 20, 21 and 22 = equivalent as above but for atmospheres ladenwith dusts like mines, grain silos…
ATEX 94/9/EC defines the Essential Safety Requirements for products
into groups and categories:• Group I = mining applications.
- Category M1 = suitable for very high risks; Category M2 = suitable forhigh risks.
• Group II = non-mining applications- Category 1 = suitable for very high risks; Category 2 = suitable forhigh risks; Category 3 = suitable for normal risks.
Putting the 2 directives together:• Products certified in Category 1 can be used in any zone 0, 1, or 2.
• Products certified in Category 2 can be used in zones 1 or 2. Products
certified in Category 3 can be used only in zone 2.
• And similarly with the category M1 for zones 20, 21 or 22 and M2 for 21
or 22.
Like for the PED, certification of a product in accordance to the ATEX
94/9/EC is done by the notified bodies. When a product is certified, its
nameplate will bear the CE mark, plus the symbol followed by its group,
its category and a “G” if the atmosphere is gas or “D” if the atmospherecan be laden with dust.

API Standard 521 (Sixth Edition January 2014) - Guide to Pressure
Relieving and Depressuring Systems
This document provides the process engineer with guidance on how toanalyze potential sources of overpressure and to determine the required
relieving loads necessary to mitigate the potentially unsafe scenario.
There is no minimum pressure in the scope, but most of the information
presented will deal with process equipment that have design pressuresequal to or above 15 psig (1.03 barg).
The document will refer to API Recommended Practice 520 part I for thesizing of the pressure relief device orifice. These sizing procedures will
be discussed in Chapters 5 and 6.
The standard will also provide information on determining the required
specifications for the fluid disposal systems downstream of the pressure
relieving device. For example, the design basis for determining therelieving loads into this downstream piping are listed in Table 3-3. The
definition of a lateral pipe is that section where a single pressure relief
device is attached, as shown in Figure 3-14. It should be noted that if
the required relieving rate is used for the pressure drop calculation andthe requirements should change, then the lateral piping pressure dropshould be recalculated.
This document will also describe guidelines used to estimate the noiseproduced by an open PRV vent to atmosphere via a vent stack. Thismethodology is found in Chapters 5 and 6.

V. American Petroleum Institute (API)API Standard/Recommended Practice 520 - Sizing, Selection andInstallation of Pressure Relieving Devices
This document is divided into two parts. Part I is denoted as a standard
and is in its ninth edition that is dated July of 2014, which focuses on thesizing and selection of the devices. Part II is denoted as a recommended
practice and is in its fifth edition and is dated August of 2003. Part II
provides guidance for the proper installation of the pressure relieving
devices.
The scope of API 520 deals with pressure vessels with a MAWP of 15 psig
(1.03 barg) and above. The relieving devices discussed are designed for
unfired vessels, such as those listed as ASME Section VIII. The power
boiler safety valves are not part of the scope.
The ASME Section VIII Code is heavily written around valve design andcertification requirements. There is little information on advantages
and disadvantages of using one type of pressure relieving device versus
another for a particular set of conditions. API 520 part I fills in this type of
information and much of the discussion in Chapter 4 of this handbook istaken from this standard.
The sizing techniques listed in API 520 part I ninth edition will be
discussed in Chapters 5 and 6.
We will review some of the installation guidelines of part II in later
chapters of this handbook.

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Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
API Standard 526 (Sixth Edition April 2009) - Flanged Steel Pressure
Relief Valves
This is really a purchasing standard that is commonly used to specify
process industry pressure relief valves. When a company requires avendor to build a valve to this standard, then known standardized piping
envelope dimensions and minimum orifice sizes will be provided. This
document is probably best known for the “lettered” orifice designations
that are listed, such as a “J”, “P”, or “T” orifice. Once a letter designationis specified, the manufacturer knows what minimum orifice size isrequired. The use of these lettered orifices in sizing valves will be
discussed in several upcoming sections of the handbook.
API 526 will also list bills of materials that would be valid for certain
set pressures and temperatures for different valve designs and sizes.
Of course, the process fluid would have to be considered for a final
material selection.
The scope of API 526 covers flanged direct acting and pilot operatedpressure relief valves. There can be dimensional differences between
similar inlet, outlet and orifice sizes of these two different valve designs.
Table 4-3 in Chapter 4 will illustrate these differences.
API Standard 527 (Fourth Edition November 2014) - Seat Tightness of
Pressure Relief Valves
This standard has been and will be mentioned several times in thishandbook. It is one method listed in ASME Section VIII to test for seatleakage at certain operating pressure levels. The requirements in thisstandard are not used for ASME Section I valves. Manufacturers may
have alternative methods to check for leakage, so it is advisable to havea common understanding of what the expectations will be with regard
to this test. The scope of this document begins with valves that have set
pressure of 15 psig (1.03 barg) and above.
In API 527, a typical test set up is shown in Figure 3-15 for closed bonnet
valves used in compressible media. If the set pressure of the valve isgreater than 50 psig (3.45 barg) then the pressure at the inlet of the valve
is brought up to 90% of the set pressure. Depending upon the valve size,
this pressure is held anywhere from 1 to 5 minutes. An outlet flange
cover or membrane that would rupture if a PRV would accidentally openis then installed. A port from the cover will provide a conduit for any seatleakage to be read one-half inch (13 mm) below the surface of the waterin its container.
A metal seated valve is allowed to leak in these operating conditions asshown in Table 3-4. A soft seated valve is required to have no bubble
leakage at any set pressure and orifice. Chapter 4 will discuss these twodifferent seat designs.

The leakage rates in Table 3-4 would be similar to 0.60 standard cubicfeet (0.017 standard cubic meters) per day for the 40 bubbles per minuterate up to 1.5 standard cubic feet (0.043 standard cubic meters) per day
at 100 bubbles per minute.
Prior to testing steam safety valves, any condensate should be removed
from its outlet. Once the test pressure is reached, there should be no
visible or audible leakage during the one minute hold time.
For incompressible service relief valves, water used as the test media
is collected after the one minute test. The acceptance criteria for metal
seated valves is that there be no more than 0.610 cubic inches or 10
cubic centimeters per hour of leakage for any valve with an inlet lessthan 1 inch (25 mm). For larger valves, the criteria is that the collectedwater not exceed 0.610 cubic inches or 10 cubic centimeters per hour
per inch of the nominal inlet size. All soft seated valves are to have zeroleakage.
API Standard 2000 (Seventh Edition March 2014) - Venting
Atmospheric and Low Pressure Storage Tanks
This standard is similar to, but not exact as, the ISO 28300 document
discussed earlier. These two documents were actually co-branded at
one time but are now stand alone.
The scope of API 2000 is the overpressure and vacuum protection of
fixed roof storage tanks that have a design from full vacuum to 15 psig
(1.03 barg). This document is very complete in that it encompasses
the process of examining what can cause the tank design pressure or
vacuum to be exceeded, much like the API 521 standard, all the way tomethods of certifying and testing relief devices. There are techniques
described in the standard to provide required relieving rates forthese pressure and vacuum contingencies and sizing procedures arepresented to select the required flow orifices of the relieving devices.
The types of devices discussed in the standard are simple open pipe
vents, direct acting weight loaded and spring loaded vent valves, andpilot operated pressure relief valves. In addition, there is guidance on
the proper installation of these devices.

Figure 3-15
API 527 Leak Test for Gas Service½”(13 mm)¼” (6.4 mm)

90% of Set Pressure

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Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
During the process of co-branding the document with ISO, there
were some notable changes from earlier editions of API 2000 that are
now in the seventh edition. Specifically, the venting loads caused byatmospheric temperature changes that cause vapors in the tank to
expand or contract may be quite different than previous editions. In theseearlier editions of API 2000 these venting rates were narrowly based
upon a service fluid similar to gasoline with limitation on tank size andoperating temperatures. Input from European Norms (EN) Standard
14015 that allows for the calculation of thermal venting for any service
fluid, tank size or physical location are now reflected in the seventh
edition. One of the most notable differences is that there may be a muchgreater requirement for the inbreathing rates that affect the size of
vacuum vents. There is an Annex A in the seventh edition that allows forthe calculation of venting requirements per previous editions. The user
should be aware of the applicability of using this Annex.
API Recommended Practice 576 (Third Edition November 2009) Inspection of Pressure Relieving Devices
This document provides guidance for the inspection and repair of all
direct acting (spring and weight) loaded PRVs, and pilot operated PRVs.
It will also discuss the inspection of non-reclosing devices. The root
causes that affect the performance of the devices is an important section
to review.
One common question asked is how often should the pressure relieving
device be inspected. API 576 and many other publications may givesome maximum intervals which may be as long as ten years. However,there will always be some caveat that these intervals will depend upon
the particular service conditions and performance history of the device.
These intervals should be adjusted according to this historical record.

The use of vacuum breather vents is called out to provide an incoming
source of pressure, typically ambient, be provided so that the tank will
not exceed its vacuum design rating.
As with many of the documents, this standard requires that the opening
from the tank to the relieving device be at least as large as the nominal
pipe size of the device. If there is any discharge piping, it must be atleast as large as the area of the valve outlet. Block or isolation valves
are allowed but there must be a lock or seal to prevent tampering. If theblock valve is closed and the tank is in service, an authorized individual
must remain at the installation to monitor the tank pressure.
API Standard 625 (First Edition August 2010) - Tank Systems forRefrigerated Liquefied Gas Storage
This new standard expands upon the tank construction details outlined in
API 620. The API 625 document discusses the entire storage tank system
requirements that can be unique to products that require refrigeration
temperatures down to and below 40°F (5°C) so that the fluid remains ina liquid state. These products may be liquefied oxygen, nitrogen, naturalgas, ethylene, propane, or ammonia. API 625 discusses the possible
need for items such as foundation heating, secondary containment
areas, insulation spaces, and instrumentation to monitor level,temperature and leakage in order to ensure safe and reliable operation.

API Standard 620 (Twelfth Edition November 2014) Design andConstruction of Large, Welded, Low Pressure Storage Tanks
API 620 deals primarily with carbon steel above ground storage tankswith a gas or vapor space not exceeding 15 psig (1.03 barg). In addition
to the carbon steel tank construction practices, there is an Appendix Q
that provides design guidance for refrigerated storage tanks requiring
more special materials and testing to store liquefied hydrocarbon gases
to -270°F (-168°C).
The standard will direct the designer to refer to API 2000 to determine
the required relieving capacities and for guidance on device selection.
Each tank is required to have pressure relieving devices to flow thisrequired capacity at a pressure no higher than 10% above the tank’s
design pressure. If there is a fire contingency, then the device can besized for a tank accumulation of 20% above its design pressure.

Table 3-4 - API 527 Leakage Rate Acceptance for Metal Seated PRV (Gas Service)
Set Pressure range
Up to 1000 psig (70.0 barg)
1500 psig (103 barg)2000 psig (138 barg)2500 psig (172 barg)
3000 psig (207 barg)
4000 psig (276 barg)
6000 psig (414 barg)

Effective Orifice 0.307 in2 (198 mm2) or lessleakage in bubbles per minute
40
60
80
100
100
100
100

Effective Orifice greater than 0.307 in2 (198 mm2)
leakage in bubbles per minute
20
30
40
50
60
80
100
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Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
With regards to overpressure and vacuum relief devices, the standard
will refer the user to API 2000 which was discussed previously. The
standard also points out that there may be local requirements, such as
the NFPA documents to be reviewed in the next section, that may be
applicable.
One additional requirement for the relief devices in API 625 that is notfound in API 2000 is that one spare pressure and vacuum vent valve isrequired to be mounted on the tank for maintenance needs.
If there is insulation installed via what is called a suspended deck, thenthe inlet piping of the pressure vent valve mounted to the roof of the tankmust run through the deck into the cold space of the tank system. Thispiping will channel the cold vapors directly to ambient and not allow the
low temperature vapor to contact locations of the tank system that may
not be designed for exposure to these cold conditions.
API Standard 650 (Twelfth Edition March 2013) - Welded Steel Tanks
for Oil Storage
Tanks designed to this standard normally have design pressures very
close to atmospheric conditions. The standard will allow the use of a
fixed roof, where venting devices are required, or a roof that floats on
the inventory of the fluid being stored, which typically will not requirerelieving devices. There is little information in the standard regarding
sizing and selection of relieving devices other than referring the designer
to API 2000. One interesting feature of some of the fixed roof designs
discussed in the standard is that the attachment of the roof to the walls
or shell of the tank can be designed to break or give way at a certain
known pressure. This is called a frangible roof joint and this literal
damage of the tank can provide adequate opening for emergency firerelief scenarios.
VI. National Fire Protection Agency (NFPA)
This US-based organization was established over 100 years ago toprovide fire prevention guidance via recommended practices, codes,
and standards. Since external fire or heat input is often a source of
overpressure for vessels and equipment, there are several of these
NFPA codes that may be used to size and select pressure relieving
devices.

NFPA 58 - Liquefied Petroleum Gas Code (2014 Edition)
This code can apply to tanks and piping that are used to provide propane,or similar hydrocarbon having a vapor pressure not exceeding that ofpropane, to a building as use for fuel gas. The scope also applies tothe over-the-road transportation, many marine terminals and onshore
pipeline storage tanks that handle this type of liquid petroleum. Marine
terminals tied to refineries, petrochemical plants and gas plants are notconsidered in the scope. The user should refer to the latest edition forother exceptions. These vessels or storage tanks can be refrigerated or
non-refrigerated.
Where storage vessels are built for use at 15 psig (1.03 barg) and above,
then these vessels are to be designed per ASME Section VIII. These
vessels are to have pressure relief valves designed to open on vapor
service. Direct acting spring loaded valves are required up to a vessel
volume of 40,000 gallons (151 m3) of water capacity. Above this volume,
either direct acting or pilot operated pressure relief devices are allowed.
Storage vessels built for use below 15 psig (1.03 barg) are to be designed
per API Standard 620.
There is methodology to determine the required relieving rates for
unrefrigerated and refrigerated tanks. In Chapter 5, we will review thefire sizing steps for refrigerated tanks. There are also requirements forthe use of thermal relief valves for piping systems.
For non-refrigerated tanks, isolation valves are not allowed unless two
or more pressure relief valves are installed on a manifold and only onepressure relief valve can be isolated. The remaining active pressure
relief valve must have adequate capacity to meet the requirements.
There are to be no isolation valves on the outlet piping. Any stress thatis excessive on the discharge piping is to be mitigated by failure on the
discharge side of the pressure relief valve without affecting the PRV
performance.
For refrigerated tanks, an isolation valve can be provided but it must bea full bore design that is lockable in the open position. There must be
another pressure relief valve on line, either via a three way diverter valve(see Figure 3-5) or via a separate tank penetration.
Pressure relief or vacuum valves on refrigerated tanks must be replacedor tested a minimum of once every five years. The minimum testing
interval for non-refrigerated tanks is ten years.

NFPA 30 - Flammable and Combustible Liquids Code (2014 Edition)
This document deals with recommendations for the safe handling andstorage of combustible or flammable liquids. These liquids would havea possibility of generating vapors that, when mixed with air, could forma mixture that could ignite. The document will provide guidance for thedetermination of vapor generation due to external fire to bulk storage
tanks. Most of the focus is on low pressure storage tank applications
less than 15 psig (1.03 barg). This technique will be discussed in Chapter
5. The sizing of the venting devices is to be in accordance with API 2000
or other locally accepted standards.

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Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
NFPA 59A - Standard for the Production, Storage, and Handling ofLiquefied Natural Gas (LNG) (2013 Edition)
As with NFPA 58, this standard also states that the vessel or tank is to be
designed per ASME Section VIII or API 620 depending upon the pressure
conditions.
Any location that liquefies natural gas, or subsequently stores andvaporizes, is subject to this standard. Portable storage or LNG fueledvehicles or vehicle fueling stations are not in the scope.
As with the other NFPA documents above, there is guidance on how toestimate the required relieving rates for various overpressure or vacuum
contingencies. The fire sizing methodology is discussed in Chapter 5. The
same isolation valve requirements for refrigerated tanks shown in NFPA
58 is repeated in NFPA 59A. Pressure relief valves on LNG storage tanks
or vessels are required to be tested a minimum of every two years.
VII. National Board of Boiler and Pressure Vessel
Inspectors
This organization was established in 1919 to provide standardization inwhat the organization calls “post-construction” activities such as theinstallation, repair, and inspection of boilers and pressure vessels. As
we noted above, the ASME Codes are used for the new construction ofboilers and pressure vessels. Commonly referred to as the “National
Board,” the organization is primarily comprised of US state or local
chief inspectors, Canadian province chief inspectors, insurancecompanies, end users and original equipment manufacturers.
National Board Inspection Code (NBIC) 23 (December 2013)
NBIC 23 is provided to give guidance to inspect and repair pressure
containing equipment for their safe, continued use. The code is written
in three parts, the first dealing with proper installation, the second
describes inspection practices and the third provides guidance for the
repair and alterations of the equipment.
In the installation portion (part one) of the code, the pressure relief valve
items to be reviewed during installation of the equipment are listed for
power boilers (such as ASME Section I design), 15 psig (1.03 barg) steam
hot water heaters (ASME Section IV), pressure vessels (ASME Section
VIII), and piping. In addition to installation guidelines, many of these
items are design related and echo the ASME Code requirements we havediscussed. For the boilers and heaters, the NBIC code displays proper
documentation to be completed prior to commissioning the system.
Part two of NBIC 23 will list items necessary to inspect the conditionof a pressure relieving device that is currently in use. There is also achecklist of installation items to review such as proper draining of thedischarge piping or hazards to personnel from a valve discharge. There
are also recommended inspection and testing intervals listed in part two.

The safety valves on power boilers less than 400 psig (27.6 barg) design,
hot water boilers, steam heating boilers, and steam service process
vessels are recommended to be pressure tested every year. For power
boilers with designs greater than 400 psig (27.6 barg) the safety valves
are recommended to be pressure tested every three years. Most process
vessel inspection frequency recommendations are to be based upon thehistorical performance due to the numerous unknowns of service andoperating conditions. In fact, all of the recommended intervals discussed
above and in part two should be evaluated and altered based upon thisoperating experience. There are items listed in the document to help
evaluate the service history. If there are any jurisdictional requirements
as to when a pressure relief valve is to be tested, then these outweigh
the recommendations in NBIC 23.
The third part of NBIC 23 defines the work process to repair or modify
equipment such as pressure relief devices. The document is very
similar to certification processes discussed above to manufacture newpressure relieving devices via the ASME Code. In part three, there are
instructions to be followed by prospective companies, whether it bean original manufacturer, assembler, repair organization and even
operating companies that wish to repair pressure relief devices under
the accreditation process of NBIC 23.
The National Board will issue what is called a Certificate of Authorization
to a facility once their quality system is reviewed and approved andverification testing is successfully completed. The quality manual
will describe in detail the scope of the repair facility’s desire andcapability of the repair, such as testing media and which ASME Codesections will be used to bring the valve back to as new conditions.
This Certificate of Authorization can be for a physical facility, a mobile“in field” repair capability or both. Unlike the assembler certificationin ASME Code which authorizes assembly of specific certified devices,
the NB repair program (VR) authorizes the certificate holder to repair
any manufacturer’s certified device if it fits into the repair scope of thecertificate holder.

3.27

Pressure relief valve engineering handbook
Chapter 3 - Codes and Standards
The verification testing includes taking a minimum of two repaired
valves for each ASME Code section and subjecting them to capacity andoperational testing per the applicable ASME Code requirements. This
verification test can be done with any type of acceptable valve design
or manufacturer per the ASME Code. Once the prospective repair
organization passes this verification testing, then any pressure relief
valve built to that part of the ASME Code can be repaired.
Once the approvals are received from the National Board, the repair
organization will be allowed to identify repaired valves with the “VR”
stamp on the nameplate. NBIC 23 includes required elements that must
be on any VR stamped repair tag, which is attached to the valve after
repair. The VR nameplate does not replace the original ASME nameplate
which must remain attached to the valve. If any information such as set
pressure, media, model number and so on, changes during the repair ormodification, then this information on the original nameplate should bemarked out but should still be legible.
If the original nameplate is lost from the valve to be repaired, then a“VR” nameplate cannot be provided. The exception is that if assurance
can be provided, perhaps via a valve serial number provided to the
manufacturer, that the valve was originally provided with an ASME Codestamp, then a duplicate nameplate can be attached along with the “VR”
nameplate.
All external adjustments are sealed and marked with an identification
tag traceable to the repair organization.
The use of the “VR” stamp is valid for three years from approval whenanother National Board audit and verification test is required.
NB-18 Pressure Relief Device Certifications
The National Board has been designated by the ASME to provide theinspection, review and acceptance of pressure relief devices to meetthe various sections of the ASME Code. The NB-18 document lists allof the original manufacturers and their assemblers that are certified toprovide new pressure relief devices per the ASME Code. These devices
are listed with their applicable ASME Code section, certified capacities
and media in which they were tested. This document is available on line
at http://www.nationalboard.org. The information is generally updated ona monthly basis.
NB-18 also lists those organizations who are certified to repair pressure
relieving devices per the NB-23 requirements discussed above.

3.28

Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
The following data is included in this chapter
I.
II.

III.

IV.

Introduction������������������������������������������������������������������������������������������������������������������������������������4.3
Direct Acting Pressure Relief Valves�������������������������������������������������������������������������������������������4.3
Weight Loaded PRV Operation�����������������������������������������������������������������������������������������������������4.3
Direct Spring Safety Valve Operation - Gas/Vapor Trim Designs����������������������������������������������4.5
Direct Spring Relief Valve Operation - Liquid Trim Design��������������������������������������������������������4.7
Direct Spring Safety Relief Valve Operation - Gas and Liquid Trim Design������������������������������4.8
Direct Spring Pressure Relief Valve Seat Designs����������������������������������������������������������������������4.8
Direct Spring Pressure Relief Valve Components����������������������������������������������������������������������4.9
Direct Spring Pressure Relief Valve - Portable Design������������������������������������������������������������4.10
Inlet Piping Considerations���������������������������������������������������������������������������������������������������������4.11
Discharge Piping Considerations�����������������������������������������������������������������������������������������������4.13
Direct Spring Safety Relief Valve Operation - Balanced Designs��������������������������������������������4.14
Pilot Operated Pressure Relief Valves���������������������������������������������������������������������������������������4.15
Snap Action Pilot Design�������������������������������������������������������������������������������������������������������������4.17
Modulating Action Pilot Design��������������������������������������������������������������������������������������������������4.19
Pilot Operated Pressure Relief Valve Seat Designs������������������������������������������������������������������4.22
Pilot Operated Main Valve Components������������������������������������������������������������������������������������4.23
Inlet Piping Considerations���������������������������������������������������������������������������������������������������������4.24
Discharge Piping Considerations�����������������������������������������������������������������������������������������������4.25
Advantages and Limitations of Valve Types�������������������������������������������������������������������������������4.27

4.1

Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
The following Figures are included in this chapter
Figure 4-1
Figure 4-2
Figure 4-3

Weight Loaded Pressure Relief Valve���������������������������������������������������������������������������4.3
Direct Acting Pressure/Vacuum Vent���������������������������������������������������������������������������4.4
Typical Weight Loaded Pressure/Vacuum Vent Capacity vs Overpressure
Characteristic�����������������������������������������������������������������������������������������������������������������4.4
Figure 4-4	Emergency Pressure Relief Device������������������������������������������������������������������������������4.5
Figure 4-5
Generic Direct Spring PRV Trim�����������������������������������������������������������������������������������4.5
Figure 4-6
Section VIII Safety Valve Trim����������������������������������������������������������������������������������������4.6
Figure 4-7
Typical Section VIII Safety Valve Lift Characteristics���������������������������������������������������4.6
Figure 4-8
Section I Design Safety Valve Trim�������������������������������������������������������������������������������4.7
Figure 4-9	Effect of Nozzle Ring for ASME Section I Design Safety Valve������������������������������������4.7
Figure 4-10	Effect of Guide Ring for ASME Section I Design Safety Valve�������������������������������������4.7
Figure 4-11 Relief Valve Trim������������������������������������������������������������������������������������������������������������4.8
Figure 4-12 Typical Relief Valve Lift Characteristic�������������������������������������������������������������������������4.8
Figure 4-13 Metal Seated Direct Spring PRV�����������������������������������������������������������������������������������4.9
Figure 4-14 API 526 Direct Spring PRV���������������������������������������������������������������������������������������������4.9
Figure 4-15 Open Bonnet Direct Spring PRV���������������������������������������������������������������������������������4.10
Figure 4-16 Threaded Portable Direct Spring PRV������������������������������������������������������������������������4.11
Figure 4-17	Flanged Portable Direct Spring PRV��������������������������������������������������������������������������4.11
Figure 4-18 Inlet Piping Effects on Direct Acting PRV�������������������������������������������������������������������4.11
Figure 4-19 Tailpipe Discharge Piping��������������������������������������������������������������������������������������������4.12
Figure 4-20 Closed Header System Discharge Piping������������������������������������������������������������������4.12
Figure 4-21	Effect of Built-up Back Pressure on Conventional PRV��������������������������������������������4.13
Figure 4-22 Superimposed Back Pressure in a Conventional Direct Spring Loaded PRV���������4.13
Figure 4-23 Balanced Bellows Direct Spring PRV�������������������������������������������������������������������������4.14
Figure 4-24 Balanced Piston Direct Spring PRV����������������������������������������������������������������������������4.15
Figure 4-25 Piston Type Pilot Operated PRV����������������������������������������������������������������������������������4.15
Figure 4-26 Diaphragm Type Pilot Operated PRV �������������������������������������������������������������������������4.16
Figure 4-27	Height Comparison of Direct Spring vs Pilot Operated PRV�������������������������������������4.17
Figure 4-28 Pop Action Pilot Operated PRV (closed)���������������������������������������������������������������������4.18
Figure 4-29 Pop Action Pilot Operated PRV (open)������������������������������������������������������������������������4.18
Figure 4-30 Main Valve Lift vs Set Pressure for Pop Action Pilot Operated PRV�������������������������4.19
Figure 4-31 Modulating Action Pilot Operated PRV (closed)���������������������������������������������������������4.20
Figure 4-32 Modulating Action Pilot Operated PRV (open)������������������������������������������������������������4.21
Figure 4-33 Modulating Action Pilot Operated PRV (open and in full lift)������������������������������������4.21
Figure 4-34 Main Valve Lift vs Set Pressure for Modulating Action Pilot Operated PRV������������4.22
Figure 4-35 Piston Type Main Valve Components��������������������������������������������������������������������������4.22
Figure 4-36 Remote Sense Pilot Operated PRV�����������������������������������������������������������������������������4.23
Figure 4-37 Balanced Modulating Pilot Operated PRV������������������������������������������������������������������4.23
Figure 4-38 Sonic to Subsonic Flow Transition������������������������������������������������������������������������������4.25
Figure 4-39 Superimposed Back Pressure in Piston Type Pilot Operated PRV��������������������������4.25
Figure 4.40 Backflow Preventer������������������������������������������������������������������������������������������������������4.25
The following Tables are included in this chapter
Table 4-1	Full Bore vs API Orifices����������������������������������������������������������������������������������������������4.16
Table 4-2
Weight Comparisons����������������������������������������������������������������������������������������������������4.16
Table 4-3
API 526 Direct Spring vs Pilot Operated PRV Dimensions����������������������������������������4.24

4.2

Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
I. Introduction
A pressure relief valve (PRV) is a safety device designed to protect apressurized vessel, system or piping during an overpressure event. An
overpressure event refers to any condition which would cause pressure
in a vessel, system, pipe or storage tank to increase beyond the specified
design pressure or maximum allowable working pressure (MAWP).
Since pressure relief valves are safety devices, there are many codesand standards written to control their design and application. (See
Chapter 3.) The purpose of this discussion is to familiarize you with thevarious parameters involved in the design of a pressure relief valve.
Many electronic, pneumatic and hydraulic systems exist today to control
fluid system variables, such as pressure, temperature and flow. Each ofthese systems requires a power source of some type, such as electricity
or compressed air, in order to operate. A pressure relief valve must be
capable of operating at all times, especially during a period of power
failure when system controls are non-functional. The sole source ofpower for the pressure relief valve, therefore, is the process fluid.
Once a condition occurs that causes the pressure in a system or vessel
to increase to a dangerous level, the pressure relief valve may be theonly device remaining to prevent a catastrophic failure.
The pressure relief valve must open at a predetermined set pressure,flow a rated capacity at a specified overpressure, and close when thesystem pressure has returned to a safe level. Pressure relief valvesmust be designed with materials compatible with many process fluids
from simple air and water to the most corrosive media. They must also
be designed to operate in a consistently smooth and stable manner ona variety of fluids and fluid phases. These design parameters lead to
the wide array of Emerson products available in the market today andprovide the challenge for future product development.
II. Direct Acting Pressure Relief Valves
The oldest and most commonly used type of PRV is the direct acting type.
They are designated as direct acting because the force element keeping
the valve closed is either a weight or a spring or a combination of both.
The process to be relieved acts directly on the seat pallet or disc, which
is held closed by the weight or spring opposing the lifting force that is
generated by the process pressure.
There are two kinds of direct acting pressure relief valves, weight loaded
and spring loaded.

Weight Loaded PRV Operation
The weight loaded PRV is one of the simplest and least complex typeof PRV. It is a direct acting valve because the weight of the valve’s
internal moving parts (see Figure 4-1) keep the valve closed until
the tank pressure equals this weight. These valves are often calledweighted pallet valves because the set pressure can be varied byadding or removing weights on the top of a trim part called a pallet(see Figure 4-2).
These weighted pallet valves are also known as conservation vents or
breather vents. This is because one of the primary uses of these devices
is to protect low pressure storage tanks that have fixed roofs. These
storage tanks are often designed per API Standard 620 or 650 and havevery low design pressures in the inches of water column (mbar) range.
Since the design pressures are very low, the simple pumping in of
product or increased ambient temperatures can raise vapor pressuresin the tank and cause these weight loaded valves to “breathe” anddischarge the pressure. The sizing and selection of these weight loaded
valves is often done per API 2000 or ISO 28300 which will be discussed in
Chapters 5 and 6.
One of the advantages of the weighted pallet valve is its ability to be set
to open as low as 0.5 ounces per square inch (0.865 inches of watercolumn). This is only 1/32 of a pound per square inch or just over 2 mbar.
In order to mount these devices to the storage tank, the nozzle, pallet
and weights are contained within a body. The material of construction
for the body and the internals can be of many types to provide
compatibility with the contents of the storage tank. In addition toductile iron, aluminum, carbon and stainless steel, these devices canbe manufactured from fiberglass reinforced plastic components. The
simple nature of the device lends itself to being a good economic choicein many applications.
In Figure 4-2 you will note another pallet denoted for vacuum protection.
Recall that the common application for these devices is for low designpressure storage tanks and as such can be subject to collapsing inward
due to vacuum conditions. These vacuum conditions can be caused
simply by pumping out product or a lowering of ambient temperature tocause condensation of the vapor in the tank. The weight loaded vacuum
pallet will provide a route for ambient pressure to break the vacuum andprevent implosion of the tank.

Figure 4-1
Weight Loaded Pressure Relief Valve
WeightP

Tank Pressure
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Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
Figure 4-2
Direct Acting Pressure/Vacuum Vent
Overpressure Protection

PalletPalletVacuum Protection
Tank Connection

Figure 4-3
Typical Weight Loaded Pressure/Vacuum Vent Capacity vsOverpressure Characteristic
10075% Rated Capacity

A weight loaded vent valve normally uses a soft resilient film material,
such as PTFE, to provide the seal between the pallet and the nozzle.
This helps to prevent sticking between these components. When
properly maintained, this film seat design will provide an acceptable seal
up to 75% of the valve’s set pressure. API 2000 and ISO 28300 provides
the acceptance criteria for seat tightness at 75% of the set pressure.
This allowable leakage can range from 0.5 SCFH (0.014 Nm3/hr) of air
for 6 inch (150 mm) and smaller valves to 5 SCFH (0.142 Nm3/hr) of air
for valve sizes up to 12 inches (300 mm). Many of these valves with thePTFE film seal are designed to provide a maximum leak rate of 1 SCFH
(0.028 Nm3/hr) of air at 90% of the set pressure.
The storage tanks that are being protected by these devices areoftentimes designed to hold large volumes of fluids and thus canrequire a pressure or vacuum relieving device which needs to be able
to deliver high venting capacities. Therefore, the size of these devices
can be as big as 12” (300 mm) for their tank connection size. Since theforce of the weights keeping the seat or pallet closed must be higher
than the tank pressure multiplied by the exposed seat area, there arephysical limitations to the maximum set pressure that can be provided.
For example, the 12” (300 mm) weighted pallet valve may have a seator nozzle area of approximately 90 in2 (581 cm2) that is exposed to theprocess. In order to obtain a set pressure of 1 psig (69 mbar), 90 lb(41 kg) of weight is required. Adding these weights to the body and trimof the valve can then require the entire valve to physically weigh several
hundred pounds which can be difficult to support on a thin roof designof an atmospheric storage tank. Therefore, the typical maximum set
pressure for these devices will range from 1 to 2 psig (69 to 138 mbar)
depending upon the size and design.
One operational characteristic of these weighted pallet valves isthe amount of overpressure required to obtain lift of the pallet andweights, and provide the required capacity through the valve. There islittle available energy at these low set pressures to provide this lift so
additional storage tank pressure above the set pressure is needed. For
example, it is not uncommon for a valve that is set in the inches of wateror millibar range to require almost 100% or more overpressure to obtainits full lift (see Figure 4-3). The same characteristic is also found during
the opening operation of the vacuum pallet.
The high overpressure that may be required for the pressure or thevacuum required capacity will often necessitate the set pressure or
vacuum setting to be closer to atmospheric conditions. This can cause
continual emissions to atmosphere or contaminate the contents ofthe storage tank with the ambient inbreathing from the vacuum pallet.
Therefore, either the operating conditions must be lowered below
the setting of the weighted pallet valve or the tank must be built witha higher design rating. In either case, the efficient use of the tank iscompromised.
In many storage tank applications there is a requirement for a pressure
relief valve to be provided for what is called emergency overpressure
relief. This overpressure capacity contingency is often caused by an
external source of heat such as a fire that boils the liquid contents. Thisparticular contingency may require higher relieving rates than can beobtained with the weighted pallet valve.

50

25

0
0

50
% Overpressure

100

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Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
In these situations, an emergency pressure relief valve can beconsidered to provide additional capacity. These devices are simply
tank hatches, also called “manways,” that normally have hinged covers.
The covers have a calibrated weight, moment arm and possibly a
counterweight to provide the required set pressure. See Figure 4-4.
These emergency relief devices are set at higher pressures than theweighted pallet valves. If called upon to open and relieve pressure, they
are designed to stay open until manually closed.
Direct Spring Safety Valve Operation - Gas/Vapor Trim Designs
Since the weighted pallet valve has limited set pressures of no morethan one or two pounds per square inch (69 to 138 mbar), a compressed
spring that opposes the force provided by the process pressure is themost widely used method of increasing the set pressure. An advantage
of using a compressed spring is the wide number of applications forthis design from approximately 5 psig (0.345 barg) to over 20,000 psig
(1380 barg) set pressures.
Since we are using what is called a direct acting spring loaded valve at
or above 15 psig (1.03 barg), this design is more often than not subject
to the requirements of the ASME Boiler and Pressure Vessel Code. As
we learned in Chapter 3, among the Code requirements are values ofmaximum accumulation that vessels can see during an overpressure
event. For ASME Section I vessels, the maximum accumulation canbe from 3% to 6% over the MAWP and for ASME Section VIII vessels,
the maximum accumulation can be from 10% to 21% over the MAWP.
Since it is desirable to not open pressure relief valves unless absolutely
necessary, most users will set them to open at the highest pressure
allowed by the relevant section of the Code. This set pressure is typicallyat or near the MAWP of the vessel. Therefore, the design of an ASME
Code certified valve must provide lifting characteristics to prevent theallowable accumulation pressure from being exceeded. In other words,
these direct spring valves must have sufficient lift with overpressures as
low as 3% for Section I vessels and 10% for Section VIII vessels.
For a weighted pallet valve, the pressure force required to lift the palletmust exceed only the weight of the pallet. This weight remains constant,regardless of the lift. In a spring loaded valve, the pressure force
required to initially lift the seat is determined by the pre-loaded spring
compression. As the spring is being compressed more during lift, theupward force required to obtain lift increases. The overpressure provides
some of this additional upward force but it is not enough to obtain theneeded lift within Code mandated accumulation pressures.
Figure 4-5 is a simple sketch showing the seat, also called a disc, heldin the closed position by the spring. When system pressure reaches thedesired opening pressure, the force of the pressure acting over Area
A1 equals the force of the spring, and the disc will lift and allow fluid
to flow out through the valve. In order to obtain the lift within the Coderequired accumulated pressure, most direct acting spring valves have asecondary control chamber to enhance lift. This secondary chamber isbetter known as the huddling chamber. The geometry of this huddlingchamber helps to determine whether the direct spring pressure relief
valve is suitable for use in compressible, non-compressible or the
possible mix of either of these fluid states.

Figure 4-4
Emergency Pressure Relief Device

Figure 4-5
Generic Direct Spring PRV Trim
SpringDisc
A2

Huddling
Chamber

A1

Nozzle Ring

Nozzle4.5

Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
Figure 4-6
Section VIII Design Safety Valve Trim

Disc
Skirt Area

Figure 4-7
Typical Section VIII Design Safety Valve Lift Characteristics
100
75% Lift Capacity

For direct spring pressure relief valves designed to work exclusively oncompressible media such as gases or steam, a typical trim is illustrated
in Figure 4-6. This type of valve is called a safety valve. As the disc begins
to lift, fluid enters the huddling chamber exposing a larger area (this is
illustrated as A2 in Figure 4-5) to the gas or vapor pressure. This causes
an incremental change in force, sometimes called the expansive force,
which overcompensates for the increase in the downward spring force
and allows the valve to open at a rapid rate. This effect allows the valve
to achieve maximum lift and capacity within overpressures that will let
this valve be set at the MAWP and prohibit the accumulation pressure
from exceeding Code mandated levels.
Because of the larger disc area A2 (Figure 4-5) exposed to the systempressure after the valve achieves lift, the valve will not close until systempressure has been reduced to some level below the set pressure. The
difference between the set pressure and the closing point, or reseatpressure, is called blowdown and is usually expressed as a percentage
of set pressure. For example, if the valve is set at 100 psig (6.90 barg)
and the reseat pressure is 92 psig (6.34 barg), then the blowdown for
the valve is 8%.
Some valves built for ASME Section VIII have non-adjustable, or fixed,
blowdowns. The valve will reseat depending upon the spring being used
and the tolerance of the moving parts. The blowdown for this type ofvalve could be 20% or more.
It may not be desirable to use a safety valve with a non-adjustable
blowdown design when the operating pressure is close to the set
pressure or if there is a need to minimize the amount of service fluid
being released during an opening cycle. In these cases a manufacturer
can provide a design with an adjustable blowdown by adding a part calleda nozzle ring. This ring is threaded around the outside diameter of thenozzle and can be adjusted up or down. See Figure 4-5.
The position of the nozzle ring controls the restriction to flow between
the disc and ring in the huddling chamber when the disc just begins
to lift at set pressure. The smaller the clearance between these twoparts, the higher the expansive force that builds up to minimize thespread between the simmer pressure and the set pressure. Thishigher expansive force provides a rapid audible pop on compressible
service but the force holding the disc open is high and there is morepressure reduction (i.e. longer blowdown) required in the vessel toreseat the valve. Therefore the nozzle ring can be lowered thus opening
the distance between the disc and ring. This increases the difference
between the simmer pressure and the set pressure, but shortens theblowdown.
For these gas/vapor safety valves, the typical performance curve isshown in Figure 4-7.
These safety valves achieve over 50% of their required lift at the set
pressure and use the overpressure to open fully when the processpressure is 10% above the set pressure of the valve. For a single safetyvalve that is set to open at the vessel MAWP for an ASME Section VIII
non-fire case overpressure contingency, if the valve is sized properly,
the vessel pressure will not exceed 10% accumulation and the valve will
reseat with a blowdown of 7% to 10%. Direct acting spring loaded safetyvalves that are built per API Standard 526 normally perform as describedabove.
As mentioned in Chapter 3, safety valves that are designed for use in
ASME Section I need to have adequate lifting characteristics at lower
overpressures than ASME Section VIII safety valve designs. It is common
to have only 3% overpressure available to obtain full lift for a Section I
safety valve. This more stringent performance requirement necessitates
the addition of a second control ring located on the outside of the valve
guide. This is called the guide ring, and this additional part permits more
precise “tuning” of the huddling chamber. See Figure 4-8.

50
25
Simmer
0
90

100
110
% Set Pressure

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Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
Operation of the two ring valve is very similar to that of the single ring
design. When inlet pressure exceeds the pressure exerted by the valve
spring, the disc moves off the nozzle and the escaping steam builds
the upward force via the nozzle ring that is directed to the disc face tominimize the spread between simmer and set pressure (see Figure 4-9).
The position of the nozzle ring determines the difference between
simmer pressure and set pressure. As with the ASME Section VIII safetyvalve design, the higher the nozzle ring position, the less simmer orleakage occurs prior to lifting the disc.
As the lift continues, the steam starts to impinge on the guide ring as
shown in Figure 4-10 to provide even more lifting force so that adequate
lift is achieved with 3% overpressure. The guide ring position determines
the blowdown setting. The lower the guide ring position, the higher the
opening force delivered by the flowing steam. This causes an increasein the blowdown. In order to decrease the blowdown, the guide ring is
raised.
Recall that the requirement during provisional certification for a Section I
safety valve is to demonstrate a blowdown of no more than 4%. This valve
performance criteria, along with the minimal overpressure, necessitates
the two ring trim design.
Direct Spring Relief Valve Operation - Liquid Trim Design
For incompressible fluids, there is no expansive force available toassist in the lifting of a relief valve seat. ASME Section VIII has the same
maximum accumulation pressure requirement for liquid service as
there is for gas service, typically 10% over the MAWP, which cannot
be exceeded during a relieving event. A direct acting liquid relief valve
initially uses reactive forces and then momentum forces to obtain its lift
within the Code requirements.
Figure 4-11 shows the trim parts for a direct spring relief valve designfor liquid service. If you compare the shape of the skirt area of the discholder in this figure to the skirt area of the disc holder in Figure 4-6 you
will notice a contour difference. This different geometry in the liquid
relief valve skirt will allow the liquid to be directed downward as theflow begins. The upward reaction force on the disc increases slowly atminimal overpressure.
As the flow of the liquid stream increases with the lift, the increased
velocity head provides the momentum force to be additive with thereactive force to open the valve disc substantially so that a surge, orgush, of liquid flow is observed at the outlet of the valve. This flow profile
happens with less than 10% overpressure and full lift, and rated capacity
is obtained with a maximum of 10% overpressure. See Figure 4-12.

Figure 4-8
Section I Design Safety Valve Trim
Guide Ring

Disc
NozzleNozzle Ring

Figure 4-9
Effect of Nozzle Ring for ASME Section I Design Safety Valve

Nozzle Ring

Figure 4-10
Effect of Guide Ring for ASME Section I Design Safety Valve

Guide Ring

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Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals

Direct Spring Safety Relief Valve Operation - Gas and Liquid Trim
DesignThere may be installations where the service fluid may be either
compressible, incompressible or a combination of phases when thepressure relief valve is called upon to operate. For instance, there may
be multiple overpressure contingencies to consider for a pressure relief
valve where the valve may see liquid in one case and a gas in another.
Another example is that there might also be a fluid that can be in a liquidstate under pressure and while it is relieving, the fluid may transition tosome quality of vapor causing a multi-phase relieving state.
There are direct acting valves available that have been certified tomeet ASME Section VIII requirements for gas or liquid. This is normallyaccomplished using the liquid trim design (Figure 4-11) which will
provide an adequate huddling chamber to obtain rated capacity by 10%
overpressure on vapor service. This trim geometry will provide a stable
lifting characteristic on gas or liquid or mixed phase fluids. One item tonote is that the blowdown is fixed and could be as long as 20% for thisgas/liquid trim design when discharging a compressible fluid.
Direct Spring Pressure Relief Valve Seat Designs
Since the normal operation condition for a pressure relief valve is closed,
one important consideration is the valve’s ability to maintain a tight seal.
The disc to nozzle interface is most commonly metal to metal as shownin Figure 4-13.
The advantage of the metal to metal seal is a wide range of chemical
and temperature compatibility with the process fluid. This is especially
important for the high pressure and temperature steam drum andsuperheater safety valves found in Section I applications.
The surface of the disc and nozzle that come in contact with each otherare polished to an exact finish. Another term used for this seat andnozzle surface treatment is called “lapping.” The valve part alignment
and the selection of the materials of construction all play a key role tomeet industry seat leakage standards such as API 527 which is often
used for process PRVs built per ASME Section VIII.
API Standard 527 requires the valve seat to be tested for tightness
normally at 90% of the set pressure. The API standard acceptancecriteria allows minor bubble leakage at this operating pressure but
this allowed leakage is many orders of magnitude more stringent thanrequired for other types of valves. A diligent maintenance schedule must
be carried out in the field to maintain the seating integrity of the valve,especially on a system where the pressure relief valve may have cycled.

Figure 4-11
Relief Valve Trim

Skirt Area

Figure 4-12
Typical Relief Valve Lift Characteristic
100% Full Lift

These liquid trim relief valves also have a blowdown ring. The ring helps
to provide the upward reactive force to assist opening, however the
position of the ring is not used to determine when the valve will reseat.
These relief valves have a fixed blowdown that will allow the device toreclose at approximately 10% to 15% below set pressure.
Prior to 1985, the ASME Section VIII code did not require certified
liquid capacities at 10% overpressure. Many manufacturers provided
PRVs with trim designs similar to that shown in Figure 4-6 for use inincompressible medias. This trim design often required up to 25%
overpressure for the valve to obtain full lift because the lower reactive
forces generated by the frim design. It was also noted that the loss ofliquid velocity during opening might cause an undesirable fluttering oroscillating action.

50

0
90

95

100

107

110

% Set Pressure

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Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
Since these metal to metal seated valves are prone to some leakage, theuse of a rupture disc upstream and in series with the direct acting valve
can provide a zero leak point during normal system operating conditions.
Recall from Chapter 3 that there is a requirement in ASME Section VIII
that mandates a capacity reduction of 10% of the certified valve capacity
if the specific rupture disc has not been capacity certified in combination
with the specific pressure relief valve. The rupture disc should also
be non-fragmenting to avoid parts of the disc impinging componentsof the pressure relief valve. As noted in Chapter 3, the pressure in thespace between the rupture disc and the pressure relief valve should bemonitored so that a burst or leaking rupture disc can be identified andreplaced.
The ASME Section I valve metal seat design can often allow for a slightly
higher operating pressure than 90% of set pressure. The leak test forthese valves is performed with steam and no visible leakage of the steam
is permitted.
When process conditions warrant, a soft seat, such as an elastomer orplastic material might be substituted for the metal to metal design. The
advantage of such a seat is that API 527 does not allow any leakage at
90% of the valve’s set pressure. In fact, some soft seated valve designs
allow the operating pressure to go as high as 95% of the set point. Thesesoft seats are easily replaceable and require no special lapping during
maintenance.
Care must be taken in selecting the proper soft seat compound based
upon the pressure, temperature and process fluid conditions that thesesoft seat materials may see.
Direct Spring Pressure Relief Valve Components
Figure 4-14 shows additional trim components for a process pressure
relief valve that often falls under the API 526 standard purchasing
specification for flanged steel valves.
These valves have what is called a full nozzle. A full nozzle has theadvantage in that the process fluid will not be in contact with the valve
body when the valve is in the closed position. You will note that the inletof the full nozzle itself forms the raised face or ring joint portion of theinlet flange connection. The full nozzle is threaded into the valve bodyand can be removed for maintenance. It is the bore of the nozzle thatwill determine the actual flow area for most of these API 526 standard
lettered orifice sizes.

Figure 4-13
Metal Seated Direct Spring PRV

DiscDisc/Nozzle Seal

NozzleFigure 4-14
API 526 Direct Spring PRV
Cap

Set Pressure
Adjustment Screw

SpringWasherSpringSpring Bonnet
Spindle

Guide

Disc Holder
Disc

Full Nozzle

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Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
The disc, whether it is metal to metal or soft seated, is contained by
the disc holder. The disc is often held in place via a loss thread typeconnection or retention clip. When the disc is contained in the holder
with either method, the disc has some articulation of movement to help
reseat properly to the nozzle.
The guide is used just as its name implies, it allows for the proper
opening and closing of the disc holder. The tolerance and the materialsof construction in the design of the guide and disc holder are vital toproviding repeatable performance. The guide and disc holder typicallyare made from different materials, that in turn have different hardness
characteristics, in order to prevent a seizing or galling of parts during
normal expected valve operation.
The spindle provides a bearing point for the spring compression to press
the disc holder against the nozzle. As with the disc and its holder, theinterface between the spindle and the top of the disc holder help to alignthe disc to properly allow the disc to contact the nozzle during repeated
operations or cycles.
The set pressure adjusting screw contacts the top spring washer to raiseor lower the set point of the valve by adding or reducing the amount ofspring compression that is imparted to the spindle/disc holder. Thisadjustment screw is located under a cap. The cap can be lock wired after
being attached to the bonnet to provide administrative controls prior toaltering the set pressure of the valve. The cap can be screwed or bolted
onto the bonnet.
One note about the spring bonnet is that it may be either a closed oropen design. The bonnet shown in Figure 4-14 illustrates a cutaway
view of a closed bonnet. The closed bonnet provides the feature ofisolating the spring from ambient conditions. One item of note for thevalve in Figure 4-14 is that when the valve is called upon to relievepressure, the service fluid will not only be routed out the discharge
flange but it will also expose all of the internal trim parts to the process.
When the service fluid is exposed to all of the PRV trim parts it can becalled a conventional direct spring operated pressure relief valve. The
closed bonnet will contain this service fluid and prevent exposing theenvironment and personnel to a possible hazardous condition.
One other type of conventional direct spring PRV uses what is called
an open spring bonnet as shown in Figure 4-15. This type of bonnet
configuration is most commonly found on boiler safety valves built tomeet ASME Section I but they can also be used in process applications.
An open spring bonnet can also be called a yoke. Exposing the spring to
the ambient will allow for the radiation of heat from the spring. Various
spring materials at elevated operating temperatures can allow a shift of
the in situ set pressure to a value that is lower than the test bench setpressure, which is done at ambient temperature. The open bonnet helps
to cool the spring to minimize the effect on the set pressure caused by
the high temperature service fluid. The disadvantage of the open bonnet
PRV is that the service fluid will not only exhaust out the discharge of thevalve but also out of its open bonnet. These open bonnet valves should
be located away from areas where personnel could be present. For open
bonnet valves that may be located outside, there are weather hoods
available to protect the exposed spring.

Figure 4-15
Open Bonnet Direct Spring PRVOpen Spring
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Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
Direct Spring Pressure Relief Valve - Portable Design
Pressure relief valves that are designed to meet the ASME Section VIII
requirements do not necessarily have to meet the API 526 purchasing
standard. There are many other types of connections, other than flanged,that are available for use. For example, many of the valves that are lessthan 2” (50 mm) in size for their inlet are often provided with threaded
connections. A common term for these valve types is “portable valves”
because of their small physical size and weight. See Figure 4-16.
These portable valves may also be adapted to weld on inlet and outlet
flanges (see Figure 4-17) to meet a particular piping specification. One
should note that the dimensions, materials, inlet/outlet sizes and orificedesignation normally do not necessarily meet the API 526 purchasing
standard.
There is no requirement in ASME Section VIII that specifies a particular
type of connection. In addition to the threaded connections, you can findthese process valves with socket weld, hubbed, union, tubing or several
other types of fittings for the inlet and/or outlet of the valve.
Inlet Piping Considerations
The proper design of inlet piping to pressure relief valves is extremely
important. It is not unusual to find these process pressure relief
valves mounted away from the equipment to be protected in order tobe more accessible, to be closer to the effluent disposal system or for
maintenance purposes. There may be a considerable length of inlet
piping with bends that may cause significant non-recoverable (loss due
primarily by friction caused by flow within the piping) pressure loss
during the valve’s operation.
Depending upon the size, geometry, and inside surface condition of theinlet piping, this pressure loss may be large (10%, 20% or even 30%of the set pressure) or small (less than 5% of the set pressure). API
recommended practice 520 part II and the non-mandatory appendix M
of ASME Section VIII guide thwe engineer to design for a maximum inlet
pressure loss of only 3% of the valve set pressure. The recommended
practice will tell the user to calculate these losses using the rated
capacity for the device.
The importance of this inlet piping evaluation is illustrated in Figure
4-18 which shows that when a direct acting PRV is closed, the pressure
throughout the system being protected is essentially the same. The inlet
piping configuration will not change the set pressure of the PRV.
However, when the valve opens, the frictional losses created by the inlet
pipe will cause a difference in the pressure from the system (Ps) and thevalve inlet (Pv). A direct acting PRV will reseat when the Pv value reaches
the blowdown for the valve. It is important that the non-recoverable inlet
line losses be less than the blowdown of the valve so there is a higher
probability of a stable lift and a single reclosure cycle. Non-recoverable
inlet losses in excess of the blowdown can be a cause of valve chatter or
rapid cycling.

Figure 4-16
Threaded Portable Direct
Spring PRV

Figure 4-17
Flanged Portable Direct
Spring PRV

FlangedFigure 4-18
Inlet Piping Effects on Direct Acting PRV

SV OpenSV Closed
PValvePValvePSystem

PSystem

PV = PSFlow

PV = PS - Inlet Loss

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Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
To provide a clearer picture of this valve chatter possibility, consider thefollowing example using Figure 4-18.
Set Pressure = 100 psig (6.90 barg)
Reseat pressure = 93 psig (6.41 barg) (7% blowdown)
Non-recoverable inlet loss at rated flow = 3 psig (0.207 barg) (3% of theset pressure)
After the valve opens, it will reseat when Pv is equal to 93 psig
(6.41 barg). The pressure in the system, Ps, will approximately be 93 psig
+ 3 psig or 96 psig (6.41 barg + 0.207 barg or 6.62 barg). The valve closes
and the system pressure stabilizes at 96 psig (6.62 barg). The valve
remains closed at this point.
Consider a second scenario where the same valve is installed where theinlet loss is increased to 10 psig (0.690 barg). In this case, when the valve
reseats at 93 psig (6.41 barg) the pressure in the system is approximately
93 + 10 psig or 103 psig (6.41 + 0.690 barg or 7.10 barg). Immediately
upon reseating, the valve must open because the system pressure is
above its set pressure. This is what can cause rapid cycling or chatter.
This unstable operation reduces capacity and is destructive to the valve
internals and possibly to the piping supporting the valve. Even the bestof tolerances between the guide and disc holder and even the bestmaterials of construction may not prevent galling on these parts.
As mentioned previously, for a compressible fluid, the single blowdown
ring API 526 direct acting safety valve will typically have a 7% to 10%
blowdown. For compressible services, the recommendation in API 520
part II and ASME Section VIII of limiting the non-recoverable line losses
to 3% provides for a needed spread from the blowdown to reduce thepossibility of valve chatter. As pointed out in Chapter Three, ISO 4126-9
provides similar cautions with regards to the maximum allowablepressure loss in the inlet line.
It is a requirement in both Section I and Section VIII that the area of thetank connection, fitting, and all inlet piping be equal to or greater than
the area of the PRV inlet.
You will recall that there is a blowdown requirement to be a maximum
of 4% when applying for ASME Section I certification. Because of this,
there are strict guidelines in Section I for boiler safety valve installations
regarding the length of the inlet piping, and to avoid sharp transitions on
the connection from the steam drum to the inlet piping. The inlet loss
should be an absolute minimum for these valves. See Figure 3-6 formore details relating to recommended piping practices for Section I.
On existing installations, the corrective action to alleviate valve instability
due to excessive inlet losses can be somewhat limited for direct acting
spring loaded PRVs. For a process pressure relief valve, if the length or
fitting restrictions of the inlet pipe to the PRV cause losses that exceed
3%, then it is recommended to increase the line size of the pipe or
straighten the pipe. Another possibility is to change the blowdown settingof the PRV. When the blowdown is greater than the non-recoverable
losses, the chance of valve instability decreases. Unfortunately, it issometimes not economically or technically feasible to do a dynamic
blowdown test, as described in Chapter 2, that is necessary to accurately
set the ring in the proper position.

Figure 4-19
Tailpipe Discharge Piping
Atmosphere

Figure 4-20
Closed Header System Discharge Piping

ClosedHeader
System

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Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
Discharge Piping Considerations
As with inlet piping, pressure losses occur in piping configurations thatare attached to the outlet connection of a pressure relief valve that has
opened and is discharging the service fluid. This occurs whether the PRV
is discharging to atmosphere via a tailpipe as shown in Figure 4-19 or
into a closed header system in Figure 4-20.
Either type of discharge piping causes built-up back pressure and thisback pressure can affect the performance of the conventional direct
acting valves discussed up to this point. The balance of forces for aconventional direct spring PRV is critical. Any change in pressure
within the valve body downstream of the seat, disc holder and huddlingchamber can disturb the lift forces. When the conventional valve is open,
the built-up back pressure will assist in reseating the valve as additional
downward force is applied on the top of the disc holder and from thespring bonnet as shown in Figure 4-21.
Most manufacturers of conventional pressure relief valves and many
recommended practices such as API 520 Part I suggest that theback pressure, calculated at the discharge flange, not exceed theoverpressure allowed for proper lift and capacity. For example, if theconventional PRV is operating with 10% overpressure, then the gauge
built-up back pressure at the outlet flange should not exceed the gauge
set pressure by more than 10%. If the built-up back pressure exceeds
this allowable overpressure, the conventional valve could operate in an
unstable fashion.
Open spring bonnet conventional valves (see Figure 4-15) help todissipate the back pressure that builds up above the moving trim
components. Therefore, an open bonnet Section I safety valve which
may only have 3% overpressure to operate, will be able to maintain its
stability when built-up back pressures exceed 3%. Some manufacturers
will allow 20% built-up back pressure or higher for some open bonnet
Section I designs.
API Standard 521 or ISO 23251 will tell the user to calculate the built-up
back pressure for most direct acting valves by using the rated capacity
for the device (see Table 3-3 for more details). If the calculated builtup back pressure is higher than the overpressure used to size theconventional direct acting PRV, then the discharge piping should beshortened, straightened or enlarged.
When the PRV outlet is connected to a closed disposal system, it is agood possibility that there will be pressure in this outlet piping before
the PRV may be called upon to relieve. This type of back pressure iscalled superimposed. A PRV datasheet will normally have two fields forsuperimposed back pressure, one that would list “constant” and onethat would list “variable.”
An example of constant superimposed back pressure might be a relief
valve protecting the discharge of a pump. It is common to send thedischarge piping of a pump relief valve back into the suction side of theequipment. This suction pressure may be the same at all times.
In Figure 4-22, the green color depicts a constant superimposed backpressure for a closed conventional direct acting PRV. There is a portionof the upper disc area located within the nozzle inside diameter where
the superimposed back pressure will add downward force to the disc.
This adds to the sealing force provided by the spring. The effect to mostconventional direct acting spring loaded valves is that for every one unitof superimposed back pressure that the valve is exposed to, it will add
one unit to the in situ opening pressure.

Figure 4-21
Effect of Built-up Back Pressure on Conventional PRV

Built-up
Back Pressure

Built-up
Back Pressure

Built-up
Back Pressure

Figure 4-22
Superimposed Back Pressure in a Conventional Direct Spring
Loaded PRV
Upper
Disc Area

Upper Disc
Area
Superimposed
Back Pressure

Superimposed
Back Pressure

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Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
Therefore, if the suction pressure of the pump (i.e. superimposed backpressure) is constant, then the test bench pressure for the conventional
PRV can be biased to open the valve at the proper pressure.
Using the pump relief valve as an example, let us say the pump outlet
design will require the relief valve to open at 100 psig (6.90 barg). The
suction pressure is 25 psig (1.72 barg). The test bench setting of the valve
would be 75 psig (5.17 barg). This adjustment, due to superimposed backpressure, is one element of the cold differential test pressure (CDTP)
used to set pressure relief valves on the test bench.
In many instances, the superimposed back pressure is not a constantvalue. This is especially true when a PRV is ultimately discharging intoa flare header (see Figure 3-14). There may be other PRVs, system
blowdown valves, purge lines and so on that may also be connectedto these header systems. The superimposed back pressure will varydepending upon what device is supplying pressure to the header. Since
the superimposed back pressure increases the opening pressureof a conventional pressure relief valve, safety may be compromised.
Therefore, it may be prudent to consider a different design type for a
direct acting PRV.

Figure 4-23
Balanced Bellows Direct Spring PRV

GuideMetal Bellows

Disc Holder

Direct Spring Safety Relief Valve Operation - Balanced Designs
A balanced direct acting spring loaded valve is designed to open at
its test bench pressure setting no matter what the magnitude ofsuperimposed back pressure. A balanced valve has essentially the same
trim components as a conventional valve. In order to provide immunity tothe opening pressure change caused by the variable superimposed backpressure, there are some additional parts added to a balanced valve.
One way to make the top of the disc (holder) area that is exposed tothe back pressure equal to its bottom area that is exposed to the backpressure is to add a bellows to the trim as shown in Figure 4-23. The
bottom portion of the bellows is normally screwed onto the disc holder
and sealed with a gasket. The top of the bellows has a flange surface
that is sandwiched between the guide and the valve body to hold thebellows. The bellows isolates the area on the top of the disc holder thatis equal to the nozzle inside diameter. The superimposed back pressure
is now exposed to the same area on the top and bottom of the disc holder
and thus there is no change in the opening pressure. The valve now isbalanced.
The bellows will also allow the direct acting PRV to be exposed to higher
built-up back pressure values because of the area balance. The springbonnet is isolated which helps to minimize the effect that back pressure
provides to reclose the PRV. Most API 526 balanced bellows valves for
gas/vapor applications will allow for a total back pressure (built-up plus
any constant and variable superimposed back pressure) to be up to 30%of the gauge set pressure in applications where 10% overpressure isused to provide full lift of the disc, with no reduction in capacity. If theallowable overpressure is higher than 10%, then the total back pressure
can be higher than 30% of the gauge set pressure with no capacity
reduction.
An additional feature of a balanced bellows PRV design is that the guideand disc holder interface, as well as the spring, are always isolated fromthe process being relieved and from the media in the discharge piping.
As earlier mentioned, the tolerance between the dynamic disc holder andstatic guide is important for the performance of the valve. The bellows
will keep this area isolated from any debris that may interfere withproper movement of the disc holder. Another benefit of isolating thespring via the bellows may be an economic one when the media may
necessitate a higher alloy spring material due to compatibility.

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Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
Please note the spring bonnet vent in Figure 4-23. All balanced valves
will have a vented bonnet to provide a release port for any downstream
media that might leak past the bellows or its gaskets. Recall that thesuperimposed back pressure increases the opening pressure so it
is important to not trap this back pressure in the spring bonnet. The
vent also provides an indication of damage to the bellows. If the fluid
is hazardous, then this vent may be ported to a safe location but it stillshould be referenced to atmospheric pressure.
It should be noted that the bellows has a limit to the total magnitude ofback pressure that may be in the downstream piping. Keep in mind thatthe bellows has to be flexible enough to still allow the proper lift of thedisc at 10% overpressure and this tends to limit the burst rating of thebellows. For example, API 526 shows a typical balanced bellows valve
to have a maximum total back pressure of 230 psig (15.9 barg) for manyof the smaller size valves and can be as low as 30 psig (2.07 barg) for
certain 8” x 10” (200 mm x 250 mm) configurations.
Some manufacturers offer an alternative to the bellows design toprovide a balanced valve. Figure 4-24 shows what is called a balanced
piston trim for a direct acting spring loaded PRV. This is also known as
a balanced spindle trim where the disc holder and guide have seals thatisolate the spring bonnet from any back pressure. The disc holder is now
a sealed piston or spindle component that, if properly designed, will not
let any service fluid escape into the spring bonnet during operation. The
diameter of the spindle seal is equal to the inside diameter of the nozzle
so that any superimposed back pressure will act on equal areas on the
top and bottom of the disc holder and thus have no effect on the openingpressure.
An advantage of the balanced piston or spindle valve is that it can
typically accommodate higher back pressures because there is no
bellows to burst. As with a balanced bellows valve, the balanced spindle
valve has a vent in the spring bonnet to provide an indication that backpressure has entered the spring area of the PRV. A disadvantage is thatthe guide seal and spindle seals are normally elastomers or plastic sotemperature and chemical compatibility with the service fluid needs tobe considered.
There are some direct acting spring loaded valves that use a combinationof a bellows and balanced piston. The bellows provides the primarydevice to balance the trim and a balanced piston is located inside thebellows to provide a back-up if the bellows becomes compromised.

Figure 4-24
Balanced Piston Direct Spring PRV

Bonnet Vent
Guide Seal
Spindle Seal
Disc Holder

NozzleFigure 4-25
Piston Type Pilot Operated PRV
Pressure Sense Line

Pilot
Dome
Main Valve
P1

PistonSeal

Out
Unbalanced
Moving Member(Piston)

P1
Seat

Pitot Tube
In

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Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
III. Pilot Operated Pressure Relief Valves
Pilot operated pressure relief valves use process pressure, instead of aspring or weight, to keep its primary seat disc closed at pressures below
set. Figure 4-25 shows two major components of the pilot operated PRV,
the main valve and pilot valve.
The main valve is attached to the vessel or system being protected anddetermines the available relieving capacity. The pilot valve controls
the opening and closing of the main valve. The process pressure (P1)enters the main valve and exerts an upward force on the seat which
is similar to a direct acting valve. This same process pressure in
Figure 4-25 is also transmitted up the pilot valve via the pitot tube andpressure sense line into the pilot. The pilot is essentially a direct acting
spring loaded device where a compressed spring holds down a seat.
This compressed spring determines the set pressure of the entire
pilot operated pressure relief valve. The process pressure entering
the pilot acts upon a pilot seat. When the P1 value is less than the set
pressure, the process fluid is allowed to exit the pilot and re-enter themain valve on top of an unbalanced trim member. Figure 4-25 showsa piston to be the unbalanced component. When pilot valves are usedat set pressures less than 15 psig (1.03 barg), or in vacuum protection
applications, it is common to use a lighter weight unbalanced member,such as a diaphragm (Figure 4-26). The volume above the piston ordiaphragm is often called the dome. The piston is called “unbalanced”
because the piston seal (Figure 4-25) has a larger exposed area to theprocess pressure (P1) than the seat area. Since force is equal to thepressure times the area being acted upon, the higher the process oroperating pressure, the tighter and tighter the main valve seat becomes.
This feature is completely the opposite of a direct spring valve where
the minimal seating force is just before the valve must open. The
benefit of the pilot operated PRV is that it may be possible to operatea system closer to the valve’s set pressure and not have leakage or
unwanted opening cycles. Most pilot operated PRV designs will allow
for an operating pressure to be 95% of the set pressure with no process
leakage. Some pilot designs even allow an operating pressure up to 98%
of the set pressure. This increased operating pressure can optimize
the equipment design and allow for the maximum throughput for theprocess.
The opening and closing operation of the pilot operated PRV will be
discussed later in this section.
The size of the pilot valve itself remains constant no matter what size
main valve is required to deliver the capacity. This feature can provide
several advantages over the direct acting PRV. Since the pilot seatremains small relative to the seat of a direct acting spring PRV, one
advantage is that the pilot operated PRV can be used in higher set
pressure applications in comparable line sizes. Once again, back to theforce equals pressure times area relationship, the larger the seat in a
direct acting spring PRV, the higher rate the spring design must be toproperly seal the valve. The spring and its bonnet required become verylarge, the total mass of the valve very heavy and the total cost more
expensive. The same exact pilot that is set, for example, at 1000 psig
(69.0 barg) can be used on a 1D2 through 8T10 main valve. API Standard
526 for instance will show an upper set pressure of 300 psig (20.7 barg)
for the spring loaded 8T10 valve. If the set pressure is over 300 psig (20.7
barg) and the required capacity dictates a “T” orifice valve, the user may
have to install multiple, smaller orifice, spring loaded PRVs, where one
pilot operated PRV would be able to provide the needed capacity.

Figure 4-26
Diaphragm Type Pilot Operated PRV

Pilot

Dome

P1

Diaphragm
P1
NozzleMain ValveOne other way to optimize the available relieving capacity is to install thiscommon pilot onto what is called a “full bore” orifice main valve. Table
4-1 shows the typical API 526 orifice sizes compared to the full bore
orifice. The use of the full bore main valve can save money on not only
the valve itself but on the total installation cost.
The height and weight savings from using a common pilot through themain valve size range may also hold some benefit when clearances aretight in a pipe rack or to minimize weight. Table 4-2 gives a comparisonof weights and Figure 4-27 illustrates the size difference in two 6R8valves.

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Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
Table 4-1 - Full Bore vs API Orifices
Valve Sizein (mm)
1.5 x 2 (40 x 50)
2 x 3 (50 x 80)
3 x 4 (80 x 100)
4 x 6 (100 x 150)
6 x 8 (150 x 200)
8 x 10 (200 x 250)API
in2 (mm2)0.785 (506.5)
1.287 (830.3)
2.853 (1841)
6.380 (4116)
16.00 (10,320)
26.00 (16,770)

Table 4-2 - Weight ComparisonsValvein (mm)8" x 10" (200 x 250)
4" x 6" (100 x 150)
3" x 4" (80 x 100)2" x 3" (50 x 80)
1.5" x 2" (40 x 50)

Inlet
Flange
150#
300#
600#
600#
900#

Typical Direct
Spring PRV
lbs. (kg)
600 (272)
230 (104)160 (73)70 (32)50 (23)Full Borein2 (mm2)
1.496 (965.2)
2.895 (1868)
6.733 (4344)
10.75 (6941)
23.32 (15,050)
44.17 (28,500)

Typical
POPRV
lbs. (kg)421 (191)
160 (73)
92 (42)53 (24)45 (20)Figure 4-27
Height Comparison of Direct Spring vs Pilot Operated PRV(6” x 8” valves shown)
+90%
+125%
+135%
+68%
+45%
+70%

WeightSavings
30%
30%
42%
24%
10%

Provided that conditions are suitable, we learned in Chapter 3 that thepilot operated PRV can be used for any ASME Section I or VIII installation.
In order for a pilot operated PRV design to be certified per the ASME
Code, the design must be shown to be fail-safe if any essential part ofthe valve is compromised. For example, if the piston seal is damaged
and cannot hold pressure in the dome volume, the main valve will failopen. One other provision of the Code is that the pilot valve must be
self actuated and use the process itself and not an external source tooperate. The pilot will use the process pressure to either snap open or
modulate the main valve during a relief cycle.
Snap Action Pilot Design
The first pilot design that was developed was a “pop” or snap action typethat allowed the main valve to obtain significant lift at the set pressure.
Figure 4-28 shows the snap action pilot operated PRV in the normal
closed position. The red color represents the process pressure and when
the pilot relief seat is closed, this pressure is ported to the dome areaof the main valve. The pressure is the same at the main valve inlet anddome and the larger piston seal provides the downward seating load.
The pilot relief seat is held closed by the compression of the pilot spring.
The pilot relief seat is the outlet for the trapped pressure being heldin the dome of the main valve. When the process pressure beneaththe relief seat overcomes the spring compression, the pilot will open
allowing the dome pressure to exhaust to atmosphere as shown in
Figure 4-29. A snap action pilot will reduce the dome pressure to nearlyatmospheric immediately upon opening and this will allow full travel
or lift of the main valve piston or diaphragm at set pressure. Recall
that API Standard 526 direct acting spring loaded safety valves are also
pop action type valves but they only go into a partial lift at set pressure,usually no higher than 70% of full lift. Overpressure is required to obtainthe full lift for a direct acting spring PRV but no overpressure is needed
for full main valve lift when using a snap action pilot operated PRV.
You will notice in Figure 4-29 that the pressure at the inlet of thepilot remains at the set pressure when the main valve is opening
and relieving. This is indicative of what is termed a “non-flowing”
pilot operation. A non-flowing pilot design prohibits the process
fluid from circulating through the pilot during the relieving cycle.
This is accomplished in Figure 4-29 by using a second pilot seat called
the blowdown seat which will seal off when the pilot relief seat opens.

This block and bleed type of operation keeps the pressure static in thesense line from the process being protected to the pilot valve. The pilot
relief seat and blowdown seat cannot be open at the same time, thus
eliminating flow through the pilot when the main valve is open.
! that
The first pilot valves that were designed were “flowing” pilot designs
did not have this blowdown seat feature. These pilots were limited toservices where there was minimal debris or moisture in the system sothat the pilot could reliably reload the dome pressure in the main valve toclose the assembly. If there was a blockage in the sense line or pilot, themain valve would remain open. The non-flowing pilot designs were first
introduced over 40 years ago and have proven that, if properly designed
and applied, they can be used where dirt, hydrates and high moisture
content occur in the fluid media.

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Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
Figure 4-28
Pop Action Pilot Operated PRV (closed)

100% Set
Dome Area

Relief Seat

Blowdown Seat
100% Set

Figure 4-29
Pop Action Pilot Operated PRV (open)
100% Set

Exhaust Vent

0%

Relief Seat

Blowdown Seat

100% Set

Blowdown
Adjustment
Sense Line

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Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals

Modulating Action Pilot Design
As with direct acting spring loaded PRVs, there are design alterations
that need to be made to enable a pilot operated PRV to operate suitably
in incompressible media. In order to provide stability during lift in aliquid application, the pilot design should not allow the immediate fullevacuation of dome pressure to occur at the set point. A regulated
amount of the dome pressure should be reduced by the pilot during
operation so that the main valve unbalanced member, such as thepiston, is at some partial lift position to throttle or modulate the releaseof the service fluid. Most manufacturers will physically change thecomplete pilot assembly from a snap action design to this modulating
action design and use the same main valve assembly.
There are flowing and non-flowing modulating pilot designs that areprovided today. The most common style is the non-flowing designs thatkeep the velocity in the pilot low to minimize the exposure that the pilot
may have to debris in the system.
Figure 4-31 shows a similar main valve assembly that was discussed forsnap action pilot designs. The snap action pilot has now been replacedwith a modulating action pilot. The green color illustrates the processpressure entering the main valve and exerting an upward force on themain valve seat. This same pressure is allowed to enter the modulating
action pilot via the pitot tube and is ported to a sense diaphragm in thisparticular pilot design. The simmer and set pressure of the complete
pilot operated PRV assembly are determined by the spring compression
and the exposed area of the sense diaphragm to the process pressure.
The sense diaphragm is mechanically attached to the feedback piston
component and these two trim parts will move as one when necessary.
When the process pressure is below the simmer point of the pilot, thepilot spring positions the feedback piston via the sense diaphragm toallow the pilot inlet seat shown in Figure 4-31 to be open. The inlet
seat communicates the process pressure from the inlet of the pilot tothe dome of the main valve. The process pressure is then providing adownward force to the piston via the piston seal, keeping the main seatclosed.

Figure 4-30
Main Valve Lift vs Set Pressure for Pop Action Pilot Operated PRV
100
75% Full Lift

The main valve recloses when the pilot valve senses a reduced
process pressure and the relief seat closes and blowdown seat opens
simultaneously thus reloading the dome pressure. The dome volumes
are minimal, for instance a 2” x 3” (50 mm x 80 mm) main valve typicallyhas just over 3 cubic inches (50 cubic cm) above the piston.
The blowdown is adjustable for most snap action, non-flowing, pilot
valve designs. Unlike a direct acting spring loaded valve, there areno blowdown rings used for the adjustment. It is the physical upward
travel of the pilot relief and blowdown seats that determines thereseat pressure of the pilot and subsequently the main valve. The
blowdown seat is contained in a component that has a spacer between
the blowdown seat and relief seat. This spacer provides a directcommunication between the two seats when pilot relief seat is open as
shown in Figure 4-29. The more compression imparted to the spring
during its opening cycle, the higher the reseat pressure or the shorterthe blowdown. The blowdown adjustment screw shown in Figure 4-29
can be rotated into or out of the bottom of the pilot body. This threaded
component changes the position of the blowdown seat stop. The higher
the stop, the more spring compression and the shorter the blowdown.
The typical performance curve for a snap action pilot operated PRV
is shown in Figure 4-30. Since these pilots open the main valve to fulllift, a snap action pilot is considered a safety valve and suitable only
for compressible media. The pilot relief seat needs the gas or vapor
expansion properties to remain open during the relief cycle so thatthe dome pressure can exhaust immediately. This pilot design may
be unstable in its lift when used in liquid applications.

50

Blowdown

25
0
90

100

% Set Pressure

105

110

The difference in the sealing area of the piston seal and the exposedarea of the main seat to the process pressure is a notable parameter.
This difference will be important in calibrating the pilot to open the main
valve at the proper pressure. As an example, many of the piston main
valve designs have approximately a 30% larger piston seal area versus
the main valve seat area. Since the modulating pilot is regulating theexhaust of the dome pressure when it operates, the main valve will notopen until a minimum 30% of the dome pressure is released. If we havean installation where the required set pressure is 100 psig (6.90 barg),
then the pilot must reduce the dome pressure to 70 psig (4.83 barg) with
100 psig (6.90 barg) at the pilot sense diaphragm.

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Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
The sequence of events that occurs in the pilot to allow the main valve
to modulate open is as follows. Referring to Figure 4-32, as the processpressure nears the set point, the sense diaphragm begins to move
upward to compress the spring. Recall that the feedback piston moves
with the sense diaphragm, therefore the inlet seat of the pilot willclose. If the process pressure continues to rise, the feedback piston willcontinue to move upward and will literally pick up the spool shown in
the pilot trim detail portion of Figure 4-33. Once this occurs, the outlet
seat opens and allows the dome pressure to begin to bleed out. Thisinitial release of process from the pilot will not signify the set pressure
because there is still enough pressure in the dome to keep the main
valve closed.
If the process pressure stabilizes during this initial pilot opening, thefeedback piston and spool piece areas are designed to allow the pilot
spring to close the outlet seat. When the inlet seat and outlet seat areclosed this is called the “null” position. There is no pilot flow in this nullposition. This is shown in the pilot trim detail of Figure 4-32.
If the process pressure continues to increase, then the outlet seat willremain open and the dome pressure will reduce to a level (oftentimes
this is 30% below the main valve inlet pressure) where the main valve
opens. This is the set pressure of the complete pilot operated PRV
assembly.
As the process pressure rises above this set pressure, dome pressure
reduction will continue and this will position the main valve piston lift
to a point where the process pressure stabilizes. The feedback pistonand spool will position the pilot inlet and outlet seat to close to provide
no flow through the pilot when the main valve is open. This is shown in
Figure 4-32.

If the process pressure continues to rise, the pilot outlet seat will open
until the main valve obtains the needed lift to deliver only the required
capacity of the overpressure contingency. Once the process pressure
begins to decay, the inlet seat will open and dome pressure will be
restored to begin the closing cycle of the main valve. At no time during
its operation will the pilot inlet seat AND outlet seat be open at the sametime. This prevents a continuous flow in the pilot.
A graphical representation of the modulating pilot open and closing
cycle is shown in Figure 4-34. Unlike the snap action pilot design, butlike the direct acting spring operated PRV design, there is overpressure
required to obtain lift. It should be noted that the modulating action pilot
operated PRV, in addition to liquid service, is also suitable for use ingas applications or a mixture of gas and liquids. It is a true safety relief
valve and can be ASME certified for either service. In compressible,
incompressible, or multi-phase flow, the performance curve is as
depicted in Figure 4-34.

Figure 4-31
Modulating Action Pilot Operated PRV (closed)
Inlet Seat

Set Pressure
Adjustment

Spool

Sense Diaphragm
Dome

Tubed to Main
Valve Outlet

Outlet Seat

Feedback Piston

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Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
Figure 4-32
Modulating Action Pilot Operated PRV (open)

Pilot Spring

Sense
Diaphragm

Piston Seal

Inlet Seat

Feedback
PistonMain Seat

Tubed to Main
Valve Outlet
Pitot Tube

Spool
Outlet Seat

Figure 4-33
Modulating Action Pilot Operated PRV (open and in full lift)
Pilot Spring

Sense
Diaphragm

Piston Seal

Inlet Seat

Feedback
PistonMain Seat

Tubed to Main
Valve Outlet
Pitot Tube

Spool
Outlet Seat

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Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
Figure 4-34
Main Valve Lift vs Set Pressure for Modulating Action
Pilot Operated PRV

Closing

Opening

100% Lift

Main Valve Piston Lift

Unlike the direct spring operated PRV, the main valve will only open toa lift needed to flow the required capacity. Recall in Figure 4-7 or 4-12,
there is an abrupt lift (but not a full lift) at set pressure for gases or agush of liquid during the overpressure of a direct acting relief valve.
If there is any capacitance in the inlet piping to the valve, these direct
acting devices may initially flow more than the source of overpressure
can provide. The modulating action pilot operated PRV operation willconserve the product during an upset, minimize interaction with control
valves trying to balance the system, lower reaction forces in the piping
supporting the PRV, reduce noise levels and provide stability at low
required capacity relieving contingencies. API Standard 521 and ISO
23251 will allow piping that is immediately downstream of a modulating
PRV to be sized based upon the required capacity rather than the rated
capacity. This can optimize the pipe size and save installation costs.
The blowdown for the modulating pilot valve assembly is minimal.
Most modulating pilot designs will have a worst case blowdown of 3%
to 4% for either gas, liquid or multi-phase applications. In multi-phaseapplications this can provide an advantage over the direct acting springloaded valve especially when the quality of the gas portion of the fluid ishigh which can create a blowdown as much as 20%.

Set
Pressure

Pilot Operated Pressure Relief Valve Seat Designs
The majority of pilot operated pressure relief valves provided today utilize
soft seats and seals in the main valve and the pilot. This helps to provide
the tight shutoff of the process as the operating pressure approaches
the set pressure. There is no difference in the API Standard 527 leakage
standards between direct spring or pilot operated pressure relief valves.
As noted previously, a soft seated valve design is normally tested at
90% of the set pressure and the requirement is zero leakage. Many
manufacturers will do a leak test of their pilot valve assemblies up to
95% of the set pressure in many instances.
Figure 4-35
Piston Type Main Valve Components
Tube Fitting

Cap Bolt

Cap

Dome Spring

Lift Sop Bolt

Piston Seal

Liner

PistonRetainer
Screw

Seat Retainer

NozzleSeat

Body

Retainer
Tube Firtting
Nozzle Seal

Pilot Tube

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Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
Recall that the service fluid and temperature play an even more
important role in selecting a soft seat material versus metal seatmaterial. Since pilot operated valves are often selected for high pressure
applications, the design of the main valve seat containment and thematerial is of importance for the reliability of the seat during operation.
There are main valves with metal seats available for use with softseated pilots that may provide durability in the high velocity flow area of
the assembly. However, when the pressure, temperature or chemical
property of the process fluid is not suitable for an elastomer or plastic
material, there are also full metal seated and sealed main valves andpilots available.

Figure 4-36
Remote Sense Pilot Operated PRV

IntegralPressure
Sensing

RemotePressure
Sensing

Pitot Tube

Pilot Operated Main Valve Components
A typical piston style main valve is shown in Figure 4-35.
The pitot tube orientation is important for proper valve operation. This
component should be oriented to face directly at the flow of fluid that will
occur during start-up to assist in loading pressure into the dome, andfor relieving conditions to provide the pilot with an accurate stagnationpressure reading for operation.
Most main valve assemblies use semi-nozzle designs as shown in Figure
4-35. This is due to the needed intrusion of the pitot tube into the inlet
bore of the main valve. It is difficult to orient a threaded-in full nozzle toalign its porting with the pitot tube porting. Full nozzle main valves areavailable, but many designs require the pressure pick-up to be remotesensed. The remote sensing of pilot valves will be discussed later. Thesesemi-nozzles could be screwed, swadged, welded or internally retained
to stay in place in the main valve.

Figure 4-37
Balanced Modulating Pilot Operated PRV
Set Pressure Adjustment

Sense Diaphragm
Main Valve

Outlet Seat

Dome

PistonFeedback Piston

Spool Seat
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Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
The valve shown in Figure 4-35 is soft seated. The seat is held in placewith a retainer and retainer screw assembly. As mentioned previously,
the containment of this seat is important to hold it in place during a
relieving cycle. Some manufacturers will drill a hole in the seat retainerso that pressure can equalize across the soft seat. This hole will provide
an exhaust of trapped process pressure during an opening cycle so theseat will not extrude and blow out.
Just as with the disc holder and guide of a direct acting spring loaded
valve, the materials of construction of the dynamic piston assembly
and the static liner part are important factors to prevent galling. These
components should have a differential hardness to provide reliable cycle
life during normal operation.
The orifice of the main valve can be changed by the physical replacementof one nozzle to another nozzle with a different bore size. Anothermethod is to use the same nozzle and to change the maximum lift of thepiston during a relieving case via the lift stop bolt that is shown in Figure
4-35. If this bolt is screwed further into the top of the piston, there will be
more available lift and thus more capacity through the main valve.
The dome spring shown does not have a bearing on the opening andclosing operation of the complete valve assembly. Some manufacturers
install this spring above the piston to help keep the piston closed during
shipping and handling. This helps to seal the main valve seat to thenozzle during initial start-up of the process.

The most common inlet and outlet configuration for the main valve
is flanged. The API 526 purchasing standard lists the dimensional
information for pilot operated valves in one section and direct springloaded valves in another. It should be noted that some, but NOT all,combinations of similar pipe size, orifice size and pressure class will
be interchangeable between the two different types of pressure relief
valves.
Use Table 4-3 below as a reference for the comparison of the data in
API 526.
The full bore pilot operated valve orifices do not fall under the scope of
API 526.
There are also threaded and hubbed connections that are common forpilot operated valves.

Table 4-3 - API 526 Direct Spring vs Pilot Operated PRV Dimensions
150#
A

B

300# Light Weight
A
B

300#
A

B

Inlet Flange Rating
600#
A
B

900#
A

1500#
B

A

2500#
B

A

B

1D2
1.5D2
1E2
1.5E2
1F2
1.5F2
1.5F3
1G2
1.5G3
2G3
1H2
1.5H2
1.5H3
2H3
2J3
3J4
3K4
3K6
3L4
4L6
4M6
4N6
4P6
6Q8
6R8
6R10
8T10
Note: light weight flange drilled for 300# ASME class bolt pattern but not rated for full 300# ASME class pressure (for DSV only)
DSV and POPRV Dimensions are the Same

OutletDSV and POPRV are Different
Valve Size and Flange Rating not in API 526 for DSV
Valve Size and Flange Rating not in API 526 for POPRV
Valve Size and Flange Rating not in API 526 for either DSV or POPRV

A

Inlet
B

4.24

Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
Figure 4-38
Sonic to Subsonic Flow Transition100

k = 1.380% Rated Capacity

Inlet Piping Considerations
Pilot operated pressure relief valves with the pilot pressure sensing
line connected to the pitot tube at the main valve inlet can experience
rapid cycling or chatter due to high non-recoverable inlet losses just as
the direct acting spring loaded valve design. This is especially true withsnap action pilots.
Figure 4-36 shows the option of remote pressure sensing for the pilot
valve. Since the pilot is controlling the opening and closing of the main
valve, the remote sense line will allow the pilot to see the true systemor vessel pressure without being subjected to inlet pressure losses.
This remote sense line will allow the pilot to operate satisfactory, but itshould be noted that the capacity that the main valve can deliver will still
be affected by the inlet line losses.
There is not a maximum distance limitation for this remote sense line
when a non-flowing pilot is selected. The remote sense line remains
static when the main valve is opened by a non-flowing pilot. It is
recommended to examine any transient conditions that may occur
between the main valve location and the remote sense location. Any
remote sense line should be self draining back to the process, andpiping, instead of tubing, may be warranted for rigidity when these nonflowing pilot sense lines exceed 100 feet (30.5 meters).
If a flowing pilot is to be used in a remote sensing application, themanufacturer should be consulted for guidance on maximum remotesense distances.

60

40

20

0
0

20

40
60
% Back Pressure P2/P1

80

100

Discharge Piping Considerations
Pilot operated PRVs can be subjected to built-up, superimposed,
or a combination of both types of back pressure. Most pilot valves
manufactured today have spring bonnets and trim that are isolated
from the built-up back pressure that can be developed during a relief
cycle. This allows the pilot to properly control the dome pressure in themain valve to provide stable lift. If you refer back to Figure 4-29, you willrecall that the snap action pilot exhausts to atmosphere so there will
be minimal built-up back pressure present at the relief seat or springbonnet.

Figure 4-40
Backflow Preventer

Figure 4-39
Superimposed Back Pressure in
Piston Type Pilot Operated PRV

P1

P2
Piston

Ap

P2

As

P2

P2

P2
P1

P2

P2
P1

4.25

Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
Since modulating pilot valves are often used for liquid applications andsince many users do not want any emissions to atmosphere, either
liquids or gases, the exhaust port of these valves is typically routed tothe main valve outlet. In this case, the trim of the pilot is exposed to thedownstream piping and will see the built-up back pressure. In order tonot change the force balance in the trim of the modulating pilot, mostmanufacturers will design the pilot with a balanced spindle. In Figure
4-37, the outlet port of the pilot will see the built-up back pressure. In
this area of the pilot, the outlet seat, where the back pressure can act
upward and the spool seal, where the back pressure can act downward,
are the same effective sealing areas. Therefore, there is no change in thepilot operating characteristics and ultimately main valve lift.
Because of these design features described for a snap action or
modulating action pilot, the presence of superimposed back pressure
does not affect the opening pressure when the valve is in service. The
snap action pilot relief seat will not see the downstream pressure atany time. The modulating action pilot has the balanced spindle design.
Since this balanced design is obtained without using a bellows, the pilot
operated PRV can be used in applications where the total back pressure
exceeds the burst pressure rating of the bellows. It is not unusual for themain valve bodies to have their inlet and outlet flange ratings in a unique
combination for a pressure relief valve, such as 900# ANSI x 900# ANSI.
As with balanced bellows valves, there are back pressure correction
curves that adjust the available capacity from pilot operated PRVs thatare exposed to back pressure. Because this type of valve can operate in
installations with high back pressure, it is not unusual, in compressible
flow, for the back pressure to exceed what is called the critical pressureof the gas. The critical pressure determines whether the gas velocity issonic or subsonic. Once the gas flow transitions from sonic or chokedflow to subsonic, the amount of total back pressure will factor into thecapacity calculation. Please note that there is no loss of actual lift in themain valve that causes this capacity reduction, it simply is a result of
the gas flowing subsonically. This transition from sonic to subsonic flow
occurs when the total back pressure is approximately 50% of the set
pressure. The actual value of the critical pressure is determined by the
specific heat ratio of the gas. Figure 4-38 shows the sonic to subsonicflow transition for a gas with a specific heat ratio of 1.3.
Outlet piping pressure drop calculations provide the total back pressure
up to the outlet flange of the PRV. Since the evaluation of the criticalpressure should be at the exit of the area (i.e, the main valve nozzle)
that determines the flow capacity, the capacity correction curve for aparticular pilot valve may differ than that shown in Figure 4-38. The
downstream body geometry of the main valve can add additional built-up
back pressure and decrease the available capacity. This is discussed
more in Chapters 7 and 8.
As we learned earlier, one feature of the pilot operated PRV is that as
the operating pressure increases, the seating force increases. This isan advantage when the operating pressure approaches the set pressure
but can be a disadvantage when the superimposed back pressure could
exceed the operating pressure. To illustrate, see Figure 4-39. This
figure shows how the back pressure (P2) causes an upward force on themain valve piston. If the operating pressure (P1) is not high enough toovercome this upward force, the piston could open and flow backwards
from the outlet piping into the system being protected. This backflow
could be considered a hazard. This backflow could even occur if theoutlet of the main valve is open to atmosphere and there is a normal
vacuum condition on the inlet.

To prevent the possibility of reverse flow in the main valve, an accessory
called a backflow preventer is required whenever the back pressure
could exceed the operating pressure. Figure 4-40 shows a simple shuttle
check valve that is mounted on the main valve. The figure illustrates
that when the back pressure exceeds the inlet pressure that the shuttle
check will allow back pressure to enter the dome of the main valve andprovide a net downward loading force to keep the main valve seat tight.
The use of the backflow preventer is also recommended whenever themain valve outlet is attached to a piping lateral even though the backpressure may never exceed the operating pressure. There may becases when the system that the pilot operated PRV is protecting is outof service and there is no operating pressure. If there is back pressure,there is a possibility of the main valve opening in this scenario. The
main valve, even with a backflow preventer, should not be considered
an isolation device, and blinds or closed isolation valves should be used
when the system being protected is idle.

4.26

Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
IV. Advantages and Limitations of Valve Types
The following summarizes the advantages and limitations of the pressure relief types discussed in this chapter. The summary is not intended to
be an absolute list but a generalization of the pros and cons of each design type. The specific application and prior user experience will play animportant role to determine the best recommendation.
Weighted Pallet Type
Advantages
Low initial cost
Very low set pressures available
SimpleConventional Metal Seated Type
Advantages
Low initial cost
Wide chemical compatibility
High temperature compatibility
Standardized flanged center to face dimensions
Accepted for ASME Section I and VIII
Balanced Bellows Metal Seated Type
Advantages
Wide chemical compatibility
High temperature compatibility
Standardized flanged center to face dimensions
Protected guiding surfaces and spring
No change in opening pressure at any superimposed back pressure
Withstand higher built-up back pressures
Conventional Soft Seated Type
Advantages
Low initial costStandardized flanged center to face dimensions
Good seat tightness before relieving and after reseating
Low maintenance costs

Limitations
Set pressure not readily adjustable
Long simmer and poor tightness
High overpressures required for full lift
Cryogenic fluids can freeze seat close
Set pressure limited to 1 or 2 psi (69 mbar or 138 mbar)
Limitations
Seat leakage
Simmer and blowdown adjustment interactive
Vulnerable to inlet pressure losses
Opening pressure changes with superimposed back pressure
In situ testing can be inaccurate
Built-up back pressure limitations
Limitations
Seat leakage
Simmer and blowdown adjustment interactive
Vulnerable to inlet pressure losses
In situ testing can be inaccurate
Bellows can limit amount of superimposed back pressure
High initial cost
High maintenance costs
Limitations
Simmer and blowdown adjustment interactive
Vulnerable to inlet pressure losses
Opening pressure changes with superimposed back pressure
Built-up back pressure limitations
High process fluid temperatures
Chemical compatibility

4.27

Pressure relief valve engineering handbook
Chapter 4 - Design Fundamentals
Balanced Bellows Soft Seated Type
Advantages
Standardized flanged center to face dimensions
Protected guiding surfaces and spring
No change in opening pressure at any superimposed back pressure
Withstand higher built-up back pressures
Good seat tightness before relieving and after reseating

Balanced Piston Soft Seated Type
Advantages
No change in opening pressure at any superimposed back pressure
Withstand higher built-up back pressures
Good seat tightness before relieving and after reseating
Low initial cost
Low maintenance cost
Pilot Operated Soft Seated Type
Advantages
Standardized flanged center to face dimensions
No change in opening pressure at any superimposed back pressure
Withstand higher built-up back pressures
Good seat tightness before relieving and after reseating
Higher set pressures available
Maximum capacity per inlet valve connection
Smaller and lighter valves in higher pressure classes and sizes
In-line maintenance of main valve
Pop or modulating action
Remote pressure sensing
Accurate in situ testing
Full lift at zero overpressure available

Limitations
Simmer and blowdown adjustment interactive
Vulnerable to inlet pressure losses
Bellows can limit amount of superimposed back pressure
High initial cost
High maintenance costs
High process fluid temperatures
Chemical compatibility
Limitations
Simmer and blowdown adjustment interactive
Vulnerable to inlet pressure losses
High process fluid temperatures
Chemical compatibility

Limitations
High initial cost
High process fluid temperatures
Chemical compatibility
Polymer or viscous fluids
Complexity

4.28

Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
The following data is included in this chapter:
I.	Introduction.....................................................................................................................................5.3
Procedure.........................................................................................................................................5.3
Pressure Relief Valve Nozzle Coefficient of Discharge.................................................................5.3
	API vs ASME Nozzle Coefficient of Discharge...............................................................................5.3
II.	Gas/Vapor Sizing – Sonic Flow......................................................................................................5.4
	Equations, Variables, Unit of Measures
III.	Gas/Vapor Sizing – Subsonic Flow................................................................................................5.5
	Equations, Variables, Unit of MeasuresIV.	Steam Sizing....................................................................................................................................5.5
	Equations, Variables, Unit of Measures
	ASME Section VIII.............................................................................................................................5.5
	ASME Section I`...............................................................................................................................5.6
V.	Liquid Sizing..................................................................................................................................5.11
	Equations, Variables, Unit of Measures
Thermal Relief
VI.	Fire Sizing......................................................................................................................................5.11
Liquid Filled Vessels....................................................................................................................5.11
	API 521 (ISO 23251)........................................................................................................................5.12
	API 2000 (ISO 28300)......................................................................................................................5.14
	NFPA 30..........................................................................................................................................5.15
	NFPA 58/59A..................................................................................................................................5.16
Gas Filled Vessels.........................................................................................................................5.17
	API 521 (ISO 23251)........................................................................................................................5.17
VII.	Two-Phase Flow Sizing................................................................................................................5.17
API 520 Part I (8th Edition)...........................................................................................................5.18
Two-Phase Flow Mixture Procedure............................................................................................5.18
	Subcooled or Saturated All Liquid Flashes..................................................................................5.20
ASME Section VIII..........................................................................................................................5.24
	Flashing of Saturated Water.........................................................................................................5.24
VIII.	Noise Level Calculations..............................................................................................................5.25
IX.	Reaction Forces............................................................................................................................5.26
	Gas Service.....................................................................................................................................5.26
	Steam Service................................................................................................................................5.26
	Liquid Service.................................................................................................................................5.27
Two-Phase Service........................................................................................................................5.27

5.1

Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
The following Figures are included in this chapter:
Figure 5-1 Varec Brand Pressure Vent Capacity Chart..................................................................... 5.6
Figure 5-2 Closed Simple Rankine Steam Cycle............................................................................... 5.7
Figure 5-3 Critical Pressure Ratio for Two-Phase Flow................................................................. 5.20
Figure 5-4 Critical Pressure Ratio for Low Subcooled Region Flashing Liquids.......................... 5.22
Figure 5-5	ASME Section VIII Appendix 11 – Available Mass Flux - Saturated Water................... 5.25
Figure 5-6	Sound Pressure Level at 100 Feet from Point of Discharge........................................ 5.26
Figure 5-7	Open Discharge Reaction Force..................................................................................... 5.27
The following Tables are included in this chapter:
Table 5-1	ASME Section I Drum/Superheater Sizing Example Summary..................................... 5.9
Table 5-2	ASME Section I Reheater Sizing Example Summary.................................................... 5.10
Table 5-3	Saturated Steam Capacities - Set Pressures 2760 - 3090 psig................................... 5.12
Table 5-4	Saturated Steam Capacities: Set Pressures 560 - 1100 psig....................................... 5.13
Table 5-5	API 520 (ISO 23251)/API 2000 (ISO 28300) Environmental Factor................................ 5.14
Table 5-6	API 2000 (ISO 28300) Heat Input..................................................................................... 5.14
Table 5-7	NFPA 30 Equivalent Air Capacity Requirement............................................................. 5.15
Table 5-8	NFPA 30 Equivalent Air Capacity Requirement (Tanks with Wetted Area > 2800 ft2
and Design Pressures > 1 psig)...................................................................................... 5.16
Table 5-9	NFPA 58/59A Environmental Factor.............................................................................. 5.16
Table 5-10	Anderson Greenwood Crosby Recommended Valve Design for Two-Phase Flow..... 5.21
Table 5-11	Noise Intensity (at 100 feet from the discharge............................................................ 5.25

5.2

Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
I. Introduction
This section of the Emerson Pressure Relief Valve Engineering
Handbook is laid out to assist the user in the sizing and selection ofpressure relief valves when system parameters are expressed in UnitedStates Customary System (USCS) units. The procedures and equationsin this chapter are consistent with the requirements of the ASME
Boiler and Pressure Vessel Code and API Recommended Practices
and Standards. Please refer to Chapter 6 for sizing using metric unit
formulations.
Please visit the Emerson Sizing Website for access to PRV2SIZE. The
address is valvesizing.emerson.com. This sizing program will performmany of the sizing techniques discussed in this chapter.
Procedure
Before the determination can be made of the required pressure relief
valve orifice area, an in-depth analysis of various overpressure scenarios
for the equipment being protected must be completed. API Standard 521
is oftentimes used as a guide to determine what possible causes ofoverpressure could occur and what subsequent required relieving capacity
is necessary to mitigate the system upset. These standards will help theprocess engineer determine the worst case scenario from unexpected
system conditions such as blocked outlets, reflux failures, power failures,
overfilling, exchanger tube damage, and external fire. There are manyother possible overpressure conditions listed in the standard.
API Standard 2000 contains similar information on causes ofoverpressure and vacuum, and the required relieving capacity for theprotection of atmospheric or low pressure storage tanks.
One key piece of information for the sizing of the pressure relief valve isthe knowledge of the largest required capacity that results from one ofthese overpressure conditions. This required capacity is often referred
to as the “worst case scenario.” This chapter will help you with the sizing
techniques to obtain the proper pressure relief valve orifice for this worst
case scenario.
It should be noted however, that the final selection of the pressure relief
valve type and its materials of construction may be based upon otheroverpressure contingencies. For example, a worst case scenario may bewhen a liquid is boiled off into a vapor due to an external fire. A pressure
relief valve is sized based upon this vapor flow rate. There may be
another overpressure condition where the liquid could overfill and thisliquid flow rate requires a smaller orifice. As we learned in Chapter 4,
not all pressure relief valve trims designed for vapor flow work well on
liquid flow. If the lesser contingency is ignored during the pressure relief
device selection, then an improper valve might be installed.

If you look further in either Section I or Section VIII of the ASME Code,there is one procedure where the manufacturer is required to test threevalves in three different sizes, for a total of nine tests. The Kd value foreach of these nine tests is calculated and averaged. The requirement isthat none of these nine Kd values can vary any more than plus or minus
5% of the average Kd.
Most gas or steam certified safety valves that use the nozzle bore as
the flow limiting dimension are quite efficient as compared to the ideal
nozzle. It is not unusual to have a Kd value of 0.950 or higher for thesevalves. The Kd value for liquid certified relief valves is much lower or in
the range of 0.750.
An additional requirement in the ASME Code (both Section I and Section
VIII) is to reduce the flow tested Kd value by 10%. This requirement isalso found in ISO 4126. This reduced coefficient provides an additionalsafety factor when calculating the required flow area for a pressure
relief valve. For example, if a safety valve is tested to have a Kd equal
to 0.950 then what is called the “ASME rated” nozzle coefficient ofdischarge is 0.950 x 0.900 or 0.855. This ASME rated nozzle coefficientis typically denoted as K (K = Kd x 0.9). The valve sizing formulas outsideof the scope of ASME (below 15 psig) will use the actual flow tested Kdvalues.

Pressure Relief Valve Nozzle Coefficient of Discharge
As you review the various orifice sizing formulas in this chapter, you willnote that there will almost always be one variable that will be listed as
the valve coefficient of discharge. This value is specific to a particularvalve design and illustrates the imperfect flow characteristics of thedevice. The best nozzle coefficient of discharge (Kd) would be that of an
ideal nozzle. The value of the Kd is the quotient of the actual measured
flow divided by the theoretical flow of an ideal nozzle. Therefore, the Kd
for a particular valve can be no larger than 1.0.
There are various codes and standards that require actual flow tests tobe performed to establish the flow efficiency of a pressure relief valve.
For example, there are testing procedures described in documents, suchas the ASME Boiler and Pressure Vessel Code, ISO 4126, and API 2000,
that will establish the Kd of a particular valve design.

5.3

Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
API Effective vs ASME Section VIII Rated Nozzle Coefficient ofDischarge
The ASME Section VIII rated nozzle coefficient of discharge (K) willvary from one valve design to the other, one service (i.e. compressible
versus incompressible) to the other, and one manufacturer to theother. Therefore, if the valve manufacturer and/or the valve design is
not yet selected, and a preliminary pressure relief valve size for an
ASME Section VIII valve is needed, many users will refer to API Standard
520 part I to obtain what are called effective nozzle coefficients. This
recommended practice publishes one common nozzle coefficient ofdischarge for gases, steam and liquids to be used for preliminary sizingof the flow orifice area.
When selecting the preliminary flow orifice size, API 520 part I will point
the user to the API Standard 526. This API 526 standard is where you willfind the effective flow orifice sizes for what are more commonly called
the “lettered” orifice designations. The scope of the API 526 standard is a1” x 2” (D orifice designation) through an 8” x 10” (T orifice designation).
The scope of API 526 is limited to flanged direct spring loaded andflanged pilot operated pressure relief valves.
Once the manufacturer and specific design are decided, API 520 part I
will instruct the user to recalculate the required flow orifice size using
the ASME rated nozzle coefficient of discharge (K). The actual flow orificearea of the valve selected should be compared to meet or exceed thecalculated orifice area value.
The API effective coefficient of discharge and effective orifice areas areillustrated with the applicable Anderson Greenwood Crosby models thatmeet API Standard 526. The direct spring valves are shown in Table 7-6
and the pilot operated valves are shown in Table 7-11. The preliminary
sizing per API can be completed using these values. You will note thatthe information for the effective nozzle coefficients and orifice areas areexactly the same for the two different valve designs.
The ASME rated coefficient of discharge (K) and the actual flow orificearea for these same valve designs are shown in Table 7-7 for the direct
spring valves and Table 7-12 for the pilot operated valves. You will now
notice the ASME rated coefficient of discharge and actual flow orificeareas are different because these values are specific to the valve design.
The user should be aware that the use of the API effective values insizing these particular Anderson Greenwood Crosby brand products will
always be conservative. The recalculation of the required orifice size
using rated coefficient of discharge (K) and comparing the answer to theactual orifice area will always allow for the same valve size, or smaller,
to that identified in the preliminary API sizing.

Set Pressures ≥ 15 psig
The following formulas can be used for sizing valves when the set
pressure is at or above 15 psig.
Weight Flow (lb/hr)

A =W
C K P1Kb Kc

TZ
M

Volumetric Flow (scfm)

A =

V MTZ
6.32 C K P1Kb Kc

Where:A	
= Minimum required discharge area, square inchesC= Gas constant based upon the ratio of specific heats of the gas
or vapor at standard conditions. See Chapter 7 Section VI. Use
C = 315 if ratio of specific heats is unknown
K	 = Coefficient of discharge. See Chapter 7 Section IX
Kb = Back pressure correction factor for gas. See Chapter 7 Section II
Kc = Combination factor for installations with a rupture disc upstreamof the valve. See Chapter 7 Section XI for flow certified factors.
Use a 0.9 value for any rupture disc/pressure relief valve
combination not listed in Chapter 7 Section XI. Use a 1.0 value
when a rupture disc is not installedM= Molecular weight of the gas or vapor. See Chapter 7 Section VII
for common gases
P1 = Relieving pressure, pounds per square inch absolute. This is theset pressure (psig) + overpressure (psig) + atmospheric pressure(psia) – inlet pressure piping loss (psig)
T= Absolute relieving temperature of the gas or vapor at the valve
inlet, degree Rankine (degree Fahrenheit + 460)
W
= Required relieving capacity, pounds per hour (lb/hr)
V= Required relieving capacity, standard cubic feet per minute (scfm)
Z= Compressibility factor. See Chapter 7 Section I

In no case should an api effective coefficient of discharge or effective area
be used with the rated coefficient of discharge or actual area to perform any
calculation. Sizing errors can be made if the effective values are mixed withthe actual values.
For Anderson Greenwood Crosby valve designs that do not fall within
the scope of API 526, such as portable valves, ASME Section I valves, orfull bore pilot operated valves, it is suggested to always use the rated
coefficient of discharge and actual orifice area for any sizing.
II. Gas/Vapor Sizing – Sonic Flow
The orifice sizing for vapors or gases can be done either by capacity
weight or by volumetric flow rates. The formulas used are based on theperfect gas laws. These laws assume that the gas neither gains nor
loses heat (adiabatic), and that the energy of expansion is converted into
kinetic energy. However, few gases behave this way and the deviationfrom the perfect gas laws becomes greater as the gas approaches
saturated conditions. Therefore, the sizing equations will contain various
correction factors, such as the gas constant (C) and the compressibility
factor (Z), that illustrate deviation from the perfect gas law.
5.4

Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
III. Gas/Vapor Sizing – Subsonic Flow
Set Pressures < 15 psig or Vacuum Conditions
The following formulas can be used for sizing valves when the set
pressure is below 15 psig. When pressure relief valves operate on gases
or vapors below 15 psig, the speed at which the service fluid travels isalways less than the speed of sound or subsonic. Under these conditions,the flow decreases with increasing back pressure as the upstream
flowing pressure stays the same.
These equations can be used to size the Anderson Greenwood low
pressure pilot operated valves listed in Chapter 7, Tables 7-14 and 7-15.
Weight Flow (lb/hr)

W TZ
A =
735 Kd P1 F M
Volumetric Flow (scfm)

V MTZ
4645 Kd P1 F

F=

k
k- 1

(

[(

2

P2 k
P1 -

(

P2
P1

(

Where:

A =

k+1
k

]

This formula is based upon the empirical Napier formula for steam flow.
The nozzle coefficient of discharge and the back pressure correction
factors are the same as those in the previous gas/vapor section. There isa new factor for steam that is above its saturation temperature, or is in
a superheated condition. The more superheated the steam, the greater
the required orifice area. A second, but rarely used input, is the Napierequation correction factor. This factor is only used for dry saturated
steam when the set pressure is 1500 psia or greater.

A =W
51.5 KP1Ksh Kn Kb

Where:A	 = Minimum required discharge area, square inches
W = Required relieving capacity, (lb/hr)
K	 = Coefficient of discharge. See Chapter 7 Section IX
P1 = Relieving pressure, pounds per square inch absolute. This is theset pressure (psig) + overpressure (psig) + atmospheric pressure(psia) – inlet pressure piping loss (psig)
Ksh = Capacity correction factor due to the degree of superheat in thesteam. For saturated steam use 1.0. See Chapter 7 Section V
Kn = Capacity correction factor for dry saturated steam at set
pressures above 1500 psia. See Chapter 7 Section III
Kb = Back pressure correction factor for gas. See Chapter 7 Section II

Where:A	 = Minimum required discharge area, square inches
Kd = Coefficient of discharge. See Chapter 7 (Tables 7-14 and 7-15)
k= Specific heat ratio. See Chapter 7 Section VII for common gases
M = Molecular weight of the gas or vapor. See Chapter 7 Section VII for
common gases
P1 = Relieving pressure, pounds per square inch absolute. This is theset pressure (psig) + overpressure (psig) + atmospheric pressure(psia) – inlet pressure piping loss (psig)
P2 = Pressure at the valve outlet during flow, pounds per square inch
absolute. This is the total back pressure (psig) + atmosphericpressure (psia)T= Absolute relieving temperature of the gas or vapor at the valve
inlet, degree Rankine (degree Fahrenheit + 460)
W = Required relieving capacity, pounds per hour (lb/hr)
V= Required relieving capacity, standard cubic feet per minute (scfm)
Z= Compressibility factor. See Chapter 7 Section I
The flow characteristics for the Varec brand weight loaded pressure andvacuum vents are unique, not only for each model, but also for each sizeof a particular model. The coefficient of discharge method is different foreach of these many combinations and is not easy to select an orifice sizewith equations. It is suggested to use flow capacity charts from the Varec
catalog to manually select the valve size. The example shown in Figure
5-1 below shows the available flow capacity for a vent with a set pressureof 4 inches of water column. One point on this chart shows that a 3 inch
vent with 2 inches of water column overpressure (i.e. 6 inches w.c.
flowing pressure) will flow 20,000 standard cubic feet of air per hour.
IV. Steam Sizing
ASME Section VIII (Set Pressures ≥ 15 psig)
The following formula is used for sizing safety valves for process vessel
applications that normally are not considered fired vessels. Examples offired vessels are economizers, steam drums, superheaters and reheatersthat fall under the ASME Section I scope. As discussed in the previous
gas/vapor section, the determination of the required steam relieving rateis needed before sizing can begin. Once again the use of API Standard 521
can be helpful to determine the required steam flow due to sources ofoverpressure such as a split exchanger tube or reflux failures.
5.5

Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)

40
30

Pressure
Inches of Water

20																		100						700		
900
1
2
3
4
5 6 7 8 9 102030
40 50 60 70 80 90		
200
300 400 500 600		 800		 1000
Curve Data by Actual Tests in Accordance With
API Std. 2000. Certified Test Data by Independent
Laboratory Available on Request.

2”

3”

4”

6”

8”

10” 12”

10
9
8
7
6
5
4
3
2

1
0.9
0.8
0.7

1
2
3
4
5 6 7 8 9 102030
40 50 60 70 80 90		
200
300 400 500 600		 800		1000																		100						700		
900

Thousands of Cubic Feet/Hour at 60°F and 14.7 psia, Air Flow (SCFH)
Figure 5-1
Varec Brand Pressure Vent Capacity Chart

ASME Section I (Set Pressures ≥ 15 psig)
The sizing and selection of steam safety valves for fired pressure vesselsthat fall under the scope of ASME Section I has a different procedurethan an ASME Section VIII steam sizing case. The steam sizing equation
listed above could be used, but there are certain valve selection rules
within ASME Section I where the use of valve capacity charts provides fora simpler procedure.
Steam drum safety valve sizing
The steam drum is one such fired pressure vessel that receives thesaturated steam from water that has been heated by burning an externalfuel source such as coal or natural gas. The boiler system may consistof only this steam drum or may have other vessels used to add heat that
we will discuss below. For the purposes of this initial discussion, let
us assume the boiler system has only a steam drum. As with the sizing
procedures discussed previously, the required steam relieving rate
must be determined to size the drum safety valve. This is fairly straightforward as, in most instances, the required capacity shall not be smaller
than the maximum designed steaming output of the boiler at its MAWP.
The user should refer to the catalog where the saturated steam capacity
tables are located. The following link will provide access to the Crosby
HL, HSJ, HCI and HE steam safety valves:(http://www.valves.emerson.com/valves/Images/CROMC-0295-US.pdf)

or greater of that of the larger valve. Beyond this requirement, there
are no other rules on how the overall required capacity is to be divided
between multiple valves but it is often found that the capacity be evenly
split between the multiple valves. This will allow the valves to be of
the same configuration which can optimize the use of spare parts formaintenance.
These same selection rules in Section I apply when the boiler system
has additional vessels in its train. However, there are additional
requirements to consider that will be discussed next for the superheater,
reheater and economizer.

Although the determination of required capacity is often simple, theselection process is more involved as there are rules to be followed in
the ASME Section I Code. One such requirement is that if a boiler system
has a combined bare tube and extended heating surface exceeding
500 square feet, and a design steaming generation capacity exceeding
4000 lb/hr, then the drum must have two or more safety valves. In thesecases where two valves are to be used, there is a requirement in ASME
Section I that the relieving capacity of the smaller valve be at least 50%
5.6

Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
Superheater safety valve sizing
As shown in Figure 5-2, when the steam created in the drum is being
used to turn a turbine to create work, the steam drum outlet is often, but
not always, attached to a heat exchanger vessel called a superheater.
The moisture in saturated steam coming from the drum can cause
corrosion and damage to the turbine blades. Therefore, the use of asuperheater allows the hot flue gases from the boiler to continue to heat
the wet steam to temperatures above saturation thus drying the fluid.
The rules in ASME Section I state that all superheaters must have one ormore safety valves located on the outlet of the superheater and prior to
the first downstream isolation valve.
The Code goes further to state that if there are no intervening stop valves
between the steam drum and the superheater, then the superheater
safety valve can be included in providing the relieving capacity for theentire system. This superheater safety valve, along with the drum valve,
will satisfy the Code requirement for multiple valves for the larger boiler
systems outlined in the previous steam drum discussion. What changes
is the allowable split of the required capacity to be delivered by these
multiple valves. ASME Section I mandates that for a boiler system, thedrum safety valve provide a minimum of 75% of the available relieving
capacity. The reason the Code limits the superheater safety valve
available capacity is to protect this exchanger. Damage to the tubes inthe exchanger can occur if the incoming saturated steam from the drum

cannot make up the flow from the superheater safety valves that may
have opened. The tubes in the superheater can overheat and fatigue
because of the lack of heat transfer. This is an important consideration
since the superheater valves are set to open before the drum valvesbecause of inlet pressure losses between the upstream drum and thedownstream superheater. If an overpressure event occurs, the openingof the safety valves on dry superheated steam is preferable to opening
the drum valves on wet steam.

SecondarySuperheater

Furnace Wall

Turbine Deck

Steam
Drum

HP

IP

Generator

Superheater

Reheater

Water
Pump

LP

DEA
Condenser

Deaerator
Feed Pump

HP Heaters

LP
Heaters

Economizer

CW
Cooling
Water Pump

Feed Water
RecircDemineralizer

Feed Water
Regulator

BoilerFeed
Pump

Condensate
Booster Pump

Figure 5-2
Closed Simple Rankine Steam Cycle
5.7

Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
Emerson engineering recommends the use of multiple steam drum valves
when a superheater is part of the boiler system. These multiple drum
valves should be set with the staggered values allowed by the Code andselected using the capacity mandate where a smaller orifice valve shouldhave at least 50% or greater capacity of the larger orifice valve. This staged
relief of steam pressure can help prevent the ingress of water into thesteam trim safety valves during an opening cycle. Please note this two or
more drum valve arrangement is not required by the Code but in many
instances the required capacity will simply be too large for one safety valve.
A sample calculation and selection of drum and superheater safetyvalves follows:
Step One
Determine the boiler specifications.
1. Total boiler steam generation: 1,450,000 lb/hr
2.	Boiler drum and superheater design pressure (MAWP): 3000 psig
3.	Drum operating pressure: 2835 psig
4.	Superheater outlet temperature: 1000°F
5.	Superheater outlet operating pressure: 2680 psig
6.	Boiler system bare tube and extended heating surface exceeds 500 sq.ft.

Step Four
Determine the superheater safety valve set pressure.
1.	Subtract the superheater outlet operating pressure from the drum
outlet operating pressure to obtain the pressure loss in the piping
between these devices: 2835 psig – 2680 psig = 155 psig.
2.	As mentioned above, it is desirable to open the superheater safetyvalve first followed by the drum safety valve if necessary. Emerson
engineering recommends that an additional 20 psi be included in the
drum to superheater pressure drop to allow this to occur. It should be
noted that this 20 psi additional pressure difference is not mandated
by the ASME Section I Code. Therefore, the total superheater pressure
differential from the drum is the pressure loss plus the Emerson
recommended 20 psi factor: 155 psig + 20 psig = 175 psig.
3. Calculate the superheater set pressure by subtracting the total
drum to superheater pressure differential from the design (MAWP)
pressure: 3000 psig – 175 psig = 2825 psig.

Step Two
Determine the capacity of the drum safety valves.
1.	A minimum of 75% of the boiling steaming capacity must be relieved
from the drum safety valves: 1,450,000 lb/hr x 0.75 = 1,087,500 lb/hr.
Step Three
Select the drum safety valves with primary valve set at the MAWP.
1.	Since we have more than 500 square feet of bare tube and heating
surface and our steam generation is greater than 4000 lb/hr,
Emerson engineering recommends to use a minimum of two drum
valves.
2.	As you recall, ASME Section I allows for 6% accumulation when
multiple valves are used. The first, or primary, valve can be set nohigher than MAWP of 3000 psig in this example. The secondary valve
can be set 3% higher than MAWP or 3090 psig.
3.	As mentioned earlier, it may be preferable, but not required, to havethe same size drum valves to facilitate effective use of spare parts.
Therefore, for this example we will split the drum capacity evenly
between two safety valves: 1,087,500 lb/hr ÷ 2 = 543,750 lb/hr.
4.	Refer to Crosby Safety Valve catalogs for capacity charts to selectthe drum safety valves. See Table 5-3 for an example of the Crosby
capacity chart. The Crosby HE valve is suitable for drum applications
at these set pressures. “M” orifice valves will provide 590,497 lb/hr
at 3000 psig set and 619,478 lb/hr at 3090 psig set. It should be notedthat the capacity charts will show the capacity in saturated steam
already adjusted using the Kn high pressure (1500 psia and above set
pressures) factor.

5.8

Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
Step Five
Determine the superheater safety valve required relieving capacity.
1. The remaining capacity to be provided by the superheater safetyvalve is the difference between the total steam generation and
rated capacity of the drum safety valves that have been selected:
1,450,000 lb/hr – 590,497 lb/hr – 619,478 lb/hr = 240,025 lb/hr.
2. The superheat correction factor must be determined because thesuperheater safety valves are operating, and will be flowing, above
the saturation point.
a. The superheater safety flowing pressure (psia) will be the set point +overpressure + atmospheric: 2825 psig x 1.03 + 14.7 psia = 2924 psiab. At 1000°F and 2924 psia, the superheat correction factor is 0.711
from Chapter 7 Section V
3.	In order to use the saturated steam capacity chart in Table 5-3 toselect the superheater safety valve, we must convert the remaining
required capacity to saturated conditions. Therefore, equivalentsaturated steam required capacity at the superheated condition at
1000°F is: 240,025 lb/hr ÷ 0.711 = 337,588 lb/hr saturated steam.
Step Six
Select the superheater safety valve.
1.	Refer to Crosby Safety Valve catalogs for capacity charts to select thesuperheater safety valve. Again, Table 5-3 provides an example. The
Crosby HCI valve is suitable for superheater outlet applications at a
set pressure of 2825 psig.
2.	From the chart, interpolation will show that a “K2” orifice will provide
381,229 lb/hr of saturated steam and meet the requirement from stepfive.
3.	At the 1000°F superheat condition, this Crosby HCI “K2” orifice valvethat has been selected will flow: 381,229 lb/hr saturated steam
x 0.711 = 271,054 lb/hr superheated steam.

The ASME Section I Code requires each reheater to have one or more
safety valves. The overall required capacity must be equal to or greaterthan the maximum steam flow for which the reheater is designed.
Unlike the superheater safety valves there can be no credit taken for
the reheater safety valves capacity in providing protection for the steam
drum.
The reheater safety valves can be located either on the reheater inlet
line that returns the spent saturated steam back from the high pressure
turbine or on the outlet line that delivers superheated steam back to theintermediate pressure turbine. One rule in ASME Section I will state thatat least one safety valve be located on the outlet of the reheater prior to
the first isolation valve and that this reheater outlet safety valve provide aminimum of 15% of the total required capacity. As with the superheater,
this requirement will protect the tubes of the reheater when the safetyvalves must lift.
Similar to the superheater outlet safety valve, the reheater outlet safetyvalve is set lower than the reheater inlet valve to allow for the pressure drop
through the exchanger and allow for the exhaust of the dry superheatedsteam to occur first. One might rightly assume it to be a good practice tohave 100% of the required relieving capacity be from the reheater outlet
valve as there is no restriction in ASME Section I for this type of installation.
One reason for not installing the safety valves in this fashion is that thesereheater outlet valves are more expensive devices than the reheater inlet
valves. This is because the high superheated temperatures on the reheater
outlet require a high alloy steel bill of materials so most specifications try
and keep this valve as small as possible.
An example of a reheater sizing and selection follows.

Step Seven
Check to ensure we meet the ASME Section I requirement that drum
safety valves flow at least 75% of the total boiler steaming capacity
and that the combined relieving capacity of all safety valves meet or
exceed the required steaming capacity of the boiler. See Table 5-1 thatsummarizes the drum and superheater safety valve selection.
Reheater safety valve sizing
In Figure 5-2, there is another heat exchanger, called a reheater, thatwill add efficiency to the steam cycle by taking spent, near saturated,
steam from the turbine and adding more heat from the exhaust gasesof the boiler to the spent steam. A closed steam cycle may, or may not,have a reheater.
The reheater operates similar to the superheater exchanger to superheat
this incoming spent steam. This superheated steam exiting the reheater
is at a much lower pressure than that at the superheater outlet but its
temperature is virtually the same. This lower pressure, superheated steam
from the reheater outlet is then sent back to the turbine deck where anintermediate pressure turbine will expand the steam and do additional work.

Table 5-1 – ASME Section I Drum/Superheater Sizing Example Summary
Location
Low Set Drum Safety Valve
High Set Drum Safety Valve
Total Flow thru Drum Safety Valve
Superheater Outlet Safety Valve
Total Flow thru all Safety Valves

Orifice SizeM
M

Set Pressure(psig)
3000
3030

Temperature
Saturated Steam
Saturated Steam

K2

2825

1000°F

Rated Relieving Capacity(lb/hr steam)
590,497
619,478
1,209,975
271,054
1,481,029% of Total Required Capacity(1,450,000 lb/hr)

83%
19%
102%

5.9

Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
Step Five
Determine the reheater inlet safety valve required relieving capacity.
1. The remaining capacity to be provided by the reheater inlet
safety valves is the difference between the design steam flow
and rated capacity of the reheater outlet that has been selected:
1,000,000 lb/hr – 163,128 lb/hr = 836,872 lb/hr.

Step One
Determine the reheater specifications.
1.	Reheater maximum design steam flow: 1,000,000 lb/hr
2.	Design pressure: 750 psig
3.	Reheater inlet operating pressure: 675 psig
4.	Reheater outlet operating pressure: 650 psig
5.	Reheater outlet operating temperature: 1000°F
Step Two
Determine the reheater outlet safety valve set pressure.
1.	Subtract the reheater outlet operating pressure from the reheater
inlet operating pressure to obtain the pressure loss between theselocations: 675 – 650 psig = 25 psig.
2.	As mentioned above, it is desirable to open the reheater outlet safetyvalve first followed by the reheater inlet safety valve if necessary.
Emerson engineering recommends that an additional 15 psi be
included in the pressure drop to allow this to occur. It should be
noted that this 15 psi additional pressure difference is not mandated
by the ASME Section I Code. Therefore, the total reheater inlet tooutlet pressure differential is the pressure loss plus the Emerson
recommended 15 psi factor: 25 psig + 15 psig = 40 psig.
3. Calculate the reheater outlet safety valve set pressure by subtracting
the reheater pressure differential from the design pressure:
750 psig – 40 psig = 710 psig.

Step Six
1.	Refer again to Crosby Safety Valve catalogs (see Table 5-4) for
capacity charts to select the reheater inlet safety valves. The Crosby
HCI valve is suitable for reheater inlet applications at these set
pressures. You will note there is not one valve that can provide theremaining required capacity. Therefore, we need to consider multiple
valves. As mentioned previously, many specifying engineers will
select identical valves to optimize spare parts.
2.	Divide the remaining required capacity for the reheater in half:
836,872 lb/hr ÷ 2 = 418,436 lb/hr.
3.	As you recall, ASME Section I allows for 6% accumulation when
multiple valves are used. The first or primary reheater inlet valve
can be set no higher than MAWP of 750 psig in this example. The
secondary valve can be set 3% higher than MAWP or 773 psig.
4.	“Q2” orifice valves will provide 436,037 lb/hr at 750 psig set and
448,873 lb/hr at 773 psig set.

Step Three
Determine the capacity of the reheater outlet safety valve.
1.	A minimum of 15% of the relieving capacity must come from thereheater outlet safety valve: 1,000,000 lb/hr x 0.15 = 150,000 lb/hr.
2. The superheat correction factor must be determined because thereheater outlet safety valves are operating, and will be flowing, above
the saturation point.
a. T he reheater safety valve flowing pressure (psia) will be the set point+ overpressure + atmospheric: 710 psig x 1.03 + 14.7 = 746 psiab. At 1000°F and 746 psia, the superheat correction factor is 0.758
from Chapter 7 Section V
3.	In order to use the saturated steam capacity chart such as that shownin Table 5-4 to select the reheater outlet safety valve, we must convert
the required capacity to saturated conditions. Therefore, equivalentsaturated steam required capacity at the superheated condition at
1000°F is: 150,000 lb/hr ÷ 0.758 = 197,889 lb/hr saturated steam.
Step Four
Select the reheater outlet safety valve.
1.	From Table 5-4 one “P” orifice Crosby HCI safety valve will provide
215,209 lb/hr at 710 psig.
2.	At the 1000°F superheat condition this HCI Crosby HCI “P” orificevalve that has been selected will flow: 215,209 lb/hr saturated steam
x 0.758 = 163,128 lb/hr superheated steam.

Table 5-2 – ASME Section I Reheater Sizing Example Summary
Location
Low set reheater inlet safety valve
High set reheater inlet safety valve

Size
Q2
Q2

Set Orifice (psig)
750
773

Pressure
Temperature
Saturated Steam
Saturated Steam

Rated Relieving Capacity(lb/hr steam)
436,037
448,873

Total Flow Thru Reheater inlet Safety Valves
Reheater Outlet Safety Valve
Total Flow Thru all Safety Valves

P

710

1000°F884,910
163,128
1,048,038% of Total Required Capacity(1,000,000 lb/hr)

16%104% of required capacity

5.10

Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
Step Seven
Check to ensure we meet the ASME Section I requirement that reheater
outlet safety valve will flow at least 15% of the total reheater design
steam flow, and that the combined relieving capacity of the reheater inlet
and reheater outlet safety valves meet or exceed the total steam flowof the reheater. See Table 5-2 that summarizes the reheater inlet andoutlet safety valve selection.
Economizer safety valve sizing
You will note in Figure 5-2 that there is one other heat exchanger vessel
located upstream of the steam drum portion of the steam cycle. As withthe superheater and reheater sections of the cycle, hot flue gases areused to add heat to the incoming boiler feedwater. This helps to reduce
the amount of energy needed to raise the temperature of the water as it
travels to the steam drum.
In many installations there is no intervening isolation valve between theeconomizer and the steam drum. When this is the case, the safety valveson the steam drum, sized and selected as described above, can be usedas overpressure protection for the economizer.
In some steam cycles, such as combined cycle type plants, it may benecessary to regulate the output of the economizer into the boiler
to meet varying needs. This requirement now adds valves that could
potentially isolate the economizer from the boiler. In this case the
ASME Section I Code mandates that the economizer have one or more
safety relief valves. The rated capacity of these safety relief valves isdetermined by the economizer manufacturer based upon the maximum
heat absorption rate. For USCS units of measure the heat absorption
rate in Btu/hr is divided by 1000 to obtain the required steam capacity
in lb/hr. Once again, use the saturated steam tables to select a safetyvalve that will have a rated capacity equal to or larger than the required
capacity.
V. Liquid Sizing

Thermal relief sizing
One very common application for a liquid service relief valve is protecting
equipment, such as piping from hydraulic expansion of the service fluid.
This overpressure contingency is commonly referred to as thermal relief
and can be caused by heat transfer from one process media to anotheror by solar radiation. API Standard 521 states that the required relievingrate is difficult to calculate and that a portable ¾” x 1” valve is very
commonly installed to provide protection.
The standard does give some cautions with regards to large diameterliquid pipelines where the distance between isolation devices may belong or where the application deals with liquid filled heat exchangers andvessels. If physical properties are known, the required relieving capacity
for thermal relief can be calculated as follows. This flow rate can then be
used in the liquid sizing formula above.

VL =

αv φ
500 G cWhere:
VL = U.S. gallons per minuteαν = Cubic expansion coefficient of the trapped liquid at the expected
temperature, expressed in 1/°F
φ = Total heat transfer rate, Btu/hG	 = Specific gravity of service liquid at flowing temperature referred towater at standard conditionsc= Specific heat capacity of the trapped fluid, Btu/lb-°F
VI. Fire Sizing
One common overpressure contingency to be considered is subjecting
a storage tank or process vessel to an external fire that could raise thetemperature of the contents in the tank or vessel. This subsequently
could increase the system pressure due to a liquid inside the vessel
vaporizing or a gas inside the vessel expanding.

The following formula is used for sizing relief valves for liquid service
at any set pressure. The flow of an incompressible fluid (that does not
flash) through an orifice is based upon the square root of the differential
pressure across that orifice. There is a correction factor for highly
viscous liquids as well as a back pressure correction factor for balanced
bellows and balanced piston direct acting relief valves.

A =

VL G
38 K KV KW P 1 - P2

Where:A	 = Minimum required discharge area, square inches
VL = Required relieving capacity, U.S. gallons per minute at flowing
temperatureK	 = Coefficient of discharge. See Chapter 7 Section IX
KV = Capacity correction factor due to viscosity of the fluid at flowing
conditions. For most applications viscosity will not affect the area
calculation so KV will be equal to 1.0. See Chapter 7 Section IV for
more information
KW = Capacity correction factor for balanced bellows and balanced
piston direct acting relief valves due to back pressure. Use KW
equal to 1.0 for conventional direct spring and pilot operated relief
valves. See Figures 7-11, 7-14 and 7-19
G	 = Specific gravity of service liquid at flowing temperature referred towater at standard conditions
P1 = Inlet pressure during flow, set pressure (psig) + overpressure(psig) – inlet pressure loss (psig)
P2 = Total back pressure during flow (psig)

5.11

Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
Liquid Filled Tanks/Vessels
The procedure that is normally used in determining the required relieving
capacity will directly or indirectly calculate the estimated heat transferfrom an external fire to the contents of the vessel. This calculated heat
input value will vary from one code, standard, recommended practice, or
statute to another. One reason for this difference in heat input values isthat one particular publication may have a different definition to anotherfor what is called the “wetted” surface area of the vessel exposed to thefire. There are also different assumptions made in the documents withregard to tank insulation, prompt fire fighting availability and drainage
that can also alter the heat input calculations.
The exposed wetted surface is that part of the vessel or tank where theliquid contents reside and where a fire can input heat to vaporize thecontents. The greater the exposed wetted surface area the greater theheat input, the greater the heat input the more vaporization can occur,
the more vaporization the larger the required relief device orifice.
Since this exposed wetted surface definition and various assumptions
as noted above can vary from one engineering practice to another, it isimportant that the user be aware of what document is to be referenced

for a particular installation and location. Some of the more common
documents that are referenced and their calculation of exposed wetted
surface area, required capacity, and required orifice area are as follows.
It is recommended to review these documents, in full, for their scopeof use and a more complete explanation of the assumptions made inproviding this guidance.

Saturated Steam Capacities: Styles HE, HCI and HSJ - USCS (United States Customary System) Units
Pounds per hour at 3% overpressure
Table 5-3 – Saturated Steam Capacities - Set Pressures 2760-3090 psig
HE
HCI
HSJ

Set Pres.

•
•
•

•

•

F
G
H
0.307 0.503 0.785

•
•

H2
0.994

J
1.288

•
J2
1.431

K
1.840

Orifice Designation and Area (sq. in.)
•
•
•
•
•
•
•
•
Orifice (sq in)(psig)
K2
L
L2
M
M2
2.545 2.853 3.341
3.60
3.976

•
•
•

•

N
4.341

P
6.38

•

•

•

•
P2
7.07

Q
Q2
R
RR
11.045 12.25 16.00 19.29

2760

-

-

-

144025-207344266606368757-484093521620576101--1024404

-

-

-

-

2770

-

-

-

144763-208406267971370645-486572524292579051--1029651

-

-

-

-

2780

-

-

-

145505-209474269344372544-489065526978582018--1034927

-

-

-

-

2790

-

-

-

146251-210548270725374454-491572529680585002--1040232

-

-

-

-

2800

-

-

-

147001-211628272114376375-494094532397588003--1045569

-

-

-

-

2810

-

-

-

147755-212714273511378307-496631535130591022--1050936

-

-

-

-

2820

-

-

-

148515-213807274916380251-499182537880594058--1056336

-

-

-

-

2830

-

-

-

149278-214906276330382206-501749540645597113--1061767

-

-

-

-

2840

-

-

-

150046-216012277752384173-504331543428600186--1067231

-

-

-

-

2850

-

-

-

150819-217125279183386152-506929546227603277--1072729

-

-

-

-

2860

-

-

-

151597-218245280622388143-509543549044606388--1078261

-

-

-

-

2870

-

-

-

152380-219372282071390147-512174551878609519--1083827

-

-

-

-

2880

-

-

-

153167-220505283529392164-514821554731

612669--1089429

-

-

-

-

2890

-

-

-

153960-221647284996394193-517485557601615840--1095067

-

-

-

-

2900

-

-

-

154758-222795286473396236-520167560491619031--1100742

-

-

-

-

2910

-

-

-

155561-223951287960398292-522866563399622243--1106454

-

-

-

-

2920

-

-

-

156369-225115289456400362-525583566327625477--1112204

-

-

-

-

2930

-

-

-

157183-226287290963402446-528319569275628733--1117993

-

-

-

-

2940

-

-

-

158003-227467292480404544-531073572243632011--1123822

-

-

-

-

2950

-

-

-

158828-228654294007

406657-533847575231635311--1129691

-

-

-

-

2960

-

-

-

159659-229851295545408784-536640578241

638635--1135601

-

-

-

-

2970

-

-

-

160496-231055297094410927-539452581272641982--1141553

-

-

-

-

2980

-

-

-

161338-232269298655413085-542285584324645354--1147548

-

-

-

-

2990

-

-

-

162187-233491300226415259-545139587399648750--1153587

-

-

-

-

3000

-

-

-

163043-234722301809417448-548014590497652171--1159670

-

-

-

-

3010

-

-

-

-

-

-

303405419655--593618655618--1165799

-

-

-

-

3020

-

-

-

-

-

-

305012421878--596762659091--1171974

-

-

-

-

3030

-

-

-

-

-

-

306631424118--599931662590--1178197

-

-

-

-

3040

-

-

-

-

-

-

308263426375--603124666117--1184468

-

-

-

-

3050

-

-

-

-

-

-

309908428650--606342669671--1190789

-

-

-

-

3060

-

-

-

-

-

-

311566430944--609586673254--1197160

-

-

-

-

3070

-

-

-

-

-

-

313238433256--612857676866--1203582

-

-

-

-

3080

-

-

-

-

-

-

314923435586--616154680508--1210057

-

-

-

-

3090

-

-

-

-

-

-

316622437937--619478684179--1216586

-

-

-

-

5.12

Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
Saturated Steam Capacities: Styles HE, HCI and HSJ - USCS (United States Customary System) Units
Pounds per hour at 3% overpressure

Table 5-4 – Saturated Steam Capacities - Set Pressures 560-1100 psig
HE
HCI
HSJ
Set
Pres.
560
570
580
590
600
610
620
630
640
650
660
670
680
690
700
710
720
730
740
750
760
770
780
790
800
810
820
830
840
850
860
870
880
890
900
910
920
930
940
950
960
970
980
990
1000
1010
1020
1030
1040
1050
1060
1070
1080
1090
1100

•

F
0.307
8211
8354
8497
8640
8783
8926
9069
9212
9355
9498
9641
9784
9927
10070
10213
10356
10499
10642
10785
10928
11071
11214
11357
11500
11643
11785
11928
12071
12214
12357
12500
12643
12786
12929
13072
13215
13358
13501
13644
13787
13930
14073
14216
14359
14502
14645
14788
14931
15074
15217
15360
15503
15646
15789
15932

G
0.503
13453
13687
13922
14156
14390
14624
14859
15093
15327
15562
15796
16030
16264
16499
16733
16967
17201
17436
17670
17904
18138
18373
18607
18841
19076
19310
19544
19778
20013
20247
20481
20715
20950
21184
21418
21652
21887
22121
22355
22589
22824
23058
23292
23527
23761
23995
24229
24464
24698
24932
25166
25401
25635
25869
26103

H
H2
0.785 0.994
20995 26585
21361 27048
21727 27511
22092 27974
22458 28437
22824 28900
23189 29363
23555 29826
23920 30289
24286 30752
24652 31215
25017 31678
25383 32141
25748 32604
26114 33067
26480 33529
26845 33992
27211 34455
27576 34918
27942 35381
28308 35844
28673 36307
29039 36770
29404 37233
29770 37696
30136 38159
30501 38622
30867 39085
31232 39548
31598 40011
31964 40474
32329 40937
32695 41399
33060 41862
33426 42325
33792 42788
34157 43251
34523 43714
34888 44177
35254 44640
35620 45103
35985 45566
36351 46029
36716 46492
37082 46955
37448 47418
37813 47881
38179 48344
38544 48807
38910 49269
39276 49732
39641 50195
40007 50658
40372 51121
40738 51584

J
1.288
34449
35049
35648
36248
36848
37448
38048
38648
39248
39847
40447
41047
41647
42247
42847
43447
44047
44646
45246
45846
46446
47046
47646
48246
48845
49445
50045
50645
51245
51845
52445
53045
53644
54244
54844
55444
56044
56644
57244
57843
58443
59043
59643
60243
60843
61443
62043
62642
63242
63842
64442
65042
65642
66242
66841

J2
1.431
38273
38940
39606
40273
40939
41606
42272
42939
43605
44272
44938
45604
46271
46937
47604
48270
48937
49603
50270
50936
51603
52269
52936
53602
54269
54935
55601
56268
56934
57601
58267
58934
59600
60267
60933
61600
62266
62933
63599
64266
64932
65598
66265
66931
67598
68264
68931
69597
70264
70930
71597
72263
72930
73596
74263

K
1.840
49212
50069
50926
51783
52640
53497
54354
55211
56068
56925
57782
58639
59496
60353
61210
62067
62924
63781
64638
65494
66351
67208
68065
68922
69779
70636
71493
72350
73207
74064
74921
75778
76635
77492
78349
79206
80063
80920
81777
82634
83490
84347
85204
86061
86918
87775
88632
89489
90346
91203
92060
92917
93774
94631
95488

•

Orifice Designation and Area (sq. in.)••Orifice (sq in)(psig)
K2
L
L2
M
M2
2.545 2.853 3.341
3.60
3.976
68068 76306 89358 96285 106342
69253 77635 90914 97962 108193
70439 78963 92470 99638 110045
71624 80292 94026 101315 111897
72809 81621 95582 102992 113749
73995 82950 97138 104668 115600
75180 84278 98694 106345 117452
76365 85607 100250 108022 119304
77551 86936 101806 109698 121156
78736 88265 103362 111375 123007
79921 89593 104918 113052 124859
81106 90922 106474 114728 126711
82292 92251 108030 116405 128563
83477 93580 109586 118081 130414
84662 94908 111142 119758 132266
85848 96237 112698 121435 134118
87033 97566 114254 123111 135970
88218 98895 115810 124788 137821
89404 100223 117366 126465 139673
90589 101552 118922 128141 141525
91774 102881 120478 129818 143377
92959 104210 122034 131495 145229
94145 105538 123590 133171 147080
95330 106867 125146 134848 148932
96515 108196 126702 136525 150784
97701 109524 128258 138201 152636
98886 110853 129814 139878 154487
100071 112182 131370 141555 156339
101256 113511 132926 143231 158191
102442 114839 134483 144908 160043
103627 116168 136039 146584 161894
104812 117497 137595 148261 163746
105998 118826 139151 149938 165598
107183 120154 140707 151614 167450
108368 121483 142263 153291 169301
109554143819171153110739145375173005111924146931
174857113109148487176709114295150043178560115480151599180412
116665153155182264117851154711184116119036156267
185967120221
157823187819121407159379189671122592160935
191523123777162491
193374124962164047
195226126148165603197078127333167159
198930128518168715200781129704170271202633130889171827204485132074173383206337

•

N
P
P2
4.341
6.38
7.07
116104 170639 189093
118126 173610 192386
120147 176581 195679
122169 179553 198971
124191 182524 202264
126213 185496 205557
128234 188467 208850
130256 191438 212142
132278 194410 215435
134300 197381 218728
136321 200352 222021
138343 203324 225313
140365 206295 228606
142387 209267 231899
144408 212238 235192
146430 215209 238484
148452 218181 241777
150474 221152 245070
152495 224124 248363
154517 227095 251655
156539 230066 254948
158561 233038 258241
160582 236009 261534
162604 238981 264826
164626 241952 268119
166648 244923 271412
168669 247895 274705
170691 250866 277997
172713 253837 281290
174735 256809 284583
176756 259780 287876
178778 262752 291168
180800 265723 294461
182822 268694 297754
184843 271666 301047
304339307632310925314218317510320803324096327389330681333974337267340560343852347145350438353731357023360316363609366902Q
Q2
R
RR
11.045 12.25 16.00 19.29
295408 327637 427934 515928
300552 333342 435385 524912
305696 339047 442837 533896
310840 344752 450289 542880
315984 350458 457741 551864
356163 465192 560848
361868 472644 569832
367574 480096 578816
373279 487548 587800
378984 495000 596784
384689 502451 605768
390395 509903 614752
396100 517355 623736
401805 524807 632720
407510 532258 641704
413216 539710 650688
418921 547162 659672
424626 554614 668656
430331 562065 677640
436037 569517 686624
441742 576969 695608
447447 584421 704592
453152 591872 713576
458858 599324 722560
464563 606776 731544
470268 614228 740528
475973 621679 749512
481679 629131 758496
487384 636583 767480
493089 644035 776464
498794 651487 785448
504500 658938 794432
510205 666390 803417
515910 673842 812401
521615 681294 821385
527321 688745
533026 696197
538731 703649
544436 711101
550142 718552
555847 726004
561552 733456
567257 740908
572963 748359
578668 755811
584373 763263
590078 770715
595784 778166
601489 785618
607194 793070
612899 800522
618605 807974
624310 815425
630015 822877
635720 830329-

5.13

Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
API Standard 521 – Pressure Relieving and Depressuring Systems
Step One
Calculate the wetted surface area.
•	Liquid Filled Vessels – calculate the wetted area as the exposed areaof the vessel up to a maximum height of 25 feet from the location of thefire• Process Vessels – calculate the wetted area as the exposed area up to
the normal liquid operating level. If the normal operating level exceeds
25 feet from the location of the fire, use 25 feet as the height for thewetted area calculation•	Spheres – calculate the exposed area up to the maximum horizontal
diameter (i.e. the equator) of the sphere and then calculate theexposed area up to a height of 25 feet from the location of the fire. Use
the larger of the two areas as the wetted area
Step Two
Calculate the heat absorption or input to the liquid.
•	If there is deemed to be prompt firefighting and drainage of the
flammable fuel of the fire away from the vessel, use the following
equation:
0,82
Q 21,000 FAW
Where:
Q= Total heat absorption (input) to the wetted surface (Btu/h)F	= Environmental factor (see Table 5-5)
Aw = Wetted surface area from step one above, square feet

Step Three
Determine the required relieving capacity.

W

Q
—
L

Where:Q= Total heat absorption (input) to the wetted surface, from step two
above, Btu/hL	= Latent heat of vaporization, Btu/lb
W= Required relieving capacity, lb/hr
Step Four
Since the primary scope of API 521 is for applications at or above 15
psig design pressures, size for the required orifice using the weight flow
vapor equation from page 5.4.
Weight Flow (lb/hr)

A=

W
CKP1 Kb Kc

TZ
M

Use the physical properties of the service fluid in the equation. Pleaserecall that for ASME Section VIII vessels, the overpressure for fire sizing
can be 21% if the valve is set at the MAWP.

Table 5-5 – API 520/API 2000 Environmental Factor

Equipment Type
Factor F1
Bare Vessel
1.0
Insulated Vessel2 (These arbitrary insulation conductance values are shown as
examples and are in BTU’s per hour per square foot per degree Fahrenheit):
4
0.3
2
0.15
1
0.075
0.67
0.05
0.50
0.0376
0.40
0.03
0.33
0.026
Water application facilities, on bare
1.0
vessels
Depressurizing and emptying facilities 1.0

Notes1	All values are based upon assumptions listed in the API documents. Review
these documents for details.
2	Insulation shall resist dislodgement by fire hose streams. Reference
the API documents for details.

•	If there is not prompt firefighting and not drainage of the flammable
fuel of the fire away from the vessel, use the following equation:

Q

0,82

34,500 FAW

Where:Q= Total heat absorption (input) to the wetted surface (Btu/h)F	= Environmental factor (see Table 5-5)
Aw = Wetted surface area from step one above, square feet

5.14

Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
API Standard 2000 – Venting of Atmospheric and Low Pressure
Storage Tanks
Step One
Calculate the wetted surface area.
•	Spheres – calculate an area of 55% of the total exposed spherical area
and then calculate the exposed area up to a height of 30 feet above
grade. Use the larger of the two areas as the wetted area.
• Horizontal Tank – calculate an area of 75% of the total exposed area
and then calculate the exposed area up to a height of 30 feet above
grade. Use the larger of the two areas as the wetted area.
• Vertical Tank – calculate the wetted area as the exposed area up to aheight of 30 feet above grade.
Step Two
Calculate the heat absorption or input to the liquid per Table 5-6. The
formula used for this calculation will vary based upon the wetted surface
area calculated in step one.

Table 5-6 – API 2000 Heat Input
Wetted Surface Area,
Aw (ft2)< 200

Design Pressure (psig)≤ 15≥ 200 and < 1000

≤ 15

≥ 1000 and < 2800

≤ 15

963,400A0.338
w≥ 2800Between 1 and 15

21,000A0.82
w≥ 2800≤1

14,090,000

Heat Input, Q (BTU/h)
20,000Aw
199,300A0.566
w

Where:Aw
Q= wetted surface area from step one above, square feet= total heat absorption (input) to the wetted surface (Btu/h)

Step Three
Calculate the required venting capacity in SCFH of equivalent air capacity
using the following formula:q=3.091QF
L

T
M

Where:q= Required relieving capacity in equivalent air, SCFH.
Q= Total heat absorption (input) to the wetted surface from step two,
Btu/hF	= Environmental factor (see Table 5-5)L	
= Latent heat of vaporization, Btu/lb
T= Absolute temperature of the relieving vapor, °R
M= Molecular weight of the relieving vapor
Step Four
API 2000 deals with storage tanks with design pressures less than 15
psig. Therefore, the equivalent air capacity in SCFH calculated in step
three can be directly used in the Varec flow capacity charts to select thevent size. For Anderson Greenwood brand pilot operated valves, use thesubsonic formula and inputs as discussed on page 5.5.
Volumetric Flow (scfm)

A =

V MTZ
12,515 Kd P1 F

Note that the capacity calculated in step three is SCFH of equivalentair. The volumetric flow equation uses SCFM. Since the capacity is in
equivalent air, use M = 29, T = 60 + 460 = 520°R, Z =1.0 and V = q/60 in thevolumetric formula. Note that F in the volumetric flow equation is theflow factor and NOT the environmental factor from Table 5-5.
5.15

Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
NFPA 30 – Flammable and Combustible Liquids Code
Step One
Calculate the wetted surface area.
•	Spheres – calculate the wetted area by taking 55% of the total exposed
spherical area.
• Horizontal Tank – calculate the wetted area by taking 75% of the total
exposed area of the horizontal tank.
•	Rectangular Tank – calculate the wetted area by taking 100% of thetotal exposed shell and floor area of the tank (exclude the top surfaceof the tank).
• Vertical Tank – calculate the wetted area as the exposed area up to aheight of 30 feet above grade.
Step Two
Determine the required equivalent air capacity based upon the following
criteria. This criteria and tables are similar, but not exact, to an
acceptable, but less accurate sizing procedure available for use to meet
API 2000.
•	For tank design pressures equal to 1 psig and below interpolate inTable 5-7 for the equivalent air capacity in SCFH. Use 742,000 SCFH for
any storage tank with a wetted surface area greater than 2800 square
feet.
Table 5-7 – NFPA 30 Equivalent Air Capacity Requirement
AWET (ft2)
0
20
30
40
50
60
70
80
90
100
120
140
160
180
200
250
300

V (SCFH)
021,10031,60042,10052,70063,20073,70084,20094,800105,000
126,000
147,000
168,000
190,000
211,000
239,000
265,000

AWET(ft2)
350
400
500
600
700
800
900
1,000
1,200
1,400
1,600
1,800
2,000
2,400
2,800
>2,800V (SCFH)
288,000
312,000
354,000
392,000
428,000
462,000
493,000
524,000
557,000
587,000
614,000
639,000
662,000
704,000
742,000-

Table 5-8 – NFPA 30 Equivalent Air Capacity Requirement(Tanks with Wetted Area > 2800 ft2 and Design Pressures > 1 psig)
Wetted Area (ft2)
Equiv Air (SCFH) Wetted Area (ft2)
Equiv Air (SCFH)
2800
742,000
9000
1,930,000
3000
786,000
10,0002,110,000
3500
892,000
15,0002,940,000
4000
995,000
20,0003,720,000
4500
1,100,000
25,0004,470,000
5000
1,250,000
30,0005,190,000
6000
1,390,000
25,0005,900,000
7000
1,570,000
40,0006,570,000
8000
1,760,000– Use the larger of the calculated or interpolated equivalent air capacity as therequired relieving rate.

Step Three
Note that NFPA 30 may allow a reduction factor to be applied to
the required equivalent air capacity (similar to the F factor in otherpractices) for drainage, water spray systems and insulation. Refer to the
publication for details.
Step FourNFPA 30 deals with storage tanks with design pressures less than 15
psig. Therefore, the equivalent air capacity in SCFH calculated in step
two can be directly used in the Varec flow capacity charts to select thevent size. For Anderson Greenwood brand pilot operated valves, use thesubsonic formula and inputs as discussed on page 5.5.
Volumetric Flow (scfm)

A =

V MTZ
12,515 Kd P1 F

Note that the capacity calculated in step two is SCFH of equivalentair. The volumetric flow equation uses SCFM. Since the capacity is in
equivalent air, use M = 29, T = 60 + 460 = 520°R, Z = 1.0 and V = q/60 in
the volumetric formula. Note that F in the volumetric flow equation is theflow factor and NOT the environmental factor from Table 5-5.

•	For tank design pressures greater than 1 psig with a wetted surface
area of 2800 square feet or less use Table 5-7 for the equivalent air
capacity in SCFH.
•	For tank design pressures greater than 1 psig with a wetted surface
area of 2800 square feet or more–C alculate the equivalent air capacity as follows:

q

0,82

1107 A W

Where:q= required relieving capacity in equivalent air, SCFH
Aw = wetted surface area from step one above, square feet.
– Compare calculated equivalent air capacity to interpolated value
from Table 5-8.

5.16

Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
NFPA 58 – Liquefied Petroleum Gas Code and NFPA 59A – Standard for
the Production, Storage and Handling of Liquefied Natural Gas (LNG)
Step One
Calculate the wetted surface area
•	All Vessels – calculate the wetted area as the exposed area of thevessel up to a maximum height of 30 feet above grade.
Step Two
Calculate the heat absorption or input to the liquid.

0.82

Q

34,500 F A W + Hn

Where:Q= Total heat absorption (input) to the wetted surface, Btu/hF	= Environmental factor (see Table 5-9)
Aw = Wetted surface area from step one above, square feet
Hn = Normal heat leak (refrigerated tanks), Btu/lb
Step Three
Determine the rate of vaporization from the liquid for the required
relieving capacity.

W

Step Five
NFPA 58 and 59A deals with containers or vessels with design pressures
above and below 15 psig
– For sonic flow (15 psig and above) use the sonic volumetric sizing
formula discussed on page 5.4.
Volumetric Flow (scfm)

A =

V MTZ
6.32 CK P1Kb Kc- For subsonic flow (below 15 psig) one can directly use the equivalent
air capacity in SCFH in the Varec flow capacity charts to select thevent size. For Anderson Greenwood pilot operated valves, use thesubsonic volumetric sizing formula discussed on page 5.5.
Volumetric Flow (scfm)

A =

V MTZ
12,515 Kd P1 F

Note that the capacity calculated in step four is SCFH of equivalentair. The volumetric flow equation uses SCFM. Since the capacity is in
equivalent air, use M = 29, T = 60 + 460 = 520°R, Z = 1.0 and V = q/60 in
the volumetric formula. Note that F in the volumetric flow equation is theflow factor and NOT the environmental factor from Table 5-9.

Q—
L

Where:
W= Required relieving capacity, lb/hr
L	= Latent heat of vaporization, Btu/lb
Q= Total heat absorption (input) to wetted surface from step two,
Btu/hTable 5-9 – NFPA58/59A Environmental Factor
Basis
Base Container
Water Application Facilities
Depressuring Facilities
Underground Container
Insulation Factor

F Factor
1.0
1.0
1.0
0.0
F= U (1660 – Tf)
34,500Note: U is the heat transfer coefficient Btu/(hr x ft2 x °F) of the insulation using themean value for the temperature range from Tf to +1660F. Tf is the temperatureof the vessel content at relieving conditions (°F). The insulation should not be
dislodged by fire hose streams nor be combustible or decompose below 1000°F.

Step Four
Calculate the required venting capacity in SCFH of equivalent air capacity
using the following formula:q =3.09W TZ
M

Where:q= Required relieving capacity in equivalent air, SCFH
W
= Required relieving capacity of the service fluid from step three,lb/hrT= Absolute temperature of the relieving vapor, °R
M= Molecular weight of the relieving vapor
Z= Compressibility factor of the relieving vapor

5.17

Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
Gas Filled Vessels
API Standard 521 provides a recommended procedurefor determining the required pressure relief valve area due to a gas filled
vessel being exposed to external flames.
Step One
Calculate the total exposed surface area. This is the complete surface
area of the gas filled vessel that is exposed to the ambient.
Step Two
Calculate what is termed the vapor fire sizing factor using the following:

F’ =

0.146
CK

[

1.25

(Tw - T1 )T1 0.6506]

Where:
C= Gas constant based upon the ratio of specific heats of the gas orvapor at standard conditions. See Chapter 7 Section VI. Use C =315 if ratio of specific heats is unknown
K	 = Coefficient of discharge. See Chapter 7 Section IX
TW = Recommended maximum wall temperature of vessel material, °R
T1 = Gas temperature at the upstream relieving pressure, °R
This gas temperature can be found using

T =

P1

Tn

1
Where:Pn
P1 = the upstream relieving pressure, set pressure + overpressure +atmospheric pressure, psia – inlet pressure piping loss (psia)
Pn = the normal operating gas pressure, psia
Tn = the normal operating gas temperature, psia
If the calculated value of F’ is less than 0.01, then use a recommended
minimum value of F’ = 0.01.
If insufficient data exists to calculate the F’, use F’ = 0.045

Step Three
Calculate the minimum required pressure relief valve discharge area
using:A =VII. Two-Phase Flow Sizing
Two-phase flow describes a condition whereby a flow stream contains a
fluid whose physical state is part liquid and part gas. For pressure relief
applications it can be common for all or part of the liquid portion of thefluid to change to vapor, or flash, as the pressure drops. The ratio of gas
to liquid in the flowing media can be a significant factor in determining
the required orifice flow area of a pressure relief valve.
It is important to note that there are no codes such as ASME, that requirea certain methodology to be used to size PRVs for two-phase flow
regimes. The selection of the method for a particular case lies solely
with the user that has the full knowledge of the process conditions.
There are several publications, written by various process relief experts,
that will provide guidance in calculating the required relief load and thesubsequent minimum required orifice area of the pressure relief valve.
What is evident from these publications is that the subject is complex
and that there is no single universally accepted calculation method thatwill handle every application. Some methods give what are considered
to be accurate results over certain ranges of fluid quality, temperatureand pressure. The inlet and outlet conditions of the pressure relief valve
must be considered in more detail than what has been discussed up tonow, where we have been dealing with a single phase fluid flow that does
not change state.
It is therefore necessary that those responsible for the selection ofpressure relief valves used for two-phase or flashing flow applications
be knowledgeable of the total system and current on the latest bestpractices for multi-phase sizing techniques. The user should note thatsome of these sizing methods have not been substantiated by actual
tests and there is no universally recognized procedure for certifyingpressure relief valve capacities in two-phase flows.
This engineering handbook will discuss two of these sizing techniques.
One is outlined in API 520 Part I (9th Edition – July 2014) Annex C and theother, from ASME Section VIII Appendix 11, which is specifically used forsaturated water applications.

F’ Aw
P1

Where,A	
= Minimum required discharge area, square inches
Aw = Total exposed surface area from step one, square feet
P1 = U pstream relieving pressure, set pressure + overpressure +atmospheric pressure, psia – inlet pressure piping loss (psia)
F’= Vapor fire sizing factor from step two
This equation in step three does not account for any insulation. Credit for
insulation may be taken per Table 5-5.

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Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)

API Standard 520 Part I (9th Edition) – Two-Phase Flow Mixture
Procedure

Step Five
Calculate the required mass flux for the service fluid if in critical flow.

G = 68.09 ηc

P1
ν1 ω

Where:
G	 = Mass flux required, lb/s-ft2
ηc = Critical pressure ratio from step three
P1 = Flowing pressure, i.e. set pressure + allowable overpressure +atmospheric pressure – inlet piping losses, psia
ν1
= Specific volume of the two-phase service fluid at the flowingpressure, ft3/lbω= Omega parameter from step one
Go to Step Seven
Step Six
Calculate the required mass flux for the service fluid if in subcritical flow.

G =

68.09
ω

( η1 −1 +1
2

P1∕ν1

(

API Standard 520 Part I (9th Edition)
One sizing procedure in Annex C is a part of what is commonly known as
the “Omega Method” which was developed by Dr. J. Leung. The Omega
Method is a simplified version of a more rigorous procedure called the
Homogeneous Equilibrium Method (HEM) which assumes that the fluid
is well mixed, and the gas and liquid portions of the fluid are flowing at
the same velocity through the nozzle of the pressure relief valve. The
fluid is also assumed to remain in thermodynamic equilibrium, whichsimply means that any flashing that occurs will take place when thepressure drops below the vapor pressure of the mixture.
What is called the “reduced” Omega method in API Standard 520 part
I is a simplified technique in that one can take the process conditions
at the pressure relief valve inlet and compare them to the process
conditions at a lower pressure. This two process point comparison will
represent the behavior of the mixture as the pressure drops during the
opening of a pressure relief valve. The process conditions, such as thedensity or specific volume, at the inlet of the valve are known parametersfrom those on the PRV datasheet at set pressure. The second processdata point required is the density or specific volume of the mixture at
90% of the flowing pressure or, in the case of 100% liquid that flashes
it would be the saturation pressure at the relieving temperature. Notethat the flowing pressure is taken as an absolute value. This data point
is normally obtained from the fluid property database or from a processsimulation flash calculation.
API 520 Part I will illustrate the use of the reduced Omega Method fortwo conditions. One condition is a two-phase mixture at the inlet of the
PRV that may or may not flash during relief and the other condition iswhere a 100% liquid fluid at the inlet of the PRV flashes during relief.

Where:
G	 = Required Mass flux, lb/s-ft2
P2 = Total expected back pressure, psia
P1 = Flowing pressure, i.e. set pressure + allowable overpressure +atmospheric – inlet piping losses, psia
η2 = Back pressure ratio, P2/P1
ω= Omega parameter from step one
ν1= Specific volume of the two-phase service fluid at the inlet of thevalve at the flowing pressure, ft3/lb

Step One
Calculate the Omega parameter.

((νω = 9 ν9 −1
1

Where:
ν9= specific volume of the two-phase fluid at 90% of the absolute
flowing pressure, ft3/lb
ν1= specific volume of the two-phase fluid at the absolute flowing at
the prv inlet, ft3/lb
Step Two
Determine the critical pressure ratio from Figure 5-3 using w from stepone. As you will note in the figure, the value of the Omega parameter will
indicate whether the mixture will or will not flash.
Step Three
Calculate the critical pressure.

Pc ηc P1
Where:
Pc = Critical pressure, psia
ηc = Critical pressure ratio from step two
P1 = Set pressure + allowable overpressure + atmospheric pressure –inlet piping losses, psia
Step Four
Determine if flow is critical or subcritical by comparing critical pressure
from step three to the expected total back pressure (P2) in psia.
If Pc ≥ P2 then flow is critical, go to step five.
If Pc < P2 then flow is subcritical, go to step six.

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Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
Step Seven
In order to help with the two-phase nozzle discharge coefficients andback pressure correction factors for the desired Anderson Greenwood
Crosby brand product, we must first determine the mass fraction (χ1) of
the gas/ vapor portion of the two-phase mixture. From the mass fraction
we can determine what is called the void fraction (α1), or volume ratioof the gas/vapor to the total volume of the two-phase mixture. This void
fraction will be used to calculate the two-phase nozzle coefficient andback pressure correction factors.

χ =
1

WG
WL + WG

Where:
χ1= Mass fraction of gas/vapor portion of two-phase mixture
WG = Required gas/vapor mass flow, lb/hr
WL = Required liquid mass flow, lb/hr

α =1

χ1 νν1
ν1

Where:
α1 = Void fraction of two-phase mixture
χ1= Mass fraction from above calculationνν1 = Specific volume of gas/vapor at the inlet of the pressure relief
valve at the flowing pressure, ft3/lb
ν1= specific volume of the two-phase fluid at the inlet of the valve at
the flowing pressure, ft3/lb
Step Eight
Select the pressure relief valve type based upon the conditions of theapplication. Emerson recommends the use of a safety relief valve for
two-phase applications. As we learned in Chapter 3, the trim of a safety
relief valve provides stable operation on either gas and/or liquid flow.
Anderson Greenwood Crosby safety relief valves have certified nozzle
coefficients for gas and liquid media that are used to calculate a twophase coefficient of discharge in the next step of this procedure.
It is also advisable that the safety relief valve selected to be of a balanced
design for these applications. It is oftentimes difficult to accurately
predict the actual magnitude of built-up back pressure that will be
developed by the flow of a flashing mixture of gas and liquid. You recall
that a balanced direct spring valve or pilot operated valve will maintainlift and stability at higher built-up back pressures when compared toconventional pressure relief valves.
See Table 5-10 for a summary of the recommended valve designs for use
in two-phase flow applications.
Step Nine
Determine the coefficient of discharge for the selected valve.

K 2φ α1 KG + (1−α1) KL
Where:
K2φ = Two-phase coefficient of discharge
α1 = Void fraction of two phase mixture from step seven
KG = Gas/vapor coefficient of discharge. See Chapter 7 Section IX
KL = Liquid coefficient of discharge. See Chapter 7 Section IX

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Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)

1.2
Non-Flashing Flow

Flashing Flow

Critical Pressure Ratio, (ηc)

1.0

0.8

0.6

0.4

ηc = (1 + (1.0446 – 0.0093431 • ω0.5) • ω-0.56261) (-0.70356 + 0.014685 • ln ω)

0.2

0.0
0.01

0.1

1

10

100

Figure 5-3
Omega Parameter, ωCritical Pressure Ratio for Two-Phase Flow
Step TenIf built-up or superimposed back pressure is evident, calculate the backpressure correction factor.

K bw α1 Kb + (1−α1) Kw
Where:
Kbw = Two-phase back pressure correction factor
α1 = Void fraction of two-phase mixture from step seven
Kb = Back pressure correction factor for gas. See Chapter 7 Section II
Kw = Capacity correction factor for balanced relief valves due to backpressure. Use Kw equal to 1.0 pilot operated or conventionalsafety relief valves. See Figure 7-11 for direct acting balanced
safety relief valves.
Step Eleven
Calculate the minimum required discharge area.

A=

0.04W

K2φ KbwKc KυGWhere:A	
= Minimum required discharge area, square inches
W= Required mass flow rate of the mixture, lb/hr
K2φ = Two-phase coefficient of discharge from step nine
Kbw = Two-phase back pressure correction factor from step ten
Kc = Combination factor for installations with a rupture disc upstreamof the valve. See Chapter 7 Section XI for flow certified factors.
Use a 0.9 value for any rupture disc/pressure relief valve
combination not listed in Chapter 7. Use a 1.0 value when arupture disc is not installedKν = Capacity correction factor due to viscosity of the fluid at flowing
conditions. For most applications viscosity will not affect the area
calculation so Kν will be equal to 1.0. See Chapter 7 Section IV for
more information
G	 = Required Mass flux from step five or six, lb/s-ft2
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Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
API Standard 520 Part I (9th Edition) – Subcooled or Saturated all
Liquid Flashes
Where a 100% process liquid flashes when the relief valve opens, thereduced Omega Method presented in API Standard 520 part I can also
be used to predict the behavior of the new mixture of the liquid and its
vapor created by the pressure drop across the valve nozzle. A liquid iscalled “subcooled” when it is at a temperature that is lower than its
saturation temperature for a particular pressure. To use this procedure,
no condensable vapor or non-condensable gas should be present in theliquid at the relief valve inlet. If these vapors or gases are in the mixture
use the two-phase flow procedure discussed previously. If the service
fluid is saturated water, use the ASME Section VIII Appendix 11 method
below.
Step One
Calculate the Omega parameter.

((ρωs = 9 ρl1 −1
9Where:
ωs = Saturated Omega parameterρ9 = Density of the mixture at 90% of the saturation or vapor pressure(Ps) at the relieving temperature at the relief valve inlet. For
multi-component liquids this represents the bubble point at therelieving temperature at the relief valve inlet, lb/ft3
ρl1 = Density of the liquid at the flowing pressure at the relief valve
inlet, lb/ft3

Step Four
Determine the ratio of the saturation pressure to the set pressure.

ηs =

Ps
P1

Where:
Ps = Saturation or vapor pressure at the relieving temperature at therelief valve inlet, psia
P1 = Flowing pressure, i.e. set pressure + allowable overpressure +atmospheric – inlet piping losses, psia
From the calculation in step three, if the flash occurs upstream of the nozzle
bore diameter (low subcooling region) then move to step five.
From the calculation in step three, if the flash occurs at the nozzle bore
diameter (high subcooling region) skip to step ten.
Step Five (low subcooled liquid region)
Determine the critical pressure ratio (ηc) of the service fluid from Figure5-4. Use the saturation pressure ratio (ηs) from step four above and thesaturated Omega (ωs) value from step one above.

Step Two
The Omega parameter is now used to predict if the subcooled liquid
will flash upstream of the bore diameter (minimum diameter) of thenozzle or at the nozzle bore diameter. This behavior is determined by thevalue of what is called the transition saturation pressure ratio which is
calculated as follows.

ηst=

2ωs
1+ 2ωs

Where:
ηst = Transition saturation pressure ratio
ωs = Saturated Omega parameter from step one
StepThree
Determine where the flash of the subcooled liquid occurs as follows:
If Ps ≥ ηstP1 then the flash occurs upstream of the nozzle bore diameterof the prv (also called the low subcooling region).
If Ps < ηstP1 then the flash occurs at the nozzle bore diameter of the prv(also called the high subcooling region).
Where:
Ps = Saturation or vapor pressure at the relieving temperature at therelief valve inlet, psia
P1 = Flowing pressure, i.e. set pressure + allowable overpressure +atmospheric – inlet pressure piping losses, psia
ηst = Transition saturation pressure ratio from step two

Table 5-10 – Anderson Greenwood Crosby Recommended Valve Designs for Two-Phase Flow

Conventional Direct Spring PRV1Balanced Direct Spring PRV
Pilot Operated PRV
JLT-JOS-E Series 900
JLT-JBS-E
Series 400/500/800
Note 1 – The magnitude of the built-up back pressure can be difficult to predict for two-phase flow. It is advisable to use either a balanced direct spring or pilot operated
PRV if this value is uncertain.

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Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
1.00

0.90

40

Critical Pressure Ratio (hc)

15
10
0.80

7
5
4
3

0.70

2

Saturated Omega Parameter (ws)

20

1.50
0.60
1.00
0.75
0.50

0.50

0.75

0.80

0.85

0.90

0.95

1.00

Figure 5-4
Saturated Pressure Ratio (hs)
Critical Pressure Ratio for Low Subcooled Region Flashing Liquids

Step Six (low subcooled liquid region)
Calculate the critical pressure (Pc) using the critical pressure ratio anddetermine whether the flow is critical or subcritical.

Pc = ηc P1
Where:
Pc = Critical pressure, psia
ηc = Critical pressure ratio from step five
P1 = Set pressure + allowable overpressure + atmospheric – inlet
piping losses, psia
If Pc ≥ P2 then flow is critical (go to step seven).
If Pc < P2 then flow is subcritical (skip step seven and go to step eight).
Where:
P2 = The total expected built-up and superimposed back pressure, psia
Step Seven (low subcooled liquid region in critical flow)
Calculate the required mass flux.

s s ln(( ηη −
s
c

s−1

s− c

P1 ρl1ηωs ηs − 1 +
c

(

− s

(

G=

68.09

Where:
G	 = Required mass flux, lb/s-ft2
ηs = Saturated pressure ratio from step fourωs = Saturated Omega parameter from step one
ηc = Critical pressure ratio from step five
P1 = Flowing pressure, i.e. set pressure + allowable overpressure +atmospheric – inlet piping losses, psia
ρl1 = Density of the liquid at the set pressure at the relief valve inlet, lb/ft3
Skip steps eight and nine and go to step thirteen.
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Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
Step Eight (low subcooled liquid region in subcritical flow)
Calculate the subcritical pressure ratio.

η2 =

P2
P1

Where:
P2 = The total expected built-up and superimposed back pressure, psia
P1 = Flowing pressure, i.e. set pressure + allowable overpressure +atmospheric – inlet piping losses, psia
Step Nine (low subcooled liquid region in subcritical flow)
Calculate the mass flux.

− s

(

(

2

(

68.09

(

G=

ηs
s s ln η −2ηωs ηs − 1 +

s−1

s− 2

P1 ρl1

Where:
G	 = Required mass flux, lb/s-ft2
ηs = Saturated pressure ratio from step fourωs = Saturated Omega parameter from step one
η2 = Subcritical pressure ratio from step eight
P1 = Set pressure + allowable overpressure + atmospheric – inlet
piping losses, psia
ρl1 = Density of the liquid at the set pressure at the relief valve inlet, lbs/ft3
Skip to step thirteen.
Step Ten (high subcooled liquid region)
Determine if flow is critical or subcritical.
If Ps ≥ P2 then flow is critical (go to step eleven).
If Ps < P2 then flow is subcritical (skip step eleven and go to step twelve).
Where:
Ps = Saturation or vapor pressure at the relieving temperature at therelief valve inlet, psia
P2 = The total expected built-up and superimposed back pressure, psia

Step Thirteen
Select the proper pressure relief valve type based upon the conditionsof the application. Since the liquid is flashing to give a certain amount of
two-phase flow through the pressure relief valve, Emerson recommends
that a safety relief valve (operates in a stable fashion on either
compressible or incompressible media) be selected. Since there will be
flashing, Emerson recommends a balanced type pressure relief valve
due to pressure variations that can occur in the valve body outlet.
See Table 5-10 for a summary of recommended valve designs for use in
two-phase flow applications.
Step Fourteen
Calculate the minimum required discharge area.

A =0.3208VL ρ l1
K Kv Kw G

Where:
A	= Minimum required discharge area, square inches
VL = Required relieving capacity, U.S. gallons per minute at flowing
temperatureρl1 = Density of the liquid at the flowing pressure at the relief valve
inlet, lb/ft3
K	 = Coefficient of discharge for liquid service. See Chapter 7 Section IX
Kv = Capacity correction factor due to viscosity of the fluid at flowing
conditions. For most applications viscosity will not affect the area
calculation so Kv will be equal to 1.0. See Chapter 7 Section IV
Kw = Capacity correction factor for balanced relief valves due to backpressure. Use Kw equal to 1.0 for pilot operated and conventionalsafety relief valves. See Figure 7-11 for direct acting balanced
safety relief valvesG	 = Required mass flux from either steps 7, 9, 11, or 12, lb/s-ft2

Step Eleven (high subcooled liquid region in critical flow)
Calculate the mass flux.

G 96.3 ρ l1 P1 − Ps
Where:
G	 = Required mass flux, lb/s-ft2
ρl1 = Density of the liquid at the set pressure at the relief valve inlet, lb/ft3
P1 = Flowing pressure, i.e. set pressure + allowable overpressure +atmospheric – inlet piping losses, psia
Ps = Saturation or vapor pressure at the relieving temperature at therelief valve inlet, psia
Skip to step thirteen.
Step Twelve (high subcooled liquid region in subcritical flow)
Calculate the mass flux.

G 96.3 ρ l1 P1 − P2
Where:
G	 = Required mass flux, lb/s-ft2
ρl1 = Density of the liquid at the flowing pressure at the relief valve
inlet, lb/ft3
P1 = Flowing pressure, i.e. set pressure + allowable overpressure +atmospheric – inlet piping losses, psia
P2 = The total expected built-up and superimposed back pressure, psia

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Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
ASME Section VIII, Appendix 11 – Flashing of Saturated Water
When the process fluid at the pressure relief valve inlet is entirely
saturated water one can refer to ASME Section VIII Appendix 11 to
estimate the available mass flux for specific valve designs. Figure
5-5 is taken from Appendix 11 of the Code. The specific valve design
requirements in order to use Figure 5-5 are:
• The ratio of the nozzle bore diameter (smallest cross section) to prv
inlet diameter must fall between 0.25 and 0.80.
• The actual (not rated) nozzle coefficient for gas/vapor service must
exceed 0.90.
Step One
Determine the available mass flux for a pressure relief valve that meetsthe above design requirements at the required set pressure from Figure5-5. The curve in Figure 5-5 is based upon 10% overpressure. Use thisavailable mass flux if sizing allows for higher overpressures as this will
be a conservative value.
An example would be that a saturated water installation is requiring a
prv to be set at 400 psig with a required capacity of 100,000 lb/hr. Please
note that the ordinate axis has the available mass flux denoted in 10-4
units so the available mass flux at 400 psig would be 6 x 10,000 or 60,000
lb/hr/in2.
Step Two
Divide the required saturated water capacity by the available mass flux
determined in step one to obtain the minimum required discharge areaof the valve.

A =

W
G

Where:
W= Required relieving capacity of saturated water, lb/hr
G	 = Available prv mass flux from step one
So following with the example above, if the required saturated watercapacity is 100,000 lb/hr, the required discharge or orifice area of thevalve would be 100,000 (lb/hr) ÷ 60,000 (lb/hr/in2) = 1.667 in2.
Step Three
Select the proper pressure relief valve type based upon the conditions of
the application and meet the design requirements required by the ASME
Code that are listed above. Emerson recommends the use of a balanced
type pressure relief valve due to pressure variations that can occur in thevalve body outlet.
The following Crosby and Anderson Greenwood balanced valves meetthe design requirements and may be considered:•	Balanced Direct Spring (JLT-JBS-E)• Modulating POPRV (Series 400/500/800) in 1F2, 1.5H3, 2J3, 3L4, 4P6,
6R8, 8T10 or any full bore (FB) orifice configuration
Go to Chapter 7 and review the ASME (do not use the API tables) actual
orifices for gas service listed in Tables 7-7 and 7-12 that are available forthe valve types listed above.
Therefore, to complete the example where we have a minimum orificearea requirement of 1.667 in2 we can look at Table 7-7 for a JLTJBS-E configuration. This table will show a 3” inlet valve, with a “K”
orifice designation, will have 2.076 in2 available. Provided the otherrequirements of the application meet this valve’s specifications, thisconfiguration would be an appropriate choice.

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Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
VIII. Noise Level Calculations((

L100 L + 10log10 0.29354 WkT
M

Where:
L100 = Sound level at 100 feet from the point of discharge in decibels
L	= Sound level from Figure 5-6 in decibels
P1 = Pressure at the valve inlet during flow, psia. This is the set
pressure (psig) + overpressure (psig) + atmospheric pressure(psia) – inlet pressure piping losses (psig)
P2 = Pressure at the valve outlet during flow, psia [bara]. This is total
back pressure (psig) + atmospheric pressure (psia)k= Specific heat ratio. See Chapter 7 Section VII
M= Molecular weight of the gas or vapor. See Chapter 7 Section VII
T= Absolute temperature of the fluid at the valve inlet, degrees
Rankine (°F + 460)
W
= Maximum relieving capacity, lb/hr
The noise level should be calculated using the maximum or total flow
through the pressure relief valve at the specified overpressure. Thisvalue can be calculated by using the sizing formulas on page 5.4 forweight flow and solving for “W”. Use the “actual” area and “actual”
coefficient of discharge for the specific valve from tables in Chapter 7.
The actual coefficient is the “rated coefficient” divided by 0.90.
When the noise level is required at a distance of other than 100 feet, thefollowing equation shall be used:

L100 − 20log10

r
(100

(

Lp

Where:
Lp = Sound level at a distance, r, from the point of discharge indecibelsr= Distance from the point of discharge, feet
Table 5-11 lists some relative noise intensity levels

Flow Capacity x 10-4 (lb/hr/in2)

24

The following formulas are used for calculating noise level of gases,
vapors and steam as a result of the discharge of a pressure relief valve
to atmosphere.

20
16
12
8
4
0

0

400

800

1200

1600

2000

2400

2800

3200

Set Pressure, psig

Figure 5-5
ASME Section VIII Appendix 11
Available Mass Flux - Saturated Water
Table 5-11 – Noise Intensity (at 100 feet from the discharge)
Relative Noise Levels
130
Decibels
120
Decibels
110
Decibels
100
Decibels
90
Decibels
80
Decibels
70
Decibels
60
Decibels
50
Decibels

Jet Aircraft on Takeoff
Threshold of Feeling
Elevated Train
Loud Highway
Loud Truck
Plant Site
Vacuum cleaner
Conversation
Offices

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Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
70

The discharge from a pressure relief valve exerts a reaction force on thevalve, vessel and/or piping as a result of the flowing fluid. Determinationof outlet reaction forces and the design of an appropriate support systemis the responsibility of the designer of the vessel and/or piping. The
following is published as technical advice or assistance.

60

Reaction Force for Open Discharge – Gas Service
The following formulas are used for the calculation of reaction forces fora pressure relief valve discharging gas or vapor directly to atmosphere.
It is assumed that critical flow of the gas or vapor is obtained at thedischarge outlet. Under conditions of subcritical flow the reaction forces
will be less than that calculated. These equations are found in API
Recommended Practice 520 Part II.

F=

W
366

kTi
+ Ao P2− PA
k +1 M

Where:
F	= Reaction force at the point of discharge to atmosphere, lbf.
See Figure 5-7
Ao = Area at discharge, square inchesk= Specific heat ratio at the outlet conditions
M= Molecular weight of the gas or vapor obtained from standard
tables. See Chapter 7 Section XI
P2 = Static pressure at discharge, psia calculated below:

P2= 0.001924
PA
Ti

W
Ao

Decibels

IX. Reaction Forces

50
40

30
20

1.5

2

3

4

5

6

7

8

9 10

P1
P2

Figure 5-6
Sound Pressure Level at 100 Feet from Point of DischargeTokM= Ambient pressure, psia
= Absolute temperature of the fluid at the valve inlet, degrees
Rankin (°F + 460)
To = Absolute temperature of the fluid at the discharge, degrees
Rankin (°F + 460)
W
= Actual relieving capacity, lb/hr. This value may be calculated byusing the sizing formula on page 5.4 for weight flow. Use the
ASME actual area and the rated coefficient divided by 0.9 to get
the actual capacity of the valve.
The above equations account for static thrust force only and do notconsider a force multiplier required for rapid application of the reactive
thrust force. ASME B31.1
Non-Mandatory Appendix II includes a method of analysis for dynamic
load factors. Force multipliers up to two times F are possible. This is
only necessary for open discharge with rapid opening valves. A minimum
value of 1.1 x F is recommended for other installation types.

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Pressure Relief Valve Engineering Handbook
Chapter 5 – Valve Sizing and Selection – USCS Units (United States Customary System)
Reaction Force for Open Discharge – Steam Service
The following formula is used for the calculation of reaction forces
for a pressure relief valve discharging steam directly to atmosphere.
The equations are based on equations in ASME B31.1 Non-mandatory
Appendix II.
F = 6.98 x 10 -4 W ho − 823 + Ao P2 − PA
Where:
F	= Reaction force at the point of discharge to atmosphere, lbf. See
Figure 5-7
ho = Stagnation enthalpy at the valve inlet, Btu/lbm
Ao = Area at discharge, square inches
P2 = Static pressure at discharge, psia
PA = Ambient pressure, psiaW
= Actual relieving capacity, lb/hr. This value may be calculated byusing the sizing formula on page 5.5 for weight flow. Use the
ASME actual area and the rated coefficient divided by 0.9 to get
the actual capacity of the valve
The bove equations account for static thrust force only and do notconsider a force multiplier required for rapid application of the reactive
thrust force. ASME B31.1
Non-Mandatory Appendix II includes a method of analysis for dynamic
load factors. Force multipliers up to two times F are possible. This is
only necessary for open discharge with rapid opening valves (i.e. ASME
Section I safety valves).

F

Figure 5-7 – Open Discharge Reaction Force

Reaction Force for Open Discharge – Liquid Service
The following formula is used for the calculation of reaction forces fora pressure relief valve discharging liquid directly to atmosphere. The
equations are based on fluid momentum. Liquid flow is assumed to be
non-flashing.

F=

3.44 x 10 -7 W 2
ρAo

Where:
F	= Reaction force at the point of discharge to atmosphere, lbf. See
Figure 5-7
Ao = Area at discharge, square inchesW= Actual relieving capacity, lb/hr. This value may be calculated byusing the sizing formula on page 5.11 for volumetric flow andthen converting to weight flow. Use the ASME actual area andthe rated coefficient divided by 0.9 to get the actual volumetriccapacity of the valve. To obtain the actual capacity, W, in lb/hr use
the conversions in Table 7-20
ρ= Density of the fluid, lbm/ft3
Reaction Force for Open Discharge – Two-Phase Flow
The following formula is found in API Recommended Practice 520 Part II.
This formula assumes the two-phase flow is in a homogeneous condition(well mixed and both phases flowing at the same velocity).
χ
1−χ
W2

F=

[ + ρ ] +A P − P

2.898 x 10 6 A o ρg

1

E

A

W
Ao
Χ

= Actual relieving capacity, lb/hr= Area at discharge outlet to atmosphere, square inches= Mass fraction of gas/vapor portion WG

WG
ρg
ρl
PE
PA

= Actual relieving capacity of gas, lb/hr= Vapor density at exit conditions, lbm/ft3= Liquid density at exit conditions, lbm/ft3= Pressure at pipe exit, psia
= Ambient pressure, psia

W

5.28

Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric Units
The following data with charts and tables are included in this chapter:I.	Introduction.....................................................................................................................................6.3
Procedure.........................................................................................................................................6.3
Pressure Relief Valve Nozzle Coefficient of Discharge.................................................................6.3
	API vs ASME Nozzle Coefficient of Discharge...............................................................................6.3
II.	Gas/Vapor Sizing – Sonic Flow......................................................................................................6.4
	Equations, Variables, Unit of Measures
III.	Gas/Vapor Sizing – Subsonic Flow................................................................................................6.5
	Equations, Variables, Unit of MeasuresIV.	Steam Sizing....................................................................................................................................6.6
	Equations, Variables, Unit of Measures
	ASME Section VIII.............................................................................................................................6.6
	ASME Section I.................................................................................................................................6.6
V.	Liquid Sizing..................................................................................................................................6.11
	Equations, Variables, Unit of Measures
Thermal Relief
VI.	Fire Sizing......................................................................................................................................6.13
	Liquid Filled Vessels......................................................................................................................6.13
	API 521 (ISO 23251)........................................................................................................................6.13
	API 2000 (ISO 28300)......................................................................................................................6.14
	Gas Filled Vessels..........................................................................................................................6.15
VII.	Two-Phase Sizing.........................................................................................................................6.15
	API 520 Part I (8th Edition)............................................................................................................6.16
Two-Phase Flow Mixture Procedure............................................................................................6.17
	Subcooled or Saturated All Liquid Flashes..................................................................................6.19
	ASME Section VIII...........................................................................................................................6.22
	Flashes at Saturated Water..........................................................................................................6.22
VIII.	Noise Level Calculations..............................................................................................................6.23
IX.	Reaction Forces............................................................................................................................6.24
	Gas Service.....................................................................................................................................6.24
	Steam Service................................................................................................................................6.25
	Liquid Service.................................................................................................................................6.25
Two-Phase Service........................................................................................................................6.25

6.1

Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric Units
The following Figures are included in this chapter:
Figure 6-1 Closed Simple Rankine Steam Cycle............................................................................... 6.7
Figure 6-2 Critical Pressure Ratio for Two-Phase Flow................................................................. 6.16
Figure 6-3 Critical Pressure Ratio for Low Subcooled Region Flashing Liquids.......................... 6.19
Figure 6-4	ASME Section VIII Appendix 11 Available Mass Flux - Saturated Water..................... 6.23
Figure 6-5	Sound Pressure Level at 30 Meters from Point of Discharge...................................... 6.23
Figure 6-6	Open Discharge Reaction Force..................................................................................... 6.25
The following Tables are included in this chapter:
Table 6-1 – 4020 A Weight Loaded Pressure Flow Capacity (Pipeaway Version).............................. 6.5
Table 6-2 – ASME Section I Drum/Superheater Sizing Example Summary...................................... 6.8
Table 6-3 – Saturated Steam Capacities - Set Pressures 173-213 barg......................................... 6.10
Table 6-4 – ASME Section I Reheater Sizing Example Summary.................................................... 6.11
Table 6-5 – Saturated Steam Capacities - Set Pressures 44-86 barg............................................. 6.12
Table 6-6 – Environmental Factor...................................................................................................... 6.14
Table 6-7 – ISO 28300 Heat Input Equations...................................................................................... 6.14
Table 6-8 – Anderson Greenwood Crosby Selection for Two-Phase Flow...................................... 6.17
Table 6-9 – Noise Intensity (at 30 Meters from the Discharge)........................................................ 6.24

6.2

Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric Units
I. Introduction
This section of the Emerson Pressure Relief Valve Engineering
Handbook is laid out to assist the user in the sizing and selection ofpressure relief valves when system parameters are expressed in metricunits. The procedures and equations in this chapter are consistent withthe requirements of the ASME Boiler and Pressure Vessel Code, API andISO recommended practices and standards. Please refer to Chapter 5
for sizing using United States Customary System (USCS).
Please visit the Emerson Sizing Website for access to PRV2SIZE. The
address is valvesizing.emerson.com. This sizing program will performmany of the sizing techniques discussed in this chapter.
Procedure
Before the determination can be made of the required pressure relief
valve orifice area, an in-depth analysis of various overpressure scenarios
for the equipment being protected must be completed. ISO 23251 or API
Standard 521 is oftentimes used as a guide to determine what possible
causes of overpressure could occur and what subsequent required
relieving capacity is necessary to mitigate the system upset. Thesestandards will help the process engineer determine the worst casescenario from unexpected system conditions such as blocked outlets,
reflux failures, power failures, overfilling, exchanger tube damage, andexternal fire. There are many other possible overpressure conditions
listed in the standard.
ISO 28300 or API Standard 2000 contains similar information on causesof overpressure and vacuum, and the required relieving capacity for theprotection of atmospheric or low pressure storage tanks.
One key piece of information for the sizing of the pressure relief valve isthe knowledge of the largest required capacity that results from one ofthese overpressure conditions. This required capacity is often referred
to as the “worst case scenario.” This chapter will help you with the sizing
techniques to obtain the proper pressure relief valve orifice for this worst
case scenario.
It should be noted however, that the final selection of the pressure relief
valve type and its materials of construction may be based upon otheroverpressure contingencies. For example, a worst case scenario may bewhen a liquid is boiled off into a vapor due to an external fire. A pressure
relief valve is sized based upon this vapor flow rate. There may be
another overpressure condition where the liquid could overfill and thisliquid flow rate requires a smaller orifice. As we learned in Chapter 4,
not all pressure relief valve trims designed for vapor flow work well on
liquid flow. If the lesser contingency is ignored during the pressure relief
device selection then an improper valve might be installed.

The requirement is that none of these nine Kd values can vary any morethan plus or minus 5% of the average Kd.
Most gas or steam certified safety valves that use the nozzle bore as
the flow limiting dimension are quite efficient as compared to the ideal
nozzle. It is not unusual to have a Kd value of 0.950 or higher for thesevalves. The Kd value for liquid certified relief valves is much lower or in
the range of 0.750.
An additional requirement in the ASME Code (both Section I and Section
VIII) and ISO 4126 is to reduce the flow tested Kd value by 10%. This
reduced coefficient provides an additional safety factor when calculating
the required flow area for a pressure relief valve. For example, if a safetyvalve is tested to have a Kd equal to 0.950 then the ASME or ISO 4126
rated nozzle coefficient of discharge is 0.950 x 0.900 or 0.855. This ASME
rated nozzle coefficient is typically denoted as K (K = Kd x 0.9) and theISO 4126 rated nozzle coefficient is typically denoted as Kdr (Kdr = Kd x 0.9).
For this chapter, the rated nozzle coefficient is shown as “K”. The valve
sizing formulas outside of the scope of ASME (below 1.03 barg) will use
the actual flow tested Kd values.

Pressure Relief Valve Nozzle Coefficient of Discharge
As you review the various orifice sizing formulas in this chapter, you willnote that there will almost always be one variable that will be listed as
the valve coefficient of discharge. This value is specific to a particularvalve design and illustrates the imperfect flow characteristics of thedevice. The best nozzle coefficient of discharge (Kd) would be that of an
ideal nozzle. The value of the Kd is the quotient of the actual measured
flow divided by the theoretical flow of an ideal nozzle. Therefore, the Kd
for a particular valve can be no larger than 1.0.
There are various codes and standards that require actual flow tests tobe performed to establish the flow efficiency of a pressure relief valve.
For example, there are testing procedures described in documents, suchas the ASME Boiler and Pressure Vessel Code, ISO 4126, and ISO 28300
that will establish the Kd of a particular valve design.
If you look further in either Section I or Section VIII of the ASME Code or
ISO 4126, there is one procedure where the manufacturer is required
to test three valves in three different sizes, for a total of nine tests.
The Kd value for each of these nine tests is calculated and averaged.

6.3

Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric Units
API Effective vs ASME Section VIII Rated Nozzle Coefficient ofDischarge
The ASME Section VIII (and ISO 4126) rated nozzle coefficient ofdischarge (K) will vary from one valve design to the other, one service(i.e. compressible versus incompressible) to the other, and one
manufacturer to the other. Therefore, if the valve manufacturer and/or the valve design is not yet selected, and a preliminary pressure relief
valve size for a ASME Section VIII valve is needed, many users will refer
to API Standard 520 part I to obtain what are called effective nozzle
coefficients. This recommended practice publishes one common nozzle
coefficient of discharge for gases, steam and liquids to be used forpreliminary sizing of the flow orifice area.
When selecting the preliminary flow orifice size, API 520 part I will point
the user to the API Standard 526. This API 526 standard is where you willfind the effective flow orifice sizes for what are more commonly called
the “lettered” orifice designations. The scope of the API 526 standard is
DN25 x DN50 (D orifice designation) through a DN200 x DN250 (T orificedesignation). The scope of API 526 is limited to flanged direct springloaded and flanged pilot operated pressure relief valves.
Once the manufacturer and specific design are decided, API 520 part I
will instruct the user to recalculate the required flow orifice size using
the ASME rated nozzle coefficient of discharge (K). The actual flow orificearea of the valve selected should be compared to meet or exceed thecalculated orifice area value.
The API effective coefficient of discharge and effective orifice areas areillustrated with the applicable Anderson Greenwood Crosby models thatmeet API Standard 526. The direct spring valves are shown in Table 8-6
and the pilot operated valves are shown in Table 8-11. The preliminary
sizing per API can be completed using these values. You will note thatthe information for the effective nozzle coefficients and orifice areas areexactly the same for the two different valve designs.
The ASME rated coefficient of discharge (K) and the actual flow orificearea for these same valve designs are shown in Table 8-7 for the direct
spring valves and Table 8-12 for the pilot operated valves. You will now
notice the rated coefficient of discharge and actual flow orifice areas aredifferent because these values are specific to the valve design.
The user should be aware that the use of the API effective values insizing these particular Anderson Greenwood Crosby products will always
be conservative. The recalculation of the required orifice size using rated
coefficient of discharge (K) and comparing the answer to the actual
orifice area will always allow for the same valve size, or smaller, to that
identified in the preliminary API sizing.

Set Pressures ≥ 1.03 barg
The following formulas can be used for sizing valves when the set
pressure is at or above 1.03 barg.
Weight Flow (kg/hr)

A =W
C K P1Kb Kc

A =

V MTZ
22.42C K P1Kb Kc

Volumetric Flow (Nm3/hr)

TZ
M

Where:A	
= Minimum required discharge area, square millimetersC= Gas constant based upon the ratio of specific heats of the gas orvapor at standard conditions. See Chapter 8 Section VI. Use C =2.390 if ratio of specific heats is unknown
K	 = Coefficient of discharge. See Chapter 8 Section IX
Kb = Back pressure correction factor for gas. See Chapter 8 Section II
Kc = Combination factor for installations with a rupture disc upstreamof the valve. See Chapter 8 Section XI for flow certified factors.
Use a 0.9 value for any rupture disc/pressure relief valve
combination not listed in Chapter 8 Section XI. Use a 1.0 value
when a rupture disc is not installedM= Molecular weight of the gas or vapor. See Chapter 8 Section VII
for common gases
P1 = Relieving pressure, bars absolute. This is the set pressure (barg)+ overpressure (barg) + atmospheric pressure (bara) – inlet
pressure piping loss (barg)T= Absolute relieving temperature of the gas or vapor at the valve
inlet, degree Kelvin (degree Celsius + 273)
W
= Required relieving capacity, pounds per hour (kg/hr)
V= Required relieving capacity, normal cubic meters per hour (Nm3/hr)
Z= Compressibility factor. See Chapter 8 Section I

In no case should an API effective coefficient of discharge or effective area
be used with the rated coefficient of discharge or actual area to perform any
calculation. Sizing errors can be made if the effective values are mixed withthe actual values.
For Anderson Greenwood Crosby valve designs that do not fall within
the scope of API 526, such as portable valves, ASME Section I valves, orfull bore pilot operated valves, it is suggested to always use the rated
coefficient of discharge and actual orifice area for any sizing.
II. Gas/Vapor Sizing – Sonic Flow
The orifice sizing for vapors or gases can be done either by capacity
weight or by volumetric flow rates. The formulas used are based on theperfect gas laws. These laws assume that the gas neither gains nor
loses heat (adiabatic), and that the energy of expansion is converted into
kinetic energy. However, few gases behave this way and the deviationfrom the perfect gas laws becomes greater as the gas approaches
saturated conditions. Therefore, the sizing equations will contain various
correction factors, such as the gas constant (C) and the compressibility
factor (Z), that illustrate deviation from the perfect gas law.
6.4

Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric Units
III. Gas/Vapor Sizing – Subsonic Flow
Set Pressures < 1.03 barg or Vacuum Conditions
The following formulas can be used for sizing valves when the set
pressure is below 1.03 barg. When pressure relief valves operate on
gases or vapors below 1.03 barg the speed at which the service fluid
travels is always less than the speed of sound or subsonic. Under these
conditions, the flow decreases with increasing back pressure eventhough the upstream flowing pressure stays the same.
These equations can be used to size the Anderson Greenwood pilot
operated valves listed in Chapter 8 (Tables 8-14 and 8-15).

The flow characteristics for the Whessoe Varec brand weight loadedpressure and vacuum vents are unique, not only for each model, butalso for each size of a particular model. The coefficient of discharge
method is different for each of these many combinations and is noteasy to select an orifice size with equations. It is suggested to use flow
capacity charts from the Whessoe and Varec catalog to manually selectthe valve size. The example shown in Table 6-1 shows the available
flow capacity for a vent with a set pressure of 10 mbar. One point on
this chart shows that a 6 inch vent with 50% overpressure (i.e. 15 mbar
flowing pressure) will flow 1289 Nm3/hr.

Weight Flow (kg/hr)

A =

W TZ
5.6 Kd P1 F M

Volumetric Flow (Nm3/hr

V MTZ
125.15 Kd P1 F

F=

k
k- 1

[(

2

P2 k
P1 -

(

Where:

(

P2
P1

(

A=

k+1
k

]

Where:A	 = Minimum required discharge area, square millimeters
Kd = Coefficient of discharge. See Chapter 8 (Tables 8-14 and 8-15)
k= Specific heat ratio. See Chapter 8 Section VII for common gases
M = Molecular weight of the gas or vapor. See Chapter 8 Section VII for
common gases
P1 = Relieving pressure, bars absolute. This is the set pressure (barg)+ overpressure (barg) + atmospheric pressure (bara) – inlet
pressure piping loss (barg)
P2 = Pressure at the valve outlet during flow, bars absolute. This is thetotal back pressure (barg) + atmospheric pressure (bara)T= Absolute relieving temperature of the gas or vapor at the valve
inlet, degree Kelvin (degree Celsius + 273)
W = Required relieving capacity, kilograms per hour (kg/hr)
V= Required relieving capacity, normal cubic meters per hour (Nm3/hr)
Z= Compressibility factor. See Chapter 8 Section I

Table 6-1 – 4020A Weight Loaded Pressure Flow Capacity (Pipeaway Version)
Set
Press. mbarg
2.5
10
25
50
60

2”
3”
20% 25% 50% 20% 25% 50% 20%
3652
72
84 116 161 150
75 103 143 169 232 322 300
119 163 226 267 367 509 475
168 231 320 378 519 720 672
184 253 351 414 568 789 736

4”
25%
206
412
652
922
1010

50%
286
573
906
1281
1403

20%
338
676
1069
1511
1656

6”
25%
464
928
1467
2075
2273

50%
644
1289
2038
2882
3157

20%
601
202
1900
2687
2943

8”
25%
825
1650
2608
3689
4041

50%
1146
2291
3622
5123
5612

20%
939
1877
2969
4198
4599

10”
25%
1289
2578
4076
5764
6314

50%
1790
3580
5660
8005
8769

20%
1352
2704
4275
6045
6622

12”
25%
1856
3712
5869
8300
9092

50%
2577
5155
8151
11527
12627

Flow in m3/hr
6.5

Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric Units
IV. Steam Sizing
ASME Section VIII (Set Pressures ≥ 15 psig)
The following formula is used for sizing safety valves for process vessel
applications that normally are not considered fired vessels. Examplesof fired vessels are economizers, steam drums, superheaters andreheaters that fall under the ASME Section I scope. As discussed in theprevious gas/vapor section, the determination of the required steam
relieving rate is needed before sizing can begin. Once again the use of
ISO 23251 (API Standard 521) can be helpful to determine the required
steam flow due to sources of overpressure such as a split exchanger
tube or reflux failures.
This formula is based upon the empirical Napier formula for steam flow.
The nozzle coefficient of discharge and the back pressure correction
factors are the same as those in the previous gas/vapor section. There isa new factor for steam that is above its saturation temperature, or is in
a superheated condition. The more superheated the steam, the greater
the required orifice area. A second, but rarely used input, is the Napierequation correction factor. This factor is only used for dry saturated
steam when the set pressure is 103.5 bara or greater.

A =47 square meters, and a design steaming generation capacity exceeding
1800 kg/hr, then the drum must have two or more safety valves. If there
are to be no more than two safety valves installed, there is a requirementin ASME Section I that the relieving capacity of the smaller valve be atleast 50% or greater of that of the larger valve. Beyond this requirement,
there are no other rules on how the overall required capacity is to be
divided between multiple valves but it is often found that the capacity beevenly split between the multiple valves. This will allow the valves to beof the same configuration which can optimize the use of spare parts formaintenance.
These same selection rules in Section I apply when the boiler system
has additional vessels in its train. However, there are additional
requirements to consider that well be discussed next for thesuperheater, reheater and economizer.

W
0.525KP1Ksh Kn Kb

Where:A	 = Minimum required discharge area, square millimeters
W = Required relieving capacity, (kg/hr)
K	 = Coefficient of discharge. See Chapter 8 Section IX
P1 = Relieving pressure, bars absolute. This is the set pressure (barg)+ overpressure (barg) + atmospheric pressure (bara) – inlet
pressure piping loss (barg)
Ksh = Capacity correction factor due to the degree of superheat in thesteam. For saturated steam use 1.0. See Chapter 8 Section V
Kn = Capacity correction factor for dry saturated steam at set
pressures above 103 bara. See Chapter 8 Section III
Kb = Back pressure correction factor for gas. See Chapter 8 Section II
ASME Section I (Set Pressures ≥ 1.03 barg)
The sizing and selection of steam safety valves for fired pressure vesselsthat fall under the scope of ASME Section I has a different procedurethan an ASME Section VIII steam sizing case. The steam sizing equation
listed above could be used, but there are certain valve selection rules
within ASME Section I where the use of valve capacity charts provides fora simpler procedure.
Steam drum safety valve sizing
The steam drum is one such fired pressure vessel that receives thesaturated steam from water that has been heated by burning an externalfuel source such as coal or natural gas. The boiler system may consist of
only this steam drum or may have other vessels used to heat that we will
discuss below. For the purposes of this initial discussion, let us assume
the boiler system has only a steam drum. As with the sizing procedures
discussed previously, the required steam relieving rate must be
determined to size the drum safety valve. This is fairly straight forward
as, in most instances, the required capacity shall not be smaller than the
maximum designed steaming output of the boiler at its MAWP.
The user should refer to the catalog where the saturated steam capacity
tables are located. The following link will provide access to the Crosby
HL, HSJ, HCI and HE steam safety valves:
http://www.valves.emerson.com/Images/CROMC-0295-US.pdf
Although the determination of required capacity is often simple, theselection process is more involved as there are rules to be followed in
the ASME Section I Code. One such requirement is that if a boiler system
has a combined bare tube and extended heating surface exceeding
6.6

Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric Units
Superheater safety valve sizing
As shown in Figure 6-1, when the steam created in the drum is being
used to turn a turbine to create work, the steam drum outlet is often, but
not always, attached to a heat exchanger vessel called a superheater.
The moisture in saturated steam coming from the drum can cause
corrosion and damage to the turbine blades. Therefore, the use of asuperheater allows the hot flue gases from the boiler to continue to heat
the wet steam to temperatures above saturation thus drying the fluid.
The rules in ASME Section I state that all superheaters must have one ormore safety valves located on the outlet of the superheater and prior to
the first downstream isolation valve.
The Code goes further to state that if there are no intervening stop valves
between the steam drum and the superheater, then the superheater
safety valve can be included in providing the relieving capacity for theentire system. This superheater safety valve, along with the drum valve,
will satisfy the Code requirement for multiple valves for the larger boiler
systems outlined in the previous steam drum discussion. What changes
is the allowable split of the required capacity to be delivered by these
multiple valves. ASME Section I mandates that for a boiler system, thedrum safety valve provide a minimum of 75% of the available relieving
capacity. The reason the Code limits the superheater safety valve
available capacity is to protect this exchanger. Damage to the tubes inthe exchanger can occur if the incoming saturated steam from the drum
cannot make up the flow from the superheater safety valves that may
have opened. The tubes in the superheater can overheat and fatigue
because of the lack of heat transfer. This is an important consideration

since the superheater valves are set to open before the drum valvesbecause of inlet pressure losses between the upstream drum and thedownstream superheater. If an overpressure event occurs, the openingof the safety valves on dry superheated steam is preferable to opening
the drum valves on wet steam.

SecondarySuperheater

Furnace Wall

Turbine Deck

Steam
Drum

HP

IP

LP
Generator

Superheater

Reheater

DEA
Condenser

Water
Pump

Deaerator
Feed Pump

HP Heaters

LP
Heaters

Economizer

CW
Cooling
Water Pump

Feed Water
RecircDemineralizer

Feed Water
Regulator

BoilerFeed
Pump

Condensate
Booster Pump

Figure 6 - 1
Closed Simple Rankine Steam Cycle
6.7

Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric Units
Emerson engineering reommends the use of multiple steam drum
valves when a superheater is part of the boiler system. These multiple
drum valves should be set with the staggered values allowed by the Codeand selected using the capacity mandate where a smaller orifice valve
should have at least 50% or greater capacity of the larger orifice valve.
This staged relief of steam pressure can help prevent the ingress ofwater into the steam trim safety valves during an opening cycle. Please
note that this two or more drum valve arrangement is not required by
Code but in many instances the required capacity will simply be too large
for one drum safety valve.
A sample calculation and selection of drum and superheater safetyvalves follows:
Step One
Determine the boiler specifications.
1.	Total boiler steam generation: 660,000 kg/hr.
2.	Boiler drum and superheater design pressure (MAWP): 206.0 barg
3.	Drum operating pressure: 196.0 barg
4.	Superheater outlet temperature: 550°C
5.	Superheater outlet operating pressure: 185.0 barg
6.	Boiler system bare tube and extended heating surface exceeds
47 sq.m.

Step Four
Determine the superheater safety valve set pressure.
1.	Subtract the superheater outlet operating pressure from the drum
outlet operating pressure to obtain the pressure loss in the piping
between these devices:196.0 barg – 185.0 barg = 11.0 barg.
2.	As mentioned above, it is desirable to open the superheater safetyvalve first followed by the drum safety valve if necessary. Emerson
engineering recommends that an additional 1.40 barg be includedin the drum to superheater pressure drop to allow this to occur. It
should be noted that this 1.40 barg additional pressure difference
is not mandated by the ASME Section I Code but it is strongly
recommended by Emerson. Therefore the total superheater pressure
differential from the drum is the pressure loss plus the Emerson
recommended 1.40 barg factor: 11.0 barg + 1.40 barg = 12.4 barg.
3. Calculate the superheater set pressure by subtracting the total
drum to superheater pressure differential from the design (MAWP)
pressure: 206.0 barg – 12.4 barg = 193.6 barg.

Step Two
Determine the capacity of the drum safety valves.
1.	A minimum of 75% of the boiling steaming capacity must be relieved
from the drum safety valves: 660,000 kg/hr x 0.75 = 495,000 kg/hr.
Step Three
Select the drum safety valves with primary valve set at the MAWP.
1.	Since we have more than 47 square meters of bare tube and heating
surface and our steam generation is greater than 1800 kg/hr
Emerson engineering recommends to use a minimum of two drum
valves.
2.	As you recall, ASME Section I allows for 6% accumulation when
multiple valves are used. The first or primary valve can be set nohigher than MAWP of 206.0 barg in this example. The secondary valve
can be set 3% higher than MAWP or 212.2 barg.
3.	As mentioned earlier it may be preferable, but not required, to havethe same size drum valves to facilitate effective use of spare parts.
Therefore, for this example we will split the drum capacity evenly
between two safety valves: 495,000 kg/hr ÷ 2 = 247,500 kg/hr.
4.	Refer to Crosby Safety Valve catalogs for capacity charts to selectthe drum safety valves. Table 6-3 provides an example. The Crosby
HE valve is suitable for drum applications at these set pressures.
“M” orifice valves will provide 266,107 kg/hr at 206 barg set.
Interpolation between 212 and 213 barg in the capacity chart will
provide the available capacity at 212.2 barg for the secondary valve.
The capacity at 212.2 barg will be 279,115 kg/hr. It should be notedthat the capacity charts will show the capacity in saturated steam
already adjusted using the Kn high pressure (103 bara and above set
pressures) factor.

Table 6-2 – ASME Section I Drum and Superheater Sizing Example Summary
Location
Low Set Drum Safety Valve
High Set Drum Safety Valve
Total Flow thru Drum Safety Valve
Superheater Outlet Safety Valve
Total Flow thru all Safety Valves

Orifice SizeM
M

Set Pressure(barg)
206.0
212.2

Temperature
Saturated Steam
Saturated Steam

K2

193.6

550°C

Rated Relieving Capacity(kg/hr steam)
266,107279,115545,222
120,668
665,890% of Total Required Capacity(660,000 kg/hr)

83%
18%
101%
6.8

Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric Units
Step Five
Determine the superheater required relieving capacity.
1.	The remaining capacity to be provided by the superheater isthe difference between the total steam generation and rated
capacity of the drum safety valves that have been selected:
660,000 kg/hr – 266,107 kg/hr – 279,511 kg/hr = 114,382 kg/hr.
2.	The superheat correction factor must be determined because thesuperheater safety valves are operating and will be flowing above thesaturation point.
a. T he superheater safety flowing pressure (bara) will be the set point+ overpressure + atmospheric:
193.6 barg x 1.03 + 1 bara = 200.4 bara
b. At 550°C and 200.4 bara, the superheat correction factor is 0.704
from Chapter 8 Section V
3.	In order to use the saturated steam capacity chart in Table 6-3 toselect the superheater safety valve, we must convert the remaining
required capacity to saturated conditions. Therefore, equivalentsaturated steam required capacity at the superheated condition at
550°C is: 114,382 kg/hr ÷ 0.704 = 162,474 kg/hr saturated steam.
Step Six
Select the superheater safety valve.
1.	Refer to Crosby Safety Valve catalogs for capacity charts to select thesuperheater safety valve. Table 6-3 provides an example. The Crosby
HCI valve is suitable for superheater outlet applications at a set
pressure of 193.6 barg.
2.	From the chart, interpolation will show that a “K2” orifice will provide
171,404 kg/hr of saturated steam and meet the requirement fromstep five.
3.	At the 550°C superheat condition, this Crosby HCI “K2” orifice valvethat has been selected will flow: 171,404 kg/hr saturated steam x
0.704 = 120,668 kg/hr superheated steam.
Step Seven
Check to ensure we meet the ASME Section I requirement that drum
safety valves flow at least 75% of the total boiler steaming capacity
and that the combined relieving capacity of all safety valves meet or
exceed the required steaming capacity of the boiler. See Table 6-2 thatsummarizes the drum and superheater safety valve selection.
Reheater safety valve sizing
In Figure 6-1, there is another heat exchanger, called a reheater, thatwill add efficiency to the steam cycle by taking spent, near saturated
steam from the turbine and adding more heat from the exhaust gases ofthe boiler. A closed steam cycle may or may not have a reheater.
The reheater operates similar to the superheater exchanger tosuperheat this incoming steam. This superheated steam exiting thereheater is at a much lower pressure than that at the superheater
outlet but its temperature is virtually the same. This lower pressure,superheated steam from the reheater outlet is then sent back to theturbine deck where an intermediate pressure turbine will expand thesteam and do additional work.
The ASME Section I Code requires each reheater to have one or more
safety valves. The overall required capacity must be equal or greaterthan the maximum steam flow for which the reheater is designed.
Unlike the superheater safety valves, there can be no credit taken for
the reheater safety valves capacity in providing protection for the steam
drum.

The reheater safety valves can be located either on the reheater inlet linethat returns saturated steam back from the high pressure turbine or on
the outlet line that delivers superheated steam back to the intermediatepressure turbine. One rule in ASME Section I will state that at least
one safety valve be located on the outlet of the reheater prior to thefirst isolation valve and that this reheater outlet safety valve provide aminimum of 15% of the total required capacity. This requirement willprotect the tubes of the reheater when the safety valves must lift.
Similar to the superheater outlet safety valve, the reheater outlet safetyvalve is set lower than the reheater inlet valve to allow for the pressure drop
through the exchanger and allow for the exhaust of the dry superheatedsteam to occur first. One might rightly assume it to be a good practice tohave 100% of the required relieving capacity be from the reheater outlet
valve as there is no restriction in ASME Section I for this type of installation.
One reason for not installing the safety valves in this fashion is that thesereheater outlet valves are more expensive devices than the reheater inlet
valves. This is because the high superheated temperatures on the reheater
outlet require a high alloy steel bill of materials so most specifications try
and keep this valve as small as possible.
An example of a reheater sizing and selection follows.
Step One
Determine the reheater specifications.
1.	Reheater maximum design steam flow: 450,000 kg/hr
2.	Design pressure: 50.0 barg
3.	Reheater inlet operating pressure: 46.5 barg
4.	Reheater outlet operating pressure: 44.8 barg
5.	Reheater outlet operating temperature: 550°C
Step Two
Determine the reheater outlet safety valve set pressure.
1.	Subtract the reheater outlet operating pressure from the reheater
inlet operating pressure to obtain the pressure loss between theselocations: 46.5 barg – 44.8 barg = 1.70 barg.
2. As mentioned above, it is desirable to open the reheater outlet safetyvalve first followed by the reheater inlet safety valve if necessary.
Emerson engineering recommends that an additional 1.03 barg be
included in the pressure drop to allow this to occur. It should be notedthat this 1.03 barg additional pressure difference is not mandated
by the ASME Section I Code. Therefore the total reheater inlet tooutlet pressure differential is the pressure loss plus the Emerson
recommended 1.03 barg factor: 1.70 barg + 1.03 barg = 2.73 barg.
3. Calculate the reheater outlet safety valve set pressure by subtracting
the reheater pressure differential from the design pressure:50.0 barg – 2.73 barg = 47.3 barg.

6.9

Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric Units
Saturated Steam Capacities: Styles HE, HCI and HSJ - Metric Units
Kilograms per hour at 3% overpressure
Table 6-3 – Saturated Steam Capacities - Set Pressures 173-213 barg
HE
HCI
HSJ

•
•
•

•

•

F
G
H
Set Pres. 198.1 324.5 506.5
17317748 29078 45381
174
17881 29296 45721
175
18014 29515 46062
176
18149 29736 46406
177
18284 29957 46753
178
18420 30181 47101
179
18557 30405 47452
180
18696 30631 47805
181
18835 30859 48160
182
18974 31088 48518
183
19115 31319 48878
184
19257 31552 49241
185
19400 31786 49607
186
19544 32022
187
19689 32260
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
-

•
•

•

H2
J
J2
641.3 830.0 923.2
57463
82726
57893
83346
58326
83969
58762
84596
59200
85227
59641
85862
60085
86501
60532
87144
60982
87792
61435
88445
61892
89101
62351
89763
62814
90429
63280
91101
63750
91777
64223
92458
64700
93145
65181
93837
65666
94535
66154
95238
66647
95948
67144
96663
67645
97384
68150
98112
68660
98846
69175
99587
69694
10033570219
10109070748
10185271282
102621
71822
10339872368
10418372918
10497673475
10577774038
106587-K
1187.1106371107167107968108774109586110402111224112051112884113723114568115418116275117138118008118884119767120657121554122459123371124290125218126154
127098128050129012
129982130962131951132951133960134980136010137051138104139169140245141334142436143551

Orifice Designation and Area (sq. mm.)
•
•
•
•
•
•
•
•
•
•
Orifice (sq mm)(barg)
K2
L
L2
M
M2
N
P
1641.9 1840.6 2155.5 2322.6 2565.2 2800.6 4116.1
147127
193143 208116 229853
148228194589 209674 231574
149336
196044 211242 233305
150451197508 212819 235047
151574
198981 214407 236800
152703
200464 216004 238565
153840201956 217612 240341
154984203459 219231 242129
156136204971 220861 243929
157297206494 222502 245741
158465208028 224154 247566
159641209572 225819 249404
160826211128 227495 251256
162020212695 229184 253121
163223214274 230885 255000
164435215865 232600 256893
165656217469 234327 258801
166887219085 236068 260724
168128220714 237824 262663
169379222356 239593 264617
170640224012 241377 266588
171913225682 243177 268575
173196227366 244992 270580
174490229065 246822 272602
175796230779 248670 274642
177113232509 250533 276700
178443234255 252415 278778
179785236017 254313 280875
181141237796 256230 282992
182509239592 258166 285130
183891241407 260121 287289
185287243239 262096 289470
186697245091 264091 291673
188123246962 266107 293900
189563248853 268144 296150
191019270204 298425
192492272287 300725
193981274393 303052
195487276523 305405
197011
278679 307785
198553280860 310195-

Step Three
Determine the capacity of the reheater outlet safety valve.
1.	A minimum of 15% of the relieving capacity must come from thereheater outlet safety valve: 450,000 kg/hr x 0.15 = 67,500 kg/hr.
2.	The superheat correction factor must be determined because thereheater outlet safety valves are operating, and will be flowing, above
the saturation point.
a. T he reheater safety valve flowing pressure (bara)
will be the set point + overpressure + atmospheric:47.3 barg x 1.03 + 1.01 bara = 49.7 bara
b. At 550°C and 49.7 bara the superheat correction factor is 0.751
from Chapter 8 Section V

•
•

•

•

•

•
P2
Q
Q2
R
RR
4561.3 7125.8 7903.2 10322.6 12445.1
408717411777414856417953421071424208427366430546433746436969440214443483446775450092453433456800460192463612467059470534474039477572481137484732488359492020495714499443503208507009510849514727518645522604526605530651534741538877543061547294551578-

3.	In order to use the saturated steam capacity chart such as thatshown in Table 6-5 to select the reheater outlet safety valve, we must
convert the required capacity to saturated conditions. Therefore,
equivalent saturated steam required capacity at the superheatedcondition at 550°C is: 67,500 kg/hr ÷ 0.751 = 89,880 kg/hr saturated
steam.

6.10

Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric Units
Step Four
Select the reheater outlet safety valve.
1.	From Table 6-5 one “P” orifice Crosby HCI safety valve will provide
99,073 kg/hr at 49.7 barg.
2.	At the 550°C superheat condition, this Crosby HCI “P” orifice valvethat has been selected will flow: 99,073 kg/hr saturated steam x
0.751 = 74,404 kg/hr superheated steam.
Step Five
Determine the reheater inlet safety valve required relieving capacity.
1. The remaining capacity to be provided by the reheater inlet
safety valves is the difference between the design steam flow
and rated capacity of the reheater outlet that has been selected:
450,000 kg/hr – 74,404 kg/hr = 375,596 kg/hr.
Step Six
1.	Refer again to Crosby Safety Valve catalogs (see Table 6-5) for
capacity charts to select the reheater inlet safety valves. The Crosby
HCI valve is suitable for reheater inlet applications at these set
pressures. You will note there is not one valve that can provide theremaining required capacity. Therefore, we need to consider multiple
valves. Many specifying engineers will select identical valves tooptimize spare parts.
2.	Divide the remaining required capacity for the reheater in half:
375,596 kg/hr ÷ 2 = 187,798 kg/hr.
3.	As you recall, ASME Section I allows for 6% accumulation when
multiple valves are used. The first or primary reheater inlet valve
can be set no higher than MAWP of 50.0 barg in this example. The
secondary valve can be set 3% higher than MAWP or 51.5 barg.
4. “Q2” orifice valves will provide 191,352 kg/hr at 50.0 barg set and
196,982 kg/hr at 51.5 barg set. See Table 6-5.

Economizer safety valve sizing
You will note in Figure 6-1 that there is one other heat exchanger vesselthat is located upstream of the steam drum portion of the steam cycle.
As with the superheater and reheater sections of the cycle, hot fluegases are used to add heat to the incoming boiler feedwater. This helps
to reduce the amount of energy needed to raise the temperature of thewater as it travels to the steam drum.
In many installations there is no intervening isolation valve between theeconomizer and the steam drum. When this is the case, the safety valveson the steam drum, sized and selected as described above, can be usedas overpressure protection for the economizer.
In some steam cycles, such as combined cycle type plants, it may benecessary to regulate the output of the economizer into the boiler
to meet varying needs. This requirement now adds valves that could
potentially isolate the economizer from the boiler. In this case the
ASME Section I Code mandates that the economizer have one or more
safety relief valves. The rated capacity of these safety relief valves isdetermined by the economizer manufacturer based upon the maximum
heat absorption rate. For metric units of measure, the heat absorption
rate in Watts is divided by 1.6 to obtain the required steam capacity in
kg/hr. Once again, use the saturated steam tables to select a safetyvalve that will have a rated capacity equal to or larger than the required
capacity.

Step Seven
Check to ensure we meet the ASME Section I requirement that reheater
outlet safety valve will flow at least 15% of the total reheater design
steam flow, and that the combined relieving capacity of the reheater inlet
and reheater outlet safety valves meet or exceed the total steam flow of
the reheater. Table 6-4 summarizes the reheater inlet and outlet safetyvalve selection.

Table 6-4 – ASME Section I Reheater Sizing Example Summary
Location
Low set reheater inlet safety valve
High set reheater inlet safety valve
Total Flow Thru Reheater inlet Safety Valves
Reheater Outlet Safety Valve
Total Flow Thru all Safety Valves

Orifice Size
Q2
Q2

Set Pressure(barg)
50.0
51.5

Temperature
Saturated Steam
Saturated Steam

P

47.3

550°C

Rated Relieving Capacity(kg/hr steam)
191,352
196,982
388,334
74,404
462,738% of Total Required Capacity(450,000 kg/hr)

17%
103%
6.11

Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric Units
Saturated Steam Capacities: Styles HE, HCI and HSJ – Metric Units
Kilograms per hour at 3% overpressure

Table 6-5 – Saturated Steam Capacities - Set Pressures 44-86 barg
HE
HCI
HSJ

•

Set
Pres.
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86

F
198.1
4231
4325
4419
4513
4607
4701
4796
4890
4984
5078
5172
5266
5360
5454
5548
5642
5736
5830
5924
6018
6112
6206
6300
6395
6489
6583
6677
6771
6865
6959
7053
7147
7241
7335
7429
7523
7617
7711
7805
7899
7994
8088
8182

Orifice Designation and Area (sq. mm.)
•
•
•
•
•
•
•
•
•
•
•
•
•
Orifice (sq mm)(barg)
G
H
H2
J
J2
K
K2
L
L2
M
M2
N
324.5 506.5 641.3 830.0 923.2 1187.1 1641.9 1840.6 2155.5 2322.6 2565.2 2800.66932 10819 13700 17752 19722 25359 35076 39321 46047 49616
54798 59829
7087 11060 14004 18146 20161 25923 35856 40195 47070 50719
56017 61159
7241 11300 14309 18541 20599 26487 36635 41069 48094 51822
57235 62489
7395 11541 14613 18935 21038 27051 37415 41943 49117 52925
58453 63819
7549 11781 14918 19330 21476 27614 38195 42817 50141 54028
59671 65149
7703 12022 15222 19725 21915 28178 38975 43691 51165 55131
60889 66479
7857 12262 15527 20119 22353 28742 39754 44566 52188 56234
62107 67809
8011 12503 15831 20514 22791 29306 40534 45440 53212 57337
63326 69139
8165 12743 16136 20909 23230 29869 41314 46314 54236 58440
64544 70469
8319 12984 16440 21303 23668 30433 42094 47188 55259 59543
65762 71799
8474 13224 16745 21698 24107 30997 42873 48062 56283 60646
66980 73129
8628 13465 17050 22092 24545 31561 43653 48936 57306 61749
68198 74459
8782 13705 17354 22487 24984 32124 44433 49810 58330 62852
69416 75789
8936 13946 17659 22882 25422 32688 45213 50684 59354 63955
70635 77119
9090 14186 17963 23276 25860 33252 45992 51558 60377 65058
71853 78449
9244 14427 18268 23671 26299 33816 46772 52432 61401 66161
73071 79779
9398 14667 18572 24065 26737 34379 47552 53307 62425 67264
74289 81109
9552 14908 18877 24460 27176 34943 48332 54181 63448 68367
75507 82439
9706 15148 19181 24855 27614 35507 49111 55055 64472 69470
76725 83769
9861 15389 19486 25249 28053 36071 49891 55929 65495 70573
77944 85099
10015 15629 19790 25644 28491 36634 50671
66519
79162
10169 15870 20095 26039 28930 37198 51450
67543
80380
10323 16110 20400 26433 29368 37762 52230
68566
81598
10477 16351 20704 26828 29806 38325 53010
69590
82816
10631 16591 21009 27222 30245 38889 53790
70614
84035
10785 16832 21313 27617 30683 39453 54569
71637
85253
10939 17072 21618 28012 31122 40017 55349
72661
86471
11093 17313 21922 28406 31560 40580 56129
73684
87689
11248 17553 22227 28801 31999 41144 56909
74708
88907
11402 17794 22531 29196 32437 41708 57688
75732
90125
11556 18034 22836 29590 32875 42272 58468
76755
91344
11710 18275 23140 29985 33314 42835 59248
77779
92562
11864 18515 23445 30379 33752 43399 60028
78802
93780
12018 18756 23750 30774 34191
60807
79826
94998
12172 18996 24054 31169 34629
61587
80850
96216
12326 19237 24359 31563 35068
62367
81873
97434
12480 19477 24663 31958 35506
63147
82897
98653
12635 19718 24968 32353 35944
63926
83921
99871
12789 19958 25272 32747 36383
64706
84944
10108912943 20199 25577 33142 36821
65486
85968
10230713097 20439 25881 33536 37260
66266
86991
10352513251 20680 26186 33931 37698
67045
88015
10474313405 20920 26490 34326 38137
67825
89039
105962
•

•
•

•
•
•
P
4116.1
87931
89886
91840
93795
95750
97705
99659
101614103569105523107478109433111388113342115297117252119206121161123116125071
-

•

•

•

•
P2
Q
Q2
R
RR
4561.3 7125.8 7903.2 10322.6 12445.1
97441
168833 220517 265860
99607
172586 225419 271770
101773
176339 230321 277680
103939180093 235223 283591
106105183846 240125 289501
108271
187599 245027 295411
191352 249929 301321
110437
195105 254831 307231
112604198858 259733 313141
114770116936202612 264636 319051
119102206365 269538 324961
121268210118 27443: 330871
123434213871 279342 336781
125600217624 284244 342692
127766221377 289146 348602
129933225131 294048 354512
132099228884 298950 360422
134265232637 303852 366332
136431236390 308754 372242
138597240143 313657 378152
140763243896 318559
142929247650 323461
145095251403 328363
147261255156 333265
149428258909 338167
151594262662 343069
153760266416 347971
155926270169 352873
158092273922 357775
160258277675 362678
162424281428 367580
164590285181 372482
166757288935 377384
168923292688 382286
171089296441 387188
173255300194 392090
175421303947 396992
177587307700 401894
179753311454 406797
181919315207 411699
184085318960 186252
322713 188418
326466 -6.12

Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric Units
V. Liquid Sizing

VI. Fire Sizing

The following formula is used for sizing relief valves
for liquid service at any set pressure. The flow of an incompressible fluid(that does not flash) through an orifice is based upon the square root of
the differential pressure across that orifice. There is a correction factor
for highly viscous liquids as well as a back pressure correction factor forbalanced bellows relief valves.

In the first part of this chapter it was noted that one of the starting
points in sizing pressure relief devices is to determine the required
capacity for various possible causes of overpressure. One common
overpressure contingency to be considered is subjecting a storage tankor process vessel to an external fire that could raise the temperatureof the contents in the tank or vessel. This subsequently could increase
the system pressure due to a liquid inside the vessel vaporizing or a gas
inside the vessel expanding.

A =19.633 VL GK KV KW P 1 - P2

Where:A	
= Minimum required discharge area, square millimeters
VL= Required relieving capacity, m3/hr at flowing temperatureK	= Coefficient of discharge. See Chapter 8 Section IX
KV= Capacity correction factor due to viscosity of the fluid at flowing
conditions. For most applications viscosity will not affect the area
calculation so KV will be equal to 1.0. See Chapter 8 Section IV
for more information
KW = Capacity correction factor for balanced direct spring relief valves
due to back pressure. Use KW equal to 1.0 for conventional
direct spring and pilot operated relief valves. See Figures 8-11,
8-14 and 8-19
G	= Specific gravity of service liquid at flowing temperature referred
to water at standard conditions (15.5°C and 1.013 bara)
P1= Inlet pressure during flow, set pressure (barg) + overpressure(barg) – inlet pressure loss (barg)
P2= Total back pressure during flow (barg)
Thermal Relief Sizing
One very common application for a liquid service relief valve is protecting
equipment, such as piping, from hydraulic expansion of the service fluid.
This overpressure contingency is commonly referred to as thermal relief
and can be caused by heat transfer from one process media to anotheror by solar radiation. ISO 23251 states that the required relieving rate isdifficult to calculate and that a portable ¾” x 1” valve is very commonly
installed to provide protection.
The standard does give some cautions with regards to large diameterliquid pipelines where the distance between isolation devices may belong or where the application concerns liquid filled heat exchangers andvessels. If physical properties are known, the required relieving capacity
for thermal relief can be calculated as follows. This flow rate can then be
used in the liquid sizing formula above.

VL =Liquid Filled Tanks/Vessels
The procedure that is normally used in determining the required relieving
capacity will directly or indirectly calculate the estimated heat transferfrom an external fire to the contents of the vessel. This calculated heat
input value will vary from one code, standard, recommended practice, or
statute to another. One reason for this difference in heat input values isthat one particular publication may have a different definition to anotherfor what is called the “wetted” surface area of the vessel exposed to thefire. There are also different assumptions made in the documents withregard to tank insulation, prompt fire fighting availability and drainage
that can also alter the heat input calculations.
The exposed wetted surface is that part of the vessel or tank where theliquid contents reside and where a fire can input heat to vaporize thecontents. The greater the exposed wetted surface area the greater theheat input, the greater the heat input the more vaporization can occur,
the more vaporization the larger the required relief device orifice.
Since this exposed wetted surface definition and various assumptions
as noted above can vary from one engineering practice to another, it isimportant that the user be aware of what document is to be referenced
for a particular installation and location. Some of the more common
documents that are referenced and their calculation of exposed wetted
surface area, required capacity, and required orifice area are as follows.
It is recommended to review these documents, in full, for their scopeof use and a more complete explanation of the assumptions made inproviding this guidance.

3.6 αv φ
Gc

Where:
VL= Volume flow rate at the flowing temperature, m3/hrαν= Cubic expansion coefficient of the trapped liquid at the expected
temperature, expressed in 1/°C
φ= Total heat transfer rate, W
G	= Specific gravity of service liquid at flowing temperature referred
to water at standard conditionsc= Specific heat capacity of the trapped fluid, J/kg-K

6.13

Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric Units
ISO 23251 – Pressure Relieving and Depressuring Systems
Step One
Calculate the wetted surface area.
•	Liquid Filled Vessels – calculate the wetted area as the exposed area ofthe vessel up to a maximum height of 7.6 meters from the location of
the fire.
• Process Vessels – calculate the wetted area as the exposed area up to
the normal liquid operating level. If the normal operating level exceeds
7.6 meters from the location of the fire, use 7.6 meters as the height
for the wetted area calculation.
•	Spheres – calculate the exposed area up to the maximum horizontal
diameter (i.e. the equator) of the sphere and then calculate theexposed area up to a height of 7.6 meters from the location of the fire.
Use the larger of the two areas as the wetted area.

Table 6-6 – Environmental Factor
Tank Design/Configuration
Bare Metal Tank
Insulated Tank

Water-Application
Facilities
Depressuring and
Emptying Facilities
Underground Storage
Earth-Covered Storage
Above Grade

Insulation
Conductance(W/m2 • K)
22.71
11.36
5.68
3.80
2.84
2.27
1.87
-

Insulation
Thickness
(cm)
0
2.5
5
10
15
20
25
30
-

F
1.0
0.3
0.15
0.075
0.05
0.0375
0.03
0.025
1.0

-

-

1.0

-

-

0
0.03

Step Three
Determine the required relieving capacity.

Q
W 3600 —L

Where:
Q= Total heat absorption (input) to the wetted surface, W
L	= Latent heat of vaporization, J/kg
W
= Required relieving capacity, kg/hr
Step Four
Since the primary scope of ISO 23251 is used for applications at or above
1.03 barg design pressures, size for the required orifice using the weightflow vapor equation from page 6.4.
Weight Flow (kg/hr)
W
TZ

A=

CKP1 Kb Kc

M

Use the physical properties of the service fluid in the equation. Pleaserecall that for ASME Section VIII applications, the overpressure forfire sizing can be 21% if the valve is set at the MAWP. The allowable
accumulation for PED applications is determined by the designer based
upon good engineering practice.

Note: The insulation should not be dislodged by fire hose streams nor be
combustible or decompose below 538°C.

Step Two
Calculate the heat absorption or input to the liquid.
•	If there is deemed to be prompt firefighting and drainage of the
flammable fuel of the fire away from the vessel, use the following
equation:
0,82
Q 43,200 FAW
Where:
Q= Total heat absorption (input) to the wetted surface, WF	= Environmental factor (see Table 6-6)
Aw = Wetted surface area from step one above, square meters• Where there is not prompt firefighting and not drainage of the
flammable fuel of the fire away from the vessel, use the following
equation:
0,82
Q 70,900 FAW
Where:
Q= Total heat absorption (input) to the wetted surface, WF	= Environmental factor (see Table 6-6)
Aw = Wetted surface area from step one above, square meters

6.14

Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric Units
ISO 28300 – Venting of Atmospheric and Low Pressure Storage Tanks
Step One
Calculate the wetted surface area.
•	Spheres – calculate an area of 55% of the total exposed spherical area
and then calculate the exposed area up to a height of 9.14 meters
above grade. Use the larger of the two areas as the wetted area.
•	Horizontal Tank – calculate an area of 75% of the total exposed area
and then calculate the exposed area up to a height of 9.14 meters
above grade. Use the larger of the two areas as the wetted area.
•	Vertical Tank – calculate the wetted area as the exposed area up to aheight of 9.14 meters above grade.
Step Two
Calculate the heat absorption or input to the liquid per Table 6-7. The
formula used for this calculation will vary based upon the wetted surface
area calculated in step one.
Table 6-7 – ISO 28300 Heat Input Equations
Wetted Surface Area,
Aw (m2)< 18.6

Design Pressure (barg)≤ 1.03≥ 18.6 and < 93

≤ 1.03≥ 93 and < 260≤ 1.03

630,4000.338
w

≥ 260

Between 0,07 and 1.03

43,200A0.82
w

≥ 260

≤ 0.074,129,700

Heat Input, Q (W)
63,150Aw
224,200A0.566
w

Where:Aw
Q= wetted surface area from step one above, square meters= total heat absorption (input) to the wetted surface (W)Step Three
Calculate the required venting capacity in Nm3/hr of equivalent air
capacity using the following formula:q=906.6QF
LT
M

Gas Filled Vessels
ISO 23251 provides a recommended procedure for determining therequired pressure relief area due to a gas filled vessel being exposed toexternal flames.
Step One
Calculate the total exposed surface area. This is the complete surface
area of the gas filled vessel that is exposed to the ambient.
Step Two
Calculate what is termed the vapor fire sizing factor using the following:

F’ =

[

1.25

0.001518 (Tw - T1 )CK
T1 0.6506]

Where:
C= Gas constant based upon the ratio of specific heats of the gas
or vapor at standard conditions. See Chapter 8 Section VI. Use
C = 2.390 if unknownK	= Coefficient of discharge. See Chapter 8 Section IX
TW = Recommended maximum wall temperature of vessel material, °K
T1= Gas temperature at the upstream relieving pressure, °K
This gas temperature can be found using

T1 =

P1

Tn

Pn
Where:
P1= the upstream relieving pressure, set pressure + overpressure +atmospheric pressure, bara
Pn = the normal operating gas pressure, bara
Tn = the normal operating gas temperature, °K
If insufficient data exists to calculate the F’, use F’ = 0.045
Step Three
Calculate the minimum required pressure relief valve discharge area
using:
1823.5 F’ Aw
A =P1
Where:
A	= Minimum required discharge area, square millimeters
AW = Wetted surface area from step one, square meters

Where:q= Required relieving capacity in equivalent air, Nm3/hr.
Q= Total heat absorption (input) to the wetted surface from step two, W
F	= Environmental factor (see Table 6-6)L	
= Latent heat of vaporization, J/kg
T
= Absolute temperature of the relieving vapor, °K
M= Molecular weight of the relieving vapor
Step FourISO 28300 deals with storage tanks with design pressures less than
1.03 barg. Therefore, the equivalent air capacity in Nm3/hr calculated instep three can be directly used in the Whessoe Varec flow capacity charts
to select the vent size. For Anderson Greenwood brand pilot operated
valves, use the subsonic formula and inputs discussed on page 6.5.
Volumetric Flow (Nm3/hr)

A =

V MTZ
12,515 Kd P1 F

Note that the capacity calculated in step three is Nm3/hr of equivalentair. The volumetric flow equation uses m3/hr. Since the capacity is in
equivalent air, use M = 29, T = 0 + 273 = 273°K, Z = 1.0 and V = q from step
3 in the volumetric formula. Note that F in the volumetric flow equation
is not the environmental factor from Table 6-6.

6.15

Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric Units
VII. Two-Phase Flow Sizing
Two-phase flow describes a condition whereby a flow stream contains a
fluid whose physical state is part liquid and part gas. For pressure relief
applications it can be common for all or part of the liquid portion of thefluid to change to vapor, or flash, as the pressure drops. The ratio of gas
to liquid in the flowing media can be a significant factor in determining
the required orifice flow area of a pressure relief valve.
It is important to note that there are no codes such as ASME or PED, that
require a certain methodology to be used to size PRVs for two phase flow
regimes. The selection of the method for a particular case lies solely
with the user that has the full knowledge of the process conditions.
There are several publications, written by various process relief experts,
that will provide guidance in calculating the required relief load and thesubsequent minimum required orifice area of the pressure relief valve.
What is evident from these publications is that the subject is complex
and that there is no single universally accepted calculation method thatwill handle every application. Some methods give what are considered
to be accurate results over certain ranges of fluid quality, temperatureand pressure. The inlet and outlet conditions of the pressure relief valve
must be considered in more detail than what has been discussed up tonow, where we have been dealing with a single phase fluid flow that does
not change state.

It is therefore necessary that those responsible for the selection ofpressure relief valves used for two-phase or flashing flow applications
be knowledgeable of the total system and current on the latest bestpractices for multi-phase sizing techniques. The user should note thatsome of these sizing methods have not been substantiated by actual
tests and there is no universally recognized procedure for certifyingpressure relief valve capacities in two-phase flows.
This engineering handbook will discuss two of these sizing techniques.
One is outlined in API 520 Part I (9th Edition – July 2014) Annex C and theother, from ASME Section VIII Appendix 11, which is specifically used forsaturated water applications.

1.2
Non-Flashing Flow

Flashing Flow

Critical Pressure Ratio, (ηc)

1.0

0.8

0.6

0.4

ηc = (1 + (1.0446 – 0.0093431 • ω0.5) • ω-0.56261) (-0.70356 + 0.014685 • ln ω)
0.2

0.0
0.01

0.1

1

10

100

Figure 6-2
Omega Parameter, ωCritical Pressure Ratio for Two-Phase Flow

6.16

Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric UnitsAPI Standard 520 Part I (9th Edition)
One sizing procedure in Annex C is a part of what is commonly known as
the “Omega Method” which was developed by Dr. J. Leung. The Omega
Method is a simplified version of a more rigorous procedure called the
Homogeneous Equilibrium Method (HEM) which assumes that the fluid
is well mixed, and the gas and liquid portions of the fluid are flowing at
the same velocity through the nozzle of the pressure relief valve. The
fluid is also assumed to remain in thermodynamic equilibrium, whichsimply means that any flashing that occurs will take place when thepressure drops below the vapor pressure of the mixture.
What is called the “reduced” Omega method in API Standard 520 Part
I is a simplified technique in that one can take the process conditions
at the pressure relief valve inlet and compare them to the process
conditions at a lower pressure. This two process point comparison will
represent the behavior of the mixture as the pressure drops during the
opening of a pressure relief valve. The process conditions, such as thedensity or specific volume, at the inlet of the valve are known parametersfrom those on the PRV datasheet at set pressure. The second processdata point required is the density or specific volume of the mixture at
90% of the flowing pressure or, in the case of 100% liquid that flashes
it would be the saturation pressure at the relieving temperature. Notethat the flowing pressure is taken as an absolute value. This data point
is normally obtained from the fluid property database or from a processsimulation flash calculation.
API 520 Part I will illustrate the use of the reduced Omega Method fortwo conditions. One condition is a two-phase mixture at the inlet of the
PRV that may or may not flash during relief and the other condition iswhere a 100% liquid fluid at the inlet of the PRV flashes during relief.

Step Four
Determine if flow is critical or subcritical by comparing critical pressure
from step three to the expected total back pressure (P2) in bara.
If Pc ≥ P2 then flow is critical, go to step five.
If Pc < P2 then flow is subcritical, go to step six.
Step Five
Calculate the required mass flux for the service fluid if in critical flow.

G = 1.138 ηc

P1
ν1 ω

Where:G	= Mass flux required, kg/hr-mm2
ηc= Critical pressure ratio from step three
P1= Flowing pressure, i.e. set pressure + allowable overpressure +atmospheric pressure – inlet piping losses, bara
ν1= Specific volume of the two-phase service fluid at the set
pressure, m3/kg
ω= Omega parameter from step one
Go to step seven.

API Standard 520 Part I (9th Edition) – Two-Phase Flow Mixture
Procedure(νω = 9 ν9 −1
1(

Step One
Calculate the Omega parameter.

Where:
ν9= specific volume of the two-phase fluid at 90% of the absolute
flowing pressure, m3/kg
ν1= specific volume of the two-phase fluid at the absolute flowingpressure at the prv inlet, m3/kg
Step Two
Determine the critical pressure ratio from Figure 6-2 using w from stepone. As you will note in the figure, the value of the Omega parameter will
indicate whether the mixture will or will not flash.
Step Three
Calculate the critical pressure.

Pc ηc P1
Where:
Pc= Critical pressure, bara
ηc
= Critical pressure ratio from step two
P1= Set pressure + allowable overpressure + atmospheric pressure –inlet piping losses, bara

Table 6-8 – Anderson Greenwood Crosby Selection for Two-Phase Flow

Conventional Direct Spring PRV1Balanced Direct Spring PRV
Pilot Operated PRV
JLT-JOS-E Series 900
JLT-JBS-E
Series 400/500/800
Note 1 – The magnitude of the built-up back pressure can be difficult to predict for two-phase flow. It is advisable to use either a balanced direct spring or pilot operated
PRV if this value is uncertain.

6.17

Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric Units
Step Six
Calculate the required mass flux for the service fluid if in subcritical flow.

1.138

(

ω 1 −1 +1
η2

P1∕ν1

(

G =

Where:
G	= Required Mass flux, kg/hr-mm2
P2 = Total expected back pressure, bara
P1 = Flowing pressure, i.e. set pressure + allowable overpressure +atmospheric – inlet piping losses, bara
η2 = Back pressure ratio, P2/P1
ω= Omega parameter from step one
ν1= Specific volume of the two-phase service fluid at the inlet of thevalve at the flowing pressure, m3/kg

Step Nine
Determine the coefficient of discharge for the selected valve.

K 2φ α1 KG + (1−α1) KL

Where:
K2φ = Two-phase coefficient of discharge
α1 = Void fraction of two phase mixture from step seven
KG = Gas/vapor coefficient of discharge. See Chapter 8 Section IX
KL = Liquid coefficient of discharge. See Chapter 8 Section IX

Step Seven
In order to help obtain the two-phase nozzle discharge coefficients andback pressure correction factors for the desired Anderson Greenwood
Crosby brand product, we must first determine the mass fraction (χ1) of
the gas/ vapor portion of the two-phase mixture. From the mass fraction,
we can determine what is called the void fraction (a1), or volume ratioof the gas/vapor to the total volume of the two-phase mixture. This void
fraction will be used to calculate the two-phase nozzle coefficient andback pressure correction factors.

χ =
1

WG
WL + WGα =χ1 νν1
ν1

Where:
χ1= Mass fraction of gas/vapor portion of two-phase mixture
WG = Required gas/vapor mass flow, kg/hr
WL = Required liquid mass flow, kg/hr

1

Where:
α1 = Void fraction of two-phase mixture
χ1= Mass fraction from above calculationνν1 = Specific volume of gas/vapor at the inlet of the pressure relief
valve at the flowing pressure, m3/kg
ν1= specific volume of the two-phase fluid at the inlet of the valve at
the flowing pressure, m3/kg
Step Eight
Select the proper pressure relief valve type based upon the conditions of
the application. Emerson recommends the use of a safety relief valve for
two-phase applications. As we learned in Chapter 3, the trim of a safety
relief valve provides stable operation on either gas and/or liquid flow.
Anderson Greenwood Crosby safety relief valves have certified nozzle
coefficients for gas and liquid media that are used to calculate a twophase coefficient of discharge in the next step of this procedure.
It is also advisable that the safety relief valve selected be of a balanced
design for these applications. It is oftentimes difficult to accurately
predict the actual magnitude of built-up back pressure that will be
developed by the flow of a flashing mixture of gas and liquid. You recall
that a balanced direct spring valve or pilot operated valve will maintainlift and stability at higher built-up back pressures when compared toconventional pressure relief valves.
See Table 6-8 for a summary of the recommended valve designs for use
in two-phase flow applications.

6.18

Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric Units
Step TenIf built-up or superimposed back pressure is evident, calculate the backpressure correction factor.

K bw α1 Kb + (1−α1) Kw
Where:
Kbw = Two-phase back pressure correction factor
α1 = Void fraction of two-phase mixture from step seven
Kb = Back pressure correction factor for gas. See Chapter 8 Section II
Kw = Capacity correction factor for balanced relief valves due to backpressure. Use Kw equal to 1.0 pilot operated or conventionalsafety relief valves. See Figure 8-11 for direct acting safety relief
valves.

Kc

Kν

G	

= Combination factor for installations with a rupture disc upstreamof the valve. See Chapter 8 Section XI for flow certified factors.
Use a 0.9 value for any rupture disc/pressure relief valve
combination not listed in Chapter 8. Use a 1.0 value when arupture disc is not installed= Capacity correction factor due to viscosity of the fluid at flowing
conditions. For most applications viscosity will not affect the area
calculation so Kν will be equal to 1.0. See Chapter 8 Section IV for
more information= Required Mass flux from step five or six, kg/hr-mm2

Step Eleven
Calculate the minimum required discharge area.

A=W

K2φ KbwKc KυGWhere:A	
= Minimum required discharge area, square millimetersW= Required mass flow rate of the mixture, kg/hr
K2φ = Two-phase coefficient of discharge from step nine
Kbw = Two-phase back pressure correction factor from step ten

1.00

0.90

40

Critical Pressure Ratio (ηc)

15
10
0.80

7
5
4
3

0.70

2

Saturated Omega Parameter (ωs)

20

1.50
0.60
1.00
0.75
0.50
0.75

0.50
0.80

0.85

0.90

0.95

1.00

Figure 6-3
Saturated Pressure Ratio (hs)
Critical Pressure Ratio for Low Subcooled Region Flashing Liquids
6.19

Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric UnitsAPI Standard 520 Part I (9th Edition) – Subcooled or Saturated all
Liquid Flashes
Where a 100% liquid process fluid flashes when the relief valve opens,
the reduced Omega Method presented in API Standard 520 Part I canalso be used to predict the behavior of the new mixture of liquid andits vapor created by the pressure drop across the valve nozzle. A liquid
is called “subcooled” when it is at a temperature that is lower than its
saturation temperature for a particular pressure. To use this procedure,
no condensable vapor or non-condensable gas should be present in theliquid at the relief valve inlet. If these vapors or gases are in the mixture,use the two-phase flow procedure above. If the service fluid is saturated
water, use the ASME Section VIII Appendix 11 method below.
Step One
Calculate the Omega parameter.

((ρωs = 9 ρl1 −1
9Where:
ωs = Saturated Omega parameterρ9 = Density of the mixture at 90% of the saturation or vapor pressure(Ps) at the relieving temperature at the relief valve inlet. For
multi-component liquids this represents the bubble point at therelieving temperature at the relief valve inlet, kg/m3
ρl1 = Density of the liquid at the flowing pressure at the relief valve
inlet, kg/m3

Step Four
Determine the ratio of the saturation pressure to the set pressure.

ηs =

Ps
P1

Where:
Ps = Saturation or vapor pressure at the relieving temperature at therelief valve inlet, bara
P1 = Flowing pressure, i.e. set pressure + allowable overpressure +atmospheric – inlet piping losses, bara
From the calculation in step three, if the flash occurs upstream of the nozzle
bore diameter (low subcooling region) then move to step five.
From the calculation in step three, if the flash occurs at the nozzle bore
diameter (high subcooling region) skip to step ten.
Step Five (low subcooled liquid region)
Determine the critical pressure ratio (hc) of the service fluid from Figure6-3. Use the saturation pressure ratio (hs) from step four above and thesaturated Omega (ωs) value from step one above.

Step Two
The Omega parameter is now used to predict if the subcooled liquid
will flash upstream of the bore diameter (minimum diameter) of thenozzle or at the nozzle bore diameter. This behavior is determined by thevalue of what is called the transition saturation pressure ratio which is
calculated as follows.

ηst=

2ωs
1+ 2ωs

Where:
ηst = Transition saturation pressure ratio
ωs = Saturated Omega parameter from step one
StepThree
Determine where the flash of the subcooled liquid occurs as follows:
If Ps ≥ ηstP1 then the flash occurs upstream of the nozzle bore diameterof the PRV (also called the low subcooling region).
If Ps < ηstP1 then the flash occurs at the nozzle bore diameter of the PRV(also called the high subcooling region).
Where:
Ps = Saturation or vapor pressure at the relieving temperature at therelief valve inlet, bara
P1 = Flowing pressure, i.e. set pressure + allowable overpressure +atmospheric – inlet pressure piping losses, baraηst = Transition saturation pressure ratio from step two

6.20

Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric Units
Step Six (low subcooled liquid region)
Calculate the critical pressure (Pc) using the critical pressure ratio anddetermine whether the flow is critical or subcritical.

Pc = ηc P1

Where:
Pc = Critical pressure, baraηc = Critical pressure ratio from step five
P1 = Flowing pressure, i.e. set pressure + allowable overpressure +atmospheric – inlet piping losses, bara
If Pc ≥ P2 then flow is critical (go to step seven).
If Pc < P2 then flow is subcritical (skip step seven and go to step eight).
Where:
P2 = The total expected built-up and superimposed back pressure, bara
Step Seven (low subcooled liquid region in critical flow)
Calculate the required mass flux.

− ss s ln(( ηη −
s
c

(

s−1

s− c

P1 ρl1ηωs ηs − 1 +
c

(

G=

1.138

Where:
G	 = Required mass flux, kg/hr mm2
ηs = Saturated pressure ratio from step fourωs = Saturated Omega parameter from step one
ηc = Critical pressure ratio from step five
P1 = Flowing pressure, i.e. set pressure + allowable overpressure +atmospheric – inlet piping losses, bara
ρl1 = Density of the liquid at the set pressure at the relief valve inlet, kg/m3
Skip to step fourteen.
Step Eight (low subcooled liquid region in subcritical flow)
Calculate the subcritical pressure ratio.

η2 =Step Ten (high subcooled liquid region)
Determine if flow is critical or subcritical.
If Ps ≥ P2 then flow is critical (go to step eleven).
If Ps < P2 then flow is subcritical (skip step eleven and go to step twelve).
Where:
Ps = Saturation or vapor pressure at the relieving temperature at therelief valve inlet, bara
P2 = The total expected built-up and superimposed back pressure, bara
Step Eleven (high subcooled liquid region in critical flow)
Calculate the mass flux.

G 1.61 ρ l1 P1 − Ps
Where:
G	 = Required mass flux, kg/hr mm2
ρl1 = Density of the liquid at the set pressure at the relief valve inlet, kg/m3
P1 = Flowing pressure, i.e. set pressure + allowable overpressure +atmospheric – inlet piping losses, bara
Ps = Saturation or vapor pressure at the relieving temperature at therelief valve inlet, bara
Skip to step fourteen.
Step Twelve (high subcooled liquid region in subcritical flow)
Calculate the mass flux.

G 1.61 ρ l1 P1 − P2
Where:
G	 = Required mass flux, lb/hr-mm2
ρl1 = Density of the liquid at the flowing pressure at the relief valve
inlet, kg/m3
P1 = Flowing pressure, i.e. set pressure + allowable overpressure +atmospheric – inlet piping losses, bara
P2 = The total expected built-up and superimposed back pressure, bara

P2
P1

Where:
P2= The total expected built-up and superimposed back pressure,bara
P1= Flowing pressure, i.e. set pressure + allowable overpressure +atmospheric – inlet piping losses, bara
Step Nine (low subcooled liquid region in subcritical flow)
Calculate the mass flux.

s s ln(( ηη −
s
2

s−1

s− 2

P1 ρl1ηωs ηs − 1 +
2

(

− s

(

G=

1.138

Where:
G	 = Required mass flux, kg/hr-mm2
ηs = Saturated pressure ratio from step fourωs = Saturated Omega parameter from step one
η2 = Subcritical pressure ratio from step eight
P1 = Flowing pressure, i.e set pressure + allowable overpressure +atmospheric – inlet piping losses, bara
ρl1 = Density of the liquid at the set pressure at the relief valve inlet, kg/m3
Skip to step fourteen.

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Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric Units

Step Fourteen
Calculate the minimum required discharge area.

VL ρ l1
A =K Kv Kw G

Where:
A	= Minimum required discharge area, square millimeters
VL = Required relieving capacity, m3/hr at flowing temperatureρl1 = Density of the liquid at the flowing pressure at the relief valve
inlet, kg/m3
K	 = Coefficient of discharge for liquid service. See Chapter 8 Section IX
Kv = Capacity correction factor due to viscosity of the fluid at flowing
conditions. For most applications viscosity will not affect the area
calculation so Kv will be equal to 1.0. See Chapter 8 Section IV for
more information
Kw = Capacity correction factor for balanced relief valves due to backpressure. Use Kw equal to 1.0 for pilot operated and conventionalsafety relief valves. See Figure 8-11 for balanced direct acting
safety relief valvesG	 = Required mass flux from either steps 7, 9, 11, or 12, kg/hr-mm2
ASME Section VIII, Appendix 11 – Flashing of Saturated Water
When the process fluid at the pressure relief valve inlet is entirely
saturated water one can refer to ASME Section VIII Appendix 11 to
estimate the available mass flux for specific valve designs. Figure
6-4 is taken from Appendix 11 of the Code. The specific valve design
requirements in order to use Figure 6-4 are:•	The ratio of the nozzle bore diameter (smallest cross section) to PRV
inlet diameter must fall between 0.25 and 0.80.
•	The actual (not rated) nozzle coefficient for gas/ vapor service must
exceed 0.90.

200
180
160
Flow Capacity, kg/(hr/mm2)

Step Thirteen
Select the proper pressure relief valve type based upon the conditionsof the application. Since the liquid is flashing to give a certain amount of
two-phase flow through the pressure relief valve, Emerson recommends
that a safety relief valve (operates in a stable fashion on either
compressible or incompressible media) be selected. Since there will be
flashing, Emerson recommends a balanced type pressure relief valve
due to pressure variations that can occur in the valve body outlet.
See Table 6-8 for a summary of recommended valve designs for use in
two-phase applications.

140
120
100
80
60
40
20
0
0

50

100

150

200

250

Set Pressure, barg

Figure 6-4
ASME Section VIII Appendix 11
Available Mass Flux - Saturated Water

Step Three
Select the proper pressure relief valve type based upon the conditions of
the application and meet the design requirements required by the ASME
Code that are listed above. Emerson recommends the use of a balanced
type pressure relief valve due to pressure variations that can occur in thevalve body outlet.
The following Crosby and Anderson Greenwood balanced valves meetthe design requirements and may be considered:•	Balanced Direct Spring (JLT-JBS-E)• Modulating POPRV (Series 400/500/800) in 25F50, 40H75, 100P150,
150R200, 200T250 or any full bore (FB) orifice configuration
Go to Chapter 8 and review the ASME (do not use the API tables) actual
orifices for gas service listed in Tables 8-7, 8-8, 8-9, and Table 8-12 thatare available for the valve types listed above.
Therefore, to complete the example where we have a minimum orificearea requirement of 714.3 mm2 we can look at Table 8-7 for a JLTJBS-E configuration. This table will show a 50 mm inlet valve, with a “J”
orifice designation, will have 937.4 mm2 available. Provided the otherrequirements of the application meet this valve’s specifications, thisconfiguration would be an appropriate choice.

Step One
Determine the available mass flux for a pressure relief valve that meetsthe above design requirements at the required set pressure from Figure
6-4. The curve in Figure 6-4 is based upon 10% overpressure. Use thisavailable mass flux if sizing allows for higher overpressures as this will
be a conservative value.
An example would be that a saturated water installation is requiring a prv
to be set at 50 barg with a required capacity of 50,000 kg/hr of saturated
water. The ordinate axis shows the available mass flux at 50 barg to be
70 kg/hr/mm2.
Step Two
Divide the required saturated water capacity by the available mass flux
determined in step one to obtain the minimum required discharge areaof the valve.

A =

W
G

Where:
W= Required relieving capacity of saturated water, kg/hr
G	 = Available PRV mass flux from step one
So following with the example above, if the required saturated watercapacity is 50,000 kg/hr, the required discharge or orifice area of thevalve would be 50,000 (kg/hr) ÷ 70 (kg/hr-mm2) = 714.3 mm2.
6.22

Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric Units
VIII. Noise Level Calculations

70

The following formula is used for calculating noise level of gases, vapors
and steam as a result of the discharge of a pressure relief valve toatmosphere.

60

Where:
L30 = Sound level at 30 meters from the point of discharge in decibels
L	= Sound level from Figure 6-5 in decibels
P1 = Pressure at the valve inlet during flow, bara. This is the set
pressure (barg) + overpressure (barg) + atmospheric pressure(bara)
P2 = Pressure at the valve outlet during flow, psia [bara]. This is backpressure (barg) + atmospheric pressure (bara)k= Specific heat ratio. See Chapter 8 Section VII
M= Molecular weight of the gas or vapor. See Chapter 8 Section VII
T= Absolute temperature of the fluid at the valve inlet, degrees
Kelvin (°C + 273)
W
= Maximum relieving capacity, kg/hr
The noise level should be calculated using the maximum or total flow
through the pressure relief valve at the specified overpressure. Thisvalue can be calculated by using the sizing formulas on page 6.4 forweight flow and solving for “W”. Use the “actual” area and “actual”
coefficient of discharge for the specific valve from tables in Chapter 8
Section IX. The actual coefficient is the “rated coefficient” divided by 0.90.
When the noise level is required at a distance of other than 30 meters,
the following equation shall be used:(

L30 − 20log10 r
30

40
30
20
1.5

2

3

4

5

6

7

8

9

10

P1
P2

Figure 6-5
Sound Pressure Level at 30 meters from Point of Discharge

(

Lp

50
Decibels((

L30 L + 10log10 1.1552 WkT
M

Where:
Lp = Sound level at a distance, r, from the point of discharge indecibelsr= Distance from the point of discharge, meters
Table 6-9 lists some relative noise intensity levels

Table 6-9 – Noise Intensity (at 30 meters from the discharge)
Relative Noise Levels
130
Decibels
120
Decibels
110
Decibels
100
Decibels
90
Decibels
80
Decibels
70
Decibels
60
Decibels
50
Decibels

Jet Aircraft on Takeoff
Threshold of Feeling
Elevated Train
Loud Highway
Loud Truck
Plant Site
Vacuum cleaner
Conversation
Offices

6.23

Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric Units
IX. Reaction Forces
The discharge from a pressure relief valve exerts a reaction force on thevalve, vessel and/or piping as a result of the flowing fluid. Determination ofoutlet reaction forces and the design of an appropriate support system isthe responsibility of the designer of the vessel and/or piping. The following
is published as technical advice or assistance.
Reaction Force for Open Discharge – Gas Service
The following formulas are used for the calculation of reaction forces fora pressure relief valve discharging gas or vapor directly to atmosphere.
It is assumed that critical flow of the gas or vapor is obtained at thedischarge outlet. Under conditions of subcritical flow the reaction
forces will be less than that calculated. The equations are based on API
Recommended Practice 520 Part 2.

F = 0.03583

kTi+ 0.10Ao P2− PA
k +1 M

Where:
F	= Reaction force at the point of discharge to atmosphere, N. See
Figure 6-6
Ao = Area at discharge, square millimetersk= Specific heat ratio at the outlet conditions
M= Molecular weight of the gas or vapor obtained from standard
tables or see Chapter 8 Section II
P2 = Static pressure at discharge, bara calculated below:

P2= 0.25329
PA
Ti

W
Ao

To
kM

= Ambient pressure, bara
= Absolute temperature of the fluid at the valve inlet, degrees
Kelvin [°C + 273)
To = Absolute temperature of the fluid at the discharge, degrees
Kelvin [°C + 273]
W
= Actual relieving capacity, kg/hr. This value may be calculated
by using the sizing formula on page 6.4 for weight flow. Use the
ASME actual area and the rated coefficient divided by 0.9 to get
the actual capacity of the valve.
The above equations account for static thrust force only and do notconsider a force multiplier required for rapid application of the reactive
thrust force. ASME B31.1 Non-Mandatory Appendix II includes a methodof analysis for dynamic load factors. Force multipliers up to 2 times F are
possible. This is only necessary for open discharge with rapid opening
valves (i.e. ASME Section I safety valves).

6.24

Pressure Relief Valve Engineering Handbook
Chapter 6 – Valve Sizing and Selection – Metric Units
Reaction Force for Open Discharge – Steam Service
The following formula is used for the calculation of reaction forces
for a pressure relief valve discharging steam directly to atmosphere.
The equations are based on equations in ASME B31.1 Non-mandatory
Appendix II.

F = 0.0068453 W ho − 823 + 0.10Ao P2 − PA
Where:
F	= Reaction force at the point of discharge to atmosphere, N
ho = Stagnation enthalpy at the valve inlet, kJ/kg
Ao = Area at discharge, square millimeters
P2 = Static pressure at discharge, bara
PA = Ambient pressure, baraW
= Actual relieving capacity, kg/hr. This value may be calculated byusing the sizing formula on page 6.6. Use the ASME actual area
and the rated coefficient divided by 0.9 to get the actual capacityof the valve.
The above equations account for static thrust force only and do notconsider a force multiplier required for rapid application of the reactive
thrust force. ASME B31.1
Non-Mandatory Appendix II includes a method of analysis for dynamic
load factors. Force multipliers up to 2 times F are possible. This is
only necessary for open discharge with rapid opening valves (i.e. ASME
Section I safety valves).

F

Figure 6-6Open Discharge Reaction Force

Reaction Force for Open Discharge – Liquid Service
The following formula is used for the calculation of reaction forces fora pressure relief valve discharging liquid directly to atmosphere. The
equations are based on fluid momentum. Liquid flow is assumed to be
non-flashing.

F=

0.07689 W 2
ρAo

Where:
F	= Reaction force at the point of discharge to atmosphere, N
Ao = Area at discharge, square millimetersW= Actual relieving capacity, kg/hr. This value may be calculated byusing the sizing formula on page 6.11. Use the ASME actual area
and the rated coefficient divided by 0.9 to get the actual capacityof the valve.
ρ= Density of the fluid, kg/m3
Reaction Force for Open Discharge – Two-Phase Flow
The following formula is found in API 520 Part 2. This formula assumes
the two-phase flow is in a homogeneous condition (well mixed and bothphases flowing at the same velocity).

F=

W2
12.96 A o[ ρχ + 1ρ− χ ] +10A P − P
g

o

1

E

A

W
Ao
Χ

= Actual relieving capacity, kg/hr= Area at discharge outlet to atmosphere, square millimeters= Mass fraction of gas/vapor portion
WG

WG
ρg
ρl
PE
PA

= Actual relieving capacity of gas, kg/hr= Vapor density at exit conditions, kg/m3= Liquid density at exit conditions, kg/m3= Pressure at pipe exit, bara
= Ambient pressure, bara

W

6.25

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
The following data with charts and tables are included in this chapter
I. Compressibility Factor, Z���������������������������������������������������������������������������������������������������������������7.3
II. Capacity Correction Factor for Back Pressure, Kb����������������������������������������������������������������������7.4
III. Capacity Correction Factor for High Pressure Steam, Kn���������������������������������������������������������7.31
IV. Capacity Correction Factor for Viscosity, Kv�������������������������������������������������������������������������������7.31
V. Capacity Correction Factor for Superheat, Ksh��������������������������������������������������������������������������7.33
VI.	Ratio of Specific Heats, k, and Coefficient, C������������������������������������������������������������������������������7.35
VII. Typical Fluid Properties����������������������������������������������������������������������������������������������������������������7.36
VIII.	Steam Saturation Pressure/Temperature����������������������������������������������������������������������������������7.40
IX.	Orifice Area and Coefficient of Discharge forAnderson Greenwood and Crosby Pressure Relief Valve����������������������������������������������������������7.41
X.	Equivalents and Conversion Factors�������������������������������������������������������������������������������������������7.48
XI. Capacity Correction Factor for Rupture Disc/Pressure Relief Device Combination, Kc���������7.54

7.1

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
The following Figures are included in this chapter
Figure 7-1	Nelson Obert Compressibility Chart������������������������������������������������������������������������������������������������������������������������������������������������������������������7.3
Figure 7-2	Effect of Built-up Back Pressure on Conventional PRV�����������������������������������������������������������������������������������������������������������������������������������7.4
Figure 7-3	Relationship of P2 and P2’�����������������������������������������������������������������������������������������������������������������������������������������������������������������������������������7.6
Figure 7-4 Valve Selection Recommendations for Built-up Back Pressure Installations������������������������������������������������������������������������������������������������7.7
Figure 7-5	Valve Selection Recommendations for Constant Superimposed Back Pressure Installations (no built-up back pressure)����������������������7.8
Figure 7-6 Valve Selection Recommendations for Variable Superimposed Back Pressure Installations����������������������������������������������������������������������7.9
Figure 7-7	Valve Selection Recommendations for Constant Superimposed Back Pressure Installations (with built-up back pressure)�����������������7.10
Figure 7-8 Crosby Series JOS-E Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) Gas/Steam Service����������������������������������7.11
Figure 7-9 Crosby Series JLT-JOS-E Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) Gas/Steam Service �������������������������7.12
Figure 7-10	Crosby Series JBS-E/JLT-JBS-E Balanced Bellows Direct Spring PRV Back Pressure Correction Factor (Kb) Gas/Steam Service ����7.13
Figure 7-11 Crosby Series JLT-JBS-E Balanced Bellows Direct Spring PRV Back Pressure Correction Factor (Kw) Liquid Service ������������������������7.14
Figure 7-12 Crosby Series 800 Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) Gas/Steam Service ��������������������������������������7.15
Figure 7-13 Crosby Series 900 Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) Gas/Steam Service ��������������������������������������7.16
Figure 7-14 Crosby Series BP Back Pressure Correction Factor (Kw) Liquid Service ����������������������������������������������������������������������������������������������������7.17
Figure 7-15 Crosby Series BP Back Pressure Correction Factor (Kb) Gas Service ���������������������������������������������������������������������������������������������������������7.18
Figure 7-16	Anderson Greenwood Series 61 Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) Gas Service ���������������������������7.19
Figure 7-17	Anderson Greenwood Series 63B (-5 Orifice Only) Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) Gas������������7.20
Figure 7-18	Anderson Greenwood Series 63B (-7 Orifice Only) Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) Gas������������7.21
Figure 7-19	Anderson Greenwood Series 81P Back Pressure Correction Factor (Kw) Liquid Service ��������������������������������������������������������������������������7.22
Figure 7-20	Anderson Greenwood Series 81/83/86 Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) Gas/Steam ������������������7.23
Figure 7-21	Anderson Greenwood Series 81P (-8 Orifice Only) Balanced Piston Direct Spring PRV Back Pressure Correction Factor (Kb) Gas������7.24
Figure 7-22	Anderson Greenwood Series 90/9000 POPRV Back Pressure Correction Factor (Kb) Gas Service ���������������������������������������������������������7.25
Figure 7-23	Anderson Greenwood Series 40 Pilot Operated PRV Back Pressure Correction Factor (Kb) Gas/Steam Service ������������������������������������7.26
Figure 7-24	Anderson Greenwood Series 50 Pilot Operated PRV Back Pressure Correction Factor (Kb) Gas/Steam Service ������������������������������������7.27
Figure 7-25	Anderson Greenwood Series 60 Pilot Operated PRV Back Pressure Correction Factor (Kb) Gas/Steam Service ������������������������������������7.28
Figure 7-26	Anderson Greenwood Series LCP Pilot Operated PRV Back Pressure Correction Factor (Kb) Gas Service ���������������������������������������������7.29
Figure 7-27	Anderson Greenwood Series 727 Pilot Operated PRV Back Pressure Correction Factor (Kb) Gas/Steam Service ����������������������������������7.30
Figure 7-28 Correction Factor for High Pressure Steam (Kn)��������������������������������������������������������������������������������������������������������������������������������������������7.31
Figure 7-29 Viscosity Correction Factor (Kv)������������������������������������������������������������������������������������������������������������������������������������������������������������������������7.32
Figure 7-30	Ratio of Specific Heats, k, and Coefficient, C���������������������������������������������������������������������������������������������������������������������������������������������������7.35
The following Tables are included in this chapter
Table 7-1	Superheat Correction Factor����������������������������������������������������������������������������������������������������������������������������������������������������������������������������7.33
Table 7-2	Gas Constant������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������7.35
Table 7-3
Physical Properties for Selected Gases�����������������������������������������������������������������������������������������������������������������������������������������������������������7.36
Table 7-4
Physical Properties for Selected Liquids���������������������������������������������������������������������������������������������������������������������������������������������������������7.38
Table 7-5	Saturated Steam Pressure Table���������������������������������������������������������������������������������������������������������������������������������������������������������������������7.40
Table 7-6
JOS-E/JBS-E/JLT-E Full Nozzle Direct Acting Spring Valves (API Effective Orifice Areas/API Effective Coefficient of Discharge)���������7.42
Table 7-7
JOS-E/JBS-E/JLT-E Full Nozzle Direct Acting Spring Valves (ASME Areas/ASME Coefficient of Discharge)�������������������������������������������7.42
Table 7-8	OMNI 800/900/BP Portable Direct Acting Spring Valves Orifice Areas/Coefficient of Discharge����������������������������������������������������������������7.43
Table 7-9	Series 60 and Series 80 Portable Direct Acting Spring Valves Orifice Areas/API Effective Coefficient of Discharge��������������������������������7.43
Table 7-10 H Series Direct Acting Spring Safety Valves Orifice Areas/Coefficient of Discharge������������������������������������������������������������������������������������7.44
Table 7-11 High Pressure Pilot Operated Valves (API Effective Orifice Areas/Coefficient of Discharge)����������������������������������������������������������������������7.44
Table 7-12 High Pressure Pilot Operated Valves (ASME Areas/ASME Coefficient of Discharge)����������������������������������������������������������������������������������7.45
Table 7-13	Low Pressure Pilot Operated Valves (Set Pressure ≥15 psig) Orifice Areas/Coefficient of Discharge�������������������������������������������������������7.46
Table 7-14	Low Pressure Pilot Operated Valves (Set Pressure <15 psig) Orifice Areas/Coefficient of Discharge�������������������������������������������������������7.46
Table 7-15	Low Pressure Pilot Operated Valves Orifice Areas/Coefficient of Discharge�����������������������������������������������������������������������������������������������7.47
Table 7-16 JB-TD Direct Acting Spring Valves Orifice Areas/Coefficient of Discharge��������������������������������������������������������������������������������������������������7.47
Table 7-17	Equivalents and Conversion Factors����������������������������������������������������������������������������������������������������������������������������������������������������������������7.48
Table 7-18 Pressure Conversions���������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������7.50
Table 7-19	Gas Flow Conversions���������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������7.51
Table 7-20	Liquid Flow Conversions�����������������������������������������������������������������������������������������������������������������������������������������������������������������������������������7.52
Table 7-21 Viscosity Conversion������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������7.53
Table 7-22 Capacity Correction Factor for Rupture Disc/PRV Combination��������������������������������������������������������������������������������������������������������������������7.55

7.2

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
I. Compressibility Factor, Z
The gas and vapor formulas of this handbook are based on perfect
T gas
Tr a perfectlaws. Many real gases and vapors, however, deviate fromgas.
Tc
The compressibility factor Z is used to compensate for the deviationsofreal gases from the ideal gas.
The compressibility factor may be determined from thermodynamic chartssuch as the Nelson Obert compressibility chart shown in Figure 7-1.
Z is a function of the reduced pressure and the reduced temperatureof the gas. The reduced temperature is equal to the ratio of the actual
absolute inlet gas temperature to the absolute critical temperature of
the gas.

Tr

T
Tc

Pr

P
Pc

Where:
Tr =	Reduced temperature
T = 	Inlet fluid temperature, °F + 460
Tc = Critical temperature, °F + 460
The reduced pressure is equal to the ratio of the actual absolute inlet
pressure to the critical pressure of the gas.

Pr

P
Pc

Where:
Pr =	Reduced pressure
P =	Relieving pressure(set pressure + overpressure + atmospheric pressure), psia
Pc = Critical pressure, psia
Enter the chart at the value of reduced pressure, move verticallyto the appropriate line of constant reduced temperature.
From this point, move horizontally to the left to read the value of Z.
In the event the compressibility factor for a gas or vapor cannot be
determined, a conservative value of Z = 1 is commonly used.

Figure 7-1 - Nelson Obert Compressibility Chart
1.0
1.0
0.9
0.9
0.8
0.8

Compressibility Factor Z

0.7
0.7
0.6

0.5

0.4

0.3

0.2

0.1
0

0.5

1.0

1.5

2.0

2.5

3.0

Reduced Pressure Pr
7.3

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)

General
Back pressure can exist in any location that is downstream from theactual discharge area of a pressure relief valve. This pressure canbe due to piping causing resistance to flow, pressures from otherequipment discharging to a common header system, or from a flashingfluid being relieved. Without proper consideration of the effects of backpressure, the PRV may experience one, some or all of the following.
• Change in set pressure• Change in reseating pressure• Lift instability• Decrease in actual relieving capacity
This particular section of the engineering handbook will deal with thesizing capacity correction factors that need to be considered for various
types of pressure relief valves.
Built-up Back Pressure
As you recall from Chapter Three, a pressure relief valve whose outlet
is discharging to atmosphere or into a piping system will experiencebuilt-up back pressure. This type of back pressure is only evident after
the valve has opened and is relieving, it does not affect the set pressureof the PRV.
For a conventional PRV, the change in the force balance of the discholder due to back pressure will hinder the upward lifting force. The
conservative rule of thumb is that if the built-up back pressure exceeds
the available overpressure to lift the valve, then a conventional valve
should not be used because the lifting force may not be sufficient for
the valve to operate in a stable fashion. Figure 7-2 illustrates the effectbuilt-up back pressure has upon a conventional PRV design where thereis 10% overpressure available. If there was a fire case contingency where
there may be 21% overpressure, then a curve similar to Figure 7-2 would
show full capacity up to 21% built-up back pressure.
An exception to this conventional valve built-up back pressure andoverpressure relationship is the Crosby brand H series product that isnormally provided for ASME Section I applications. The H series valve isnormally provided with the open spring bonnet design. This opening toatmosphere dramatically decreases the built-up back pressure amountthat acts down on the disc holder. For this valve design, when the H
series valve is in lift with 3% overpressure, the calculated built-up backpressure at the outlet flange of the valve can be up to a maximum of
27.5% of the set pressure.
There is no capacity correction factor in either gas/vapor or liquid
applications for a suitable conventional prv where the valve is exposed tobuilt-up back pressure which is less than the available overpressure. In
other words, the Kb or Kw will be 1.0.
When a balanced direct spring or pilot operated PRV is open and flowing
against a built-up back pressure, the lift of the device should be stable if
properly designed. The built-up back pressure can exceed the available
overpressure for these devices. However, the capacity that the PRV is
able to deliver may be less than expected due to a reduced, but stable,
lift and/or a compressible fluid flow transitions from critical to subcriticalconditions.
The calculation of the magnitude of the built-up back pressure, andthe subsequent design of the outlet piping for a new installation, isoftentimes an iterative process.
• The PRV is initially sized with the assumption of a maximum built-up
back pressure. For instance, in an application that may require theprocess fluid exhaust to be routed via a simple tail pipe discharge
to atmosphere, the sizing for the PRV may assume a built-up backpressure to be 10% of the flowing pressure. This assumption would
allow the use of a conventional direct spring PRV.

• The PRV required minimum orifice is then selected based upon a
Kb = 1.0.
• Once the PRV is selected, the engineer should perform a pressure
drop calculation for the proposed size and style of discharge pipe. In
the example above, the pressure drop through the tailpipe should bedetermined.
• The API Standard 521 will guide the engineer to use the rated capacity
for some types of direct spring operated PRV (recall that safety valvesobtain substantial lift at the set pressure) or the required capacity for a
modulating action pilot operated PRV to calculate the pressure loss inthe discharge piping. This will provide the magnitude of built-up backpressure at the outlet flange of the PRV.
• If this calculated built-up back pressure exceeds 10% then, for thisexample, the tailpipe may need to be redesigned to provide lessresistance to flow. Perhaps enlarging or straightening this fitting ispossible to reduce the built-up back pressure.
• If the outlet piping cannot be changed, then a balanced or pilot
operated PRV may need to be considered and the iterative process
begins again. We will discuss the correction factors for balanced andpilot operated PRVs below.

Figure 7-2
Effect of Built-up Back Pressure on Conventional PRV
100

80
10% Overpressure% Rated Capacity

II. Capacity Correction Factors for Back Pressure, Kb

60

40

20

0
0

10

20

30

40

50

% Built-up Back Pressure to Set Pressure
Per API RP520 and ASME Section VIII BPBU ≤ 10%

7.4

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Superimposed Back Pressure
When the outlet of a PRV is connected to a closed discharge system,
it may not only be exposed to built-up back pressure but may also see
superimposed back pressure. The superimposed back pressure isevident on the downstream side of the PRV before the valve has opened.
This is very common in process plant environments where effluentsare captured or thermally oxidized via common header systems. This
superimposed back pressure may be a constant value but it could vary inthese types of installations.
A conventional, unbalanced PRV can be considered if the superimposedback pressure is a constant value. As we learned in Chapter Three, one
can set the conventional PRV with a bias on the test bench to accountfor this constant superimposed back pressure. All unbalanced Crosby
and Anderson Greenwood brand PRVs have a force balance that willcause a unit-for-unit increase in the in situ opening pressure whensuperimposed back pressure is present. In other words, if there is
50 psig of superimposed back pressure the unbalanced valve will open
50 psig higher than the opening pressure allowed by just the spring
compression. For this example, the spring compression can be set
50 psig lower to compensate for the constant superimposed backpressure. As you recall, this bias is one element of the cold differential
set pressure (CDTP) setting.
A balanced direct acting or pilot operated PRV does not need any test
bench correction for superimposed back pressure. Therefore, when the
superimposed back pressure is variable it is recommended to use theseparticular valve designs.
The calculation of superimposed back pressure is performed byexamining the entire pressure relief disposal system and making
determinations regarding whether or not other devices attached to thesystem may be operating at the time the PRV is to open and then relieve.
These effluent flows are then used with the disposal system piping
geometry to determine what the superimposed back pressure may be at
the outlet flange of the PRV. The maximum superimposed back pressure
should be listed on the PRV data sheet.

The transition from critical to subcritical flow is based upon the criticalpressure of the gas service. This critical pressure is calculated asfollows:

Pcritical

P1

[ ]
2
K+1

k
k-1

Where:
P1 = 	Pset + Overpressure + atmospheric - inlet pressure piping loss,psia
k = ratio of specific heat
If the sum of the built-up back pressure and superimposed backpressure exceed this critical pressure then the capacity will be reduced.
As an example, let us consider the gas as air with a ratio of specificheats equal to 1.4. Let us assume that the absolute relieving pressure(P1) is 100 psia. After performing the calculation above, the criticalpressure will be equal to 52.8 psia. This means that capacity will be
reduced when the total back pressure at the outlet of the discharge
area is greater than 52.8 psia.

Compressible Fluid Back Pressure Correction Charts
There are several figures in this chapter that show back pressure
correction factors for various series of Emerson products used incompressible media service. For example, Figure 7-8 shows the Kb
factor for the Crosby JOS-E conventional PRV and we will use thischart to help explain why these back pressure capacity corrections areneeded.
Properly setting a conventional PRV, such as the Crosby JOS-E, with a
CDTP will provide an adequate lift to meet its certified capacity. This iscontingent upon any built-up back pressure that is developed will not
exceed the available overpressure at the set pressure. In gas service,
there may be a capacity correction factor required for conventional PRVs.
The Kb factor in this case is a result of the flow becoming what is calledsubcritical at the discharge area of the PRV.
When the flow is critical at the discharge area of the PRV it can also
be called “choked flow.” This means that even if the back pressure is
reduced there can be no more flow capacity provided through the PRV.
Once the flow becomes subcritical then any change in back pressure willchange the capacity.

7.5

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
As mentioned above, the calculation of the superimposed and built-up
back pressure gives a value for these pressures at the PRV outlet flange.
The capacity of the PRV is determined by the conditions at the location of
the actual discharge area. For the Crosby JOS-E series valve this is thenozzle outlet. If you look at Figure 7-3, the total calculated superimposedand built-up back pressure is denoted by P2 while it is the P2’ pressure
at the nozzle outlet that determines whether the flow is critical or
subcritical. The outlet of the body of the JOS-E creates additional builtup back pressure that is not accounted for in the total (built-up plus
superimposed) back pressure calculations at the outlet flange, making
the value of P2’ higher than P2.
Therefore, using Figure 7-8 and our previous example where the criticalpressure is 52.8 psia, you will note in the figure that when the calculated
total back pressure is approximately 20% of the flowing pressure we
begin to adjust the capacity with the Kb value. This is well below the
expected 0.528 critical pressure ratio or 52.8 psia critical pressure. Thisis due to the P2’ and P2 relationship. The P2’ is actually above the criticalpressure when the calculated total back pressure at the outlet flange(P2) is reaching 20% of the flowing pressure.
This same P2’ and P2 relationship holds for other valve designs such as
the Crosby balanced bellows and most of the Anderson Greenwood pilot
operated PRVs. This relationship is also a contributor to the liquid Kw
correction factors for various valve designs.
Use the following flow charts (Figures 7-4 through 7-7) to assist withselecting an appropriate Emerson model recommendation and backpressure capacity correction factor.

Figure 7-3
Relationship of P2’ and P2’

Total back pressure at nozzle outlet (P2’)

Calculated superimposed and built-up
back pressure (P2)

7.6

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Figure 7-4
Valve Selection Recommendations for Built-up Back Pressure Installations (No Superimposed Back Pressure)

Built-up Back Pressure Only

POPRV

JLT-JBS-E
Fig. 7-11

81P
Fig. 7-19

BP
Fig. 7-14

Available
overpressure morethan built-up backpressure?

No

PRV
DesignJOS-E, JOS-JLT-E 800, 900,
61/63B 81/83/86 Kb = 1.0

Balanced DSV
Gas/Steam

Yes
LiquidBalanced
DSV

ServiceBP
Fig. 7-15

JBS-E,JLT-JBS-E
JB-TD
Fig. 7-10

Conventional
DSV

ServiceLiquid81P
Fig. 7-21
Gas/Steam

POPRV

PRV
DesignPOPRVService

900, 81P, JOS-JLT-E
Kw = 1.0

LiquidKw = 1.0

Gas/SteamFig. 7-22, 7-23, 7-24, 7-25, 7-26, 7-27

7.7

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Figure 7-5
Valve Selection Recommendations for Constant Superimposed Back Pressure Installations (No Built-up Back Pressure)

Constant Superimposed Only

Conventional
DSV

Set w/CDTP

Balanced
DSV

PRV
DesignServiceLiquidGas/Steam
Gas/Steam

JOS-E
Fig. 7-8

ServiceLiquidPOPRV

JLT-JOS-E, 900
Kw = 1.0

800
Fig. 7-12

81P
Fig. 7-21

JBS-E, JLT-JBS-E,
JB-TD
Fig. 7-10

BP
Fig. 7-15

BP
Fig. 7-14

61/63B
Fig. 7-16,17,18

81/83/86
Fig. 7-20

JLT-JOS-E
Fig. 7-9

900
Fig. 7-13
ServiceLiquid81P
Fig. 7-19

JLT-JBS-E
Fig. 7-11

Kw = 1.0

Gas/SteamFig. 7-22, 7-23, 7-24, 7-25, 7-26

7.8

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Figure 7-6
Valve Selection Recommendations for Variable Superimposed Back Pressure Installations

Built-up with Variable Superimposed

Variable Superimposed Only

Variable and Constant Superimposed

Built-up with Constantand Variable Superimposed

Gas/Steam

Service

Balanced
DSV

PRV
DesignPOPRV

Liquid81P
Fig. 7-21

BP
Fig. 7-15

ServiceLiquidKw = 1.0

Gas/SteamFig. 7-22, 7-23, 7-24, 7-25, 7-26

JBS-E, JLT-JBS-E,
JB-TD
Fig. 7-10

BP
Fig. 7-14

81P
Fig. 7-19

JLT-JBS-E
Fig. 7-11

7.9

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Figure 7-7
Valve Selection Recommendations for Constant Superimposed Back Pressure Installations with Built-up Back Pressure

Built-up with Constant Superimposed

61/63B
Fig. 7-16,
17,18

POPRV

JLT-JBS-E
Fig. 7-11

BP
Fig. 7-14

PRV
Design81P
Fig. 7-19

No

Available
overpressure
more thanbuilt-up backpressure?

JOS-E
Fig. 7-8

POPRV

ServicePOPRV

JBS-E,JLT-JBS-E,
JB-TD
Fig. 7-10

JLT-JOS-E
Fig. 7-9

800
Fig. 7-12

Gas/Steam

LiquidBP
Fig. 7-15

900
Fig. 7-13

Balanced DSV
Yes

81P
Fig. 7-21

81/83/86
Fig. 7-20

Balanced
DSV

PRV
DesignConv.
DSV

Set w/
CDTP

LiquidPOPRV

JLT-JOS-E, 900
Kw = 1.0

Gas/Steam
Service

ServiceLiquidKw = 1.0

Gas/SteamFig. 7-22, 7-23, 7-24, 7-25, 7-26

7.10

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Figure 7-8
Crosby Series JOS-E Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) - Gas/Steam Service
1.00
k = 1.0

k = Cp/Cv

k = 1.2
0.90

k = 1.4

0.80

k = 1.6
k = 1.8
k = 2.0

0.70

0.60

Kb

0.50

0.40

0.30

0.20

735
Kb =
C

[

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = 0.561* (P2/P1) + 0.443

0.00
0.00

0.10

0.20

0.30

0.40

0.50
P2
P1

0.60

0.70

0.80

0.90

1.00

Where:
P2 = Pressure at valve outlet during flow, psia. This is total back pressure (psig) + atmospheric pressure (psia)
P1 = Relieving pressure, psia. This is the set pressure (psig) + overpressure (psig) + atmospheric pressure (psia) - inlet pressure piping loss (psig)
7.11

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Figure 7-9
Crosby Series JLT-JOS-E Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) - Gas Service
1.00
k = 1.0
k = Cp/Cv

k = 1.2

0.90

k = 1.4

k = 1.6

0.80

k = 1.8
k = 2.0
0.70

0.60

Kb

0.50

0.40

0.30

0.20

[

735
Kb =
C

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = 0.617* (P2/P1) + 0.389

0.00
0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

P2
P1
Where:
P2 = Pressure at valve outlet during flow, psia. This is total back pressure (psig) + atmospheric pressure (psia)
P1 = Relieving pressure, psia. This is the set pressure (psig) + overpressure (psig) + atmospheric pressure (psia) - inlet pressure piping loss (psig)
7.12

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Figure 7-10
Crosby Series JBS-E/JLT-JBS-E/JB-TD Balanced Bellows Direct Spring PRV Back Pressure Correction Factor (Kb) - Gas/Steam Service
1.00

Pset ≥ 50 psig
All Orifices

0.90

0.80

Kb

Pset < 50 psig J thru BB2
Orifices

0.70

Pset < 50 psig D thru H
Orifices

0.60

0.50

0.40
0.00

0.10

0.20

0.30

0.40

0.50

0.60

P2
Pset
Where:
P2 = P ressure at valve outlet during flow, psig. This is the total back pressure (psig).
Pset = Set pressure (psig)

Note
This figure is based upon 10% overpressure.
The Kb factor shown will be conservative for higher overpressure values.

7.13

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Figure 7-11
Crosby Series JLT-JBS-E Balanced Bellows Direct Spring PRV Back Pressure Correction Factor (Kw) - Liquid Service
1.00

0.90

Kw

0.80

0.70

0.60

0.50
0.00

0.10

0.20

0.30
P2
Pset

Where:
P2 = Pressure at valve outlet during flow, psig. This is the total back pressure (psig).
Pset = Set pressure (psig)

0.40

0.50

0.60

Note
This figure is based upon 10% overpressure.
The Kw factor shown will be conservative for higher overpressure values.

7.14

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Figure 7-12
Crosby Series 800 Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) - Gas/Steam Service
1.00
k = 1.0
k = Cp/Cv

k = 1.2

0.90

k = 1.4

k = 1.6

0.80

k = 1.8
k = 2.0
0.70

Kb

0.60

0.50

0.40

0.30

0.20

735
Kb =
C

[

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = 0.639* (P2/P1) + 0.365

0.00
0.00

0.10

0.20

0.30

0.40

0.50
P2
P1

0.60

0.70

0.80

0.90

1.00

Where:
P2 = Pressure at valve outlet during flow, psia. This is total back pressure (psig) + atmospheric pressure (psia)
P1 = Relieving pressure, psia. This is the set pressure (psig) + overpressure (psig) + atmospheric pressure (psia) - inlet pressure piping loss (psig)

7.15

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Figure 7-13
Crosby Series 900 Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) - Gas/Steam Service
1.00
k = 1.0

k = Cp/Cv

k = 1.2

0.90

k = 1.4

k = 1.6

0.80

k = 1.8
k = 2.0
0.70

Kb

0.60

0.50

0.40

0.30

0.20

735
Kb =
C

[

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = 0.736* (P2/P1) + 0.266
0.00
0.00

0.10

0.20

0.30

0.40

0.50
P2
P1

0.60

0.70

0.80

0.90

1.00

Where:
P2 = Pressure at valve outlet during flow, psia. This is total back pressure (psig) + atmospheric pressure (psia)
P1 = Relieving pressure, psia. This is the set pressure (psig) + overpressure (psig) + atmospheric pressure (psia) - inlet pressure piping loss (psig)

7.16

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Figure 7-14
Crosby Series BP Back Pressure Correction Factor (Kw) - Liquid Service
1.00

Kw

0.90

0.80

0.70
0.00

0.10

0.20

0.30
P2
Pset

0.40

Where:
P2 = Pressure at valve outlet during flow, psig. This is the total back pressure (psig)
Pset = Set pressure (psig)

0.50

0.60

0.70

Note
This figure is based upon 10% overpressure.
The Kw factor shown will be conservative for higher overpressure values.

7.17

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Figure 7-15
Crosby Series BP Back Pressure Correction Factor (Kb) - Gas Service
1.00

0.90

Kb

0.80

0.70

0.60

0.50

0.40
0.00

0.10

0.20

P2
Pset

Where:
P2 = Pressure at valve outlet during flow, psig. This is the total back pressure (psig)
Pset = Set pressure (psig)

0.30

0.40

0.50

Note
This figure is based upon 10% overpressure.
The Kb factor shown will be conservative for higher overpressure values.

7.18

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Figure 7-16
Anderson Greenwood Series 61 Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) - Gas Service
1.00
k = 1.0
k = Cp/Cv

k = 1.2
k = 1.4

0.90

k = 1.6

0.80

k = 1.8
k = 2.0
0.70

Kb

0.60

0.50

0.40

0.30

0.20

735
Kb =
C

[

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = 0.727* (P2/P1) + 0.2674
0.00
0.00

0.10

0.20

0.30

0.40

0.50
P2
P1

0.60

0.70

0.80

0.90

1.00

Where:
P2 = Pressure at valve outlet during flow, psia. This is total back pressure (psig) + atmospheric pressure (psia)
P1 = Relieving pressure, psia. This is the set pressure (psig) + overpressure (psig) + atmospheric pressure (psia) - inlet pressure piping loss (psig)

7.19

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Figure 7-17
Anderson Greenwood Series 63B (-5 Orifice Only) Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) - Gas Service
1.00
k = 1.0
k = Cp/Cv

k = 1.2

0.90

k = 1.4

k = 1.6

0.80

k = 1.8
k = 2.0
0.70

Kb

0.60

0.50

0.40

0.30

0.20

[

735
Kb =
C

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = 0.723* (P2/P1) + 0.2793
0.00
0.00

0.10

0.20

0.30

0.40

0.50
P2
P1

0.60

0.70

0.80

0.90

1.00

Where:
P2 = Pressure at valve outlet during flow, psia. This is total back pressure (psig) + atmospheric pressure (psia)
P1 = Relieving pressure, psia. This is the set pressure (psig) + overpressure (psig) + atmospheric pressure (psia) - inlet pressure piping loss (psig)

7.20

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Figure 7-18
Anderson Greenwood Series 63B (-7 Orifice Only) Conventional Direct Acting PRV Back Pressure Correction Factor (Kb) - Gas Service
1.00
k = 1.0
k = Cp/Cv

k = 1.2

0.90

k = 1.4

k = 1.6

0.80

k = 1.8
k = 2.0
0.70

Kb

0.60

0.50

0.40

0.30

0.20

735
Kb =
C

[

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = 0.6381* (P2/P1) + 0.3668

0.00
0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

P2
P1

Where:
P2 = Pressure at valve outlet during flow, psia. This is total back pressure (psig) + atmospheric pressure (psia)
P1 = Relieving pressure, psia. This is the set pressure (psig) + overpressure (psig) + atmospheric pressure (psia) - inlet pressure piping loss (psig)

7.21

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Figure 7-19
Anderson Greenwood Series 81P Back Pressure Correction Factor (Kw) - Liquid Service
1.00

0.90

0.80

Numbered Orifices

0.70

Kw

0.60

0.50
Lettered Orifices

0.40

0.30

0.20

0.10

0.00
0.00

0.10

0.20

0.30
P2
Pset

Where:
P2 = Pressure at valve outlet during flow, psig. This is the total back pressure (psig)
Pset = Set pressure (psig)

0.40

0.50

0.60

Note
This figure is based upon 10% overpressure.
The Kw factor shown will be conservative for higher overpressure values.

7.22

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Figure 7-20
Anderson Greenwood Series 81/83/86 Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) - Gas/Steam Service
1.00
k = 1.0
k = Cp/Cv

k = 1.2

0.90

k = 1.4

k = 1.6

0.80

k = 1.8
k = 2.0
0.70

Kb

0.60

0.50

0.40

0.30

0.20

735
Kb =
C

0.10

[

2

k+1

k
k
k
(PR’) - (PR’ )k- 1]

PR’ = 0.735* (P2/P1) + 0.259

0.00
0.00

0.10

0.20

0.30

0.40

0.50
P2
P1

0.60

0.70

0.80

0.90

1.00

Where:
P2 = Pressure at valve outlet during flow, psia. This is total back pressure (psig) + atmospheric pressure (psia)
P1 = Relieving pressure, psia. This is the set pressure (psig) + overpressure (psig) + atmospheric pressure (psia) - inlet pressure piping loss (psig)

7.23

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Figure 7-21
Anderson Greenwood Series 81P (-8 Orifice Only) Balanced Piston Direct Spring PRV Back Pressure Correction Factor (Kb) - Gas Service
1.00
k = 1.0
k = Cp/Cv

k = 1.2

0.90

k = 1.4

k = 1.6

0.80

k = 1.8
k = 2.0

0.70

Kb

0.60

0.50

0.40

0.30

0.20

735
Kb =
C

[

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = 0.7974* (P2/P1) + 0.1969
0.00
0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

P2
P1
Where:
P2 = Pressure at valve outlet during flow, psia. This is total back pressure (psig) + atmospheric pressure (psia)
P1 = Relieving pressure, psia. This is the set pressure (psig) + overpressure (psig) + atmospheric pressure (psia) - inlet pressure piping loss (psig)

7.24

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Figure 7-22
Anderson Greenwood Series 90/9000 - POPRV Back Pressure Correction Factor (Kb) - Gas Service
1.00
k = 1.0
k = Cp/Cv

k = 1.2

0.90

k = 1.4

k = 1.6

0.80

k = 1.8
k = 2.0
0.70

Kb

0.60

0.50

0.40

0.30

0.20

735
Kb =
C

[

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = P2/P1
0.00
0.00

0.10

0.20

0.30

0.40

0.50
P2
P1

0.60

0.70

0.80

0.90

1.00

Where:
P2 = Pressure at valve outlet during flow, psia. This is total back pressure (psig) + atmospheric pressure (psia)
P1 = Relieving pressure, psia. This is the set pressure (psig) + overpressure (psig) + atmospheric pressure (psia) - inlet pressure piping loss (psig)

7.25

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Figure 7-23
Anderson Greenwood Series 40 Pilot Operated PRV Back Pressure Correction Factor (Kb) - Gas/Steam Service

1.00
k = 1.0
k = Cp/Cv

k = 1.2

0.90

k = 1.4

k = 1.6

0.80

k = 1.8

k = 2.0

0.70

Kb

0.60

0.50

0.40

0.30

0.20

735
Kb =
C

[

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = 0.813* (P2/P1) + 0.197
0.00
0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

P2
P1

Where:
P2 = Pressure at valve outlet during flow, psia. This is total back pressure (psig) + atmospheric pressure (psia)
P1 = Relieving pressure, psia. This is the set pressure (psig) + overpressure (psig) + atmospheric pressure (psia) - inlet pressure piping loss (psig)

7.26

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)

Figure 7-24
Anderson Greenwood Series 50 Pilot Operated PRV Back Pressure Correction Factor (Kb) - Gas/Steam Service
1.00
k = 1.0
k = Cp/Cv

k = 1.2

0.90

k = 1.4

k = 1.6

0.80

k = 1.8
k = 2.0

0.70

0.60

Kb

0.50

0.40

0.30

0.20

735
Kb =
C

[

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = 0.584* (P2/P1) + 0.416
0.00
0.00

0.10

0.20

0.30

0.40

0.50
P2
P1

0.60

0.70

0.80

0.90

1.00

Where:
P2 = Pressure at valve outlet during flow, psia. This is total back pressure (psig) + atmospheric pressure (psia)
P1 = Relieving pressure, psia. This is the set pressure (psig) + overpressure (psig) + atmospheric pressure (psia) - inlet pressure piping loss (psig)

7.27

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Figure 7-25
Anderson Greenwood Series 60 Pilot Operated PRV Back Pressure Correction Factor (Kb) - Gas/Steam Service
1.00
k = 1.0
k = 1.2

k = Cp/Cv
0.90

k = 1.4

k = 1.6

0.80

k = 1.8
k = 2.0
0.70

Kb

0.60

0.50

0.40

0.30

0.20

735
Kb =
C

[

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = 0.755* (P2/P1) + 0.249
0.00
0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

P2
P1
Where:
P2 = Pressure at valve outlet during flow, psia. This is total back pressure (psig) + atmospheric pressure (psia)
P1 = Relieving pressure, psia. This is the set pressure (psig) + overpressure (psig) + atmospheric pressure (psia) - inlet pressure piping loss (psig)

7.28

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Figure 7-26
Anderson Greenwood Series LCP Pilot Operated PRV Back Pressure Correction Factor (Kb) - Gas Service

1.00
k = 1.0
k = Cp/Cv

k = 1.2

0.90

k = 1.4

k = 1.6

0.80

k = 1.8
k = 2.0

0.70

0.60

Kb

0.50

0.40

0.30

0.20

735
Kb =
C

[

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = 0.680* (P2/P1) + 0.323

0.00
0.00

0.10

0.20

0.30

0.40

0.50
P2
P1

0.60

0.70

0.80

0.90

1.00

Where:
P2 = Pressure at valve outlet during flow, psia. This is total back pressure (psig) + atmospheric pressure (psia)
P1 = Relieving pressure, psia. This is the set pressure (psig) + overpressure (psig) + atmospheric pressure (psia) - inlet pressure piping loss (psig)

7.29

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Figure 7-27
Anderson Greenwood Series 727 Pilot Operated PRV Back Pressure Correction Factor (Kb) - Gas/Steam Service

1.00
k = 1.0
k = 1.2

k = Cp/Cv

0.98

k = 1.4
k = 1.6
k = 1.8

0.96

k = 2.0

0.94

Kb

0.92

0.90

0.88

735
Kb =
C
0.86

[

2

k+1

k
k
k
(PR’) - (PR’ )k- 1]

PR’ = 0.929* (P2/P1) + 0.07

0.84
0.35

0.40

0.45

0.50

P2
P1

0.55

0.60

0.65

0.70

Where:
P2 = Pressure at valve outlet during flow, psia. This is total back pressure (psig) + atmospheric pressure (psia)
P1 = Relieving pressure, psia. This is the set pressure (psig) + overpressure (psig) + atmospheric pressure (psia) - inlet pressure piping loss (psig)

7.30

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
III. Capacity Correction Factor for High Pressure Steam, Kn
The high pressure steam correction factor Kn is used when steam
relieving pressure P1 is greater than 1500 psia and less than or equal
to 3200 psia. This factor has been adopted by ASME to account for thedeviation between steam flow as determined by Napier’s equation andactual saturated steam flow at high pressures. Kn may be calculated by
the following equation or may be taken from Figure 7-28.
USCS Units:Kn =0.1906P1 - 1000
0.2292P1 - 1061

Where:
Kn = High pressure steam correction factor.
P1 =	Relieving pressure, psia. This is the set pressure + overpressure +atmospheric pressure.

IV. Capacity Correction Factors for Viscosity, Kv
When a liquid relief valve is required to flow a viscous fluid there may
be the occasion to adjust the required orifice area for a laminar flow
regime. If the Reynolds Number is less than 100,000 then there will be a
viscosity correction factor, Kv. The procedure to determine the Kv factor
is as follows:
Step One
Calculate the minimum required discharge area using the liquid sizing
formula in Chapter 5 Section V. Assume the Kv factor is equal to 1.0.
Step Two
Select the actual orifice area that will equal or exceed the minimum area
calculated in step one from an appropriate valve in Chapter 7 Section IX
in square inches.
Step Three
Calculate the Reynolds Number.

R=

2800VLGµ A’

Where:
R	 =	Reynolds Number
VL =	Required relieving capacity, U.S. gallons per minute at flowing
temperatureG	 =	Specific gravity of process fluid at flowing temperature referred towater at standard conditionsµ =	Absolute viscosity at the flow temperature, centipoises
A’ =	Actual orifice area selected in step two
Step Four
Use the Reynolds Number from step three and obtain the Kv factor from
Figure 7-29.
Step Five
Repeat step one calculation using the Kv from step four. If the minimum
required discharge area is equal to or less than the selected actual
orifice area, A’, from step two, the procedure is complete. If not, chosethe next largest available actual orifice area and repeat steps three
through five.

Figure 7-28
Correction Factor for High Pressure Steam, Kn

psia
3200
3100
3000
2900
2800
2700
2600
2500
2400
2300
2200
2100
2000
1900
1800
1700
1600
1500
.95

1.00

1.05

1.10

1.15

1.20

Correction Factor, Kn
All Crosby steam capacity charts will reflect the Kn factor
where applicable.

7.31

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Figure 7-29
Viscosity Correction Factor (Kv)

1

0.9

0.8

Viscosity Correction Factor, Kv

0.7

0.6

0.5

0.4

0.3
101

102

103

104

105

106

Re = Reynold’s Number

7.32

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
V. Capacity Correction Factor for Superheat, Ksh
The steam sizing formulas are based on the flow of dry saturated steam. To size for superheated steam, the superheat correction factor is used
to correct the calculated saturated steam flow to superheated steam flow. For saturated steam Ksh = 1.0. When the steam is superheated, enter
Table 7-1 at the required relieving pressure and read the superheat correction factor under the total steam temperature column.

Table 7-1 - Superheat correction factors
Flowing pressure(psia)
50
100
150
200
250
300
350
400
450
500
550
600
650
700
750
800
850
900
950
1000
1050
1100
1150
1200
1250
1300
1350
1400
1450
1500
1550
1600
1650
1700
1750
1800
1850
1900
1950
2000

400
0.987
0.998
0.984
0.979
-

450
0.957
0.963
0.970
0.977
0.972
0.968
0.968
-

500
0.930
0.935
0.940
0.945
0.951
0.957
0.963
0.963
0.961
0.961
0.962
0.964
0.968
-

550
0.905
0.909
0.913
0.917
0.921
0.926
0.930
0.935
0.940
0.946
0.952
0.958
0.958
0.958
0.958
0.960
0.962
0.965
0.969
0.974
-

600
0.882
0.885
0.888
0.892
0.895
0.898
0.902
0.906
0.909
0.914
0.918
0.922
0.927
0.931
0.936
0.942
0.947
0.953
0.958
0.959
0.960
0.962
0.964
0.966
0.969
0.973
0.977
0.982
0.987
0.993
-

Total temperature of superheated steam, °F
650
700
750
800
850
900
950
0.861 0.841 0.823 0.805 0.789 0.774 0.759
0.864 0.843 0.825 0.807 0.790 0.775 0.760
0.866 0.846 0.826 0.808 0.792 0.776 0.761
0.869 0.848 0.828 0.810 0.793 0.777 0.762
0.871 0.850 0.830 0.812 0.794 0.778 0.763
0.874 0.852 0.832 0.813 0.796 0.780 0.764
0.877 0.854 0.834 0.815 0.797 0.781 0.765
0.880 0.857 0.836 0.816 0.798 0.782 0.766
0.883 0.859 0.838 0.818 0.800 0.783 0.767
0.886 0.862 0.840 0.820 0.801 0.784 0.768
0.889 0.864 0.842 0.822 0.803 0.785 0.769
0.892 0.867 0.844 0.823 0.804 0.787 0.770
0.896 0.869 0.846 0.825 0.806 0.788 0.771
0.899 0.872 0.848 0.827 0.807 0.789 0.772
0.903 0.875 0.850 0.828 0.809 0.790 0.774
0.906 0.878 0.852 0.830 0.810 0.792 0.774
0.910 0.880 0.855 0.832 0.812 0.793 0.776
0.914 0.883 0.857 0.834 0.813 0.794 0.777
0.918 0.886 0.860 0.836 0.815 0.796 0.778
0.923 0.890 0.862 0.838 0.816 0.797 0.779
0.927 0.893 0.864 0.840 0.818 0.798 0.780
0.931 0.896 0.867 0.842 0.820 0.800 0.781
0.936 0.899 0.870 0.844 0.821 0.801 0.782
0.941 0.903 0.872 0.846 0.823 0.802 0.784
0.946 0.906 0.875 0.848 0.825 0.804 0.785
0.952 0.910 0.878 0.850 0.826 0.805 0.786
0.958 0.914 0.880 0.852 0.828 0.807 0.787
0.963 0.918 0.883 0.854 0.830 0.808 0.788
0.968 0.922 0.886 0.857 0.832 0.809 0.790
0.970 0.926 0.889 0.859 0.833 0.811 0.791
0.972 0.930 0.892 0.861 0.835 0.812 0.792
0.973 0.934 0.894 0.863 0.836 0.813 0.792
0.973 0.936 0.895 0.863 0.836 0.812 0.791
0.973 0.938 0.895 0.863 0.835 0.811 0.790
0.974 0.940 0.896 0.862 0.835 0.810 0.789
0.975 0.942 0.897 0.862 0.834 0.810 0.788
0.976 0.944 0.897 0.862 0.833 0.809 0.787
0.977 0.946 0.898 0.862 0.832 0.807 0.785
0.979 0.949 0.898 0.861 0.832 0.806 0.784
0.982 0.952 0.899 0.861 0.831 0.805 0.782

1000
0.745
0.746
0.747
0.748
0.749
0.750
0.750
0.751
0.752
0.753
0.754
0.755
0.756
0.757
0.758
0.759
0.760
0.760
0.761
0.762
0.763
0.764
0.765
0.766
0.767
0.768
0.769
0.770
0.771
0.772
0.773
0.774
0.772
0.771
0.770
0.768
0.767
0.766
0.764
0.762

1050
0.732
0.733
0.733
0.734
0.735
0.736
0.736
0.737
0.738
0.739
0.740
0.740
0.741
0.742
0.743
0.744
0.744
0.745
0.746
0.747
0.748
0.749
0.749
0.750
0.751
0.752
0.753
0.754
0.754
0.755
0.756
0.756
0.755
0.754
0.752
0.751
0.749
0.748
0.746
0.744

1100
0.719
0.720
0.721
0.721
0.722
0.723
0.723
0.724
0.725
0.725
0.726
0.727
0.728
0.728
0.729
0.730
0.730
0.731
0.732
0.732
0.733
0.734
0.735
0.735
0.736
0.737
0.737
0.738
0.739
0.740
0.740
0.740
0.739
0.738
0.736
0.735
0.733
0.731
0.729
0.728

1150
0.708
0.708
0.709
0.709
0.710
0.710
0.711
0.712
0.712
0.713
0.713
0.714
0.715
0.715
0.716
0.716
0.717
0.718
0.718
0.719
0.719
0.720
0.721
0.721
0.722
0.723
0.723
0.724
0.724
0.725
0.726
0.726
0.724
0.723
0.721
0.720
0.718
0.716
0.714
0.712

1200
0.696
0.697
0.697
0.698
0.698
0.699
0.699
0.700
0.700
0.701
0.701
0.702
0.702
0.703
0.703
0.704
0.704
0.705
0.705
0.706
0.707
0.707
0.708
0.708
0.709
0.709
0.710
0.710
0.711
0.711
0.712
0.712
0.710
0.709
0.707
0.705
0.704
0.702
0.700
0.698

7.33

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Table 7-1 - Superheat correction factors (continued)
Flowing pressure(psia)
2050
2100
2150
2200
2250
2300
2350
2400
2450
2500
2550
2600
2650
2700
2750
2800
2850
2900
2950
3000
3050
3100
3150
3200

400
-

450
-

500
-

550
-

600
-

Total temperature of superheated steam, °F
650
700
750
800
850
900
950
0.985 0.954 0.899 0.860 0.830 0.804 0.781
0.988 0.956 0.900 0.860 0.828 0.802 0.779
0.956 0.900 0.859 0.827 0.801 0.778
0.955 0.901 0.859 0.826 0.799 0.776
0.954 0.901 0.858 0.825 0.797 0.774
0.953 0.901 0.857 0.823 0.795 0.772
0.952 0.902 0.856 0.822 0.794 0.769
0.952 0.902 0.855 0.820 0.791 0.767
0.951 0.902 0.854 0.818 0.789 0.765
0.951 0.902 0.852 0.816 0.787 0.762
0.951 0.902 0.851 0.814 0.784 0.759
0.951 0.903 0.849 0.812 0.782 0.756
0.952 0.903 0.848 0.809 0.779 0.754
0.952 0.903 0.846 0.807 0.776 0.750
0.953 0.903 0.844 0.804 0.773 0.747
0.956 0.903 0.842 0.801 0.769 0.743
0.959 0.902 0.839 0.798 0.766 0.739
0.963 0.902 0.836 0.794 0.762 0.735
0.902 0.834 0.790 0.758 0.731
0.901 0.831 0.786 0.753 0.726
0.899 0.827 0.782 0.749 0.722
0.896 0.823 0.777 0.744 0.716
0.894 0.819 0.772 0.738 0.711
0.889 0.815 0.767 0.733 0.705

1000
0.761
0.759
0.757
0.755
0.753
0.751
0.748
0.746
0.743
0.740
0.738
0.735
0.731
0.728
0.724
0.721
0.717
0.713
0.708
0.704
0.699
0.693
0.688
0.682

1050
0.742
0.740
0.738
0.736
0.734
0.732
0.729
0.727
0.724
0.721
0.718
0.715
0.712
0.708
0.705
0.701
0.697
0.693
0.688
0.684
0.679
0.673
0.668
0.662

1100
0.726
0.724
0.722
0.720
0.717
0.715
0.712
0.710
0.707
0.704
0.701
0.698
0.695
0.691
0.687
0.684
0.679
0.675
0.671
0.666
0.661
0.656
0.650
0.644

1150
0.710
0.708
0.706
0.704
0.702
0.699
0.697
0.694
0.691
0.688
0.685
0.682
0.679
0.675
0.671
0.668
0.663
0.659
0.655
0.650
0.645
0.640
0.634
0.628

1200
0.696
0.694
0.692
0.690
0.687
0.685
0.682
0.679
0.677
0.674
0.671
0.664
0.664
0.661
0.657
0.653
0.649
0.645
0.640
0.635
0.630
0.625
0.620
0.614

7.34

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
VI. Ratio of Specific Heats, k, and Coefficient, C
The following formula equates the ratio of specific heats (k) to the coefficient, C, used in sizing methods for gases and vapors. Figure 7-30 and
Table 7-2 provide the calculated solution to this formula. If k is not known, use C = 315.

( )
2
k +1

C = 520 k

k+1
k-1

Where:
k = Ratio of specific heats

Figure 7-30
Gas Constant
410
400
390
380

Coeficient, C

370
360
350
340
330
320
310
1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

Ratio of Specific Heats, k

Table 7-2 - Gas Constant Values
k

C

k

C

k

C

k

C

k

C

1.01
1.02
1.03
1.04
1.05
1.06
1.07
1.08
1.09
1.10
1.11
1.12
1.13
1.14
1.15
1.16
1.17
1.18
1.19
1.20

317
318
319
320
321
322
323
325
326
327
328
329
330
331
332
333
334
335
336
337

1.21
1.22
1.23
1.24
1.25
1.26
1.27
1.28
1.29
1.30
1.31
1.32
1.33
1.34
1.35
1.36
1.37
1.38
1.39
1.40

338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
353
354
355
356

1.41
1.42
1.43
1.44
1.45
1.46
1.47
1.48
1.49
1.50
1.51
1.52
1.53
1.54
1.55
1.56
1.57
1.58
1.59
1.60

357
358
359
360
360
361
362
363
364
365
365
366
367
368
369
369
370
371
372
373

1.61
1.62
1.63
1.64
1.65
1.66
1.67
1.68
1.69
1.70
1.71
1.72
1.73
1.74
1.75
1.76
1.77
1.78
1.79
1.80

373
374
375
376
376
377
378
379
379
380
381
382
382
383
384
384
385
386
386
387

1.81
1.82
1.83
1.84
1.85
1.86
1.87
1.88
1.89
1.90
1.91
1.92
1.93
1.94
1.95
1.96
1.97
1.98
1.99
2.00

388
389
389
390
391
391
392
393
393
394
395
395
396
397
397
398
398
399
400
400

7.35

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)VII. Typical Fluid Properties
The specific heat ratios listed herein have been obtained from numerous sources. They may vary from values available to the reader.
Exercise caution when selecting the specific heat ratio.

Table 7-3 - Physical Properties for Selected Gases
Gas

Empirical formula

Molecular weightM

Specific heat ratiok

Gas constantC

Acetone

C3H6O

58.08

1.12

329

Acetylene (Ethyne)

C2H2
-

26.04

1.26

343

28.97

1.40

356

17.03

1.31

348

Argon

NH3
Ar

39.95

1.67

378

Benzene (Benzol or Benzole)

C6H6

78.11

1.12

329

Boron Trifluoride

BF3

67.82

1.2

337

Butadiene-1,3 (Divinyl)

C4H6

54.09

1.12

329

Butane (Normal Butane)

C4H10

58.12

1.09

326

Butylene (1-Butene)

56.11

1.11

328

Carbon Dioxide

C4H8
CO2

Carbon Disulfide (C. Bisulfide)
Carbon Monoxide

CS2
CO

44.01
76.13
28.01

1.29
1.21
1.40

346
338
356

Carbon Tetrachloride

CCI4

153.82

1.11

328

Chlorine

Cl2

70.91

1.36

353

Chloromethane (Methyl Chloride)

CH3Cl

50.49

1.28

345

Cyclohexane

C6H12

84.16

1.09

326

Cyclopropane (Trimethylene)

C3H6

42.08

1.11

328

Decane-n

C10H22

142.291.04

320Diethylene Glycol (DEG)

C4H10 O3

106.17

1.07

323

Diethyl Ether (Methyl Ether)

C2H6O
-

46.07

1.11

328

165.00147.00

1.05
1.00

321
315
336

Air
Ammonia, Anhydrous

Dowtherm A
Dowtherm E
Ethane

C2H6

30.07

1.19

Ethyl Alcohol (Ethanol)

C2H6O

46.07

1.13

330

Ethylene (Ethene)

C2H4

28.05

1.24

341

Ethylene Glycol

C2H6O2

62.07

1.09

326

Ethylene Oxide

C2H4 O

44.05

1.21

338

12, Dichlorodifluoromethane

CCI2F2120.93

1.14

331

13, Chlorotrifluoromethane

CCIF3

104.471.17

334

13B1, Bromotrifluoromethane

CBrF3

148.93

1.14

331

22, Chlorodifluoromethane

CHCIF2

86.48

1.18

335

115, Chloropentafluoroethane

C2CIF5154.481.08

324

Glycerine (Glycerin or Glycerol)

C3H8 O3

92.10

1.06

322

Fluorocarbons:

7.36

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Table 7-3 - Physical properties for selected gases (continued)
Gas
Helium

Empirical formulaHe

Molecular weightM
4.00

Specific heat ratiok
1.67

Gas constant
C
378
321

Heptane

C7H16

100.21

1.05

Hexane

C6H14

86.18

1.06

322

Hydrogen

2.02

1.41

357

Hydrogen Chloride, Anhydrous

H2
HCl

36.46

1.41

357

Hydrogen Sulfide

H2 S

34.08

1.32

349

Isobutane (2-Methylpropane)

C4H10

58.12

1.10

327

Isobutane (2-Methyl-1,3butadiene)

C5H8

68.12

1.09

326

Isopropyl Alcohol (Isopropanol)

C3H8 O
Kr

60.10

1.09

326

83.80

1.71

380

CH4
CH4O

16.04

1.31

348

32.04

1.20

337

KryptonMethane
Methyl Alcohol (Methanol)
Methylanmines, Anhydrous:
Monomethylamine (Methylamine)

CH5N

31.06

1.02

317

Dimethylamine

C2H7N

45.08

1.15

332
335

Triethylamine

C3H9N

59.11

1.18

Methyl Mercapton (Methylamine)

CH4 S

48.11

1.20

337

Naphthalene (Naphthaline)

128.171.07

323

Natural Gas (Relative Density = 0.60)
Neon

C10H8
Ne

17.40
20.18

1.27
1.64

344
375
356

Nitrogen

N2

28.01

1.40

Nitrous Oxide

N2O

44.01

1.30

347

OctaneC8H18

114.23

1.05

321

Oxygen

O2

32.00

1.40

356

Pentane

C5H12

72.15

1.07

323

Propadiene (Allene)

C3H4

40.07

1.69

379

Propane

C3H8

44.10

1.13

330

Propylene (Propene)

C3H6

42.08

1.15

332

Propylene Oxide

C3H6O

58.08

1.13

330

Styrene

C8H8

104.151.07

323

Sulfur Dioxide

SO2

64.06

1.28

345

Sulfur Hexafluoride

SF6

146.05

1.09

326

Steam

H2O

18.02

1.31

348

Toluene (Toluol or Methylbenzene)

C7H8

92.14

1.09

326

Triethylene Glycol (TEG)

C6H14 O4

150.18

1.04

320

Vinyl Chloride Monomer (VCM)

62.50

1.19

336

Xenon

C2H3ClXe

131.301.65

376

Xylene (p-Xylene)

C8H10

106.17

1.07

323

7.37

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Table 7-4 - Physical Properties for Selected Liquids

Relative Density G
Water = 1

Fluid Temperature
°F

C2H4

0.779

68

C2H4 O2

1.051

68

Acetone

C3H6O

0.792

68

Ammonia, Anhydrous

NH3

0.666

68

0.88-0.94
1.01

60
60

Fluid

Empirical Formula

Acetaldehyde
Acetic Acid

Automotive Crankcase and Gear Oils:
SAE-5W Through SAE 150
Beer-

Benzene (Benzol)

C6H6

0.880

68

Boron Trifluoride

BF3

1.57

-148
68

Butadiene-1,3

C4H6

0.622

Butane-n (Normal Butane)

C4H10

0.579

68

Butylene (1-Butene)

C4H8

0.600

68

Carbon Dioxide

CO2

1.03

-4

Carbon Disulphide (C. Bisulphide)

CS2

1.27

68

Carbon Tetrachloride

CCl4

1.60

68

Chlorine

Cl2

1.42

68

Chloromethane (Methyl Chloride)

0.921

68

Crude Oils:
32.6 Deg API
35.6 Deg API
40 Deg API
48 Deg API

CH3Cl
-

0.862
0.847
0.825
0.79

60
60
60
60

Cyclohexane

C6H12

0.780

68

Cyclopropane (Trimethylene)

C3H6

0.621

68

Decane-n

C10H22
-

0.731

68

0.82-0.95

60

Diesel Fuel Oils
Diethylene Glycol (DEG)

C4H10 O3

1.12

68

Dimethyl Ether (Methyl Ether)

0.663

68

Dowtherm A
Dowtherm E

C2H6O
-

0.998
1.087

68
68

Ethane

C2H6

0.336

68

Ethyl Alcohol (Ethanol)

C2H6O

0.79

68

Ethylene (Ethene)

C2H4

0.569

-155

Ethylene Glycol

C2H6O2

1.115

68

Ethylene Oxide

C2H4 O

0.901

68
68

Fluorocarbons:
R12, Dichlorodifluoromethane

CCl2F2

1.34

R13, Chlorotrifluoromethane

CClF3

0.916

68

R13B1, Bromtrifluoromethane

CBrF3

1.58

68

R22, Chlorodifluoromethane

CHClF2

1.21

68

R115, Chloropentafluoroethane

C2ClF5

1.31

68

7.38

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Table 7-4 - Physical Properties for Selected Liquids (continued)

Relative Density G:
Water = 1
0.82-0.95
0.68-0.74

Fluid Temperature
°F
60
60

Fluid
Fuel Oils, Nos. 1, 2, 3, 5 and 6
Gasolines

Empirical formula-

Glycerine (Glycerin or Glycerol)

C3H8 O3

1.26

68

Heptane

C7H16

0.685

68

Hexane0.660

68

Hydrochloric Acid

C6H14
HCl

1.64

60

Hydrogen Sulphide

H2 S

0.78

68

Isobutane (2-Methylpropane)

C4H10

0.558

68

Isoprene (2-Methyl-1,3-Butadiene)

C5H8

0.682

68

Isopropyl Alcohol (Isopropanol)

0.786

68

Jet Fuel (average)
Kerosene

C3H8 O-

0.82
0.78-0.82

60
60

Methyl Alcohol (Methanol)

CH4 O

0.792

68

Methylamines, Anhydrous:
Monomethylamine (Methylamine)

CH5N

0.663

68

Dimethylamine

C2H7N

0.656

68

Trimethylamine

C3H9N

0.634

68

Methyl Mercapton (Methanethiol)

CH4 S

0.870

68

Nitric Acid

HNO3

1.5

60

Nitrous Oxide

N2O

1.23

-127
68

Octane

C8H18

0.703

Pentane

C5H12

0.627

68

Propadiene (Allene)

C3H4

0.659

-30
68

Propane

C3H8

0.501

Propylene (Propene)

C3H6

0.514

68

Propylene Oxide

C3H6O

0.830

68

Styrene

C8H8

0.908

68

Sulfur Dioxide

SO2

1.43

68

Sulphur Hexafluoride

SF6

1.37

68

95-100%60%
20%

H2 SO4
-

1.839
1.50
1.14

68
68
68

Toluene (Toluol or Methylbenzene)

C7H8

0.868

68

Triethylene Glycol (TEG)

C6H12O4

1.126

68

Vinyl Chloride Monomer (VCM)

C2H3Cl

0.985-4

Water, fresh

1.00

68

Water, sea

H2O
-

1.03

68

Xylene (p-Xylene)

C8H10

0.862

68

Sulphur Acid:

7.39

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
VIII. Saturated Steam Pressure Table
Table 7-5 - Saturation Pressure (psia)/Temperature (°F)
Pressure
psia
14.7
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
115
120
125
130
135
140
145
150
160
170
180
190
200
210
220
230
240
250
260
270
280
290
300
320
340
360
380
400
420
440
460
480
500

Temperature
deg F
212.0
213.3
228.0
240.1
250.3
259.3
267.3
274.4
281.0
287.1
292.7
298.0
302.9
307.6
312.0
316.3
320.3
324.1
327.8
331.4
334.8
338.1
341.3
344.4
347.3
350.2
353.0
355.8
358.4
363.6
368.4
373.1
377.5
381.8
385.9
389.9
393.7
397.4
401.0
404.4
407.8
411.1
414.3
417.4
423.3
429.0
434.9
439.6
444.6
449.4
454.0
458.5
462.8
467.0

Pressure
psia
520
540
560
580
600
620
640
660
680
700
720
740
760
780
800
820
840
860
880
900
920
940
960
980
1000
1050
1100
1150
1200
1250
1300
1350
1400
1450
1500
1600
1700
1800
1900
2000
2100
2200
2300
2400
2500
2600
2700
2800
2900
3000
3100
3200
3208

Temperature
deg F
471.1
475.0
478.8
482.6
486.2
489.7
493.2
496.6
499.9
503.1
506.2
509.3
512.3
515.3
518.2
521.1
523.9
526.6
529.3
532.0
534.6
537.1
539.7
542.1
544.6
550.5
556.3
561.8
567.2
572.4
577.4
582.3
587.1
591.7
596.2
604.9
613.1
621.0
628.6
635.8
642.8
649.6
655.9
662.1
668.1
673.9
679.5
685.0
690.2
695.3
700.3
705.1
705.5

7.40

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
IX. Anderson Greenwood and Crosby Pressure Relief Valves
Orifice Area and Coefficient of Discharge
As mentioned in Chapter Three, the use of the proper orifice area (A) and coefficient of discharge(K) in the sizing formulas presented in this handbook are critical to determining the correct valve
size. For some valve designs, two sets of values are published.
One set, the effective area and effective coefficient of discharge, are published by API in
Standard 526, Flanged Steel Pressure Relief Valves and Standard 520 part I, Sizing, Selection
and Installation of Pressure Relieving Devices. These values are independent of any specific valve
design and are used to determine a preliminary pressure relief valve size.
Where applicable, a second set of areas and discharge coefficients is shown to determine the“rated” capacity of a valve using the “actual” orifice area and “rated” coefficient of discharge.
Rated coefficients are established by regulatory bodies like ASME and “actual” areas are
published by the manufacturer.
It is important to remember that the effective area and effective coefficient of discharge are used
only for the initial selection. The actual orifice area and rated coefficient of discharge must always
be used to verify the actual capacity of the pressure relief valve.
In no case should an effective area or effective coefficient of discharge be used with actual area or
rated coefficient of discharge. Sizing errors can be made if the effective values are mixed with theactual values.
The following tables provide orifice areas and coefficient of discharge for Anderson Greenwood
and Crosby pressure relief valves. Once again, where applicable, there is a table with API“effective” values and a separate table with ASME “rated” and “actual” values.
Do not mix values from these tables.

7.41

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Table 7-6 - JOS-E/JBS-E/JLT-E Full Nozzle Direct Acting Spring Valves

API Effective Orifice Area and Coefficient of Discharge

Minimum
inlet size
1”
1”
1.5”
1.5”
1.5”
2”
3”
3”
4”
4”
4”
6”
6”
8”

Orificedesignation
D
E
F
G
H
J
K
L
M
N
P
Q
R
T

Gas (in2)
SeriesJOS-E, JBS-E, JLT-JOS-E, JLT-JBS-E
K = 0.975
0.110
0.196
0.307
0.503
0.785
1.287
1.838
2.853
3.600
4.340
6.380
11.05016.00026.000Liquid (in2)
SeriesJLT-JOS-E, JLT-JBS-E
K = 0.650
0.110
0.196
0.307
0.503
0.785
1.287
1.838
2.853
3.600
4.340
6.380
11.05016.00026.000

Steam (in2)
SeriesJOS-E, JBS-E
K = 0.975
0.110
0.196
0.307
0.503
0.785
1.287
1.838
2.853
3.600
4.340
6.380
11.05016.00026.000

Table 7-7 - JOS-E/JBS-E/JLT-E Full Nozzle Direct Acting Spring Valves

ASME Actual Orifice Area and Rated Coefficient of Discharge

Minimum
inlet size
1”
1”
1.5”
1.5”
1.5”
2”
3”
3”
4”
4”
4”
6”
6”
8”
8”

Orificedesignation
D
E
F
G
H
J
K
L
M
N
P
Q
R
T
T2

Air/gas (in2)
SeriesJOS-E, JBS-E
K = 0.865
0.124
0.221
0.347
0.567
0.887
1.453
2.076
3.221
4.065
4.900
7.206
12.47018.06029.36031.470Air/gas (in2)
SeriesJLT-JOS-E, JLT-JBS-E
K = 0.870
0.124
0.221
0.347
0.567
0.887
1.453
2.076
3.221
4.065
4.900
7.206
12.47018.06029.36031.470Liquid (in2)
SeriesJLT-JOS-E, JLT-JBS-E
K = 0.656
0.124
0.221
0.347
0.567
0.887
1.453
2.076
3.221
4.065
4.900
7.206
12.47018.06029.36031.470

Steam (in2)
SeriesJOS-E, JBS-E
K = 0.865
0.124
0.221
0.347
0.567
0.887
1.453
2.076
3.221
4.065
4.900
7.206
12.47018.06029.36031.470

7.42

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Table 7-8 - OMNI 800/900/BP Portable Direct Acting Spring Valves

ASME Actual Orifice Area and Rated Coefficient of Discharge

Minimum
Inlet Size
½”
½”
¾”
¾”
1”
1.5”
1.5”

Orifice
Desig nation
-5
-6
-5
-6
-7
-8
-9

Series 800
K = 0.877

Gas (in2)
Section VIII
Series 900
K = 0.878

Series BP
K = 0.841

0.124
0.220
0.344
0.567

0.085
0.124
0.220
0.347
0.567

0.093
0.136
-

Liquid (in2)
Section VIII
Series 900
Series BP
K = 0.662
K = 0.631
0.085
0.124
0.220
0.347
0.567

0.093
0.136
-

Steam (in2)
Section VIII
Series 800
Series 900
K = 0.877
K = 0.878
0.124
0.220
0.344
0.567

0.085
0.124
0.220
0.347
0.567

Table 7-9 - Series 60 and Series 80 Portable Direct Acting Spring Valves

Minimum
Inlet Size
½”
½”
½”
¾”
¾”
¾”
1.5”
1.5”
2”
2”

Orifice
Designation
-4
-5
-6
-4
-7
-8
F
G
H
J

ASME Actual Orifice Area and Rated Coefficient of Discharge
Gas (in2)
Section VIII
81/83
81P
61
63B
K = 0.816
K = 0.816
K = 0.877
K = 0.847
0.049
0.076
0.110
0.110
0.149
0.196
0.196
0.307
0.503
0.785
1.287
-

Liquid (in2)
Section VIII
81P
K = 0.720
0.049
0.196
0.503
1.287

Steam (in2)
Section VIII86
K = 0.816
0.049
0.049
0.196
0.503
1.287

7.43

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Table 7-10 - H Series Direct Acting Spring Safety Valves

Minimum
Inlet size
1.25”
1.25”
1.5”
1.5”
1.5”
1.5”
1.5”
2”
2”
2”
2”
2.5”
2.5”
2.5”
3”
3”
3”
3”
3”
4”
4”
4”
6”
6”
6”
6”

ASME Actual Orifice Area and Rated Coefficient of Discharge
Steam (in2)
Section I/Section VIIIHCIHSJ
ISOFLEX
Orifice
K = 0.878
K = 0.878
Designation
F
G
F
0.307
G
0.503
H
0.785
H2
0.994
J
H
0.785
J
1.288
J2
1.431
K
K
1.840
K2
2.545
L
K
1.840
L
2.853
L2
3.341
M
3.600
3.976
M2
N
4.341
P
6.380
P2
7.070
Q
11.040Q2
12.250R
16.000RR
19.290HE
ISOFLEXK = 0.877
1.840
2.545
3.600
3.976
7.069
-

Table 7-11 - High Pressure Pilot Operated Valves

Minimum
Inlet size
1”
1”
1”
1.5”
1.5”
2”
2”
2”
3”
3”
4”
4”
4”
6”
6”
8”

Orifice
Designation
D
E
F
G
H
G
H
J
K
L
M
N
P
Q
R
T

API Effective Orifice Area and Coefficient of Discharge
Gas (in2)
Liquid (in2)
SeriesSeries200/400/800
500
727
400/800
500
K = 0.975
K = 0.975
K = 0.975
K = 0.650
K = 0.650
0.110
0.110
0.196
0.196
0.307
0.307
0.307
0.307
0.503
0.503
0.785
0.785
0.785
0.785
0.503
0.503
0.503
0.785
0.785
0.785
1.287
1.287
1.287
1.287
1.287
1.838
1.838
1.838
2.853
2.853
2.853
2.853
2.853
3.600
3.600
3.600
4.340
4.340
4.340
6.380
6.380
6.380
6.380
6.380
11.05011.05011.05016.00016.00016.00016.00016.00026.00026.00026.00026.00026.000

Steam (in2)
Series500
K = 0.975
0.785
0.785
1.287
2.853
6.380
16.00026.000727
K = 0.975
0.503
0.785
1.287
1.838
2.853
3.600
4.340
6.380
11.05016.00026.000

7.44

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Table 7-12 - High Pressure Pilot Operated Valves

ASME Actual Orifice Area and Rated Coefficient of Discharge
Gas
Min.
Section VIII
Inlet
Orifice
500
LCP
Size
Designation 200/400/800
1"D
A = 0.205 in2
K = 0.627
1"E
A = 0.356 in2
K = 0.627
1"
F
A = 0.357 in2
A = 0.357 in2
K = 0.877
K = 0.877
1"
A = 0.785 in2
K = 0.860
1.5"G
A = 0.831 in2
K = 0.627
1.5"
H
A = 0.913 in2
A = 0.913 in2
K = 0.877
K = 0.877
1.5"
A = 1.767 in2
K = 0.860
1.5"FBA = 1.496 in2
A = 1.496 in2
K = 0.860
K = 0.860
2"G
A = 0.850 in2
K = 0.627
2"
H
A = 1.312 in2
K = 0.627
2"
J
A = 1.496 in2
A = 1.496 in2
K = 0.877
K = 0.877
2"
A = 3.142 in2
K = 0.860
2"FBA = 2.895 in2
A = 2.895 in2
K = 0.860
K = 0.860
3"J
A = 2.132 in2
K = 0.627
3"
K
A = 3.043 in2
K = 0.627
3"
L
A = 3.317 in2
A = 3.317 in2
K = 0.877
K = 0.877
3"
A = 7.069 in2
K = 0.860
3"FBA = 6.733 in2
A = 6.733 in2
K = 0.860
K = 0.860
4"L
A = 4.729 in2
K = 0.627
4"M
A = 5.959 in2
K = 0.627
4"N
A = 7.188 in2
K = 0.627
4"P
A = 7.645 in2
A = 7.645 in2
K = 0.877
K = 0.877
4"FB
A = 10.75 in2
A = 10.75 in2
K = 0.860
K = 0.860-727
A = 0.629 in2
K = 0.788
A = 0.981 in2
K = 0.788
A = 1.635 in2
K = 0.788
A = 2.298 in2
K = 0.788
A = 3.557 in2
K = 0.788
A = 4.505 in2
K = 0.788
A = 5.425 in2
K = 0.788
A = 7.911 in2
K = 0.788-

LiquidSection VIII
400/800
500
A = 0.221 in2
K = 0.491
A = 0.356 in2
K = 0.491
A = 0.357 in2
A = 0.357 in2
K = 0.766
K = 0.766
A = 0.911 in2
K = 0.491
A = 0.913 in2
A = 0.913 in2
K = 0.766
K = 0.766
A = 1.496 in2
A = 1.496 in2
K = 0.712
K = 0.712
A = 1.005 in2
K = 0.491
A = 1.495 in2
K = 0.491
A = 1.496 in2
A = 1.496 in2
K = 0.766
K = 0.766
A = 2.895 in2
A = 2.895 in2
K = 0.712
K = 0.712
A = 2.574 in2
K = 0.491
A = 3.313 in2
K = 0.491
A = 3.317 in2
K = 0.766
A = 6.733 in2
A = 6.733 in2
K = 0.712
K = 0.712
A = 5.711 in2
K = 0.491
A = 6.385 in2
K = 0.491
A = 7.059 in2
K = 0.491
A = 7.069 in2
A = 7.069 in2
K = 0.766
K = 0.766
A = 10.75 in2
A = 10.75 in2
K = 0.712
K = 0.712-

Steam
Section VIII500
A = 0.357 in2
K = 0.877
A = 0.913 in2
K = 0.877
A = 1.496 in2
K = 0.860
A = 1.496 in2
K = 0.877
A = 2.895 in2
K = 0.860
A = 3.317 in2
K = 0.877
A = 6.733 in2
K = 0.860
A = 7.645 in2
K = 0.877
A = 10.75 in2
K = 0.860-727
K = 0.759 (water)
A = 0.629 in2
K = 0.788
A = 0.981 in2
K = 0.788
A = 1.635 in2
K = 0.788
A = 2.298 in2
K = 0.788
A = 3.557 in2
K = 0.788
A = 4.505 in2
K = 0.788
A = 5.425 in2
K = 0.788
A = 7.911 in2
K = 0.788-

Economizer
Section I
5100
A = 0.357 in2
K = 0.876 (steam)A = 0.913 in2
K = 0.876 (steam)
K = 0.759 (water)
A = 1.496 in2
K = 0.849 (steam)
K = 0.709 (water)
A = 1.496 in2
K = 0.876 (steam)
K = 0.759 (water)
A = 2.895 in2
K = 0.849 (steam)
K = 0.709 (water)
A = 3.317 in2
K = 0.876 (steam)
K = 0.759 (water)
A = 6.733 in2
K = 0.849 (steam)
K = 0.709 (water)
A = 10.75 in2
K = 0.849 (steam)
K = 0.709 (water)

7.45

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Table 7-12 - High Pressure Pilot Operated Valves (continued)
ASME Actual Orifice Area and Rated Coefficient of Discharge
Gas
Min.
Section VIII
Inlet
Orifice
500
LCP
Size
Designation 200/400/800
6"Q
A = 18.294 in2
K = 0.627
6"R
A = 18.597 in2 A = 18.597 in2
K = 0.877
K = 0.877
6"FB
A = 23.32 in2
A = 23.32 in2
K = 0.860
K = 0.860
8"T
A = 30.58 in2 A = 30.58 in2
K = 0.877
K = 0.877
8"
8FB 8x8
A = 32.17 in2 A = 32.17 in2
K = 0.860
K = 0.860
8"
8FB 10A = 44.18 in2 A = 44.18 in2
K = 0.860
K = 0.860
10"FBA = 72.01 in2 A = 72.01 in2
K = 0.860
K = 0.860-

727
A = 13.81 in2
K = 0.788
A = 20.00 in2
K = 0.788
A = 32.50 in2
K = 0.788-

LiquidSection VIII
400/800
500
A = 15.88 in2
K = 0.491
A = 15.90 in2
A = 15.90 in2
K = 0.766
K = 0.766
A = 23.32 in2
A = 23.32 in2
K = 0.712
K = 0.712
A = 28.27 in2 A = 28.27 in2
K = 0.766
K = 0.766
A = 31.17 in2 A = 31.17 in2
K = 0.712
K = 0.712
A = 44.18 in2 A = 44.18 in2
K = 0.712
K = 0.712-

Steam
Section VIII500
A = 18.59 in2
K = 0.877
A = 23.32 in2
K = 0.860
A = 30.58 in2
K = 0.877
A = 32.17 in2
K = 0.860
A = 44.18 in2
K = 0.860
A = 72.01 in2
K = 0.860

727
A = 13.81 in2
K = 0.788
A = 20.00 in2
K = 0.788
A = 32.50 in2
K = 0.788-

Economizer
Section I
5100
A = 23.32 in2
K = 0.849 (steam)
K = 0.709 (water)
A = 44.18 in2
K = 0.849 (steam)
K = 0.709 (water)-

Table 7-13 - Low Pressure Pilot Operated Valves

ASME Actual Orifice Area and Rated Coefficient of Discharge (Set pressure ≥15 psig)
Gas (in2)
91/94
K = 0.770

93
K = 0.845

95
K = 0.8529300
K = 0.629

*

2.924

2.290

2.926

3.350

*
*
*
*
*
*

6.243
10.33022.22039.57056.75089.8705.160
8.740
19.56036.40051.00084.000

6.246
10.32022.150-7.390
12.73028.89050.00078.850113.000

9200

9300

Min.
Inlet Size

Orifice
Designation

2”
3”
4”
6”
8”
10”
12”

Table 7-14 - Low Pressure Pilot Operated Valves

Actual Orifice Area and Rated Coefficient of Discharge (Set pressure < 15 psig)
Min.
Inlet Size
2”
3”
4”
6”
8”
10”
12”

Orifice
Designation
*
*
*
*
*
*
*

91/94

93

Kd = 0.678 (P2 /P1)-0.285
2.924
6.243
10.33022.22039.57056.75089.870Kd = 0.700 (P1/P2)-0.265
2.290
5.160
8.740
19.56036.40051.00084.000Gas (in2)
95

Kd = 0.678 (P1/P2)-0.285 Kd = 0.756 (P1/P2) 0.0517 Kd = 0.650 (P1/P2) -0.349
2.926
3.350
3.350
6.246
7.390
7.390
10.32012.73012.73022.15028.89028.89050.00050.00078.85078.850113.000
113.000* Full bore
Where:
P2 = Pressure at valve outlet during flow, psia. This is total back pressure (psig) + atmospheric pressure (psia).
P1 = Relieving pressure, psia. This is the set pressure (psig) + overpressure (psig) + atmospheric pressure (psia) - inlet pressure piping loss (psig).
PA = Atmospheric pressure (psia).

7.46

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)

Table 7-15 - Low Pressure Pilot Operated Valves

Actual Orifice Area and Rated Coefficient of Discharge - Vacuum Flow
Air/Gas (in2)
Minimum
Inlet Size
2”
3”
4”
6”
8”
10”
12”

Orifice
Designation
*
*
*
*
*
*
*

9200

9300

Kd = 0.667
3.350
7.390
12.73028.89050.00078.850113.000

Kd = 0.55
3.350
7.390
12.73028.89050.00078.850113.000* Full bore

Table 7-16 - JB-TD Direct Acting Spring Valves

ASME Actual Orifice Area and Rated Coefficient of Discharge
Air/Gas/Steam (in2)
JB-TD
K = 0.856

Inlet x Outlet
Size
10x14
12x16

Orifice
Designation
V
W

12x16
14x18
16x18

W1
Y
Z

93.78
103.2016x18

Z1

110.00

16x20

Z2
AA
BB

155.00191.40

18x24
20x24

47.85
68.90
72.00

123.50

7.47

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
X. Equivalents and Conversion Factors
Table 7-17 - Equivalents and Conversion Factors
A

B

C

Multiply
Atmospheres
Atmospheres
Atmospheres
Atmospheres
Atmospheres
Atmospheres
Atmospheres
Atmospheres
Barrels
Bars
Bars
Bars
Centimeters
Centimeters
Centimeters
Centimeters
Cubic centimeters
Cubic feet
Cubic feet
Cubic feet per minute

By
14.70
1.033
29.92
760.0
407.5
33.96
1.013
101.3
42.00
14.50
1.020
100.0
0.39370.032810.010
0.01094
0.061027.481
0.17810.02832

ObtainPounds per square inch
Kilograms per sq. cm
Inches of mercury
Millimeters of mercury
Inches of water
Feet of water
Bars
Kilo Pascals
Gallons (U.S.)
Pounds per square inch
Kilograms per sq. cm
Kilo Pascals
InchesFeet
Meters
Yards
Cubic inches
Gallons
Barrels
Cubic meters per minute

Cubic feet per second
Cubic inches
Cubic inches
Cubic meters
Cubic meters per hour
Cubic meters per minuteStandard cubic feet per min.
Standard cubic feet per min.
Standard cubic feet per min.

448.8
16.39
0.004329
264.2
4.403
35.31
60.00
1440.00.02716

Gallons per minute
Cubic centimeters
Gallons
Gallons
Gallons per minute
Cubic feet per minuteStandard cubic ft. per hrStandard cubic ft. per day
Nm3/min. (0°C, 1 Bara)

Standard cubic feet per min.
Standard cubic feet per min.
Standard cubic feet per min.
Standard cubic feet per min.
Standard cubic feet per min.
Feet
Feet
Feet
Feet of water (68°F)
Feet of water (68°F)
Gallons (U.S.)
Gallons (U.S.)
Gallons (U.S.)
Gallons (Imperial)
Gallons (U.S.)
Gallons (U.S.)
Gallons of water (60°F)
Gallons of liquid
Gallons per minute
Gallons per minute (60°F)
Gallons per minute
Gallons per minute
Gallons per minute
Grams
InchesInchesInchesInches

1.630
39.11
0.02832
1.699
40.78
0.30480.333330.48
0.88120.43283785.00.1337231.0
277.4
0.83273.785
8.337
500 x Sp. Gr.
0.002228
227.0 x SG
0.06309
3.785
0.22710.035272.540
0.08333
0.02540.02778

Nm3/hr. (0°C, 1 Bara)
Nm3/day (0°C, 1 Bara)
Nm3/minNm3/hr
Nm3/day
MetersYards
Centimeters
Inches of mercury (0°C)
Pounds per square inch
Cubic centimeters
Cubic feet
Cubic inches
Cubic inches
Gallons (Imperial)
LitersPoundsPounds per hour liquid per minute
Cubic feet per second
Kilograms per hour
Liters per second
Liters per minute
M3/hr
OuncesCentimeters
Feet
Meters
Yards

Notes
This table may be used in two ways:
1. Multiply the unit under column A by the figure
under column B, the result is the unit undercolumn C.
2. Divide the unit under column C by the figure undercolumn B, the result is then the unit under column A.

7.48

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Table 7-17 - Equivalents and Conversion Factors (continued)
A
Multiply
Inches of mercury (0°C)
Inches of mercury (0°C)
Inches of mercury (0°C)
Inches of mercury (0°C)
Inches of water (68°F)
Inches of water (68°F)
Kilograms
Kilograms
Kilograms
Kilograms per minute
Kilograms per sq. cm
Kilograms per sq. cm
Kilograms per sq. cm
Kilograms per cubic meter
Kilo Pascals
Kilo Pascals
Kilo Pascals
LitersLitersLitersLiters per hour
MetersMetersMetersMetersPoundsPoundsPoundsPoundsPoundsPoundsPounds per hour
Pounds per hour
Pounds per hour liquid
Pounds per sq. inch
Pounds per sq. inch
Pounds per sq. inch
Pounds per sq. inch
Pounds per sq. inch
Pounds per sq. inch
Pounds per sq. inch
Pounds per sq. inch
Pounds per sq. inch
Specific gravity (of gas or vapors)
Square centimeter
Square inch
Square inch
SSU
SSU
Water (cubic feet at 60°F)
Temperature:
Centigrade
KelvinFahrenheit
Fahrenheit
Fahrenheit

B
By
1.135
0.49120.033420.03453
0.036070.073432.205
0.001102
35.27
132.3
14.22
0.967828.96
0.06240.14500.01000.010200.035311000.00.26420.004403
3.281
1.094
100.0
39.37
0.1199453.6
0.00050.45360.0004536
16.00
6.324/M.W.
0.45360.002/Sp.Gr.
27.73
2.311
2.036
0.070310.068051.71
0.70430.06895
6.895
28.97
0.15506.4516645.160.2205 x SG
0.2162
62.37

CObtainFeet of water (68°F)
Pounds per square inch
Atmospheres
Kilograms per sq. cm
Pounds per sq. in.
Inches of mercury (0°C)
PoundsShort tons (2000 lbs.)
OuncesPounds per hour
Pounds per sq. in.
Atmospheres
Inches of mercury
Pounds per cubic footPounds per sq. in.
Bars
Kilograms per sq. cm
Cubic feet
Cubic centimeters
Gallons
Gallons per minute
Feet
Yards
Centimeters
InchesGallons H2O at 60°F (U.S.)
Grams
Short tons (2000 lbs.)
Kilograms
Metric tons
Ounces
SCFM
Kilograms per hour
Gallons per minute liquid (at 60°F)
Inches of water (68°F)
Feet of water (68°F)
Inches of mercury (0°C)
Kilograms per sq. cm
Atmospheres
Millimeters of mercury (0°C)
Meters of water (68°F)
Bar
Kilo Pascals
Molecular weight (of gas or vapors)
Square inch
Square centimeter
Square millimeter
Centipoise
Centistoke
Pounds=====5/9 (Fahrenheit -32)
Centigrade + 273
9/5 (Centigrade) +32
Rankine -460
(9/5 Kelvin) -460

Notes
This table may be used in two ways:
1. Multiply the unit under column A by the figure
under column B, the result is the unit undercolumn C.
2. Divide the unit under column C by the figure undercolumn B, the result is then the unit under column A.

7.49

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Conversion Factors
Table 7-18 - Pressure Conversions[1]
Given
mm wc
(mm water column)(60°F or 15.6°C)
mbar (millibars)
mm Hg[2](mm Mercury)(32°F or 0°C)
in wc
(in. water column)(60°F or 15.6°C)
oz (oz/in2)

mm wc
-

mbar
0.0980To Find (To find desired value, multiply “Given” value by factor below)
mm Hg
in wc
oz
kPa
in Hg
psig
0.735
0.03940.02270.00980
0.002900.001421

10.21
13.61

1.333

0.750
-

0.40190.53580.23200.30940.10000.13330.02960.03948

25.40

2.488

1.866

-

0.57750.2488

43.99

4.309

3.232

1.732

-

kPa (kilopascal)in Hg (in. Mercury)(60°F or 15.6°C)
psig (lbs/in2)
kg/cm2

102.1
344.7

10.00
33.77

7.501
25.33

4.019
13.57

703.8

68.95

51.72

10010
10207

980.7
1000

735.6
750.1

bars

kg/cm2
1
10010

bars
1
10207

0.01450
0.019340.001020.001360.00100
0.00133

0.0737

0.03609

0.002540.002490.43090.12760.004390.00431

2.321
7.836

3.377

0.2961-

0.0625 or
1/16
0.14500.48980.01020.03440.01000.0338

27.71

16.00

6.895

2.042

-

0.07030.0689

394.1
401.9

227.6
232.1

98.07
100.0

29.04
29.61

14.22
14.50

1.020

0.9807-

Notes
1.	When pressure is stated in liquid column height, conversions are valid only for listed temperature.
2.	Also expressed as torr.
3.	Inch-Pound Standard Conditions are at sea level, equal to 14.7 psia (pounds force per square inch absolute),
rounded from 14.696 psia, at a base temperature of 60°F (15.6°C).

7.50

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Conversion Factors
Table 7-19 - Gas Flow Conversions
Given
scfm

Notes

scfh

[1]

lb/hr

[2]

kg/hr

[3]

Nm3/hr[4]

Nm3/min[4][1]

To Find (To find desired value, multiply “Given” value by factor below)
scfm
scfh
lb/hr
kg/hr
Nm3/hr
Nm3/min
60
M
M
1.608
0.02686.32
13.93
0.01677
M
M
0.02680.000447
379.2
836.1
6.32
379.2
0.453610.17
0.1695
M
M
M
M
13.93
836.1
2.205
22.40
0.3733
M
M
M
M
0.621637.30
MM
0.01667
10.17
22.40
37.30
2238
5.901 M
2.676 M60-

Notes
M = molecular weight of gas.
1.	Volumetric flow(per time unit of hour or minute as shown) instandard cubic feet per minute at 14.7 psia, 60°F.
2.	Weight flow in pounds per hour.
3.	Weight flow in kilograms per hour.
4.	Volumetric flow (per time unit of hour or
minute as shown) at 1.013 bars absolute, 0°C.
This represents the commercial standard, known
as the Normal Temperature and Pressure (NTP).

Conversions from volumetric to volumetric or to weight flow (and vice versa) may only be done
when the volumetric flow is expressed in the standard conditions shown above. If flows are
expressed at temperature or pressure bases that differ from those listed above, they must first be
converted to the standard base.
If flow is expressed in actual volume, such as cfm (cubic feet per minute) or acfm (actual cfm) as
is often done for compressors, where the flow is described as displacement or swept volume, theflow may be converted to scfm as follows.

USCS Units

scfm = (cfm or acfm) x 14.7 + p x 520
460 + t14.7
Where:
p= gauge pressure of gas in psig
t= temperature of gas in °F
cfm or acfm= displacement or swept volume in cubic feet or actual cubic feet per minute

7.51

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Conversion Factors

Table 7-20 - Liquid Flow Conversions
Given
l/hr
liters/hour
gpm (US)
US gallons per minutegpm (Imp)
Imperial gallons per minutebarrels/day (petroleum)(42 US gallons)m3/hr
cubic meters per hourm3/s
cubic meters per second
kg/hr
kilograms per hourlb/hrpounds per hour

To Find (To find desired value, multiply “Given” value by factor below)
l/hr
gpm (US)gpm (Imp)barrels/ day
m3/hr
0.004400.003666
0.15100.0010227.1-

0.832734.29

0.2271

272.8

1.201

-

41.18

0.27286.624

0.029170.02429-

0.006624

1000

4.403

3.666

151.0

-

3.6 x 10615.85013.200543.400

3600

1
G
1
2.205G1
227.1G1
500.8G

1
272.8G1
601.5G

0.151
G
1
14.61G

1
1000G
1
2205G

Note
G = relative density of liquid at its relieving
temperature to that of water at 68°F where
Gwater = 1.00.

7.52

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)

Conversion Factors
Viscosity Units and Their Conversion
When a correction for the effects of viscosity
in the liquid orifice sizing formula is needed,
the value of viscosity, expressed in centipoise,is required. Since most liquid data for viscosity
uses other expressions, a convenient method
for conversion is presented below.
The viscosity, µ (Greek mu), in centipoise,
is correctly known as absolute or dynamic
viscosity. This is related to the kinematic
viscosity expression, ν (Greek nu), in
centistokes as follows:µ=νxG
Where:µ = absolute viscosity, centipoiseν = kinematic viscosity, centistokes
G = relative density (water = 1.00)
Most other viscosity units in common usage
are also kinematic units and can be related to
the kinematic viscosity in centistokes, via theaccompanying table. To use this table, obtainthe viscosity from data furnished. Convert thisto ν, in centistokes, then convert to absoluteviscosity µ, in centipoise.
The conversions are approximate but
satisfactory for viscosity correction in liquidsafety valve sizing.

Table 7-21 - Viscosity Conversion
Seconds
Viscosity
Centistokes
ν
1.00
2.56
4.30
7.40
10.3
13.1
15.7
18.2
20.6
32.1
43.2
54.0
65.0
87.60
110.0
132.0
154.0
176.0
198.0
220.0
330.0
440.0
550.0
660.0
880.0
1100.01320.01540.01760.01980.02200.03300.04400.0

Seconds
Saybolt
Universal
ssu
31
35
40
50
60
70
80
90
100
150
200
250
300
400
500
600
700
800
900
1000
1500
2000
2500
3000
4000
5000
6000
7000
8000
9000
10000
15000
20000

Seconds
Saybolt
Furol
ssf
12.95
13.70
14.40
15.24
19.30
23.50
28.00
32.50
41.90
51.60
61.40
71.10
81.00
91.00
100.70150.00200.00250.00300.00400.00500.00600.00700.00800.00900.001000.00
1500.00
2000.00

Seconds
Redwood1(standard)
29.0
32.1
36.2
44.3
52.3
60.9
69.2
77.6
85.6
128.0
170.0
212.0
254.0
338.0
423.0
508.0
592.0
677.0
462.0
896.0
1270.01690.02120.02540.03380.04230.05080.05920.06770.07620.08460.013700.018400.0

Seconds
Redwood2(Admiralty)
5.10
5.83
6.77
7.60
8.44
9.30
10.12
14.48
18.90
23.45
28.00
37.10
46.20
55.40
64.60
73.80
83.00
92.10
138.20184.20230.00276.00368.00461.00553.00645.00737.00829.00921.00-

7.53

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
XI - Capacity Correction Factor for Rupture Disc/Pressure Relief Valve
Combination, Kc
It may be desirable to isolate a pressure relief valve from the process fluid in the vessel that itis protecting. A non-reclosing device such as a rupture disc can be installed upstream of thepressure relief valve to provide this isolation. For example, it may be more economical to install arupture disc made from Inconel and then mount a standard stainless steel pressure relief valve inseries with the disc where the service conditions require such a high alloy material. This rupture
disc/pressure relief valve combination may also be beneficial when the fluid may have entrainedsolids or is viscous. The rupture disc can also provide for a zero leak point during normal vessel
operation.
Since the rupture disc is in the flow path of the pressure relief valve, the ASME Section VIII Codemandates that the pressure relief valve rated capacity be adjusted with a capacity combination
factor (Kc). This correction factor is determined by performing actual flow tests with specificrupture disc and pressure relief valve designs. The materials of construction, minimum size, andminimum burst pressure of the rupture disc must be specified to use this measured correction
factor.
If there has been no combination testing performed then the Kc factor is equal to 0.90.
Table 7-22 lists the combination tests performed with the Crosby J series direct acting springloaded valves. For any other Crosby brand or Anderson Greenwood brand pressure relief valve
product used in series with a rupture disc, use a Kc factor equal to 0.90.

7.54

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Table 7-22 - Capacity Correction Factor for Rupture Disc/PRV Combination (Kv)
Emerson PRV Series
JOS-E/JBS-E

JOS-E/JBS-E

Rupture disc
manufacturer
BS&B

Continental Disc

Disc type
CSR

Minimum disc size(inches)
1.5

Minimum burst pressure(psig)
50

Disc material
Inconel®

JRS
JRS
RLS
RLS
RLS
S90
S90
CDC
CDC-FS
CDCV FSCDCV FSCDCV FSCDCV FSCDCV FSCDCV FSCDCV LL
CDCV LL
CDCV LL
CDCV LL
CDCV LL
CDCV LL
CDCV LL
CDCV LL
DCV
DCV
KBA
Micro X
Micro X
Micro X
Micro X
Micro X
Micro XULTRXMINTRX
STARX
STARX
STARX
STARX
STARX
STARX
STARX
STARX
STARX
STARX
STARX
STARX
STARX
STARX

1
1.5
1
1
2
1
2
1
3
1
1
1.5
1.5
3
3
1
1
1
1.5
1.5
1.5
3
3
3
3
1
1
1
2
2
2
2
1
1
1
1
1
1.5
1.5
1.5
1.5
1.5
1.5
3
3
3
3
3

60
23
138
172
84.5
125
75
60
15
60
60
30
30
15
15
60
60
60
30
30
30
15
15
35
35
60
150
150
80
80
80
80
60
60
60
60
60
30
30
30
30
30
30
15
15
15
15
15

316 SS
Monel®
Monel®
Hastelloy®
Monel®
NickelNickelMonel®/Teflon®
Monel®/Teflon®
316 SS/Teflon®
Hastelloy®/Teflon®
316 SS/Teflon®
Hastelloy®/Teflon®
316 SS/Teflon®
Hastelloy®/Teflon®
316 SS/Teflon®
Hastelloy®/Teflon®
Monel®/Teflon®
316 SS/Teflon®
Monel®/Teflon®
Nickel/Teflon®
316 SS/Teflon®
Monel®/Teflon®
Monel®/Teflon®
316 SS/Teflon®
Monel®
Monel®
Nickel316 SS
Inconel®
Monel®
Nickel316 SS
Hastelloy®
Inconel®
Monel®
Nickel316 SS
Hastelloy®
Inconel®
Monel®
NickelTantalum
316 SS
Hastelloy®
Inconel®
Monel®
NickelKc Factor
0.986
0.993
0.981
0.981
0.972
0.981
0.995
0.994
0.971
0.986
0.985
0.983
0.976
0.973
0.982
0.981
0.978
0.960
0.961
0.959
0.958
0.953
0.953
0.979
0.994
0.978
0.984
0.984
0.990
0.991
0.997
0.988
0.992
0.980
0.987
0.984
0.980
0.981
0.984
0.986
0.989
0.987
0.981
0.978
0.985
0.992
0.991
0.987
0.981

7.55

Pressure Relief Valve Engineering Handbook
Chapter 7 - Engineering Support Information - USCS Units (United States Customary System)
Table 7-22 - Capacity correction factor for rupture disc/PRV combination (Kv) (continued)
Emerson PRV series

Rupture disc
manufacturer

JOS-E/JBS-E

Fike

JOS-E/JBS-E

OSECO

Disc type
ZAP

Minimum disc size(inches)1

Minimum burst pressure(psig)
60

Disc material
Monel®

ZAP
ZAP
ZAP
ZAP
ZAP
ZAP
ZAP
ZAP
ZAP
ZAP
Axius
MRK
MRK
MRK
MRK
Poly-SD CS
Poly-SD DH
SRL
SRX
COV
FAS
PCR

1
1
1
1.5
1.5
1.5
3
3
3
3
1
1
1
3
3
1
1
1
1
2
1.5
1.5

60
60
60
30
30
30
35
35
35
35
15
60
60
35
35
124
32
27
95
31
90
90

316 SS
Inconel®
Nickel316 SS
Monel®
NickelInconel®
Monel®
Nickel316 SS
316 SS
316 SS
Nickel316 SS
NickelAluminum
Aluminum
SS Nickel
NickelMonel®/Teflon®
NickelNickelKc Factor
0.985
0.985
0.988
0.992
0.955
0.955
0.992
0.992
0.982
1.000
0.970
0.987
0.967
0.977
0.982
0.995
0.970
0.997
0.979
0.996
0.979
0.975
0.967

7.56

Pressure Relief Valve Engineering Handbook
Chapter 8 – Engineering Support Information – Metric Units
The following data with charts and tables are included in this chapter
I. Compressibility Factor, Z���������������������������������������������������������������������������������������������������������������8.3
II. Capacity Correction Factor for Back Pressure, Kb����������������������������������������������������������������������8.4
III. Capacity Correction Factor for High Pressure Steam, Kn���������������������������������������������������������8.31
IV. Capacity Correction Factor for Viscosity, Kv�������������������������������������������������������������������������������8.31
V. Capacity Correction Factor for Superheat, Ksh��������������������������������������������������������������������������8.33
VI.	Ratio of Specific Heats, k, and Coefficient, C������������������������������������������������������������������������������8.35
VII. Typical Fluid Properties����������������������������������������������������������������������������������������������������������������8.36
VIII.	Steam Saturation Pressure/Temperature����������������������������������������������������������������������������������8.40
IX.	Orifice Area and Coefficient of Discharge forAnderson Greenwood and Crosby Pressure Relief Valve����������������������������������������������������������8.41
X.	Equivalents and Conversion Factors�������������������������������������������������������������������������������������������8.48
XI. Capacity Correction Factor for Rupture Disc/Pressure Relief Device Combination, Kc���������8.54

8.1

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
The following Figures are included in this chapter
Figure 8-1	Nelson Obert Compressibility Chart������������������������������������������������������������������������������������������������������������������������������������������������������������������8.3
Figure 8-2	Effect of Built-up Back Pressure on Conventional PRV�����������������������������������������������������������������������������������������������������������������������������������8.4
Figure 8-3	Relationship of P2 and P2’�����������������������������������������������������������������������������������������������������������������������������������������������������������������������������������8.6
Figure 8-4 Valve Selection Recommendations for Built-up Back Pressure Installations������������������������������������������������������������������������������������������������8.7
Figure 8-5	Valve Selection Recommendations for Constant Superimposed Back Pressure Installations (no built-up back pressure)����������������������8.8
Figure 8-6 Valve Selection Recommendations for Variable Superimposed Back Pressure Installations����������������������������������������������������������������������8.9
Figure 8-7	Valve Selection Recommendations for Constant Superimposed Back Pressure Installations (with built-up back pressure)�����������������8.10
Figure 8-8 Crosby Series JOS-E Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) Gas/Steam Service����������������������������������8.11
Figure 8-9 Crosby Series JLT-JOS-E Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) Gas/Steam Service �������������������������8.12
Figure 8-10	Crosby Series JBS-E/JLT-JBS-E Balanced Bellows Direct Spring PRV Back Pressure Correction Factor (Kb) Gas/Steam Service ����8.13
Figure 8-11 Crosby Series JLT-JBS-E Balanced Bellows Direct Spring PRV Back Pressure Correction Factor (Kw) Liquid Service ������������������������8.14
Figure 8-12 Crosby Series 800 Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) Gas/Steam Service ��������������������������������������8.15
Figure 8-13 Crosby Series 900 Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) Gas/Steam Service ��������������������������������������8.16
Figure 8-14 Crosby Series BP Back Pressure Correction Factor (Kw) Liquid Service ����������������������������������������������������������������������������������������������������8.17
Figure 8-15 Crosby Series BP Back Pressure Correction Factor (Kb) Gas Service ���������������������������������������������������������������������������������������������������������8.18
Figure 8-16	Anderson Greenwood Series 61 Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) Gas Service ���������������������������8.19
Figure 8-17	Anderson Greenwood Series 63B (-5 Orifice Only) Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) Gas������������8.20
Figure 8-18	Anderson Greenwood Series 63B (-7 Orifice Only) Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) Gas������������8.21
Figure 8-19	Anderson Greenwood Series 81P Back Pressure Correction Factor (Kw) Liquid Service ��������������������������������������������������������������������������8.22
Figure 8-20	Anderson Greenwood Series 81/83/86 Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) Gas/Steam ������������������8.23
Figure 8-21	Anderson Greenwood Series 81P (-8 Orifice Only) Balanced Piston Direct Spring PRV Back Pressure Correction Factor (Kb) Gas������8.24
Figure 8-22	Anderson Greenwood Series 90/9000 POPRV Back Pressure Correction Factor (Kb) Gas Service ���������������������������������������������������������8.25
Figure 8-23	Anderson Greenwood Series 40 Pilot Operated PRV Back Pressure Correction Factor (Kb) Gas/Steam Service ������������������������������������8.26
Figure 8-24	Anderson Greenwood Series 50 Pilot Operated PRV Back Pressure Correction Factor (Kb) Gas/Steam Service ������������������������������������8.27
Figure 8-25	Anderson Greenwood Series 60 Pilot Operated PRV Back Pressure Correction Factor (Kb) Gas/Steam Service ������������������������������������8.28
Figure 8-26	Anderson Greenwood Series LCP Pilot Operated PRV Back Pressure Correction Factor (Kb) Gas Service ���������������������������������������������8.29
Figure 8-27	Anderson Greenwood Series 727 Pilot Operated PRV Back Pressure Correction Factor (Kb) Gas/Steam Service ����������������������������������8.30
Figure 8-28 Correction Factor for High Pressure Steam (Kn)��������������������������������������������������������������������������������������������������������������������������������������������8.31
Figure 8-29 Viscosity Correction Factor (Kv)������������������������������������������������������������������������������������������������������������������������������������������������������������������������8.32
Figure 8-30	Ratio of Specific Heats, k, and Coefficient, C���������������������������������������������������������������������������������������������������������������������������������������������������8.35
The following Tables are included in this chapter
Table 8-1	Superheat Correction Factor����������������������������������������������������������������������������������������������������������������������������������������������������������������������������8.33
Table 8-2	Gas Constant������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������8.35
Table 8-3
Physical Properties for Selected Gases�����������������������������������������������������������������������������������������������������������������������������������������������������������8.36
Table 8-4
Physical Properties for Selected Liquids���������������������������������������������������������������������������������������������������������������������������������������������������������8.38
Table 8-5	Saturated Steam Pressure Table���������������������������������������������������������������������������������������������������������������������������������������������������������������������8.40
Table 8-6
JOS-E/JBS-E/JLT-E Full Nozzle Direct Acting Spring Valves (API Effective Orifice Areas/API Effective Coefficient of Discharge)���������8.42
Table 8-7
JOS-E/JBS-E/JLT-E Full Nozzle Direct Acting Spring Valves (ASME Areas/ASME Coefficient of Discharge)�������������������������������������������8.42
Table 8-8	OMNI 800/900/BP Portable Direct Acting Spring Valves Orifice Areas/Coefficient of Discharge����������������������������������������������������������������8.43
Table 8-9	Series 60 and Series 80 Portable Direct Acting Spring Valves Orifice Areas/API Effective Coefficient of Discharge��������������������������������8.43
Table 8-10 H Series Direct Acting Spring Safety Valves Orifice Areas/Coefficient of Discharge������������������������������������������������������������������������������������8.44
Table 8-11 High Pressure Pilot Operated Valves (API Effective Orifice Areas/Coefficient of Discharge)����������������������������������������������������������������������8.44
Table 8-12 High Pressure Pilot Operated Valves (ASME Areas/ASME Coefficient of Discharge)����������������������������������������������������������������������������������8.45
Table 8-13	Low Pressure Pilot Operated Valves (Set Pressure ≥15 psig) Orifice Areas/Coefficient of Discharge�������������������������������������������������������8.46
Table 8-14	Low Pressure Pilot Operated Valves (Set Pressure <15 psig) Orifice Areas/Coefficient of Discharge�������������������������������������������������������8.46
Table 8-15	Low Pressure Pilot Operated Valves Orifice Areas/Coefficient of Discharge�����������������������������������������������������������������������������������������������8.47
Table 8-16 JB-TD Direct Acting Spring Valves Orifice Areas/Coefficient of Discharge��������������������������������������������������������������������������������������������������8.47
Table 8-17	Equivalents and Conversion Factors����������������������������������������������������������������������������������������������������������������������������������������������������������������8.48
Table 8-18 Pressure Conversions���������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������8.50
Table 8-19	Gas Flow Conversions���������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������8.51
Table 8-20	Liquid Flow Conversions�����������������������������������������������������������������������������������������������������������������������������������������������������������������������������������8.52
Table 8-21 Viscosity Conversion������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������8.53
Table 8-22 Capacity Correction Factor for Rupture Disc/PRV Combination��������������������������������������������������������������������������������������������������������������������8.55

8.2

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
I. Compressibility Factor, Z
The gas and vapor formulas of this handbook are based on perfect
T gas
Tr a perfectlaws. Many real gases and vapors, however, deviate fromgas.
Tc
The compressibility factor Z is used to compensate for the deviationsofreal gases from the ideal gas.
The compressibility factor may be determined from thermodynamic chartssuch as the Nelson Obert compressibility chart shown in Figure 8-1.
Z is a function of the reduced pressure and the reduced temperatureof the gas. The reduced temperature is equal to the ratio of the actual
absolute inlet gas temperature to the absolute critical temperature of
the gas.

Tr

T
Tc

Pr

P
Pc

Where:
Tr =	Reduced temperature
T = 	Inlet fluid temperature, °C + 273
Tc = Critical temperature, °C + 273
The reduced pressure is equal to the ratio of the actual absolute inlet
pressure to the critical pressure of the gas.

Pr

P
Pc

Where:
Pr =	Reduced pressure
P =	Relieving pressure(set pressure + overpressure + atmospheric pressure), bara
Pc = Critical pressure, baraEnter the chart at the value of reduced pressure, move verticallyto the appropriate line of constant reduced temperature.
From this point, move horizontally to the left to read the value of Z.
In the event the compressibility factor for a gas or vapor cannot be
determined, a conservative value of Z = 1 is commonly used.

Figure 8-1 - Nelson Obert Compressibility Chart
1.0
1.0
0.9
0.9
0.8
0.8

Compressibility Factor Z

0.7
0.7
0.6

0.5

0.4

0.3

0.2

0.1
0

0.5

1.0

1.5

2.0

2.5

3.0

Reduced Pressure Pr
8.3

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units

General
Back pressure can exist in any location that is downstream from theactual discharge area of a pressure relief valve. This pressure canbe due to piping causing resistance to flow, pressures from otherequipment discharging to a common header system, or from a flashingfluid being relieved. Without proper consideration of the effects of backpressure, the PRV may experience one, some or all of the following.
• Change in set pressure• Change in reseating pressure• Lift instability• Decrease in actual relieving capacity
This particular section of the engineering handbook will deal with thesizing capacity correction factors that need to be considered for various
types of pressure relief valves.
Built-up Back Pressure
As you recall from Chapter Three, a pressure relief valve whose outlet
is discharging to atmosphere or into a piping system will experiencebuilt-up back pressure. This type of back pressure is only evident after
the valve has opened and is relieving, it does not affect the set pressureof the PRV.
For a conventional PRV, the change in the force balance of the discholder due to back pressure will hinder the upward lifting force. The
conservative rule of thumb is that if the built-up back pressure exceeds
the available overpressure to lift the valve, then a conventional valve
should not be used because the lifting force may not be sufficient for
the valve to operate in a stable fashion. Figure 8-2 illustrates the effectbuilt-up back pressure has upon a conventional PRV design where thereis 10% overpressure available. If there was a fire case contingency where
there may be 21% overpressure, then a curve similar to Figure 8-2 would
show full capacity up to 21% built-up back pressure.
An exception to this conventional valve built-up back pressure andoverpressure relationship is the Crosby brand H series product that isnormally provided for ASME Section I applications. The H series valve isnormally provided with the open spring bonnet design. This opening toatmosphere dramatically decreases the built-up back pressure amountthat acts down on the disc holder. For this valve design, when the H
series valve is in lift with 3% overpressure, the calculated built-up backpressure at the outlet flange of the valve can be up to a maximum of
27.5% of the set pressure.
There is no capacity correction factor in either gas/vapor or liquid
applications for a suitable conventional prv where the valve is exposed tobuilt-up back pressure which is less than the available overpressure. In
other words, the Kb or Kw will be 1.0.
When a balanced direct spring or pilot operated PRV is open and flowing
against a built-up back pressure, the lift of the device should be stable if
properly designed. The built-up back pressure can exceed the available
overpressure for these devices. However, the capacity that the PRV is
able to deliver may be less than expected due to a reduced, but stable,
lift and/or a compressible fluid flow transitions from critical to subcriticalconditions.
The calculation of the magnitude of the built-up back pressure, andthe subsequent design of the outlet piping for a new installation, isoftentimes an iterative process.
• The PRV is initially sized with the assumption of a maximum built-up
back pressure. For instance, in an application that may require theprocess fluid exhaust to be routed via a simple tail pipe discharge
to atmosphere, the sizing for the PRV may assume a built-up backpressure to be 10% of the flowing pressure. This assumption would
allow the use of a conventional direct spring PRV.

• The PRV required minimum orifice is then selected based upon a
Kb = 1.0.
• Once the PRV is selected, the engineer should perform a pressure
drop calculation for the proposed size and style of discharge pipe. In
the example above, the pressure drop through the tailpipe should bedetermined.
• The API Standard 521 will guide the engineer to use the rated capacity
for some types of direct spring operated PRV (recall that safety valvesobtain substantial lift at the set pressure) or the required capacity for a
modulating action pilot operated PRV to calculate the pressure loss inthe discharge piping. This will provide the magnitude of built-up backpressure at the outlet flange of the PRV.
• If this calculated built-up back pressure exceeds 10% then, for thisexample, the tailpipe may need to be redesigned to provide lessresistance to flow. Perhaps enlarging or straightening this fitting ispossible to reduce the built-up back pressure.
• If the outlet piping cannot be changed, then a balanced or pilot
operated PRV may need to be considered and the iterative process
begins again. We will discuss the correction factors for balanced andpilot operated PRVs below.

Figure 8-2
Effect of Built-up Back Pressure on Conventional PRV
100

80
10% Overpressure% Rated Capacity

II. Capacity Correction Factors for Back Pressure, Kb

60

40

20

0
0

10

20

30

40

50

% Built-up Back Pressure to Set Pressure
Per API RP520 and ASME Section VIII BPBU ≤ 10%

8.4

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Superimposed Back Pressure
When the outlet of a PRV is connected to a closed discharge system,
it may not only be exposed to built-up back pressure but may also see
superimposed back pressure. The superimposed back pressure isevident on the downstream side of the PRV before the valve has opened.
This is very common in process plant environments where effluentsare captured or thermally oxidized via common header systems. This
superimposed back pressure may be a constant value but it could vary inthese types of installations.
A conventional, unbalanced PRV can be considered if the superimposedback pressure is a constant value. As we learned in Chapter Three, one
can set the conventional PRV with a bias on the test bench to accountfor this constant superimposed back pressure. All unbalanced Crosby
and Anderson Greenwood brand PRVs have a force balance that willcause a unit-for-unit increase in the in situ opening pressure whensuperimposed back pressure is present. In other words, if there is
3 barg of superimposed back pressure the unbalanced valve will open
3 barg higher than the opening pressure allowed by just the spring
compression. For this example, the spring compression can be set
3 barg lower to compensate for the constant superimposed backpressure. As you recall, this bias is one element of the cold differential
set pressure (CDTP) setting.
A balanced direct acting or pilot operated PRV does not need any test
bench correction for superimposed back pressure. Therefore, when the
superimposed back pressure is variable it is recommended to use theseparticular valve designs.
The calculation of superimposed back pressure is performed byexamining the entire pressure relief disposal system and making
determinations regarding whether or not other devices attached to thesystem may be operating at the time the PRV is to open and then relieve.
These effluent flows are then used with the disposal system piping
geometry to determine what the superimposed back pressure may be at
the outlet flange of the PRV. The maximum superimposed back pressure
should be listed on the PRV data sheet.

The transition from critical to subcritical flow is based upon the criticalpressure of the gas service. This critical pressure is calculated asfollows:

Pcritical

P1

[ ]
2
K+1

k
k-1

Where:
P1 = 	Pset + Overpressure + atmospheric - inlet pressure piping loss,bara
k = ratio of specific heat
If the sum of the built-up back pressure and superimposed backpressure exceed this critical pressure then the capacity will be reduced.
As an example, let us consider the gas as air with a ratio of specificheats equal to 1.4. Let us assume that the absolute relieving pressure(P1) is 10 barg. After performing the calculation above, the criticalpressure will be equal to 5.28 barg. This means that capacity will be
reduced when the total back pressure at the outlet of the discharge
area is greater than 5.28 barg.

Compressible Fluid Back Pressure Correction Charts
There are several figures in this chapter that show back pressure
correction factors for various series of Emerson products used incompressible media service. For example, Figure 8-8 shows the Kb
factor for the Crosby JOS-E conventional PRV and we will use thischart to help explain why these back pressure capacity corrections areneeded.
Properly setting a conventional PRV, such as the Crosby JOS-E, with a
CDTP will provide an adequate lift to meet its certified capacity. This iscontingent upon any built-up back pressure that is developed will not
exceed the available overpressure at the set pressure. In gas service,
there may be a capacity correction factor required for conventional PRVs.
The Kb factor in this case is a result of the flow becoming what is calledsubcritical at the discharge area of the PRV.
When the flow is critical at the discharge area of the PRV it can also
be called “choked flow.” This means that even if the back pressure is
reduced there can be no more flow capacity provided through the PRV.
Once the flow becomes subcritical then any change in back pressure willchange the capacity.

8.5

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
As mentioned above, the calculation of the superimposed and built-up
back pressure gives a value for these pressures at the PRV outlet flange.
The capacity of the PRV is determined by the conditions at the location of
the actual discharge area. For the Crosby JOS-E series valve this is thenozzle outlet. If you look at Figure 8-3, the total calculated superimposedand built-up back pressure is denoted by P2 while it is the P2’ pressure
at the nozzle outlet that determines whether the flow is critical or
subcritical. The outlet of the body of the JOS-E creates additional builtup back pressure that is not accounted for in the total (built-up plus
superimposed) back pressure calculations at the outlet flange, making
the value of P2’ higher than P2.
Therefore, using Figure 8-8 and our previous example where the criticalpressure is 5.28 barg, you will note in the figure that when the calculated
total back pressure is approximately 20% of the flowing pressure we
begin to adjust the capacity with the Kb value. This is well below the
expected 0.528 critical pressure ratio or 5.28 barg critical pressure. Thisis due to the P2’ and P2 relationship. The P2’ is actually above the criticalpressure when the calculated total back pressure at the outlet flange(P2) is reaching 20% of the flowing pressure.
This same P2’ and P2 relationship holds for other valve designs such as
the Crosby balanced bellows and most of the Anderson Greenwood pilot
operated PRVs. This relationship is also a contributor to the liquid Kw
correction factors for various valve designs.
Use the following flow charts (Figures 8-4 through 8-7) to assist withselecting an appropriate Emerson model recommendation and backpressure capacity correction factor.

Figure 8-3
Relationship of P2’ and P2’

Total back pressure at nozzle outlet (P2’)

Calculated superimposed and built-up
back pressure (P2)

8.6

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Figure 8-4
Valve Selection Recommendations for Built-up Back Pressure Installations

Built-up Back Pressure Only

POPRV

JLT-JBS-E
Fig. 8-11

81P
Fig. 8-19

BP
Fig. 8-14

Available
overpressure morethan built-up backpressure?

No

PRV
DesignJOS-E, JOS-JLT-E 800, 900,
61/63B 81/83/86 Kb = 1.0

Balanced DSV
Gas/Steam

Yes
LiquidBalanced
DSV

Service
61P
Fig. 8-21

JBS-E,JLT-JBS-E
JB-TD
Fig. 8-10

Conventional
DSV

ServiceLiquidBP
Fig. 8-15
Gas/Steam

POPRV

PRV
DesignPOPRVService

900, 81P, JOS-JLT-E
Kw = 1.0

LiquidKw = 1.0

Gas/SteamFig. 8-22, 8-23, 8-24, 8-25, 8-26, 8-27

8.7

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Figure 8-5
Valve Selection Recommendations for Constant Superimposed Back Pressure Installations (No Built-up Back Pressure)

Constant Superimposed Only

Conventional
DSV

Set w/CDTP

Balanced
DSV

PRV
DesignServiceLiquidGas/Steam
Gas/Steam

JOS-E
Fig. 8-8Service

800
Fig. 8-12

LiquidPOPRV

JLT-JOS-E, 800, 900
Kw = 1.0

81P
Fig. 8-21

JBS-E, JLT-JBS-E,
JB-TD
Fig. 8-10

BP
Fig. 8-15

BP
Fig. 8-14

61/63B
Fig. 8-16,17,18

81/83/86
Fig. 8-20

JLT-JOS-E
Fig. 8-9900
Fig. 8-13
ServiceLiquid81P
Fig. 8-19

JLT-JBS-E
Fig. 8-11

Kw = 1.0

Gas/SteamFig. 8-22, 8-23, 8-24, 8-25, 8-26

8.8

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Figure 8-6
Valve Selection Recommendations for Variable Superimposed Back Pressure Installations

Built-up with Variable Superimposed

Variable Superimposed Only

Variable and Constant Superimposed

Built-up with Constantand Variable Superimposed

Gas/Steam

Service

Balanced
DSV

PRV
DesignPOPRV

Liquid81P
Fig. 8-21

BP
Fig. 8-15

ServiceLiquidKw = 1.0

Gas/SteamFig. 8-22, 8-23, 8-24, 8-25, 8-26

JBS-E, JLT-JBS-E,
JB-TD
Fig. 8-10

BP
Fig. 8-14

81P
Fig. 8-19

JLT-JBS-E
Fig. 8-11

8.9

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Figure 8-7
Valve Selection Recommendations for Constant Superimposed Back Pressure Installations (with Built-up Back Pressure)

Built-up with Constant Superimposed

61/63B
Fig. 8-16,
17,18

POPRV

JLT-JBS-E
Fig. 8-11

BP
Fig. 8-14

PRV
Design81P
Fig. 8-19

No

Available
overpressure
more thanbuilt-up backpressure?

JOS-E
Fig. 8-8

POPRV

ServicePOPRV

JBS-E,JLT-JBS-E,
JB-TD
Fig. 8-10

JLT-JOS-E
Fig. 8-9800
Fig. 8-12

Gas/Steam

LiquidBP
Fig. 8-15

900
Fig. 8-13

Balanced DSV
Yes

81P
Fig. 8-21

81/83/86
Fig. 8-20

Balanced
DSV

PRV
DesignConv.
DSV

Set w/
CDTP

LiquidPOPRV

JLT-JOS-E, 900
Kw = 1.0

Gas/Steam
Service

ServiceLiquidKw = 1.0

Gas/SteamFig. 8-22, 8-23, 8-24, 8-25, 8-26

8.10

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Figure 8-8
Crosby Series JOS-E Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) - Gas/Steam Service
1.00
k = 1.0

k = Cp/Cv

k = 1.2
0.90

k = 1.4

0.80

k = 1.6
k = 1.8
k = 2.0

0.70

0.60

Kb

0.50

0.40

0.30

0.20

5.58
Kb =
C

[

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = 0.561* (P2/P1) + 0.443

0.00
0.00

0.10

0.20

0.30

0.40

0.50
P2
P1

0.60

0.70

0.80

0.90

1.00

Where:
P2 = Pressure at valve outlet during flow, bara. This is total back pressure (barg) + atmospheric pressure (bara)
P1 = Relieving pressure, bara. This is the set pressure (barg) + overpressure (barg) + atmospheric pressure (bara) - inlet pressure piping loss (barg)
8.11

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Figure 8-9
Crosby Series JLT-JOS-E Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) - Gas Service
1.00
k = 1.0
k = Cp/Cv

k = 1.2

0.90

k = 1.4

k = 1.6

0.80

k = 1.8
k = 2.0
0.70

0.60

Kb

0.50

0.40

0.30

0.20

[

5.58
Kb =
C

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = 0.617* (P2/P1) + 0.389

0.00
0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

P2
P1
Where:
P2 = Pressure at valve outlet during flow, bara. This is total back pressure (barg) + atmospheric pressure (bara)
P1 = Relieving pressure, bara. This is the set pressure (barg) + overpressure (barg) + atmospheric pressure (bara) - inlet pressure piping loss (barg)
8.12

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Figure 8-10
Crosby Series JBS-E/JLT-JBS-E/JB-TD Balanced Bellows Direct Spring PRV Back Pressure Correction Factor (Kb) - Gas/Steam Service
1.00

Pset ≥ 3.45 barg
All Orifices

0.90

0.80

Kb

Pset < 3.45 barg J thru BB2
Orifices

0.70

Pset < 3.45 barg D thru H
Orifices

0.60

0.50

0.40
0.00

0.10

0.20

0.30

0.40

0.50

0.60

P2
Pset
Where:
P2 = P ressure at valve outlet during flow, psig. This is the total back pressure (barg).
Pset = Set pressure (barg)

Note
This figure is based upon 10% overpressure.
The Kb factor shown will be conservative for higher overpressure values.

8.13

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Figure 8-11
Crosby Series JLT-JBS-E Balanced Bellows Direct Spring PRV Back Pressure Correction Factor (Kw) - Liquid Service
1.00

0.90

Kw

0.80

0.70

0.60

0.50
0.00

0.10

0.20

0.30
P2
Pset

0.40

0.50

0.60

Where:
Note
P2 = Pressure at valve outlet during flow, psig. This is the total back pressure (barg). This figure is based upon 10% overpressure.
Pset = Set pressure (barg)
The Kw factor shown will be conservative for higher overpressure values.

8.14

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Figure 8-12
Crosby Series 800 Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) - Gas/Steam Service
1.00
k = 1.0
k = Cp/Cv

k = 1.2

0.90

k = 1.4

k = 1.6

0.80

k = 1.8
k = 2.0
0.70

Kb

0.60

0.50

0.40

0.30

0.20

5.58
Kb =
C

[

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = 0.639* (P2/P1) + 0.365

0.00
0.00

0.10

0.20

0.30

0.40

0.50
P2
P1

0.60

0.70

0.80

0.90

1.00

Where:
P2 = Pressure at valve outlet during flow, bara. This is total back pressure (barg) + atmospheric pressure (bara)
P1 = Relieving pressure, bara. This is the set pressure (barg) + overpressure (barg) + atmospheric pressure (bara) - inlet pressure piping loss (barg)

8.15

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Figure 8-13
Crosby Series 900 Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) - Gas/Steam Service
1.00
k = 1.0

k = Cp/Cv

k = 1.2

0.90

k = 1.4

k = 1.6

0.80

k = 1.8
k = 2.0
0.70

Kb

0.60

0.50

0.40

0.30

0.20

5.58
Kb =
C

[

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = 0.736* (P2/P1) + 0.266
0.00
0.00

0.10

0.20

0.30

0.40

0.50
P2
P1

0.60

0.70

0.80

0.90

1.00

Where:
P2 = Pressure at valve outlet during flow, bara. This is total back pressure (barg) + atmospheric pressure (bara)
P1 = Relieving pressure, bara. This is the set pressure (barg) + overpressure (barg) + atmospheric pressure (bara) - inlet pressure piping loss (barg)

8.16

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Figure 8-14
Crosby Series BP Back Pressure Correction Factor (Kw) - Liquid Service
1.00

Kw

0.90

0.80

0.70
0.00

0.10

0.20

0.30
P2
Pset

0.40

Where:
P2 = Pressure at valve outlet during flow, barg. This is the total back pressure (barg)
Pset = Set pressure (barg)

0.50

0.60

0.70

Note
This figure is based upon 10% overpressure.
The Kw factor shown will be conservative for higher overpressure values.

8.17

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Figure 8-15
Crosby Series BP Back Pressure Correction Factor (Kb) - Gas Service
1.00

0.90

Kb

0.80

0.70

0.60

0.50

0.40
0.00

0.10

0.20

P2
Pset

0.30

Where:
P2 = Pressure at valve outlet during flow, barg. This is the total back pressure (barg)
Pset = Set pressure (barg)

0.40

0.50

Note
This figure is based upon 10% overpressure.
The Kb factor shown will be conservative for higher overpressure values.

8.18

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Figure 8-16
Anderson Greenwood Series 61 Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) - Gas Service
1.00
k = 1.0
k = Cp/Cv

k = 1.2
k = 1.4

0.90

k = 1.6

0.80

k = 1.8
k = 2.0
0.70

Kb

0.60

0.50

0.40

0.30

0.20

5.58
Kb =
C

[

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = 0.727* (P2/P1) + 0.2674
0.00
0.00

0.10

0.20

0.30

0.40

0.50
P2
P1

0.60

0.70

0.80

0.90

1.00

Where:
P2 = Pressure at valve outlet during flow, bara. This is total back pressure (barg) + atmospheric pressure (bara)
P1 = Relieving pressure, bara. This is the set pressure (barg) + overpressure (barg) + atmospheric pressure (bara) - inlet pressure piping loss (barg)

8.19

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Figure 8-17
Anderson Greenwood Series 63B (-5 Orifice Only) Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) - Gas Service
1.00
k = 1.0
k = Cp/Cv

k = 1.2

0.90

k = 1.4

k = 1.6

0.80

k = 1.8
k = 2.0
0.70

Kb

0.60

0.50

0.40

0.30

0.20

[

5.58
Kb =
C

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = 0.723* (P2/P1) + 0.2793
0.00
0.00

0.10

0.20

0.30

0.40

0.50
P2
P1

0.60

0.70

0.80

0.90

1.00

Where:
P2 = Pressure at valve outlet during flow, bara. This is total back pressure (barg) + atmospheric pressure (bara)
P1 = Relieving pressure, bara. This is the set pressure (barg) + overpressure (barg) + atmospheric pressure (bara) - inlet pressure piping loss (barg)

8.20

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Figure 8-18
Anderson Greenwood Series 63B (-7 Orifice Only) Conventional Direct Acting PRV Back Pressure Correction Factor (Kb) - Gas Service
1.00
k = 1.0
k = Cp/Cv

k = 1.2

0.90

k = 1.4

k = 1.6

0.80

k = 1.8
k = 2.0
0.70

Kb

0.60

0.50

0.40

0.30

0.20

5.58
Kb =
C

[

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = 0.723* (P2/P1) + 0.2793

0.00
0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

P2
P1

Where:
P2 = Pressure at valve outlet during flow, bara. This is total back pressure (barg) + atmospheric pressure (bara)
P1 = Relieving pressure, bara. This is the set pressure (barg) + overpressure (barg) + atmospheric pressure (bara) - inlet pressure piping loss (barg)

8.21

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Figure 8-19
Anderson Greenwood Series 81P Back Pressure Correction Factor (Kw) - Liquid Service
1.00

0.90

0.80

Numbered Orifices

0.70

Kw

0.60

0.50
Lettered Orifices

0.40

0.30

0.20

0.10

0.00
0.00

0.10

0.20

0.30
P2
Pset

0.40

0.50

0.60

Where:
Note
P2 = Pressure at valve outlet during flow, barg. This is the total back pressure (barg) This figure is based upon 10% overpressure.
Pset = Set pressure (barg)
The KW factor shown will be conservative for higher overpressure values.

8.22

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Figure 8-20
Anderson Greenwood Series 81/83/86 Conventional Direct Spring PRV Back Pressure Correction Factor (Kb) - Gas/Steam Service
1.00
k = 1.0
k = Cp/Cv

k = 1.2

0.90

k = 1.4

k = 1.6

0.80

k = 1.8
k = 2.0
0.70

Kb

0.60

0.50

0.40

0.30

0.20

5.58
Kb =
C

0.10

[

2

k+1

k
k
k
(PR’) - (PR’ )k- 1]

PR’ = 0.735* (P2/P1) + 0.259

0.00
0.00

0.10

0.20

0.30

0.40

0.50
P2
P1

0.60

0.70

0.80

0.90

1.00

Where:
P2 = Pressure at valve outlet during flow, bara. This is total back pressure (barg) + atmospheric pressure (bara)
P1 = Relieving pressure, bara. This is the set pressure (barg) + overpressure (barg) + atmospheric pressure (bara) - inlet pressure piping loss (barg)

8.23

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Figure 8-21
Anderson Greenwood Series 81P (-8 Orifice Only) Balanced Piston Direct Spring PRV Back Pressure Correction Factor (Kb) - Gas Service
1.00
k = 1.0
k = Cp/Cv

k = 1.2

0.90

k = 1.4

k = 1.6

0.80

k = 1.8
k = 2.0

0.70

Kb

0.60

0.50

0.40

0.30

0.20

5.58
Kb =
C

[

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = 0.7974* (P2/P1) + 0.1969
0.00
0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

P2
P1
Where:
P2 = Pressure at valve outlet during flow, bara. This is total back pressure (barg) + atmospheric pressure (bara)
P1 = Relieving pressure, bara. This is the set pressure (barg) + overpressure (barg) + atmospheric pressure (bara) - inlet pressure piping loss (barg)

8.24

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Figure 8-22
Anderson Greenwood Series 90/9000 - POPRV Back Pressure Correction Factor (Kb) - Gas Service
1.00
k = 1.0
k = Cp/Cv

k = 1.2

0.90

k = 1.4

k = 1.6

0.80

k = 1.8
k = 2.0
0.70

Kb

0.60

0.50

0.40

0.30

0.20

5.58
Kb =
C

[

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = P2/P1
0.00
0.00

0.10

0.20

0.30

0.40

0.50
P2
P1

0.60

0.70

0.80

0.90

1.00

Where:
P2 = Pressure at valve outlet during flow, bara. This is total back pressure (barg) + atmospheric pressure (bara)
P1 = Relieving pressure, bara. This is the set pressure (barg) + overpressure (barg) + atmospheric pressure (bara) - inlet pressure piping loss (barg)

8.25

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Figure 8-23
Anderson Greenwood Series 40 Pilot Operated PRV Back Pressure Correction Factor (Kb) - Gas/Steam Service

1.00
k = 1.0
k = Cp/Cv

k = 1.2

0.90

k = 1.4

k = 1.6

0.80

k = 1.8

k = 2.0

0.70

Kb

0.60

0.50

0.40

0.30

0.20

5.58
Kb =
C

[

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = 0.813* (P2/P1) + 0.197
0.00
0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

P2
P1

Where:
P2 = Pressure at valve outlet during flow, bara. This is total back pressure (barg) + atmospheric pressure (bara)
P1 = Relieving pressure, bara. This is the set pressure (barg) + overpressure (barg) + atmospheric pressure (bara) - inlet pressure piping loss (barg)

8.26

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units

Figure 8-24
Anderson Greenwood Series 50 Pilot Operated PRV Back Pressure Correction Factor (Kb) - Gas/Steam Service
1.00
k = 1.0
k = Cp/Cv

k = 1.2

0.90

k = 1.4

k = 1.6

0.80

k = 1.8
k = 2.0

0.70

0.60

Kb

0.50

0.40

0.30

0.20

5.58
Kb =
C

[

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = 0.584* (P2/P1) + 0.416
0.00
0.00

0.10

0.20

0.30

0.40

0.50
P2
P1

0.60

0.70

0.80

0.90

1.00

Where:
P2 = Pressure at valve outlet during flow, bara. This is total back pressure (barg) + atmospheric pressure (bara)
P1 = Relieving pressure, bara. This is the set pressure (barg) + overpressure (barg) + atmospheric pressure (bara) - inlet pressure piping loss (barg)

8.27

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Figure 8-25
Anderson Greenwood Series 60 Pilot Operated PRV Back Pressure Correction Factor (Kb) - Gas/Steam Service
1.00
k = 1.0
k = 1.2

k = Cp/Cv
0.90

k = 1.4

k = 1.6

0.80

k = 1.8
k = 2.0
0.70

Kb

0.60

0.50

0.40

0.30

0.20

5.58
Kb =
C

[

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = 0.755* (P2/P1) + 0.249
0.00
0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

P2
P1
Where:
P2 = Pressure at valve outlet during flow, bara. This is total back pressure (barg) + atmospheric pressure (bara)
P1 = Relieving pressure, bara. This is the set pressure (barg) + overpressure (barg) + atmospheric pressure (bara) - inlet pressure piping loss (barg)

8.28

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Figure 8-26
Anderson Greenwood Series LCP Pilot Operated PRV Back Pressure Correction Factor (Kb) - Gas Service

1.00
k = 1.0
k = Cp/Cv

k = 1.2

0.90

k = 1.4

k = 1.6

0.80

k = 1.8
k = 2.0

0.70

0.60

Kb

0.50

0.40

0.30

0.20

5.58
Kb =
C

[

2

k+1

k
k
k
(PR’) - (PR’ )
k- 1

]

0.10
PR’ = 0.680* (P2/P1) + 0.323

0.00
0.00

0.10

0.20

0.30

0.40

0.50
P2
P1

0.60

0.70

0.80

0.90

1.00

Where:
P2 = Pressure at valve outlet during flow, bara. This is total back pressure (barg) + atmospheric pressure (bara)
P1 = Relieving pressure, bara. This is the set pressure (barg) + overpressure (barg) + atmospheric pressure (bara) - inlet pressure piping loss (barg)

8.29

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Figure 8-27
Anderson Greenwood Series 727 Pilot Operated PRV Back Pressure Correction Factor (Kb) - Gas/Steam Service

1.00
k = 1.0
k = 1.2

k = Cp/Cv

0.98

k = 1.4
k = 1.6
k = 1.8

0.96

k = 2.0

0.94

Kb

0.92

0.90

0.88

5.58
Kb =
C
0.86

[

2

k+1

k
k
k
(PR’) - (PR’ )k- 1]

PR’ = 0.929* (P2/P1) + 0.07

0.84
0.35

0.40

0.45

0.50

P2
P1

0.55

0.60

0.65

0.70

Where:
P2 = Pressure at valve outlet during flow, bara. This is total back pressure (barg) + atmospheric pressure (bara)
P1 = Relieving pressure, bara. This is the set pressure (barg) + overpressure (barg) + atmospheric pressure (bara) - inlet pressure piping loss (barg)

8.30

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
III. Capacity Correction Factor for High Pressure Steam, Kn
The high pressure steam correction factor Kn is used when steam
relieving pressure P1 is greater than 103.4 bara and less than or equal
to 220.6 bara. This factor has been adopted by ASME to account for thedeviation between steam flow as determined by Napier’s equation andactual saturated steam flow at high pressures. Kn may be calculated by
the following equation or may be taken from Figure 8-28.
USCS Units:Kn =2.763P1 - 1000
3.324P1 - 1061

Where:
Kn = High pressure steam correction factor.
P1 =	Relieving pressure, bara. This is the set pressure + overpressure+ atmospheric pressure.

Figure 8-28
Correction Factor for High Pressure Steam, Kn

bara
220.6
213.7
206.9
200.0
193.1
186.2
179.3
172.4
165.5

IV. Capacity Correction Factors for Viscosity, Kv
When a liquid relief valve is required to flow a viscous fluid there may
be the occasion to adjust the required orifice area for a laminar flow
regime. If the Reynolds Number is less than 100,000 then there will be a
viscosity correction factor, Kv. The procedure to determine the Kv factor
is as follows:
Step One
Calculate the minimum required discharge area using the liquid sizing
formula in Chapter 5 Section V. Assume the Kv factor is equal to 1.0.
Step Two
Select the actual orifice area that will equal or exceed the minimum area
calculated in step one from an appropriate valve in Chapter 8 Section IX
in square centimeters.
Step Three
Calculate the Reynolds Number.

R=(3,133,300)VLGµ A’

Where:
R	 =	Reynolds Number
VL =	Required relieving capacity, m3/h at flowing temperatureG	 =	Specific gravity of process fluid at flowing temperature referred towater at standard conditionsµ =	Absolute viscosity at the flow temperature, centipoises
A’ =	Actual orifice area selected in step two, square millimeters
Step Four
Use the Reynolds Number from step three and obtain the Kv factor from
Figure 8-29.
Step Five
Repeat step one calculation using the Kv from step four. If the minimum
required discharge area is equal to or less than the selected actual
orifice area, A’, from step two, the procedure is complete. If not, chosethe next largest available actual orifice area and repeat steps three
through five.

158.6
151.7
144.8
137.9
131.0
124.1
117.2
110.3

103.4

.95

1.00

1.05

1.10

1.15

1.20

Correction Factor, Kn
All Crosby steam capacity charts will reflect the Kn factor
where applicable.

8.31

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Figure 8-29
Viscosity Correction Factor (Kv)

1

0.9

0.8

Viscosity Correction Factor, Kv

0.7

0.6

0.5

0.4

0.3
101

102

103

104

105

106

Re = Reynold’s Number

8.32

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
V. Capacity Correction Factor for Superheat, Ksh
The steam sizing formulas are based on the flow of dry saturated steam. To size for superheated steam, the superheat correction factor is used
to correct the calculated saturated steam flow to superheated steam flow. For saturated steam Ksh = 1.0. When the steam is superheated, enter
Table 8-1 at the required relieving pressure and read the superheat correction factor under the total steam temperature column.

Table 8-1 - Superheat correction factors
Flowing pressure(bara)
5.0
7.5
10.0
12.5
15.0
17.5
20.0
22.5
25.0
27.5
30.0
32.5
35.0
37.5
40.0
42.5
45.0
47.5
50.0
52.5
55.0
57.5
60.0
62.5
65.0
67.5
70.0
72.5
75.0
77.5
80.0
82.5
85.0
87.5
90.0
92.5
95.0
97.5
100.0
102.5

205
0.991
0.995
0.985
0.981
-

225
0.968
0.972
0.973
0.976
-

250
0.942
0.946
0.950
0.954
0.957
0.959
0.960
0.963
-

275
0.919
0.922
0.925
0.928
0.932
0.935
0.939
0.943
0.946
0.948
0.949
0.951
0.953
0.956
0.959
0.961
-

300
0.896
0.899
0.902
0.905
0.907
0.910
0.913
0.916
0.919
0.922
0.925
0.929
0.933
0.936
0.940
0.943
0.944
0.946
0.947
0.949
0.952

325
0.876
0.878
0.880
0.883
0.885
0.887
0.889
0.892
0.894
0.897
0.899
0.902
0.905
0.908
0.910
0.913
0.917
0.919
0.922
0.926
0.930

Total temperature of superheated steam, °C
350
375
400
425
450
475
0.857 0.839 0.823 0.807 0.792 0.778
0.859 0.841 0.824 0.808 0.793 0.779
0.861 0.843 0.825 0.809 0.794 0.780
0.863 0.844 0.827 0.810 0.795 0.781
0.865 0.846 0.828 0.812 0.796 0.782
0.866 0.847 0.829 0.813 0.797 0.782
0.868 0.849 0.831 0.814 0.798 0.784
0.870 0.850 0.832 0.815 0.799 0.785
0.872 0.852 0.834 0.816 0.800 0.785
0.874 0.854 0.835 0.817 0.801 0.786
0.876 0.855 0.837 0.819 0.802 0.787
0.879 0.857 0.838 0.820 0.803 0.788
0.881 0.859 0.840 0.822 0.804 0.789
0.883 0.861 0.841 0.823 0.806 0.790
0.885 0.863 0.842 0.824 0.807 0.791
0.887 0.864 0.844 0.825 0.808 0.792
0.890 0.866 0.845 0.826 0.809 0.793
0.892 0.868 0.847 0.828 0.810 0.793
0.894 0.870 0.848 0.829 0.811 0.794
0.897 0.872 0.850 0.830 0.812 0.795
0.899 0.874 0.851 0.831 0.813 0.797

500
0.765
0.766
0.766
0.767
0.768
0.769
0.769
0.770
0.771
0.772
0.772
0.773
0.774
0.775
0.776
0.776
0.777
0.778
0.779
0.780
0.780

525
0.752
0.753
0.753
0.754
0.755
0.756
0.756
0.757
0.757
0.758
0.759
0.759
0.760
0.761
0.762
0.762
0.763
0.764
0.765
0.765
0.766

550
0.740
0.740
0.741
0.741
0.742
0.743
0.744
0.744
0.744
0.745
0.746
0.746
0.747
0.748
0.748
0.749
0.749
0.750
0.751
0.752
0.752

575
0.728
0.729
0.729
0.729
0.730
0.731
0.731
0.732
0.732
0.733
0.733
0.734
0.734
0.735
0.735
0.736
0.737
0.737
0.738
0.738
0.739

600
0.717
0.717
0.718
0.718
0.718
0.719
0.720
0.720
0.720
0.721
0.722
0.722
0.722
0.723
0.723
0.724
0.725
0.725
0.725
0.726
0.727

625
0.706
0.707
0.707
0.707
0.708
0.708
0.708
0.709
0.710
0.710
0.710
0.711
0.711
0.711
0.712
0.713
0.713
0.713
0.714
0.714
0.714

-

-

-

-

0.954
0.957
0.960
0.964
0.966
-

0.933
0.937
0.940
0.944
0.946
0.947
0.949
0.951
0.953
0.955
0.957
0.960
0.963
0.966
0.970
0.973
0.977
0.981
0.984

0.902
0.904
0.907
0.910
0.913
0.916
0.919
0.922
0.925
0.928
0.932
0.935
0.939
0.943
0.947
0.950
0.954
0.957
0.959

0.782
0.783
0.783
0.784
0.785
0.786
0.787
0.788
0.788
0.789
0.790
0.791
0.792
0.793
0.794
0.795
0.796
0.797
0.798

0.767
0.768
0.768
0.769
0.769
0.770
0.771
0.772
0.772
0.773
0.774
0.775
0.776
0.776
0.777
0.778
0.779
0.780
0.780

0.753
0.753
0.754
0.754
0.755
0.756
0.756
0.757
0.758
0.758
0.759
0.760
0.760
0.761
0.762
0.763
0.763
0.764
0.764

0.739
0.740
0.740
0.741
0.742
0.742
0.743
0.744
0.744
0.744
0.745
0.746
0.746
0.747
0.747
0.748
0.749
0.749
0.750

0.727
0.727
0.728
0.729
0.729
0.729
0.730
0.730
0.731
0.732
0.732
0.732
0.733
0.734
0.734
0.734
0.735
0.735
0.736

0.715
0.716
0.716
0.716
0.717
0.717
0.717
0.718
0.719
0.719
0.719
0.720
0.721
0.721
0.721
0.722
0.722
0.722
0.723

0.876
0.878
0.880
0.882
0.885
0.887
0.889
0.891
0.893
0.896
0.898
0.901
0.903
0.906
0.909
0.911
0.914
0.917
0.920

0.853
0.855
0.856
0.859
0.860
0.862
0.863
0.865
0.867
0.869
0.871
0.873
0.875
0.877
0.879
0.881
0.883
0.885
0.887

0.833
0.834
0.836
0.837
0.839
0.840
0.842
0.843
0.844
0.846
0.847
0.849
0.850
0.852
0.853
0.855
0.857
0.859
0.860

0.815
0.816
0.817
0.818
0.819
0.820
0.822
0.823
0.824
0.825
0.827
0.828
0.829
0.830
0.832
0.833
0.834
0.836
0.837

0.798
0.798
0.799
0.801
0.802
0.802
0.803
0.805
0.806
0.806
0.807
0.809
0.810
0.811
0.812
0.813
0.814
0.815
0.816

8.33

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Table 8-1 - Superheat correction factors (continued)
Flowing pressure(bara)
105.0
107.5
110.0
112.5
115.0
117.5
120.0
122.5
125.0
127.5
130.0
132.5
135.0
140.0
142.5
145.0
147.5
150.0
152.5
155.0
157.5
160.0
162.5
165.0
167.5
170.0
172.5
175.0
177.5
180.0
182.5
185.0
187.5
190.0
192.5
195.0
197.5
200.0
202.5
205.0
207.5
210.0
212.5
215.0
217.5
220.0

Total temperature of superheated steam, °C
350
375
400
425
450
475
0.961 0.923 0.889 0.862 0.838 0.817
0.962 0.925 0.891 0.863 0.839 0.818
0.963 0.928 0.893 0.865 0.840 0.819
0.964 0.930 0.893 0.865 0.840 0.819
0.964 0.931 0.894 0.865 0.840 0.818
0.965 0.932 0.894 0.865 0.839 0.817
0.966 0.933 0.894 0.864 0.839 0.817
0.967 0.935 0.895 0.864 0.839 0.816
0.967 0.936 0.896 0.864 0.838 0.816
0.968 0.937 0.896 0.864 0.838 0.815
0.969 0.939 0.896 0.864 0.837 0.814
0.971 0.940 0.897 0.864 0.837 0.813
0.972 0.942 0.897 0.863 0.837 0.813
0.976 0.946 0.897 0.863 0.835 0.811
0.978 0.947 0.898 0.862 0.834 0.810
0.948 0.898 0.862 0.833 0.809
0.948 0.898 0.862 0.832 0.808
0.948 0.899 0.861 0.832 0.807
0.947 0.899 0.861 0.831 0.806

205
-

225
-

250
-

275
-

300
-

325
-

-

-

-

-

-

-

-

0.947
0.946

0.899
0.899

0.861
0.860

0.830
0.829

-

-

-

-

-

-

-

0.945
0.945
0.945
0.944
0.944
0.944
0.944
0.944
0.944
0.945
0.945
0.945
0.946
0.948
0.950
0.952
-

0.900
0.900
0.900
0.900
0.900
0.900
0.900
0.900
0.901
0.901
0.901
0.901
0.901
0.901
0.900
0.899
0.899
0.899
0.899
0.898
0.896
0.894
0.892
0.891
0.887

0.859
0.859
0.858
0.857
0.856
0.855
0.854
0.853
0.852
0.851
0.850
0.849
0.847
0.846
0.844
0.842
0.840
0.839
0.837
0.834
0.832
0.829
0.826
0.823
0.820

0.828
0.827
0.826
0.825
0.823
0.822
0.820
0.819
0.817
0.815
0.814
0.812
0.810
0.808
0.806
0.803
0.801
0.798
0.795
0.792
0.790
0.786
0.783
0.779
0.776

500
0.799
0.799
0.800
0.799
0.798
0.797
0.797
0.796
0.796
0.795
0.794
0.792
0.792
0.790
0.789
0.787
0.786
0.785
0.784

525
0.781
0.782
0.782
0.781
0.780
0.780
0.779
0.778
0.777
0.776
0.775
0.774
0.773
0.771
0.770
0.768
0.767
0.766
0.764

550
0.765
0.766
0.766
0.765
0.764
0.763
0.762
0.761
0.760
0.759
0.758
0.757
0.756
0.753
0.752
0.751
0.749
0.748
0.746

575
0.750
0.751
0.751
0.750
0.749
0.748
0.747
0.746
0.745
0.744
0.743
0.741
0.740
0.737
0.736
0.734
0.733
0.732
0.730

600
0.737
0.737
0.737
0.736
0.735
0.734
0.733
0.732
0.731
0.729
0.728
0.727
0.725
0.723
0.721
0.720
0.719
0.717
0.716

625
0.723
0.724
0.724
0.723
0.722
0.721
0.719
0.718
0.717
0.716
0.715
0.713
0.712
0.709
0.707
0.706
0.704
0.703
0.702

0.804
0.803

0.782
0.781

0.763
0.761

0.745
0.743

0.728
0.727

0.714
0.712

0.700
0.698

0.802
0.801
0.799
0.797
0.796
0.794
0.792
0.791
0.789
0.787
0.785
0.783
0.781
0.778
0.776
0.773
0.770
0.767
0.764
0.761
0.758
0.754
0.750
0.746
0.743

0.779
0.778
0.776
0.774
0.773
0.771
0.769
0.767
0.765
0.763
0.761
0.758
0.756
0.753
0.750
0.748
0.745
0.742
0.738
0.735
0.732
0.728
0.724
0.720
0.716

0.759
0.757
0.756
0.754
0.752
0.750
0.748
0.746
0.744
0.742
0.739
0.737
0.734
0.732
0.729
0.726
0.723
0.720
0.717
0.713
0.710
0.706
0.702
0.698
0.694

0.741
0.739
0.738
0.736
0.734
0.732
0.730
0.728
0.725
0.723
0.720
0.718
0.715
0.713
0.710
0.707
0.704
0.701
0.697
0.694
0.691
0.686
0.682
0.679
0.674

0.725
0.723
0.721
0.719
0.717
0.715
0.713
0.711
0.709
0.706
0.704
0.701
0.698
0.696
0.693
0.690
0.687
0.683
0.680
0.677
0.673
0.669
0.665
0.661
0.657

0.710
0.708
0.706
0.704
0.702
0.700
0.698
0.696
0.694
0.691
0.689
0.686
0.683
0.681
0.677
0.674
0.671
0.668
0.665
0.661
0.658
0.654
0.650
0.646
0.641

0.696
0.694
0.692
0.690
0.688
0.686
0.684
0.681
0.679
0.677
0.674
0.671
0.669
0.666
0.663
0.660
0.657
0.654
0.651
0.647
0.643
0.640
0.636
0.631
0.627

8.34

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
VI. Ratio of Specific Heats, k, and Coefficient, C
The following formula equates the ratio of specific heats (k) to the coefficient, C, used in sizing methods for gases and vapors.
Figure 8-30 and Table 8-2 provide the calculated solution to this formula.

( )
2
k +1

C = 3.948 k

k+1
k-1

Where:
k = Ratio of specific heats

Figure 8-30
Gas Constant
3.1

3.0

Coeficient, C

2.9

2.8

2.7

2.6

2.5

2.4
1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

Ratio of Specific Heats, k

Table 8-2 - Gas Constant Values
k

C

k

C

k

C

k

C

k

C

1.01
1.02
1.03
1.04
1.05
1.06
1.07
1.08
1.09
1.10
1.11
1.12
1.13
1.14
1.15
1.16
1.17
1.18
1.19
1.20

2.404
2.412
2.421
2.430
2.439
2.447
2.456
2.464
2.472
2.481
2.489
2.497
2.505
2.513
2.521
2.529
2.537
2.545
2.553
2.560

1.21
1.22
1.23
1.24
1.25
1.26
1.27
1.28
1.29
1.30
1.31
1.32
1.33
1.34
1.35
1.36
1.37
1.38
1.39
1.40

2.568
2.570
2.583
2.591
2.598
2.605
2.613
2.620
2.627
2.634
2.641
2.649
2.656
2.663
2.669
2.676
2.683
2.690
2.697
2.703

1.41
1.42
1.43
1.44
1.45
1.46
1.47
1.48
1.49
1.50
1.51
1.52
1.53
1.54
1.55
1.56
1.57
1.58
1.59
1.60

2.710
2.717
2.723
2.730
2.736
2.743
2.749
2.755
2.762
2.768
2.774
2.780
2.786
2.793
2.799
2.805
2.811
2.817
2.823
2.829

1.61
1.62
1.63
1.64
1.65
1.66
1.67
1.68
1.69
1.70
1.71
1.72
1.73
1.74
1.75
1.76
1.77
1.78
1.79
1.80

2.833
2.840
2.846
2.852
2.858
2.863
2.869
2.874
2.880
2.886
2.891
2.897
2.902
2.908
2.913
2.918
2.924
2.929
2.934
2.940

1.81
1.82
1.83
1.84
1.85
1.86
1.87
1.88
1.89
1.90
1.91
1.92
1.93
1.94
1.95
1.96
1.97
1.98
1.99
2.00

2.945
2.950
2.955
2.960
2.965
2.971
2.976
2.981
2.986
2.991
2.996
3.001
3.006
3.010
3.015
3.020
3.025
3.030
3.034
3.039

8.35

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
VII. Typical Fluid Properties
The specific heat ratios listed herein have been obtained from numerous sources. They may vary from values available to the reader.
Exercise caution when selecting the specific heat ratio.

Table 8-3 - Physical Properties for Selected Gases
Gas

Empirical formula

Molecular weightM

Specific heat ratiok

Gas constantC

Acetone

C3H6O

58.08

1.12

2.497

Acetylene (Ethyne)

C2H2
-

26.04

1.26

2.605

28.97

1.40

2.703

17.03

1.31

2.641

Argon

NH3
Ar

39.95

1.67

2.869

Benzene (Benzol or Benzole)

C6H6

78.11

1.12

2.497

Boron Trifluoride

BF3

67.82

1.2

2.560

Butadiene-1,3 (Divinyl)

C4H6

54.09

1.12

2.497

Butane (Normal Butane)

C4H10

58.12

1.09

2.472

Butylene (1-Butene)

56.11

1.11

2.489

Carbon Dioxide

C4H8
CO2

Carbon Disulfide (C. Bisulfide)
Carbon Monoxide

CS2
CO

44.01
76.13
28.01

1.29
1.21
1.40

2.627
2.568
2.703

Carbon Tetrachloride

CCI4

153.82

1.11

2.489

Chlorine

Cl2

70.91

1.36

2.676

Chloromethane (Methyl Chloride)

CH3Cl

50.49

1.28

2.620

Cyclohexane

C6H12

84.16

1.09

2.472

Cyclopropane (Trimethylene)

C3H6

42.08

1.11

2.489

Decane-n

C10H22

142.29

1.04

2.430

Diethylene Glycol (DEG)

C4H10 O3

106.17

1.07

2.456

Diethyl Ether (Methyl Ether)

C2H6O
-

46.07

1.11

2.489

165.00147.00

1.05
1.00

2.439
2.401
2.553

Air
Ammonia, Anhydrous

Dowtherm A
Dowtherm E
Ethane

C2H6

30.07

1.19

Ethyl Alcohol (Ethanol)

C2H6O

46.07

1.13

2.505

Ethylene (Ethene)

C2H4

28.05

1.24

2.591

Ethylene Glycol

C2H6O2

62.07

1.09

2.472

Ethylene Oxide

C2H4 O

44.05

1.21

2.568

12, Dichlorodifluoromethane

CCI2F2120.93

1.14

2.513

13, Chlorotrifluoromethane

CCIF3

104.47

1.17

2.537

13B1, Bromotrifluoromethane

CBrF3

148.93

1.14

2.513

22, Chlorodifluoromethane

CHCIF2

86.48

1.18

2.545

115, Chloropentafluoroethane

C2CIF5154.48

1.08

2.464

Glycerine (Glycerin or Glycerol)

C3H8 O3

92.10

1.06

2.447

Fluorocarbons:

8.36

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Table 8-3 - Physical properties for selected gases (continued)
Gas
Helium

Empirical formulaHe

Molecular weightM
4.00

Specific heat ratiok
1.67

Gas constant
C
2.869
2.439

Heptane

C7H16

100.21

1.05

Hexane

C6H14

86.18

1.06

2.447

Hydrogen

2.02

1.41

2.710

Hydrogen Chloride, Anhydrous

H2
HCl

36.46

1.41

2.710

Hydrogen Sulfide

H2 S

34.08

1.32

2.649

Isobutane (2-Methylpropane)

C4H10

58.12

1.10

2.481

Isobutane (2-Methyl-1,3butadiene)

C5H8

68.12

1.09

2.472

Isopropyl Alcohol (Isopropanol)

C3H8 O
Kr

60.10

1.09

2.472

83.80

1.71

2.891

CH4
CH4O

16.04

1.31

2.641

32.04

1.20

2.560

KryptonMethane
Methyl Alcohol (Methanol)
Methylanmines, Anhydrous:
Monomethylamine (Methylamine)

CH5N

31.06

1.02

2.412

Dimethylamine

C2H7N

45.08

1.15

2.521
2.545

Triethylamine

C3H9N

59.11

1.18

Methyl Mercapton (Methylamine)

CH4 S

48.11

1.20

2.560

Naphthalene (Naphthaline)

128.17

1.07

2.456

Natural Gas (Relative Density = 0.60)
Neon

C10H8
Ne

17.40
20.18

1.27
1.64

2.613
2.852

Nitrogen

N2

28.01

1.40

2.703

Nitrous Oxide

N2O

44.01

1.30

2.634

OctaneC8H18

114.23

1.05

2.439

Oxygen

O2

32.00

1.40

2.703

Pentane

C5H12

72.15

1.07

2.456

Propadiene (Allene)

C3H4

40.07

1.69

2.880

Propane

C3H8

44.10

1.13

2.505

Propylene (Propene)

C3H6

42.08

1.15

2.521

Propylene Oxide

C3H6O

58.08

1.13

2.505

Styrene

C8H8

104.15

1.07

2.456

Sulfur Dioxide

SO2

64.06

1.28

2.620

Sulfur Hexafluoride

SF6

146.05

1.09

2.472

Steam

H2O

18.02

1.31

2.641

Toluene (Toluol or Methylbenzene)

C7H8

92.14

1.09

2.472

Triethylene Glycol (TEG)

C6H14 O4

150.18

1.04

2.430

Vinyl Chloride Monomer (VCM)

62.50

1.19

2.553

Xenon

C2H3ClXe

131.30

1.65

2.858

Xylene (p-Xylene)

C8H10

106.17

1.07

2.456

8.37

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Table 8-4 - Physical Properties for Selected Liquids

Relative Density G
Water = 1

Fluid Temperature
°C

C2H4

0.779

20

C2H4 O2

1.051

20

Acetone

C3H6O

0.792

20

Ammonia, Anhydrous

NH3

0.666

20

0.88-0.94
1.01

15.6
15.6

Fluid

Empirical Formula

Acetaldehyde
Acetic Acid

Automotive Crankcase and Gear Oils:
SAE-5W Through SAE 150
Beer-

Benzene (Benzol)

C6H6

0.880

20

Boron Trifluoride

BF3

1.57

-100
20

Butadiene-1,3

C4H6

0.622

Butane-n (Normal Butane)

C4H10

0.579

20

Butylene (1-Butene)

C4H8

0.600

20

Carbon Dioxide

CO2

1.03

-20

Carbon Disulphide (C. Bisulphide)

CS2

1.27

20

Carbon Tetrachloride

CCl4

1.60

20

Chlorine

Cl2

1.42

20

Chloromethane (Methyl Chloride)

0.921

20

Crude Oils:
32.6 Deg API
35.6 Deg API
40 Deg API
48 Deg API

CH3Cl
-

0.862
0.847
0.825
0.79

15.6
15.6
15.6
15.6

Cyclohexane

C6H12

0.780

20

Cyclopropane (Trimethylene)

C3H6

0.621

20

Decane-n

C10H22
-

0.731

20

0.82-0.95

15.6

Diesel Fuel Oils
Diethylene Glycol (DEG)

C4H10 O3

1.12

20

Dimethyl Ether (Methyl Ether)

0.663

20

Dowtherm A
Dowtherm E

C2H6O
-

0.998
1.087

20
20

Ethane

C2H6

0.336

20

Ethyl Alcohol (Ethanol)

C2H6O

0.79

20

Ethylene (Ethene)

C2H4

0.569

-104

Ethylene Glycol

C2H6O21.115

20Ethylene Oxide

C2H4 O

0.901

20
20

Fluorocarbons:
R12, Dichlorodifluoromethane

CCl2F2

1.34

R13, Chlorotrifluoromethane

CClF3

0.916

20

R13B1, Bromtrifluoromethane

CBrF3

1.58

20

R22, Chlorodifluoromethane

CHClF2

1.21

20

R115, Chloropentafluoroethane

C2ClF5-1.31

20

0.82-0.95

15.6

Fuel Oils, Nos. 1, 2, 3, 5 and 6

8.38

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Table 8-4 - Physical Properties for Selected Liquids (continued)

Relative Density G:
Water = 1
0.68-0.74

Fluid Temperature
°C
15.6

Fluid
Gasolines

Empirical formula-

Glycerine (Glycerin or Glycerol)

C3H8 O3

1.26

20

Heptane

C7H16

0.685

20

Hexane0.660

20

Hydrochloric Acid

C6H14
HCl

1.64

15.6

Hydrogen Sulphide

H2 S

0.78

20

Isobutane (2-Methylpropane)

C4H10

0.558

20

Isoprene (2-Methyl-1,3-Butadiene)

C5H8

0.682

20

Isopropyl Alcohol (Isopropanol)

0.786

20

Jet Fuel (average)
Kerosene

C3H8 O-

0.82
0.78-0.82

15.6
15.6

Methyl Alcohol (Methanol)

CH4 O

0.792

20

Methylamines, Anhydrous:
Monomethylamine (Methylamine)

CH5N

0.663

20

Dimethylamine

C2H7N

0.656

20
20

Trimethylamine

C3H9N

0.634

Methyl Mercapton (Methanethiol)

CH4 S

0.870

20

Nitric Acid

HNO3

1.5

15.6

Nitrous Oxide

N2O

1.23

-88.5

Octane

C8H18

0.703

20

Pentane

C5H12

0.627

20

Propadiene (Allene)

C3H4

0.659

-34.4
20

Propane

C3H8

0.501

Propylene (Propene)

C3H6

0.514

20

Propylene Oxide

C3H6O

0.830

20

Styrene

C8H8

0.908

20

Sulfur Dioxide

SO2

1.43

20

Sulphur Hexafluoride

SF6

1.37

20

95-100%60%
20%

H2 SO4
-

1.839
1.50
1.14

20
20
20

Toluene (Toluol or Methylbenzene)

C7H8

0.868

20

Triethylene Glycol (TEG)

C6H12O4

1.126

20

Vinyl Chloride Monomer (VCM)

C2H3Cl0.985-20

Water, fresh

H2O
-

1.00

20

Water, sea

1.03

20

Xylene (p-Xylene)

C8H10

0.862

20

Sulphur Acid:

8.39

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
VIII. Saturated Steam Pressure Table
Table 8-5 - Saturation Pressure (bara)/Temperature (°C)
Pressure
bara
1.01
1.03
1.38
1.72
2.07
2.41
2.76
3.10
3.45
3.79
4.14
4.48
4.83
5.17
5.52
5.86
6.21
6.55
6.90
7.24
7.58
7.93
8.27
8.62
8.96
9.31
9.65
10.00
10.30
11.00
11.70
12.40
13.10
13.80
14.50
15.20
15.90
16.50
17.20
17.90
18.60
19.30
20.00
20.70
22.10
23.40
24.80
26.20
27.60
29.00
30.30
31.70
33.10
34.50

Temperature
deg C
100.0
100.7
108.9
115.6
121.3
126.3
130.7
134.7
138.3
141.7
144.8
147.8
150.5
153.1
155.6
157.9
160.2
162.3
165.3
166.3
168.2
170.1
171.8
173.6
175.2
176.8
178.3
179.9
181.3
184.2
186.9
189.5
191.9
194.3
196.6
198.8
200.9
203.0
205.0
206.9
208.8
210.6
212.4
214.1
217.4
220.6
223.8
226.4
229.2
231.9
23.40
236.9
239.3
241.7

Pressure
bara
35.9
37.2
38.6
40.0
41.4
42.7
44.1
45.5
46.9
48.3
49.6
51.0
52.4
53.8
55.2
56.5
57.9
59.3
60.7
62.1
63.4
64.8
66.2
67.6
69.0
72.4
75.8
79.3
82.7
86.2
89.6
93.1
96.5
100.0
103.0
110.0
117.0
124.0
131.0
137.0
144.0
151.0
158.0
165.0
172.0
179.0
186.0
193.0
200.0
207.0
213.0
220.0
221.0

Temperature
deg C
243.9
246.1
248.2
250.3
252.3
254.3
256.2
258.1
259.9
261.7
263.4
265.2
266.8
268.5
270.1
271.7
273.3
274.8
276.3
277.8
279.2
280.6
282.1
283.4
284.8
288.1
291.3
294.3
297.3
300.2
303.0
305.7
308.4
310.9
313.4
318.3
322.8
327.2
331.4
335.4
339.3
343.1
346.6
350.1
353.4
356.6
359.7
362.8
365.7
368.5
371.3
373.9
374.2

8.40

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
IX. Anderson Greenwood and Crosby Pressure Relief Valves
Orifice Area and Coefficient of Discharge
As mentioned in Chapter Three and Six, the use of the proper orifice area (A) and coefficient ofdischarge (K) in the sizing formulas presented in this handbook are critical to determining thecorrect valve size. For some valve designs, two sets of values are published.
One set, the effective area and effective coefficient of discharge, are published by API in
Standard 526, Flanged Steel Pressure Relief Valves and Standard 520 part I, Sizing, Selection
and Installation of Pressure Relieving Devices. These values are independent of any specificvalve design and are used to determine a preliminary pressure relief valve size. The “effective”
coefficient of discharge is 0.975 for gases, vapors and steam, and 0.650 for liquids.
Where applicable, a second set of areas and discharge coefficients is used to determine the“rated” capacity of a valve using the “actual” orifice area and “rated” coefficient of discharge.
Rated coefficients are established by regulatory bodies like ASME and “actual” areas are
published by the manufacturer.
It is important to remember that the effective area and effective coefficient of discharge are used
only for the initial selection. The actual orifice area and rated coefficient of discharge must always
be used to verify the actual capacity of the pressure relief valve.
In no case should an effective area or effective coefficient of discharge be used with actual area or
rated coefficient of discharge. Sizing errors can be made if the effective values are mixed with theactual values.
The following tables provide orifice areas and coefficient of discharge for Anderson Greenwood
and Crosby pressure relief valves. Once again, where applicable, there is a table with API“effective” values and a separate table with ASME “rated” and “actual” values.
Do not mix values from these tables.

8.41

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Table 8-6 - JOS-E/JBS-E/JLT-E Full Nozzle Direct Acting Spring Valves

API Effective Orifice Area and Coefficient of Discharge

Minimum
inlet size
mm
25
25
40
40
40
50
80
80
100
100
100
150
150
200

Orificedesignation
D
E
F
G
H
J
K
L
M
N
P
Q
R
T

Gas (mm2)
SeriesJOS-E, JBS-E, JLT-JOS-E, JLT-JBS-E
K = 0.975
71.00
126.50198.10324.50506.50830.301186.00
1841.00
2333.00
2800.00
4116.00
7129.00
10320.00
16770.00

Liquid (mm2)SeriesJLT-JOS-E, JLT-JBS-E
K = 0.650
71.00
126.50198.10324.50506.50830.301186.00
1841.00
2333.00
2800.00
4116.00
7129.00
10320.00
16770.00

Steam (mm2)SeriesJOS-E, JBS-E
K = 0.975
71.00
126.50198.10324.50506.50830.301186.00
1841.00
2333.00
2800.00
4116.00
7129.00
10320.00
16770.00

Table 8-7 - JOS-E/JBS-E/JLT-E Full Nozzle Direct Acting Spring Valves

ASME Actual Orifice Area and Rated Coefficient of Discharge

Minimum
inlet size
mm
25
25
40
40
40
50
80
80
100
100
100
150
150
200
200

Orificedesignation
D
E
F
G
H
J
K
L
M
N
P
Q
R
T
T2

Air/gas (mm2)SeriesJOS-E, JBS-E
K = 0.865
80.00
142.80223.90366.10572.20937.401339.00
2078.00
2622.00
3161.00
4649.00
8046.00
11650.00
18940.00
20300.00

Air/gas (mm2)SeriesJLT-JOS-E, JLT-JBS-E
K = 0.870
80.00
142.80223.90366.10572.20937.401339.00
2078.00
2622.00
3161.00
4649.00
8046.00
11650.00
18940.00
20300.00

Liquid (mm2)SeriesJLT-JOS-E, JLT-JBS-E
K = 0.656
80.00
142.80223.90366.10572.20937.401339.00
2078.00
2622.00
3161.00
4649.00
8046.00
11650.00
18940.00
20300.00

Steam (mm2)SeriesJOS-E, JBS-E
K = 0.865
80.00
142.80223.90366.10572.20937.401339.00
2078.00
2622.00
3161.00
4649.00
8046.00
11650.00
18940.00
20300.00

8.42

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Table 8-8 - OMNI 800/900/BP Portable Direct Acting Spring Valves

ASME Actual Orifice Area and Rated Coefficient of Discharge

Minimum
Inlet Size
mm
15
15
18
18
25
40
40

Orifice
Desig nation
-5
-6
-5
-6
-7
-8
-9

Series 800
K = 0.877

Gas (mm2)
Section VIII
Series 900
K = 0.878

Series BP
K = 0.841

80.20
141.90222.00366.1054.84
80.20
141.90223.90366.10

60.00
87.74
-

Liquid (mm2)Section VIII
Series 900
Series BP
K = 0.662
K = 0.631
54.84
80.20
141.90223.90366.10

60.00
87.74
-

Steam (mm2)Section VIII
Series 800
Series 900
K = 0.877
K = 0.878
80.20
141.90222.00366.1054.84
80.20
141.90223.90366.10

Table 8-9 - Series 60 and Series 80 Portable Direct Acting Spring Valves

Minimum
Inlet Size
mm
15
15
15
18
18
18
40
40
50
50

Orifice
Designation
-4
-5
-6
-4
-7
-8
F
G
H
J

ASME Actual Orifice Area and Rated Coefficient of Discharge
Gas (mm2)
Section VIII
81/83
81P
61
63B
K = 0.816
K = 0.816
K = 0.877
K = 0.847
31.61
49.03
71.00
71.00
96.17
126.50126.5
198.10324.50506.50830.30-Liquid (mm2)Section VIII
81P
K = 0.720
31.61
126.50324.50830.30

Steam (mm2)Section VIII86
K = 0.816
31.61
31.61
126.50830.30

8.43

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Table 8-10 - H Series Direct Acting Spring Safety Valves

Minimum
Inlet size
mm
30
30
40
40
40
40
40
50
50
50
50
60
60
60
80
80
80
80
80
100
100
100
150
150
150
150

ASME Actual Orifice Area and Rated Coefficient of Discharge
Steam (mm2)Section I/Section VIIIHCIHSJ
ISOFLEX
Orifice
K = 0.878
K = 0.878
Designation
F
G
F
198.1
G
324.5
H
506.5
H2
641.3
J
H
506.5
J
830.3
J2
923.2
K
K
1186.0K2
1642.0L
K
1186.0L
1841.0L22156.0M
2323.02565.0M2
N2800.0P4116.0P24561.0Q
7129.0Q27903.0R
10320.0
RR
12440.0

HE
ISOFLEXK = 0.877
1186
1642
2323
2565
4559
-

Table 8w-11 - High Pressure Pilot Operated Valves

Minimum
Inlet
mm
25
25
25
40
40
50
50
50
80
80
100
100
100
150
150
200

Orifice
Designation
D
E
F
G
H
G
H
J
K
L
M
N
P
Q
R
T

API Effective Orifice Area and Coefficient of Discharge
Gas (mm2)
Liquid (mm2)SeriesSeries200/400/800
500
727
400/800
500
K = 0.975
K = 0.975
K = 0.975
K = 0.650
K = 0.650
71.0
71.0
126.5
126.5
198.1
198.1
198.1
198.1
324.5
324.5
506.5
506.5
506.5
506.5
324.5
324.5
324.5
506.5
506.5
506.5
830.3
830.3
830.3
830.3
830.3
1186.01186.01186.01841.01841.01841.01841.01841.02323.02323.02323.02800.02800.02800.04116.04116.04116.04116.04116.07129.07129.07129.010320.010320.010320.010320.010320.016770.016770.016770.016770.016770.0

Steam (mm2)Series500727
K = 0.975
K = 0.975
198.1
506.5
324.5
506.5
830.3
830.3
1186.01841.01841.02323.02800.04116.04116.07129.010320.010320.016770.016770.0

8.44

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Table 8-12 - High Pressure Pilot Operated Valves

ASME Actual Orifice Area and Rated Coefficient of Discharge
Gas
Min.
Section VIII
Inlet
Orifice
500
LCP
Size
Designation 200/400/800
25D
A = 132.2 mm2
K = 0.627
25E
A = 229.7 mm2
K = 0.627
25
F
A = 230.3 mm2
K = 0.877
25
A = 506.4 mm2
K = 0.860
40G
A = 536.1 mm2
K = 0.627
40
H
A = 589.0 mm2 A = 589.0 mm2
K = 0.877
K = 0.877
40
A = 1140 mm2
K = 0.860
40FB
A = 965.2 mm2 A = 965.2 mm2
K = 0.860
K = 0.860
50G
A = 548.4 mm2
K = 0.627
50
H
A = 846.4 mm2
K = 0.627
50J
A = 965.2 mm2 A = 965.2 mm2
K = 0.877
K = 0.877
50
A = 2027 mm2
K = 0.860
50FBA = 1868 mm2 A = 1868 mm2
K = 0.860
K = 0.86080J
A = 1376 mm2
K = 0.627
80
K
A = 1963 mm2
K = 0.627
80
L
A = 2140 mm2 A = 2140 mm2
K = 0.877
K = 0.877
80
A = 4561 mm2
K = 0.860
80FBA = 4344 mm2 A = 4344 mm2
K = 0.860
K = 0.860
100
L
A = 3051 mm2
K = 0.627
100
M
A = 3845 mm2
K = 0.627
100
N
A = 4637 mm2
K = 0.627
100
P
A = 4932 mm2 A = 4932 mm2
K = 0.877
K = 0.877
100
FB
A = 6941 mm2 A = 6941 mm2
K = 0.860
K = 0.860-

LiquidSection VIII
727
400/800
500
A = 142.6 mm2
K = 0.491
A = 229.7 mm2
K = 0.491
A = 230.3 mm2
K = 0.766
A = 587.7 mm2
K = 0.491
A = 589.0 mm2 A = 589.0 mm2
K = 0.766
K = 0.766
A = 965.2 mm2 A = 965.2 mm2
K = 0.712
K = 0.712
A = 405.8 mm2 A = 648.4 mm2
K = 0.788
K = 0.491
A = 632.9 mm2 A = 964.5 mm2
K = 0.788
K = 0.491
A = 1055 mm2 A = 965.2 mm2 A = 965.2 mm2
K = 0.788
K = 0.766
K = 0.766
A = 1868 mm2 A = 1868 mm2
K = 0.712
K = 0.712
A = 1661 mm2
K = 0.491
A = 1482 mm2 A = 2137 mm2
K = 0.788
K = 0.491
A = 2295 mm2 A = 2140 mm2 A = 2140 mm2
K = 0.788
K = 0.766
K = 0.766
A = 4344 mm2 A = 4344 mm2
K = 0.712
K = 0.712
A = 3685 mm2
K = 0.491
A = 2906 mm2 A = 4119 mm2
K = 0.788
K = 0.491
A = 3500 mm2 A = 4554 mm2
K = 0.788
K = 0.491
A = 5104 mm2 A = 4561 mm2 A = 4561 mm2
K = 0.788
K = 0.766
K = 0.766
A = 6941 mm2 A = 6941 mm2
K = 0.712
K = 0.712-

Steam
Section VIII500
A = 589.0 mm2
K = 0.877
A = 965.2 mm2
K = 0.860
A = 965.2 mm2
K = 0.877
A = 1868 mm2
K = 0.860
A = 2140 mm2
K = 0.877
A = 4344 mm2
K = 0.860
A = 4932 mm2
K = 0.877
A = 6941 mm2
K = 0.860-

727
A = 405.8 mm2
K = 0.788
A = 632.9 mm2
K = 0.788
A = 1055 mm2
K = 0.788
A = 1482 mm2
K = 0.788
A = 2295 mm2
K = 0.788
A = 2906 mm2
K = 0.788
A = 3500 mm2
K = 0.788
A = 5104 mm2
K = 0.788-

Economizer
Section I
5100
A = 589.0 mm2
K = 0.876 (steam)
K = 0.759 (water)
A = 965.2 mm2
K = 0.849 (steam)
K = 0.709 (water)
A = 965.2 mm2
K = 0.876 (steam)
K = 0.759 (water)
A = 1868 mm2
K = 0.849 (steam)
K = 0.709 (water)
A = 2140 mm2
K = 0.876 (steam)
K = 0.759 (water)
A = 4344 mm2
K = 0.849 (steam)
K = 0.709 (water)
A = 6941 mm2
K = 0.849 (steam)
K = 0.709 (water)

8.45

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Table 8-12 - High Pressure Pilot Operated Valves (continued)
ASME Actual Orifice Area and Rated Coefficient of Discharge
Gas
Min.
Section VIII
Inlet
Orifice200/400/800
500
LCP
Size
Designation
150
Q
K = 0.627
A =12000 mm2 A =12000 mm2
150
R
K = 0.877
K = 0.877
A =15050 mm2 A =15050 mm2
150
FB
K = 0.860
K = 0.860
150
T
A = 19730 mm2 A = 19730 mm2
K = 0.877
K = 0.877
150
8FB 8x8
A = 20750 mm2 A = 20750 mm2
K = 0.860
K = 0.860
150
8FB 10A = 28500 mm2 A = 28500 mm2
K = 0.860
K = 0.860
200FBA = 46460 mm2 A = 46460 mm2
K = 0.860
K = 0.860-

LiquidSection VIII
727
400/800
500
K = 0.788
K = 0.491
A =12900 mm2 A = 10260 mm2 A = 10260 mm2
K = 0.788
K = 0.766
K = 0.766
A = 15050 mm2 A = 15050 mm2
K = 0.712
K = 0.712
A = 20970 mm2 A = 18240 mm2 A = 18240 mm2
K = 0.788
K = 0.766
K = 0.766
A = 20110 mm2 A = 20110 mm2
K = 0.712
K = 0.712
A = 28500 mm2 A = 28500 mm2
K = 0.712
K = 0.712-

Steam
Section VIII500
A =12000 mm2
K = 0.877
A = 15050 mm2
K = 0.860
A = 19730 mm2
K = 0.877
A = 20750 mm2
K = 0.860
A = 28500 mm2
K = 0.860
A = 46460 mm2
K = 0.860727
K = 0.788
A =10260 mm2
K = 0.788
A =20970 mm2
K = 0.788-

Economizer
Section I
5100
A = 15050 mm2
K = 0.849 (steam)
K = 0.709 (water)
A = 28500 mm2
K = 0.849 (steam)
K = 0.709 (water)-

Table 7-13 - Low Pressure Pilot Operated Valves

ASME Actual Orifice Area and Rated Coefficient of Discharge (Set pressure ≥1.03 barg)
Gas (mm2)
Min.
91/94
93
Inlet Size
Orifice
K = 0.770
K = 0.845
mm
Designation95
K = 0.8529300
K = 0.629

50

*

1884

1477

1890

2161

80
100
150
200
250
300

*
*
*
*
*
*

4026
6666
14340
25530
36610
57980

3329
5639
12620
23480
32900
54190

4032
6658
14290
-

4768
8213
18640
32260
50870
72900

Table 7-14 - Low Pressure Pilot Operated Valves

Actual Orifice Area and Rated Coefficient of Discharge (Set pressure < 1.03 barg)
Min.
Inlet Size
mm
50
80
100
150
200
250
300

Orifice
Designation
*
*
*
*
*
*
*

91/94

93

Gas (mm2)
95

9200

9300

Kd = 0.678 (P2 /P1)-0.285
1884
4026
6666
14340
25530
36610
57980

Kd = 0.700 (P1/P2)-0.265
1477
3329
5639
12620
23480
32900
54190

Kd = 0.678 (P1/P2)-0.285
1890
4032
6653
14290
-

Kd = 0.756 (P1/P2) 0.0517
2161
4768
8213
18640
32260
50870
72900

Kd = 0.650 (P1/P2) -0.349
2161
4763
8213
18640
32260
50870
72900

* Full bore
Where:
P2 = Pressure at valve outlet during flow, bara. This is total back pressure (barg) + atmospheric pressure (bara).
P1 = Relieving pressure, bara. This is the set pressure (barg) + overpressure (barg) + atmospheric pressure (bara) - inlet pressure piping loss (barg).
PA = Atmospheric pressure (bara).

8.46

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units

Table 8-15 - Low Pressure Pilot Operated Valves

Actual Orifice Area and Rated Coefficient of Discharge - Vacuum Flow
Minimum
Inlet Size
mm
50
80
100
150
200
250
300

Air/Gas (mm2)
Orifice
Designation
*
*
*
*
*
*
*

9200

9300

Kd = 0.667
2161
4768
8213
18640
32260
50870
72900

Kd = 0.55
2161
4768
8213
18640
32260
50870
72900

* Full bore

Table 8-16 - JB-TD Direct Acting Spring Valves

ASME Actual Orifice Area and Rated Coefficient of Discharge
Inlet x Outlet
Size
mm
10x14
12x16
12x16

Orifice
Designation
V
W

Air/Gas/Steam (mm2)JB-TD
K = 0.856
30870
44450

W1
Y
Z

46450

14x18
16x18
16x18

Z1

70970

16x20

Z2
AA
BB

100000123500

18x24
20x24

60500
66550
79660

8.47

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
X. Equivalents and Conversion Factors
Table 8-17 - Equivalents and Conversion Factors
A

B

C

Multiply
Atmospheres
Atmospheres
Atmospheres
Atmospheres
Atmospheres
Atmospheres
Atmospheres
Atmospheres
Barrels
Bars
Bars
Bars
Centimeters
Centimeters
Centimeters
Centimeters
Cubic centimeters
Cubic feet
Cubic feet
Cubic feet per minute

By
14.70
1.033
29.92
760.0
407.5
33.96
1.013
101.3
42.00
14.50
1.020
100.0
0.39370.032810.010
0.01094
0.061027.481
0.17810.02832

ObtainPounds per square inch
Kilograms per sq. cm
Inches of mercury
Millimeters of mercury
Inches of water
Feet of water
Bars
Kilo Pascals
Gallons (U.S.)
Pounds per square inch
Kilograms per sq. cm
Kilo Pascals
InchesFeet
Meters
Yards
Cubic inches
Gallons
Barrels
Cubic meters per minute

Cubic feet per second
Cubic inches
Cubic inches
Cubic meters
Cubic meters per hour
Cubic meters per minuteStandard cubic feet per min.
Standard cubic feet per min.
Standard cubic feet per min.

448.8
16.39
0.004329
264.2
4.403
35.31
60.00
1440.00.02716

Gallons per minute
Cubic centimeters
Gallons
Gallons
Gallons per minute
Cubic feet per minuteStandard cubic ft. per hrStandard cubic ft. per day
Nm3/min. (0°C, 1 Bara)

Standard cubic feet per min.
Standard cubic feet per min.
Standard cubic feet per min.
Standard cubic feet per min.
Standard cubic feet per min.
Feet
Feet
Feet
Feet of water (68°F)
Feet of water (68°F)
Gallons (U.S.)
Gallons (U.S.)
Gallons (U.S.)
Gallons (Imperial)
Gallons (U.S.)
Gallons (U.S.)
Gallons of water (60°F)
Gallons of liquid
Gallons per minute
Gallons per minute (60°F)
Gallons per minute
Gallons per minute
Gallons per minute
Grams
InchesInchesInchesInches

1.630
39.11
0.02832
1.699
40.78
0.30480.333330.48
0.88120.43283785.00.1337231.0
277.4
0.83273.785
8.337
500 x Sp. Gr.
0.002228
227.0 x SG
0.06309
3.785
0.22710.035272.540
0.08333
0.02540.02778

Nm3/hr. (0°C, 1 Bara)
Nm3/day (0°C, 1 Bara)
Nm3/minNm3/hr
Nm3/day
MetersYards
Centimeters
Inches of mercury (0°C)
Pounds per square inch
Cubic centimeters
Cubic feet
Cubic inches
Cubic inches
Gallons (Imperial)
LitersPoundsPounds per hour liquid per minute
Cubic feet per second
Kilograms per hour
Liters per second
Liters per minute
M3/hr
OuncesCentimeters
Feet
Meters
Yards

Notes
This table may be used in two ways:
1. Multiply the unit under column A by the figure
under column B, the result is the unit undercolumn C.
2. Divide the unit under column C by the figure undercolumn B, the result is then the unit under column A.

8.48

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Table 8-17 - Equivalents and Conversion Factors (continued)
A
Multiply
Inches of mercury (0°C)
Inches of mercury (0°C)
Inches of mercury (0°C)
Inches of mercury (0°C)
Inches of water (68°F)
Inches of water (68°F)
Kilograms
Kilograms
Kilograms
Kilograms per minute
Kilograms per sq. cm
Kilograms per sq. cm
Kilograms per sq. cm
Kilograms per cubic meter
Kilo Pascals
Kilo Pascals
Kilo Pascals
LitersLitersLitersLiters per hour
MetersMetersMetersMetersPoundsPoundsPoundsPoundsPoundsPoundsPounds per hour
Pounds per hour
Pounds per hour liquid
Pounds per sq. inch
Pounds per sq. inch
Pounds per sq. inch
Pounds per sq. inch
Pounds per sq. inch
Pounds per sq. inch
Pounds per sq. inch
Pounds per sq. inch
Pounds per sq. inch
Specific gravity (of gas or vapors)
Square centimeter
Square inch
Square inch
SSU
SSU
Water (cubic feet at 60°F)
Temperature:
Centigrade
KelvinFahrenheit
Fahrenheit
Fahrenheit

B
By
1.135
0.49120.033420.03453
0.036070.073432.205
0.001102
35.27
132.3
14.22
0.967828.96
0.06240.14500.01000.010200.035311000.00.26420.004403
3.281
1.094
100.0
39.37
0.1199453.6
0.00050.45360.0004536
16.00
6.324/M.W.
0.45360.002/Sp.Gr.
27.73
2.311
2.036
0.070310.068051.71
0.70430.06895
6.895
28.97
0.15506.4516645.160.2205 x SG
0.2162
62.37

CObtainFeet of water (68°F)
Pounds per square inch
Atmospheres
Kilograms per sq. cm
Pounds per sq. in.
Inches of mercury (0°C)
PoundsShort tons (2000 lbs.)
OuncesPounds per hour
Pounds per sq. in.
Atmospheres
Inches of mercury
Pounds per cubic footPounds per sq. in.
Bars
Kilograms per sq. cm
Cubic feet
Cubic centimeters
Gallons
Gallons per minute
Feet
Yards
Centimeters
InchesGallons H2O at 60°F (U.S.)
Grams
Short tons (2000 lbs.)
Kilograms
Metric tons
Ounces
SCFM
Kilograms per hour
Gallons per minute liquid (at 60°F)
Inches of water (68°F)
Feet of water (68°F)
Inches of mercury (0°C)
Kilograms per sq. cm
Atmospheres
Millimeters of mercury (0°C)
Meters of water (68°F)
Bar
Kilo Pascals
Molecular weight (of gas or vapors)
Square inch
Square centimeter
Square millimeter
Centipoise
Centistoke
Pounds=====5/9 (Fahrenheit -32)
Centigrade + 273
9/5 (Centigrade) +32
Rankine -460
(9/5 Kelvin) -460

Notes
This table may be used in two ways:
1. Multiply the unit under column A by the figure
under column B, the result is the unit undercolumn C.
2. Divide the unit under column C by the figure undercolumn B, the result is then the unit under column A.

8.49

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Conversion Factors
Table 8-18 - Pressure Conversions[1]
Given
mm wc
(mm water column)(60°F or 15.6°C)
mbar (millibars)
mm Hg[2](mm Mercury)(32°F or 0°C)
in wc
(in. water column)(60°F or 15.6°C)
oz (oz/in2)

mm wc
-

mbar
0.0980To Find (To find desired value, multiply “Given” value by factor below)
mm Hg
in wc
oz
kPa
in Hg
psig
0.735
0.03940.02270.00980
0.002900.001421

10.21
13.61

1.333

0.750
-

0.40190.53580.23200.30940.10000.13330.02960.03948

25.40

2.488

1.866

-

0.57750.2488

43.99

4.309

3.232

1.732

-

kPa (kilopascal)in Hg (in. Mercury)(60°F or 15.6°C)
psig (lbs/in2)
kg/cm2

102.1
344.7

10.00
33.77

7.501
25.33

4.019
13.57

703.8

68.95

51.72

10010
10207

980.7
1000

735.6
750.1

bars

kg/cm2
1
10010

bars
1
10207

0.01450
0.019340.001020.001360.00100
0.00133

0.0737

0.03609

0.002540.002490.43090.12760.004390.00431

2.321
7.836

3.377

0.2961-

0.0625 or
1/16
0.14500.48980.01020.03440.01000.0338

27.71

16.00

6.895

2.042

-

0.07030.0689

394.1
401.9

227.6
232.1

98.07
100.0

29.04
29.61

14.22
14.50

1.020

0.9807-

Notes
1.	When pressure is stated in liquid column height, conversions are valid only for listed temperature.
2.	Also expressed as torr.
3.	Normal Temperature and Pressure (NTP) conditions, are at sea level, equal to 1.013 bars (absolute) or
1.033 kg/cm2 (kilograms force per square centimeter absolute) at base temperature of 0°C. This differs
slightly from Metric Standard conditions, (MSC), which uses 15°C for the base temperature.

8.50

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Conversion Factors
Table 8-19 - Gas Flow Conversions
Given
scfm

Notes

scfh

[1]

lb/hr

[2]

kg/hr

[3]

Nm3/hr[4]

Nm3/min[4][1]

To Find (To find desired value, multiply “Given” value by factor below)
scfm
scfh
lb/hr
kg/hr
Nm3/hr
Nm3/min
60
M
M
1.608
0.02686.32
13.93
0.01677
M
M
0.02680.000447
379.2
836.1
6.32
379.2
0.453610.17
0.1695
M
M
M
M
13.93
836.1
2.205
22.40
0.3733
M
M
M
M
0.621637.30
MM
0.01667
10.17
22.40
37.30
2238
5.901 M
2.676 M60-

Notes
M = molecular weight of gas.
1.	Volumetric flow(per time unit of hour or minute as shown) instandard cubic feet per minute at 14.7 psia, 60°F.
2.	Weight flow in pounds per hour.
3.	Weight flow in kilograms per hour.
4.	Volumetric flow (per time unit of hour or
minute as shown) at 1.013 bars absolute, 0°C.
This represents the commercial standard, known
as the Normal Temperature and Pressure (NTP).

Conversions from volumetric to volumetric or to weight flow (and vice versa) may only be done
when the volumetric flow is expressed in the standard conditions shown above. If flows are
expressed at temperature or pressure bases that differ from those listed above, they must first be
converted to the standard base.
If flow is expressed in actual volume, such as m3/hr (cubic meters per hour) as is often done forcompressors, where the flow is described as displacement or swept volume, the flow may be
converted to Nm3/hr as follows.

USCS Units

Nm3/hr = m3/hr x 1.013 + p x 273
273 + t1.013
Where:
p= gauge pressure of gas in barg
t= temperature of gas in °C
m3/hr= displacement or swept volume in cubic meters/hour

8.51

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Conversion Factors

Table 8-20 - Liquid Flow Conversions
Given
l/hr
liters/hour
gpm (US)
US gallons per minutegpm (Imp)
Imperial gallons per minutebarrels/day (petroleum)(42 US gallons)m3/hr
cubic meters per hourm3/s
cubic meters per second
kg/hr
kilograms per hourlb/hrpounds per hour

To Find (To find desired value, multiply “Given” value by factor below)
l/hr
gpm (US)gpm (Imp)barrels/ day
m3/hr
0.004400.003666
0.15100.0010227.1-

0.832734.29

0.2271

272.8

1.201

-

41.18

0.27286.624

0.029170.02429-

0.006624

1000

4.403

3.666

151.0

-

3.6 x 10615.85013.200543.400

3600

1
G
1
2.205G1
227.1G1
500.8G

1
272.8G1
601.5G

0.151
G
1
14.61G

1
1000G
1
2205G

Note
G = relative density of liquid at its relieving
temperature to that of water at 20°C where
Gwater = 1.00.

8.52

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units

Conversion Factors
Viscosity Units and Their Conversion
When a correction for the effects of viscosity
in the liquid orifice sizing formula is needed,
the value of viscosity, expressed in centipoise,is required. Since most liquid data for viscosity
uses other expressions, a convenient method
for conversion is presented below.
The viscosity, µ (Greek mu), in centipoise,
is correctly known as absolute or dynamic
viscosity. This is related to the kinematic
viscosity expression, ν (Greek nu), in
centistokes as follows:µ=νxG
Where:µ = absolute viscosity, centipoiseν = kinematic viscosity, centistokes
G = relative density (water = 1.00)
Most other viscosity units in common usage
are also kinematic units and can be related to
the kinematic viscosity in centistokes, via theaccompanying table. To use this table, obtainthe viscosity from data furnished. Convert thisto ν, in centistokes, then convert to absoluteviscosity µ, in centipoise.
The conversions are approximate but
satisfactory for viscosity correction in liquidsafety valve sizing.

Table 7-21 - Viscosity Conversion
Seconds
Viscosity
Centistokes
ν
1.00
2.56
4.30
7.40
10.3
13.1
15.7
18.2
20.6
32.1
43.2
54.0
65.0
87.60
110.0
132.0
154.0
176.0
198.0
220.0
330.0
440.0
550.0
660.0
880.0
1100.01320.01540.01760.01980.02200.03300.04400.0

Seconds
Saybolt
Universal
ssu
31
35
40
50
60
70
80
90
100
150
200
250
300
400
500
600
700
800
900
1000
1500
2000
2500
3000
4000
5000
6000
7000
8000
9000
10000
15000
20000

Seconds
Saybolt
Furol
ssf
12.95
13.70
14.40
15.24
19.30
23.50
28.00
32.50
41.90
51.60
61.40
71.10
81.00
91.00
100.70150.00200.00250.00300.00400.00500.00600.00700.00800.00900.001000.00
1500.00
2000.00

Seconds
Redwood1(standard)
29.0
32.1
36.2
44.3
52.3
60.9
69.2
77.6
85.6
128.0
170.0
212.0
254.0
338.0
423.0
508.0
592.0
677.0
462.0
896.0
1270.01690.02120.02540.03380.04230.05080.05920.06770.07620.08460.013700.018400.0

Seconds
Redwood2(Admiralty)
5.10
5.83
6.77
7.60
8.44
9.30
10.12
14.48
18.90
23.45
28.00
37.10
46.20
55.40
64.60
73.80
83.00
92.10
138.20184.20230.00276.00368.00461.00553.00645.00737.00829.00921.00-

8.53

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
XI - Capacity Correction Factor for Rupture Disc/Pressure Relief Valve
Combination, Kc
It may be desirable to isolate a pressure relief valve from the process fluid in the vessel that itis protecting. A non-reclosing device such as a rupture disc can be installed upstream of thepressure relief valve to provide this isolation. For example, it may be more economical to install arupture disc made from Inconel and then mount a standard stainless steel pressure relief valve inseries with the disc where the service conditions require such a high alloy material. This rupture
disc/pressure relief valve combination may also be beneficial when the fluid may have entrainedsolids or is viscous. The rupture disc can also provide for a zero leak point during normal vessel
operation.
Since the rupture disc is in the flow path of the pressure relief valve, the ASME Section VIII Codemandates that the pressure relief valve rated capacity be adjusted with a capacity combination
factor (Kc). This correction factor is determined by performing actual flow tests with specificrupture disc and pressure relief valve designs. The materials of construction, minimum size, andminimum burst pressure of the rupture disc must be specified to use this measured correction
factor.
If there has been no combination testing performed then the Kc factor is equal to 0.90.
Table 8-22 lists the combination tests performed with the Crosby J series direct acting springloaded valves. For any other Crosby brand or Anderson Greenwood brand pressure relief valve
product used in series with a rupture disc, use a Kc factor equal to 0.90.

8.54

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Table 8-22 - Capacity Correction Factor for Rupture Disc/PRV Combination (Kv)
Emerson PRV Series
JOS-E/JBS-E

JOS-E/JBS-E

Rupture disc
manufacturer
BS&B

Continental Disc

Disc type
CSR

Minimum disc size(mm)
40

Minimum burst pressure(barg)
3.45

Disc material
Inconel®

JRS
JRS
RLS
RLS
RLS
S90
S90
CDC
CDC-FS
CDCV FSCDCV FSCDCV FSCDCV FSCDCV FSCDCV FSCDCV LL
CDCV LL
CDCV LL
CDCV LL
CDCV LL
CDCV LL
CDCV LL
CDCV LL
DCV
DCV
KBA
Micro X
Micro X
Micro X
Micro X
Micro X
Micro XULTRXMINTRX
STARX
STARX
STARX
STARX
STARX
STARX
STARX
STARX
STARX
STARX
STARX
STARX
STARX
STARX

25
40
25
25
50
25
50
25
80
25
25
40
40
80
80
25
25
25
40
40
40
80
80
80
80
25
25
25
50
50
50
50
25
25
25
25
25
40
40
40
40
40
40
80
80
80
80
80

4.14
1.59
9.52
11.9
5.83
8.62
5.17
4.14
1.03
4.14
4.14
2.07
2.07
1.03
1.03
4.14
4.14
4.14
2.07
2.07
2.07
1.03
1.03
2.41
2.41
4.14
10.3
10.3
5.52
5.52
5.52
5.52
4.14
4.14
4.14
4.14
4.14
2.07
2.07
2.07
2.07
2.07
2.07
1.03
1.03
1.03
1.03
1.03

316 SS
Monel®
Monel®
Hastelloy®
Monel®
NickelNickelMonel®/Teflon®
Monel®/Teflon®
316 SS/Teflon®
Hastelloy®/Teflon®
316 SS/Teflon®
Hastelloy®/Teflon®
316 SS/Teflon®
Hastelloy®/Teflon®
316 SS/Teflon®
Hastelloy®/Teflon®
Monel®/Teflon®
316 SS/Teflon®
Monel®/Teflon®
Nickel/Teflon®
316 SS/Teflon®
Monel®/Teflon®
Monel®/Teflon®
316 SS/Teflon®
Monel®
Monel®
Nickel316 SS
Inconel®
Monel®
Nickel316 SS
Hastelloy®
Inconel®
Monel®
Nickel316 SS
Hastelloy®
Inconel®
Monel®
NickelTantalum
316 SS
Hastelloy®
Inconel®
Monel®
NickelKc Factor
0.986
0.993
0.981
0.981
0.972
0.981
0.995
0.994
0.971
0.986
0.985
0.983
0.976
0.973
0.982
0.981
0.978
0.960
0.961
0.959
0.958
0.953
0.953
0.979
0.994
0.978
0.984
0.984
0.990
0.991
0.997
0.988
0.992
0.980
0.987
0.984
0.980
0.981
0.984
0.986
0.989
0.987
0.981
0.978
0.985
0.992
0.991
0.987
0.981

8.55

Pressure Relief Valve Engineering Handbook
Chapter 8 - Engineering Support Information - Metric Units
Table 8-22 - Capacity correction factor for rupture disc/PRV combination (Kv) (continued)
Emerson PRV series
JOS-E/JBS-E

Rupture disc
manufacturer
Continental DiscJOS-E/JBS-E

Fike

JOS-E/JBS-E

OSECO

Disc type
ZAP

Minimum disc size(mm)
25

Minimum burst pressure(barg)
4.14

Disc material
Monel®

ZAP
ZAP
ZAP
ZAP
ZAP
ZAP
ZAP
ZAP
ZAP
ZAP
Axius
MRK
MRK
MRK
MRK
Poly-SD CS
Poly-SD DH
SRL
SRX
COV
FAS
PCR

25
25
25
40
40
40
80
80
80
80
25
25
25
80
80
25
25
25
25
50
80
80

4.14
4.14
4.14
2.07
2.07
2.07
2.41
2.41
2.41
2.41
1.03
4.14
4.14
2.41
2.41
8.55
2.21
1.86
6.55
2.14
6.21
6.21

316 SS
Inconel®
Nickel316 SS
Monel®
NickelInconel®
Monel®
Nickel316 SS
316 SS
316 SS
Nickel316 SS
NickelAluminum
Aluminum
SS Nickel
NickelMonel®/Teflon®
NickelNickelKc Factor
0.985
0.985
0.988
0.992
0.955
0.955
0.992
0.992
0.982
1.000
0.970
0.987
0.967
0.977
0.982
0.995
0.970
0.997
0.979
0.996
0.979
0.975
0.967

8.56

The Emerson logo is a trademark and service mark of emerson electric co.
All other marks are property of their respective owners.