BS-EN-ISO-4126-1-2013-A1-2016

EUROPEAN STANDARD

EN ISO ISO4126-1 4126-1:2013+A1

NORME EUROPÉENNE EUROPÄISCHE NORM

July 2013 June 2016

ICS 13.240

Supersedes EN ISO 4126-1:2004

English Version

Safety devices for protection against excessive pressure - Part 1: Safety valves (ISO 4126-1:2013) Dispositifs de sécurité pour protection contre les pressions excessives - Partie 1: Soupapes de sûreté (ISO 41261:2013)

Sicherheitseinrichtungen gegen unzulässigen Überdruck Teil 1: Sicherheitsventile (ISO 4126-1:2013)

This European Standard was approved by CEN on 28 December 2012. CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN member. This European Standard exists in three official versions (English, French, German). A version in any other language made by translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management Centre has the same status as the official versions. CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, I taly, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and United Kingdom.

EUROPEAN COMMITTEE FOR STANDARDIZATION COMITÉ EUROPÉEN DE NORMALISATION EUROPÄISCHES KOMITEE FÜR NORMUNG

Management Centre: Avenue Marnix 17, B-1000 Brussels

© 2013 CEN

All rights of exploitation exploitation in any form and by any any means reserved worldwide for CEN national Members.

Ref. No. EN ISO 4126-1:2013: E

BS EN ISO 4126-1:2013+A1:2016

EN )SO -:+A: E

Foreword This document (EN ISO 4126-1:2013) has been prepared by Technical Committee ISO/TC 185 "Safety devices for protection against excessive pressure" in collaboration with Technical Committee CEN/TC 69 “Industrial valves” the secretariat of which is held by AFNOR. This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by January 2014, and conflicting national standards shall be withdrawn at the latest by January 2014. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights. This document document supersedes supersedes EN ISO 4126-1:2004. 4126-1:2004. This document has been prepared under a mandate given to CEN by the European Commission and the European Free Trade Association, and supports essential requirements of EU Directive(s). For relationship with EU Directive(s), see informative Annex ZA, which is an integral part of this document. According to the CEN-CENELEC Internal Regulations, the national standards organizations of the following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom.

Endorsement notice The text of ISO 4126-1:2013 4126-1:2013 has been approved approved by CEN as EN ISO 4126-1:2013 4126-1:2013 without without any modification modification..

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BS EN ISO 4126-1:2013+A1:2016

EN )SO -:+A: E

Foreword to amendment A This document (EN ISO 4126-1:2013/A1:2016) has been prepared by Technical Committee ISO/TC 185 "Safety devices for protection against excessive pressure" in collaboration with Technical Committee CEN/TC 69 “Industrial valves” the secretariat of which is held by AFNOR. This Amendment to the European Standard EN ISO 4126-1:2013 shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by December 2016, and conflicting national standards shall be withdrawn at the latest by December 2016. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights. This document has been prepared under a mandate given to CEN by the European Commission and the European Free Trade Association, and supports essential requirements of EU Directive. For relationship with EU Directive, see informative Annex ZA, which is an integral part of EN ISO 4126-1:2013. According to the CEN-CENELEC Internal Regulations, the national standards organizations of the following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom.

Endorsement notice The text of ISO 4126-1:2013/Amd 1:2016 has been approved by CEN as EN ISO 4126-1:2013/A1:2016 without any modification.

iii

BS EN ISO 4126-1:2013+A1:2016

EN )SO -:+A: E

Annex ZA (informative)

Relationship between this International Standard and the Essential Requirements of EU Directive 97/23/EC (PED)

By agreement between ISO and CEN, this CEN annex is included in the DIS and the FDIS but will not appear in the published ISO standard. This International Standard has been prepared under a mandate given to CEN by the European Commission and the European Free Trade Association to provide one means of conforming to Essential Requirements of the New Approach Directive 97/23/EC (PED). Once this standard is cited in the Official Journal of the European Communities under that Directive and has been implemented as a national standard in at least one Member State, compliance with the normative clauses of this standard given in Table ZA.1 confers, within the limits of the scope of this standard, a presumption of conformity with the corresponding Essential Requirements of that Directive and associated EFTA regulations. Table ZA.1 — Correspondence between this International Standard and Directive 97/23/EC (PED) Sub-clauses of this International Standard

Essential Requirements of Directive 97/23/EC (PED) Essential Requirements

Annex I of PED

5,6,7,8,9

Safety accessories

2.11.1

5.1.5

Safety of operation

2.3

5.1.6

Drain and venting

2.5

6.3

Proof test

3.2.2

10

Marking and labelling

3.3

WARNING: Other requirements and other EU Directives may be applicable to the products falling within the scope of this standard.

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BS EN ISO 4126-1:2013+A1:2016 BS EN ISO 4126-1:2013

)SO -:+A:E ISO 4126-1:2013(E)

Contents

Page

Foreword ........................................................................................................................................................................................................................................ivi v 1

Scope .................................................................................................................................................................................................................................1

2

Normative references ......................................................................................................................................................................................1



Terms and deinitions .....................................................................................................................................................................................1

4

Symbols and units ...............................................................................................................................................................................................4

5

Design ..............................................................................................................................................................................................................................4 . General ...........................................................................................................................................................................................................4 5.2 Valve end connections ......................................................................................................................................................................5 5.3 Minimum requirements for springs .....................................................................................................................................5 5.4 Materials .......................................................................................................................................................................................................6

6

Production testing ..............................................................................................................................................................................................6 6.1 Purpose ..........................................................................................................................................................................................................6 . General ...........................................................................................................................................................................................................6 . (ydrostatic testing ..............................................................................................................................................................................6 6.4 Pneumatic testing .................................................................................................................................................................................7 6.5 Adjustment of set or cold differential test pressure ............................................................................................... 8 6.6 Seat leakage test .....................................................................................................................................................................................8

7

Type testing................................................................................................................................................................................................................8 . General ...........................................................................................................................................................................................................8 7.2 Tests to determine operating characteristics ...............................................................................................................9 . Tests to determine low characteristics..........................................................................................................................11 . Determination of the coeficient of discharge ...........................................................................................................12 . Certiication of coeficient of discharge ..........................................................................................................................12

8

Determination of safety valve performance .........................................................................................................................13

9

Sizing of safety valves ...................................................................................................................................................................................13

10

Marking and sealing. ......................................................................................................................................................................................13 10.1 Marking ......................................................................................................................................................................................................13 . Sealing of a safety valve ................................................................................................................................................................13

Bibliography .............................................................................................................................................................................................................................14

© ISO 2013 – All rights reserved

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BS EN EN ISO ISO 4126-1:2013 4126-1:2013+A1:2016 BS

)SO ISO -:+A:E 4126-1:2013(E)

Foreword )SO the )nternational Organization for Standardi zation is a worldwide federation of national standards bodies )SO member bodies. The work of preparing )nternational Standards is normally carried out through )SO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with )SO, also take part in the work. )SO collaborates closely with the )nternational Electrotechnical Commission )EC on all matters of electrotechnical standardization. )nternational Standards are draf ted in accordance with the rules given in t he )SO/)EC Directives, Part . The main task of technical committees is to prepare International Standards. Draft International

Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an )nternational Standard requires approval by at least  % of the member bodies casting a vote. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. )SO shall not be held responsible for identifying any or all such patent rights. )SO - was prepared by Technical Committee )SO/TC , Safety devices for protection against excessive pressure. This third edit ion cancels and replaces the second edition )SO -:, which has been technically revised. It also incorporates the Technical Corrigendum ISO 4126-1:2004/Cor.1:2007. )SO  consists of the following parts, under the general title Safety devices for protection against excessive pressure: — Part 1: Safety valves — Part 2: Bursting disc safety devices — Part 3: Safety valves and bursting disc safety devices in combination — Part 4: Pilot operated safety valves — Part 5: Controlled safety pressure relief systems (CSPRS) — Part 6: Application, selection and installation of bursting disc safety dev ices — Part 7: Common data — Part 9: Application and installation of safety devices excluding stand-alone bursting disc safety devices — Part : Sizing of safety valves for gas/liquid two-phase low — Part 11: Performance testing1) Part 7 contains data that is common to more than one of the parts of ISO 4126 to avoid unnecessary repetition.

1)

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Under preparation. © ISO 2013 – All rights reserved


INTERNATIONAL STANDARD

ISO 4126-1:2013(E) )SO -:+A:E

Safety devices for protection against excessive pressure —

Part 1: Safety valves 1 Scope This part of )SO  speciies general requirements for safety valves irrespect ive of the luid for which they are designed. )t is applicable to safety valves having a low diameter of  mm and above which are for use at set pressures of , bar gauge and above. No limitation is placed on temperature. This is a product standard and is not applicable to applications of safety valves.

2

Normative references

The following referenced documents are indispensable for the application of this document. For dated

references, only the edition cited applies. For undated references, the latest edition of the referenced document including any amendments applies. )SO -:, Safety devices for protection against excessive pressure — Part 7: Common data

 Terms and deinitions For the purposes of this document, the following terms and deinitions apply. 3.1 safety valve

valve which automatically, without the assistance of any energy other than that of the  luid concerned, discharges a quantity of the luid so as to prevent a predetermined safe pressure being exceeded, and which is designed to re-close and prevent further low of luid after normal pressure conditions of service have been restored Note  to entry: The valve can be character ized either by pop action rapid opening or by opening in proportion not necessarily linear to the increase in pressure over the set pressure.

3.2 direct loaded safety valve

safety valve in which the loading due to the luid pressure underneath the valve disc is opposed only by a direct mechanical loading device such as a weight, lever and weight, or spring 3.3 assisted safety valve

safety valve which, by means of a powered assistance mechanism, may additionally be lif ted at a pressure lower than the set pressure and will, even in the event of failure of the assistance mechanism, comply with all the requirements for safety valves given in )SO 


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)SO ISO -:+A:E 4126-1:2013(E)

3.4 supplementary loaded safety valve

safety valve which has, until the pressure at the inlet to the safety valve reaches the set pressure, an additional force which increases the sealing force Note  to entry: This additional force supplementary load, which may be provided by means of an extraneous power source, is reliably released when the pressure at the inlet of the safety valve reaches the set pressure. T he amount of supplementary loading is so arranged that if such supplementary loading is not released, the safety valve will attain its certiied discharge capacity at a pressure not greater than , times the maximum allowable pressure of the equipment to be protected.

Note  to entry: Other ty pes of supplementary loaded safet y devices are dealt w ith in )SO -.

3.5 set pressure

predetermined pressure at which a safety valve under operating conditions commences to open Note  to entry: )t is the gauge pressure measured at the valve inlet at which the pressure forces tending to open the valve for the speciic service conditions are in equilibrium with the forces retaining the va lve disc on its seat.

3.6 maximum allowable pressure PS maximum pressure for which the protected equipment is designed 3.7 overpressure pressure increase over the set pressure Note  to entry: Overpressure is usua lly expressed as a percentage of the set pressure.

3.8 reseating pressure value of the inlet static pressure at which the disc re-establishes contact with the seat or at which the

lift becomes zero 3.9 cold differential test pressure

inlet static pressure at which a safety valve is set to commence to open on the test bench Note  to entry: This test pressure includes corrections for service conditions, e.g. back pressure and/or temperature.

3.10 relieving pressure

pressure used for the sizing of a safety valve which is greater than or equal to the set pressure plus overpressure 3.11 back pressure

pressure that exists at t he outlet of a safety valve as a result of the pressure in the discharge system Note  to entry: The back pressure is the sum of the superimposed and built-up back pressures.

3.12 built-up back pressure

pressure existing at the outlet of a safety valve caused by low through the valve and the discharge system 3.13 superimposed back pressure

pressure existing at the outlet of a safety valve at the time when the device is required to operate Note  to entry: )t is the result of pressure in t he discharge system from other sources.

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)SO -:+A:E ISO 4126-1:2013(E)

3.14 balanced bellows

device which minimizes the effect of back pressure on the set pressure and/or the operation of a safety valve 3.15 blowdown difference between set and reseating pressures Note  to entry: Blowdown is normally stated as a percentage of set pressure except for pressures of less than 3 bar when the blowdown is expressed in bar.

3.16 lift

actual travel of the valve disc away from the closed position 3.17 low area

minimum cross-sectional low area  but not the smallest area between disc and seat between inlet and seat which is used to calculate the theoretical  low capacity, with no deduction for any obstruction 3.18 low diameter

diameter corresponding to the low area 3.19 theoretical discharge capacity

calculated capacity expressed in mass or volumetric units of a theoretically perfect nozzle having a cross-sectional low area equal to the  low area of a safety valve 3.20 coeficient of discharge

value of actual lowing capacity from tests divided by the theoretical lowing capacity from ca lculation 3.21 certiied discharge capacity

that portion of the measured capacity permit ted to be used as a basis for the application of a safety valve Note  to entry: )t may, for example, equal the: a measured low rate times the derating f actor; or b theoretical low rate times the coeficient of discharge times the derating factor; or c theoretical low rate t imes the certiied de-rated coeficient of discharge.

3.22 DN (nominal size)

alphanumeric designation of size that is common for components used in a piping system, used for reference purposes, comprising the letters DN followed by a dimensionless number having an indirect correspondence to the physical size of the bore or outside diameter of the component end connection Note  to entry: The dimensionless number does not represent a measurable value and is not used for calculation purposes.

Note  to entry: Prei x DN usage is applicable to components bearing PN designations according to )SO . Note  to entry: Adapted from )SO :, deinition ..


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)SO ISO -:+A:E 4126-1:2013(E)

4 Symbols and units Table 1 — Symbols and their descriptions Symbol

Unit

A

Flow area of a safety valve not smallest area between seat and disc

K d

Coeficient of dischargea

—

K dr

Certiied de-rated coef icient of discharge K d × ,a

—

Number of tests

—

n

a

Description

mm2

qm

Theoretical speciic discharge capacity

kg/h⋅mm2)

q′m

Speciic discharge capacity determined by tests

kg/h⋅mm2)

K d and K dr are expressed as ,xxx.

5 Design 5.1

General

5.1.1 The design shall incorporate guiding arrangements necessary to ensure consistent operation and seat tightness.

The seat of a safety valve, other than when it is an integral part of the valve shell, shall be fastened securely to prevent the seat becoming loose in service. 5.1.2

5.1.3 )n the case of valves where the lift can be reduced to conform to the required discharge capacity, restriction of the lift shall not interfere with the operation of the valve. The lift restricting device shall be

designed so that, if adjustable, the adjustable feature can be mechanically locked and access sealed. The lift restricting device shall be installed and sealed in accordance with the design of the manufacturer. Valve lift shall not be restricted to a value less than  % of unrestricted lift or  mm, whichever is the greater. 5.1.4

Means shall be provided to lock and/or to seal all ex ternal adjustments in such a manner so as to

prevent or reveal unauthorized adjustments of the safet y valve. Safety valves for toxic or lammable luids shall be of the closed bonnet type to prevent leakage to atmosphere, or if vented, it shall be disposed of in a safe place. 5.1.5 5.1.6

Provision shall be made to prevent liquid collecting on the discharge side of the safety valve shell.

5.1.7 The design stress of pressure-retaining shells shall not exceed that speciied in the appropriate st andards. NOTE

5.1.8

For example, EN  or ANS)/ASME B . may be used as reference.

The materials for adjacent sliding surfaces such as guide(s) and disc/disc holder/spindle shall be

selected to ensure corrosion resistance and to minimize wear and avoid galling. The materials for the seat and disc of safety valves shall be selected to ensure resistance to metallic bonding between these two surfaces in order to prevent an increase of set pressure, e.g. sticking or cold working. 5.1.9

5.1.10 Sealing elements which may adversely affect the operating characteristics by frictional forces are not permitted. 5.1.11 Easing gear shall be provided when speciied. 5.1.12 Safety valves shall be constructed so that breakage of any part , or failure of any device, will not obstruct free and full discharge through the valve. 4 4



)SO -:+A:E ISO 4126-1:2013(E)

5.2

Valve end connections

The inlet design of valve end connections, regardless of type, shall be such that the internal area of the external pipe or stub connection at the safety valve inlet is at least equal to that of the valve inlet connection [see Figure 1 a)]. The outlet design of valve end connections, regardless of type, shall be such that the internal area of the external pipe connection at the safet y valve outlet is at least equal to that of the valve outlet, except those valves with female threaded outlet connections [see Figure 1 b)]. See Clause 7 regarding type testing.

NOTE

a) Inlet

b) Outlet Key

1 valve

3 unsatisfactory

2 satisfactory

4 required internal diameter of the safety valve for the

a

valve to function properly )f the nominal diameter of the pipe is not equal to the nominal diameter of the valve outlet, as shown, then a suitable pipe shall be itted during testing as speciied in 7.1.5.

Figure 1 — Design of end connections

5.3

Minimum requirements for springs

Springs shall be in accordance with ISO 4126-7.


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)SO ISO -:+A:E 4126-1:2013(E)

5.4 Materials Only approved materials shall be used for pressure-retaining shells. NOTE For example, EN  or any other published national or international material standards e.g. ASME, ASTM, J)S, etc. may be used a s reference.

These materials and their temperature limitations shall be suitable for pressure-containing function.

6 Production testing 6.1

Purpose

The purpose of these tests is to ensure that all safety valves meet the requirements for which they have been designed without exhibiting any form of leakage from pressure-retaining components or joints.

6.2

General

)t is permissible to adopt an alternative test of equal validity e.g. proof of design test s associated with statistical sampling to the hydrostatic test for valve shells with: — threaded ends; and — a maximum inlet diameter of  mm; and — a ratio of bursting pressure to design pressure of at least ; and — a design pressure equal to or less than  bar; and — for use with non-hazardous luids; and also for valves as above but with:

— a design pressure greater than  bar; and — a ratio of bursting pressure to design pressure of at least ; and — material which is either wrought or forged.

All temporary pipes and connections and blanking devices shall be adequate to safely withstand the test pressure. Any temporary welded-on attachments shall be carefully removed and the resulting weld scars shall be ground lush with the parent material. After grinding, all such scars shall be inspected by using magnetic particle or luid penetrant techniques.

6.3 6.3.1

Hydrostatic testing Application

The portion of the valve from the inlet to the seat shall be tested to a pressure , t imes the manufacturer’s stated maximum pressure for which the safety valve is designed. The shell on the discharge side of the seat shall be tested to , times the manufacturer’s stated maximum back pressure for which the valve is designed. This pressure can be lower than that given by the outlet lange rating.

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)SO -:+A:E ISO 4126-1:2013(E)

6.3.2

Duration

The test pressure shall be applied and maintained at the required magnitude for a suficient length of time to permit a visual examination to be made of all surfaces and joints, but in any case for not less than the times given in Table 2. For tests on the discharge side of the seat, the test ing time shall be based on the pressure speciied in 6.3.1 and the discharge size. Table 2 — Minimum duration of hydrostatic test

6.3.3

Nominal size

Minimum duration

DN

in seconds

DN  

15

  DN  

60

DN  

180

Acceptance criteria

No leakage from the tested parts as deined in 6.3.1 is accepted. 6.3.4

Safety requirements

Water of suitable purity shall normally be used as the test medium. Where other liquids are used, additional precautions may be necessar y. Valve bodies shall be properly vented to remove entrapped air. )f materials which are liable to failure by brittle fract ure are incorporated in that part of the safet y valve which is to be hydrostatically tested, then both the safety valve, or part thereof, and the testing medium shall be at a suf icient temperature to prevent the possibility of such failure. No valve or part thereof undergoing pressure testing shall be subjected to any form of shock loading, for example hammer testing.

6.4 6.4.1

Pneumatic testing Application and duration of test

Pressure testing with air or other suitable gas should be avoided but may be carried out in place of the standard shell hydrostatic test with the agreement of all parties involved in the following cases: a valves of such design and construct ion that make it not practicable for them to be illed with liquid; and/or b) valves that are to be used in service where even small traces of water cannot be tolerated.

The portions of the valve to be tested, test pressure and duration of application shall be as speciied in 6.3. 6.4.2

Safety requirements

The hazards involved in pneumatic pressure testing shall be considered and adequate precautions taken. Particular attention is drawn to some relevant factors as follows:

a if a major rupture of the valve should occur at some stage during application of pressure, considerable energy will be released; hence no personnel should be in the immediate vicinity during pressure raising for example a given volume of air contains  times the amount of energy that a similar volume of water contains, when both are at the same pressure; b the risk of brittle failure under test conditions shall have been critically assessed at the design stage and the choice of materials for valves that are to be pneumatically tested shall be such as to avoid © ISO 2013 – All rights reserved

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)SO ISO -:+A:E 4126-1:2013(E)

the risk of brittle failure during test . This necessitates provision of an adequate margin between the

transition temperature of all parts and the metal temperature during testing ; c)

attention is drawn to the fact that if there is a reduction in gas pressure between the high-pressure

storage and the valve under test, the temperature will decrease. Valves undergoing pneumatic test should not be approached for close inspection until after the pressure increase has been completed.

No valve undergoing pneumatic test shall be subject to any form of shock loading. Precautions shall be taken against pressures generated in excess of test pressure.

6.5

Adjustment of set or cold differential test pressure

Each safety valve shall be adjusted to its designated set or cold differential test pressure. Before adjusting a safety valve to the set or, if applicable, cold differential test pressure using air or other gas as the test medium, it shall previously be subjected to a hydrostatic test see 6.3).

6.6

Seat leakage test

The seat leakage test of a safety valve shall be carried out after adjustment of the set or cold differential test pressure. The test procedure and leakage rate shall be agreed between the manufact urer and the purchaser. NOTE

For example, AP)  can be used.

7 Type testing 7.1

General

7.1.1

Application

The operating and low characteristics of safety valves shall be determined by type tests in conformity with this clause. — This clause applies to the types of safety devices deined in 3.1. 7.1.2

Tests

The tests to determine the operating characteristics shall be in accordance with 7.2 and the tests to determine the low characteristics shall be in accordance with 7.3.

When these tests are carried out separately, the parts of the valve which inluence luid low shall be complete and installed in the valve. The testing procedure, test rig and equipment shall be such that the operability and capacity at the relieving pressure can be established. 7.1.3

Objective of tests

The objective of the tests is to determine under speciic testing conditions, as a minimum, t he following characteristics of the valves before opening, while discharging and at reseating: a set pressure; b overpressure; c 8 8

reseating pressure; © ISO 2013 – All rights reserved


)SO -:+A:E ISO 4126-1:2013(E)

d reproducibility of valve performance; e mechanical characteristics of the valves determined by sight or hearing such as: — ability to reseat satisfactorily; — absence of chatter, lutter, sticking and/or harmful vibration; f

lift at overpressure;

g actual mass low rate. 7.1.4

Procedure for testing

The tests shall provide suitable data from which the operational and low characteristics may be determined. For valves with internally screwed connections on the outlet with a coniguration as shown in Figure 1 b, a pipe, of appropriate thickness, at least ive diameters long, shall be it ted during the test . 7.1.5

Results calculated from the tests

The theoretical lowing capacity is calculated in accordance with )SO -, as applicable, and, using this value together with t he actual  lowing capacity at relieving pressure, the coeficient of discharge of the valve is calculated in accordance with ISO 4126-7. 7.1.6

Design changes

When changes are made in the design of a safety valve in such a manner as to affect the low path, lift or performance characteristics, new tests shall be car ried out in accordance with Clause 7.

7.2

Tests to determine operating characteristics

7.2.1

General Requirements

The set pressures at which the operating characteristics are determined shall be the minimum and maximum set pressures for which the spring is designed. Valves for air or other gas service shall be

tested using air or any other gas of k nown characteristics or superheated steam with a minimum of  °C of superheat. Valves for steam serv ice shall be tested on steam, air or other gas of known characterist ics. Valves for liquid service shall be tested on water or other liquids of known characteristics. The allowable tolerances or limits as applicable on the operating characteristics are as follows: a)

set pressure: ±3 % of set pressure or ±0,1 bar, whichever is the greater;

b lift at overpressure: not less than the values stated by the manufacturer; c

overpressure: the value stated by the manufacturer but not exceeding  % of set pressure or , bar, whichever is greater;

d blowdown: not greater than the value stated by the manufacturer, but within the following limits: — compressible luids: minimum: , % [not applicable to safety valves with proportional opening characteristics according to f)], maximum: 15 % or 0,3 bar, whichever is greater; — incompressible luids: minimum: , % [not applicable to safety valves with proport ional opening characteristics according to f)], maximum: 20 % or 0,6 bar, whichever is greater; e)

overpressure and blowdown of restricted lift valves shall have the same tolerances or limits as the unrestricted lift valves;


9

BS EN ISO 4126-1:2013+A1:2016

)SO -:+A:E

f

overpressure and blowdown of valves with proportional opening characteristics shall be veriied and be stable for various lifts between the minimum and maximum stated by the manufacturer. A curve shall be established for valve lift versus overpressure.

7.2.2

Safety valve opening characteristics

The manufacturer shall specify the lift characteristic for all safety valves. 7.2.3

Test equipment

The uncertainty of pressure measurement shall be within ± , %. 7.2.4

Valves used in the test programme

The safety valves tested shall be representative of the design, pressure and size range of valves for which operating characteristics are determined w ithin the capability of the test laboratory. The ratio of valve inlet to low area and t he ratio of low area to valve outlet shall be taken into account. For size ranges containing seven or more sizes, tests shall be carr ied out on three sizes. )f the size range contains not more than six sizes, t he number of sizes tested may be reduced to two. When a size range is extended so that the safety valves tested previously are no longer representative of the range, further tests on the appropriate number of sizes shall be carried out. The tests shall be carried out using three signi icantly different springs for each size of valve tested. This may be achieved by testing either one valve with t hree signiicantly different springs or three valves of the same size with three signiicantly different springs. Each test shall be carried out a minimum of three times in order to establish and conirm acceptable reproducibility of performance. Tests at the minimum design set pressure shall be carried out. )n the case of valves of which one size only at var ious pressure ratings is being manufactured, tests shall be carried out using four dif ferent springs, which shall cover the range of pressures for which the valve is to be used. Where the size range cannot be adequately covered then scale models shall be used having a lowdiameter not less than 40 mm. All dimensions of the low path in the model shall be strictly to scale with the corresponding dimensions of the actual valve. All dimensions of the parts that can affect the overall thrust exercised by the medium on the moving parts shall be to scale. )n the case of balanced bellows, it is permitted that the effec tive area be only to scale. NOTE

Effective area is the area of the balanced bellows from which end loads are calculated (piston area).

The overall spring rate of spring plus bellows, if any, of the model shall be to scale with the overall rate of the actual valve. The roughness of all surfaces of the low path of the model shall not be less than that of the corresponding surfaces of the actual valve. Before tests are carried out it shall be veriied that the model conforms with the above.





)SO -:+A:E ISO 4126-1:2013(E)

.

Tests to determine low characteristics

7.3.1

Test requirements

After the operating characteristics (see 7.2 have been satisfactorily established, it is acceptable to use

steam, air or other gas of known characteristics as the luid for  low characteristic tests except for valves designed for liquid service. Valves for use with liquids shall be test ed with water or other liquid of known characteristics. When  low characteristics are determined independently from operating characteristics, the valve disc shall be held at the lift as determined by the operating characteristics tests. 7.3.2

Valves used in the test programme

The safety valves tested shall be the same as, or identical to, those used for the operating characteristics tests (see 7.2.4). 7.3.3 7.3.3.1

Test procedure Test conditions

The testing procedure, test rig and equipment shall be approved before testing is undertaken. The testing procedure, test rig and equipment shall be such that the capacity at the overpressure can be established. A travel stop may be itted to limit the lift to t hat determined in accordance with .. b. The tests can be carried out with or without the spring itted. When the spring is in the low path, the test shall be carried out with the spring itted. Tests shall be conducted at various pressures to establish that no variations of t he coeficient of discharge with the relevant positionss of the adjusting rings, if any, occur. 7.3.3.2

Number of test valves

The tests shall be carried out at three different pressures for each of three sizes of a given valve design unless the size range contains not more than si x sizes, in which case the number of sizes tested may be reduced to two. When a size range is extended from one containing fewer than seven sizes to one containing seven or more sizes, then tests on three sizes of valves a total of nine tests shall be carried out. )n the case of valves of either novel or special design of which one size only at various pressure ratings is being manufactured, tests shall be carried out at four different set pressures which shall cover the range of pressures for which the valves will be used, or as determined by the limit s of the test facility. 7.3.3.3

Restricted lift valves

For restricted lift valves the capacity at restricted lift may be determined immediately following the tests to determine low characteristics at full lift or determined later. )n the case of restricted lift, a curve shall be established, using a minimum of three points at all test pressures, for the coeficient of discharge versus valve lift. 7.3.3.4

Value of test pressure

Three tests shall be carried out on each safety valve size at test pressures whereby the ratio of absolute back pressure to absolute relieving pressure is less than ,. These tests shall be carried out with atmospheric back pressure. © ISO 2013 – All rights reserved

11 11


)SO ISO -:+A:E 4126-1:2013(E)

For compressible luids when the ratio of absolute back pressure to absolute relieving pressure exceeds ,, the coeficient of discharge can be largely dependent upon this ratio. Then tests shall be conducted at ratios between the pressure ratio of , and the max imum pressure ratio required to obtain c urves or tables of coeficient of discharge K d versus the ratio of absolute back pressure to absolute relieving pressure. This curve may be extended to cover the tests with pressure ratios less than ,. This curve shall be used for establishing the coeficient of discharge at any set pressure and overpressure. )t shall also be used for establishing the coeficient of discharge under back pressure conditions. 7.3.3.5

Flow testing acceptance tolerance

)n all the methods described for low characteristics testing, all inal results shall be within ±  % of the arithmetic average to certify one common discharge coeficient. Where these tolerances are not achieved when testing, to produce the curve of coeficient of discharge versus the ratio of absolute back pressure to absolute relieving pressure greater than ,, the curve illustrating the lowest coeficient of discharge versus this ratio shall be accepted for the range of valves tested. 7.3.4

Adjustment during test

No adjustment to the valve shall be made during the tests. Following any changes or deviation in the test conditions, a suficient period of time shall be allowed to permit the rate of  low, temperature and pressure to reach stable conditions before readings are taken. 7.3.5

Records and test results

The test records shall include all observations, measurements, instrument readings and instrument calibration records (if required) for the objective(s) of the tests. Original test records shall remain in the custody of the test establishment that conducted the tests. Copies of all test records shall be supplied to each of the parties concerned with the tests. Corrections and corrected values shall be entered separately in the test record. The manufacturer or his authorized representative shall keep a copy of the test records and their additions for a period of  years after t he last of the safety valves has been manufactured. 7.3.6

Flow test equipment

The test equipment shall be designed and operated such that the actual test lowing capacity measurements are accurate to within ±  %.

. Determination of the coeficient of discharge The coeficient of discharge K d is calculated using Formula (1):

∑   ′

  qm 

n

K

d

=

qm

1

(1)

n

. Certiication of coeficient of discharge The certi ied de-rated coeficient of discharge K dr of the safety valve shall be not greater than  % of the coeficient of discharge K d determined by Formula : K dr

= 0, 9 K d

(2)

Neither the coeficient of discharge nor the certiied de-rated coeficient of discharge can be used to calculate the capacity at a lower relieving pressure than that at which the tests to determine the low 12 12



)SO -:+A:E ISO 4126-1:2013(E)

characteristics (see 7.3 were carried out, although they can be used to calculate the capacity at any higher relieving pressure.

8

Determination of safety valve performance

Refer to ISO 4126-7.

9

Sizing of safety valves

Refer to ISO 4126-7.

10 Marking and sealing 10.1 Marking 10.1.1 Marking on the shell of a safety valve

Marking on the shell of a safety valve may be integral with the shell or on a plate securely ixed on the shell. The following minimum information shall be marked on all safety valves: a size designat ion inlet, for example DN xxx; b material designation of the shell; c

manufacturer’s name or trademark;

d an arrow showing the direction of low where the inlet and outlet connections have the same dimensions or the same pressure rating. .. Marking on an identiication plate

The following information shall be given on an identiication plate securely ixed to the safety valve: a set pressure, in bar gauge or other internationally recognized unit; b reference to this part of )SO , i.e. )SO -:; c

manufacturer’s type reference;

d certiied de-rated coeficient of discharge indicating reference luid: G for gas, S for steam and L for liquid; NOTE The designation of the luid can be placed either before or after the certiied de-rated coeficient of discharge, e.g. G-,.

e

low area, in square millimetres or any other internationally recognized unit;

f

minimum value of the lift, in millimetres or any other internationally recognized unit, and corresponding overpressure, expressed as, for example, a percentage of set pressure;

g cold differential test pressure if applicable in bar gauge or other internationally recognized unit; h serial number or alternative coding to indicate year of manufacture.

10.2 Sealing of a safety valve All external adjustments shall be sealed.


 13


)SO ISO -:+A:E 4126-1:2013(E)

Bibliography []

)SO :, Pipework components — Deinition and selection of DN nominal size

[]

)SO , Pipe components — Deinition of nominal pressure

[]

ANS)/ASME B ., Valves — Flanged, threaded, and welding end

[]

AP) , Seat tightness of pressure relief valves

[]

EN  all parts, Industrial valves — Shell design strength

14 14



ISO 4126-1:2013(E)

National Annex NA (informative) Performance testing of safety valves for hot water duty NA.1 Type test recommendations for hot water duty NA.1.1 General (see BS EN ISO 4126-1, clause 7)

The statement in BS EN ISO 4126-1, 7.2.1 and 7.3.1, concerning valves for liquid service has been unanimously interpreted by Technical Committee PVE/12 as referring to liquids that do not change phase during discharge. This does not cover hot water above 100 C and in the absence of any specific requirements in BS EN ISO 4126-1 the committee recommends that the operating and flow characteristics for safety valves for use on hot water duty above 100 C are determined as follows: ˚

˚

NOTE Technical Committee PVE/12 has been replaced by Technical Committee PSE/18/6.

NA.1.2 Tests to determine the operating characteristics NA.1.2.1 Test fluid

Valves for use on hot water at or above 100 °C should be tested using steam. NA.1.3 Tests to determine flow characteristics NA.1.3.1 Test recommendations

It is not permissible to use water or any other liquid for the flow characteristic testing of a safety valve for use on hot water at or above 100 °C. NA.1.3.1.1 Safety valves for use on hot water at or above 100 °C

After operational characteristics have been established satisfactorily using steam as the test fluid, it is acceptable to use steam, air or another gas of known characteristics as the fluid for flow characteristic tests. When discharged quantities are being assessed using fluids other than steam, the valve disc should be held mechanically at the same lift as that obtained with steam at the same overpressure. NA.2 Determination of certified (discharge) capacity NA.2.1 Safety valve certified (discharge) capacity NA.2.1.1 Hot water at or above 100 °C

The certified (discharge) capacity of a safety valve, i.e. the rating of the safety valve, operating on water at or above 100 °C should be calculated a s follows: R kw = 0,329 p o AK dr

where R kw = The rating of the safety valve p o = The relieving pressure = set pressure + overpressure A = The safety valve flow area (not curtain area)

kW bar (abs.) mm2

K dr = The certified derated coefficient of discharge ( K d × 0,9)

15 15


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ISO 4126-1:2013(E)

BS EN ISO 4126−1:2004

NOTE 1 The kW, while not being the usual unit for the rating of a safety valve, has been used because the rated output of a hot water boiler is given in kW in the appropriate standards and for ease in selecting the correct complement of safety valves, identical units are essential. NOTE 2 The formula is based upon all the water flashing to steam and this will always size the largest flow area. The kW rating can be converted to steam flow in kg/h by multiplying kW by 1,6.

NA.3 Determination of coefficient of discharge NA.3.1 Coefficient of discharge

The coefficient of discharge K d should be calculated from the following equation:

K d =

Actual flowing capacity from test Theoretical flowing capacity from calculation

The certified derated coefficient of discharge K dr should be calculated from the following equation:

K dr = ( K d × 0,9)

NA.4 Sizing examples

Sizing examples are given in this subclause. A calculation for the sizing of a safety valve operating at temperatures at or above 100 ºC is given in Example 1. A similar calculation for a safety valve operating at temperatures below 100 ºC is given in Example 2 for comparison.

EXAMPLE 1 — Safety valves operating at or above 100 ºC

Calculate the required flow area of a safety valve to discharge hot water at 10 % overpressure under the following conditions: Safety valve certified derated discharge coefficient at 10 % overpressure = 0,8

16 16

Boiler PS

= 11,2 bar gauge

Set pressure

= 11,2 bar gauge

Water temperature

= 160 °C

Boiler rating

= 3 300 kW

p o = (11,2 × 1,1) + 1

= 13,32 bar (abs.)


Page 31

ISO 4126-1:2013(E)

BS EN ISO 4126−1:2004

The required area = A =

A =

R

0,329 po K dr

3300 2 = 941,29 mm 0,329 × 13,32× 0,8

A safety valve with a flow area ( A) greater than 941,29 mm 2 should be selected for this flow capacity.

EXAMPLE 2 — Safety valves operating below 100 ºC

Calculate the required flow area of a safety valve to discharge hot water at 10 % overpressure under the following conditions:

Safety valve certified derated discharge coefficient at 10 % overpressure = 0,5

Pressure vessel PS

= 11,2 bar gauge

Set pressure

= 11,2 bar gauge

Back pressure

= Atmospheric

Volumetric mass

= 925 kg/m3

Required capacity

= 5 000 kg/h

Water temperature

= 50 °C

p o = (11,2 × 1,1) + 1

= 13,32 bar (abs.)

Required flow area = A =

A =

Q m

1,61K dr

ρ

( po − p b )

5000 1,61 × 0,5 925(13,32 − 1)

2

= 58,18 mm

A safety valve with a flow area ( A) greater than 58,18 mm 2 should be selected for this flow capacity.

17 17


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ISO 4126-1:2013(E)

BS EN ISO 4126−1:2004

National Annex NB (informative) Supplementary information NB.1 Material recommendations and limitations NB.1.1 Cast iron and bronze NB.1.1.1 General

When cast iron, or bronze, materials are used in the manufacture of safety valve bodies or other pressure containing components the following limits should be met: NB.1.1.2 Flake graphite cast iron NB.1.1.1

This should only be used with minimum mechanical properties equal to BS EN 1561 grade EN-JL1030 and should not be used above 13 bar and 220 °C. This material should not be used for valve sizes above 200 mm when connected directly to a boiler. NB.1.1.3 Spheroidal graphite cast iron NB.1.1.2

This should only be used with minimum mechanical properties equal to BS EN 1563 grade EN-JS1030 and should not be used above 350 °C. NB.1.1.3 NB.1.1.4 Cast bronze

Copper alloy should only be used with minimum mechanical properties equal to BS EN 1982 CC491K and should not be used above 260 °C. NB.1.2 Steel NB.1.2.1 General

When steel materials are used in the manufacture of safety valve bodies or pressure containing components, they should comply with whichever of the following is applicable: No limits are recommended for pressure. NB.1.2.2 NB.1.2.1

Cast carbon steel

The following material specification should not be used at temperatures in excess of those stated. a) Carbon steel casting to BS EN 10213-2: (GP280GH, 1.0625). Max temperature 450 ºC. NB.1.2.3 NB.1.2.2

Cast low alloy steel

The following material specifications should not be used at temperatures in excess of those stated. a) Low alloy steel casting to BS EN 10213-2: (G20Mo5, 1.5419). Max temperature 500 ºC. b) Low alloy steel casting to BS EN 10213-2: (G17CrMo5-5, 1.7357). Max temperature 550 ºC. c) Low alloy steel casting to BS EN 10213-2: (G17CrMo9-10, 1.7379). Max temperature 550 ºC. NB.1.2.4 NB.1.2.3 Forged

18 18

carbon manganese steel


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ISO 4126-1:2013(E)

BS EN ISO 4126−1:2004

The following material specification should not be used at a temperature exceeding 400 ºC. a) BS EN 10222-2: (P245GH, 1.0352). Max temperature 400 ºC. NB.1.2.5 NB.1.2.4 Forged

chromium molybdenum steel

The following material specifications should not be used at temperatures in excess of 600 ºC. a) Chromium molybdenum alloy steel forging to BS EN 10222-2: (13CrMo4-5, 1.7335). b) Chromium molybdenum alloy steel forging to BS EN 10222-2: (14MoV6-3, 1.7715). c) Chromium molybdenum alloy steel forging to BS EN 10222-2: (15MnCrMoNiV5-3, 1.6920). d) Chromium molybdenum alloy steel forging to BS EN 10222-2: (14MoV6-3, 1.7715). Cast and forged austenitic chromium nickel steel, and also forged austenitic chromium nickel molybdenum steel

NB.1.2.6 NB.1.2.5

The following material specifications should not be used at temperatures in excess of those stated. a) Austenitic stainless steel casting to BS EN 10213-4: (GX5CrNiNb19-11, 1.4552) or (GX5CrNiMoNb19-11-2, 1.4581). Max temperature 550 ºC. b) Austenitic stainless steel forging to BS EN 10222-5: (X6 CrNi 18-10, 1.4948). Max temperature 600 ºC . Or (X6 CrNiNb 18-10, 1.4550) or (X3 CrNiMo 17-13-3, 1.4436). Max temperature of 500 ºC. NB.2 Spring recommendations NB.2.1 Helical compression springs

When the valve is at its maximum design lift, defined as that lift necessary to achieve rated capacity, the spring compression should be no greater than 80 % of the total deflection to solid length. NB.2.2 Torsion bar springs

Torsion bar springs should be made from steel, which has a tensile yield stress not less than 1 350 N/mm 2. When selecting the material from which the spring is manufactured, due account should be taken of the additional deflection and hence increased stresses that apply when the valve is at its design lift. The shear stress in the springs, when the safety valve is loaded to its set pressure, should not exceed 550 N/mm 2 when calculated in accordance with the following equation:

19 19


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ISO 4126-1:2013(E)

BS EN ISO 4126−1:2004

q =

16T 3 πd

Where q = shear stress

N/mm2

T = torque

N mm

d = bar diameter

mm

NB.3 Lifting lever recommendation

Every safety valve when fitted to a steam boiler, hot water boiler or compressed air pressure vessel should be fitted with a lifting lever to enable the valve disc to be eased off the seat while under a pressure of not less than 75 % of the safety valve set pressure. NB.4 Guidance on safety valve mounting and installation NB.4.1 Inlet pipework

Excessive pressure loss at the inlet of a safety valve will cause extreme rapid opening and closing of the valve, which is known as chatter or hammering. This may result in reduced capacity, damage to seating faces and other parts of the valve. The pressure loss at the inlet to the safety valve, under flowing conditions, should not exceed 3 % of the set pressure of the safety valve. The total pressure drop is calculated using the certified capacity divided by 0,9 and should include all pipework and fittings between the safety valve and the source of pressure. The adoption of the following recommendations will help to reduce these factors. a) Safety valves should be installed at least eight to ten pipe diameters downstream from any converging or diverging ‘Y’ fitting or any bend in the pipework. This distance should be increased if the direction or change of the fluid flow is from vertically upwards to horizontal in such a manner as to increase the density of the flow in the area directly beneath the safety valve nozzle. b) A safety valve branch should never be installed in pipework in a position directly opposite a branch on the lower side. c) Inlet branches should have: 1) Corner radiuses radii to be tonot be not lessless thanthan one-quarter one-quarter of the of bore, the bore, or or 2) A taper bore with an inlet area approximately twice that of the outlet, or 3) The branch entrance rounded at the downstream corner to a radius of not less than one-quarter of the bore. This radius should be reduced gradually, leaving only a small portion of the upstream corner sharp.

 20


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ISO 4126-1:2013(E)

BS EN ISO 4126−1:2004

NB.4.2 Pipework vibrations

Excessive pipework vibrations are known to produce inconsistencies in safety valve set pressures and induce chatter. On installations where there is pressure pulsation, it is advisable to locate the safety valve in a region where the effects of the pulsation are minimized consistent with maintaining correct operation of the safety valve. For assisted opening and supplementary loaded safety valves, to minimize the effect of localized pulsation and system vibrations in the pressure tapping line when the valves are operating, the connection for the pressure t apping line at the pressure source should be located as remotely as possible from the safety valve mounting. The line should incorporate a siphon, where applicable, and stop valves with a t est connection so that the pressure settings and the correct functioning of the instrumentation can be checked. NB.4.3 Valve location and position

If a vessel contains both liquid and gas, and gas is to be relieved, a safety valve for use with gas should be connected to the vessel in the gas space or to pipework connected to this space and located in a position chosen to minimize the entrapment of liquid when the valve discharges. Alternativel y, if liquid is to be relieved, a safet y valve for use with liquid should be connected to the vessel or pipework below the liquid level at a point chosen to prevent ingress of gas. NB.4.4 Discharge pipework

The discharge pipe should be at least equal to the bore of the safety valve outlet and should be designed to limit the built-up back pressure to 10 % of the safety valve set pressure, when the valve is discharging the certified (discharge) capacity. If the calculated back pressure is greater than that specified above, this should only be accepted with agreement of the safety valve manufacturer. All associated discharge pipework should be installed in such a way that it will not impose undue stresses on the safety valve that could result in distortion and leakage at pressures below the set point. Discharge piping should not be supported by the safety valve. The maximum mass on the outlet of the valve should not exceed the mass of the valve exhaust pipe, i.e. flange elbow and short pipe and drip pan where fitted. The distance from the discharge piping centre line to the centre line of the valve should be kept to a minimum. Clearances between the valve exhaust piping and the discharge pipework should be sufficient to avoid any additional forces being applied to the valve due to thermal expansion of the boiler and discharge pipework and/or discharge pipework vibration. Where there is possibility of a liquid head forming in the discharge pipe, a drain should be provided and should lead to a safe location. If discharge pipes ar e fitted with devices to prevent ingress of rainwater or foreign objects, these devices should not obstruct the full discharge of the safety valve. NB.4.5 Multiple valves

When multiple valves have a common discharge pipe, it should be large enough so that when one or more valves are discharging, the superimposed back pressure on the remaining valves does not have a significant effect on their operational and flow characteristic.

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ISO 4126-1:2013(E)

BS EN ISO 4126−1:2004

A superimposed back pressure will increase the set pressure of a conventional valve. If the back pressure is both known and constant, its effect should be compensated for by reducing the spring load by an amount equal to the constant back pressure. If the back pressure is unknown or is variable, a balanced bellows/piston valve should be used; in which case adequate venting of the bonnet space should be provided. NB.5 Operating system NB.5.1 Recommendations of the operating system to be protected

The design of a vessel or pipework system is dictated by the operating pressure of the process, the relieving characteristics of the safety valve used to protect the vessel and the maximum accumulated pressure permitted by the pressure vessel standard to which the vessel is designed and manufactured. The normal operating pressure of the system should be below the reseating pressure of the safety valve, the difference being chosen on the basis of the probable variations in operating pressure due to process factors and the tolerance on cold differential test pressure. The valve seat/nozzle sealing characteristics are reduced as the operating pressure gets closer to the valve set pressure. For this reason a minimum of 10 % should be achieved between valve set pressure and the s ystem operating pressure, except when the reseating pressure is greater than 10 % of the set pressure. With assisted and supplementary loaded safety valves, failure of the assist mechanism or of the supplementary load to be released may result in the valve remaining closed until a higher than desired set pressure is reached. If the integrity of the assist mechanism or release of the supplementary load cannot be assured, the set pressure of the valve should be such that, in the event of failure, the required capacity will still be achieved at the desired overpressure. Where the safety valve requires a greater overpressure than 10 % to achieve its rated capacity, this should be taken into account when determining the safety valve set pressure. The set pressure must be chosen to ensure that the maximum permitted accumulation pressure does not exceed 10 % of the design pressure of the vessel to comply with regulatory limitation.

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BS-EN-ISO-4126-1-2013-A1-2016

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