Design Requirements for Pressure Relief Valves

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SAFETY VALVES

Design requirements for pressure safety relief valves In this paper, the authors review the standards applicable to pressure safety relief valves, such as EN (PED), AD/TRD and ASME. They also reveal some of the testing and evaluation work that is required when designing a new pressure safety relief valve, using a Bopp & Reuther valve as an example. By Dr. Ing. Bernhard Föllmer and Dr. Ing.Armin Schnettler, Bopp & Reuther, Mannheim

Introduction

Codes: structure & comparison

In Europe, only pressure safety relief valves with settings above 0.5 barg which conform with the Pressure Equipment Directive (PED) 97/23 EC may be used. They are classified PED Category IV.A Notified Body validates the fulfilment of the PED requirements in accordance with a selected conformity evaluation procedure also drawn from the PED. The harmonized standards or other technical reference works are stated in a manufacturer’s declaration of conformity, which is supplied with the pressure safety relief valve at delivery. Only this ultimately makes it possible to establish the basis used for CE certification and the certified properties which can be derived therefrom. It should be noted that the CE symbol on the identification plate alone does not supply sufficient information for this purpose. A comparative assessment of the harmonized EN standards compared against the AD and TRD technical rules in this article discloses differences in the certified properties and the applications for springloaded pressure safety relief valves.This is explained in section 5 of the recently developed new Bopp & Reuther series of pressure safety relief valves.The ASME code is also included in the assessment, since it plays a significant role at least outside Europe.

Harmonized European Standards It is compulsory that the CEN – members allocate and are obliged to give the new European standard without any modification the full same status as the national standard.The national standardisation institute CEN – members are: Belgium, Denmark, Germany, Finland, France, Greece, Ireland, Island, Italy, Luxemburg, Malta,The Netherlands, Norway,Austria, Portugal, Sweden, Switzerland, Spain, the Czech Republic and the United Kingdom. For systems that require an over pressure protection device the following classification is valid: a) Product standards. Contain definitions, design requirements, production tests, type tests, functional characteristics, definition of size, identification.The sections are: - EN ISO 4126-1 Pressure safety relief valves - EN ISO 4126-2 Bursting disc safety devices - EN ISO 4126-3 Safety valves and bursting disc safety devices in combination - EN ISO 4126-4 Pilot operated safety valves - EN ISO 4126-5 Controlled safety pressure relief systems (CSPRS) - EN ISO 4126-6 www.valve-world.net

Fig.1: New type Si 43 EN-conformant pressure safety relief valve (closed bonnet)

Application, selection and installation of bursting disc safety devices - EN ISO 4126-7 Common Data b) Application standards. Contain the selection of the safety devices, mounting, requirements pressure drop inlet pipe and back pressure, recurrent test.The sections are: - DIN EN 12952 -10 Water-tube boilers, safety installations against excessive pressure - DIN EN 12953 - 8 Shell boilers, safety devices against excessive pressure - DIN EN 764 - 7 Unfired pressure vessels Section 7: Safety devices Applicable Technical Rules for Germany AD 2000 - A2 for pressure vessels and TRD 421 for steam boilers deal with pressure safety relief valves, assisted pressure safety relief valves and safety shut-off valves.They contain definitions, functional characteristics, types, design requirements, determination of size, mounting, requirements pressure loss inlet pipe and back pressure, recurrent test, identification.That means they cover a wide range. Note:The AD 2000 – rule may be applied for the implementation of the basic safety requirements of the Pressure Equipment Directive. DECEMBER 2004

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“VdTÜV Merkblatt Sicherheitsventil 100” comprises requirements as to the execution of the type test approval for pressure safety relief valves and assisted pressure safety relief valves. ASME - Code In the ASME-Code, pressure safety relief valves are classified into: - ASME Sec.I Steam generators (V- Stamp) - ASME Sec.VIII Pressure vessels (UV- Stamp) - ASME Sec. III Nuclear power stations (NV- Stamp) Comparison of performance characteristics for pressure safety relief valves

Functional characteristic The functional characteristic of springloaded pressure safety relief valves is the result of the combination of flow force and spring force. In relation with the set overpressure where the pressure safety relief valve start to open, the percental increase in pressure (opening pressure difference = accumulation %) until reaching the pre-determined lift, is defined as opening. For closing the decrease in pressure (closing pressure difference = blow down %) is defined accordingly. Looking at Tables 1 and 2, in all rules the function values are fixed at a maximum, with the exception of EN ISO 4126-1! Here, those function values which are stated by the manufacturer are checked and certified.There is only an upper limit determined compared to which the manufacturer indication may be smaller.The consequence is that the customer, normally the project engineer, must always specify the desired function values and insist on the test reports from the manufacturer! The following example shows how the closing pressure difference for gas/steam can be addressed. In accordance with ADA2 / TRD 421 it is fixed to 10%, and according to EN ISO 4126-1 it may also be max 15%. If the Pressure safety relief valves originally had an approval according to ASME VIII – and has an adjusting ring, the manufacturer would also declare 7% according to EN ISO 4126-1.That means that it is possible with the certification according to EN ISO 4126-1 that the closing 2 Valve World

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AD A2 / TRD 421

Open Close

EN 4126-1

ASME VIII

ASME I

Gas/vapour/steam

Gas/vapour/steam

Gas/vapour/steam

Steam

max. 5% / 10% or 0.1 bar# Max. 10% or 0.3 bar

Manufacturer indication max. 10% or 0.1 bar Manufacturer indication max. 15% or 0.3 bar

max. 10% or 0.2 bar

3%*

max. 7%* or 0.2 bar

max. 4%* or 0.28 bar

Table 1: Comparison: Required function values for pressure safety relief valves, gas and steam

Open

AD A2 / TRD 421 Liquids max. 10%or 0.1 bar

Close

max. 20% or 0.6 bar

EN 4126-1 Liquids Manufacturer indication max. 10% or 0.1 bar Manufacturer indication max. 20% or 0.6 bar

ASME VIII Liquids max. 10% or 0.2 bar

ASME I Liquids N/A

Not specified

N/A

Table 2: Comparison: Required function values for pressure safety relief valves, liquids

Notes: # Generally,the indication “max.10% or 0.1 bar“,means that the greater value is always applicable. * obtained only with adjusting ring(s)

pressure difference with the different manufacturers is 7%, 10% or under certain conditions up to 15%.This results in considerable differences relating to the operating pressure which has to be smaller than the closing pressure of the pressure safety relief valve. It is necessary for the project engineer to specify the desired function values in the future and demand them from the manufacturer! Flow capacity comparison The flow capacity of a pressure safety relief valve is determined by the product
w*A0. In this connection
w is the certified discharge coefficient (or flow coefficient) and A0 is the smallest cross-section area inside the inlet nozzle ahead of the valve seat.The certified discharge coefficient (or flow coefficient) may also have the formula symbol Kdr (discharge coefficient). It is always reduced by approximately 10%, either
w =
/1.1 is valid or Kdr = Kd * 0.9. However, there are differences in the determination of the discharge coefficients. Generally, the conditional equations according to AD-A2 / TRD 421 and EN ISO 4126-1 correspond to each other, even if the forms look different. For saturated steam the sizing coefficient x (“Druckmittelbeiwert”) is formed with the isentropic exponents of the “wet side“ www.valve-world.net

in AD-A2 and TRD 421 in the sizing equations.This proceeding is conservative and was adopted by EN ISO 4126-1 or –7, respectively! However, a modification to the application was carried out:As because of some degrees of superheating above the saturated steam condition, the isentropic exponent increases from approximately 1.1 to 1.3, there is also a sudden increase in the calculated mass flow within a range smaller than 200 bar by approximately 5% until 10%! Of course, this cannot happen and therefore, in the EN ISO 4126-7 (Common Data) the steam from the saturated line until 10°C superheating was treated as dry saturated steam. For steam with a superheating from 10°C until 30°C superheating, the values v (spec. volume),  (isentropic exponent) were linearized and adapted! ASME sizing differs for steam. Here, an empiric equation, the so-called Napierformula, is applied which results for saturated steam within a pressure range smaller than 200 bar a mass flow approximately 3% greater and for a pressure range exceeding 210 bar a mass flow which is up to 5% smaller. For superheated steam there is then a correction factor KSH indicated in table form in the “API Recommended Practice 520“, the accuracy of which may be checked in comparison!

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Determining discharge coefficients according to ASME The discharge coefficients of a maximum of 3 x 3 test valves must be within ± 5% of their average value at 10% above the set pressure.Then, the average value, multiplied by 0.9, will be declared as the certified discharge coefficient. By this, the discharge coefficient is independent of the pressure ratio for gas/vapour/steam and it is accepted that the safety margin of the discharge capacity is between 5% and 15%! Note:The dependence of the discharge coefficient on the lift is not determined so that the possibility of a lift restriction in case of over dimensioning is not included. The test condition is with low pressure loss in the inlet (well rounded connection) and relief into the atmosphere without back pressure.A detailed report concerning the requirements, development and approval of a new pressure safety relief valve series according to ASME VIII and at the same time TÜV – type approval in 1995 is contained in [1]. Determining discharge coefficients according toVdTÜV 100 for AD and TRD Instead of 3 x 3 individual tests according to ASME the function values are determined by means of function records according to VdTÜV 100 for AD and TRD not only for fixed spring adjustments but also over the adjusting range of the individual spring. In this connection the obtained lift is demonstrated.These measurements are carried out with representative valve sizes and pressures. Then, the discharge coefficient of representative valve sizes is measured versus the lift and for gas / vapour / steam also versus the pressure ratio pa/p.The certified
w-curves are defined from the measured
-curves by division by 1.1.Thus, a capacity reserve of at least 10% in contrast to ASME is always assured and by determination of the discharge coefficient versus the lift, precise capacity limitations by lift restrictions can be applied if necessary. The test conditions may be well rounded inlet (with low pressure loss) or max. sharp-edged inlet. In the last time the influence of the built-up back pressure is also measured.

The test conditions are recorded in the test report and conclusions for the application are specified.The essential results of the type approval are published in a VdTÜV-Merkblatt under the so-called BKZ-number (type approval number) and are freely accessible. Determining discharge coefficients according to EN ISO 4126-1 In principle the procedure according to VdTÜV 100 or ASME is possible, but it must be identified or asked for, respectively. However, there is no publication intended as for the TÜV type approval for instance in a VdTÜV – Merkblatt. Sizing and spring setting according to DIN EN 764-7 The discharge pressure for the sizing, i. e. the calculation of the smallest flow area of a Pressure safety relief valve, is generally 1.1 times the maximum allowable operating pressure.This is also valid if the valve reaches the required lift at a smaller overpressure than 10% above set pressure. Even if the set pressure is smaller than the maximum allowable operating pressure, the discharge pressure for the sizing would be 1.1 times the maximum allowable operating pressure. Furthermore, it is possible in accordance with DIN EN 764-7 that in cases of overpressure smaller than/equal to 5% the set pressure of the pressure safety relief valve may be setted to up to 105% of the maximum allowable operating pressure if at least one pressure safety relief valve out of several is adjusted and set to the maximum allowable operating pressure.Another option is that the set pressure of even one pressure safety relief valve may be higher than the maximum allowable operating pressure if an additional “pressure limiting device” is provided. Controlled safety pressure relief systems (CSPRS)

Commonly known as “Gesteuerte Sicherheitsventile” (assisted safety valves).These safety valve systems have only been approved in accordance with the Technical Rules AD-A2 or TRD 421 or by special authorization (Stoomwezen).With the Pressure Equipment Directive and the www.valve-world.net

harmonized European Standards, CSPRSs are authorized in the CEN – member countries.The “Assisted pressure safety relief valves“ are dealt with in the product standard EN ISO 4126-5 and have the designation in the English version “Controlled safety pressure relief system“ (CSPRS). For an extensive treatment of the assisted pressure safety relief valves and a comparison with pilot operated pressure safety relief valves according to EN ISO 4126-4 we refer to the publication[2]. It was published in 1995 and at that time the product standards were called EN 1268-1 until –7 although they are now EN ISO 4126-1 until –7, all other parts from the professional statements of the publication [2] are entirely valid. ”New” designed pressure safety relief valve Type Si 41/43/44 according to EN Standard

As early as 1995 Bopp & Reuther demonstrated how a pressure safety relief valve (Series Si 81/83/84 according to design API 526) could be generated (without the use of the rings) using a systematic development method [1].Approval was simultaneously obtained according to ASME VIII by N.B. (national board) and according to TRD and AD by TÜV. In accordance with the same criteria, the development of a new pressure safety relief valve type Si 41/43/44 (Figure 1) was launched and approved in 2002 according to (i) PED category IV by conformity evaluation procedure module B, i.e. EG design test, in combination with module D, i.e. quality assurance production and (ii) by TÜV - type test with type test approval number SV.02-1094 according to VdTÜV-Merkblatt Pressure safety relief valve 100/1 (10.2002) with requirements according to prEN ISO 4126-1 (12.2002) and AD 2000 – A2 ,TRD 421. In accordance with the statements of the previous chapters a number of characteristics were stated and specified: * Conservatively, the lowest discharge coefficient curve
w(h/do) of several representative valve sizes was reduced by division by 1.1.Thus, a capacity reserve of 10% is always safeguarded and if necessary, capacity limitations may be applied by lift restrictions (Figure 2). DECEMBER 2004

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SAFETY VALVES

* Opening pressure differences 5% and closing pressure difference 7% for gas/vapour/steam (follows from the manufacturer’s indication with EN ISO 4126-1 documented according to VdTÜV-Merkblatt SV 100). This generates advantages for the efficiency of the process conditions The opening pressure difference 5% permits, according to DIN EN 764-7, that one or more pressure safety relief valves may be adjusted to until 105% of the maximum allowable operation pressure if at least one out of several pres-

sure safety relief valves is set to the maximum allowable operation pressure. Deviating from this, the adjusting pressure of a pressure safety relief valve may be higher than the maximum allowable operation pressure, if an additional “pressure limiting device” is installed. * The closing pressure difference of 7% permits a higher operating pressure than with greater closing pressure differences as the service pressure must always be considerably lower than the closing pressure of the pressure safety relief valve. * The new Bopp & Reuther pressure safe-

Sharp-edged inlet

Fig.3: Test condition with slip-through sharpedged inlet L= 5 * D and nozzle set up for testing with different built up back pressure in the exhaust pipe; certification of performance at back pressure Pg = 20% of the set pressure

In order to justify that the pressure safety relief valve, under back pressure conditions, functions and provides the required lift, these were measured at a built-up back pressure of 20% of the set pressure for gas/vapour/steam.The test condition in Figure 3 shows how the back pressure in the discharge pipe of the pressure safety relief valve was obtained by throttling by a nozzle at the end of the pipe. In Figure 4 a functional test with air is shown where the back pressure was not additionally increased by the throttle.As a consequence of the large discharge capacity of the type Si 41/43/44 a built-up back pressure of 15% without throttling is already generat-

= Length of the inlet line, mm = Diameter of the inlet line, mm = lift, mm = flow diameter, mm = back pressure, bara = system pressure, bara

Fig.2: Discharge coefficient
w DECEMBER 2004

Nozzle set up for testing with different back pressure Pg

Back pressure conditions

If the inlet line fulfils the requirement L/D < 5 or L < 200 mm (holds for the larger value) then no design report for pressure loss less than 3% required. For longer inlet line a design report for pressure loss is required not taking into account the pressure loss of the sharp edged inlet.

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Exhaust pipe

ty relief valve was checked according to “VdTÜV Merkblatt SV 100“ with well rounded inlet (low pressure loss inlet). Under these conditions the
w–value is 0.81 for gas/vapour/steam. * The pressure safety relief valve was also stringently tested according to VdTÜV Merkblatt SV 100 with sharp-edged inlet. Under these conditions the
w–value is 0.78 for gas/vapour/steam (Figure 3).Than the verification of the pressure loss less than 3% by the project engineer is not necessary if the sharpedged inlet pipe has a length of L/D < 5 or L < 200 mm in accordance with the test condition in Figure 3. Otherwise in the required pressure loss verification the resistance coefficient of the sharpedged inlet (usually  = 0.5) need not be taken into consideration.

Steam / gas:
w D/G = 0.81* /0.78** * with low pressure loss inlet (well rounded) ** with pressure loss at sharp edged inlet Liquids:
w F = 0.57 Dotted line: with sharp edged inlet (
w = 0.78 for D/G):

L D h do pa p

Pg

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SAFETY VALVES

ed.The lift-pressure-diagram shows sudden opening at an increase in pressure of only 2% and a stable lift stop with great force reserve until closing at approximately 10% blow down. Figure 5 shows the functional test where the only modification is the throttle at the end of the pipe, with which a built-up back pressure was accumulated until 29.6%.The result is a still stable functional behaviour with full lift, i. e. without capacity reduction. In this connection, the closing pressure difference has decreased until approximately 3% and furnishes information with regard to the quality of the flow force curves and

an optimal balancing with the spring which is a result of the development method with the force-characteristicmeasurement [1]. Finally, the nameplate was produced according to PED, i.e. CE marked and additionally indication of the type approval symbol “TÜV.SV.021094.do.D/G(F).
w.p”.Thus, the access to the published VdTÜV – Merkblatt is also possible. Final observation

Anyone who acts as a manufacturer in the industrial power and process markets in

Lift versus time

Test – No FUL02384.BSN With exhaust pipe DN 40 Without nozzle set-up

Pressure versus time

Test specimen Si 4302 A DN 25 x 40 do = 22mm Results

Lift and back pressure versus system pressure

Germany as well as the rest of Europe, as well as in the field of pressure safety relief valves outside of Europe, has to fulfil different requirements. However, the project engineer, too, has to specify with better care and more precision what is required and has to consider these aspects accordingly during the plant-planning. The new pressure safety relief valve type Si 41/43/44 presented here combines advantages for the user which are “New” in this combination. Furthermore, requirements according to PED and AD-/TRDCode are fulfilled at the same time. ■

Set pressure

P

Opening pressure Pc

14.5 bar_g 14.8 bar_g

Closing pressure

Ps

13 bar_g

Accumulation

dPc

2%

Blow down

dPs

10.3%

Back pressure

Pg

2.3 bar_g

Suggested reading [1] Föllmer, B., Schnettler, A. Challenges in designing API safety relief valves, Valve World, October 2003, p 39. [2] Bung, W., Föllmer, B. Assisted safety valves in power stations according to the German Rules. VGB Kraftwerkstechnik 75, (1995), issue 9, p. 771 – 776. [3] Lester Millard. Safety relief valves protecting life and property, Valve World, June 2002, p. 39.

Pg / P 15%

Fig. 4: Function test with air without nozzle set up for built up back pressure in the exhaust pipe according to Fig. 3. Lift versus time

Test – No FUL02381.BSN With Exhaust pipe DN 40

About the authors

With nozzle set-up

Pressure versus time

Dr. Ing. Bernhard Follmer, born 17.05.1950. Head of Design and Development of Bopp & Reuther Sicherheits- und Regelarmaturen GmbH.

Test specimen Si 4302 A DN 25 x 40 do = 22mm Results

Lift and back pressure versus system pressure

Set pressure

P

14.5 bar_g

Opening pressure Pc

14.8 bar_g

Closing pressure Ps

14.1 bar_g

Accumulation

dPc

2%

Blow down

dPs

2.8%

Back pressure

Pg

4.3 bar_g

Pg / P 29.6%

Dr. Ing. Armin Schnettler, born 08.09.1950. Head of Development/ Test Laboratory of Bopp & Reuther Sicherheits- und Regelarmaturen GmbH.

Fig. 5: Function test with air with nozzle set up for built up back pressure in the exhaust pipe according to Figure 3.

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