Introduction
In Europe, only pressure safety relief valves with settings above 0.5 barg which conform with the Pressure Equipment Di-rective (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 proce-dure also drawn from the PED.
The harmonized standards or other tech-nical reference works are stated in a man-ufacturer’s declaration of conformity, which is supplied with the pressure safety relief valve at delivery. Only this ultimate-ly makes it possible to establish the basis used for CE certification and the certified properties which can be derived there-from. It should be noted that the CE sym-bol on the identification plate alone does not supply sufficient information for this purpose.
A comparative assessment of the harmo-nized EN standards compared against the AD and TRD technical rules in this article discloses differences in the certified prop-erties and the applications for spring-loaded pressure safety relief valves.This is explained in section 5 of the recently de-veloped 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 out-side Europe.
Codes: structure & comparison
Harmonized European Standards It is compulsory that the CEN – members allocate and are obliged to give the new European standard without any modifica-tion the full same status as the namodifica-tional standard.The national standardisation in-stitute 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 classifica-tion 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 de-vices in combination
- EN ISO 4126-4
Pilot operated safety valves - EN ISO 4126-5
Controlled safety pressure relief sys-tems (CSPRS)
- EN ISO 4126-6
Fig.1: New type Si 43 EN-conformant pressure safety relief valve (closed bonnet)
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
Application, selection and installation of bursting disc safety devices
- EN ISO 4126-7 Common Data
b) Application standards. Contain the se-lection 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 ex-cessive 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 pres-sure safety relief valves, assisted prespres-sure safety relief valves and safety shut-off valves.They contain definitions, functional characteristics, types, design require-ments, determination of size, mounting, requirements pressure loss inlet pipe and back pressure, recurrent test, identifica-tion.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.
Design requirements
for pressure safety
“VdTÜV Merkblatt Sicherheitsventil 100” comprises requirements as to the execu-tion 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 character-istics for pressure safety relief valves
Functional characteristic
The functional characteristic of spring-loaded 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 re-lief valve start to open, the percental in-crease in pressure (opening pressure dif-ference = accumulation %) until reaching the pre-determined lift, is defined as opening. For closing the decrease in pres-sure (closing prespres-sure 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 stat-ed by the manufacturer are checkstat-ed and certified.There is only an upper limit de-termined compared to which the manu-facturer indication may be smaller.The consequence is that the customer, normal-ly the project engineer, must always speci-fy 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 AD-A2 / TRD 421 it is fixed to 10%, and ac-cording 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 ac-cording to EN ISO 4126-1 that the closing
pressure difference with the different manufacturers is 7%, 10% or under cer-tain 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 re-lief valve is determined by the product w*A0. In this connection wis the
certi-fied discharge coefficient (or flow coeffi-cient) and A0is the smallest cross-section
area inside the inlet nozzle ahead of the valve seat.The certified discharge coeffi-cient (or flow coefficoeffi-cient) may also have the formula symbol Kdr(discharge
coeffi-cient). It is always reduced by approxi-mately 10%, either w= /1.1 is valid or
Kdr= Kd* 0.9.
However, there are differences in the de-termination of the discharge coefficients. Generally, the conditional equations ac-cording 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“
in AD-A2 and TRD 421 in the sizing equa-tions.This proceeding is conservative and was adopted by EN ISO 4126-1 or –7, re-spectively! 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 satu-rated 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 Napier-formula, is applied which results for satu-rated steam within a pressure range small-er than 200 bar a mass flow approximately 3% greater and for a pressure range ex-ceeding 210 bar a mass flow which is up to 5% smaller. For superheated steam there is then a correction factor KSHindicated in
table form in the “API Recommended Practice 520“, the accuracy of which may be checked in comparison!
AD A2 / TRD 421 EN 4126-1 ASME VIII ASME I
Gas/vapour/steam Gas/vapour/steam Gas/vapour/steam Steam Open max. 5% / 10% Manufacturer indication max. 10% or 0.2 bar 3%*
or 0.1 bar# max. 10% or 0.1 bar
Close Max. 10% or 0.3 bar Manufacturer indication max. 7%* or 0.2 bar max. 4%*
max. 15% or 0.3 bar or 0.28 bar
AD A2 / TRD 421 EN 4126-1 ASME VIII ASME I
Liquids Liquids Liquids Liquids
Open max. 10%or 0.1 bar Manufacturer indication max. 10% or 0.2 bar N/A max. 10% or 0.1 bar
Close max. 20% or 0.6 bar Manufacturer indication Not specified N/A max. 20% or 0.6 bar
Table 1: Comparison: Required function values for pressure safety relief valves, gas and steam
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)
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, multi-plied by 0.9, will be declared as the certi-fied discharge coefficient. By this, the dis-charge 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 dis-charge coefficient on the lift is not deter-mined so that the possibility of a lift re-striction 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 concern-ing 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 ac-cording to VdTÜV 100 for AD and TRD Instead of 3 x 3 individual tests according to ASME the function values are deter-mined by means of function records ac-cording to VdTÜV 100 for AD and TRD not only for fixed spring adjustments but also over the adjusting range of the indi-vidual spring. In this connection the ob-tained lift is demonstrated.These meas-urements are carried out with
representative valve sizes and pressures. Then, the discharge coefficient of repre-sentative valve sizes is measured versus the lift and for gas / vapour / steam also ver-sus the pressure ratio pa/p.The certified
w-curves are defined from the measured
-curves by division by 1.1.Thus, a capac-ity reserve of at least 10% in contrast to ASME is always assured and by determina-tion of the discharge coefficient versus the lift, precise capacity limitations by lift re-strictions 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 in-fluence of the built-up back pressure is also measured.
The test conditions are recorded in the test report and conclusions for the appli-cation 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, respective-ly. However, there is no publication in-tended 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 operat-ing pressure.This is also valid if the valve reaches the required lift at a smaller over-pressure than 10% above set over-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 op-erating pressure.
Furthermore, it is possible in accordance with DIN EN 764-7 that in cases of over-pressure smaller than/equal to 5% the set pressure of the pressure safety relief valve may be setted to up to 105% of the maxi-mum allowable operating pressure if at least one pressure safety relief valve out of several is adjusted and set to the maxi-mum allowable operating pressure. Anoth-er option is that the set pressure of even one pressure safety relief valve may be higher than the maximum allowable oper-ating pressure if an additional “pressure limiting device” is provided.
Controlled safety pressure relief systems (CSPRS)
Commonly known as “Gesteuerte Sicher-heitsventile” (assisted safety valves).These safety valve systems have only been ap-proved in accordance with the Technical Rules AD-A2 or TRD 421 or by special authorization (Stoomwezen).With the Pressure Equipment Directive and the
harmonized European Standards, CSPRSs are authorized in the CEN – member countries.The “Assisted pressure safety re-lief valves“ are dealt with in the product standard EN ISO 4126-5 and have the des-ignation in the English version “Controlled safety pressure relief system“ (CSPRS). For an extensive treatment of the assisted pressure safety relief valves and a compar-ison with pilot operated pressure safety relief valves according to EN ISO 4126-4 we refer to the publication[2]. It was pub-lished 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 profes-sional 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 demon-strated 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 devel-opment method [1]. Approval was simul-taneously obtained according to ASME VIII by N.B. (national board) and accord-ing to TRD and AD by TÜV.
In accordance with the same criteria, the development of a new pressure safety re-lief 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 ap-proval 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 character-istics were stated and specified:
* Conservatively, the lowest discharge co-efficient curve w(h/do) of several
rep-resentative valve sizes was reduced by division by 1.1.Thus, a capacity reserve of 10% is always safeguarded and if nec-essary, capacity limitations may be ap-plied by lift restrictions (Figure 2).
* 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 effi-ciency 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 sure if at least one out of several
pres-sure safety relief valves is set to the max-imum allowable operation pressure. De-viating from this, the adjusting pressure of a pressure safety relief valve may be higher than the maximum allowable op-eration pressure, if an additional “pres-sure 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 pres-sure of the prespres-sure safety relief valve. * The new Bopp & Reuther pressure
safe-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 sharp-edged 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 sharp-edged inlet (usually = 0.5) need not be taken into consideration.
Back pressure conditions
In order to justify that the pressure safety relief valve, under back pressure condi-tions, 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 in-creased 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-Steam / gas: wD/G = 0.81* /0.78**
* with low pressure loss inlet (well rounded) ** with pressure loss at sharp edged inlet Liquids: wF = 0.57
Dotted line: with sharp edged inlet (w= 0.78 for D/G): 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.
L = Length of the inlet line, mm D = Diameter of the inlet line, mm h = lift, mm
do = flow diameter, mm pa = back pressure, bara p = system pressure, bara
Fig.3: Test condition with slip-through sharp-edged 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
Sharp-edged inlet Nozzle set up for testing with
different back pressure Pg
Pg Exhaust pipe
ed.The lift-pressure-diagram shows sud-den opening at an increase in pressure of only 2% and a stable lift stop with great force reserve until closing at approximate-ly 10% blow down. Figure 5 shows the functional test where the only modifica-tion 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 dif-ference 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-characteristic-measurement [1].
Finally, the nameplate was produced ac-cording to PED, i.e. CE marked and addi-tionally indication of the type approval symbol
“TÜV.SV.02-1094.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
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 dif-ferent requirements. However, the project engineer, too, has to specify with better care and more precision what is required and has to consider these aspects accord-ingly during the plant-planning.
The new pressure safety relief valve type Si 41/43/44 presented here combines ad-vantages for the user which are “New” in this combination. Furthermore, require-ments according to PED and AD-/TRD-Code are fulfilled at the same time. ■
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.
Fig. 4: Function test with air without nozzle set up for built up back pressure in the exhaust pipe according to Fig. 3.
Lift and back pressure versus system pressure Lift versus time
Pressure versus time
Lift and back pressure versus system pressure
Lift versus time
Pressure versus time
Fig. 5: Function test with air with nozzle set up for built up back pressure in the exhaust pipe according to Figure 3.
About the authors
Dr. Ing. Bernhard Follmer, born 17.05.1950. Head of Design and Development of Bopp & Reuther Sicherheits- und Regelarmaturen GmbH.
Dr. Ing. Armin Schnettler, born 08.09.1950. Head of Development/ Test Laboratory of Bopp & Reuther Sicherheits- und Regelarmaturen GmbH.
Test – No FUL02381.BSN
With Exhaust pipe DN 40 With nozzle set-up
Test specimen Si 4302 A DN 25 x 40 do = 22mm
Results
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% Test – No FUL02384.BSN
With exhaust pipe DN 40 Without nozzle set-up
Test specimen Si 4302 A DN 25 x 40 do = 22mm
Results
Set pressure P 14.5 bar_g Opening pressure Pc 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
