oSIST prEN ISO 24664:2021

SLOVENSKI STANDARD
oSIST prEN ISO 24664:2021
01-junij-2021
Hladilni sistemi in toplotne črpalke - Tlačne varnostne naprave in njihove
napeljave - Metode za izračun (ISO/DIS 24664:2021)
Refrigerating systems and heat pumps - Pressure relief devices and their associated
piping - Methods for calculation (ISO/DIS 24664:2021)
Kälteanlagen und Wärmepumpen - Druckentlastungseinrichtungen und zugehörige
Leitungen - Berechnungsverfahren (ISO/DIS 24664:2021)
Systèmes frigorifiques et pompes à chaleur - Dispositifs de limitation de pression et
tuyauteries associées - Méthodes de calcul (ISO/DIS 24664:2021)
Ta slovenski standard je istoveten z: prEN ISO 24664
ICS:
27.080 Toplotne črpalke Heat pumps
27.200 Hladilna tehnologija Refrigerating technology
oSIST prEN ISO 24664:2021 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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oSIST prEN ISO 24664:2021

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oSIST prEN ISO 24664:2021
DRAFT INTERNATIONAL STANDARD
ISO/DIS 24664
ISO/TC 86/SC 1 Secretariat: ANSI
Voting begins on: Voting terminates on:
2021-03-25 2021-06-17
Refrigerating systems and heat pumps — Pressure
relief devices and their associated piping — Methods for
calculation
Systèmes de réfrigeration et pompes à chaleur — Dispositifs de surpression et tuyauteries associèes —
Méthodes de calcul
ICS: 27.200; 27.080
THIS DOCUMENT IS A DRAFT CIRCULATED
This document is circulated as received from the committee secretariat.
FOR COMMENT AND APPROVAL. IT IS
THEREFORE SUBJECT TO CHANGE AND MAY
NOT BE REFERRED TO AS AN INTERNATIONAL
STANDARD UNTIL PUBLISHED AS SUCH.
IN ADDITION TO THEIR EVALUATION AS
ISO/CEN PARALLEL PROCESSING
BEING ACCEPTABLE FOR INDUSTRIAL,
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
STANDARDS MAY ON OCCASION HAVE TO
BE CONSIDERED IN THE LIGHT OF THEIR
POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN
Reference number
NATIONAL REGULATIONS.
ISO/DIS 24664:2021(E)
RECIPIENTS OF THIS DRAFT ARE INVITED
TO SUBMIT, WITH THEIR COMMENTS,
NOTIFICATION OF ANY RELEVANT PATENT
RIGHTS OF WHICH THEY ARE AWARE AND TO
©
PROVIDE SUPPORTING DOCUMENTATION. ISO 2021

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oSIST prEN ISO 24664:2021
ISO/DIS 24664:2021 (E)
ISO/DIS 24664:2021(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national
standards bodies (ISO member bodies). The work of preparing International Standards is normally
carried out through ISO 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 ISO, also take part in
the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all
matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see
www.iso.org/iso/foreword.html.
This document was prepared by the European Committee for Standardization (CEN) Technical
Committee CEN/TC 182, Refrigerating systems, safety and environmental requirements, in collaboration
with ISO Technical Committee ISO/TC 86, Refrigeration and air-conditioning, Subcommittee SC 1, Safety
and environmental requirements for refrigerating systems, in accordance with the Agreement on
technical cooperation between ISO and CEN (Vienna Agreement). Any feedback or questions on this
document should be directed to the user’s national standards body. A complete listing of these bodies
can be found at www.iso.org/members.html.
This first edition cancels and replaces EN 13136:2013+A1:2018, which has been technically revised.
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
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Email: copyright@iso.org
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3
Published in Switzerland
ii © ISO 2021 – All rights reserved

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oSIST prEN ISO 24664:2021
ISO/DIS 24664:2021 (E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national
standards bodies (ISO member bodies). The work of preparing International Standards is normally
carried out through ISO 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 ISO, also take part in
the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all
matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see
www.iso.org/iso/foreword.html.
This document was prepared by the European Committee for Standardization (CEN) Technical
Committee CEN/TC 182, Refrigerating systems, safety and environmental requirements, in collaboration
with ISO Technical Committee ISO/TC 86, Refrigeration and air-conditioning, Subcommittee SC 1, Safety
and environmental requirements for refrigerating systems, in accordance with the Agreement on
technical cooperation between ISO and CEN (Vienna Agreement). Any feedback or questions on this
document should be directed to the user’s national standards body. A complete listing of these bodies
can be found at www.iso.org/members.html.
This first edition cancels and replaces EN 13136:2013+A1:2018, which has been technically revised.
3

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oSIST prEN ISO 24664:2021
ISO/DIS 24664:2021 (E)
Introduction
This International Standard is based on applicable parts of ISO 4126-1:2013, ISO 4126-2:2018 and
ISO/FDIS 21922:2021.
It is suited to the specific requirements, and includes the data, of refrigerating systems. It provides
means of satisfying the pressure relief devices requirements of EN 378-2:2016 and ISO 5149-2:2014.

4

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oSIST prEN ISO 24664:2021
ISO/DIS 24664:2021 (E)
1 Scope
1.1 This International Standard describes the calculation of mass flow for sizing pressure relief
devices for parts of refrigerating systems.
NOTE The term "refrigerating system" used in this International Standard includes heat pumps.
1.2 This International Standard describes the calculation of discharge capacities for pressure relief
valves and other pressure relief devices in refrigerating systems including the necessary data for sizing
these when relieving to atmosphere or to part of the refrigerating system at lower pressure.
1.3 This International Standard specifies the requirements for selection of pressure relief devices to
prevent excessive pressure due to internal and external heat sources, the sources of increasing pressure
(e.g. compressor, heaters, etc.) and thermal expansion of trapped liquid.
1.4 This International Standard describes the calculation of the pressure loss in the inlet and outlet
lines of pressure relief valves and other pressure relief devices and includes the necessary data.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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.
ISO/FDIS 21922:2020, Refrigerating systems and heat pumps — Valves — Requirements, testing and
marking
ISO 4126-1:2013+Amd 1:2016, Safety devices for protection against excessive pressure — Part 1: Safety
valves
ISO 4126-2:2018, Safety devices for protection against excessive pressure — Part 2: Bursting disc safety
devices
ISO 817, Refrigerants — Designation system
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oSIST prEN ISO 24664:2021
ISO/DIS 24664:2021 (E)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO/FDIS 21922:2020, ISO 4126-
1:2013 and ISO 4126-2:2018 apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
refrigerant
fluid used for heat transfer in a refrigerating system, which absorbs heat at a low temperature and a low pressure
of the fluid and rejects heat at a higher temperature and a higher pressure usually involving changes of the state of
the fluid
3.2
part of the refrigerating system
several components assembled together and exposed to the same pressure in operation or pressure source,
respectively, as determined by the manufacturer
3.3
pressure relief device
pressure relief valve or bursting disc device designed to relieve excessive pressure automatically
3.4
pressure relief valve
pressure actuated valve held shut by a spring or other means and designed to relieve excessive pressure
automatically by starting to open at a set pressure and re-closing after the pressure has fallen below the set
pressure
Note 1 to entry: For the purpose of this standard, the definition of a safety valve as given in ISO 4126-1 is regarded
equivalent to a pressure relief valve.
3.5
pressure vessel
any refrigerant-containing component of a refrigerating system other than:
• coils (including their headers) consisting of pipes with air as secondary fluid;
• piping and its valves, joints and fittings;
• control devices;
• pressure switches, gauges, liquid indicators;
• safety valves, fusible plugs, bursting discs;
• equipment comprising casings or machinery where the dimensioning, choice of material and
manufacturing rules are based primarily on requirements for sufficient strength, rigidity and stability to
meet the static and dynamic operational effects or other operational characteristics and for which
pressure is not a significant design factor. Such equipment may include: pumps and compressors.
Note 1 to entry: The semi-hermetic and open type compressors used in refrigerating systems may be subject to the
exclusion article 1.2.j of the EU Directive 2014/68/EU by referring to the working party group guidelines WPG 1/11, 1/12 and
6

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oSIST prEN ISO 24664:2021
ISO/DIS 24664:2021 (E)
2/34. The compressor manufacturer has to decide on the basis of a case by case assessment, if the exclusion article 1.2.j of the
EU Directive 2014/68/EU is applicable.
Note 2 to entry: This definition is aligned to EU Directive 2014/68 EU.
4 Symbols
For the purposes of this document, the following apply:
Symbol Designation Unit
Actual flow area of the pressure relief device. The flow area at the most
2
A mm
actual
narrow cross section when the pressure relief device is fully open
2
A Effective area of the pressure relief device mm
effective
2
A Calculated flow area of liquid after expansion mm
liq
2
A Inside area of tube mm
R
2
A External surface area of the vessel m
surf
2
A Calculated flow area of vapour after expansion mm
vap
DN Nominal size (see ISO 6708:1995) –
d Actual most narrow flow diameter of the pressure relief device mm
d Inside diameter of tube mm
R
f Darcy friction factor -
Δh Heat of vaporisation kJ/kg
vap
K Capacity correction factor –
cap
Certified coefficient of discharge considering the backpressure ratio
K –
d
p /p and the possible reduced stroke of the pressure relief valve
b 0
K De-rated coefficient of discharge –
dr
K De-rated coefficient of discharge for liquid  –
drl
Valve constant (the rate of water flow for a pressure loss of 1 bar at the
3
K m /h
vs
rated full opening)
K Viscosity correction factor –
visc
L Length of pipe mm
- 1
n Rotational frequency min
p Atmospheric pressure (1,01325 bar) bar
atm
p Back pressure at outlet of pressure relief device, absolute bar
b
p Critical absolute pressure bar
c
p Pressure in connection point bar
connection
p Actual absolute relieving pressure bar
0
Absolute pressure at the inlet to the outlet line of the pressure relief
p bar
1
device
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oSIST prEN ISO 24664:2021
ISO/DIS 24664:2021 (E)
Symbol Designation Unit
p Choked pressure ratio –
r,choked
Set pressure, gauge (the pre-determined pressure at which a pressure
p bar
set
relief device under operation starts to open)
Δp Pressure loss bar
Δp Pressure loss in common outlet line bar
common
Δpin Pressure loss in the inlet line to the pressure relief device bar
Δp Pressure loss in the outlet line from the pressure relief device bar
out
Q Rate of heat production, internal heat source kW
h
2
q Theoretical discharge capacity kg/h ⋅ mm
m
2
q’ Actual discharge capacity determined by tests kg/h ⋅ mm
m
Adjusted discharge capacity, of the pressure relief device. Used for
Q kg/h
m, adjusted
pressure drop calculation in piping
Q Mass flow in common outlet pipe kg/h
m, common
Q Flow of liquid after expansion kg/h
m, liq
Q Calculated refrigerant mass flow rate of the pressure relief device kg/h
m, relief
Minimum required discharge capacity, of refrigerant, of the pressure
Q kg/h
m, required
relief device
Q Flow of vapour after expansion kg/h
m, vap
R Bending radius of bend mm
Re Reynolds number –
s Thickness of insulation m
u Velocity in pipe m/s
3
V Theoretical displacement m
3
v Specific volume of vapour or liquid m /kg
3
v Specific volume of vapour in inlet line m /kg
0
3
v Specific volume at the inlet to the outlet line of the pressure relief m /kg
1
device
Actual flow speed of liquid in the smallest section of pressure relief
w m/s
0
valve
w Speed at the inlet into the outlet line m/s
1
x Vapour fraction of refrigerant at p –
b
α Flush connection angle °
Heat capacity ratio –
γ
ε Pipe roughness mm
R
ζ Pressure loss coefficient –
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oSIST prEN ISO 24664:2021
ISO/DIS 24664:2021 (E)
Symbol Designation Unit
Pressure loss coefficient related to DN –
ζ
DN
Pressure loss coefficient of fittings –
ζ
fittings
Pressure loss coefficient of pipes in outlet line –
ζ
pipes
Total pressure loss coefficient in outlet line –
ζ
total
η Volumetric efficiency estimated at suction pressure and discharge –
v
pressure equivalent to the pressure relief device setting
2
ν Kinematic viscosity m /s
3
ρ Density of vapour or liquid kg/m
3
ρ Density of vapour in inlet line kg/m
0
3
ρ Vapour density at refrigerant saturation pressure/dew point at 10 °C kg/m
10
2
Density of heat flow rate kW/m
φ
2
φ Reduced density of heat flow rate kW/m
red
5 General
This international standard describes the calculation of:
• The required discharge capacity of a pressure relief device.
• The actual capacity of a pressure relief device.
• Pressure losses in inlet and outlet lines from the pressure relief device.
The capacity of the pressure relief device (calculated in Clause 7), shall be larger than the required
capacity (calculated in Clause 6), and the pressure losses (calculated in Clause 8) shall be within given
limits for the pressure relief device to operate correctly.
The equations in Clause 7 are only valid for discharge of refrigerant gas or vapour.
NOTE 1 Calculations of flow areas for pressure relief devices for non-flashing and flashing liquids are given in
Annex B. Example calculations with corresponding piping are given in Annex C.
NOTE 2 Requirements for protection against excessive pressure in refrigerating systems and heat pumps are
given in EN 378-2 and ISO 5149-2.
For design and manufacturing of bodies, bonnets and bolts for pressure relief devices — safety valves
and bursting discs — specification of strength pressure test, ISO/FDIS 21922:2020 applies.
For other aspects, the requirements of ISO 4126-1:2013, Clause 3, Terms and definitions, Clause 5,
Design, Clause 7, Type tests and Clause 10, Marking and sealing and ISO 4126-2:2018 Bursting Disc
Safety Devices, Clause 17 Marking, 17.2 Bursting discs/bursting disc assemblies and 17.3 Bursting disc
holders apply.
5.1 Refrigerant properties used in calculations
The actual absolute relieving pressure of a pressure relief device is calculated as:
p=1,1⋅+pp [bar] (1)
0 set atm
For calculation of the required discharge capacity of a pressure relief device, knowledge of the heat of
vaporisation Δh of the refrigerant is required.
vap
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oSIST prEN ISO 24664:2021
ISO/DIS 24664:2021 (E)
For calculation of the actual discharge capacity of a pressure relief device, knowledge of the density ρ
0
(or specific volume v ) and the heat capacity ratio γ of the refrigerant is required.
0
For calculation of pressure losses in inlet and outlet piping, knowledge of the density ρ (or specific
0
volume v ) is required.
0
The values are found at the following conditions:
• If the pressure p is less than the critical pressure of the refrigerant:
0
o If the saturated gas temperature corresponding to p is higher than the critical
0
temperature minus 5 K, then ρ , v and Δh are found at saturated gas at critical
0 0 vap
temperature minus 5 K.
o Else ρ , v and Δh are found at saturated gas at pressure p . If the inlet temperature is
0 0 vap 0
given (superheated gas), then ρ , v and Δh are found at pressure p and the inlet
0 0 vap 0
temperature.
• If the pressure p is higher than the critical pressure of the refrigerant, then ρ , v and Δh are
0 0 0 vap
found at saturated gas at critical temperature minus 5 K.
The value of the heat capacity ratio γ shall be found at 25 °C and 1,01325 bar. Values of γ for different
refrigerants can be found in Table A.1.
6 Minimum required discharge capacity for protection of parts of a refrigerating
system
6.1 General
Calculations are based on known or assumed processes, which result in an increase in pressure. All
foreseeable processes shall be considered including those covered in 6.2, 6.3 and 6.4.
In case of supercritical pressure, the pressure relief valve shall be suitable for both gas and liquid.
In case of relieving CO to a pressure below the triple point (e.g. atmospheric pressure), there is a
2
possibility to create solid CO2. Necessary precautions shall be taken to ensure safe operation.
Even if a vessel contains only gas, it might in some situations contain liquid and should therefore for the
purpose of this standard be treated as a vessel containing both liquid and gas.
6.2 Excessive pressure caused by heat sources
6.2.1 External heat sources
The minimum required discharge capacity of the pressure relief device for pressure vessels is
calculated as:
3600⋅⋅φ A
surf
 [kg/h] (2)
Q =
m,required
∆h
vap
For pressure vessels in this International Standard, the density of heat flow rate is assumed to be:
2
φ= 10 kW/m (3)
but a higher value shall be used if necessary.
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oSIST prEN ISO 24664:2021
ISO/DIS 24664:2021 (E)
When the thickness (s) of the insulation of the pressure vessel is higher than 0,04 [m] and the insulation
is tested according to reaction of fire as described in EN 13501-1 and classified better than class C, a
reduced density of heat flow rate can be used:
0,04
2
φφ⋅ [kW/m ] (4)
red
s
For pressure vessels the total external surface area of the vessel is calculated depending on geometry.


Figure 1 — Plate Heat Exchanger (PHE) Figure 2 — Plate and Shell Heat Exchanger (PSHE)
For plate heat exchangers the surface area is calculated as:
2
A =2(⋅ LL⋅ + L⋅ L+⋅LL ) [m ] (5)
surf 1 2 2 3 13
For plate and shell heat exchangers the surface area is calculated:
π
2
2
[m ] (6)
A =2(⋅ ⋅ d )+(π⋅⋅dL)
surf 1 11
4
2
Higher values for density of heat flow rate than 10 kW/m may be necessary where, in case of fire, full
engulfment of the pressure vessel is to be expected and/or in the case the pressure vessel is insulated
with a flammable insulation. Other calculation methods could be necessary in case of heat radiation
with a higher heat flow directed to one side of the vessel.
6.2.2 Internal heat sources
For conditions, which arise due to an internal source of excessive heat, the minimum required discharge
capacity of the pressure relief device is calculated as:
3600⋅Q
h
Q = [kg/h] (7)
m,required
∆h
vap
6.3 Excessive pressure caused by compressors
The minimum required discharge capacity of the pressure relief device for excessive pressure caused by
compressors is calculated as:
Q = 60⋅Vn⋅⋅ρη⋅ [kg/h] (8)
m,required 10 v
For low temperature operations, where it can be established that the compressor motor cannot run
with the suction pressure corresponding to 10 °C saturated conditions, the density value at the highest
suction pressure shall be used in the calculation.
NOTE 1 In cases where discharge shut-off valves are not fitted, a high-pressure relief device will suffice,
providing there are no intermediate shut-off valves.
NOTE 2 Non-positive displacement compressors need not have a pressure relief device providing it is not
possible to exceed the maximum allowable pressure.
11
=

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oSIST prEN ISO 24664:2021
ISO/DIS 24664:2021 (E)
NOTE 3 Relieving to the low-pressure side may cause compressor overheating and / or uncontrolled internal
pressure in compressors (e.g. in screw compressors).
NOTE 4 EN 12693 covers compressors, which can run against a closed discharge valve. EN 12693 should
therefore be considered, especially the requirement covering conditions where the allowable evaporating
temperature exceeds 10 °C by more than 5 K.
6.4 Excessive pressure caused by expansion of trapped liquid
For protection against pressure caused by expansion of trapped liquid, the effective area of the pressure
relief device shall be calculated based on the volume of the trapped liquid:
2
[mm ] (9)
A KV⋅
effective volume trapped
Where:
2
K = 0,02 [mm /l] (10)
volume
And the effective area of the pressure relief device is defined as:
2
A A⋅ K [mm ] (11)
effective actual dr
If the calculated effective area results in a flow diameter less than 1 mm, then the diameter shall be
selected to 1 mm:
d≥1 mm (12)
For refrigerants where the temperature difference between relieving temperature and critical
2
temperature is less than 20 K, the value of K shall be at least 0,04 mm /l.
volume
NOTE Liquids having a temperature close to the critical temperature expand considerably.
It is advisable to consider the backpressure ratio p /p and the possibly reduced stroke of the pressure
b 0
relief valve.
The possibility of contamination by dirt should be considered.
Where practicable, the pressure relief device shall relieve to the low-pressure side of the system and the
pressure relief device shall meet the requirements even at maximum back pressure.
7 Discharge capacity of pressure relief devices
7.1 General
For the most common use of pressure relief devices in refrigerating systems, the back pressure is lower
than approximately 0,5 times the relieving pressure (p ≤ 0,5⋅ p ), which indicates that the flow through
b 0
the pressure relief device is choked.
The flow of gas or vapor through an orifice, such as the flow areas of a pressure relief device, increases
as the outlet pressure is decreased until choked flow is achieved. Further decrease in outlet pressure
will not result in any further increase in the flow.
For pressure relief valves where the lift is a function of back pressure, the manufacturer shall state the
maximum permissible back pressure ratio p /p and the related certified coefficient of discharge
b 0
considering the possibly reduced stroke of the pressure relief valve.
7.2 Discharge capacity of pressure relief valves
The mass flow of gas through a pressure relief valve can be calculated as:
12
=
=

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oSIST prEN ISO 24664:2021
ISO/DIS 24664:2021 (E)
p
0
Q 1,1384⋅⋅AK⋅ K⋅ [kg/h] (13)
m,relief dr cap
v
0
56−
NOTE 1 1,1384= 10⋅⋅10 3600
The de-rated coefficient of discharge is calculated as:
KK0,9⋅ [-] (14)
dr d
where the coefficient of discharge is calculated from:
q'
m
[-] (15)
K =
d
q
m
The factor 0.9 in equation (14) is a safety factor used when calculating the discharge capacity of
pressure relief valves.
The value of the capacity correction factor K depends on whether the flow through the pressure relief
cap
valve is choked or not.
/p is less than or equal to the choked pressure ratio:
Choked flow occurs when the pressure ratio pb 0
p
b
≤ p [–] (16)
r,choked
p
0
Where the choked pressure ratio is calculated as:
γ
γ−1
2
p = [-] (17)
r,choked 
γ+1

For choked flow the value of the capacity correction factor is:
γ+1
γ−1
2
K γ⋅ [–] (18)
cap 
γ+1

If the flow is not choked, then the capacity correction factor is calculated as:
21γ+

γγ
pp
2⋅γ
bb
[–] (19)
K= ⋅−
cap  

γ−1 pp
00


The choked pressure ratio for different refrigerant is given in Table A.1 in Annex A, and values of K at
cap
choked and non-choked flow are given in tables A.1 to A.3 in Annex A.
NOTE 2 Equations (13) to (19) give identical results to corresponding equations in EN 13136:2013+A1:2018
and ISO 4126-7:2013.
7.2.1 Adjusted discharge capacity
When calculating pressure drop in inlet or outlet piping and fittings, and the discharge capacity of the
pressure relief valve Q is considerable larger than the required capacity Q , then the
m,relief m,required
refrigerant mass flow used in the pressure drop calculations can be adjusted according to the following
rules:
if QQ< 1,25⋅ then QQ= (20)
( )
m,relief m, required m,,adjusted m required
13
=
=
=

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oSIST prEN ISO 24664:2021
ISO/DIS 24664:2021 (E)
Q
m,relief
if Q ≥ 1,25⋅Q then Q = (21)
( )
m, , relief m required m,adjusted
1,25
The adjusted mass flow rate is used when calculating pressure drop in piping and fittings.
7.3 Discharge capacity of bursting discs
Domed bursting discs shall be designed so that they burst due to tensile forces when the bursting
pressure is applied to the concave side of the bursting disc. They shall be domed such that no further
plastic flow will occur initially when the bursting disc is subject to its intended operating condition.
The discharge capacity of a bursting disc shall be calculated from the formula given in Clause 7.2. The
following values for K shall be the maximum used depending on how the pipe between the vessel and
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the bursting disc is mounted on the vessel:
a) flush or flared connection (see Table A.2): K = 0,70.
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b) inserted connection (see Table A.2): K = 0,55.
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If the K -value of the bursting disc itself is lower than the maximum value given above, then the
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smaller value shall be used in the calculation.
8 Pressure loss in inlet and
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