EN ISO 24664:2024

Refrigerating systems and heat pumps - Pressure relief devices and their associated piping - Methods for calculation (ISO 24664:2024)

This document describes the calculation of:
—     mass flow for sizing pressure relief devices for parts of refrigerating systems;
—     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;
—     the pressure loss in the inlet and outlet lines of pressure relief valves and other pressure relief devices and includes the necessary data.
This document 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.
NOTE            The term "refrigerating system" used in this document includes heat pumps.

Kälteanlagen und Wärmepumpen - Druckentlastungseinrichtungen und zugehörige Leitungen - Berechnungsverfahren (ISO 24664:2024)

Dieses Dokument beschreibt die Berechnung von Folgendem:
   Massenströme zur Auslegung von Druckentlastungseinrichtungen für Teile von Kälteanlagen;
   Abblaseleistungen von Druckentlastungsventilen und anderen Druckentlastungseinrichtungen in Kälteanlagen einschließlich der erforderlichen Daten zur Auslegung dieser Einrichtungen, wenn sie ins Freie oder in einen Teil der Kälteanlage mit niedrigerem Druck abblasen;
   Druckabfall in den Ein- und Austrittsleitungen von Druckentlastungsventilen und anderen Druckentlastungseinrichtungen einschließlich der erforderlichen Daten.
Dieses Dokument legt die Anforderungen an die Auswahl von Druckentlastungseinrichtungen zur Vermeidung von unzulässigem Druck, hervorgerufen durch innere und äußere Wärmequellen, Druckerzeuger (z. B. Verdichter, Erhitzer usw.) und die Wärmeausdehnung eingeschlossener Flüssigkeit fest.
ANMERKUNG   Der in diesem Dokument verwendete Begriff „Kälteanlage“ schließt Wärmepumpen mit ein.

Systèmes de réfrigération et pompes à chaleur - Dispositifs de limitation de pression et tuyauteries associées - Méthodes de calcul (ISO 24664:2024)

Le présent document décrit le calcul:
—     du débit massique pour le dimensionnement des dispositifs de limitation de pression pour les composants des systèmes de réfrigération;
—     des capacités de décharge des soupapes de sécurité et autres dispositifs de limitation de pression des systèmes de réfrigération, y compris les données nécessaires à leur dimensionnement lors de leur décharge dans l'atmosphère ou dans les composants du système de réfrigération à plus basse pression;
—     de la perte de charge dans la tubulure en amont et en aval des soupapes de sécurité et autres dispositifs de limitation de pression, et inclut les données nécessaires.
Le présent document spécifie les exigences relatives à la sélection des dispositifs de limitation de pression pour éviter une pression excessive due aux sources de chaleur internes et externes, aux sources de génération de pression (par exemple compresseurs, chauffages, etc.) et à la dilatation thermique du fluide piégé.
NOTE            Le terme «système de réfrigération» utilisé dans le présent document inclut les pompes à chaleur.

Hladilni sistemi in toplotne črpalke - Tlačne varnostne naprave in njihove napeljave - Metode za izračun (ISO 24664:2024)

1.1   Ta standard opisuje izračun masnega pretoka za tlačne varnostne naprave komponent hladilnih sistemov.
OPOMBA:   Izraz »hladilni sistem« v tem standardu zajema tudi toplotne črpalke.
1.2   Ta standard opisuje izračun zmogljivosti za praznjenje tlačnih varnostnih ventilov in drugih tlačnih varnostnih naprav v hladilnih sistemih, vključno s potrebnimi podatki za določanje njihove količine pri razbremenitvi v ozračje ali v komponente znotraj sistema, ki imajo nižji tlak.
1.3   Ta standard določa zahteve za izbiro tlačnih varnostnih naprav za preprečevanje prekomernega tlaka, ki nastane zaradi notranjih in zunanjih virov toplote, virov naraščajočega tlaka (npr. kompresor, grelniki itd.) ter toplotnega raztezanja ujete tekočine.
1.4   Ta standard opisuje izračun izgube tlaka na vhodni in izhodni liniji tlačnih varnostnih ventilov in drugih tlačnih varnostnih naprav ter vključuje potrebne podatke.
1.5   Ta standard se navezuje na druge ustrezne standarde iz točke 5.

General Information

• Status: Published
• Publication Date: 17-Dec-2024
• ICS: 27.080 - Heat pumps
• ICS: 27.200 - Refrigerating technology
• Technical Committee: CEN/TC 182 - Refrigerating systems, safety and environmental requirements
• Drafting Committee: CEN/TC 182/WG 2 - Design and testing
• Current Stage: 6060 - Definitive text made available (DAV) - Publishing
• Start Date: 18-Dec-2024
• Due Date: 20-Feb-2023
• Completion Date: 18-Dec-2024
• Directive: 2014/68/EU - Directive 2014/68/EU of the European Parliament and of the Council of 15 May 2014 on the harmonisation of the laws of the Member States relating to the making available on the market of pressure equipment
• Mandate: M/071 - Mandate to CEN for standardization in the field of Pressure equipment
• Mandate: M/601 - COMMISSION IMPLEMENTING DECISION of 1.3.2024 on a standardisation request to the European Committee for Standardization and the European Committee for Electrotechnical Standardization as regards pressure equipment and assemblies in support of Directive 2014/68/EU of the European Parliament and of the Council
• Ref Project: SIST EN ISO 24664:2025 - Refrigerating systems and heat pumps - Pressure relief devices and their associated piping - Methods for calculation (ISO 24664:2024)

Relations

• Replaces: EN 13136:2013+A1:2018 - Refrigerating systems and heat pumps - Pressure relief devices and their associated piping - Methods for calculation
• Effective Date: 09-Dec-2020

Overview

EN ISO 24664:2024 (ISO 24664:2024) is a CEN-endorsed international standard that provides methods for the calculation and sizing of pressure relief devices and their associated piping in refrigerating systems and heat pumps. The standard covers calculation of mass flow rates, discharge capacities for pressure relief valves and other devices (including bursting discs), and pressure losses in inlet and outlet lines. Its scope includes relieving to atmosphere or to a lower-pressure part of the refrigerating system and addresses causes of excessive pressure such as internal and external heat loads, compressor effects and thermal expansion of trapped liquid.

Key topics and requirements

• Sizing of pressure relief devices: Methods to determine the required mass flow and minimum discharge capacity for protecting parts of refrigerating systems.
• Discharge capacities: Criteria and necessary data for sizing pressure relief valves and bursting discs when discharging to atmosphere or to another part of the system.
• Piping pressure losses: Calculation of pressure loss in inlet and outlet lines, total pressure loss and guidance for connection of multiple valve outlets to a common line.
• Sources of overpressure: Identification and treatment of overpressure causes, including external fire/heat, internal heat sources (heaters, compressors), compressor-induced pressure rises and trapped-liquid expansion.
• Annexes supporting calculations:
  • Annex A: values of factors and refrigerant properties
  • Annex B: flow-area calculations for flashing and non‑flashing liquids
  • Annex C: worked example for sizing relief devices and piping
  • Annex D: guidance for outlet lines where velocity may exceed speed of sound

Practical applications

EN ISO 24664:2024 is intended for practical use in the design, safety assessment and compliance activities for refrigeration and heat pump systems. Typical applications include:

• Sizing and selection of pressure relief valves, bursting discs, and associated inlet/outlet piping.
• Engineering design checks to prevent excessive pressure under normal and abnormal heat input scenarios.
• Preparing documentation and calculations required for conformity with pressure equipment and safety regulations.
• Integrating relief device sizing into system design workflows for industrial refrigeration, commercial HVAC and heat-pump installations.

Who should use this standard

• Mechanical and refrigeration engineers designing or verifying refrigeration and heat-pump installations
• Safety and compliance professionals responsible for pressure equipment conformity
• Manufacturers and suppliers of pressure relief devices, valves and bursting discs
• Consultants and certifying bodies assessing system safety and regulatory compliance

Related standards

• ISO 4126-1:2013 / Amd 1:2016 - Safety devices for protection against excessive pressure (safety valves)
• ISO 4126-2:2018 - Bursting disc safety devices
• ISO 21922:2021 - Valves for refrigerating systems and heat pumps
• EN 378-2:2016 and ISO 5149-2:2014 - Refrigeration system safety requirements
• Directive 2014/68/EU - Pressure Equipment Directive (informative relationship in Annex ZA)

Keywords: refrigerating systems, heat pumps, pressure relief devices, pressure relief valves, bursting discs, discharge capacity, pressure loss, sizing, EN ISO 24664:2024, ISO 24664:2024.

Standards Content (Sample)

SLOVENSKI STANDARD
01-marec-2025
Hladilni sistemi in toplotne črpalke - Tlačne varnostne naprave in njihove
napeljave - Metode za izračun (ISO 24664:2024)
Refrigerating systems and heat pumps - Pressure relief devices and their associated
piping - Methods for calculation (ISO 24664:2024)
Kälteanlagen und Wärmepumpen - Druckentlastungseinrichtungen und zugehörige
Leitungen - Berechnungsverfahren (ISO 24664:2024)
Systèmes frigorifiques et pompes à chaleur - Dispositifs de limitation de pression et
tuyauteries associées - Méthodes de calcul (ISO 24664:2024)
Ta slovenski standard je istoveten z: EN ISO 24664:2024
ICS:
27.080 Toplotne črpalke Heat pumps
27.200 Hladilna tehnologija Refrigerating technology
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 24664
EUROPEAN STANDARD
NORME EUROPÉENNE
December 2024
EUROPÄISCHE NORM
ICS 27.080; 27.200 Supersedes EN 13136:2013+A1:2018
English Version
Refrigerating systems and heat pumps - Pressure relief
devices and their associated piping - Methods for
calculation (ISO 24664:2024)
Systèmes de réfrigération et pompes à chaleur - Kälteanlagen und Wärmepumpen -
Dispositifs de limitation de pression et tuyauteries Druckentlastungseinrichtungen und zugehörige
associées - Méthodes de calcul (ISO 24664:2024) Leitungen - Berechnungsverfahren (ISO 24664:2024)
This European Standard was approved by CEN on 17 September 2024.

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, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2024 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 24664:2024 E
worldwide for CEN national Members.

Contents Page
European foreword . 3
Annex ZA (informative) Relationship between this European Standard and the essential
requirements of Directive 2014/68/EU (Pressure equipment Directive) aimed to be
covered. 4

European foreword
This document (EN ISO 24664:2024) has been prepared by Technical Committee ISO/TC 86
"Refrigeration and air-conditioning" in collaboration with Technical Committee CEN/TC 182
“Refrigerating systems, safety and environmental requirements” the secretariat of which is held by DIN.
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 June 2025, and conflicting national standards shall be
withdrawn at the latest by June 2025.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN 13136:2013+A1:2018.
This document has been prepared under a standardization request addressed to CEN by the European
Commission. The Standing Committee of the EFTA States subsequently approves these requests for its
Member States.
For the relationship with EU Legislation, see informative Annex ZA, which is an integral part of this
document.
Any feedback and questions on this document should be directed to the users’ national standards
body/national committee. A complete listing of these bodies can be found on the CEN website.
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, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and the
United Kingdom.
Endorsement notice
The text of ISO 24664:2024 has been approved by CEN as EN ISO 24664:2024 without any modification.

Annex ZA
(informative)
Relationship between this European Standard and the essential
requirements of Directive 2014/68/EU (Pressure equipment Directive)
aimed to be covered
This European Standard has been prepared under a Commission’s standardization request M/601 to
provide one voluntary means of conforming to essential requirements of Directive 2014/68/EU OF THE
EUROPEAN PARLIAMENT AND OF THE COUNCIL of 15 May 2014 on the harmonization of the laws of
the Member States relating to the making available on the market pressure equipment.
Once this standard is cited in the Official Journal of the European Union under that Directive,
compliance with the normative clauses of this standard given in Table ZA.1 and application of the
edition of the normatively referenced standards as given in Table ZA.2 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 European Standard and Annex I of
Directive 2014/68/EU (Pressure equipment Directive)
Essential Requirements Clause(s)/subclause(s) of Remarks/Notes
of Directive 2014/68/EU this EN
2.10 Clause 6 Protection against exceeding the
allowable limits of pressure
equipment
2.12 Clause 6.2.1 External Fire
2.11.2 Clause 5
Clause 7 Pressure limiting devices
Clause 8
Table ZA.2 — Applicable Standards to confer presumption of conformity as described in this
Annex ZA
Reference in European Standard Title Corresponding European
Clause 2 Edition Standard Edition
ISO 4126-1:2013 ISO 4126-1:2013 Safety devices for EN ISO 4126-1:2013
/Amd 1:2016 protection against
ISO 4126-1:2013/Amd EN ISO 4126-
excessive pressure —
1:2016 1:2013/A1:2016
Part 1: Safety valves
ISO 4126-2:2018 ISO 4126-2:2018 Safety devices for EN ISO 4126-2:2019
protection against
excessive pressure —
Part 2: Bursting disc
safety devices
ISO 21922:2021 ISO 21922:2021 Refrigerating systems EN ISO 21922:2021
and heat pumps —
Valves —
Requirements, testing
and marking
The documents listed in the Column 1 of Table ZA.2, in whole or in part, are normatively referenced in
this document, i.e. are indispensable for its application. The achievement of the presumption of
conformity is subject to the application of the edition of standards as listed in Column 4 or, if no
European Standard Edition exists, the European Standard Edition given in Column 2 of Table ZA.2.
WARNING 1 — Presumption of conformity stays valid only as long as a reference to this European
Standard is maintained in the list published in the Official Journal of the European Union. Users of this
standard should consult frequently the latest list published in the Official Journal of the European
Union.
WARNING 2 — Other Union legislation may be applicable to the product(s) falling within the scope of
this standard.
International
Standard
ISO 24664
First edition
Refrigerating systems and heat
2024-11
pumps — Pressure relief devices
and their associated piping —
Methods for calculation
Systèmes de réfrigération et pompes à chaleur — Dispositifs de
limitation de pression et tuyauteries associées — Méthodes de calcul
Reference number
ISO 24664:2024(en) © ISO 2024
ISO 24664:2024(en)
© ISO 2024
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
ISO 24664:2024(en)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols . 3
5 General . 4
6 Minimum required discharge capacity for protection of parts of a refrigerating system . 5
6.1 General .5
6.2 Excessive pressure caused by heat sources.6
6.2.1 External heat sources .6
6.2.2 Internal heat sources .7
6.3 Excessive pressure caused by compressors .7
6.4 Excessive pressure caused by expansion of trapped liquid .7
7 Discharge capacity of pressure relief devices . 8
7.1 General .8
7.2 Discharge capacity of pressure relief valves .8
7.3 Discharge capacity of bursting discs .9
8 Pressure loss in inlet and outlet lines . 10
8.1 General .10
8.2 Pressure loss in inlet line .10
8.3 Pressure loss in outlet line .11
8.4 Total pressure loss . 12
8.5 Connection of outlets from several pressure relief valves to a common outlet line . 12
Annex A (normative) Values of factors and properties of refrigerants . 14
Annex B (informative) Calculation of flow areas for non-flashing and flashing liquids .21
Annex C (informative) Example calculation for sizing pressure relief devices with
corresponding pipes and fittings .24
Annex D (informative) Relief valve outlet line when velocity is higher than speed of sound .31
Bibliography .35

iii
ISO 24664:2024(en)
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 document 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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
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 Technical Committee ISO/TC 86, Refrigeration and air-conditioning,
Subcommittee SC 1, Safety and environmental requirements for refrigerating systems, in collaboration with the
European Committee for Standardization (CEN) Technical Committee CEN/TC 182, Refrigerating systems,
safety and environmental requirements, 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.

iv
ISO 24664:2024(en)
Introduction
This document is based on EN 13136:2013+A1:2018 and applicable parts of ISO 4126-1:2013, ISO 4126-2:2018
and ISO 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.

v
International Standard ISO 24664:2024(en)
Refrigerating systems and heat pumps — Pressure relief
devices and their associated piping — Methods for calculation
1 Scope
This document describes the calculation of:
— mass flow for sizing pressure relief devices for parts of refrigerating systems;
— 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;
— the pressure loss in the inlet and outlet lines of pressure relief valves and other pressure relief devices
and includes the necessary data.
This document 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.
NOTE The term "refrigerating system" used in this document includes heat pumps.
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 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 21922:2021, Refrigerating systems and heat pumps — Valves — Requirements, testing and marking
EN 13501-1:2018, Fire classification of construction products and building elements — Part 1: Classification
using data from reaction to fire tests
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 21922:2021, ISO 4126-1:2013/Amd
1:2016, and ISO 4126-2:2018 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
refrigerant
fluid used for heat transfer in a refrigerating system
Note 1 to entry: A refrigerant absorbs heat at a low temperature and a low pressure of the fluid and rejects it at a
higher temperature and a higher pressure of the fluid, usually involving changes of the phase of the fluid.

ISO 24664:2024(en)
[SOURCE: ISO 817:2024, 3.1.37]
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 purposes of this document, the definition of a safety valve as given in ISO 4126-1:2013 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 A-11,
A-12 and B-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.
3.6
nominal size
DN
alphanumeric designation of size for components of a pipework system, which is used for reference purposes
comprising the letters DN followed by a dimensionless whole number which is indirectly related to the
physical size, in millimetres, of the bore or outside diameter of the end connections
Note 1 to entry: The number following the letters DN does not represent a measurable value and should not be used for
calculation purposes except where specified in this document.
Note 2 to entry: Where the nominal size is not specified, for the purpose of this document it is assumed to be the
internal diameter of the pipe or component in mm (DN/ID).
Note 3 to entry: Nominal size is not the same as port size which is commonly used for the size of the valve seat opening.
[SOURCE: ISO 6708:1995, 2.1, modified — Note 2 and 3 to entry added.]

ISO 24664:2024(en)
4 Symbols
Symbol Designation Unit
Actual flow area of the pressure relief device. The flow area at the most narrow
A mm
actual
cross section when the pressure relief device is fully open
A Effective area of the pressure relief device mm
effective
A Calculated flow area of liquid after expansion mm
liq
A Inside area of tube mm
R
A External surface area of the vessel m
surf
A Calculated flow area of vapour after expansion mm
vap
DN Nominal size –
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 p /p and the
b 0
K –
d
possible reduced stroke of the pressure relief valve
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 rated full
K m /h
vs
opening)
K Viscosity correction factor –
visc
K Trapped liquid factor mm /l
volume
L Length of pipe mm
m mass kg
- 1
n Rotational frequency min
p Atmospheric pressure (1,013 25 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 Choked pressure ratio –
r,choked
Set pressure, gauge (the pre-determined pressure at which a pressure relief device
p bar
set
under operation starts to open)
Δp Pressure loss bar
Δp Pressure loss in common outlet line bar
common
Δp Pressure loss in the inlet line to the pressure relief device bar
in
Δp Pressure loss in the outlet line from the pressure relief device bar
out
p Actual absolute relieving pressure bar
p Absolute pressure at the inlet to the outlet line of the pressure relief device bar
Q Rate of heat production, internal heat source kW
h
Adjusted discharge capacity, of the pressure relief device. Used for pressure drop
Q kg/h
m,adjusted
calculation in piping
Q Mass flow in common outlet line 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
Q Minimum required discharge capacity, of refrigerant, of the pressure relief device kg/h
m,required
5 2
NOTE 1 bar = 0,1 MPa = 10 Pa; 1 MPa = 1 N/mm .

ISO 24664:2024(en)
Symbol Designation Unit
Q Flow of vapour after expansion kg/h
m,vap
q Theoretical discharge capacity kg/h⋅mm
m
q’ Actual discharge capacity determined by tests kg/h⋅mm
m
R Bending radius of bend mm
Re Reynolds number –
s Thickness of insulation m
u Velocity in line m/s
V Theoretical displacement (volume) m
v Specific volume of vapour or liquid m /kg
v Specific volume of vapour in inlet line m /kg
v Specific volume at the inlet to the outlet line of the pressure relief device m /kg
w Actual flow speed of liquid in the smallest section of pressure relief valve m/s
w Speed at the inlet into the outlet line m/s
x Vapour fraction of refrigerant at p –
b
α Flush connection angle °
γ Heat capacity ratio –
ε Pipe roughness mm
R
ζ Pressure loss coefficient –
ζ Pressure loss coefficient related to DN –
DN
ζ Pressure loss coefficient of fittings –
fittings
ζ Pressure loss coefficient of pipes in outlet line –
pipes
ζ Total pressure loss coefficient in outlet line –
total
η Volumetric efficiency estimated at suction pressure and discharge pressure equivalent –
v
to the pressure relief device setting
ν Kinematic viscosity m /s
ρ Density of vapour or liquid kg/m
ρ Density of vapour in inlet line kg/m
ρ Vapour density at refrigerant saturation pressure/dew point at 10 °C kg/m
ϕ Density of heat flow rate kW/m
ϕ Reduced density of heat flow rate kW/m
red
5 2
NOTE 1 bar = 0,1 MPa = 10 Pa; 1 MPa = 1 N/mm .
5 General
This document 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 Formulae 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.

ISO 24664:2024(en)
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) and for the specification of strength pressure test, ISO 21922:2021 applies.
For other aspects, the requirements of ISO 4126-1:2013/Amd 1:2016, Clause 5, Clause 7, and Clause 10, and
ISO 4126-2:2018, Clause 17, apply.
The actual absolute relieving pressure of a pressure relief device is calculated as:
pp=⋅11, + p (1)
0 setatm
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
For calculation of the actual discharge capacity of a pressure relief device, knowledge of the density ρ (or
specific volume v ) and the heat capacity ratio γ of the refrigerant is required.
For calculation of pressure losses in inlet and outlet lines, knowledge of the density ρ (or specific volume v )
0 0
is required.
The values are found at the following conditions:
a) If the pressure p is less than the critical pressure of the refrigerant:
— If the saturated gas temperature corresponding to p is higher than the critical temperature minus
5 K, then ρ , v and Δh are found at saturated gas at critical temperature minus 5 K.
0 0 vap
— Else ρ , v and Δh are found at saturated gas at pressure p . If the inlet temperature is given
0 0 vap 0
(superheated gas), then ρ , v and Δh are found at pressure p and the inlet temperature.
0 0 vap 0
b) If the pressure p is higher than the critical pressure of the refrigerant, then ρ , v and Δh are found at
0 0 0 vap
saturated gas at critical temperature minus 5 K.
The value of the heat capacity ratio γ shall be found at 25 °C and 1,013 25 bar. Values of γ for different
refrigerants can be found in Table A.1.
To check if the velocity in the outlet line is larger than the speed of sound, the density and the speed of sound
of the refrigerant at the outlet of the outlet line are needed. The refrigerant properties at the outlet of the
outlet line are found assuming isenthalpic expansion from the relieving condition (p , v ) to the pressure
0 0
at the outlet of the outlet line. If the isenthalpic expansion results in either a mixture of gas and liquid or a
mixture of gas and solid, then the density and speed of sound of saturated gas at the pressure at the outlet of
the outlet line are used.
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. The commonly relevant processes are covered in 6.2, 6.3 and 6.4.
NOTE Information about necessary protection measures against excessive pressure can be found in system safety
standards such as ISO 5149-2 and EN 378-2. For instance due to standstill pressure, pressure to internal or external
heat sources, or trapped fluid.
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 possibility
to create solid CO . Necessary precautions shall be taken to ensure safe operation.
ISO 24664:2024(en)
Even if a vessel contains only gas, it might in some situations contain liquid and should therefore for the
purpose of this document 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
in Formula (2):
3 600⋅⋅φ A
surf
Q = (2)
m,required
Δh
vap
For pressure vessels in this document, the density of heat flow rate ϕ is assumed to be 10 kW/m , but a
higher value shall be used if necessary.
When the thickness, s, of the insulation of the pressure vessel is higher than 0,04 m and the insulation is
tested for reaction to fire according to EN 13501-1:2018 and classified better than class C, a reduced density
of heat flow rate shall be used as a minimum value:
00, 4
φφ=⋅ (3)
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)

ISO 24664:2024(en)
For plate heat exchangers (see Figure 1) the surface area is calculated as per Formula (4):
AL=⋅2 ()⋅+LL ⋅+LL ⋅L (4)
surf 12 23 13
For plate and shell heat exchangers (see Figure 2) the surface area is calculated as per Formula (5):
π
Ad=⋅2 ()⋅+()π ⋅⋅dL (5)
surf 1 11
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 per Formula (6):
3 600⋅Q
h
Q = (6)
m,required
Δh
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:
QV=⋅60 ⋅⋅n ρη⋅ (7)
mr, equiredv
In Formula (7) the saturated gas density value at the highest allowable suction pressure shall be used. The
highest allowable suction pressure is defined by the compressor manufacturer.
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.
NOTE 3 Relieving to the low-pressure side can 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.
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:
AK=⋅V (8)
effectivevolumetrapped
where K is 0,02 mm /l and the effective area of the pressure relief device is defined as:
volume
AA=⋅K (9)
effectiveactualdr
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
ISO 24664:2024(en)
For refrigerants where the temperature difference between relieving temperature and critical 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 the
b 0
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 considering
b 0
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:
p
QA=⋅1,138 4 ⋅⋅KK ⋅ (10)
m,relief dr cap
v
56−
NOTE 1 The factor 1,138 41=⋅0103⋅ 600 is a result of converting between units.
The de-rated coefficient of discharge is calculated as:
KK=⋅09, (11)
dr d
where the coefficient of discharge is calculated from:
q'
m
K = (12)
d
q
m
NOTE 2 The factor 0,9 in Formula (11) 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.
ISO 24664:2024(en)
Choked flow occurs when the pressure ratio p /p is less than or equal to the choked pressure ratio:
b 0
p
b
≤p (13)
r,choked
p
Where the choked pressure ratio is calculated as:
γ
 2 γ−1
p = (14)
r,choked  
γ +1
 
For choked flow the value of the capacity correction factor is:
γ+1
 γ−1
K =⋅γ (15)
cap  
γ +1
 
If the flow is not choked, then the capacity correction factor is calculated as:
2 γ +1
 
p p
2⋅γ  γ   γ
 
bb
K = ⋅ − (16)
cap    
 
γ −1 p p
   
0 0
 
 
The choked pressure ratio for different refrigerant is given in Annex A ,Table A.1, and values of K at choked
cap
and non-choked flow are given in Annex A, Table A.1 to Table A.3.
NOTE 3 Formula (10) to Formula (16) give identical results to corresponding Formulae in EN 13136:2013+A1:2018
and ISO 4126-7:2013.
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 refrigerant
m,relief m,required
mass flow used in the pressure drop calculations can be adjusted according to the following rules:
if QQ<⋅12, 5 then QQ= (17)
()
mm,,relief required mm,,adjusted requirred
Q
m,relieef
if QQ≥⋅12, 5 then Q = (18)
()
mm,, relief required m,adjusted
12, 5
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 7.2. The following
values for K shall be the maximum used depending on how the pipe between the vessel and the bursting
dr
disc is mounted on the vessel:
a) flush or flared connection (see Table A.2): K = 0,70.
dr
b) inserted connection (see Table A.2): K = 0,55.
dr
If the K -value of the bursting disc itself is lower than the maximum value given above, then the smaller
dr
value shall be used in the calculation.

ISO 24664:2024(en)
8 Pressure loss in inlet and outlet lines
8.1 General
To ensure correct operation of a pressure relief device, the pressure loss in both inlet and outlet lines,
including any changeover device shall not exceed the following:
The values stated by the manufacturer of the pressure relief device, but not more than:
— Inlet line (including changeover device):
Δpp≤⋅00, 3 (19)
in 0
— Outlet line:
— Back pressure dependent pressure relief device:
Δpp≤⋅01, 0 (20)
out 0
— Back pressure independent pressure relief device:
Δpp≤⋅02, 0 (21)
out 0
The velocity in the inlet line shall not reach the speed of sound. If the velocity in the outlet line is larger than
the speed of sound, then either the pipe diameter shall be increased and/or the pressure drop across the
resulting shock shall be included in the total pressure drop of the outlet line. Annex D includes an example of
how to calculate this.
The flow area of piping or changeover devices in the inlet and outlet lines shall not be less than the actual
flow area A of the pressure relief device.
actual
NOTE It is expected that pipes in both inlet and outlet line are selected so that they can withstand the pressure
and temperature during discharge.
8.2 Pressure loss in inlet line
The pressure loss in the inlet line is calculated using the Darcy-Weisbach Formula:
L u
−5
Δpf=⋅ρ ⋅⋅ ⋅10 (22)
d 2
R
When using the adjusted mass flow instead of velocity, the Formula can be written as:
Q
 
L
m,adjusted
Δ pf=⋅0,3858 ⋅⋅v ⋅ (23)
 
d A
R  R 
 
1 10
−5
NOTE 1 The factor 0,385 8 =⋅ ⋅10 is a result of unit conversion and simplification.
 
 
2 3 600
 
Depending on the type of component in the inlet line, Formula (23) can be formulated differently.

ISO 24664:2024(en)
For pipes, Formula (23) is used directly. The friction factor, f, is calculated from von Karman’s equation for
fully developed turbulent flow:
f = (24)
37, 1⋅d
  
R
2⋅log
 10  
ε
  
R
Values for pipe roughness can be found in Table A.5 in Annex A.
NOTE 2 If the flow is not fully turbulent other formulae can be used to calculate the friction factor (for example
according to [Colebrook, 1939]).
For fittings the pressure loss is calculated by means of the pressure loss coefficient:
L
ζ =⋅f (25)
d
R
The pressure loss using Formula (23) is then calculated as:
Q
 
m,adjusted
Δ pv=⋅0,3858 ζ ⋅⋅ (26)
 
A
 R 
Pressure loss coefficients for typical fittings shall be according to Annex A, Table A.4 .
If the manufacturer provides the pressure loss coefficient ζ for devices (valves) related to the nominal
DN
diameter (DN), it is converted to the pressure loss coefficient ζ for the actual internal diameter of the pipe from:
d

...