ISO/FDIS 21013-3

Cryogenic vessels — Pressure-relief accessories for cryogenic service — Part 3: Sizing and capacity determination

ISO 21013-3:2016 provides separate calculation methods for determining the required mass flow to be relieved for each of the following specified conditions: - vacuum-insulated vessels with insulation system (outer jacket + insulating material) intact under normal vacuum, outer jacket at ambient temperature, inner vessel at temperature of the contents at the specified relieving pressure; - vacuum-insulated vessels with insulation system (outer jacket + insulating material) intact under normal vacuum, outer jacket at ambient temperature, inner vessel at temperature of the contents at the specified relieving pressure, pressure regulator of the pressure build-up system functioning at full potential; - vacuum or non-vacuum-insulated vessels with insulation system remaining in place, but with loss of vacuum in the case of vacuum-insulated vessels, outer jacket at ambient temperature, inner vessel at temperature of the contents at the specified relieving pressure or vacuum or non-vacuum-insulated vessels with insulation system remaining fully or partially in place, but with loss of vacuum in the case of vacuum-insulated vessels, fire engulfment, inner vessel at temperature of the contents at the specified relieving pressure; - vacuum-insulated vessels containing fluids with saturation temperature below 75 K at 1 bar with insulation system remaining in place, but with loss of vacuum with air or nitrogen in the vacuum space; - vacuum insulated vessels containing fluids with saturation temperature below 75 K at 1 bar with insulation system remaining in place, but with loss of vacuum with air or nitrogen in the vacuum space with fire engulfment; - vessels with insulation system totally lost and fire engulfment. Good engineering practice based on well-established theoretical physical science needs to be adopted to determine the required mass flow where an appropriate calculation method is not provided for an applicable condition. Recommendations for pressure relief devices for cryostats are given in Annex A.

Récipients cryogéniques — Dispositifs de sécurité pour le service cryogénique — Partie 3: Détermination de la taille et du volume

ISO 21013-3:2016 établit des méthodes de calcul distinctes permettant de déterminer le débit massique à décharger pour chacune des conditions spécifiées suivantes: - récipients isolés sous vide comportant un système d'isolation (enveloppe extérieure + matériau isolant) intact sous vide normal, enveloppe extérieure à température ambiante, récipient intérieur à la température du contenu à la pression de décharge spécifiée; - récipients isolés sous vide comportant un système d'isolation (enveloppe extérieure + matériau isolant) intact sous vide normal, enveloppe extérieure à température ambiante, récipient intérieur à la température du contenu à la pression de décharge spécifiée, régulateur de pression du système de mise en pression fonctionnant à pleine charge; - récipients isolés sous vide ou non, comportant un système d'isolation restant en place mais avec perte de vide en cas de récipients isolés sous vide, enveloppe extérieure à température ambiante, récipient intérieur à la température du contenu à la pression de décharge spécifiée, ou récipients isolés sous vide ou non, comportant un système d'isolation restant totalement ou partiellement en place, mais avec perte de vide dans le cas des récipients isolés sous vide, feu environnant, récipient intérieur à la température du contenu à la pression de décharge spécifiée; - récipients isolés sous vide contenant des fluides avec une température de saturation inférieure à 75 K à 1 bar avec un système d'isolation restant en place, mais avec perte de vide avec de l'air ou de l'azote dans l'espace sous vide; - récipients isolés sous vide contenant des fluides avec une température de saturation inférieure à 75 K à 1 bar avec un système d'isolation restant en place, mais avec perte de vide avec de l'air ou de l'azote dans l'espace sous vide avec feu environnant; - récipients avec perte totale du système d'isolation et feu environnant. Il est nécessaire qu'une bonne pratique d'ingénierie reposant sur des données de physique théorique bien établies soit adoptée afin de déterminer le débit massique requis lorsqu'aucune méthode de calcul appropriée n'est fournie pour une condition donnée. Des Recommandations concernant les dispositifs de décharge de pression pour cryostats sont données dans l'Annexe A.

General Information

• Status: Not Published
• ICS: 23.020.40 - Cryogenic vessels
• Technical Committee: ISO/TC 220 - Cryogenic vessels
• Drafting Committee: ISO/TC 220 - Cryogenic vessels
• Current Stage: 5000 - FDIS registered for formal approval
• Start Date: 16-Mar-2026
• Completion Date: 29-Sep-2025

Relations

• Consolidates: prEN ISO 21013-3 - Cryogenic vessels - Pressure-relief accessories for cryogenic service - Part 3: Sizing and capacity determination (ISO/DIS 21013-3:2025)
• Effective Date: 12-Feb-2026
• Revises: ISO 21013-3:2016 - Cryogenic vessels — Pressure-relief accessories for cryogenic service — Part 3: Sizing and capacity determination
• Effective Date: 18-Jun-2022

Standards Content (Sample)

FINAL DRAFT
International
Standard
ISO/TC 220
Cryogenic vessels — Pressure-relief
Secretariat: AFNOR
accessories for cryogenic service —
Voting begins on:
2026-06-30
Part 3:
Sizing and capacity determination
Voting terminates on:
2026-08-25
Récipients cryogéniques — Dispositifs de sécurité pour le service
cryogénique —
Partie 3: Détermination de la taille et du volume
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 SUPPOR TING DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO-
ISO/CEN PARALLEL PROCESSING LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
Reference number
FINAL DRAFT
International
Standard
ISO/TC 220
Cryogenic vessels — Pressure-relief
Secretariat: AFNOR
accessories for cryogenic service —
Voting begins on:
Part 3:
Sizing and capacity determination
Voting terminates on:
Récipients cryogéniques — Dispositifs de sécurité pour le service
cryogénique —
Partie 3: Détermination de la taille et du volume
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 SUPPOR TING DOCUMENTATION.
© ISO 2026
IN ADDITION TO THEIR EVALUATION AS
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO-
ISO/CEN PARALLEL PROCESSING
LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
or ISO’s member body in the country of the requester.
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MADE IN NATIONAL REGULATIONS.
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Published in Switzerland Reference number
ii
Contents Page
Foreword .v
1 Scope . 1
2 Normative references . 2
3 Terms, definitions and symbols . 2
3.1 Symbols .2
4 Calculation of the total quantity of heat transferred per unit time from the hot wall
(outer jacket) to the cold wall (inner vessel) . 6
4.1 General .6
4.2 Under conditions other than fire .7
4.2.1 Vacuum-insulated vessels under normal vacuum .7
4.2.2 Pressure build-up device .7
4.2.3 Vacuum-insulated vessels in the case of loss of vacuum and non-vacuum
insulated vessels . .8
4.2.4 Supports and piping.9
4.3 Under fire conditions .10
4.3.1 Insulation system remains fully or partially in place during fire conditions .10
4.3.2 Insulation system does not remain in place during fire conditions .10
4.4 Air or nitrogen condensation . .11
4.4.1 General .11
4.4.2 Loss of vacuum with air and nitrogen .11
4.4.3 Fire with loss of vacuum with air or nitrogen .11
4.5 Heat transfer per unit time (watts) . 12
4.5.1 General . 12
4.5.2 Normal operation. 12
4.5.3 Pressure build up regulator fully open . 13
4.5.4 Loss of vacuum condition . 13
4.5.5 Fire condition with loss of vacuum, vacuum jacket, and insulation fully or
partially in place .14
4.5.6 Fire condition with loss of vacuum, insulation not in place .14
4.5.7 Total heat transfer rate .14
5 Calculation of the mass flow to be relieved by pressure relief devices . 14
5.1 Relieving pressure, P, less than the critical pressure .14
5.2 Relieving pressure, P, equal to or greater than the critical pressure . 15
5.3 Example . 15
6 Piping for pressure relief devices .16
6.1 Pressure drop .16
6.1.1 General .16
6.1.2 Relief valves .16
6.1.3 Bursting discs .16
6.2 Back pressure consideration . .17
6.3 Heat transfer .17
7 Sizing of pressure relief devices .18
7.1 General .18
7.2 Sizing of pressure relief valves.18
7.2.1 Discharge capacity .18
7.2.2 Determination of critical vs. subcritical flow for gases .19
7.2.3 Critical flow .19
7.2.4 Subcritical flow .19
7.2.5 Recommended analysis method . 20
7.2.6 Example . 22
7.3 Sizing of bursting discs .27
7.3.1 Discharge capacity .27
7.3.2 Determination of critical vs. subcritical flow for gases . 28

iii
7.3.3 Critical flow . 28
7.3.4 Subcritical flow . 28
7.3.5 Recommended analysis method . 29
7.3.6 Example .32
Annex A (informative) Cryostats .36
Annex ZA (informative) Relationship between this European Standard and the essential
requirements of Directive 2014/68/UE (Pressure Equipment Directive) aimed to be
covered .37
Bibliography .38

iv
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).
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 220, Cryogenic vessels, in collaboration with
the European Committee for Standardization (CEN) Technical Committee CEN/TC 268, Cryogenic vessels, in
accordance with the Agreement on technical cooperation between ISO and CEN (Vienna Agreement).
This third edition cancels and replaces the second edition (ISO 21013-3:2016), which has been technically
revised.
The main changes are as follows:
— precision added on the total outer surface area of the pipe network containing flow of fluid;
— inclusion of the symbol of coefficient of discharge;
— precision added on the overall convective heat transfer coefficient of the ambient air vaporizer,
— additional information added on the fire case and PBU conditions;
— correction of the molar mass in kg/kmol and modification of Formulae (32) and (43);
— supplementary information added on cryostats.
A list of all parts in the ISO 21013 series can be found on the ISO website.
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.

v
FINAL DRAFT International Standard ISO/FDIS 21013-3:2026(en)
Cryogenic vessels — Pressure-relief accessories for cryogenic
service —
Part 3:
Sizing and capacity determination
1 Scope
This document specifies calculation methods for determining the required mass flow to be relieved for each
specified conditions described in Table 1.
a
Table 1 — Subclause required for calculating mass flow rate based on specified condition
Insulation
Type of system (outer Pressure
Use clauses
insu- Vacuum Inner vessel jacket + Ambient buildup
Condition including any
lation status temperature insulating conditions system
subclauses
system material) condition
condition
normal
(ambient 4.2.3,4.2.4, 4.5.1,
1 intact
tempera- 4.5.2, 4.5.7, 5, 6, 7
non-
ture)
N/A
vacuum
normal
fully or
condition 4.3.1, 4.5.1, 4.5.7, 5,
2 partially in fire (922 K)
6, 7
tempera- place
ture of the
4.2.1, 4.2.4, 4.5.1,
contents at
4.5.2, 4.5.7, 5, 6, 7
the specified
normal
normal
4.2.1, 4.2.2, 4.2.4,
relieving
(ambient
4 intact full open 4.5.1, 4.5.3, 4.5.7, 5,
pressure
tempera-
6, 7
ture)
4.2.3, 4.2.4, 4.5.1,
4.5.4, 4.5.7 ,5, 6, 7
loss of
fully or
vacuum vacuum
4.3.1, 4.4.3, 4.5.1,
6 partially in fire (922 K)
4.5.5, 4.5.7, 5, 6, 7
place
normal
(ambient 4.4.2, 4.5.1, 4.5.4,
7 normal
loss of
tempera- 4.5.7, 5, 6 ,7
condition
intact
vacuum
ture)
contents with
with air or
relieving 4.4.3, 4.5.1, 4.5.5,
8 nitrogen
temperature 4.5.7, 5, 6, 7
in the
below 75 K
Vacuum
vacuum
fire (922 K)
and 4.5.1, 4.5.6, 4.5.7, 5,
space
9 totally lost
non- 6, 7
vacuum
When no appropriate calculation method is provided for a condition, the required mass flow is determined using good engineering
practice based on well-established theoretical physical science.
Recommendations for pressure relief devices for cryostats are given in Annex A.

2 Normative references
There are no normative references in this document.
3 Terms, definitions and symbols
No terms and definitions are listed in this document.
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 Symbols
A arithmetic mean of inner and outer surface areas of vessel insulating material m
A actual flow area of a pipe element m
B
A total outer surface area of the pipe network containing flow of fluid, from the outer m
e
jacket up to the flow temperature location x under consideration; applicable only
when l > 0,6 m (See 6.1.1)
A minimum flow area (reference area) in a pipe network m
F
A minimum flow area (reference area) in the pipe network, downstream of relief valve m
Fd
A minimum flow area (reference area) in the pipe network, upstream of relief valve m
Fu
A total outside surface area of inner vessel m
i
A total outer surface area of pipe network between the inner and outer jackets m
j
(interspace)
A larger flow area of a pipe element containing two different flow area sizes m
L
A cross-sectional area, support, or pipe material m
n
A area ratio, A /A —
R S L
A smaller flow area of a pipe element containing two different flow area sizes m
S
A actual flow (orifice) area of a pressure relief valve mm
V
A actual flow (orifice) area of pressure relief valve selected for final analysis mm
Va
A minimum required relief valve flow (orifice) area mm
V1
A external heat transfer surface area of ambient air vaporizer m
C experimentally determined flow rate through a pipe element or device m /h
V
K flow resistance coefficient of a pipe element in terms of A —
A F
K flow resistance coefficient of a pipe element in terms of A —
B B
K subcritical flow coefficient —
b
K coefficient of discharge —
d
K derated coefficient of discharge —
dr
K derated coefficient of discharge of next largest available valve orifice area greater —
dr,a
than A
V1
K derated coefficient of discharge of initially analysed valve —
dr,1
K flow resistance coefficient of complete pipe network in terms of reference area, A —
R F
K flow resistance coefficient at the transition between critical and subcritical flow —
RC
K overall flow resistance coefficient of pipe network, downstream of pressure relief —
Rd
valve
K overall flow resistance coefficient of pipe network, upstream of pressure relief valve —
Ru
K total flow resistance coefficient of a series or parallel pipe network —
SUM
3 1)
K experimentally determined flow rate through a pipe element or device m /h/bar
V
L latent heat of vaporization of cryogenic liquid at relieving conditions kJ/kg
1)
L latent heat of vaporization of cryogenic liquid at a pressure of 1,013 bar kJ/kg
a
L’ enthalpy-to-volume expansion ratio for critical or all-gas fluid flow conditions kJ/kg
M molar mass kg/kmol
N normal evaporation rate (NER) %/day
P relieving pressure, inner vessel in absolute pressure bar
P pressure, safety relief valve outlet in absolute pressure bar
b
P pressure at relief valve outlet for a downstream built-up backpressure of 10 % bar
b10
P pressure at pipe network exit in absolute pressure bar
exit
P pressure, safety relief valve inlet in absolute pressure bar
i
P pressure relief valve set pressure in absolute pressure bar
S
Q mass flow rate kg/h
m
Q mass flow rate of a relief valve within a given pipe network kg/h
ma
Q mass flow rate due to the normal evaporation rate kg/h
mNER
R universal gas constant J/(mol·K)
T relieving temperature, inner vessel K
T maximum external ambient temperature, non-fire condition K
a
T temperature at relief valve outlet K
b,Pb
T temperature at relief valve outlet for a downstream built-up backpressure of 10 % K
b10
T external temperature for a given condition K
e
1) bar = 0,1 MPa = 105 Pa; 1 MPa = 1 N/mm

T temperature at pipe network exit K
exit,Pb
T external temperature, fire condition K
f
T temperature, safety relief valve inlet K
i
T temperature of fluid at a given flow start location along the pipe network K
n
T saturation temperature of fluid at a pressure of 1 bar K
sat
T temperature of fluid at a given location x along the pipe network K
x
U heat transfer coefficient of insulating material, normal vacuum, non-fire condition W/ (m ·K)
U overall convective heat transfer coefficient of ambient air vaporizer W/ (m ·K)
U heat transfer coefficient of insulating material with air or gaseous lading, non-fire W/ (m ·K)
condition
U heat transfer coefficient, air or nitrogen condensation, loss of vacuum, non-fire W/m
3a
condition
U heat transfer coefficient of insulating material with air or gaseous lading, fire W/ (m ·K)
condition
U heat transfer coefficient, air or nitrogen condensation, loss of vacuum, fire condition W/m
5a
U overall heat transfer coefficient of a pipe network for given temperature conditions W/ (m ·K)
p
W total heat transfer rate for specified conditions Watt [W]
T
W total heat transfer rate under normal operation Watt [W]
T1
W total NER heat transfer rate under normal operation Watt [W]
T1NER
W total heat transfer rate under normal operation, including pressure build-up device Watt [W]
T2
W total NER heat transfer rate under normal operation, including pressure build-up Watt [W]
T2NER
device
W total heat transfer rate, loss of vacuum, insulation in place, non-fire condition, Watt [W]
T3
T ˃ 75 K
sat
W total heat transfer rate, loss of vacuum, insulation in place, non-fire condition, Watt [W]
T3a
T ≤ 75 K
sat
W total heat transfer rate, loss of vacuum, insulation in place, fire condition, T ˃ 75 K Watt [W]
T5 sat
W total heat transfer rate, loss of vacuum, insulation in place, fire condition, T ≤ 75 K Watt [W]
T5a sat
W total heat transfer rate, loss of vacuum, insulation not in place, fire condition Watt [W]
T6
W heat transfer rate through insulation system, normal vacuum, non-fire condition Watt [W]
W heat transfer rate through pressure build-up device, fully open regulator Watt [W]
W heat transfer rate through insulation system, loss of vacuum, non-fire condition Watt [W]
W heat transfer rate through air or nitrogen condensation, loss of vacuum, Watt [W]
3a
non-fire condition
W heat transfer rate through interspace supports and piping Watt [W]
W heat transfer rate through vessel walls, insulation in place, fire condition Watt [W]
W heat transfer rate through air or nitrogen condensation, loss of vacuum, Watt [W]
5a
fire condition
W heat transfer rate through vessel walls, insulation not in place, fire condition Watt [W]
X number of insulation layers —
Y heat transfer rate U or U —
3a 5a
Z compressibility factor at pressure P and temperature T —
i i i
a
c constant pressure specific heat capacity at the average of T and T kJ/(kg·K)
p n e
e nominal insulating material thickness, in normal vacuum, non-fire condition m
e minimum insulating material thickness, considering loss of vacuum, non-fire m
condition
e insulating material thickness remaining in place during fire conditions m
f pipe flow friction coefficient —
T
h enthalpy of fluid at conditions of ν kJ/kg
h specific enthalpy at relief valve inlet and outlet kJ/kg
r
k mean thermal conductivity of insulating material, normal vacuum, W/(m·K)
non-fire condition
k mean thermal conductivity of insulating material with air or gaseous lading, W/(m·K)
non-fire condition
k mean thermal conductivity of an individual support or pipe, between T and T W/(m·K)
n a
l length, pipe element m
l length of support or pipe in vacuum interspace m
n
m maximum mass capacity of vessel kg
max
r pipe elbow transition radius m
x lengthwise location along a pipe network m
φ —
PP P
exit exit
pressure ratio 1
P P
κ isentropic exponent —
λ subcritical flow coefficient —
ν specific volume of critical or all-gas fluid at a given temperature at pressure P m /kg
ν specific volume at relief valve outlet for a downstream built-up backpressure of 10 % m /kg
b10
ν specific volume at pressure relief valve outlet, evaluated at h and a trial value of P m /kg
b,Pb r b
ν average downstream specific volume, for a downstream built-up backpressure m /kg
d10
of 10 %
ν maximum average downstream specific volume, as per desired backpressure limit m /kg
dmax
ν specific volume at pipe network exit for a downstream built-up backpressure of 10 % m /kg
exit10
ν specific volume at pipe network exit, evaluated at P and T m /kg
exit,Pb exit exit,Pb
ν specific volume of saturated gas at relieving pressure, P m /kg
g
ν specific volume of saturated gas at a pressure of 1,013 bar m /kg
ga
ν specific volume, safety relief valve inlet m /kg
i
ν specific volume of saturated liquid at relieving pressure, P m /kg
l
ν specific volume of saturated liquid at a pressure of 1,013 bar m /kg
la
ν average specific volume of flowing fluid upstream of pressure relief valve inlet m /kg
u
3/2 1/2
ψ expression for determining Q and T for critical or gas-full-vessel fluid flow m ·kg /
m
conditions kJ
4 Calculation of the total quantity of heat transferred per unit time from the hot wall
(outer jacket) to the cold wall (inner vessel)
4.1 General
P (in bar absolute) is the actual relieving pressure inside the vessel which is used for calculating the required
mass flow through pressure relief devices.
T (in K) is the maximum ambient temperature for conditions other than fire (as specified, for example, by a
a
regulation or standard).
T (in K) is the external environment temperature under fire conditions which is taken to be 922 K in this
f
document.
T (in K) is the relieving temperature in the vessel to be taken into account.
a) For subcritical fluids, T is the saturation temperature of the liquid at pressure P.
b) For critical or supercritical fluids, T is calculated from 5.2.

4.2 Under conditions other than fire
4.2.1 Vacuum-insulated vessels under normal vacuum
W is the quantity of heat transferred per unit time (in watts) by heat leak through the insulation system, as
per Formula (1).
WUAT T (1)

11 a
where
U is the overall heat transfer coefficient of the insulating material under normal vacuum, in W/
k
(m ⋅K): U = ;
e
k is the mean thermal conductivity of the insulating material under normal vacuum, between T
and T , in W/(m⋅K);
a
e is the nominal insulating material thickness, in m;
A is the arithmetic mean of the inner and outer surface areas of the vessel insulating material, in
m ;
T is the relieving temperature, inner vessel;
T is the maximum external ambient temperature, non-fire condition.
a
4.2.2 Pressure build-up device
W is the quantity of heat transferred per unit time (in watts) by the pressure build-up device circuit with
the regulator fully open. W is determined from the type (ambient air, water or steam, electrical, etc.) and
design of the pressure build-up device circuit. For example, in the case of an ambient air vaporizer, W is
calculated using Formula (2):
WUAT T (2)

22 2 a
where
U is the overall convective heat transfer coefficient of the ambient air vaporizer, in W/(m ⋅K);
A is the external heat transfer surface area of the vaporizer, in m ;
T is the relieving temperature, inner vessel;
T is the maximum external ambient temperature, non-fire condition.
a
As a first approximation, Formula (3) or Formula (4) may be used:
UT TT19 000 W/mfor 75K (3)

2 a
UT TT2 850 W/mfor 75K (4)

2 a
U shall not be lower than the values proposed but manufacturers can use greater values if their design
requires.
4.2.3 Vacuum-insulated vessels in the case of loss of vacuum and non-vacuum insulated vessels
W is the quantity of heat transferred per unit time (in watts) by heat leak through the insulating material. It
is calculated using Formula (5):
WUAT T (5)

33 a
where
U is the overall heat transfer coefficient of the insulating material when saturated with
gaseous lading or air at atmospheric pressure, whichever is greater, in W/(m ·K);
T is the relieving temperature, inner vessel;
T is the maximum external ambient temperature, non-fire condition;
a
A is the arithmetic mean of inner and outer surface areas of vessel insulating material.
If the insulation is fully effective for conduction, convection, and radiation heat transfer at 328 K, U may be
calculated using Formula (6):
k
U = (6)
e
where
k is the mean thermal conductivity of the insulating material saturated with gaseous lading
or air at atmospheric pressure, whichever provides the greater coefficient, between
T and T , in W/(m·K) (values of k for gases are listed in Table 2);
a 3
e is the minimum insulating material thickness taking into account the manufacturing
tolerances or effects of sudden loss of vacuum, in m.
NOTE This formula might not be applicable at temperatures below 75 K with a small thickness of insulating
material as the maximum heat transfer coefficient would be given by air condensation.
Vacuum space, gas space, or space occupied by the deteriorated insulation shall not be included in the
thickness of the insulation. The effectiveness of these spaces or deteriorated insulation in reducing
conduction, convection, or radiation heat transfer may be evaluated separately and included in the overall
heat transfer coefficient, U , using methods found in published heat transfer literature. Deterioration of the
insulation can be caused by the following:
— moisture condensation;
— air condensation;
— increase in the density of the insulation due to a sudden loss of vacuum.

Table 2 — Thermal conductivity for refrigerated (cryogenic) fluids at the mean temperature
between saturation and 328 K (k) and 922 K (k) at 1 bar
3 5
k k
3 5
Fluid
[W/(m·K)] [W/(m·K)]
Air 0,019 0,043
Argon 0,013 0,027
Carbon dioxide 0,017 0,039
Carbon monoxide 0,020 0,039
Helium 0,104 0,211
Hydrogen 0,116 0,217
Methane 0,024 0,074
Neon 0,034 0,067
Nitrogen 0,019 0,040
Oxygen 0,019 0,043
Krypton 0,007 0,015
Xenon 0,005 0,009
Ethane 0,016 0,064
Trifluoromethane 0,012 0,027
Ethylene (ethene) 0,015 0,056
Nitrous oxide 0,014 0,038
Values are for reference only. Use fluid properties given in current databases, such as from NIST.
4.2.4 Supports and piping
W is the quantity of heat transferred per unit time (in watts) by supports and piping located in the
interspace, given by Formula (7):
WwwwwT T (7)

41 23 na
where w is the heat leak per degree K contributed by one of the supports or the pipes, in W/K, given by
n
Formula (8):
 A 
n
wk (8)
 
nn
 
l
 
n
where
k is the mean thermal conductivity of the support or pipe material between T and T , in
n a
W/(m⋅K);
A is the support or pipe section area, in m ;
n
l is the support or pipe length in the vacuum interspace, in m.
n
4.3 Under fire conditions
4.3.1 Insulation system remains fully or partially in place during fire conditions
W is the quantity of heat transferred per unit time (in watts) by heat leak through the vessel walls, given by
Formula (9):
08, 2
WT26, 922 UA (9)

where if the insulation is fully effective for conduction, convection, and radiation heat transfer for an
external temperature of 922 K, U may be calculated using Formula (10):
k
U = (10)
e
where
U is the overall heat transfer coefficient of the container-insulating material when saturated
with gaseous lading or air at atmospheric pressure, whichever is greater, in W/(m ·K);
k is the mean thermal conductivity of the insulating material saturated with gaseous lading
or air at atmospheric pressure, whichever provides the greater coefficient, between T and
922 K, in W/(m·K) (values of k for gases are listed in Table 2);
e is the thickness of the insulating material remaining in place during fire conditions, in m;
A is the arithmetic mean of the inner and outer surface areas of the insulating material
remaining in place during fire conditions, in m ;
T is the relieving temperature, inner vessel.
Vacuum space, gas space, or space occupied by the deteriorated insulation shall not be included in the
thickness of the insulation. The effectiveness of these spaces or deteriorated insulation in reducing
conduction, convection, or radiation heat transfer may be evaluated separately and included in the overall
heat transfer coefficient, U , using methods found in published heat transfer literature. Deterioration of the
insulation can be caused by the following:
— moisture condensation;
— air condensation;
— increase in the density of the insulation due to a sudden loss of vacuum;
— degradation due to heat.
If the outer jacket remains in place during fire conditions, but the insulating material is entirely destroyed,
U is equal to the overall heat transfer coefficient with gaseous lading or air at atmospheric pressure in the
space between the outer jacket and the inner vessel, whichever provides the greater coefficient, between T
and 922 K. A is equal to the mean surface area of the interspace.
4.3.2 Insulation system does not remain in place during fire conditions
W is the quantity of heat transferred per unit time (in watts) by heat leak through the vessel walls,
calculated using Formula (11):
40,82
WA71, 10  (11)
6 i
where A is the total outside surface area of the inner vessel, in m .
i
The heat transferred by supports and piping located in the interspace can be neglected in this case.
4.4 Air or nitrogen condensation
4.4.1 General
The air or nitrogen condensation case for the loss of vacuum condition shall be considered for fluids with a
saturation temperature below 75 K at 1 bar.
Air condensation, for the case of loss of vacuum to the atmosphere on a vacuum insulated container,
is highly dependent on the type of insulation and how the insulation is designed. Since air condensation
occurs primarily below 75 K, and fluids with saturation temperatures below 75 K are generally stored and
transported in containers insulated with multi-layer insulation, this document covers air condensation on
multi-layer insulated containers only. In the absence of pertinent reliable data on perlite insulated vessels,
4.2.3, 4.3.1, and 4.3.2 shall be used with thermal conductivity values shown in Table 2 increased by a factor
of two in Formula (6) for k and Formula (10) for k , respectively.
3 5
Condensation of air on a multi-layer insulated surface below 75 K will depend on the rate of air access to the
insulated surface. On a multi-layer insulated container, the air condensation rate can vary depending on the
number of layers and the air access allowed by the design of the insulation.
Figure 1 provides heat transfer rates from air condensation to the stored fluid as a function of the number
of layers of insulation. The fire engulfment curve is an extrapolation for 922 K ambient temperature. Unless
heat transfer rates under the loss of vacuum condition from air condensation can be determined for the
same type and design of multi-layer insulation from prototype tests or actual incidents, heat transfer rates
from Figure 1 shall be used.
4.4.2 Loss of vacuum with air and nitrogen
W is the quantity of heat transferred per unit time (in watts) through air or nitrogen condensation for
3a
vacuum-insulated vessels in the case of a loss of vacuum with air or nitrogen, calculated using Formula (12).
WUA (12)
33aa i
where U is the heat transfer through air or nitrogen condensation, in watts per square metre of the inner
3a
vessel outer surface area, from Figure 1.
4.4.3 Fire with loss of vacuum with air or nitrogen
W is the quantity of heat transferred per unit time (in watts) through air or nitrogen condensation for
5a
vacuum-insulated vessels in the case of a fire and loss of vacuum with air or nitrogen, calculated using
Formula (13).
08, 2
WU19, 5 A (13)
55aa i
where U is the heat transfer through air or nitrogen condensation during fire conditions, in watts per
5a
square metre of the inner vessel surface area, from Figure 1.

Key
X number of insulation layers
Y heat transfer [W/m ]
07, 3
38 400420 X
UY
3a
07, 3
09, 6X
07, 3
92 1601 000 X
UY
5a
07, 3
09, 6X
Figure 1 — Heat transfer rate for air or nitrogen condensation
4.5 Heat transfer per unit time (watts)
4.5.1 General
The required mass flow rate, Q , to be relieved can be calculated from the heat transfer, W , for the conditions
m T
specified in 4.5.2 to 4.5.6, where W is the total heat transfer applicable to the specified condition.
T
4.5.2 Normal operation
Formula (14) applies:
WWW (14)
T1 14
Alternatively, the heat transfer rate, W , can be determined from the normal evaporation rate (NER)
T1NER
using Formula (15).
 v 
QL 
ga
mNER a
 
W  (15)
 
TN1 ER
 
 
36, v v
 
ga la
 
where
Q is the mass flow rate due to the normal evaporation rate, in kg/h;
mNER
L is the latent heat of vaporization of the cryogenic liquid at a pressure of 1,013 bar, in kJ/kg;
a
ν is the specific volume of saturated gas at a saturation pressure of 1,013 bar, in m /kg;
ga
ν is the specific volume of saturated liquid at a saturation pressure of 1,013 bar, in m /kg;
la
W is the total heat leak calculated from the experimentally determined normal evaporation
T1NER
rate, in watts.
4.5.3 Pressure build up regulator fully open
Formula (16) and Formula (17) apply:
WWW (16)
TT21 2
Or for the NER method:
WWW (17)
TN21ER TNER 2
4.5.4 Loss of vacuum condition
The heat transfer rate is to be the larger of W or W , calculated using Formula (18) and Formula (19):
T3 T3a
WWW (18)
T3 34
WWW (19)
Ta33a 4
where
W is the total heat transfer rate if the saturation temperature of the fluid is greater than or equal
T3
to 75 K at 1 bar;
W is the total heat transfer rate if the saturation temperature of the fluid is less than 75 K at 1 bar.
T3a
Alternatively, W may be calculated using Formula (20):
T3
WWW (20)
TT31 3
or approximately, using Formula (21):
WWW (21)
TT31NER 3
W may be calculated using Formula (22):
T3a
WWW (22)
Ta31Ta3
or approximately, using Formula (23):
WWW (23)
TT31aNER 3a
4.5.5 Fire condition with loss of vacuum, vacuum jacket, and insulation fully or partially in place
The heat transfer rate shall be the larger of W or W , where
T5 T5a
W is equal to the heat transfer rate, W , if the saturation temperature of the fluid is greater than or
T5 5
equal to 75 K at 1 bar;
W is equal to the heat transfer rate, W , if the saturation temperature of the fluid is less than 75 K
T5a 5a
at 1 bar.
4.5.6 Fire condition with loss of vacuum, insulation not in place
The heat transfer rate shall be the larger of W = W or W = W , where W is calculated with U for the
T6 6 T5 5a 5a 5a
bare surface condition from Figure 1.
4.5.7 Total heat transfer rate
W is the total heat transfer rate and is equal to W , W , W , W , W , W , W , W , or W , as
T T1 T1NER T2 T2NER T3 T3a T5 T5a T6
applicable.
It is possible that the fire case is not the worst case, so fire and PBU conditions shall both be checked. If PBU
is changed later in the field, the sizing and capacity of pressure relief devices shall be verified according to
new PBU capacity.
5 Calculation of the mass flow to be relieved by pressure relief devices
5.1 Relieving pressure, P, less than the critical pressure
For a relieving pressure, P, less than the critical pressure, the discharge mass flow rate Q (in kg/h) is
m
calculated on the basis of either the heat input, W , or the normal evaporation rate, Q (in kg/h), using
T mNER
Formula (24).
For heat input W of the
...

ISO/DIS FDIS 21013-3:2026(en)
ISO/TC 220/WG 3
Secretariat: AFNOR
Date: 2026-03-1204-30
Cryogenic vessels — Pressure-relief accessories for cryogenic service
— —
Part 3:
Sizing and capacity determination
Récipients cryogéniques — Dispositifs de sécurité pour le service cryogénique —
Partie 3: Détermination de la taille et du volume
FDIS stage
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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: + 41 22 749 01 11
EmailE-mail: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
ISO/DISFDIS 21013-3:2026(en)
Contents
Foreword . iv
1 Scope . 1
2 Normative references . 2
3 Terms, definitions and symbols . 2
3.1 Symbols . 2
4 Calculation of the total quantity of heat transferred per unit time from the hot wall
(outer jacket) to the cold wall (inner vessel) . 6
4.1 General . 6
4.2 Under conditions other than fire . 6
4.3 Under fire conditions . 10
4.4 Air or nitrogen condensation . 11
4.5 Heat transfer per unit time (watts) . 13
5 Calculation of the mass flow to be relieved by pressure relief devices . 15
5.1 Relieving pressure, P, less than the critical pressure . 15
5.2 Relieving pressure, P, equal to or greater than the critical pressure . 15
5.3 Example . 16
6 Piping for pressure relief devices . 17
6.1 Pressure drop . 17
6.2 Back pressure consideration . 17
6.3 Heat transfer . 17
7 Sizing of pressure relief devices . 19
7.1 General . 19
7.2 Sizing of pressure relief valves . 20
7.3 Sizing of bursting discs . 31
Annex A (informative) Cryostats . 41
Annex ZA (informative) Relationship between this European Standard and the essential
requirements of Directive 2014/68/UE (Pressure Equipment Directive) aimed to be
covered . 42
Bibliography . 43

iii
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).
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 220, Cryogenic vessels, in collaboration with the
European Committee for Standardization (CEN) Technical Committee CEN/TC 268, Cryogenic vessels, in
accordance with the Agreement on technical cooperation between ISO and CEN (Vienna Agreement).
This third edition cancels and replaces the second edition (ISO 21013-3:2016), which has been technically
revised.
The main changes are as follows:
— — precision added on the total outer surface area of the pipe network containing flow of fluid;
— — inclusion of the symbol of coefficient of discharge;
— — precision added on the overall convective heat transfer coefficient of the ambient air vaporizer,
— — additional information added on the fire case and PBU conditions;
— — correction of the molar mass in kg/kmol and modification of Formulae (32)formulae (32) and
(43)(43);;
— — supplementary information added on cryostats.
A list of all parts in the ISO 21013 series can be found on the ISO website.
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
DRAFT International Standard ISO/DIS 21013-3:2026(en)

Cryogenic vessels — Pressure-relief accessories for cryogenic service
— —
Part 3:
Sizing and capacity determination
1 Scope
This document specifies calculation methods for determining the required mass flow to be relieved for each
specified conditions described in Table 1the following table:.
a
Table 1 – — Subclause required for calculating mass flow rate based on specified condition
Insulation
system Pressure
Type of Inner Use clauses
Vacuu (outer Ambient buildup
Conditi insulati vessel including any
m jacket + conditio system
on on temperatu sub-clauses
status: insulating ns: conditio
system: re: subclauses
material) n:
condition:
normal 4.2.3,4.2.4, 4.5.1,
(ambient 4.5.2, 4.5.7, 5,
1 intact
temperatu 64.2.3,4.2.4, 4.5.1,
non-
re) 4.5.2, 4.5.7, 5, 6, 7, 7
N/A
vacuum
fully or 4.3.1, 4.5.1, 4.5.7, 5,
fire normal
2 partially in 64.3.1, 4.5.1, 4.5.7,
(922 K) condition
place 5, 6, 7, 7
4.2.1, 4.2.4, 4.5.1,
4.5.2, 4.5.7, 5,
64.2.1, 4.2.4, 4.5.1,
temperature
4.5.2, 4.5.7, 5, 6, 7, 7
of the
normal contents at
4.2.1, 4.2.2, 4.2.4,
normal
the specified
4.5.1, 4.5.3, 4.5.7, 5,
(ambient
relieving
4 intact full open 64.2.1, 4.2.2, 4.2.4,
temperatu
pressure
4.5.1, 4.5.3, 4.5.7, 5,
re)
6, 7, 7
vacuum
4.2.3, 4.2.4, 4.5.1,
4.5.4, 4.5.7 ,5,
64.2.3, 4.2.4, 4.5.1,
4.5.4, 4.5.7 ,5, 6, 7, 7
loss of
vacuum
normal
4.3.1, 4.4.3, 4.5.1,
fully or
condition
fire 4.5.5, 4.5.7, 5,
6 partially in
(922 K) 64.3.1, 4.4.3, 4.5.1,
place
4.5.5, 4.5.7, 5, 6, 7, 7
loss of contents normal 4.4.2, 4.5.1, 4.5.4,
7 intact
vacuum with (ambient 4.5.7, 5, 64.4.2,
Insulation
system Pressure
Type of Inner Use clauses
Vacuu (outer Ambient buildup
Conditi insulati vessel including any
m jacket + conditio system
on on temperatu sub-clauses
status: insulating ns: conditio
system: re: subclauses
material) n:
condition:
with air relieving temperatu 4.5.1, 4.5.4, 4.5.7, 5,
or temperature re) 6 ,7 ,7
nitrogen below 75 K
4.4.3, 4.5.1, 4.5.5,
in the
4.5.7, 5, 64.4.3,
vacuum
4.5.1, 4.5.5, 4.5.7, 5,
space
6, 7, 7
fire
vacuum
(922 K)
Vacuum 4.5.1, 4.5.6, 4.5.7, 5,
9 and totally lost 64.5.1, 4.5.6, 4.5.7,
non- 5, 6, 7, 7
vacuum
a
When no appropriate calculation method is provided for a condition, the required mass flow is determined using good engineering
practice based on well-established theoretical physical science.
Recommendations for pressure relief devices for cryostats are given in Annex AAnnex A.
2 Normative references
There are no normative references in this document.
3 Terms, definitions and symbols
No terms and definitions are listed in this document.
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 Symbols
A arithmetic mean of inner and outer surface areas of vessel insulating material m
A actual flow area of a pipe element m
B
A total outer surface area of the pipe network containing flow of fluid, from the outer m
e
jacket up to the flow temperature location x under consideration; applicable only when
l > 0,6 m (See 6.1.16.1.1))
A minimum flow area (reference area) in a pipe network m
F
A minimum flow area (reference area) in the pipe network, downstream of relief valve m
Fd
A minimum flow area (reference area) in the pipe network, upstream of relief valve m
Fu
A total outside surface area of inner vessel m
i
A total outer surface area of pipe network between the inner and outer jackets m
j
(interspace)
ISO/DISFDIS 21013-3:2026(en)
A larger flow area of a pipe element containing two different flow area sizes m
L
A cross-sectional area, support, or pipe material m
n
A area ratio, A /A —
R S L
A smaller flow area of a pipe element containing two different flow area sizes m
S
A actual flow (orifice) area of a pressure relief valve mm
V
A actual flow (orifice) area of pressure relief valve selected for final analysis mm
Va
A minimum required relief valve flow (orifice) area mm
V1
A external heat transfer surface area of ambient air vaporizer m
C experimentally determined flow rate through a pipe element or device gal/min/psi
V
m /h
K flow resistance coefficient of a pipe element in terms of A —
A F
K flow resistance coefficient of a pipe element in terms of A —
B B
K subcritical flow coefficient —
b
K coefficient of discharge —
d
K derated coefficient of discharge —
dr
K derated coefficient of discharge of next largest available valve orifice area greater —
dr,a
than A
V1
K derated coefficient of discharge of initially analysed valve —
dr,1
K flow resistance coefficient of complete pipe network in terms of reference area, A —
R F
K flow resistance coefficient at the transition between critical and subcritical flow —
RC
K overall flow resistance coefficient of pipe network, downstream of pressure relief —
Rd
valve
K overall flow resistance coefficient of pipe network, upstream of pressure relief valve —
Ru
K total flow resistance coefficient of a series or parallel pipe network —
SUM
3 11)
K experimentally determined flow rate through a pipe element or device m /h/bar
V
L latent heat of vaporization of cryogenic liquid at relieving conditions kJ/kg
1)
L latent heat of vaporization of cryogenic liquid at a pressure of 1,013 bar kJ/kg
a
L’ enthalpy-to-volume expansion ratio for critical or all-gas fluid flow conditions kJ/kg
M molar mass kg/kmol
N normal evaporation rate (NER) %/day
P relieving pressure, inner vessel in absolute pressure bar
P pressure, safety relief valve outlet in absolute pressure bar
b
P pressure at relief valve outlet for a downstream built-up backpressure of 10 % bar
b10
P pressure at pipe network exit in absolute pressure bar
exit
P pressure, safety relief valve inlet in absolute pressure bar
i
1 2
bar = 0,1 MPa = 105 Pa; 1 MPa = 1 N/mm
1) 2
bar = 0,1 MPa = 105 Pa; 1 MPa = 1 N/mm

P pressure relief valve set pressure in absolute pressure bar
S
Q mass flow rate kg/h
m
Q mass flow rate of a relief valve within a given pipe network kg/h
ma
Q mass flow rate due to the normal evaporation rate kg/h
mNER
R universal gas constant J/(mol·K)
T relieving temperature, inner vessel K
T maximum external ambient temperature, non-fire condition K
a
T temperature at relief valve outlet K
b,Pb
T temperature at relief valve outlet for a downstream built-up backpressure of 10 % K
b10
T external temperature for a given condition K
e
T temperature at pipe network exit K
exit,Pb
T external temperature, fire condition K
f
T temperature, safety relief valve inlet K
i
T temperature of fluid at a given flow start location along the pipe network K
n
T saturation temperature of fluid at a pressure of 1 bar K
sat
T temperature of fluid at a given location x along the pipe network K
x
U heat transfer coefficient of insulating material, normal vacuum, non-fire condition W/ (m ·K)
U overall convective heat transfer coefficient of ambient air vaporizer W/ (m ·K)
U heat transfer coefficient of insulating material with air or gaseous lading, non-fire W/ (m ·K)
condition
U heat transfer coefficient, air or nitrogen condensation, loss of vacuum, non-fire W/m
3a
condition
U heat transfer coefficient of insulating material with air or gaseous lading, fire W/ (m ·K)
condition
U heat transfer coefficient, air or nitrogen condensation, loss of vacuum, fire condition W/m
5a
U overall heat transfer coefficient of a pipe network for given temperature conditions W/ (m ·K)
p
W total heat transfer rate for specified conditions Watt [W]
T
W total heat transfer rate under normal operation Watt [W]
T1
W total NER heat transfer rate under normal operation Watt [W]
T1NER
W total heat transfer rate under normal operation, including pressure build-up device Watt [W]
T2
W total NER heat transfer rate under normal operation, including pressure build-up Watt [W]
T2NER
device
W total heat transfer rate, loss of vacuum, insulation in place, non-fire condition, Watt [W]
T3
T ˃ 75 K
sat
W total heat transfer rate, loss of vacuum, insulation in place, non-fire condition, Watt [W]
T3a
T ≤ 75 K
sat
W total heat transfer rate, loss of vacuum, insulation in place, fire condition, T ˃ 75 K Watt [W]
T5 sat
W total heat transfer rate, loss of vacuum, insulation in place, fire condition, T ≤ 75 K Watt [W]
T5a sat
W total heat transfer rate, loss of vacuum, insulation not in place, fire condition Watt [W]
T6
ISO/DISFDIS 21013-3:2026(en)
W heat transfer rate through insulation system, normal vacuum, non-fire condition Watt [W]
W heat transfer rate through pressure build-up device, fully open regulator Watt [W]
W heat transfer rate through insulation system, loss of vacuum, non-fire condition Watt [W]
W heat transfer rate through air or nitrogen condensation, loss of vacuum, Watt [W]
3a
non-fire condition
W heat transfer rate through interspace supports and piping Watt [W]
W heat transfer rate through vessel walls, insulation in place, fire condition Watt [W]
W heat transfer rate through air or nitrogen condensation, loss of vacuum, Watt [W]
5a
fire condition
W heat transfer rate through vessel walls, insulation not in place, fire condition Watt [W]
X number of insulation layers —
Y heat transfer rate U or U —
3a 5a
Z compressibility factor at pressure, P , and temperature, T —
i i i
a
c constant pressure specific heat capacity at the average of T and T kJ/(kg·K)
p n e
e nominal insulating material thickness, in normal vacuum, non-fire condition m
e minimum insulating material thickness, considering loss of vacuum, non-fire m
condition
e insulating material thickness remaining in place during fire conditions m
f pipe flow friction coefficient —
T
h enthalpy of fluid at conditions of ν kJ/kg
h specific enthalpy at relief valve inlet and outlet kJ/kg
r
k mean thermal conductivity of insulating material, normal vacuum, W/(m·K)
non-fire condition
k mean thermal conductivity of insulating material with air or gaseous lading, W/(m·K)
non-fire condition
k mean thermal conductivity of an individual support or pipe, between T and T W/(m·K)
n a
l length, pipe element m
l length of support or pipe in vacuum interspace m
n
m maximum mass capacity of vessel kg
max
r pipe elbow transition radius m
x lengthwise location along a pipe network m
𝑃𝑃−𝑃𝑃 𝑃𝑃−𝑃𝑃 𝑃𝑃 𝑃𝑃
𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 exit 𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 exit
φ —
pressure ratio = 1−
𝑃𝑃 𝑃𝑃 𝑃𝑃 𝑃𝑃
κ isentropic exponent —
λ subcritical flow coefficient —
ν specific volume of critical or all-gas fluid at a given temperature at pressure, P m /kg
ν specific volume at relief valve outlet for a downstream built-up backpressure of 10 % m /kg
b10
ν specific volume at pressure relief valve outlet, evaluated at h and a trial value of P m /kg
b,Pb r b
ν average downstream specific volume, for a downstream built-up backpressure of 10 % m /kg
d10
ν maximum average downstream specific volume, as per desired backpressure limit m /kg
dmax
ν specific volume at pipe network exit for a downstream built-up backpressure of 10 % m /kg
exit10
ν specific volume at pipe network exit, evaluated at P and T m /kg
exit,Pb exit exit,Pb
ν specific volume of saturated gas at relieving pressure, P m /kg
g
ν specific volume of saturated gas at a pressure of 1,013 bar m /kg
ga
ν specific volume, safety relief valve inlet m /kg
i
ν specific volume of saturated liquid at relieving pressure, P m /kg
l
ν specific volume of saturated liquid at a pressure of 1,013 bar m /kg
la
ν average specific volume of flowing fluid upstream of pressure relief valve inlet m /kg
u
3/2 1/2
ψ expression for determining Q and T for critical or gas-full-vessel fluid flow m ·kg /k
m
conditions J
4 Calculation of the total quantity of heat transferred per unit time from the hot
wall (outer jacket) to the cold wall (inner vessel)
4.1 General
P (in bar absolute) is the actual relieving pressure inside the vessel which is used for calculating the required
mass flow through pressure relief devices.
T (in K) is the maximum ambient temperature for conditions other than fire (as specified, for example, by a
a
regulation or standard).
T (in K) is the external environment temperature under fire conditions which is taken to be 922 K in this
f
document.
T (in K) is the relieving temperature in the vessel to be taken into account.
a) a) For subcritical fluids, T is the saturation temperature of the liquid at pressure, P.
b) b) For critical or supercritical fluids, T is calculated from 5.25.2.
4.2 Under conditions other than fire
4.2.1 Vacuum-insulated vessels under normal vacuum
W is the quantity of heat transferred per unit time (in watts) by heat leak through the insulation system, as
per Formula (1)Formula (1).
𝑊𝑊 = (𝑈𝑈 ⋅𝐴𝐴)(𝑇𝑇 −𝑇𝑇) (1)
1 1 𝑎𝑎
where
U is the overall heat transfer coefficient of the insulating material under normal vacuum,
k
in W/(m⋅K): U = ;
e
ISO/DISFDIS 21013-3:2026(en)
k is the mean thermal conductivity of the insulating material under normal vacuum, between
T and T , in W/(m⋅K);
a
e is the nominal insulating material thickness, in metres;
A is the arithmetic mean of the inner and outer surface areas of the vessel insulating material,
in m ;
T is the relieving temperature, inner vessel;
T is the maximum external ambient temperature, non-fire condition.
a
U is the overall heat transfer coefficient of the insulating material under normal vacuum, in
𝑘𝑘
W/(m⋅K): 𝑈𝑈 = ;
𝑒𝑒
k is the mean thermal conductivity of the insulating material under normal vacuum, between T
and T , in W/(m⋅K);
a
e is the nominal insulating material thickness, in m;
A is the arithmetic mean of the inner and outer surface areas of the vessel insulating material, in
m ;
T is the relieving temperature, inner vessel;
T is the maximum external ambient temperature, non-fire condition.
a
4.2.2 Pressure build-up device
W is the quantity of heat transferred per unit time (in watts) by the pressure build-up device circuit with the
regulator fully open. W is determined from the type (ambient air, water or steam, electrical, etc.) and design
of the pressure build-up device circuit. For example, in the case of an ambient air vaporizer, W is calculated
using Formula (2).:
𝑊𝑊 = (𝑈𝑈 ⋅𝐴𝐴 )(𝑇𝑇 −𝑇𝑇) (2)
2 2 2 𝑎𝑎
where
U is the overall convective heat transfer coefficient of the ambient air vaporizer, in W/(m⋅K);
A is the external heat transfer surface area of the vaporizer, in m ;
T is the relieving temperature, inner vessel;
T is the maximum external ambient temperature, non-fire condition.
a
U is the overall convective heat transfer coefficient of the ambient air vaporizer, in W/(m⋅K);
A is the external heat transfer surface area of the vaporizer, in m ;
T is the relieving temperature, inner vessel;
T is the maximum external ambient temperature, non-fire condition.
a
As a first approximation, Formula (3)Formula (3) or Formula (4)Formula (4) may be used:
(3)
(4)
𝑈𝑈 (𝑇𝑇 −𝑇𝑇) = 19  000  W/m  for  𝑇𝑇≤ 75 K (3)
2 a
𝑈𝑈 (𝑇𝑇 −𝑇𝑇) = 2  850  W/m  for  𝑇𝑇 > 75 K (4)
2 a
U shall not be lower than the values proposed but manufacturers can use greater values if their design
requires.
4.2.3 Vacuum-insulated vessels in the case of loss of vacuum and non-vacuum insulated vessels
W is the quantity of heat transferred per unit time (in watts) by heat leak through the insulating material. It
is calculated using Formula (5)Formula (5)::
𝑊𝑊 = (𝑈𝑈 ⋅𝐴𝐴)(𝑇𝑇 −𝑇𝑇) (5)
3 3 a
where If the insulation is fully effective for conduction, convection, and radiation heat transfer at 328 K, U
may be calculated using Formula (6).
(6)
where
U is the overall heat transfer coefficient of the insulating material when saturated with
gaseous lading or air at atmospheric pressure, whichever is greater, in W/(m ·K);
k is the mean thermal conductivity of the insulating material saturated with gaseous lading
or air at atmospheric pressure, whichever provides the greater coefficient, between
T and T , in W/(m·K). Values of k for gases are listed in Table 1;
a 3
e is the minimum insulating material thickness taking into account the manufacturing
tolerances or effects of sudden loss of vacuum, in m;
T is the relieving temperature, inner vessel;
T is the maximum external ambient temperature, non-fire condition;
a
A is the arithmetic mean of inner and outer surface areas of vessel insulating material.
If the insulation is fully effective for conduction, convection, and radiation heat transfer at 328 K, U may be
calculated using Formula (6):
𝑘𝑘
𝑈𝑈 = (6)
𝑒𝑒
where
k is the mean thermal conductivity of the insulating material saturated with gaseous lading
or air at atmospheric pressure, whichever provides the greater coefficient, between
T and T , in W/(m·K) (values of k for gases are listed in Table 2);
a 3
e is the minimum insulating material thickness taking into account the manufacturing
tolerances or effects of sudden loss of vacuum, in m.
NOTE This formula might not be applicable at temperatures below 75 K with a small thickness of insulating material
as the maximum heat transfer coefficient would be given by air condensation.
Vacuum space, gas space, or space occupied by the deteriorated insulation shall not be included in the
thickness of the insulation. The effectiveness of these spaces or deteriorated insulation in reducing
conduction, convection, or radiation heat transfer may be evaluated separately and included in the overall
heat transfer coefficient, U , using methods found in published heat transfer literature. Deterioration of the
insulation can be caused by the following:
— — moisture condensation;
ISO/DISFDIS 21013-3:2026(en)
— — air condensation;
— — increase in the density of the insulation due to a sudden loss of vacuum.
Table 2 — Thermal conductivity for refrigerated (cryogenic) fluids at the mean temperature
between saturation and 328 K (k ) and 922 K (k ) at 1 bar
3 5
k k
3 5
Fluid
[W/(m·K)] [W/(m·K)]
Air 0,019 0,043
Argon 0,013 0,027
Carbon dioxide 0,017 0,039
Carbon monoxide 0,020 0,039
Helium 0,104 0,211
Hydrogen 0,116 0,217
Methane 0,024 0,074
Neon 0,034 0,067
Nitrogen 0,019 0,040
Oxygen 0,019 0,043
Krypton 0,007 0,015
Xenon 0,005 0,009
Ethane 0,016 0,064
Trifluoromethane 0,012 0,027
Ethylene (ethene) 0,015 0,056
Nitrous oxide 0,014 0,038
Remark: Values are for reference only. Use fluid properties given in current databases, such as from NIST.
4.2.4 Supports and piping
W is the quantity of heat transferred per unit time (in watts) by supports and piping located in the interspace,
given by Formula (7).:
(7)
𝑊𝑊 = (𝑤𝑤 +𝑤𝑤 +𝑤𝑤 +⋯ +𝑤𝑤 )(𝑇𝑇 −𝑇𝑇) (7)
4 1 2 3 n a
where w is the heat leak per degree K contributed by one of the supports or the pipes, in W/K, given by
n
Formula (8).:
𝐴𝐴
n
𝑤𝑤 =𝑘𝑘 ( ) (8)
n n
𝑙𝑙
n
where
k is the mean thermal conductivity of the support or pipe material between T and T , in
n a
W/(m⋅K);
A is the support or pipe section area, in m ;
n
l is the support or pipe length in the vacuum interspace, in m.
n
4.3 Under fire conditions
4.3.1 Insulation system remains fully or partially in place during fire conditions
W is the quantity of heat transferred per unit time (in watts) by heat leak through the vessel walls, given by
Formula (9).:
(9)
0,82
𝑊𝑊 = 2,6⋅ (922−𝑇𝑇)⋅𝑈𝑈 ⋅𝐴𝐴 (9)
5 5
where if the insulation is fully effective for conduction, convection, and radiation heat transfer for an external
temperature of 922 K, U may be calculated using Formula (10)Formula (10).:
𝑘𝑘
𝑈𝑈 = (10)
𝑒𝑒
where
U is the overall heat transfer coefficient of the container-insulating material when saturated
with gaseous lading or air at atmospheric pressure, whichever is greater, in W/(m ·K);
k is the mean thermal conductivity of the insulating material saturated with gaseous lading
or air at atmospheric pressure, whichever provides the greater coefficient, between T and
922 K, in W/(m·K). Values) (values of k for gases are listed in Table 2Table 2;);
e is the thickness of the insulating material remaining in place during fire conditions, in m;
A is the arithmetic mean of the inner and outer surface areas of the insulating material
remaining in place during fire conditions, in m ;
T is the relieving temperature, inner vessel.
Vacuum space, gas space, or space occupied by the deteriorated insulation shall not be included in the
thickness of the insulation. The effectiveness of these spaces or deteriorated insulation in reducing
conduction, convection, or radiation heat transfer may be evaluated separately and included in the overall
heat transfer coefficient, U , using methods found in published heat transfer literature. Deterioration of the
insulation can be caused by the following:
— — moisture condensation;
— — air condensation;
— — increase in the density of the insulation due to a sudden loss of vacuum;
— — degradation due to heat.
If the outer jacket remains in place during fire conditions, but the insulating material is entirely destroyed, U
is equal to the overall heat transfer coefficient with gaseous lading or air at atmospheric pressure in the space
between the outer jacket and the inner vessel, whichever provides the greater coefficient, between T and
922 K. A is equal to the mean surface area of the interspace.
ISO/DISFDIS 21013-3:2026(en)
4.3.2 Insulation system does not remain in place during fire conditions
W is the quantity of heat transferred per unit time (in watts) by heat leak through the vessel walls, calculated
using Formula (11)Formula (11).:
4 0,82
𝑊𝑊 = 7,1⋅ 10 ⋅𝐴𝐴 (11)
6 i
where A is the total outside surface area of the inner vessel, in m .
i
The heat transferred by supports and piping located in the interspace can be neglected in this case.
4.4 Air or nitrogen condensation
4.4.1 General
The air or nitrogen condensation case for the loss of vacuum condition shall be considered for fluids with a
saturation temperature below 75 K at 1 bar.
Air condensation, for the case of loss of vacuum to the atmosphere on a vacuum insulated container, is highly
dependent on the type of insulation and how the insulation is designed. Since air condensation occurs
primarily below 75 K, and fluids with saturation temperatures below 75 K are generally stored and
transported in containers insulated with multi-layer insulation, this document covers air condensation on
multi-layer insulated containers only. In the absence of pertinent reliable data on perlite insulated vessels,
4.2.3, 4.3.14.2.3, 4.3.1,, and 4.3.24.3.2 shall be used with thermal conductivity values shown in Table 2Table 2
increased by a factor of two in Formula (6)Formula (6) for k and Formula (10)Formula (10) for k ,
3 5
respectively.
Condensation of air on a multi-layer insulated surface below 75 K will depend on the rate of air access to the
insulated surface. On a multi-layer insulated container, the air condensation rate can vary depending on the
number of layers and the air access allowed by the design of the insulation.
Figure 1Figure 1 provides heat transfer rates from air condensation to the stored fluid as a function of the
number of layers of insulation. The fire engulfment curve is an extrapolation for 922 K ambient temperature.
Unless heat transfer rates under the loss of vacuum condition from air condensation can be determined for
the same type and design of multi-layer insulation from prototype tests or actual incidents, heat transfer rates
from Figure 1Figure 1 shall be used.
4.4.2 Loss of vacuum with air and nitrogen
W is the quantity of heat transferred per unit time (in watts) through air or nitrogen condensation for
3a
vacuum-insulated vessels in the case of a loss of vacuum with air or nitrogen, calculated using
Formula (12)Formula (12).
𝑊𝑊 =𝑈𝑈 ⋅𝐴𝐴 (12)
3a 3a i
where 𝑈𝑈 is the heat transfer through air or nitrogen condensation, in watts per square metre of the inner
3a
vessel outer surface area, from Figure 1Figure 1.
4.4.3 Fire with loss of vacuum with air or nitrogen
W is the quantity of heat transferred per unit time (in watts) through air or nitrogen condensation for
5a
vacuum-insulated vessels in the case of a fire and loss of vacuum with air or nitrogen, calculated using
Formula (13)Formula (13).
0,82
𝑊𝑊 = 1,95⋅𝑈𝑈 ⋅𝐴𝐴 (13)
5a 5a i
where 𝑈𝑈 is the heat transfer through air or nitrogen condensation during fire conditions, in watts per square
5a
metre of the inner vessel surface area, from Figure 1Figure 1.

Key
X number of insulation layers
Y heat transfer [W/m ]
0,73
38  400 + 420⋅𝑋𝑋
𝑈𝑈 =𝑌𝑌 =
3𝑎𝑎
0,73
0,96 +𝑋𝑋
ISO/DISFDIS 21013-3:2026(en)
0,73
92  160 + 1  000⋅𝑋𝑋
𝑈𝑈 =𝑌𝑌 =
5𝑎𝑎
0,73
0,96 +𝑋𝑋
Figure 1 — Heat transfer rate for air or nitrogen condensation
4.5 Heat transfer per unit time (watts)
4.5.1 General
The required mass flow rate, Q , to be relieved can be calculated from the heat transfer, W , for the
m T
followingconditions specified in 4.5.2 to 4.5.6conditions,, where W is the total heat transfer applicable to the
T
specified condition.
4.5.2 Normal operation
Formula (14) applies:
𝑊𝑊 =𝑊𝑊 +𝑊𝑊 (14)
𝑇𝑇1 1 4
Alternatively, the heat transfer rate, W , can be determined from the normal evaporation rate (NER) using
T1NER
Formula (15)Formula (15).
(15)
𝑣𝑣
𝑄𝑄 ⋅𝐿𝐿
ga
mNER a
𝑊𝑊 = ( )( ) (15)
T1NER
3,6 𝑣𝑣 −𝑣𝑣
ga la
where
Q is the mass flow rate due to the normal evaporation rate, in kg/h;
mNER
L is the latent heat of vaporization of the cryogenic liquid at a pressure of 1,013 bar, in kJ/kg;
a
ν is the specific volume of saturated gas at a saturation pressure of 1,013 bar, in m /kg;
ga
ν is the specific volume of saturated liquid at a saturation pressure of 1,013 bar, in m /kg;
la
W is the total heat leak calculated from the experimentally determined normal evaporation
T1NER
rate, in watts.
4.5.3 Pressure build up regulator fully open
Formula (16) and Formula (17) apply:
𝑊𝑊 =𝑊𝑊 +𝑊𝑊 (16)
T2 T1 2
Or for the NER method:
𝑊𝑊 =𝑊𝑊 +𝑊𝑊 (17)
T2NER T1NER 2
4.5.4 Loss of vacuum condition
The heat transfer rate is to be the larger of W or W , calculated using Formula (18) and Formula (19).:
T3 T3a
𝑊𝑊 =𝑊𝑊 +𝑊𝑊 (18)
T3 3 4
𝑊𝑊 =𝑊𝑊 +𝑊𝑊 (19)
T3a 3a 4
where
W is the total heat transfer rate if the saturation temperature of the fluid is greater than or equal
T3
to 75 K at 1 bar;
W is the total heat transfer rate if the saturation temperature of the fluid is less than 75 K at 1 bar.
T3a
where
W is the total heat transfer rate if the saturation temperature of the fluid is greater than or equal
T3
to 75 K at 1 bar;
W is the total heat transfer rate if the saturation temperature of the fluid is less than 75 K at
T3a
1 bar.
Alternatively, W may be calculated using Formula (20)Formula (20):
T3
𝑊𝑊 =𝑊𝑊 +𝑊𝑊 (20)
𝑇𝑇3 T1 3
or approximately, using Formula (21)Formula (21):
𝑊𝑊 =𝑊𝑊 +𝑊𝑊 (21)
T3 T1NER 3
W may be calculated using Formula (22)Formula (22):
T3a
𝑊𝑊 =𝑊𝑊 +𝑊𝑊 (22)
T3a T1 3a
or approximately, using Formula (23)Formula (23):
𝑊𝑊 =𝑊𝑊 +𝑊𝑊 (23)
T3a T1NER 3a
4.5.5 Fire condition with loss of vacuum, vacuum jacket, and insulation fully or partially in place
The heat transfer rate is toshall be the larger of W or W ., where
T5 T5a
where
W is equal to the heat transfer rate, W , if the saturation temperature of the fluid is greater than or
T5 5
equal to 75 K at 1 bar;
W is equal to the heat transfer rate, W , if the saturation temperature of the fluid is less than 75 K at
T5a 5a
1 bar.
4.5.6 Fire condition with loss of vacuum, insulation not in place
The heat transfer rate shall be the larger of W = W or W = W , where W is calculated with U for the
T6 6 T5 5a 5a 5a
bare surface condition from Figure 1Figure 1.
4.5.7 Total heat transfer rate
W is the total heat transfer rate and is equal to W , W , W , W , W , W , W , W , or W , as
T T1 T1NER T2 T2NER T3 T3a T5 T5a T6
applicable.
It is possible that the fire case is not the worst case, so fire and PBU conditions shall both be checked. If PBU is
changed later in the field, the sizing and capacity of pressure relief devices shall be verified according to new
PBU capacity.
ISO/DISFDIS 21013-3:2026(en)
5 Calculation of the mass flow to be relieved by pressure relief devices
5.1 Relieving pressure, P, less than the critical pressure
For a relieving pressure, P, less than the critical pressure, the discharge mass flow rate Q (in kg/h) is
m
calculated on the basis of either the heat input, W , or the normal evaporation rate, Q (in kg/h), using
T mNER
Formula (24)Formula (24).
For heat input W of the vapor discharge:
T
(24)
where
𝑣𝑣 −𝑣𝑣
𝑊𝑊
g l
T
𝑄𝑄 = 3,6( )( ) (24)
m
𝑣𝑣 𝐿𝐿
g
where
ν is the specific volume of saturated gas at the relieving pressure, P, in m /kg;
g
ν is the specific volume of saturated liquid at the relieving pressure, P, in m /kg;
l
L is the latent heat of vaporization of the cryogenic liquid at the relieving conditions, in kJ/kg.
ν is the specific volume of saturated gas at the relieving pressure, P, in m /kg;
g
ν is the specific volume of saturated liquid at the relieving pressure, P, in m /kg;
l
L is the latent heat of vaporization of the cryogenic liquid at the relieving conditions, in kJ/kg.
For normal evaporation rate, Q :
mNER
𝑁𝑁⋅𝑚𝑚
max
𝑄𝑄 =𝑄𝑄 = (25)
m mNER
2 400
where
N is the normal evaporation rate (NER), in percent per day;
m is the maximum mass capacity of the vessel, in kg.
max
where
N is the normal evaporation rate (NER), in percent per day;
m is the maximum mass capacity of the vessel, in kg.
max
5.2 Relieving pressure, P, equal to or greater than the critical pressure
For a relieving pressure, P, equal to or greater than the critical pressure, the discharge mass flow rate Q is
m
given by Formula (26)Formula (26).:
𝑊𝑊
T
𝑄𝑄 = 3,6( ) (26)
m
𝐿𝐿′
where L' is defined by Formula (27):
𝜕𝜕ℎ
𝐿𝐿′ =𝑣𝑣[ ] (27)
𝑃𝑃
𝜕𝜕𝑣𝑣
L’ (kJ/kg) is evaluated at the relieving pressure, P, and inner vessel flow exit temperature, T (K). The value of
T and its corresponding value of L’ are found by tabulating values of ψ using thermophysical property tables
and finding the values of T and L’ associated with the maximum value found for ψ, where ψ is given by
Formula (28)Formula (28).
(28)
√𝑣𝑣 √𝑣𝑣
𝜓𝜓 = =
𝜕𝜕ℎ
𝐿𝐿′
𝑣𝑣[ ]
𝑃𝑃
𝜕𝜕𝑣𝑣
(28)
where
where
ν is the specific volume of critical or subcritical fluid (gas-full condition) at the relieving
pressure, P, in the vessel at the temperature of consideration, in m /kg;
h is the enthalpy of the fluid at the same conditions as ν, in kJ/kg.
ν is the specific volume of critical or subcritical fluid (gas-full condition) at the relieving pressure,
P, in the vessel at the temperature of consideration, in m /kg;
h is the enthalpy of the fluid at the same conditions as ν, in kJ/kg.
5.3 Example
Calculate the values of L’ and T to be used for liquid hydrogen relieving at pressure, P = 13,8 bar, as given in
Table 3Table 3.
Table 3 — Determination of relieving temperature, T
𝜕𝜕ℎ
Temperature
νm /kg
𝑣𝑣
√
𝐿𝐿′ =𝑣𝑣[ ]
𝑃𝑃 𝜓𝜓 =
K 𝜕𝜕𝑣𝑣
𝜕𝜕ℎ
𝑣𝑣[ ]
𝑃𝑃
𝜕𝜕𝑣𝑣
K m /kg
33,3 0,027 16 214,09 0,000 769 7
34,7 0,058 30 236,56 0,001 020 6
34,8 0,058 85 237,49 0,001 021 4 max
34,9 0,059 35 238,65 0,001 020 8
38,9 0,085 54 304,53 0,000 960 3
44,4 0,110 97 384,77 0,000 865 7
At P = 13,8 bar, the maximum value of ψ occurs at T = 34,8 K for hydrogen. For this condition,
L’ = 237,49 kJ/kg.
NOTE Values are for reference only. Use fluid properties given in current databases, such as from
NIST.
ISO/DISFDIS 21013-3:2026(en)
6 Piping for pressure relief devices
6.1 Pressure drop
6.1.1 General
If the piping between the outer jacket and safety relief device is longer than 0,6 m, heat transfer to the released
flow shall be taken into account. This heat transfer reduces the product density and consequently reduces the
effective discharge rate of the relief system.
When fittings and piping are used on the upstream or downstream sides of pressure relief devices, the
passages shall be designed such that the flow capacity of the pressure relief system is not reduced below the
capacity required for the container on which the pressure relief system is installed.
6.1.2 Relief valves
Pressure drops associated with the flow resistance of the pipe network shall be considered when sizing relief
valves. In order to avoid resonance (“chatter”) in conventional direct acting relief valves, it is recommended
that the maximum pressure drop across the upstream pipe network at the flow capacity, Q , of the valve
ma
within the given system configuration be less than or equal to 3 % of the relief valve set pressure (bar, gauge).
Additionally, it is recommended that the differential pressure drop between the relief valve outlet and the
system outlet that develops as a result of flow after the pressure relief device opens (“built-up back pressure”)
be limited to 10 % of the valve set pressure (bar, gauge). The sizing and balancing of the upstream and
downstream pipe network designs shall be optimized whenever possible to satisfy these pressure drop
conditions. Furthermore, internally pressure-balanced or externally pilot-operated valves may be used to
m
...