EN ISO 4126-10:2024

SLOVENSKI STANDARD
01-junij-2024
Varnostne naprave za zaščito pred prekomernim tlakom - 10. del: Velikosti
varnostnih ventilov in varnostne membrane za dvofazni pretok plina/tekočine (ISO
4126-10:2024)
Safety devices for protection against excessive pressure - Part 10: Sizing of safety
valves and bursting discs for gas/liquid two-phase flow (ISO 4126-10:2024)
Sicherheitseinrichtungen gegen unzulässigen Überdruck - Teil 10: Auslegung von
Sicherheitsventilen und Berstscheiben bei Zweiphasenströmung (flüssig/gas) (ISO 4126-
10:2024)
Dispositifs de sécurité pour protection contre les pressions excessives - Partie 10:
Dimensionnement des soupapes de sûreté et des disques de rupture pour les débits
diphasiques gaz/liquide (ISO 4126-10:2024)
Ta slovenski standard je istoveten z: EN ISO 4126-10:2024
ICS:
13.240 Varstvo pred previsokim Protection against excessive
tlakom pressure
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 4126-10
EUROPEAN STANDARD
NORME EUROPÉENNE
April 2024
EUROPÄISCHE NORM
ICS 13.240
English Version
Safety devices for protection against excessive pressure -
Part 10: Sizing of safety valves and bursting discs for
gas/liquid two-phase flow (ISO 4126-10:2024)
Dispositifs de sécurité pour protection contre les Sicherheitseinrichtungen gegen unzulässigen
pressions excessives - Partie 10: Dimensionnement des Überdruck - Teil 10: Auslegung von Sicherheitsventilen
soupapes de sûreté et des disques de rupture pour les und Berstscheiben bei Zweiphasenströmung
débits diphasiques gaz/liquide (ISO 4126-10:2024) (flüssig/gas) (ISO 4126-10:2024)
This European Standard was approved by CEN on 2 October 2023.

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 4126-10:2024 E
worldwide for CEN national Members.

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

European foreword
This document (EN ISO 4126-10:2024) has been prepared by Technical Committee ISO/TC 185 "Safety
devices for protection against excessive pressure" in collaboration with Technical Committee CEN/TC
69 “Industrial valves” the secretariat of which is held by AFNOR.
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 October 2024, and conflicting national standards shall
be withdrawn at the latest by October 2024.
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 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 4126-10:2024 has been approved by CEN as EN ISO 4126-10:2024 without any
modification.
Annex ZA
(informative)
Relationship between this European Standard and the essential safety
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 safety requirements of Directive 2014/68/EU
on the harmonisation of the laws of the Member States relating to the making available on the market of
pressure equipment.
Once this standard is cited in the Official Journal of the European Union under that Directive
2014/68/EU, 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
safety requirements of that Directive 2014/68/EU, and associated EFTA regulations.
Table ZA.1 — Correspondence between this European Standard and
Annex I of Directive 2014/68/EU
Essential Safety Requirements Clause(s)/sub-clause(s) Remarks/Notes
of Directive 2014/68/EU of this EN
2.11.2 6.3.3, Pressure limiting devices
6.5.2,
6.5.3,
6.5.4 (except last paragraph),
6.5.6,
6.7 (paragraph 1)
Table ZA.2 — Applicable Standards to confer presumption of conformity as described in this
Annex ZA
Column 1 Column 2 International Column 3 Column 4
Reference in Standard Edition
Title Corresponding European
Clause 2
Standard Edition
ISO 4126-7 ISO 4126-7:2013 Safety devices for EN ISO 4126-7:2013
protection against
ISO 4126-7:2013/Amd 1:2016 EN ISO 4126-7:2013/A1:2016
excessive pressure -
Part 7: Common data
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 International 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 4126-10
Second edition
Safety devices for protection against
2024-02
excessive pressure —
Part 10:
Sizing of safety valves and bursting
discs for gas/liquid two-phase flow
Dispositifs de sécurité pour protection contre les pressions
excessives —
Partie 10: Dimensionnement des soupapes de sûreté et des
disques de rupture pour les débits diphasiques gaz/liquide
Reference number
ISO 4126-10:2024(en) © ISO 2024

ISO 4126-10:2024(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: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
ISO 4126-10:2024(en)
Contents Page
Foreword .v
Introduction .vi
1 S c op e . 1
2 Nor m at i ve r ef er enc e s . 1
3 T erms and definitions . 1
3.1 G eneral .1
3.2 Pressure .2
3 . 3 F low r at e .4
3 .4 F low a r e a .5
3.5 F luid state .5
3.6 T emperature .5
4 S ymbols and abbreviated terms and figures . 6
4.1 Symbols .6
4.2 A bbreviated terms .8
4.3 Figures .9
5 Application range of the method .11
5.1 G eneral .11
5.2 L imitations of the method for calculating the two-phase mass flux in safety devices .11
5.2.1 Flashing flow.11
5.2.2 Condensing flow. 12
5.2.3 Flashing flow for multi-component liquids . 12
5 . 2.4 D i s s ol ve d gas e s . 12
5.2.5 Compressibility coefficient ω . 13
5.3 L imitations of the method for calculating the mass flow rate required to be discharged . 13
5.3.1 Rate of temperature and pressure increase . 13
5.3.2 Immiscible liquids . 13
6 S i z i n g s t ep s . .13
6.1 G eneral outline of sizing steps . . 13
6.2 S tep 1 — Identification of the sizing case .14
6.3 S tep 2 — Flow regime at the inlet of the vent line system . 15
6.3.1 G eneral . 15
6.3.2 Phenomenon of level swell . 15
6.3.3 Influence of liquid viscosity and foaming behaviour on the flow regime . 15
6.3.4 Prediction of the flow regime (gas/vapour or two-phase flow) .17
6.4 S tep 3 — Calculation of the mass flow rate required to be discharged . 20
6.4.1 G eneral . 20
6.4.2 Pressure increase caused by an excess in-flow . 20
6.4.3 Pressure increase due to external heating . 22
6.4.4 Pressure increase due to thermal runaway reactions . 25
6.5 S tep 4 — Calculation of the dischargeable mass flux through and pressure change in
the vent line system . 29
6.5.1 General . 29
6.5.2  Two-phase flow discharge coefficient, K .32
dr,2ph
6.5.3 Dimensionless mass flow rate, C . 33
6.5.4 Compressibility coefficient, ω (numerical method) . 34
6.5.5 Calculation of the downstream stagnation condition. 35
6.5.6 Slip correction for non-flashing two-phase flow . 35
6.5.7 Slip correction for two-phase flow in straight pipes . 36
6.6 S tep 5 — Ensure proper operation of safety valve vent line systems under plant
conditions . 36
6.7 S imultaneous calculation of the dischargeable mass flux and pressure change in the
vent line system . 36
6.8 Summary of calculation procedure .37

iii
ISO 4126-10:2024(en)
Annex A (informative) Identification of sizing scenarios .44
Annex B (informative) Example calculation of the mass flow rate to be discharged .46
Annex C (informative) Example of calculation of the dischargeable mass flux and pressure
change through connected vent line systems .50
Annex D (informative) Environmental factor. 67
Bibliography .68

iv
ISO 4126-10:2024(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 185, Safety devices for protection against
excessive pressure, in collaboration with the European Committee for Standardization (CEN) Technical
Committee CEN/TC 69, Industrial valves, in accordance with the Agreement on technical cooperation
between ISO and CEN (Vienna Agreement).
This second edition cancels and replaces the first edition (ISO 4126-10:2010), which has been technically
revised.
The main changes are as follows:
— opening of the method for sizing of bursting discs;
— more thorough iteration for the calculation of the flow rate;
— allowing for slip;
— allowing for velocity in the outlet line and pressure losses in front and after the safety device;
— added an example for flow rate to be discharged (Annex B);
— added an example for dischargeable mass flow rate added and method to estimate pressure drop in pipe
flow (Annex C);
— various correction.
A list of all parts in the ISO 4126 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
ISO 4126-10:2024(en)
Introduction
Well-established recommendations exist for the sizing of safety valves and bursting discs and the connected
inlet and outlet lines for steady-state, single-phase gas/vapour or liquid flow. However, in the case of a two-
phase vapour/liquid flow, the required relieving area to protect a system from overpressure is larger than
that required for single-phase flow when the same vessel condition and heat release are considered. The
requirement for a larger relief area results from the fact that, in two-phase flow, the liquid partially blocks
the relieving area for the vapour flow, by which most of the energy is removed by evaporation from the
vessel.
This document includes a widely applicable method for the sizing of the most typical safety valves and
bursting discs in fluid services encountered in various industrial fields (see Table 1). It is based on the
omega parameter method, which is extended by a thermodynamic non-equilibrium parameter. A balance is
attempted between the accuracy of the method and the unavoidable uncertainties in the input and property
data under the actual sizing conditions.
In case of two-phase flow, the safety device size can influence the fluid state and, hence, the mass flow
rate to be discharged. Furthermore, the two-phase mass flow rate through a safety device essentially
depends on the mass flow quality (mass fraction of vapour) of the fluid at the inlet of the device. Because
these parameters are, in most cases, not readily at hand during the design procedure of a relief device,
this document also includes a comprehensive procedure that covers the determination of the fluid-phase
composition at the safety device inlet. This fluid-phase composition depends on a scenario that leads to the
pressure increase. Therefore, the recommended sizing procedure starts with the definition of the sizing case
and includes a method for the prediction of the mass flow rate required to be discharged and the resulting
mass flow quality at the inlet of the safety device.
The formulae of ISO 4126-7:2013/Amd 1:2016 for single-phase flow up to the narrowest flow cross-section
are included in this document, modified to SI units, to calculate the flow rates at the limiting conditions of
single-phase gas and liquid flow.
In this document, the unit bar for pressures is being used 100 000 Pa = 1 bar.
Table 1 — Possible fluid state at the inlet of the safety valve or bursting disc that can result in two-
phase flow
Fluid state at
Cases Examples
device inlet
liquid subcooled (possibly flashing in the safety device) cold water
saturated boiling water
with dissolved gas CO /water
gas/vapour near saturated vapour (possibly condensing in the safety device) steam
gas/liquid vapour/liquid steam/water
non-evaporating liquid and non-condensable gas (constant quality) air/water
gas/liquid mixture, when gas is desorbed or produced

vi
International Standard ISO 4126-10:2024(en)
Safety devices for protection against excessive pressure —
Part 10:
Sizing of safety valves and bursting discs for gas/liquid two-
phase flow
1 S cope
This document specifies the sizing of safety valves and bursting discs for gas/liquid two-phase flow
in pressurized systems such as reactors, storage tanks, columns, heat exchangers, piping systems or
transportation tanks/containers, see Figure 2. The possible fluid states at the safety device inlet that can
result in two-phase flow are given in Table 1.
NOTE The pressures used in this document are absolute pressures, not gauge pressures.
2 Normat ive 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 4126-7:2013/Amd 1:2016, Safety devices for protection against excessive pressure — Part 7: Common data
3 T erms and definitions
For the purposes of this document, the terms and definitions given in ISO 4126-7:2013/Amd 1:2016 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 General
3.1.1
pressurized system
equipment being protected against excessive pressure accumulation by a safety device
EXAMPLE Equipment can be reactors, storage tanks, columns, heat exchangers, piping systems and transport
tanks/containers, etc.
3.1.2
critical filling threshold
ϕ
limit
maximum initial liquid filling threshold (liquid hold-up) in the pressurized system (3.1.1) at sizing conditions,
up to where vapour disengagement occurs and single-phase gas or vapour flow can be expected
Note 1 to entry: The critical filling threshold is expressed as a ratio of the total volume of the system.
Note 2 to entry: For filling levels above the critical filling threshold, two-phase flow is assumed to occur.

ISO 4126-10:2024(en)
3.1.3
initial liquid filling level
ϕ
liquid hold-up in the pressurized system (3.1.1) at the sizing conditions
Note 1 to entry: The initial liquid filling level is expressed as a ratio of the total volume of the system.
3.1.4
inlet line
piping and associated fittings connecting the pressurized system (3.1.1) to the safety device inlet
3.1.5
outlet line
piping and associated fittings connecting the safety device outlet to a containment system or the atmosphere
3.1.6
vent line system
combination of safety device, inlet line (3.1.4) and outlet line (3.1.5)
3.1.7
cryogenic vessel
vacuum jacketed vessel intended for application at low temperature involving liquefied gases
3.2 Pressure
3.2.1
maximum allowable working pressure
p
MAW
maximum pressure permissible at the top of a pressurized system (3.1.1) in its operating position for
designated temperature
3.2.2
maximum allowable accumulated pressure
p
MAA
sum of the maximum allowable working pressure (3.2.1) and the maximum allowable accumulation (3.2.3)
Note 1 to entry: The maximum allowable accumulation is established by applicable code for operating and fire
contingencies.
3.2.3
maximum allowable accumulation
Δp
MAA
pressure increase over the maximum allowable working pressure (3.2.1) of a pressurized system (3.1.1) during
discharge through the safety device
Note 1 to entry: The maximum allowable accumulation is expressed in pressure units or as a percentage of the
maximum allowable working pressure.
3.2.4
opening pressure
p
open
predetermined absolute pressure at which a safety valve under operating conditions at the latest commences
to open
3.2.5
absolute overpressure
Δp
over
pressure increase over the opening pressure (3.2.4), p , of the safety device
open
Note 1 to entry: The maximum absolute overpressure is the same as the maximum accumulation, Δp , when the
MAA
opening pressure of the safety valve is set at the maximum allowable working pressure (3.2.1) of the pressurized system
(3.1.1).
ISO 4126-10:2024(en)
Note 2 to entry: The absolute overpressure is expressed in pressure units or as a percentage of the opening pressure.
3.2.6
overpressure
p
over
maximum pressure in the pressurized system (3.1.1) during relief, i.e. pressure less or equal to the maximum
accumulated pressure
3.2.7
sizing pressure
p
pressure at which all property data, especially the compressibility coefficient, ω, are calculated for sizing
the safety device
Note 1 to entry: In the case of tempered and hybrid reactive systems, the sizing pressure shall be as low as reasonable
possible, but should not affect the normal operation. In the case of non-reactive and gassy systems (3.5.3), the designer
may choose a higher value for the sizing pressure, but it shall not exceed the maximum allowable accumulated pressure
(3.2.2).
3.2.8
critical pressure
p
crit
fluid-dynamic critical pressure occurring in the narrowest flow cross-section of the safety valve and/or at
an area enlargement in the outlet line (3.1.5)
Note 1 to entry: At this pressure, the mass flow rate approaches a maximum at a given sizing condition in the
pressurized system (3.1.1). Any further decrease of the downstream pressure does not increase the flow rate further.
Usually, the critical pressure occurs in the safety valve, either in the valve seat, inlet nozzle and/or valve body. In the
bursting disc, critical pressure can occur downstream of the device at a minimum flow area, at the exit of the vessel or
a change in pipe diameter. In long safety device outlet lines, multiple critical pressures can also occur.
3.2.9
stagnation condition
condition when fluid is at rest
EXAMPLE Fluid in large vessels, where the flow velocity is almost zero, even in case of a discharge of mass.
3.2.10
critical pressure ratio
η
crit
ratio of critical pressure (3.2.8) to the sizing pressure (3.2.7)
3.2.11
thermodynamic critical pressure
p
c
state property, together with thermodynamic critical temperature (3.6.1), at the thermodynamic critical
point
3.2.12
back pressure
p
b
pressure that exists at the outlet of a safety device as a result of pressure in the discharge system
Note 1 to entry: Back pressure can be either constant or variable; it is the sum of superimposed and built-up back
pressure (3.2.13).
3.2.13
built-up back pressure
pressure existing at the outlet of the safety device caused by flow through the valve or bursting disc and
discharge system
ISO 4126-10:2024(en)
3.2.14
inlet pressure loss
Δp
loss
irrecoverable pressure decrease due to flow in the piping from the equipment that is protected to the inlet
of the safety device
3.2.15
blowdown
Δp
BD
difference between opening pressure (3.2.4) and reseating pressure of a safety valve
Note 1 to entry: Blowdown is normally stated as a percentage of the opening pressure.
3.2.16
dimensionless reduced pressure
p
red
local pressure divided by the thermodynamic critical pressure (3.2.11) of the substance
3.3 Flow rate
3.3.1
mass flow rate required to be discharged from a pressurized system
Q
m,out
mass flow rate required to avoid that the pressure exceeds the maximum allowable accumulated pressure
(3.2.2) in the pressurized system (3.1.1) during relief
3.3.2
feed mass flow rate into the pressurized system
Q
m,feed
maximum mass flow rate through a feed line or control valve fed into the pressurized system (3.1.1) being
protected
3.3.3
dischargeable mass flux through the safety device

m
SD
mass flow rate per area through a safety device at the sizing conditions calculated by means of the certified
discharge coefficients for gas and liquid flow
Note 1 to entry: See Formula (48).
3.3.4
certified valve discharge coefficient for single-phase gas/vapour respectively liquid flow
K 〈gas〉
dr,g
K 〈liquid〉
dr,l
correction factor defined by the ratio of the theoretically dischargeable mass flux through the safety device
(3.3.3) to an experimentally determined mass flux through a device of the same manufacturer's type
Note 1 to entry: The discharge coefficient of a safety valve is related to the valve seat cross-section and accounts
for the imperfection of flow through the device compared to that through a reference model (ideal nozzle). Certified
values for gas and liquid flow, K , are usually supplied by valve manufacturers or determined by experiment. Rated
d
discharge coefficients K , equal to 0,9 K , are used to calculate the safety valve sizing area.
dr d
Note 2 to entry: The discharge coefficient of a bursting disc is related to the disc cross-section and accounts for the
imperfection of flow through the device compared to that through a reference model.

ISO 4126-10:2024(en)
3.4 Flow area
3.4.1
safety device sizing area
A
most essential result of the sizing procedure in accordance with this document required to select an
adequately sized safety device and defined as the minimum cross-section of flow area
Note 1 to entry: It is important that the dischargeable mass flux through the safety device (3.3.3) be related to this
specific area.
3.4.2
effective flow area of the feed line or the control valve
A
feed
discharge flow area of a feed line or control valve in the line to the pressurized system (3.1.1)
3.5 Fluid state
3.5.1
gas/liquid mixture
fluid mixture composed of both a liquid part and a gas part, in which the gas is not necessarily of the same
chemical composition as the liquid
3.5.2
tempered system
fluid system in which some energy is removed from the liquid phase by evaporation or flashing
3.5.3
gassy system
fluid system in which permanent gas is generated (e.g. by chemical reaction or by evolution from solution)
and in which no significant amount of energy is removed from the liquid by evaporation at the sizing
conditions
3.5.4
hybrid system
fluid system that exhibits characteristics of both tempered and gassy systems (3.5.3) to a significant extent
at the sizing conditions
3.5.5
thermal runaway reaction
uncontrolled or undesired exothermic chemical reaction
3.6 Temperature
3.6.1
thermodynamic critical temperature
T
c
state property, together with thermodynamic critical pressure (3.2.11), at the thermodynamic critical point
3.6.2
sizing temperature
T
temperature of the pressurized system (3.1.1) at the sizing conditions
3.6.3
overtemperature
T
over
maximum temperature in the pressurized system (3.1.1) during relief

ISO 4126-10:2024(en)
3.6.4
saturation temperature difference
ΔT
over
difference between the saturation temperature at the maximum pressure during relief, p , and the
over
saturation temperature at the sizing pressure (3.2.7), p
3.6.5
dimensionless reduced temperature
T
red
local temperature divided by the thermodynamic critical temperature (3.6.1) of the substance
4 S ymbols and abbreviated terms and figures
4.1 Symbols
Variable Definition Unit
A effective flow area of the feed line or the control valve m
feed
The wetted surface area to be considered for the heat transfer due to fire. In de-
tail, it is the partial surface area of a vertical cylindrical vessel wetted by internal
A liquid and located within 7,5 m vertically from ground or from any surface capa- m
fire
ble of sustaining a pool fire. Depending on the fire case considered it may either
include the wall of the bottom or the bottom wall of the vessel is not included.
A area of heat exchange in the pressurized system in case of external heat input m
heat
minimum required safety device area (safety device sizing area). In general, for
A safety valves it is the safety valve seat area and for bursting discs the minimum m
net flow area.
A cross-sectional area in a vertical cylindrical vessel m
R
B (maximum) overall heat transfer coefficient, see Formula (24) W/(m ·K)
heat
C dimensionless mass flow rate —
C flow conversion factor 1
C flow conversion factor 2
c specific heat capacity at constant pressure J/(kg·K)
p
D inner vessel diameter of a vertical cylindrical vessel m
d diameter m
dp
rate of pressure increase in the pressurized system Pa/s
dt
dT
reaction self-heat rate inside the pressurized system K/s
dt
F environmental factor for heat input from fire (see 6.4.3.2) —
g acceleration due to gravity m/s
height of liquid level in a vertical cylindrical vessel (bottom of vessel to liquid
H m
l
level)
H maximum height of flames above ground m
fire
H height of the bottom of the vessel flames above ground m
vessel
k correlating parameter to calculate the characteristic bubble-rise velocity —
∞
K two-phase flow valve discharge coefficient —
dr,2ph
K certified valve discharge coefficient for single-phase gas/vapour flow —
dr,g
K certified valve discharge coefficient for single-phase liquid flow —
dr,l
K velocity head loss for bursting disc —
R
ISO 4126-10:2024(en)
Variable Definition Unit
Liquid discharge factor for fully opened control valve in the feed line, which char-
K acterizes A of the feed line or control valve of a frictionless valve with the same m /h
vs feed
pressure difference for the same flow rate.
Vertical length of a flow restriction to account for potential energy change. For
safety valves and bursting discs L may be set to 0. For inlet and outlet lines the m
L
heights of the system shall be considered.
 mass flux kg/(m ·s)
m

m dischargeable mass flux through the safety device kg/(m ·s)
SD
M total liquid mass in the pressurized system at the sizing conditions kg
M molecular mass kg/kmol
N boiling delay factor accounting for thermodynamic non-equilibrium —
p pressure in the pressurized system Pa
p back pressure Pa
b
p thermodynamic critical pressure Pa
c
p fluid-dynamic critical pressure Pa
crit
p maximum allowable working pressure Pa
MAW
p maximum allowable accumulated pressure Pa
MAA
p sizing pressure Pa
p maximum pressure in a pressurized system during relief, see Figure 1 Pa
over
p opening pressure Pa
open

q dimensionless fire exposure flux —
fire
Q mass flow rate required to be discharged from a pressurized system kg/s
m,out
Q feed mass flow rate into the pressurized system kg/s
m,feed
Q dischargeable mass flow rate through the safety device kg/s
m,SD
heat input into the pressurized system, either by runaway reaction or by external

W
Q
heating
*

ratio of the sensible heat to the latent heat —
Q
acc
*

ratio of total heat input to energy flow removed by evaporation —
Q
in
R universal gas constant (8 314,2 J/(kmol·K)) J/(kmol·K)
R
two-phase multiplier —
2ph
T temperature in the pressurized system K
T thermodynamic critical temperature K
c
T maximum possible temperature of the external heat source K
heat
T temperature of the pressurized system at the sizing conditions K
T maximum temperature in the pressurized system during relief K
over
superficial gas velocity in the free-board gas volume of a vertical cylindrical ves-
u m/s
g,0
sel at the sizing conditions
u characteristic bubble-rise velocity of the gas/vapour in the liquid m/s
∞
u* dimensionless bubble-rise velocity —
v specific volume in the pressurized system m /kg
V volume of the pressurized system m
mass flow quality, i.e. the ratio of the gas mass flow rate to the total mass flow

x —
rate of a two-phase mixture
Z real gas factor —
β ratio of the vent inlet diameter to the throat diameter —

ISO 4126-10:2024(en)
Variable Definition Unit
void fraction in the pressurized system at the sizing conditions for a homogeneous
ε —
two-phase mixture
ε void fraction in the narrowest cross-section, see Formula (50) —
seat
ζ Resistance coefficient of reference, either inlet or outlet —
v,ref
η pressure ratio, either η or η —
crit b
η ratio of the safety valve back pressure to the sizing pressure —
b
η critical pressure ratio —
crit
ratio of the saturation pressure corresponding to the sizing temperature and the
η —
S
sizing pressure (measure of liquid subcooling), see Formula (64)
ϴ Angle of a vent line to the horizontal °
κ isentropic coefficient —
ρ fluid density kg/m
density of water during experiments to measure the K value at a temperature of
vs 3
ρ kg/m
H2O
5 °C
σ surface tension N/m
ϕ critical filling th
...