oSIST prEN ISO 4126-10:2021

Safety devices for protection against excessive pressure - Part 10: Sizing of safety valves and bursting discs for gas/liquid two-phase flow (ISO/DIS 4126-10:2021)

Sicherheitseinrichtungen gegen unzulässigen Überdruck - Teil 10: Auslegung von Sicherheitsventilen und Berstscheiben bei Zweiphasenströmung (flüssig/gas) (ISO/DIS 4126 10:2021)

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/DIS 4126-10:2021)

Varnostne naprave za zaščito pred prekomernim tlakom - 10. del: Velikosti varnostnih ventilov in varnostne membrane za dvofazni pretok plina/tekočine (ISO/DIS 4126-10:2021)

General Information

• Status: Not Published
• Public Enquiry End Date: 01-Sep-2021
• ICS: 13.240 - Protection against excessive pressure
• Technical Committee: IHPV - Hydraulics and pneumatics
• Current Stage: 4020 - Public enquire (PE) (Adopted Project)
• Start Date: 16-Jun-2021
• Due Date: 03-Nov-2021
• Directive: 2014/68/EU - Directive 2014/68/EU of the European Parliament and of the Council of 15 May 2014 on the harmonisation of the laws of the Member States relating to the making available on the market of pressure equipment
• Mandate: M/071 - Mandate to CEN for standardization in the field of Pressure equipment
• Ref Project: prEN ISO 4126-10 - Safety devices for protection against excessive pressure - Part 10: Sizing of safety valves and bursting discs for gas/liquid two-phase flow (ISO/DIS 4126-10:2021)

Standards Content (sample)

SLOVENSKI STANDARD
oSIST prEN ISO 4126-10:2021
01-september-2021
Varnostne naprave za zaščito pred prekomernim tlakom - 10. del: Velikosti
varnostnih ventilov in varnostne membrane za dvofazni pretok plina/tekočine
(ISO/DIS 4126-10:2021)

Safety devices for protection against excessive pressure - Part 10: Sizing of safety

valves and bursting discs for gas/liquid two-phase flow (ISO/DIS 4126-10:2021)
Sicherheitseinrichtungen gegen unzulässigen Überdruck - Teil 10: Auslegung von

Sicherheitsventilen und Berstscheiben bei Zweiphasenströmung (flüssig/gas) (ISO/DIS

4126 10:2021)

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/DIS 4126-10:2021)
Ta slovenski standard je istoveten z: prEN ISO 4126-10
ICS:
13.240 Varstvo pred previsokim Protection against excessive
tlakom pressure
oSIST prEN ISO 4126-10:2021 en,fr,de

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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oSIST prEN ISO 4126-10:2021
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oSIST prEN ISO 4126-10:2021
DRAFT INTERNATIONAL STANDARD
ISO/DIS 4126-10
ISO/TC 185 Secretariat: ANSI
Voting begins on: Voting terminates on:
2021-06-07 2021-08-30
Safety devices for protection against excessive pressure —
Part 10:
Sizing of safety valves and bursting discs for gas/liquid
two-phase flow
ICS: 13.240
THIS DOCUMENT IS A DRAFT CIRCULATED
This document is circulated as received from the committee secretariat.
FOR COMMENT AND APPROVAL. IT IS
THEREFORE SUBJECT TO CHANGE AND MAY
NOT BE REFERRED TO AS AN INTERNATIONAL
STANDARD UNTIL PUBLISHED AS SUCH.
IN ADDITION TO THEIR EVALUATION AS
ISO/CEN PARALLEL PROCESSING
BEING ACCEPTABLE FOR INDUSTRIAL,
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
STANDARDS MAY ON OCCASION HAVE TO
BE CONSIDERED IN THE LIGHT OF THEIR
POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN
Reference number
NATIONAL REGULATIONS.
ISO/DIS 4126-10:2021(E)
RECIPIENTS OF THIS DRAFT ARE INVITED
TO SUBMIT, WITH THEIR COMMENTS,
NOTIFICATION OF ANY RELEVANT PATENT
RIGHTS OF WHICH THEY ARE AWARE AND TO
PROVIDE SUPPORTING DOCUMENTATION. ISO 2021
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oSIST prEN ISO 4126-10:2021
ISO/DIS 4126-10:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

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Published in Switzerland
ii © ISO 2021 – All rights reserved
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oSIST prEN ISO 4126-10:2021
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Contents Page

Foreword ..........................................................................................................................................................................................................................................v

Introduction ................................................................................................................................................................................................................................vi

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

3.1 General ........................................................................................................................................................................................................... 2

3.2 Pressure ........................................................................................................................................................................................................ 2

3.3 Flow rate ....................................................................................................................................................................................................... 6

3.4 Flow area ...................................................................................................................................................................................................... 7

3.5 Fluid state .................................................................................................................................................................................................... 7

3.6 Temperature .............................................................................................................................................................................................. 7

4 Symbols and abbreviated terms ........................................................................................................................................................... 8

5 Application range of the method ......................................................................................................................................................12

5.1 General ........................................................................................................................................................................................................12

5.2 Limitations of the method for calculating the two-phase mass flux in safety devices ...........12

5.2.1 Flashing flow ....................................................................................................................................................................12

5.2.2 Condensing flow ............................................................................................................................................................13

5.2.3 Flashing flow for multi-component liquids ...........................................................................................13

5.2.4 Dissolved gases .................. .................................................... .........................................................................................13

5.2.5 Compressibility coefficient ω ............................................................................................................................. 14

5.3 Limitations of the method for calculating the mass flow rate required to be discharged ..14

5.3.1 Rate of temperature and pressure increase ..........................................................................................14

5.3.2 Immiscible liquids .......................................................................................................................................................14

6 Sizing steps ..............................................................................................................................................................................................................15

6.1 General outline of sizing steps ................................................................................................................................................15

6.2 Step 1 — Identification of the sizing case .....................................................................................................................16

6.3 Step 2 — Flow regime at the inlet of the vent line system .............................................................................16

6.3.1 General...................................................................................................................................................................................16

6.3.2 Phenomenon of level swell ..................................................................................................................................16

6.3.3 Influence of liquid viscosity and foaming behaviour on the flow regime ....................17

6.3.4 Prediction of the flow regime (gas/vapour or two-phase flow) ..........................................19

6.4 Step 3 — Calculation of the mass flow rate required to be discharged ..............................................22

6.4.1 General...................................................................................................................................................................................22

6.4.2 Pressure increase caused by an excess in-flow ..................................................................................22

6.4.3 Pressure increase due to external heating .............................................................................................23

6.4.4 Pressure increase due to thermal runaway reactions ..................................................................27

6.5 Step 4 — Calculation ofthe dischargeable mass flux through and pressure change

in the vent line system ...................................................................................................................................................................31

6.5.1 General...................................................................................................................................................................................31

6.5.2 Two-phase flow discharge coefficient, K .......................................................................................

dr,2ph 34

6.5.3 Dimensionless mass flow rate, C .....................................................................................................................35

6.5.4 Compressibility coefficient, ω (numerical method) .......................................................................35

6.5.5 Calculation of the downstream stagnation condition ...................................................................37

6.5.6 Slip correction for non-flashing two-phase flow ...............................................................................37

6.6 Step 5 — Ensure proper operation of safety valve vent line systems under plant

conditions .................................................................................................................................................................................................38

6.7 Simultaneous calculation of the dischargeable mass flux and pressure change in

the vent line system .........................................................................................................................................................................38

Annex A (informative) Identification of sizing scenarios .............................................................................................................51

Annex B (informative) Example calculation of the mass flow rate to be discharged ......................................53

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oSIST prEN ISO 4126-10:2021
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Annex C (informative) Example of calculation of the dischargeable mass flux and pressure

change through connected vent line systems ......................................................................................................................57

Annex ZA (informative) Relationship between this European Standard and the essential

requirements of Directive 2014/68/EU (Pressure Equipment Directive) aimed to

be covered ......... ........................................................................................................................................................................................................75

Bibliography .............................................................................................................................................................................................................................76

iv © ISO 2021 – All rights reserved
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oSIST prEN ISO 4126-10:2021
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Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/ patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation on 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 the following

URL: www .iso .org/ iso/ foreword .html.

This document was prepared by Technical Committee ISO/TC 185, Safety devices for protection against

excessive pressure.

ISO 4126 consists of the following parts, under the general title Safety devices for protection against

excessive pressure:
— Part 1: Safety valves
— Part 2: Bursting disc safety devices
— Part 3: Safety valves and bursting disc safety devices in combination
— Part 4: Pilot-operated safety valves
— Part 5: Controlled safety pressure-relief systems (CSPRS)

— Part 6: Application, selection and installation of bursting disc safety devices

— Part 7: Common data

— Part 9: Application and installation of safety devices excluding stand-alone bursting disc safety devices

— Part 10: Sizing of safety valves for gas/liquid two-phase flow
In this standard, the unit bar for pressures is being used. 100 000 Pa = 1 bar:
© ISO 2021 – All rights reserved v
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oSIST prEN ISO 4126-10:2021
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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 part of ISO 4126 includes a widely usable engineering tool for the sizing of the most typical safety

valves and bursting discs in fluid services encountered in various industrial fields. 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 part of ISO 4126 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 equations of ISO 4126-7 for single-phase flow up to the narrowest flow cross-section are included

in this part of ISO 4126, modified to SI units, to calculate the flow rates at the limiting conditions of

single-phase gas and liquid flow.
vi © ISO 2021 – All rights reserved
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oSIST prEN ISO 4126-10:2021
DRAFT INTERNATIONAL STANDARD ISO/DIS 4126-10:2021(E)
Safety devices for protection against excessive pressure —
Part 10:
Sizing of safety valves and bursting discs for gas/liquid
two-phase flow
1 Scope

This part of ISO 4126 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. The possible fluid states at the safety device inlet that can result in

two-phase flow are given in Table 1.

NOTE The expression “safety valve” is a synonym for valves as described in ISO 4126-1, ISO 4126-4 and

ISO 4126-5. The expression “bursting disc” is a synonym for bursting disc safety device as described in

ISO 4126-2, ISO 4126-3 and ISO 4126-6.

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
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any amendments) applies.

EN ISO 21013-3:2016, Cryogenic vessels - Pressure-relief accessories for cryogenic service - Part 3: Sizing

and capacity determination

ISO 4126-1, Safety devices for protection against excessive pressure — Part 1: Safety valves

ISO 4126-7, Safety devices for protection against excessive pressure — Part 7: Common data

API Standard 521:2014-01, Pressure-Relieving and Depressuring Systems
3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 4126-7 and the following apply.

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oSIST prEN ISO 4126-10:2021
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ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— IEC Electropedia: available at https:// www .electropedia .org/
— ISO Online browsing platform: available at https:// www .iso .org/ obp
3.1 General
3.1.1
pressurized system

equipment such as reactors, storage tanks, columns, heat exchangers, piping systems and transport

tanks/containers being protected against impermissible pressure accumulation by a safety device

3.1.2
critical filling threshold
limit

maximum initial liquid filling threshold (liquid hold-up) in the pressurized system 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.

3.1.3
initial liquid filling level
liquid hold-up in the pressurized system 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 to the safety device inlet

3.1.5
outlet line

piping and associated fittings connecting the safety valve outlet to a containment system or the

atmosphere
3.1.6
vent line system
combination of safety device, inlet line and outlet line
3.1.7
cryogenic vessel

vacuum jacketed vessel intended for application at low temperature involving liquefied gases

3.2 Pressure

See Figures 1 a) and 1 b) for an illustration of the relationship of the pressures defined in 3.2.

In contrast to the definition used in other parts of this International Standard (e.g. ISO 4126-7) all

pressures are absolute pressures and not gauge pressures.
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oSIST prEN ISO 4126-10:2021
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a) Pressure history of a typical tempered reaction system that is adequately sized

b) Typical pressure history for an externally heated gas vented system
Key

p maximum allowable accumulated pressure p sizing pressure equal to p as shown in

MAA 0 open
Figure 1 a) and equal to p as shown in
over
Figure 1 b)
p maximum allowable working absolute pressure p overpressure
MAW over
p opening pressure Δp maximum allowable accumulation
open MAA
p reseating pressure Δp change in overpressure
reseat over
p operating pressure
operating
1 Δp blowdown
Figure 1 — Relationship of the defined pressures
3.2.1
maximum allowable working absolute pressure
MAW

maximum pressure permissible at the top of a pressurized system in its operating position for

designated temperature
Note 1 to entry: This pressure is an absolute pressure, not a gauge pressure.
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oSIST prEN ISO 4126-10:2021
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3.2.2
maximum allowable accumulated pressure
MAA

sum of the maximum allowable working pressure and the maximum allowable accumulation

Note 1 to entry: The maximum allowable accumulation is established by applicable code for operating and fire

contingencies.
Note 2 to entry: This pressure is an absolute pressure, not a gauge pressure.
3.2.3
maximum allowable accumulation
MAA

pressure increase over the maximum allowable working pressure of a pressurized system 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.
Note 2 to entry: This pressure is an absolute pressure, not a gauge pressure.
3.2.4
opening pressure
open

predetermined absolute pressure at which a safety valve under operating conditions at the latest

commences to open
Note 1 to entry: This pressure is an absolute pressure, not a gauge pressure.
3.2.6
absolute overpressure
over
pressure increase over the opening pressure of the safety device, p
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 of the pressurized system.

Note 2 to entry: The absolute overpressure is expressed in pressure units or as a percentage of the opening

pressure.
Note 3 to entry: This pressure is an absolute pressure, not a gauge pressure.
3.2.7
overpressure
over

maximum pressure in the pressurized system during relief, i.e. pressure less or equal to the maximum

accumulated pressure
Note 1 to entry: This pressure is an absolute pressure, not a gauge pressure.
3.2.8
sizing pressure

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,

the designer may choose a higher value for the sizing pressure, but it shall not exceed the maximum allowable

accumulated pressure.
Note 2 to entry: This pressure is an absolute pressure, not a gauge pressure.
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3.2.9
critical pressure
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

Note 1 to entry: At this pressure, the mass flow rate approaches a maximum at a given sizing condition in the

pressurized system. 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.

Note 2 to entry: This pressure is an absolute pressure, not a gauge pressure.
3.2.10
critical pressure ratio
crit
ratio of critical pressure to the sizing pressure
3.2.11
thermodynamic critical pressure

state property, together with thermodynamic critical temperature, at the thermodynamic critical point

Note 1 to entry: This pressure is an absolute pressure, not a gauge pressure.
3.2.12
back pressure

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.
Note 2 to entry: This pressure is an absolute pressure, not a gauge pressure.
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
Note 1 to entry: This pressure is an absolute pressure, not a gauge pressure.
3.2.14
superimposed back pressure

pressure existing at the outlet of the safety device at the time when the device is required to operate

Note 1 to entry: Superimposed back pressure is the result of pressure in the discharge system from other sources.

Note 2 to entry: This pressure is an absolute pressure, not a gauge pressure.
3.2.15
inlet pressure loss
loss

irrecoverable pressure decrease due to flow in the piping from the equipment that is protected to the

inlet of the safety device
Note 1 to entry: This pressure is an absolute pressure, not a gauge pressure.
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3.2.16
blowdown
difference between opening pressure and reseating pressure of a safety valve

Note 1 to entry: Blowdown is normally stated as a percentage of the opening pressure.

Note 2 to entry: This pressure is an absolute pressure, not a gauge pressure.
3.2.17
dimensionless reduced pressure
red
local pressure divided by the thermodynamic critical pressure of the substance
3.3 Flow rate
3.3.1
mass flow rate required to be discharged from a pressurized system
m,out

mass flow rate required to be discharged from a pressurized system, such that the pressure does not

exceed maximum allowable accumulated pressure in the pressurized system during relief

3.3.2
feed mass flow rate into the pressurized system
m,feed

maximum mass flow rate through a feed line or control valve fed into the pressurized system being

protected
3.3.3
dischargeable mass flux through the safety device

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 Equation (49).
3.3.4
discharge coefficient for gas and liquid flow
K 〈for gas〉
dr,g
K 〈for liquid〉
dr,l

correction factor defined by the ratio of the theoretically dischargeable mass flux through the safety

device 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 referen

...