ISO 23251:2019
(Main)Petroleum, petrochemical and natural gas industries — Pressure-relieving and depressuring systems
Petroleum, petrochemical and natural gas industries — Pressure-relieving and depressuring systems
This document is applicable to pressure-relieving and vapour depressuring systems. Although intended for use primarily in oil refineries, it is also applicable to petrochemical facilities, gas plants, Liquefied Natural Gas (LNG) facilities and oil and gas production facilities. The information provided is designed to aid in the selection of the system that is most appropriate for the risks and circumstances involved in various installations. This document supplements API Std 521, 6th edition (2014), the requirements of which are applicable with the exceptions specified in this document.
Industries du pétrole, de la pétrochimie et du gaz naturel — Systèmes de dépressurisation et de protection contre les surpressions
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INTERNATIONAL ISO
STANDARD 23251
Second edition
2019-03
Petroleum, petrochemical and natural
gas industries — Pressure-relieving
and depressuring systems
Industries du pétrole, de la pétrochimie et du gaz naturel — Systèmes
de dépressurisation et de protection contre les surpressions
Reference number
©
ISO 2019
© ISO 2019
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
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2019 – All rights reserved
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Supplements to API Std 521, 6th edition (2014). 2
4.1 General requirements . 2
4.2 Chemical reaction . 2
4.3 Fires . 2
4.4 Causes for vacuum . 3
4.5 Considerations for individual PRDs . 3
4.5.1 General. 3
4.5.2 Multiple PRDs . 3
4.6 Piping . 3
4.6.1 General. 3
4.6.2 Backpressure . 3
4.6.3 Line sizing . 3
4.6.4 Multiple relief scenarios . 3
4.6.5 Reaction forces . 3
4.7 Disposal to flare . 4
4.7.1 General. 4
4.7.2 Combustion properties . 4
4.7.3 Combustion methods . 4
4.7.4 Flare systems designs . 4
4.7.5 Sizing . 4
4.7.6 Ignition of flare gases . 4
4.7.7 Liquid seal drum drums . 4
4.8 Single PRD protecting several components in a process system . 4
Bibliography . 5
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso
.org/iso/foreword .html.
This document was prepared by Technical Committee ISO/TC 67, Materials, equipment and offshore
structures for petroleum, petrochemical and natural gas industries, Subcommittee SC 6, Processing
equipment and systems.
This second edition cancels and replaces the first edition (ISO 23251:2006), which has been technically
revised. It also incorporates the Technical Corrigendum ISO 23251:2006/Cor.1:2007 and the Amendment
ISO 23251:2006/Amd.1:2008.
This document supplements API Std 521, 6th edition (2014).
The technical requirements of this document and API Std 521 used to be identical. In the meantime
API Std 521 has been technically revised as API Std 521, 6th edition (2014). The purpose of this document
is to bring it up to date, by referencing the current edition of API Std 521 and adding supplementary
content.
The main changes compared to the previous edition are as follows:
— Permission to use administrative controls such as car-sealing or chain-locking valves to prevent an
over-pressure only if the corrected hydrotest pressures are not exceeded;
— Addition of requirements to cover choke valve failures and acoustic fatigue;
— Strengthening of the requirements to evaluate check valve reverse flow failures;
— Addition of guidance on the use of alternative method for fire relief and blowdown system design.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/members .html.
iv © ISO 2019 – All rights reserved
Introduction
Users of this document are in
...
INTERNATIONAL ISO
STANDARD 23251
Second edition
2019-03
Petroleum, petrochemical and natural
gas industries — Pressure-relieving
and depressuring systems
Industries du pétrole, de la pétrochimie et du gaz naturel — Systèmes
de dépressurisation et de protection contre les surpressions
Reference number
©
ISO 2019
© ISO 2019
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
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2019 – All rights reserved
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Supplements to API Std 521, 6th edition (2014). 2
4.1 General requirements . 2
4.2 Chemical reaction . 2
4.3 Fires . 2
4.4 Causes for vacuum . 3
4.5 Considerations for individual PRDs . 3
4.5.1 General. 3
4.5.2 Multiple PRDs . 3
4.6 Piping . 3
4.6.1 General. 3
4.6.2 Backpressure . 3
4.6.3 Line sizing . 3
4.6.4 Multiple relief scenarios . 3
4.6.5 Reaction forces . 3
4.7 Disposal to flare . 4
4.7.1 General. 4
4.7.2 Combustion properties . 4
4.7.3 Combustion methods . 4
4.7.4 Flare systems designs . 4
4.7.5 Sizing . 4
4.7.6 Ignition of flare gases . 4
4.7.7 Liquid seal drum drums . 4
4.8 Single PRD protecting several components in a process system . 4
Bibliography . 5
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso
.org/iso/foreword .html.
This document was prepared by Technical Committee ISO/TC 67, Materials, equipment and offshore
structures for petroleum, petrochemical and natural gas industries, Subcommittee SC 6, Processing
equipment and systems.
This second edition cancels and replaces the first edition (ISO 23251:2006), which has been technically
revised. It also incorporates the Technical Corrigendum ISO 23251:2006/Cor.1:2007 and the Amendment
ISO 23251:2006/Amd.1:2008.
This document supplements API Std 521, 6th edition (2014).
The technical requirements of this document and API Std 521 used to be identical. In the meantime
API Std 521 has been technically revised as API Std 521, 6th edition (2014). The purpose of this document
is to bring it up to date, by referencing the current edition of API Std 521 and adding supplementary
content.
The main changes compared to the previous edition are as follows:
— Permission to use administrative controls such as car-sealing or chain-locking valves to prevent an
over-pressure only if the corrected hydrotest pressures are not exceeded;
— Addition of requirements to cover choke valve failures and acoustic fatigue;
— Strengthening of the requirements to evaluate check valve reverse flow failures;
— Addition of guidance on the use of alternative method for fire relief and blowdown system design.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/members .html.
iv © ISO 2019 – All rights reserved
Introduction
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