EN ISO 23251:2020

Petroleum, petrochemical and natural gas industries - Pressure-relieving and depressuring systems (ISO 23251:2019)

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.

Erdöl-, petrochemische und Erdgasindustrie - Druckentlastungs- und Druckausgleichssysteme (ISO 23251:2019)

Industries du pétrole, de la pétrochimie et du gaz naturel - Systèmes de dépressurisation et de protection contre les surpressions (ISO 23251:2019)

Le présent document est applicable aux systèmes de protection contre les surpressions et de dépressurisation des phases gazeuses. Bien que son utilisation première soit destinée aux raffineries, elle est aussi applicable aux installations pétrochimiques, usines à gaz, installations de gaz naturel liquéfié (GNL) et installations de production de pétrole et de gaz. Les informations fournies sont conçues pour aider au choix du système le plus adapté aux risques et circonstances impliqués dans diverses installations.
Le présent document complète l'API Std 521, 6e édition (2014), dont les exigences sont applicables sauf exceptions spécifiées dans le présent document.

Petrokemična industrija ter industrija za predelavo nafte in zemeljskega plina - Sistemi za sproščanje in izravnavanje tlaka (ISO 23251:2019)

General Information

• Status: Published
• Publication Date: 01-Sep-2020
• Withdrawal Date: 30-Mar-2021
• ICS: 75.180.20 - Processing equipment
• Technical Committee: CEN/TC 12 - Materials, equipment and offshore structures for petroleum and natural gas industries
• Drafting Committee: CEN/TC 12 - Materials, equipment and offshore structures for petroleum and natural gas industries
• Current Stage: 6060 - Definitive text made available (DAV) - Publishing
• Start Date: 02-Sep-2020
• Due Date: 20-Mar-2022
• Completion Date: 02-Sep-2020
• Ref Project: SIST EN ISO 23251:2020 - Petroleum, petrochemical and natural gas industries - Pressure-relieving and depressuring systems (ISO 23251:2019)

Overview

EN ISO 23251:2020 (ISO 23251:2019) provides guidance for the selection, design and application of pressure-relieving and vapour depressuring systems in the petroleum, petrochemical and natural gas industries. Intended primarily for oil refineries, it also applies to petrochemical plants, gas plants, LNG facilities and upstream production sites. The standard supplements API Std 521 (6th ed., 2014) and identifies additional requirements and clarifications specific to European and international practice.

Key topics and technical coverage

• Scope and applicability - pressure-relief devices (PRDs), rupture discs, safety valves and vapour depressuring systems used to protect equipment from overpressure and enable controlled depressuring.
• Supplements to API Std 521 - provisions that modify or add to API Std 521 including exceptions and additional guidance.
• Failure modes and special cases - guidance on chemical reactions, fire scenarios, causes of vacuum, choke valve failures, check valve reverse-flow evaluation and acoustic fatigue.
• PRD considerations - selection, use of multiple PRDs, interaction between devices, and requirements for individual PRDs.
• Piping and backpressure - line sizing, backpressure effects, reaction forces and routing for relief lines.
• Disposal to flare - combustion properties, flare system design and sizing, ignition of flare gases, and use of liquid-seal drums.
• Administrative and operational controls - conditions under which administrative measures (e.g., car‑sealing or chain‑locking valves) may be used, and limits tied to hydrostatic test pressures.
• Alternative methods - guidance for alternative approaches to fire relief and blowdown system design where justified.

Practical applications

EN ISO 23251:2020 is used to:

• Specify and verify design of pressure-relieving and depressuring systems for refineries, petrochemical plants, LNG terminals and gas processing facilities.
• Determine appropriate PRD selection (safety valves, rupture discs) and relief line sizing for process safety and regulatory compliance.
• Define safe flare disposal arrangements and depressuring plans for emergency scenarios (fire, runaway reactions, equipment failure).
• Inform risk assessments and engineering studies addressing overpressure protection, acoustic fatigue and valve failure modes.

Who should use this standard

• Process safety and piping engineers
• Pressure equipment designers and manufacturers (PRVs, rupture discs)
• Flare system and relief-system designers
• Plant operators, safety managers and regulatory compliance teams
• Vendors preparing equipment specifications or certifying conformance

Related standards and references

• API Std 521 (6th edition, 2014) - primary reference supplemented by EN ISO 23251
• ISO 4126 (safety devices for protection against excessive pressure)
• ISO 15649 (piping)
• ISO 25457 (flare details)
• ISO 28300 (venting of low-pressure tanks)
• EN 764-7 (safety systems for unfired pressure equipment)

EN ISO 23251:2020 is essential for organizations seeking robust, internationally aligned guidance on pressure relief and depressuring system design and operation in hydrocarbon processing facilities.

Standards Content (Sample)

SLOVENSKI STANDARD
01-november-2020
Petrokemična industrija ter industrija za predelavo nafte in zemeljskega plina -
Sistemi za sproščanje in izravnavanje tlaka (ISO 23251:2019)
Petroleum, petrochemical and natural gas industries - Pressure-relieving and
depressuring systems (ISO 23251:2019)
Erdöl-, petrochemische und Erdgasindustrie - Druckentlastungs- und
Druckausgleichssysteme (ISO 23251:2019)
Industries du pétrole, de la pétrochimie et du gaz naturel - Systèmes de dépressurisation
et de protection contre les surpressions (ISO 23251:2019)
Ta slovenski standard je istoveten z: EN ISO 23251:2020
ICS:
75.180.20 Predelovalna oprema Processing equipment
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 23251
EUROPEAN STANDARD
NORME EUROPÉENNE
September 2020
EUROPÄISCHE NORM
ICS 75.180.20
English Version
Petroleum, petrochemical and natural gas industries -
Pressure-relieving and depressuring systems (ISO
23251:2019)
Industries du pétrole, de la pétrochimie et du gaz Erdöl-, petrochemische und Erdgasindustrie -
naturel - Systèmes de dépressurisation et de protection Druckentlastungs- und Druckausgleichssysteme (ISO
contre les surpressions (ISO 23251:2019) 23251:2019)
This European Standard was approved by CEN on 19 July 2020.

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, Turkey 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
© 2020 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 23251:2020 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
The text of ISO 23251:2019 has been prepared by Technical Committee ISO/TC 67 "Materials,
equipment and offshore structures for petroleum, petrochemical and natural gas industries” of the
International Organization for Standardization (ISO) and has been taken over as EN ISO 23251:2020 by
Technical Committee CEN/TC 12 “Materials, equipment and offshore structures for petroleum,
petrochemical and natural gas industries” the secretariat of which is held by NEN.
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 March 2021, and conflicting national standards shall
be withdrawn at the latest by March 2021.
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.
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, Turkey and the
United Kingdom.
Endorsement notice
The text of ISO 23251:2019 has been approved by CEN as EN ISO 23251:2020 without any modification.

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 23251:2019(E)
©
ISO 2019
ISO 23251:2019(E)
© 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: [phone]
Fax: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii © ISO 2019 – All rights reserved

ISO 23251:2019(E)
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
ISO 23251:2019(E)
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
...