oSIST prEN ISO 23251:2011

Petroleum, petrochemical and natural gas industries - Pressure-relieving and depressuring systems (ISO/DIS 23251:2011)

2011-02-21 EMA: // ENQ draft received in ISO/CS (see notification from 2011-02-18 in dataservice).
2009-11-06 EMA: Project reinstated at ISO. VA information updated.
2009-10-23 EMA: WI deleted at ISO (ID 45509).

Erdöl-, petrochemische und Erdgasindustrie - Druckentlastungs- und Druckausgleichssysteme (ISO/DIS 23251:2011)

Industries du pétrole, de la pétrochimie et du gaz naturel - Systèmes de dépressurisation et de protection contre les surpressions (ISO/DIS 23251:2011)

Petrokemična industrija ter industrija za predelavo nafte in zemeljskega plina - Sistemi za sproščanje in izravnavanje tlaka (ISO/DIS 23251:2011)

General Information

• Status: Not Published
• Public Enquiry End Date: 24-Jun-2011
• ICS: 75.180.20 - Processing equipment
• Technical Committee: I13 - Imaginarni 13
• Current Stage: 98 - Abandoned project (Adopted Project)
• Start Date: 27-Jan-2020
• Due Date: 01-Feb-2020
• Completion Date: 27-Jan-2020
• Ref Project: prEN ISO 23251 - Petroleum, petrochemical and natural gas industries - Pressure-relieving and depressuring systems (ISO/DIS 23251:2011)

Relations

• Revised: SIST EN ISO 23251:2007 - Petroleum, petrochemical and natural gas industries - Pressure-relieving and depressuring systems (ISO 23251:2006)
• Effective Date: 18-Jan-2023
• Revised: SIST EN ISO 23251:2007 - Petroleum, petrochemical and natural gas industries - Pressure-relieving and depressuring systems (ISO 23251:2006)
• Effective Date: 22-Dec-2008

Overview

The oSIST prEN ISO 23251:2011 standard, titled Petroleum, petrochemical and natural gas industries – Pressure-relieving and depressuring systems (ISO/DIS 23251:2011), provides comprehensive guidelines for designing and implementing pressure-relieving and vapour-depressuring systems in the oil, petrochemical, and natural gas sectors. Although primarily aimed at oil refineries, this standard is also applicable to petrochemical facilities, natural gas plants, liquefied natural gas (LNG) installations, and upstream oil and gas production facilities.

This draft International Standard was developed under the collaboration of ISO Technical Committee ISO/TC 67 and CEN Technical Committee CEN/TC 12, reflecting a harmonized approach in Europe and internationally. It is intended to complement standards such as ISO 4126 and API RP 520-1, focusing on detailed system design based on risk assessment and operational contexts.

Key Topics

• Overpressure Causes and Relieving Rates
  The standard analyzes principal causes of overpressure and offers methodologies to calculate the individual relieving rates necessary to maintain system safety. It covers both general overpressure phenomena and specific scenarios including fire-related relief demands, vacuum relief guidelines, and vapour depressuring processes.

• Pressure-Relieving Devices and System Design
  Guidance is provided on selecting appropriate pressure-relieving devices and designing systems that include piping, vessels, flares, and vent stacks. Special attention is given to documentation requirements that support flare-header calculations and safety validation.

• Disposal Systems
  Extensive specifications address disposal system design including fluid characteristics affecting system selection, arrangement, system load calculations, and detailed piping design. Disposal routes covered include disposal to lower-pressure systems, flares, and atmospheric release, as well as design details for knockout and seal drums.

• Supplementary Guidance
  Informative annexes supplement core requirements, providing background on fire relief calculation methods, special system design considerations, sample engineering calculations, typical detail sketches, and integration of high integrity protection systems (HIPS).

Applications

• Oil Refineries
  Ensures safe pressure relief and depressuring operations in refinery processes to mitigate risk of equipment failure and environmental hazards.

• Petrochemical Plants
  Applies to pressure safety designs within chemical production processes involving volatile hydrocarbons and complex chemical reactions.

• Natural Gas and LNG Facilities
  Supports design of relief systems tailored to gaseous hydrocarbons and cryogenic conditions, ensuring operational continuity while managing overpressure risks.

• Upstream Oil and Gas Production
  Provides a framework for depressuring wellhead systems, separators, and processing units, critical to safe production operations and emergency response.

Related Standards

• ISO 4126 - Safety devices for protection against excessive pressure
• API RP 520-1 - Sizing, selection, and installation of pressure-relieving devices in refineries
• ISO 25457 - Mechanical design, operation, and maintenance of flare equipment
• National and regional regulations - Complemented by local mandatory requirements for pressure relief and plant safety systems

---

Adhering to oSIST prEN ISO 23251:2011 ensures reliable and standardized design practices in pressure-relieving and depressuring systems within petroleum and petrochemical industries. This standard is essential for engineers, safety professionals, and facility operators seeking robust solutions for managing pressure hazards in complex hydrocarbon processing environments.

Standards Content (Sample)

SLOVENSKI STANDARD
01-junij-2011
3HWURNHPLþQDLQGXVWULMDWHULQGXVWULMD]DSUHGHODYRQDIWHLQ]HPHOMVNHJDSOLQD
6LVWHPL]DVSURãþDQMHLQL]UDYQDYDQMHWODND,62',6
Petroleum, petrochemical and natural gas industries - Pressure-relieving and
depressuring systems (ISO/DIS 23251:2011)
Erdöl-, petrochemische und Erdgasindustrie - Druckentlastungs- und
Druckausgleichssysteme (ISO/DIS 23251:2011)
Industries du pétrole, de la pétrochimie et du gaz naturel - Systèmes de dépressurisation
et de protection contre les surpressions (ISO/DIS 23251:2011)
Ta slovenski standard je istoveten z: prEN ISO 23251
ICS:
75.180.20 Predelovalna oprema Processing equipment
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
DRAFT
NORME EUROPÉENNE
EUROPÄISCHE NORM
April 2011
ICS 75.180.20 Will supersede EN ISO 23251:2007
English Version
Petroleum, petrochemical and natural gas industries - Pressure-
relieving and depressuring systems (ISO/DIS 23251:2011)
Industries du pétrole, de la pétrochimie et du gaz naturel - Erdöl-, petrochemische und Erdgasindustrie -
Systèmes de dépressurisation et de protection contre les Druckentlastungs- und Druckausgleichssysteme (ISO/DIS
surpressions (ISO/DIS 23251:2011) 23251:2011)
This draft European Standard is submitted to CEN members for parallel enquiry. It has been drawn up by the Technical Committee
CEN/TC 12.
If this draft becomes a European Standard, 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.

This draft European Standard was established by CEN 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, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.

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.

Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without notice and
shall not be referred to as a European Standard.

EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2011 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN ISO 23251:2011: E
worldwide for CEN national Members.

Contents Page
Foreword .3

Foreword
This document (prEN ISO 23251:2011) has been prepared by Technical Committee ISO/TC 67 "Materials,
equipment and offshore structures for petroleum, petrochemical and natural gas industries" in collaboration
with Technical Committee CEN/TC 12 “Materials, equipment and offshore structures for petroleum,
petrochemical and natural gas industries” the secretariat of which is held by AFNOR.
This document is currently submitted to the parallel Enquiry.
This document will supersede EN ISO 23251:2007.
Endorsement notice
The text of ISO/DIS 23251:2011 has been approved by CEN as a prEN ISO 23251:2011 without any
modification.
DRAFT INTERNATIONAL STANDARD ISO/DIS 23251
ISO/TC 67/SC 6 Secretariat: AFNOR
Voting begins on Voting terminates on

2011-04-07 2011-09-07
INTERNATIONAL ORGANIZATION FOR STANDARDIZATION  •  МЕЖДУНАРОДНАЯ ОРГАНИЗАЦИЯ ПО СТАНДАРТИЗАЦИИ  •  ORGANISATION INTERNATIONALE DE NORMALISATION

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
[Revision of first edition (ISO 23251:2006)]
ICS 75.180.20
ISO/CEN PARALLEL PROCESSING
This draft has been developed within the International Organization for Standardization (ISO), and
processed under the ISO-lead mode of collaboration as defined in the Vienna Agreement.
This draft is hereby submitted to the ISO member bodies and to the CEN member bodies for a parallel
five-month enquiry.
Should this draft be accepted, a final draft, established on the basis of comments received, will be
submitted to a parallel two-month approval vote in ISO and formal vote in CEN.

In accordance with the provisions of Council Resolution 15/1993 this document is circulated in
the English language only.
Conformément aux dispositions de la Résolution du Conseil 15/1993, ce document est distribué
en version anglaise seulement.

To expedite distribution, this document is circulated as received from the committee
secretariat. ISO Central Secretariat work of editing and text composition will be undertaken at
publication stage.
Pour accélérer la distribution, le présent document est distribué tel qu'il est parvenu du
secrétariat du comité. Le travail de rédaction et de composition de texte sera effectué au
Secrétariat central de l'ISO au stade de publication.

THIS DOCUMENT IS A DRAFT CIRCULATED 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 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 NATIONAL REGULATIONS.
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.
©  International Organization for Standardization, 2011

ISO/DIS 23251
Copyright notice
This ISO document is a Draft International Standard and is copyright-protected by ISO. Except as permitted
under the applicable laws of the user’s country, neither this ISO draft nor any extract from it may be
reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic,
photocopying, recording or otherwise, without prior written permission being secured.
Requests for permission to reproduce should be addressed to either ISO at the address below or ISO’s
member body in the country of the requester.
ISO copyright office
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Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail [e-mail]
Web www.iso.org
Reproduction may be subject to royalty payments or a licensing agreement.
Violators may be prosecuted.
ii © ISO 2011 – All rights reserved

ISO/DIS 23251
Contents Page
Foreword .v
Introduction.vi
1 Scope.1
2 Normative references.1
3 Terms and definitions .1
4 Causes of overpressure and their relieving rates.10
4.1 General .10
4.2 Overpressure protection philosophy .11
4.3 Determination of individual relieving rates.14
4.4 Individual overpressure causes and their relieving rates.16
4.5 Guidance on vacuum relief .62
4.6 Vapour depressuring .64
4.7 Relief system design documentation.72
4.8 List of items required in flare-header calculation documentation .76
4.9 Special considerations for individual pressure-relief devices .77
5 Disposal systems .78
5.1 General .78
5.2 Fluid properties that influence selection and design of disposal systems .78
5.3 System design load.81
5.4 System arrangement.84
5.5 Piping.87
5.6 Disposal to a lower-pressure system.103
5.7 Disposal to flare.104
5.8 Disposal to atmosphere.147
5.9 Design details for seal and knockout drums.164
Annex A (informative) Determination of fire relief requirements .167
A.1 Nature of a fire .167
A.2 Background of the empirical method in 4.4.13.2.3.2 .167
A.3 Alternative Analytical Method .168
A.4 Data on latent heat of vaporization of hydrocarbons.186
Annex B (informative) Special system design considerations.188
B.1 Single pressure-relief device protecting several components in a process system .188
B.2 Description of a typical process system.188
B.3 Procedure to calculate the design pressure or MAWP of equipment components .188
Annex C (informative) Sample calculations .191
C.1 Flash calculation .191
C.2 Sizing a subsonic flare stack .193
C.3 Flare knockout drum.208
C.4 Sizing a vent stack .212
C.5 Noise calculation.213
C.6 The use of the Annex A analytical method to reproduce fire test data .215
Annex D (informative) Typical details and sketches .226
Annex E (informative) High integrity protection systems (HIPS).229
E.1 Introduction.229
E.2 Background.229
E.3 Relevant regulations and industry standards .230
E.4 Procedures for applying HIPS.233
ISO/DIS 23251
E.5 Test intervals for HIPS . 234
E.6 Documentation. 235
E.7 Training. 235
E.8 Additional source material. 235
Bibliography. 236

iv © ISO 2011 – All rights reserved

ISO/DIS 23251
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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
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.
ISO 23251 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.
ISO/DIS 23251
Introduction
The portions of this International Standard dealing with flares and flare systems are an adjunct to
[10]
ISO 25457) , which addresses mechanical design, operation and maintenance of flare equipment. It is
important for all parties involved in the design and use of a flare system to have an effective means of
communicating and preserving design information about the flare system. To this end, ISO and API has
developed a set of flare data sheets, which can be found in ISO 25457 Annex E. The use of these data sheets
is both recommended and encouraged as a concise, uniform means of recording and communicating design
information.
The Bibliography lists the documents that are referenced informatively in this International Standard, as well
as other documents that are not cited in this International Standard but that contain additional useful
information. Some of the content of the documents listed might not be suitable for all applications and
therefore needs to be assessed for each application before use.
vi © ISO 2011 – All rights reserved

DRAFT INTERNATIONAL STANDARD ISO/DIS 23251

Petroleum, petrochemical and natural gas industries —
Pressures-relieving and depressuring systems
1 Scope
This International Standard 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 International Standard is intended to supplement the practices set forth in ISO 4126 or API RP 520-1 for
establishing a basis of design.
This International Standard specifies requirements and gives guidelines for examining the principal causes of
overpressure; and determining individual relieving rates; and selecting and designing disposal systems,
including such component parts as piping, vessels, flares, and vent stacks. This International Standard does
not apply to direct-fired steam boilers.
Piping information pertinent to pressure-relieving systems is presented in 5.5.
2 Normative references
The following referenced documents are indispensable for the application 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 (all parts), Safety devices for protection against excessive pressure
API STD 520-I:2008, Sizing, Selection and Installation of Pressure-Relieving Devices in Refineries — Part I:
1)
Sizing and Selection
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
accumulation
pressure increase over the maximum allowable working pressure of the vessel allowed during discharge
through the pressure-relief device
NOTE Accumulation is expressed in units of pressure or as a percentage of MAWP or design pressure. Maximum
allowable accumulations are established by pressure-design codes for emergency operating and fire contingencies.

1)
American Petroleum Institute, 1220 L Street, N.W., Washington, D.C., 20005-4070, USA.
ISO/DIS 23251
3.2
administrative controls
procedures intended to ensure that personnel actions do not compromise the overpressure protection of the
equipment
3.3
assist gas
fuel gas that is added to relief gas prior to the flare burner or at the point of combustion in order to raise the
heating value
NOTE In some designs, the assist gas can increase turbulence for improved combustion.
3.4
atmospheric discharge
release of vapours and gases from pressure-relieving and depressuring devices to the atmosphere
3.5
availability
fraction of time that a system (e.g. safety instrumented system, atmospheric relief system or flare relief
system) is able to perform the designated service if required for use
3.6
back pressure
pressure that exists at the outlet of a pressure-relief device as a result of the pressure in the discharge system
NOTE The back pressure is the sum of the superimposed and built-up back pressures.
3.7
balanced pressure-relief valve
spring-loaded pressure-relief valve that incorporates a bellows or other means for minimizing the effect of
back pressure on the operational characteristics of the valve
3.8
blowdown drum
knockout drum with or connected to a stack open to atmosphere
NOTE 1 The term blowdown drum is sometimes also used for knockout drums connected to flare or other disposal
systems, but it is not used in this context in this International Standard.
NOTE 2 The term blowdown is sometimes used in the context of emergency depressuring of a plant or part of a plant,
but it is not used in this context in this International Standard.
3.9
blow-off
loss of a stable flame where the flame is lifted above the burner, occurring if the fuel velocity exceeds the
flame velocity
3.10
breaking-pin device
pressure-relief device actuated by static differential or static inlet pressure and designed to function by the
breakage of a load-carrying section of a pin that supports a pressure-containing member
3.11
buckling pin device
pressure-relief device actuated by static differential or static inlet pressure and designed to function by the
buckling of an axially-loaded compressive pin that supports a pressure-containing member
2 © ISO 2011 – All rights reserved

ISO/DIS 23251
3.12
built-up back pressure
increase in pressure at the outlet of a pressure-relief device that develops as a result of flow after the
pressure-relief device opens
3.13
buoyancy seal
dry vapour seal that minimizes the amount of purge gas needed to protect against air infiltration
NOTE The buoyancy seal functions by trapping a volume of light gas in an internal inverted compartment; this
prevents air from displacing buoyant light gas in the flare.
3.14
burnback
internal burning within the flare tip
NOTE Burnback can result from air backing down the flare burner at purge or low flaring rates.
3.15
burning velocity
flame velocity
speed at which a flame front travels into an unburned combustible mixture
3.16
burn-pit flare
open excavation, normally equipped with a horizontal flare burner that can handle liquid as well as vapour
hydrocarbons
3.17
burst pressure
value of the upstream static pressure minus the value of the downstream static pressure just before a rupture
disk bursts
NOTE If the downstream pressure is atmospheric, the burst pressure is the upstream static gauge pressure.
3.18
closed disposal system
disposal system capable of containing pressures that are different from atmospheric pressure
3.19
cold differential test pressure
CDTP
pressure at which a pressure-relief valve is adjusted to open on the test stand
NOTE The cold differential test pressure includes corrections for the service conditions of back pressure or
temperature or both.
3.20
combustion air
air required to combust the flare gases
3.21
confined fire
fire inside a building or a compact process module where the walls and/or surrounding equipment can re-
radiate and preheat the combustion air causing higher heat fluxes than an unconfined (i.e., open) fire
ISO/DIS 23251
3.22
conventional pressure-relief valve
spring-loaded pressure-relief valve whose operational characteristics are directly affected by changes in the
back pressure
3.23
corrected hydrotest pressure
hydrostatic test pressure multiplied by the ratio of stress value at upset temperature to the stress value at test
temperature
NOTE 1 See 4.2.2.
NOTE 2 In this definition, the hydrostatic test pressure is that specified by the pressure design code, whether or not the
equipment has actually been hydrostatically tested.
3.24
deflagration
explosion in which the flame-front of a combustible medium is advancing at less than the speed of sound
cf. detonation (3.27)
3.25
design pressure
pressure, together with the design temperature, used to determine the minimum permissible thickness or
physical characteristic of each component, as determined by the design rules of the pressure-design code
NOTE The design pressure is selected by the user to provide a suitable margin above the most severe pressure
expected during normal operation at a coincident temperature, and it is the pressure specified on the purchase order. The
design pressure is equal to or less than the MAWP (the design pressure can be used as the MAWP in cases where the
MAWP has not been established).
3.26
destruction efficiency
mass percent of the combustible vapour that is at least partially oxidized
NOTE In the case of a hydrocarbon, destruction efficiency is the mass percentage of carbon in the fluid vapour that
oxidizes to CO or CO2.
3.27
detonation
explosion in which the flame-front of a combustible medium is advancing at or above the speed of sound
cf. deflagration (3.24)
3.28
dispersion
dilution of a vent stream or products of combustion as the fluids move through the atmosphere
3.29
elevated flare
flare where the burner is raised high above ground level to reduce radiation intensity and to aid in dispersion
3.30
enclosed flare
enclosure with one or more burners arranged in such a manner that the flame is not directly visible
3.31
enrichment
process of adding assist gas to the relief gas
4 © ISO 2011 – All rights reserved

ISO/DIS 23251
3.32
flame-retention device
device used to prevent flame blow off from a flare burner
3.33
flare
device or system used to safely dispose of relief gases in an environmentally compliant manner through the
use of combustion
3.34
flare burner
flare tip
part of the flare where fuel and air are mixed at the velocities, turbulence and concentration required to
establish and maintain proper ignition and stable combustion
3.35
flare header
piping system that collects and delivers the relief gases to the flare
3.36
fuel-controlled fire
fire that always has at least enough air for combustion so the amount of fuel (i.e., the size of the pool
or the fuel leak rate) controls the heat release rate3.37
flashback
phenomenon occurring in a flammable mixture of air and gas when the local velocity of the combustible
mixture becomes less than the flame velocity, causing the flame to travel back to the point of mixture
3.38
ground flare
non-elevated flare
NOTE A ground flare is normally an enclosed flare but can also be a ground multi-burner flare or a burnpit.
3.39
heat release
total heat liberated by combustion of the relief gases based on the lower heating value
NOTE The heat release is expressed in kW (Btu/h).
3.40
hydrate
solid, crystalline compound of water and a low-boiling-point gas (e.g. methane and propane), in which the
water combines with the gas molecule to form a solid
3.41
jet fire
fire created when a leak from a pressurized system ignites and forms a burning jet
NOTE A jet fire can impinge on other equipment, causing damage.
3.42
knockout drum
vessel in the effluent handling system designed to remove and store liquids
3.43
lateral
section of pipe from outlet flange(s) of single-source relief device(s) downstream of a header connection
where relief devices from other sources are tied in
ISO/DIS 23251
NOTE The relief flow in a lateral is always from a single source, whereas the relief flow in a header can be from
either single or multiple sources simultaneously.
3.44
lift
actual travel of the disc from the closed position when a valve is relieving
3.45
liquid seal
water seal
device that directs the flow of relief gases through a liquid (normally water) on the path to the flare burner,
used to protect the flare header from air infiltration or flashback, to divert flow, or to create back pressure for
the flare header
3.46
Mach number
ratio of a fluid’s velocity, measured relative to some obstacle or geometric figure, divided by the speed at
which sound waves propagate through the fluid
3.47
manifold
piping system for the collection and/or distribution of a fluid to or from multiple flow paths
3.48
marked burst pressure
rated burst pressure
〈rupture disk〉 burst pressure, established by tests for the specified temperature and marked on the disk tag by
the manufacturer
NOTE The marked burst pressure can be any pressure within the manufacturing design range unless otherwise
specified by the customer. The marked burst pressure is applied to all of the rupture disks of the same lot.
3.49
maximum allowable working pressure
MAWP
maximum gauge pressure permissible at the top of a completed vessel in its normal operating position at the
designated coincident temperature specified for that pressure
cf. design pressure (3.25)
NOTE The MAWP is the least of the values for the internal or external pressure as determined by the vessel design
rules for each element of the vessel using actual nominal thickness, exclusive of additional metal thickness allowed for
corrosion and loadings other than pressure. The MAWP is the basis for the pressure setting of the pressure-relief devices
that protect the vessel.
3.50
non-condensable gas
gas or vapour that remains in the gaseous state at the temperature and pressure expected
3.51
open fire
fire for which the surroundings do not contribute to preheating the ventilation air or reradiation. More heat will
be lost to the surroundings as compared with confined fires
3.52
operating pressure
pressure the process system experiences during normal operation, including normal variations
6 © ISO 2011 – All rights reserved

ISO/DIS 23251
3.53
overpressure
〈general〉 condition where the MAWP, or other specified pressure, is exceeded
〈relieving device〉 pressure increase over the set pressure of a relieving device
NOTE In the latter context, overpressure is the same as accumulation (3.1) only when the relieving device is set to
open at the MAWP of the vessel.
3.54
pilot-operated pressure-relief valve
pressure-relief valve in which the major relieving device or main valve is combined with and controlled by a
self-actuated auxiliary pressure-relief valve (pilot)
3.55
pin device
non-reclosing pressure-relief device actuated by static pressure and designed to function by buckling or
breaking a pin that holds a piston or a plug in place; upon buckling or breaking of the pin, the piston or plug
instantly moves to the fully open position
3.56
pool fire
burning pool of liquid
3.57
pressure-design code
standard to which the equipment is designed and constructed
[20]
EXAMPLE ASME Section VIII, Division 1 .
3.58
pressure-relief valve
valve designed to open and relieve excess pressure and to reclose and prevent the further flow of fluid after
normal conditions have been restored
NOTE In ISO 4126-1, this is termed a safety valve.
3.59
process tank
process vessel
tank or vessel used for an integrated operation in petrochemical facilities, refineries, gas plants, oil and gas
production facilities, and other facilities
cf. storage tank (3.76)
NOTE A process tank or vessel used for an integrated operation can involve, but is not limited to, preparation,
separation, reaction, surge control, blending, purification, change in state, energy content, or composition of a material.
3.60
purge gas
fuel gas or non-condensable inert gas added to the flare header to mitigate air ingress and burnback
3.61
quenching
cooling of a fluid by mixing it with another fluid of a lower temperature
3.62
radiation intensity
local radiant heat transfer rate from the flare flame, usually considered at grade level
ISO/DIS 23251
3.63
rated relieving capacity
relieving capacity used as the basis for the application of a pressure-relief device, determined in accordance
with the pressure-design code or regulation and supplied by the manufacturer
NOTE The capacity marked on the device is the rated capacity on steam, air, gas or water as required by the
applicable code.
3.64
relief gas
flared gaswaste gas
waste vapour
all gases and vapors present just after the exit from the flare tip, not including entrained air, and consisting of
the sum of organic material, nitrogen and any other gases added to the vent gas collection system, natural
gas added as supplemental fuel, nitrogen added as purge gas, natural gas flowing to the flare pilots, and
steam added at the flare tip
3.65
relief valve
spring-loaded pressure-relief valve actuated by the static pressure upstream of the valve, due to which the
valve normally opens in proportion to the pressure increase over the opening pressure
NOTE A relief valve is normally used with incompressible fluids.
3.66
relieving conditions
inlet pressure and temperature on a pressure-relief device during an overpressure condition
NOTE The relieving pressure is equal to the valve set pressure (or rupture disk burst pressure) plus the
overpressure. The temperature of the flowing fluid at relieving conditions can be higher or lower than the operating
temperature.
3.67
rupture-disk device
non-reclosing pressure-relief device actuated by static differential pressure between the inlet and outlet of the
device and designed to function by the bursting of a rupture disk
NOTE 1 A rupture disk device includes a rupture disk and a rupture disk holder.
NOTE 2 In ISO 4126-2, this is termed a bursting-disc safety device.
3.68
safety instrumented system
SIS
emergency shutdown system
ESD, ESS
high-integrity protection system
HIPS
high-integrity pressure-protection system
HIPPS
safety-shutdown system
SSD
safety-interlock system
system composed of sensors, logic solvers and final control elements for the purpose of taking the process to
a safe state when predetermined conditions are violated
3.69
safety-integrity level
SIL
discrete integrity level of a safety instrumented function in a safety instrumented system
8 © ISO 2011 – All rights reserved

ISO/DIS 23251
NOTE SILs are categorized in terms of probability of failure; see Annex E.
3.70
safety relief valve
spring-loaded pressure-relief valve that can be used as either a safety valve or a relief valve depending on the
application
3.71
safety valve
spring-loaded pressure-relief valve actuated by the static pressure upstream of the valve and characterized by
rapid opening or pop action
NOTE 1 A safety valve is normally used with compressible fluids.
NOTE 2 This definition is different than that in ISO 4126-1; see 3.58.
3.72
set pressure
inlet gauge pressure at which a pressure-relief device is set to open under service conditions
3.73
shear pin device
non-reclosing pressure-relief device actuated by static differential or static inlet pressure and designed to
function by the shearing of a load-carrying member that supports a pressure-containing member
3.74
staged flare
group of two or more flares or burners that are controlled so that the number of flares or burners in operation
is proportional to the relief gas flow
3.75
stoichiometric air
chemically correct ratio of fuel to air capable of perfect combustion with no unused fuel or air
3.76
storage tank
storage vessel
fixed tank or vessel that is not part of the processing unit in petrochemical facilities, refineries, gas plants, oil
and gas production facilities, and other facilities
cf. process tank (3.59)
NOTE These tanks or vessels are often located in tank farms.
3.77
superimposed back pressure
static pressure that exists at the outlet of a pressure-relief device at the time the device is required to operate
NOTE It is the result of pressure in the discharge system coming from other sources and can be constant or variable.
3.78
vapour depressuring system
protective arrangement of valves and piping intended to provide for rapid reduction of pressure in equipment
by releasing vapours
NOTE The actuation of the system can be automatic or manual.
ISO/DIS 23251
3.79
velocity seal
orifice sealdry vapour seal that minimizes the required purge gas needed to protect against air infiltration into
the flare burner exit
3.80
vent gas
organic materials, nitrogen and any other gases that flow from process units, nitrogen added as purge gas,
and any other gases or vapors added to the flare system prior to the flare tip
3.81
vent header
piping system that collects and delivers the relief gases to the vent stack
3.82
vent stack
elevated vertical termination of a disposal system that discharges vapours into the atmosphere without
combustion or conversion of the relieved fluid
3.83
ventilation-controlled fire
fire that is in shortage of air; hence the available air - the ventilation - controls the heat release rate
3.84
vessel
container or structural envelope in which materials are processed, treated or stored
EXAMPLES Pressure vessels, reactor vessels and storage vessels (tanks).
3.85
windshield
device used to protect the down-wind side of an elevated flare burner from direct flame impingement
NOTE Windshields are also integral to the design of pilots to avoid flame-outs during bad/stormy weather.
4 Causes of overpressure and their relieving rates
4.1 General
Pressure vessels, heat exchangers, operating equipment and piping are designed to contain the system
pressure. The design is based on
a) the normal operating pressure at operating temperatures;
b) the effect of any combination of process upsets that are likely to occur;
c) the differential between the operating, and set pressures of the pressure-relieving device;
d) the effect of any combination of supplemental loadings such as earthquake and wind.
Clause 4 discusses the principal causes of overpressure where the MAWP, design pressure, or other
specified pressure can be exceeded. Guidance in plant design to minimize the effects of these overpressure
causes and guidance on estimating relieving rates is provided. Overpressure is the result of an unbalance or
disruption of the normal flows of material and energy that causes the material or energy, or both, to build up in
some part of the system. Analysis of the causes and magnitudes of overpressure is, therefore, a special and
complex study of material and energy balances in a process system.
10 © ISO 2011 – All rights reserved

ISO/DIS 23251
The principal causes of overpressure listed in 4.4.2 through 4.4.16 are guides to generally accepted practices.
The process-systems designer shall define the minimum pressure-relief capacity required to prevent the
pressure in any piece of equipment from exceeding the maximum allowable accumulated pressure. Annex B
provides guidance on the use of a common relief device to protect multiple pieces of equipment from
overpressure.
The application of the principles outlined in Clause 4 is unique for each processing system. Although efforts
have been made to cover all major circumstances, the user is cautioned not to consider the conditions
described as the only causes of overpressure. The treatment of overpressure in this International Standard
can be only suggestive. Any circumstance that reasonably constitutes a hazard under the prevailing
conditions for a system should be considered in the design. Pressure-relieving devices are installed to ensure
that a process system or any of its components is not subjected to pressures that exceed the maximum
allowable accumulated pressure. The practices evaluated in Clause 4 should be used in conjunction with
sound engineering judgment and with full consideration of federal, state and local rules and regulations.
4.2 Overpressure protection philosophy
4.2.1 Hierarchy of protective measures
A hierarchy of measures should be used to ensure that equipment is not subject to excess pressure. Such a
hierarchy involves firstly avoiding or reducing risks, then providing engineering controls and lastly providing
administrative controls. Avoiding risks includes, for example, setting the maximum allowable working pressure
of the equipment above the maximum pressure of all possible sources. Engineering controls include providing
pressure relief on the vessel. Administrative controls include provision of block valves of the locked-open
design. The user is cautioned that some systems may have unacceptable risk due to failure of administrative
controls and resulting consequences due to loss of containment.
4.2.2 Test pressure
Certain pressure design codes allow the use of administrative controls if the potential overpressure does not
exceed the corrected hydrotest pressure, whereas other pressure design codes do not address this subject.
Therefore, applying this for equipment built to pressure design codes that do not address the issue could
cause the equipment to be overstressed. In these cases, the user should perform mechanical analyses and/or
risk analyses. This philosophy is applied to the following scenarios:
a) Closed outlets on vessels – see 4.4.2
b) Check-valve leakage or failure – see 4.4.9.3
c) Inadvertent valve opening – see 4.4.9.2
d) Heat-transfer equipment failure – see 4.4.14
The user is cautioned that some systems can have unacceptable risk due to failure of administrative controls
and resulting consequences due to loss of containment. In these cases, limiting the overpressure to the
normally allowable overpressure can be more appropriate. Note that the entire system, including all of the
auxiliary devices (e.g. gasketed joints, instrumentation), should be considered for the overpressure during the
failure of administrative controls.
[22]
The corrected test pressure is explained as follows. For example, an ASTM A 515 Grade 70 carbon steel
vessel with a design gauge pressure of 517 kPa (75 psi) and design temperature of 343 °C (650 °F) has an
allowable stress of 130 MPa (18 800 psi) at these design conditions. Because the hydrostatic test is often
performed at a temperature less than design temperature, the hydrostatic test pressure should be specified to
account for the allowable stress differences at the two temperatures by multiplying the design pressure by the
ratio of stress at test temperature to the s
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