prEN ISO 23251

SLOVENSKI STANDARD
oSIST prEN ISO 23251:2011
01-junij-2011
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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
oSIST prEN ISO 23251:2011 en,fr
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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oSIST prEN ISO 23251:2011

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oSIST prEN ISO 23251:2011


EUROPEAN STANDARD
DRAFT
prEN ISO 23251
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.

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oSIST prEN ISO 23251:2011
prEN ISO 23251:2011 (E)
Contents Page
Foreword .3

2

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oSIST prEN ISO 23251:2011
prEN ISO 23251:2011 (E)
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.

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oSIST prEN ISO 23251:2011

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oSIST prEN ISO 23251:2011

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

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oSIST prEN ISO 23251:2011
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
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
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

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oSIST prEN ISO 23251:2011
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
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E.5 Test intervals for HIPS . 234
E.6 Documentation. 235
E.7 Training. 235
E.8 Additional source material. 235
Bibliography. 236

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oSIST prEN ISO 23251:2011
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.

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oSIST prEN ISO 23251:2011
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.
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oSIST prEN ISO 23251:2011
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.
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oSIST prEN ISO 23251:2011
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
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oSIST prEN ISO 23251:2011
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
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oSIST prEN ISO 23251:2011
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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
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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
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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 f
...