SIST EN ISO 25457:2009

SLOVENSKI STANDARD
01-marec-2009
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Petroleum, petrochemical and natural gas industries - Flare details for general refinery
and petrochemical service (ISO 25457:2008)
Erdöl-, petrochemische und Erdgasindustrie - Fackeln für den allgemeinen Betrieb in
Raffinerien und petrochemischen Service (ISO 25457:2008)
Industries du pétrole, de la pétrochimie et du gaz naturel - Détails sur les torches
d'usage général dans les raffineries et dans les usines pétrochimiques (ISO 25457:2008)
Ta slovenski standard je istoveten z: EN ISO 25457:2008
ICS:
75.180.20 Predelovalna oprema Processing equipment
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN ISO 25457
NORME EUROPÉENNE
EUROPÄISCHE NORM
December 2008
ICS 75.180.20
English Version
Petroleum, petrochemical and natural gas industries - Flare
details for general refinery and petrochemical service (ISO
25457:2008)
Industries du pétrole, de la pétrochimie et du gaz naturel - Erdöl-, petrochemische und Erdgasindustrie - Fackeln für
Détails sur les torches d'usage général dans les raffineries den allgemeinen Betrieb in Raffinerien und
et dans les usines pétrochimiques (ISO 25457:2008) petrochemischen Service (ISO 25457:2008)
This European Standard was approved by CEN on 11 December 2008.
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 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 Management Centre has the same status as the
official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, 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.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2008 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 25457:2008: E
worldwide for CEN national Members.

Contents Page
Foreword .3

Foreword
This document (EN ISO 25457:2008) has been prepared by Technical Committee ISO/TC 67 "Materials,
equipment and offshore structures for petroleum 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 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 June 2009, and conflicting national standards shall be withdrawn at
the latest by June 2009.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. CEN [and/or CENELEC] 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, 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 the United Kingdom.
Endorsement notice
The text of ISO 25457:2008 has been approved by CEN as a EN ISO 25457:2008 without any modification.

INTERNATIONAL ISO
STANDARD 25457
First edition
2008-12-15
Petroleum, petrochemical and natural gas
industries — Flare details for general
refinery and petrochemical service
Industries du pétrole, de la pétrochimie et du gaz naturel — Détails sur
les torches d'usage général dans les raffineries et dans les usines
pétrochimiques
Reference number
ISO 25457:2008(E)
©
ISO 2008
ISO 25457:2008(E)
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©  ISO 2008
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ii © ISO 2008 – All rights reserved

ISO 25457:2008(E)
Contents Page
Foreword .v
Introduction.vi
1 Scope.1
2 Normative references.1
3 Terms and definitions .2
4 Design.8
4.1 Introduction.8
4.2 System design .10
4.3 Process definition .10
4.4 Types of flares .11
4.5 Flare burners.13
4.6 Mechanical design.13
4.7 Pilots.14
4.8 Pilot-ignition systems .16
4.9 Pilot-flame detection .16
4.10 Piping.17
4.11 Auxiliary components.17
5 Mechanical details — Elevated flares.18
5.1 Mechanical design — Design loads .18
5.2 Design details .21
5.3 Materials of construction.22
5.4 Welding.22
5.5 Inspection.23
5.6 Surface preparation and protection .23
5.7 Attachments.23
5.8 Aircraft warning lighting.24
5.9 Platforms and ladders.24
6 Mechanical details — Enclosed-flame flare.25
6.1 Combustion chamber.25
6.2 Burners.25
6.3 Burner piping .26
6.4 Pilots.26
6.5 Wind fence .27
6.6 Radiation shielding .27
Annex A (informative) Flare equipment overview.28
A.1 Types of flares and components .28
A.2 Flare burner.42
A.3 Pilots.53
A.4 Ignition equipment .58
A.5 Flame-detection equipment.68
A.6 Verification test.72
A.7 Purge-gas conservation seals .73
A.8 Knockout drums and liquid seals.75
A.9 Blowers and drivers .76
A.10 Blower staging and control equipment.80
A.11 Pressure-staging equipment.84
A.12 Flow and pressure sensing equipment.88
Annex B (informative) Components of multi-burner staged flare equipment .90
ISO 25457:2008(E)
B.1 Burners .90
B.2 Pilots .91
B.3 Ignition equipment.91
B.4 Flame-detection equipment.91
B.5 Buoyancy and velocity seals.91
B.6 Manifolds .92
B.7 Operations .92
B.8 Maintenance .92
B.9 Troubleshooting.92
Annex C (informative) Enclosed-flame flares.93
C.1 Purpose.93
C.2 General description .93
C.3 Operating considerations for enclosed-flame flares .100
C.4 Maintenance .104
C.5 Troubleshooting.105
Annex D (informative) Instructions for flare data sheets .106
D.1 Introduction.106
D.2 General information forms — Instructions .108
D.3 Elevated-flare forms — Instructions.114
D.4 Enclosed-flare forms — Instructions.116
Annex E (informative) Flare data sheets.118
E.1 SI units .118
E.2 USC units.118
Bibliography .119

iv © ISO 2008 – All rights reserved

ISO 25457:2008(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.
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 25457 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 25457:2008(E)
Introduction
Users of this International Standard should be aware that further or differing requirements may be needed for
individual applications. This International Standard is not intended to inhibit a vendor from offering, or the
purchaser from accepting, alternative equipment or engineering solutions for the individual application. This
may be particularly applicable where there is innovative or developing technology. Where an alternative is
offered, the vendor should identify any variations from this International Standard and provide details.
In International Standards, the SI system of units is used. Where practical in this International Standard, US
Customary units are included in brackets for information.
A bullet (z) at the beginning of a clause or subclause indicates that either a decision is required or further
information is to be provided by the purchaser. This information should be indicated on data sheets (see
examples in Annex E) or stated in the enquiry or purchase order.

vi © ISO 2008 – All rights reserved

INTERNATIONAL STANDARD ISO 25457:2008(E)

Petroleum, petrochemical and natural gas industries — Flare
details for general refinery and petrochemical service
1 Scope
This International Standard specifies requirements and provides guidance for the selection, design,
specification, operation and maintenance of flares and related combustion and mechanical components used
in pressure-relieving and vapour-depressurizing systems for petroleum, petrochemical and natural gas
industries.
Although this International Standard is primarily intended for new flares and related equipment, it is also
possible to use it to evaluate existing flare facilities.
Annexes A, B and C provide further guidance and best practices for the selection, specification and
mechanical details for flares and on the design, operation and maintenance of flare combustion and related
equipment.
Annex D explains how to use the data sheets provided in Annex E; it is intended that these data sheets be
used to communicate and record design information.
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 1461, Hot dip galvanized coatings on fabricated iron and steel articles — Specifications and test methods
ISO 2408:2004, Steel wire ropes for general purposes — Minimum requirements
ISO 8501-1:2007, Preparation of steel substrates before application of paints and related products — Visual
assessment of surface cleanliness — Part 1: Rust grades and preparation grades of uncoated steel
substrates and of steel substrates after overall removal of previous coatings
ISO 10684, Fasteners — Hot dip galvanized coatings
ISO 13705:2006, Petroleum, petrochemical and natural gas industries — Fired heaters for general refinery
service
ISO 15156 (all parts), Petroleum and natural gas industries — Materials for use in H S-containing
environments in oil and gas production
ISO 23251, Petroleum, petrochemical and natural gas industries — Pressure-relieving and depressuring
systems
EN 1092-1:2007, Flanges and their joints — Circular flanges for pipes, valves, fittings and accessories, PN
designated — Part 1: Steel flanges
ISO 25457:2008(E)
1)
EN 10264-2:2002 , Steel wire and wire products — Steel wire for ropes — Part 2: Cold drawn non alloy steel
wire for ropes for general applications
EN 12385-10, Steel wire ropes — Safety — Part 10: Spiral ropes for general structural applications
2)
API RP 2A WSD:2000 , Recommended Practice for Planning, Designing and Constructing Fixed Offshore
Platforms — Working Stress Design
3)
ASME B16.5-2003 , Pipe Flanges and Flanged Fittings
ASME STS-1, Steel Stacks
4)
ASTM A 123/A123M , Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel
Products
ASTM A 143/A143M, Standard Practice for Safeguarding Against Embrittlement of Hot-Dip Galvanized
Structural Steel Products and Procedure for Detecting Embrittlement
ASTM A 153/A153M, Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware
ASTM A 384/A384M, Standard Practice for Safeguarding Against Warpage and Distortion During Hot-Dip
Galvanizing of Steel Assemblies
ASTM A 385, Standard Practice for Providing High-Quality Zinc Coatings (Hot-Dip)
ASTM A 475-03, Standard Specification for Zinc-Coated Steel Wire Strand
ASTM A 586-04, Standard Specification for Zinc-Coated Parallel and Helical Steel Wire Structural Strand
ASTM B 633, Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel
5)
NACE MR0103:07 , Materials Resistant to Sulfide Stress Cracking in Corrosive Petroleum Refining
Environments
6)
SSPC SP 6/NACE No. 3 , Joint Surface Preparation Standard: Commercial Blast Cleaning
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
air seal
purge reduction device
device used to minimize or eliminate the intrusion of air back into the riser from the exit
EXAMPLE Buoyancy seal, orifice seal, velocity seal.

1) Comité Européen de Normalisation, 36, rue de Stassart, B-1050 Brussels, Belgium.
2) American Petroleum Institute, 1220 L Street, N.W., Washington, D.C. 20005, USA.
3) American Society of Mechanical Engineers, 3 Park Avenue, New York, NY 10017, USA.
4) American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, USA.
5) NACE International, P.O. Box 218340, Houston, TX 77218-8340, USA.
6) The Society for Protective Coatings, 40 24th Street, 6th Floor, Pittsburgh, PA 15222-4643, USA.
2 © ISO 2008 – All rights reserved

ISO 25457:2008(E)
3.2
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.
3.3
back blowing
procedure by which the dry air seal drain line is blown back from the base of the drain into the buoyancy seal
to ensure the line is clear
3.4
blowoff
loss of a stable flame where the flame is lifted above the burner, which occurs when the fuel velocity exceeds
the flame velocity
3.5
buoyancy seal
diffusion seal
dry vapour seal that minimizes the required purge gas needed to protect from air infiltration
NOTE The buoyancy seal functions by trapping a volume of light gas in an internal inverted compartment that
prevents air from displacing buoyant light gas in the flare.
3.6
burnback
internal burning within the burner
NOTE Burnback can result from air backing down the flare burner at purge or low flaring rates.
3.7
burn-pit flare
open excavation, normally equipped with a horizontal flare burner that can handle liquid as well as gaseous
hydrocarbons
3.8
burning velocity
flame velocity
speed at which a flame front travels into an unburned combustible mixture
3.9
coanda flare
flare burner that is designed to employ the aerodynamic effect where moving fluids follow a curved or inclined
surface over which they flow
NOTE Flares of this type generally use steam or pressure to achieve smokeless performance.
3.10
combustion air
air required to combust the flare gases
3.11
combustion efficiency
percentage of the combustible fluid totally oxidized in the burner
NOTE In the case of hydrocarbons, combustion efficiency is the mass percent of carbon in the original fluid that
oxidizes completely to CO .
ISO 25457:2008(E)
3.12
condensable gas
vapour that can condense at the temperature and pressure expected in a flare header during or after a flaring
event
3.13
cryogenic service
systems that may be called upon to handle waste gas below −40 °C (−40 °F)
3.14
derrick support
support system for the elevated flare riser, normally used for very tall flares or when plot space is limited
NOTE Various derrick-supported arrangements are available: a fixed system has its riser permanently supported to
the derrick; a demountable derrick has multiple riser sections that are designed such that they can be lowered and
removed to permit lowering of the flare burner to grade; a demountable derrick with one fixed riser provides for a single-
piece design that can be lowered to grade as a single component.
3.15
design flare capacity
maximum design flow to the flare
NOTE The design flare capacity is normally expressed in kilograms per hour (pounds per hour) of a specific
composition, temperature, and pressure.
3.16
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 CO .
3.17
detached stable flame
flame that is not in contact with the flare burner itself but burns with a stable flame-front in the vicinity of the
flare burner
3.18
direct ignition
ignition of a flare flame by a high-energy source rather than by a pilot flame
3.19
dispersion
scattering of the products of combustion over a wide area to reduce ground-level concentrations of the
combustion products
3.20
enclosed flare
flare enclosure with one or more burners arranged in such a manner that the flame is not directly visible
3.21
endothermic flare
flare that utilizes outside energy, usually assist or enrichment gas, to maintain the combustion reaction
3.22
enrichment
process of adding assist gas to the relief gas
4 © ISO 2008 – All rights reserved

ISO 25457:2008(E)
3.23
elevated flare
flare where the burner is raised high above ground level to reduce radiation intensity and to aid in dispersion
3.24
excess air
air provided to a flame in excess of stoichiometric requirements
3.25
exit velocity
velocity at which the design flare capacity exits the burner
NOTE The exit velocity is usually expressed as metres per second (feet per second) or as a fraction of the Mach
number for the fluid.
3.26
flame detection system
flame monitor
system that verifies a flame is present
3.27
flame front generator
device used for lighting a pilot by means of a flame front
NOTE A combustible gas-air mixture is created and allowed to fill an ignition line connecting the flame front generator
and the pilot. Igniting the mixture allows the flame front to travel through the ignition line to the pilot.
3.28
flame retention device
physical device meant to prevent flame blowoff from a flare burner
3.29
flare
device or system used to safely dispose of relief fluids in an environmentally compliant manner through the
use of combustion
3.30
flare burner
flare tip
part of the flare where fuel and air are mixed at velocities, turbulence and concentration required to establish
and maintain proper ignition and stable combustion
NOTE The name “flare burner” is considered more appropriate than “flare tip” given the engineered nature of design
and inclusion of measures for flame stabilization, most often of proprietary design.
3.31
flare header
piping system that collects and delivers the relief gases to the flare
3.32
flare stack
flare boom
flare tower
mechanical device upon which an elevated flare burner is mounted
3.33
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
ISO 25457:2008(E)
3.34
ground flare
non-elevated flare
NOTE A ground flare is normally an enclosed flare but may also refer to a ground multi-burner flare or a burnpit.
3.35
guyed flare
elevated flare with the riser kept from overturning using cables
NOTE A typical guyed flare is shown in Figure A.2.
3.36
heat release
total heat liberated by combustion of the relief gases based on the lower heating value
NOTE The heat release is expressed in kilowatts (British thermal units per hour).
3.37
higher heating value
HHV
gross heating value
total heat obtained from the combustion of a specified fuel at 16 °C (60 °F)
NOTE The higher heating value includes the latent heat of vaporization of water in the combustion products
(including the water already present in the flare gas).
3.38
lower heating value
LHV
net heating value
higher heating value minus the latent heat of vaporization of the water (both the water formed in the
combustion products and that already present in the flare gas)
3.39
knockout drum
vessel in the flare header designed to remove and store condensed and entrained liquids from the relief gases
3.40
liquid seal
water seal
device that directs the flow of relief gases through a liquid (normally water) on their path to the flare burner,
used to protect the flare header from air infiltration or flashback, to divert flow or to create backpressure for the
flare header
3.41
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
3.42
manifold
device for the collection and/or distribution of a fluid to or from multiple flow paths
3.43
multi-burner flare
group of burners designed to burn all or a portion of the design flow capacity, which are often arranged in
stages to facilitate better burning
NOTE Multi-burner flares are capable of smokeless combustion at high flow rates with lower radiation levels.
6 © ISO 2008 – All rights reserved

ISO 25457:2008(E)
3.44
muffler
device that mitigates noise
3.45
multi-point flare
single flare burner with multiple separate exits
3.46
opacity
degree of non-transparency to rays of light
NOTE The opacity is quantified by the Ringelmann number.
3.47
pilot
small, continuously operating burner that provides ignition energy to ignite and/or stabilize combustion of the
flared gases
3.48
pin-actuated 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 plug in place; upon buckling or breaking of the pin, the piston or plug
instantly moves to its full open position
3.49
pressure design code
recognized pressure equipment standard specified or agreed by the purchaser
EXAMPLES ASME VIII or EN 13445 (all parts) for pressure vessels; ISO 15649, EN 13480 (all parts) or
ANSI/ASME B31.3 for piping.
3.50
purge gas
fuel gas or non-condensable inert gas added to the flare header to mitigate air ingress and burnback
3.51
radiation intensity
radiant heat transfer rate from the flare flame
NOTE The rate is usually considered at grade level.
3.52
relief gas
waste gas
flared gas
waste vapour
gas or vapour vented or relieved into a flare header for conveyance to a flare
3.53
Ringelmann number
visually comparative scale used to define levels of opacity, where clear is 0, black is 5 and 1 through 4 are
increasing levels of gray as used in describing smoke from combustion of hydrocarbons
NOTE The Ringelmann number is often used to describe the intensity of smoke.
3.54
riser
pipe or other conduit that conveys the relief gas, combustion air, etc. to the flare burner of an elevated flare
ISO 25457:2008(E)
3.55
smokeless capacity
range of flow to a flare burner that can be burned without smoke
NOTE The term without smoke can be quantified using the Ringelmann number (3.53).
3.56
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
3.57
stoichiometric air
chemically correct quantity of air, i.e., a quantity capable of perfect combustion with no unused fuel or air
3.58
structural design code
recognized structural standard specified or agreed by the purchaser
EXAMPLES AISC S302, ASCE 7, AWS D1.1/D1.1M.
3.59
supplemental gas
fuel gas burned external to a flare burner in order to facilitate the burning of low-heating-value relief gas
3.60
thermocouples
temperature-measuring devices used to detect whether the pilot is in operation
3.61
velocity seal
orifice seal
dry vapour seal that minimizes the required purge gas needed to protect against air infiltration into the flare
burner exit
3.62
wind fence
structure surrounding a flare at ground level to modify the effect of crosswinds on the combustion process, to
prevent unauthorized access, limit the radiation to the surroundings and/or make the flame non-visible to the
neighbors
See Figure A.7.
3.63
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 Design
4.1 Introduction
A flare is a critical mechanical component of a complete system design intended for the safe, reliable and
efficient discharge and combustion of hydrocarbons from pressure-relieving and vapour-depressurizing
systems. Being critical to the safety of an operating plant, a flare shall be continuously available with high
reliability and capable of its intended performance through all operating-plant emergency conditions, including
a site-wide general power failure. The flare and related mechanical components shall be designed to operate
and properly perform for the specified service conditions for a minimum of five years without the need for an
outage of the operating facility.
8 © ISO 2008 – All rights reserved

ISO 25457:2008(E)
Although flare availability and reliability are strongly dependent on the design of the related mechanical
equipment, proper training, installation, commissioning, and operating and maintenance practices are critical
to ensuring the safety of plant personnel, the operating facility and the general public. Management systems
shall be in place which
a) clearly document the intended capacity, performance and operational limitations of the pressure-relieving
and vapour-depressurizing systems and flare,
b) provide operating procedures and operator training, and
c) provide planned and routine maintenance of components critical to the safety and operating goals.
The high-level safety and operating goals of a flare are summarized as follows:
⎯ to provide safe, reliable and efficient discharge and combustion of hydrocarbons with a high combustion
efficiency;
⎯ to ensure that the discharged hydrocarbons burn with stable combustion over the entire defined operating
range;
⎯ to ensure a continuity of the flare flame under severe weather conditions;
⎯ to ensure that ground level concentrations of specified compounds do not exceed environmental limits;
⎯ to ensure that the back pressure does not exceed the maximum allowable;
⎯ to ensure that velocity throughout the flare piping and the flare burner does not exceed the maximum
specified;
⎯ to ensure that the opacity limit at the smokeless flow rate range does not exceed that defined;
⎯ to ensure that the flare radiation intensity does not exceed the maximum allowable;
⎯ to ensure that noise levels do not to exceed the maximum permissible.
For new designs, the development of a design can be advanced by using the guidance and examples of good
engineering practice that are identified in this document.
A flare design basis is developed in consideration of the performance expectations, the functional
requirements and mechanical details required to achieve the safety and operating goals established for each
application. In 4.2 to 4.11, objectives are identified that establish the safety and operating goals together with
the functional requirements that enable the objectives to be achieved. Clauses 5 and 6 provide functional
requirements more specific to the arrangement and mechanical details of design.
The functional requirements in this International Standard are supported by the technical guidance provided in
Annexes A, B and C. The technical guidance provided in the informative annexes addresses alternative
designs or techniques and provides good practices on the basis of which, through sound engineering
judgment, the practitioner can make appropriate design decisions and selections.
The finalized basis of design shall be recorded on data sheets (e.g., those provided in Annex E) in order to
properly communicate requirements and preserve design information. Annex D provides instructions for
completing the flare data sheets in Annex E.
ISO 25457:2008(E)
4.2 System design
4.2.1 Objective
The objective is to identify fundamental requirements, specific design criteria and information consistent with
the delivery of the critical safety and operating goals of the specific flare under design.
4.2.2 Functional requirements
Fundamental system design requirements are established primarily in accordance with ISO 23251, from which
the following aspects shall be defined on the data sheets:
⎯ design-flow cases from the pressure-relieving and vapour-depressurizing system, including maximum
continuous case and maximum intermittent case;
⎯ flare staging requirement and method;
⎯ allowable flare-burner exit velocity;
⎯ system hydraulics with respect to allowable pressure drop, static pressure and diameter of the flare;
⎯ environmental performance requirements related to smokeless capacity, opacity limits and permissible
noise limits;
⎯ operating performance, such as peak radiation intensity at grade;
⎯ functional description of the intended system operation;
⎯ selection of major system components that can be integral to the flare, such as a knockout drum, liquid
seal, buoyancy seal, purge-reduction device, etc.;
⎯ site and ambient design conditions; and
⎯ utilities available.
4.3 Process definition
4.3.1 Objective
To provide a clear process definition for the flare ensuring all capacity and process stream characterization
information relevant to the system performance and mechanical design considerations are provided.
4.3.2 Functional requirements
In addition to the functional system design requirements as defined in 4.2, complete composition, range of
temperature and hydrocarbon characterization information of the process stream(s) shall be provided. In
consideration that various operating or pressure-relieving cases can individually define various aspects of the
design, i.e. hydraulic capacity, ground-level radiation, aeration requirement for the defined smokeless capacity,
requirement for dilution gas, design metal temperature, thermal expansion, etc., multiple cases together with
expected duration and frequency should be provided to permit the designer to determine which cases control
the design.
The potential for liquid introduction or the condensation of hydrocarbons, or the formation of hydrates in the
flare header or flare riser, which can be carried to the combustion zone, shall be considered by both the user
and the flare designer. Hydrocarbon droplets entrained in the gas stream that are carried into the flame
usually burn incompletely, can produce burning liquid droplets, form soot and decrease the smokeless
capacity of the flare. The maximum liquid-droplet size that can enter the combustion zone and can be handled
10 © ISO 2008 – All rights reserved

ISO 25457:2008(E)
within achievable measures for smokeless control depends on the burner design. ISO 23251 gives further
guidance on the maximum droplet size for different types of burners.
Flare-system knockout-drum design and heat-tracing systems downstream of the knockout drum shall be
designed in consideration of these potential problems.
Process definition is primarily established in ISO 23251.
4.4 Types of flares
4.4.1 Objective
To identify and select the m
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