EN ISO 23251:2007

SLOVENSKI STANDARD
01-oktober-2007
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Petroleum, petrochemical and natural gas industries - Pressure-relieving and
depressuring systems (ISO 23251:2006)
Erdöl-, petrochemische und Erdgasindustrie - Druckentlastungs- und
Druckausgleichssysteme (ISO 23251:2006)
Industries du pétrole, de la pétrochimie et du gaz naturel - Systemes de dépressurisation
et de protection contre les surpressions (ISO 23251:2006)
Ta slovenski standard je istoveten z: EN ISO 23251:2007
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 23251
NORME EUROPÉENNE
EUROPÄISCHE NORM
July 2007
ICS 75.180.20
English Version
Petroleum, petrochemical and natural gas industries - Pressure-
relieving and depressuring systems (ISO 23251:2006)
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
surpressions (ISO 23251:2006) 23251:2006)
This European Standard was approved by CEN on 12 July 2007.
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
© 2007 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 23251:2007: E
worldwide for CEN national Members.

Foreword
The text of ISO 23251:2006 has been prepared by Technical Committee ISO/TC 67 "Materials,
equipment and offshore structures for petroleum and natural gas industries” of the International
Organization for Standardization (ISO) and has been taken over as EN ISO 23251:2007 by
Technical Committee CEN/TC 12 "Materials, equipment and offshore structures for petroleum,
petrochemical and natural gas industries", the secretariat of which is held by 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 January 2008, and conflicting national
standards shall be withdrawn at the latest by January 2008.

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 United
Kingdom.
Endorsement notice
The text of ISO 23251:2006 has been approved by CEN as EN ISO 23251:2007 without any
modifications.
INTERNATIONAL ISO
STANDARD 23251
First edition
2006-08-15
Corrected version
2006-10-01
Petroleum, petrochemical and natural gas
industries — Pressure-relieving and
depressuring systems
Industries du pétrole, de la pétrochimie et du gaz naturel — Systèmes
de dépressurisation et de protection contre les surpressions

Reference number
ISO 23251:2006(E)
©
ISO 2006
ISO 23251:2006(E)
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ii © ISO 2006 – All rights reserved

ISO 23251:2006(E)
Contents Page
Foreword. v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 1
4 Causes of overpressure. 10
4.1 General. 10
4.2 Overpressure protection philosophy. 10
4.3 Potentials for overpressure . 11
4.4 Recommended minimum relief system design content . 17
4.5 List of items required in flare-header calculation documentation . 20
4.6 Guidance on vacuum relief. 20
5 Determination of individual relieving rates. 22
5.1 Principal sources of overpressure. 22
5.2 Sources of overpressure . 24
5.3 Effects of pressure, temperature, and composition . 24
5.4 Effect of operator response. 24
5.5 Closed outlets . 24
5.6 Cooling or reflux failure . 25
5.7 Absorbent flow failure. 26
5.8 Accumulation of non-condensables. 26
5.9 Entrance of volatile material into the system . 26
5.10 Failure of process stream automatic controls. 27
5.11 Abnormal process heat input . 29
5.12 Internal explosion (excluding detonation) .30
5.13 Chemical reaction. 30
5.14 Hydraulic expansion. 31
5.15 External pool fires. 36
5.16 Jet fires . 51
5.17 Opening manual valves. 51
5.18 Electric power failure. 51
5.19 Heat-transfer equipment failure . 52
5.20 Vapour depressuring. 55
5.21 Special considerations for individual pressure-relief devices . 63
5.22 Dynamic simulation. 64
6 Selection of disposal systems . 65
6.1 General. 65
6.2 Fluid properties that influence design. 65
6.3 Atmospheric discharge. 66
6.4 Disposal by flaring. 76
6.5 Disposal to a lower-pressure system . 95
6.6 Disposal of liquids and condensable vapours . 96
7 Disposal systems. 97
7.1 Definition of system design load . 97
7.2 System arrangement . 100
7.3 Design of disposal system components. 102
7.4 Flare gas recovery systems. 137
ISO 23251:2006(E)
Annex A (informative) Determination of fire relief requirements. 142
Annex B (informative) Special system design considerations . 146
Annex C (informative) Sample calculations for sizing a subsonic flare stack. 149
Annex D (informative) Typical details and sketches. 166
Annex E (informative) High integrity protection systems (HIPS) . 169
Bibliography . 176

iv © ISO 2006 – All rights reserved

ISO 23251:2006(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 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.
This corrected version of ISO 23251:2006 incorporates corrections to Table 4, column 2, second row under
the header, and the five rows of data in column 3.
ISO 23251:2006(E)
Introduction
This International Standard is based on the draft 5th edition of API RP 521, with the intent that the 6th edition
of API RP 521 will be identical to this International Standard.
The portions of this International Standard dealing with flares and flare systems are an adjunct to
[10]
API Std 537 , 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, API has developed a
set of flare data sheets, which can be found in of API Std 537, Appendix A. The use of these data sheets is
both recommended and encouraged as a concise, uniform means of recording and communicating design
information.
vi © ISO 2006 – All rights reserved

INTERNATIONAL STANDARD ISO 23251:2006(E)

Petroleum, petrochemical and natural gas industries —
Pressure-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-I 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 7.3.1.
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 RP 520-I:2000, 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 23251:2006(E)
3.2
administrative controls
procedures intended to ensure that personnel actions do not compromise the overpressure protection of the
equipment
3.3
assist gas
combustible 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
3.4
atmospheric discharge
release of vapours and gases from pressure-relieving and depressuring devices to the atmosphere
3.5
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.
3.6
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
3.7
blowdown
depressurization of a plant or part of a plant, and equipment
NOTE Not to be confused with the difference between the set pressure and the closing pressure of a pressure-relief
valve.
3.8
blow-off
loss of a stable flame where the flame is lifted above the burner, occurring if the fuel velocity exceeds the
flame velocity
3.9
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
3.10
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
3.11
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
3.12
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.
2 © ISO 2006 – All rights reserved

ISO 23251:2006(E)
3.13
burnback
internal burning within the flare tip
NOTE Burnback can result from air backing down the flare burner at purge or low flaring rates.
3.14
burning velocity
flame velocity
speed at which a flame front travels into an unburned combustible mixture
3.15
burn-pit flare
open excavation, normally equipped with a horizontal flare burner that can handle liquid as well as vapour
hydrocarbons
3.16
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.
3.17
closed disposal system
disposal system capable of containing pressures that are different from atmospheric pressure
3.18
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.
3.19
combustion air
air required to combust the flare gases
3.20
conventional pressure-relief valve
spring-loaded pressure-relief valve whose operational characteristics are directly affected by changes in the
back pressure
3.21
corrected hydrotest pressure
hydrostatic test pressure multiplied by the ratio of stress value at design temperature to the stress value at test
temperature
NOTE See 4.3.2.
3.22
deflagration
explosion in which the flame-front of a combustible medium is advancing at less than the speed of sound
cf. detonation (3.25)
ISO 23251:2006(E)
3.23
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).
3.24
destruction efficiency
mass fraction of the fluid vapour that can be oxidized or partially oxidized
NOTE For a hydrocarbon, this is the mass fraction of carbon in the fluid vapour that oxidizes to CO or CO .
3.25
detonation
explosion in which the flame-front of a combustible medium is advancing at or above the speed of sound
cf. deflagration (3.22)
3.26
dispersion
dilution of a vent stream or products of combustion as the fluids move through the atmosphere
3.27
elevated flare
flare where the burner is raised high above ground level to reduce radiation intensity and to aid in dispersion
3.28
enclosed flare
enclosure with one or more burners arranged in such a manner that the flame is not directly visible
3.29
enrichment
process of adding assist gas to the relief gas
3.30
flame-retention device
device used to prevent flame blow off from a flare burner
3.31
flare
device or system used to safely dispose of relief gases in an environmentally compliant manner through the
use of combustion
3.32
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
3.33
flare header
piping system that collects and delivers the relief gases to the flare
4 © ISO 2006 – All rights reserved

ISO 23251:2006(E)
3.34
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
3.35
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.
3.36
heat release
total heat liberated by combustion of the relief gases based on the lower heating value
3.37
huddling chamber
annular chamber located downstream of the seat of a pressure-relief valve, which assists the valve to lift
3.38
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
3.39
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.
3.40
knockout drum
vessel in the effluent handling system designed to remove and store liquids
3.41
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
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.
3.42
lift
actual travel of the disc from the closed position when a valve is relieving
3.43
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
3.44
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
ISO 23251:2006(E)
3.45
manifold
piping system for the collection and/or distribution of a fluid to or from multiple flow paths
3.46
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.
3.47
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 (3.23)
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.
3.48
non-condensable gas
gas or vapour that remains in the gaseous state at the temperature and pressure expected
3.49
operating pressure
pressure the process system experiences during normal operation, including normal variations
3.50
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.
3.51
pilot burner
small, continuously operating burner that provides ignition energy to light the flared gases
3.52
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)
3.53
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
6 © ISO 2006 – All rights reserved

ISO 23251:2006(E)
3.54
pool fire
burning pool of liquid
3.55
pressure-design code
standard to which the equipment is designed and constructed
[20]
EXAMPLE ASME Section VIII, Division 1 .
3.56
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.
3.57
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 (3.74)
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.
3.58
purge gas
fuel gas or non-condensable inert gas added to the flare header to mitigate air ingress and burnback
3.59
quenching
cooling of a fluid by mixing it with another fluid of a lower temperature
3.60
radiation intensity
local radiant heat transfer rate from the flare flame, usually considered at grade level
3.61
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.
3.62
relief gas
flared gas
waste gas
waste vapour
gas or vapour vented or relieved into a flare header for conveyance to a flare
3.63
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.
ISO 23251:2006(E)
3.64
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.
3.65
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.
3.66
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
3.67
safety-integrity level
SIL
discrete integrity level of a safety instrumented function in a safety instrumented system
NOTE SILs are categorized in terms of probability of failure; see Annex E.
3.68
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
3.69
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 3.56.
3.70
set pressure
inlet gauge pressure at which a pressure-relief device is set to open under service conditions
3.71
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
8 © ISO 2006 – All rights reserved

ISO 23251:2006(E)
3.72
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
3.73
stoichiometric air
chemically correct ratio of fuel to air capable of perfect combustion with no unused fuel or air
3.74
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 (3.57)
NOTE These tanks or vessels are often located in tank farms.
3.75
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.
3.76
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.
3.77
velocity seal
dry vapour seal that minimizes the required purge gas needed to protect against air infiltration into the flare
burner exit
3.78
vent header
piping system that collects and delivers the relief gases to the vent stack
3.79
vent stack
elevated vertical termination of a disposal system that discharges vapours into the atmosphere without
combustion or conversion of the relieved fluid
3.80
vessel
container or structural envelope in which materials are processed, treated or stored
EXAMPLES Pressure vessels, reactor vessels and storage vessels (tanks).
3.81
windshield
device used to protect the outside of a flare burner from direct flame impingement
NOTE The windshield is so named because external flame impingement occurs on the downwind side of an elevated
flare burner.
ISO 23251:2006(E)
4 Causes of overpressure
4.1 General
Clause 4 discusses the principal causes of overpressure and offers guidance in plant design to minimize the
effects of these causes. 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.
The application of the principles outlined in Clause 4 are 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 Double jeopardy
The causes of overpressure are considered to be unrelated if no process or mechanical or electrical linkages
exist among them, or if the length of time that elapses between possible successive occurrences of these
causes is sufficient to make their classification unrelated. The simultaneous occurrence of two or more
unrelated causes of overpressure (also known as double or multiple jeopardy) is not a basis for design.
Examples of double-jeopardy scenarios are fire exposure simultaneous with exchanger internal tube failure,
fire exposure simultaneous with failure of administrative controls to drain and depressure isolated equipment,
or operator error that leads to a blocked outlet coincident with a power failure. On the other hand, instrument
air failure during fire exposure may be considered single jeopardy if the fire exposure causes local air line
failures.
This International Standard describes single-jeopardy scenarios that should be considered as a basis for
design. The user may choose to go beyond these practices and assess multiple jeopardy scenarios. Since
such assessments are outside the basis for design, the user is not required to meet accumulations allowed by
the pressure-design code for these scenarios. Acceptance criteria are the sole responsibility of the user.
4.2.2 Latent failures
Latent failures should normally be considered as an existing condition and not as a cause of overpressure
when assessing whether a scenario is single or double jeopardy. For example, latent failures can exist in
instrumentation that prevents it from functioning favourably during an overpressure condition. It is not double
jeopardy to assume the absence of beneficial instrumentation response in combination with an unrelated
overpressure cause. Likewise, it is not double jeopardy to assume a latent failure of a check valve allowing
reverse flow during a pump failure.
4.2.3 Operator error
Operator error is considered a potential source of overpressure.
4.2.4 Role of instrumentation in overpressure protection
Fail-safe devices, automatic start-up equipment and other conventional instrumentation should not be a
substitute for properly sized pressure-relieving devices as protection against single-jeopardy overpressure
scenarios. There can be circumstances, however, where the use of pressure-relief devices is impractical and
10 © ISO 2006 – All rights reserved

ISO 23251:2006(E)
reliance on instrumented safeguards is needed. Where this is the case, if permitted by local regulations, a
pressure-relieving device might not be required.
[129]
NOTE See ASME Code Case 2211 .
The design shall comply with the local regulations and the owner’s risk tolerance criteria, whichever is more
restrictive. If these risk tolerance criteria are not available, then, as a minimum, the overall system
performance including instrumented safeguards should provide safety-integrity-level 3 (SIL-3) performance.
Guidance on the application of safety instrumented systems is given in Annex E.
Although favourable response of conventional instrumentation should not be assumed when sizing individual
process-equipment pressure relief, in the design of some components of a relieving system, such as the
blowdown header, flare, and flare tip, favourable response of some instrument systems can be assumed. The
decision to base the design of such systems on excluded or reduced specific loads due to the favourable
response of instrument systems should consider the number and reliability of applicable instrument systems.
See 7.1 for more details on sizing disposal systems.
4.3 Potentials for overpressure
4.3.1 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.
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. The
principal causes of overpressure listed in 4.3.2 through 4.3.15 are guides to generally accepted practices.
Annex B provides guidance on the use of a common relief device to protect multiple pieces of equipment from
overpressure.
4.3.2 Closed outlets on vessels
The inadvertent closure of a manual block valve on the outlet of a pressure vessel while the equipment is on
stream can expose the vessel to a pressure that exceeds the maximum allowable working pressure. If closure
of an outlet-block valve can result in overpressure, a pressure-relief device is required unless administrative
controls are in place. Every valve should be considered as being subject to inadvertent operation. In general,
the omission of block valves interposed in vessels in a series can simplify pressure-relieving requirements. If
the pressure resulting from the failure of administrative controls can exceed the corrected hydrotest pressure
(see 3.21), reliance on administrative controls as the sole means to prevent overpressure might not be
appropriate. 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]
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
stress at design temperature. At ambient temperature, the allowable stress of ASTM A 515 Grade 70 carbon
steel is 138 MPa (20 000 psi). If the pressure-design code requires the hydrostatic test be performed at 130 %
of the design pressure, then the hydrostatic test pressure is as follows:
ISO 23251:2006(E)
In SI units:
517 × (138/130) × 1,3 = 713 kPa (gauge)
In USC units:
75 × (20/18,8) × 1,3 = 103,7 psig
The uncorrected hydrotest gauge pressure is 517 × 1,3 = 672 kPa (75 × 1,3 = 97,5 psi). In this example,
reliance on administrative controls as the sole means of overpressure protection might not be appropriate if
the gauge pressure caused by closure of the outlet valve exceeds 672 kPa (97,5 psi). This assumes the
overpressure occurs while the vessel is at design temperature. Within stage 1 and stage 2 creep, short-
duration pressure exceedances up to 1,5 times the design pressure at design temperature should not re
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