Pressure Relief Systems

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Process Safety Guide:
GBHE-PSG-008
Pressure Relief
Systems
BACKGROUND TO RELIEF SYSTEM
DESIGN Vol.1 of 6
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Information contained in this publication or as otherwise supplied to Users is
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Process Safety Guides: Process Safety Relief
INDEX
VOL. I BACKGROUND TO RELIEF SYSTEM DESIGN
(This includes principles of pressure relief and use of this Guide, alternatives,
statutory and mandatory requirements, and reporting).
VOL. II CAUSES OF RELIEF SITUATIONS
VOL. III CALCULATION OF REQUIRED RELIEF RATE
VOL. IV SELECTION, SIZING, AND INSTALLATION OF PRESSURE
RELIEF DEVICES
(This includes the pressure setting in relation to the design pressure of the
protected equipment).
VOL. V DISCHARGE AND DISPOSAL SYSTEM DESIGN
VOL. VI REFERENCE SECTIONS
DOCUMENTS REFERRED TO IN THIS PROCESS GUIDE
It is emphasized that this document is only a Guide, describing good practice at
the date of issue, and is not itself mandatory (although some mandatory
instructions are quoted). When used in this Guide, the words "must", "shall", and
"should" have no legal force and are not mandatory, except where they are part
of a quoted mandatory instruction from another source.
The word" must" has not been used, except when part of a quotation.
"Shall" is a strong recommendation of GBHE based upon experience or upon the
position adopted by recognized authorities, and the engineer may quote
compliance with this guide only when that recommendation has been followed.
"Should" is a recommendation based upon the judgment of experienced people
but recognizes that some discretion may be appropriate.
Note:
This Guide includes references to and quotations from external and British
Standards. The reader should always check if the Standards have been updated
since the last issue of this Guide.

Volume 1: Background To Relief System Design
SECTION 1: PURPOSE AND SCOPE OF GUIDE
1 PURPOSE
2 SCOPE
SECTION 2 : PRINCIPL.ES OF PRESSURE RELIEF AND USE OF GUIDE
3 BASIC AIMS
4 GENERAL DESIGN PHILOSOPHY
5 KEY PRINCIPLES AND DESIGN SEQUENCE
6 ELATED DESIGN ACTIVITIES "
SECTION 3: INHERENT SAFETY AND ALTERNATIVES TO PRESSURE RELIEF
7 BASIC PHILOSOPHY
8 ELIMINATION OF THE SOURCE OF OVERPRESSURE OR UNDER PRESSURE
9 REDUCTION OF THE HAZARD
10 CONTAINMENT OF THE PRESSURE THAT MIGHT OCCUR
11 ALTERNATIVES TO PRESSURE RELIEF SYSTEMS
12 MINIMIZING THE RELIEF SYSTEM REQUIRED
SECTION 4: STATUTORY AND OTHER GOVERNMENT APPROVED REQUIREMENTS.
PREFACE
13 INTRODUCTION
14 STATUTORY POSITION IN THE UNITED KINGDOM

15 CURRENT REQUIREMENTS
15.1 General
15.2 Specific
15.3 Design of Pressure Relieving Systems
15.4 Relief Devices and Ancillary Equipment
16 APPROVAL OF DESIGNS RELEVANT TO PRESSURIZED SYSTEMS
16.1 United Kingdom
16.2 Other European Countries
16.3 Canada.
SECTION 5 : MANDATORY REQUIREMENTS AND RECOMMENDED PRACTICE
17 INTRODUCTION
18 GENERAL RULES
18.1 Basic Requirements of EPI PRE 6.1
18.2 Accepted Practice from Other Sources
19 REQUIREMENTS OF EDP.lNS.00.14
19.1 Registration of Protective Devices
19.2 Inspection and Maintenance
20 RECOMMENDED PRACTICE
20.1 Use of External Standards in the United Kingdom
20.2 Steam Boilers and Associated Equipment
20.3 Air Receivers and Associated Equipment
20.4 Process Vessels and Plant
20.5 Isolation of Relief Systems
20.6 Relief Devices
20.7 Installation
21 ENGINEERING SPECIFICATIONS
SECTION 6: ECONOMIC CONSIDERATIONS
SECTION 7 : SYSTEMS AND PROCEDURES FOR REPORTING AND RECORDING
PRESSURE RELIEF STREAM DESIGN
22 INTRODUCTION
23 CODES, REGULATIONS, STANDARDS AND PROCEDURES

24 DESIGN AND DESIGN DOCUMENTATION
24.1 General
24.2 Stand-alone Pressure Relief Streams of Uncomplicated Design
24.3 Projects Involving Several Pressure Relief Streams or Interrelated Streams of
Complex Design
24.4 Packaged Units and Proprietary Equipment
25 PRESSURE RELIEF STREAM DESIGN VERIFICATION
26 SPECIFICATION REQUIREMENTS FOR PRESSURE RELIEF DEVICES
27 ADVICE
27.1 Integrated Safety and Environmental Technology
27.2 Piping Section
27.3 In-Service Inspection
27.4 International Engineering Standards and Professionalism
28 SITE CONSIDERATIONS
29 RESPONSIBILITIES
30 AVAILABILITY OF DOCUMENTS AND FORMS

APPENDICES
A BRITISH STANDARDS RELEVANT TO SAFETY VALVES AND RELIEF SYSTEMS
FITTED TO STEAM BOILERS - BS 1113 AND BS 759
B BRITISH STANDARD FOR SAFETY VALVES. GAUGES AND OTHER SAFETY
FITTINGS FOR AIR RECEIVERS AND COMPRESSED AIR INSTALLATIONS - BS 1123
C BRITISH STANDARD FOR PRESSURE RELIEF PROTECTIVE DEVICES FOR
PRESSURE VESSELS - BS 5500 APPENDIX J
D DRAFT BRITISH STANDARD: BS DOCUMENT 82170824 - SAFETY VALVES FOR
USE IN THE CHEMICAL, PETROLEUM AND ALLIED INDUSTRIES
E BRITISH STANDARDS RELEVANT TO BURSTING DISCS AND BURSTING DISC
ASSEMBLIES - BS 2915 AND GBHE ENGINEERING SPECIFICATION
F EXEMPTION FROM MANDATORY REQUIREMENTS
G RECOMMENDED PRACTICE FOR INSPECTION AND MAINTENANCE
H BIBLIOGRAPHY OF CODES, REGULATIONS. STANDARDS AND PROCEDURES
J PRESSURE RELIEF STREAM DESIGN AND DOCUMENTATION
K THE DESIGN OF PRESSURE RELIEF SYSTEMS - MAIN DESIGN STAGES AND USE OF
DATA SHEETS
L NUMBERING OF PRESSURE RELIEF STREAMS
M PRESSURE RELIEF DESIGN VERIFICATION
TABLE
PRESSURE RELIEF - SUMMARY OF CONSIDERATIONS FOR ECONOMY, INHERENT SAFETY
AND ALTERNATIVES TO PRESSURE RELIEF
FIGURES
1 STATUTORY ACTS etc., CODES OF PRACTICE, GUIDES AND STANDARDS
2 TYPICAL NUMBERING SCHEMES FOR SIMPLE RELIEF STREAMS
3 TYPICAL NUMBERING SCHEMES FOR COMPLEX HEADER RELIEF SYSTEMS

SECTION 1.0 PURPOSE AND SCOPE
1 PURPOSE
The purpose of this Guide is to help engineers to ensure that process
plant will not be subjected to excessive overpressure or under pressure
resulting from:
(a) External fire.
(b) Process abnormality or mal-operation.
(c) Equipment or service/utility failures.
(d) Changes in ambient conditions.
2 SCOPE
The Guide is concerned with pressure relief events which meet the
following conditions:
(a) They last for a period of several seconds or longer.
(b) The pressure and temperature within the protected equipment are
approximately uniform at anyone moment.
(c) Any pressure generation by reaction within the relief system is
negligible.
(d) Relief through a relief device followed by piping to a safe place is
practicable.
Under these conditions, steady state flow equations are applicable and
there is also time for either a safety valve or a bursting disc to open.
Events associated with more rapid transient pressure changes are
excluded from the Guide, except for some advice on pressure surge. In
particular, events associated with a flame front (typically completed within
less than a second) are excluded. Thus dust, vapor phase and
condensed-phase explosions (deflagrations and detonations) are
excluded; further advice is available in other GBHE Process Guides and
Reports. Similarly, consideration of prime movers such as diesels and gas
turbine drivers has been excluded.

The Guide has been written to advise those involved in the design and
engineering of pressure relief systems. It takes the user from the initial
identification of potential causes of overpressure or under pressure
through the process design of relief systems to the detailed mechanical
design. "Hazard Studies" and quantitative hazards analysis are not
described; these are seen as complementary activities. Typical users of
the Guide will use some Parts in detail and others in overview.
SECTION 2 : PRINCIPLES OF PRESSURE RELIEF AND USE OF
GUIDE
3 BASIC AIMS
The GBHE policies for Safety, Health and Environmental Protection
require that operations conform to the relevant legislation and that
additional measures are taken where necessary to protect people
and the environment. To fulfill this requirement it is necessary, among
other things, to preserve the integrity of vessels and equipment and
prevent their failure as a result of either over or under pressure.
4 GENERAL DESIGN PHILOSOPHY
To satisfy the basic aims, it is necessary to ensure that every vessel,
machine or pipe is either designed for the maximum and minimum
pressure that can arise in it during start-up, shutdown, operation or
emergency condition or that it is protected by a suitable device that will
prevent the pressure exceeding the maximum permitted by the
appropriate design code and from which it cannot be isolated whilst in use.
Mostly the protective device will be some form of relief device (e.g. safety
valve, bursting disc, vent, breather valve, lute etc.) but in some
circumstances specially designed instrumented protective systems may
be used (see Section 3).
The procedures for ensuring this are broadly termed "pressure relief" and
cover mainly:
(a) The identification of causes of over or under pressure.
(b) The specification of equipment design pressure and or the selection
and sizing of a relief system to prevent the hazard.

(c) The safe disposal of vented material.
(d) The installation and maintenance of any relief systems.
Note:
In some circumstances it may be acceptable, or even preferable, to
provide a high integrity protective system (Instrument Protective System)
instead of a pressure relief system. The design of such systems is not
within the scope of this Guide.
This Guide deals with the engineering aspects of pressure relief. The
general design philosophy is to follow the procedures recommended in
this Guide applying the appropriate professional engineering expertise,
experience and judgment, whilst observing two constraints.
Firstly, any specific statutory requirements shall be satisfied. Section 5
discusses statutory requirements.
Secondly, any Company mandatory requirements shall be fulfilled and
relevant codes and standards complied with wherever appropriate. Some
are mentioned below and others are discussed in Section 6.
5 KEY PRINCIPLES AND DESIGN SEQUENCE
A number of key principles (listed as follows, A to P) underlie the practice
of "pressure relief". They apply more or less in the following sequence.
Notes:
(1) In general, the purpose of relief systems, as discussed in this Guide, is the
ultimate pressure protection of a vessel or equipment under abnormal or
emergency conditions (with the possible exceptions of storage tank
breather valves which may commonly also serve as normal process vents,
and some pumping systems fitted with a recycle-to-suction incorporating a
relief valve). If an emergency relief system is called upon to operate
frequently, this indicates that the normal process control system is
inadequate and should be improved.
(2) A relief system may comprise one or more relief streams. A relief stream
comprises a relief device (or devices), the pipe work and fittings upstream
and downstream of the device(s) and any equipment provided for safe
disposal of vented material. The disposal arrangements may be common
to a number of relief streams.

(A) All causes of potential overpressure (or under pressure) of any plant item
or pipe that are physically possible should be identified.
To have high confidence that all causes are identified, it is desirable to
adopt a systematic procedure such as is detailed in Part 8 of this Guide.
Hazard Studies are mandatory for most projects and form a useful adjunct
to pressure relief studies as a means of checking that all overpressure and
under pressure hazards have been identified.
(B) No over or under pressure hazard that is physically possible may be
discounted unless adequate justification can be demonstrated.
Mostly in pressure relief studies, it is assumed that "if it can happen, it
will", but in some circumstances it can be demonstrated on the basis of
operating experience or by a quantified hazard analysis that the probability
of a particular hazard occurring is sufficiently small to be discounted. The
help of an experienced hazard analyst would be necessary.
In some situations the hazard may take so long to develop that it may be
reasonable to assume that preventive action will be taken. A particular
example concerns fire relief; if it can be shown that it would take, say,
several hours for the temperature of the contents of a system to be
increased sufficiently to cause a relief situation then it may be reasonable
to assume that fire fighting action would always be taken and that "fire
relief" would not be necessary. See Part 8, Section 2 and Part C, Section
1 of, this Guide.
(C) Overpressure or under pressure hazards should be "designed out"
wherever it is practicable and economic to do so.
For this reason it is necessary for pressure relief studies to start early in
the line diagram development for a project, so that ways to avoid the need
for pressure relief can be incorporated most effectively. To this end, it is
worth stressing that early agreement on operating philosophy and
procedures can frequently lead to simplification of the equipment. In the
limit, designing equipment for the maximum pressure that can occur is the
obvious and best way of avoiding the need for pressure relief when it is
economic to do so. The extra cost of higher design pressures can often be
offset by the savings on pressure relief equipment (both capital and
maintenance costs) and on research and design effort. Also the problems
possibly associated with emissions from relief systems are avoided.

Instrument Protective Systems may be used in some circumstances to
prevent an overpressure or under pressure hazard occurring. Typical
situations are:
(1) when it is necessary to avoid any emission of a material to
atmosphere;
or
(2) when a plant or process modification would otherwise make an
existing relief system inadequate.
Such Instrument Protective Systems will always have some degree of
"redundancy" to give an adequate reliability; detailed hazard analysis is
required to establish the level of reliability. See Section 3.
(D) When design pressure can be exceeded as a result of some abnormality,
a relief system of adequate capacity shall be provided to ensure that no
vessel or other equipment can be subjected to a pressure more than a
small specified amount above its design pressure (except under the
controlled conditions of a pressure test).
The maximum pressure permitted during relief is defined in the
relevant equipment design codes. See Section 6.
(E) Required relief rates for each of the relevant hazards should be calculated
using the best available methods and making safe assumptions with
respect to the state of equipment and the materials therein.
Methods to use and assumptions to make are given in Part C of
this Guide. For example, a safe assumption is made, that simple
instrumented control or trip systems are assumed to fail to work if
failure would lead to an increased relief requirement. See Section
3.
(F) The possibility of simultaneous occurrence of hazards should always be
considered.
Whilst the majority of overpressure or under pressure hazards may
be independent events of short duration, (the simultaneous
occurrence of which can be discounted) others may have a
common cause or be of sufficient duration that simultaneous
occurrence is probable.

It is not acceptable to bypass this study by making generalized
rules such as "design for single jeopardy but not for double
jeopardy". Such expressions are liable to be interpreted differently
by different people and are best
avoided.
Combinations that would give relief rates only marginally higher
than the largest individual rate (or the sum of those with a common
cause) can often be accepted as the design basis without further
study. If a combination would give a significantly higher relief rate,
then it is worth a detailed quantified hazard analysis to determine
whether or not it need be considered.
(G) Relief systems shall be sized for the largest independent or ·credible
combination of events.
Guidance on sizing is given in Part D of this Guide. Relief systems
have sometimes to be sized on the basis of preliminary information,
early in design. It is therefore important that capacities are checked
and recalculated when full information is available and when line
diagrams have been finalized. Safety valves may have been
selected on data sheet quantities, but when the nearest suitable
instrument valve, for example, is selected, it may pass (when fully
opened) considerably more than the required relief rate originally
quoted on the data sheet.
(H) It is essential that relief systems are designed, and where necessary
arrangements are made, to ensure that any material that may be
discharged from any relief system is disposed of safely without creating
additional hazards.
Guidance on the selection and design of means for safe disposal of
vented material is given in Part E of this Guide. Difficulties of
ensuring safe disposal can be a reason to avoid the need for a
relief system (see Item (C) above).
(I) Relief systems and other protective devices should be selected and
arranged to minimize disturbance to equipment and effect on the
environment.

In some cases the required relief rate can be provided in different
ways and it is worth going to some trouble to find the best one. For
example, a furnace coil could be protected by a safety valve before
or after the coil; the former may require a smaller valve but the
latter maintains flow through the coil and it may be advisable to
provide part of the required capacity at each position. Similarly, it
may be desirable to maintain flow through exchangers. Where
there are a number of safety valves at different points along a
process stream, relative capacities and locations should be chosen
so that when a disturbance occurs at the far end of the stream, the
safety valves help each other as much as possible and also that the
total discharge is no greater than really necessary. Set pressures
and design pressure can sometimes be chosen to help achieve
these objectives.
Volume 4 of this Guide series gives guidance on the choice of relief
device, when duplication is necessary and when two or more
should be used with staggered settings. Guidance on safe disposal
of vented material is given in Volume 5 of this Guide series..
(J) No isolations shall be made at any place in relief stream(s) while the
equipment to be protected is in use, if the isolations would result in the
relief capacity available being less than the required relief rate, except that
in a few special cases certain isolations may be permitted .
All relief streams shall be checked to ensure that where isolation
devices such as block valves, slip plates etc. are used, such
streams are designed and installed in accordance with the
following:
(1) Where maintenance requires that a relief stream be isolated,
either:
(i) the equipment shall be decommissioned first;
or
(ii) parallel relief streams or a single parallel relief stream
of at least the required relief capacity shall be
provided.
Where option (ii) is taken there shall be suitable mechanical
interlocking of the isolation devices to ensure the required relief
capacity is provided. See Volume 4 of this Guide series.

(2) Isolation of single relief streams may be permitted on liquid
expansion relief duties providing the conditions of J(3),
below, are satisfied. See Volume 4 of this Guide series,
Section 4 of this Guide.
(3) In some circumstances, carefully considered authorized and
supervised administrative procedures can be invoked to
prevent overpressure hazards arising from isolation of relief
streams.
It is envisaged that such Administrative Process Control
Procedures (APCPs) for isolation of pressure relief streams
will be rarely used and only in ,circumstances where
alternatives can be shown to be more hazardous, e.g.
break-ins of new equipment to existing complex header
systems.
(4) In some circumstances, an APCP may be used to prevent
overpressure or under pressure hazards arising on standby
equipment which is isolated from the process equipment
system on which the relief system that would protect
it when in use is located.
(K) All components of the relief system shall be sized to ensure that the relief
capacity equals or exceeds the required relief rate.
Piping (including valves, fittings etc.) upstream and downstream of
the protective devices shall be designed to this end, and, in the
case of safety valves, so that the manufacturers' requirements
relating to pressure drop upstream of the safety valve and
limitations on back pressure are satisfied. Advice is given in
Volume 4 of this Guide series and Volume 5 of this Guide series.
(L) A properly designed and maintained pressure relief stream whose relief
capacity is greater than the maximum required relief rate is judged to give
adequate integrity of protection to process equipment against excessive
positive or negative pressure in the majority of circumstances.
Only in situations of high risk potential, where failure of pressure
relief stream components could have very serious consequences,
need consideration be given to the provision of a "redundant" relief
stream.

Quantitative hazard analysis can be used to assess such situations
although care should be exercised in using fractional dead time
data for safety valves. A qualified hazard or reliability engineer
should be consulted. See Section 4.
Most steam boiler codes require the provision on larger boilers of at
least two safety valves with total capacity in excess of the boiler
evaporation rate. See Volume 6 of this Guide series.
On process duties, a typical installation designed to give higher
integrity protection may comprise 3 relief streams each having a
capacity of 50% or 2 each having a capacity of 100% of maximum
required relief rate. Such systems require careful selection and
Volume 4 of this Guide series should be consulted.
(M) The basis of sizing and selecting a relief stream should be fully
documented.
Having established the required relief rate and the relief capacity to
be provided, it is most important that the reasoning on which they
are based is fully documented for future reference when design
changes or plant or process modifications are made. Plant
modifications such as additional isolation valves, change of action
or of trim of control valves, etc. may affect the required relief rate
and therefore the documentation will be such that the operating
Works can assess these effects and make allowance for them. The
documentation required is described in Section 8.
(N) The design of all new and modified pressure relief streams shall be
verified.
Section 8 details the process and documentation. The purpose of
design verification is to ensure that:
(1) The relief stream design conforms with specified
requirements.
(2) Relevant Regulations, Codes, Standards etc. have been
complied with.
(3) Good engineering practice has been employed.

(4) The documentation is complete.
Design verification does not involve checking of the relief
philosophy, process engineering or any calculations.
(0) Any plant or process modifications shall not invalidate the sizing,
selection and installation of any relief stream.
Plant modifications are controlled by local documented
procedures requiring any effect on existing relief streams to
be thoroughly checked so that appropriate changes can be
made if necessary. Although not all plant modifications
involving pressure relief streams will require a Hazard Study,
formal, documented consideration shall be made by a
responsible manager with knowledge of the process.
(P) To ensure that pressure relief streams will function when required
to do so, they shall be inspected comprehensively at regular
intervals .
The registration and periodic examination of pressure relief
streams is mandatory. All new relief streams shall be
registered and a Works file set up. Also there shall be a local
procedure for ensuring inspection of relief streams and
reporting on those overdue for inspection.
6 RELATED DESIGN ACTIVITIES
A number of related design activities, some of which are mandatory, can
have a direct bearing on pressure relief design though they may be more
directly aimed at other aspects of design, construction and operation.
Included in this category are:
(a) Strict adherence to the agreed design code for pressure vessels
and piping standards at appropriate stages, e.g. BS 5500 and
ASME/ANSI 831.3.
(b) Hazard Studies (including HAZOP).
(c) Quantitative Hazard Analysis when shown to be needed by Hazard
Studies or other considerations.

(d) Examination of process materials for flammability and dust
explosion potential.
(e) Examination of process materials for toxicity.
(f) Classification of areas for electrical equipment specification.
(g) Layout studies, in particular concerning safety and access, and
permissible places of gaseous and liquid discharges.
(h) Assessment of noise emission.
(i) Fire protection studies.
(k) Specification and ordering of "Packaged Units" (e.g. compressors,
fridge sets etc.).
The engineer or team responsible for pressure relief studies should take
full account of these and any other relevant activities.
SECTION 3 : INHERENT SAFETY AND ALTERNATIVES TO PRESSURE
RELIEF
7 BASIC PHILOSOPHY
During the 19th Century it became compulsory to fit a safety valve to air
receivers and steam boilers in a successful attempt to reduce the numbers
of explosions that were occurring. Partially because of this historical
background and because of the wording of the pressure vessel design
codes there is still a strong tradition of assuming that a safety valve will be
fitted to all pressure vessels. It may often be late in the design stage of a
project before it is realized that a particular relief system will create a
problem due to its excessive size, high cost or difficulty of design.
Furthermore, the consequential consideration of safely handling the
discharged material may create another dilemma due to its toxicity,
flammability, corrosiveness or smell and environmental effects.
To help avoid these problems it is therefore important to consider the
design of the relief systems at an early stage in a project so that any
potentially troublesome relief systems are identified.

These considerations can conveniently take place at Hazard Study Stages
1 and 2 and need be only a preliminary qualitative review so that potential
alternative solutions can be identified for evaluation. It is important to
consider the relief systems as a whole (that is any relief device, the
piping, and any necessary disposal system for the discharged fluid).
If a difficult or expensive relief system design is recognized sufficiently
early in a project, then it may be possible for the process engineer to find
an alternative.
The questions to ask, therefore, in order to arrive at the most economical
solution overall and possibly achieve a more inherently safe design are as
follows:
(a) Can the overpressure hazard be eliminated?
(b) Can the overpressure hazard be reduced?
(c) Can the overpressure be contained?
(d) Can protection by alternatives to relief systems be considered?
(e) Can the relief system required be minimized?
These are summarized in Table 1 and discussed further in Clauses 8 to
12.

8 ELIMINATION OF THE SOURCE OF OVERPRESSURE OR
UNDERPRESSURE
The source of the over pressurization (or under pressurization) can
sometimes be eliminated by redesign, when it is practicable and economic
to do so. This approach is best illustrated by a few examples:
(a) The manufacture of Nitroglycerine at a European company contains
many excellent examples of eliminating sources of overpressure
and explosions. The most well known example is the use of a
mixed-acid fed injector to provide the motive force (vacuum) for the
glycerin feed.

If the mixed-acid flowrate falls off, then the glycerin flowrate drops
more than by a proportional amount without any mechanical
intervention and this automatically eliminates the dangerous
condition which arises when an excess of glycerin is present.
(b) A more common example is the use of a controlled-pressure steam
heating system as the heating service to a reactor containing an
endothermic reaction. With such a system the appropriate choice of
steam pressure limits the temperature that the reactor can reach
and avoids the dangerous exothermic decompositions that can
arise at higher temperatures.
(c) Select the maximum pressure that can be generated by a pump or
compressor to be less than the design pressure of the downstream
equipment.
(d) Set the design pressure of a distillation column at a value greater
than the vapor pressure of the feed at the maximum possible
temperature of the heating medium of the reboiler.
9 REDUCTION OF THE HAZARD
If the source of over or underpressure cannot be eliminated at source it
may be possible to reduce the magnitude of the overpressure effects. In
general, reducing processing pressures, temperatures and inventories will
contribute to that and should be considered wherever possible.
In the nitroglycerine process mentioned in Clause 8, the inventory is
significantly reduced by the use of continuous processing. This does not
eliminate the inherent chemical hazard but diminishes the potential for
explosion to a manageable level. In batch processing, several smaller
reactors rather than one large reactor effectively reduce the inventory
involved in a relief situation and can produce a more acceptable design.
10 CONTAINMENT OF THE PRESSURE THAT MIGHT OCCUR
If the source of over or underpressure cannot be eliminated then it is
worthwhile considering the possibility of containing it. It can be viable to
design the equipment for the maximum pressure (or vacuum) that can
occur and so eliminate the need for a relief stream altogether. Any
measures taken to reduce the overpressure hazard may make the
possibility of containment greater.

Sometimes the additional cost of designing for the higher pressure may, in
fact, be minimal; that may be the case, for instance, where the strength of
a vessel is determined more by a requirement to be self supporting than
by the pressure it has to withstand. In other cases, it frequently can be
found that the additional costs of the vessel itself may be more than offset
by the savings on the relief system (no longer required), particularly if
special disposal systems would have been required to cope with the
vented fluid.
11 ALTERNATIVES TO PRESSURE RELIEF SYSTEMS
In relatively recent times the use of Instrumented Protective Systems (IPS)
has gained acceptance as an alternative to a relief stream. Typically an
arrangement would comprise IPS of sufficiently high reliability to be able to
initiate shutdown systems to prevent an overpressure hazard occurring
so that a relief system is not required. They are particularly relevant where
it is desirable to avoid the emission problems that can be associated. with
relief streams. Also, with reaction systems it can be cheaper and more
practicable to prevent potential reaction runaways by IPS. This can
particularly be the case when there would otherwise be uncertainty over
the nature of the reactions occurring under abnorma.1 conditions or where
the reliability of a relief device is questionable, for example where there is
a risk of blockage. Even where relief hardware may be relatively cheap,
the need to have a safe discharge area can be expensive in terms of plant
layout and needs to be considered when comparing costs.
The use of IPS to replace a relief stream is a rational approach given that
a conventional relief stream has a finite reliability and is not infallible (see
Section 4). However, it has to be appreciated that the use of a
mechanically self-opening relief device tends to be regarded as the "norm"
for overpressure protection so that any deviation from that "norm" requires
careful justification. Detailed hazard analysis is necessary and, in some
cases, detailed scrutiny by the Health and Safety Executive can be
expected. When considering the acceptability of an IPS, a point to be
remembered is that it may only be able to protect against those hazards
for which it has been specifically designed. A relief stream, on the other
hand, would be able to give some protection against unidentified hazards
but not, of course, if the required relief rate exceeded the capacity of
the system.

IPS are expensive and require a large amount of detailed design effort in
order to be certain that adequate reliability can be achieved. They also
require regular testing and inspection to ensure their reliability, hence
maintenance costs may also be high. Where IPS are to be used instead of
relief systems it has to be properly appreciated that a system for
registration, inspection and testing has to be set up that is of comparable
integrity to the well established systems for pressure vessels and
mechanical relief devices. If that cannot be assured, then IPS cannot be
used instead of relief systems.
12 MINIMIZING THE RELIEF SYSTEM REQUIRED
Sometimes the required relief rate can be reduced by instrumented
systems or mechanical devices. An example is the wide use by British
Gas of mechanically actuated "slam shut" valves at the let down stations
in their distribution systems. The idea is that in the event of a let down
valve failure the slam shut valve will operate and prevent flow into the low
pressure system. In that way the need for a large safety valve on the low
pressure side is avoided. To allow for some leakage only a nominal
capacity relief stream (typically 5% of the relief rate otherwise required) is
provided. As with safety valves on continuously operating plant it may be
necessary to fit dual (parallel) streams so that one valve can be isolated
for testing and inspection.
This approach can be a particularly useful method of reducing the quantity
of fluid that may have to be discharged to atmosphere or into a flare,
scrubber or other disposal system. Similarly, it can be adopted where a
plant modification introduces additional overpressure hazards but it is
undesirable or impractical to change the existing relief stream.
Devices to limit relief rate need careful consideration. They are more likely
to be acceptable for fluids that are clean and free from corrosion
problems.

SECTION 4 : STATUTORY AND OTHER GOVERNMENT APPROVED
REQUIREMENTS
PREFACE
At the time of first issue of this Guide, this Section mainly addressed the statutory
requirements for protection of equipment in the U.K. The Health and Safety at
Work etc. act of 1974 was the most recent legislation. Since then, there has been
a significant change in the legislative position in the U.K. with a substantial
increase in the requirements the authorities place on operators of process plant.
Furthermore. European legislation in draft form may also influence decisions
engineers take throughout chemical company operations in Europe.
It is not possible to give clear guidance on the best means of compliance with
local requirements because of the pace of change. However, within GBHE
Engineering advice can be obtained from the appropriate functional section in the
geographical group for the particular region. In addition. Volume 5 of this Guide
series dealing with discharge and disposal system design has been written in
order to provide guidance on meeting environmental requirements.
It has been decided that any update of this Section will be withheld until
experience with U.K. legislation is assimilated and the likely effects of proposed
European legislation studied. The existing text will be retained for the time being
to allow users to see the original basis of many U.K. designs. But conformance
with pre-1990 standards should not be assumed to provide conformance with
later legislation. In many instances. this will certainly not be the case.
13 INTRODUCTION
This Section covers the statutory requirements for protection of closed
equipment against hazards caused by overpressure and underpressure. It
is primarily concerned with the requirements which have to be met to
satisfy both the Health & Safety Executive and the Local Authorities in the
U.K. Additional information which is relevant to some European countries.
Canada and the U.S.A. is also included though it is not practicable within
the scope of this Guide to cover in detail the far reaching and ever
changing requirements in other countries.
For any overseas project, all relevant regulations and technical
requirements of the country concerned should be scrutinized and adhered
to.

It should be borne in mind, however, that most of the basic technical
requirements given here are relevant anywhere in the world.
For the U.K., there is no comprehensive legislation which applies to the
protection of closed equipment of any type in all circumstances. Although
the stated statutory requirements are few and not set out in any detail, the
implications are far reaching and severe. They are backed up by
a large number of wide-ranging advisory documents which may not carry
the force of law but which will frequently be referred to by factory
inspectors and other authorities. The philosophy behind all this is to
ensure the safety of plants and equipment by the application of good
engineering practice consistent with economically practicable means of
achieving these objectives.
Guidance on the content of these advisory documents is given in Section
6 and more detailed abstracts in the Appendices A to G.
The discharge of hazardous or noxious materials to the atmosphere is
limited by statutory regulations which "apply in principle to emergency
discharge via relief systems. In practice, there may be little restriction if the
total quantity discharged in anyone emergency relief is small, the
material is not particularly hazardous and/or the frequency is expected to
be very low.
14 STATUTORY POSITION IN THE UNITED KINGDOM
The most recent guidance to what the law requires in the UK is given in
the Health and Safety at Work, etc. Act, 1974. While not specifically
mentioning pressure relief, the Act places a duty upon employers,
designers, etc. and also employees to ensure the provision, construction,
operation and maintenance of plant that is, so far as is reasonably
practicable, safe and without risk to health when properly used. This is an
Enabling Act and it is expected to be followed by Regulations to
provide detailed requirements.
The Health and Safety Commission have studied the present spread of
legislation, covering pressurized plant under working conditions, and
believe that there is a need for new legislation to take full account of the
problems common to all pressurized systems. A consultative document,
'Proposals for new legislation for pressurized systems', was issued by the
Health and Safety Commission in" 1978 and contains the following
statement with respect to design and construction:

"All parts of a pressurized system should be properly designed and be of
good construction, sound material and adequate strength. The general
requirements of Section 6 of the Health and Safety at Work etc. Act 1974
apply and the design of plant should correspond to the best current
practice for the process. The system should be designed and constructed
to permit adequate examination and testing to ensure safety and should
be provided with a sufficient number of such fittings, attachments and
protective devices as are necessary to ensure safe operation. All such
devices should be required to operate in a safe manner.
The design, construction, examination and testing requirements will vary
quite considerably over the many different types of pressurized systems. It
is proposed that there should be a main general code of regulations
supported by supplementary sets of regulations for defined groups of
pressurized systems".
A much wider interpretation of pressurized systems than that traditionally
accepted is envisaged. The proposals are aimed at increasing standards
of safety and do not in themselves suggest any immediate changes in the
procedures we now use. There is, however, more emphasis on "systems",
rather than vessels and a much wider range of equipment is included.
Inevitably, the present situation where steam and air vessels come under
different legislation from all other systems, will eventually change.
Some of the proposals have been modified in the light of comments made
by various sections of industry and are still under discussion.
15 CURRENT REQUIREMENTS
15.1 General
Statutory regulations relevant to the prevention of hazards due to
overpressure in closed vessels and equipment are:
(a) Steam boilers Factories Act 1961 Sections 32,33,38.
(b) Steam receivers Factories Act 1961 Section 35.
(c) Air receivers Factories Act 1961 Section 36.
(d) Stills and closed vessels Chemical Works Regulations 1922
No.5.

(e) Control of emissions Control of Pollution Act 1974.
Despite the limited statements contained in these statutory documents, it
is clear from the Health and Safety at Work etc. Act 1974 that the standard
of protection required for process equipment is at least as high as that for
steam and air - though the means of achieving it may be different.
These requirements are not covered and will therefore only be
summarized here. Additionally, the requirements of a number of relevant
British and ISO Standards are often quoted as defining an acceptable
practice. As stated in Clause 13, these standards do not carry the force of
law but any failure to meet their requirements could, in certain
circumstances, be construed to mean that the plant failed to meet the
statutory requirements. Adherence to BS requirements goes a long way
towards ensuring that the statutory body would approve the design.
In practice, permission to operate is granted by the Local Planning
Authority on the advice of the Health and Safety Executive (HSE)
Inspectorate (See Clause 16).
15.2 Specific
The specific minimum requirements are given in 15.2.1 to 15.2.4 together
with a brief definition of the items to which they apply. The reader should
always read the full authoritative statements either in the statutory
document or the GBHE Procedure.
15.2.1 Steam Boiler
Every steam boiler, whether separate or one of a range, shall be fitted with
(amongst other things):
A suitable safety valve fixed, without an intervening stop valve, directly to,
or as close as practicable to, the boiler and adjusted so as to prevent the
boiler from working at a pressure greater than the maximum permissible
pressure. See the Factories Act 1961, Section 33.
"Maximum permissible pressure"; means that specified in the report of the
last examination.

Note:
"Steam Boiler" means any closed vessel in which for any purpose steam
is generated under pressure greater than atmospheric pressure, and
includes any economizer used to heat water being fed to any such vessel,
and any superheater used for heating steam. It also includes vessels in
which steam is generated from boiler feed water as a means of heat
recovery but not when water is evaporated in the presence of other
substances (as in a chemical process).
15.2.2 Steam Receiver
(a) Every steam receiver not constructed so that it can safely withstand
the maximum pressure in the steam supply pipe shall be fitted with:
(1) a suitable reducing valve or other suitable automatic device
to prevent the safe working pressure of the receiver being
exceeded;
and
(2) a suitable safety valve so adjusted as to prevent the safe
working pressure from being exceeded, or a suitable device
for automatically cutting off the steam supply immediately
the safe working pressure is exceeded. The safety valve
may be fitted to the steam supply line between the receiver
and the reducing valve.
(b) Where a set of steam receivers forming part of a single machine is
supplied with steam through a single pipe, the set may be treated
as a single receiver as in 15.2.1 and the reducing valve, pressure
gauge, safety valve and stop valve may be fitted in the single
steam pipe.
(c) Where a set of steam receivers not forming part of a single
machine is supplied with steam through a single pipe, the set may
be treated as a single receiver as in (a) and the reducing valve,
pressure gauge and safety valve may be fitted in the single pipe,
but each receiver shall be provided with a stop valve.
Note:
"Steam Receiver" means any vessel or apparatus (other than a steam
boiler, steam container, a steam pipe or coil, or part of a prime mover)
used for containing steam under pressure greater than atmospheric
pressure.

15.2.3 Air Receiver
(a) Every air receiver shall (inter alia):
(1) if connected with an air compressing plant, either be so
constructed as to withstand with safety the maximum
pressure that can be obtained in the compressor or be fitted
with a suitable reducing valve or other suitable appliance to
prevent the safe working pressure of the receiver being
exceeded; (Factories Act 1961, Section 36).
(2) be fitted with a suitable safety valve so adjusted as to permit
air to escape as soon as the safe working pressure is
exceeded.
(b) A set of air receivers supplied with air through a single pipe may be
treated as one receiver so far as safety valves are concerned,
provided that the reducing valve or other appliance if required in
compliance with (1) is fitted on the single pipe.
Note:
The most relevant of several definitions of air receiver given is: any vessel
(other than a pipe or coil, or an accessory, fitting or part of a compressor)
for containing compressed air and connected with an air compressing
plant. Other definitions, relating to starters for engines, etc. are included in
the Act and can be consulted when required.
15.2.4 Process Vessels and Equipment
Closed vessels are covered by The Chemical Works Regulations, 1922:
"Every still and every closed vessel in which gas is enclosed or into which
gas is passed and in which the pressure is liable to rise to a dangerous
degree, shall have attached to it, and maintained in proper condition, a
proper safety valve or other equally efficient means to relieve the
pressure".
Thus very little guidance is provided since the liability for pressure "to rise
to a dangerous degree" is subjective. Currently, the most widely accepted
guidance is provided by the British Standard for the design of pressure
vessels. BS 5500. This Standard carries an Appendix (J) which provides a
set of rules for the provision of "an appropriate protective device". The
most relevant basic requirements of Appendix J may be summarized:

Every pressure vessel shall be protected from excessive pressure or
vacuum, excessive temperature, overfilling, corrosion, explosion or other
hazardous conditions by an appropriate protective device. Each
compartment of a subdivided vessel shall be treated as a separate vessel
and suitably connected to a protective device. Where a vessel is provided
with an impervious movable partition, as in a gas loaded hydraulic
accumulator, protective devices shall be provided for the spaces on both
sides of each partition.
An exception to this rule is that when the source of pressure (or
temperature) is external to the vessel and is under such positive control
that the pressure (or temperature) cannot exceed the design pressure (or
temperature) a pressure (or temperature) protective device need not be
provided. GBHE practice is normally to provide a pressure relief system
on all closed vessels. Subsequent clauses of Appendix J of BS 5500
(which also relates to bursting discs) give more detailed advice that will be
covered in Section 6.
15.3 Design of Pressure Relieving Systems
15.3.1 Steam Systems
Existing legislation does not cover the design requirements for pressure
relieving systems for steam. However, the British Standards which specify
the requirements of pressure relieving systems for steam boilers and
associated plant. including piping state that:
"Use of the provisions of the standards for design and construction is only
valid when the relevant requirements of the other standards listed in the
Forewords are completely satisfied. Boilers and their ancillary pressure
parts may only be marked and certified in accordance with Section 6 of
the standards when all the relevant requirements of the appropriate
standards in the list have been fulfilled".
Note:
"Section 6 of the standards" appears in both as 1113 and as 2790: Parts 1
and 2.
This means that where steam boilers are designed and certified in
accordance with as 1113 and as 2790 the pressure relieving systems are
to satisfy the requirements of the relevant standard and also as 759 and
as 806.

Detailed requirements of these standards, many of which are relevant to
process plant, are quoted later (See Section 6)
15.3.2 Air Systems
Existing legislation does not cover the design requirements for pressure
relieving systems for air.
BS 1123, whilst specifying many requirements for air receivers and
compressed air installations, does not fully cover the requirements where
equipment is associated with process plant. It does not refer to
compressed nitrogen but is frequently accepted within GBHE for use with
nitrogen systems.
15.3.3 Process systems
The most useful guidelines to· satisfy statutory bodies are given in BS
5500. See also Section 6.
15.4 Relief Devices and Ancillary Equipment
There are no statutory specifications for safety valves, bursting discs and
other relief devices but there is no doubt that the Inspectorate would not
approve an installation which relied upon relief devices of inferior or
dubious design or construction. Here again, reference is usually made to
the relevant British Standards and American Standards (See Section 6).
Forthcoming ISO Standards can be expected to become of greater
importance to the Inspectors in the future. GBHE Engineering
Specifications (for example are written to ensure that suppliers conform to
national and international requirements.
Relief pipelines, mountings, etc. shall also be designed and constructed to
withstand the maximum pressure to which they may be subject during
discharge and also conform to accepted design codes. See ANSI/ASME
B31.3 and BS 806.

16 APPROVAL OF DESIGNS RELEVANT TO PRESSURIZED SYSTEMS
16.1 United Kingdom
In the U.K., permission to operate any plant which could constitute a
hazard to the operators, employees or the public should be obtained from
the local government authority in whose area the plant is situated. The
Department concerned will usually take advice from the HSE Inspectorate.
There is no statutory requirement for pressure relief design to be
"approved" in detail. In certain cases the HSE inspectors may seek
assurance and may wish to check calculations. They have the right to
examine designs and specifications, inspect equipment and to question
the mode of operation. This may mean a study of the basic factors, the
assumptions made and the calculations used for sizing each relief device
and also any ancillary system. Thus there is, in effect. a statutory
requirement to provide full design data and to be able to justify the
assumptions and methods of calculations if demanded by the inspector.
The specification of all items of pressure relief systems should be made
with this end in view even though no demand may be made at the time of
application for a permit.
In the case of established plants, any potentially dangerous incident or
one actually leading to damage, injury or death is most likely to come
before the inspectorate. The reliability of the design for protection from any
form of overpressure (or underpressure) can be questioned and here
again detailed design information is likely to be required. Hence, the
authority may call for more than the basic statutory requirements stated
above (See Clause 14).
It may be necessary to produce evidence that the design conforms to the
best available practice in the circumstances; therefore every step taken in
establishing the basis for and the design of a pressure relief system
should be recorded in such a way that retrieval by subsequent operators
of the plant is assured.
16.2 Other European Countries
Technically good designs which meet all UK statutory (See Section 6) can
be expected to satisfy most European requirements. However, details may
be different, and the justification of designs may, however, require more
consultation in some countries. Local advice should be sought before
completing the design.

16.3 Canada
In Canada, it is necessary to secure the approval of the appropriate
provincial ministry dealing with consumer and commercial affairs whose
responsibility it is to approve the design of pressurized systems and
manufacturers' equipment for the protection of operators and employees.
The design submissions to government are made by a registered
professional engineer. Requirements for testing and operation are
codified. Proper operation of the system is the responsibility of the
owner and operator.
With respect to public safety and environmental protection, environmental
permits are necessary from appropriate government ministries if pressure
relief emissions may exceed regulated limits, or which could have an
adverse impact on the public or environment. Flaring or collection of
pressure relief emissions for treatment may be necessary. These
requirements vary depending upon provincial jurisdiction. Provision may
also be necessary to attenuate venting where applicable.
SECTION 6 : MANDATORY REQUIREMENTS AND RECOMMENDED
PRACTICE
PREFACE
At the time of the first issue of this Guide, this Section referred to legislative
requirements, British Standards and procedures which are now obsolescent or
have been substantially modified. Further policy documents, procedures and
instructions are being prepared. European Directives on pressure systems and
product standards for safety valves, bursting discs and other pressure relief
devices are in course of preparation. A European Standard (European Norm BS
EN 286) for unfired, simple pressure vessels is currently available and standards
for more complex vessels, boilers and piping will follow. These standards are
likely to have requirements which will supersede those of current national
standards throughout Europe.
Part A Section 8 of the Guide provides advice on complying with design
documentation requirements in select chemical companies. The procedures and
specifications which are referred to in Section 8 provide the basis for specifying
and designing pressure relief streams to meet mandatory requirements. Where
further advice is needed, this can be obtained within GBHE from International
Standards and Professionalism of GBHE Engineering or from the appropriate
functional section in the geographical group for the particular region.

It has been decided that any update of this Section will be withheld until it is
possible to give more specific advice on how requirements will link with national
and community legislation. The existing text will be retained for the time being to
allow users to see the basis of many U.K. designs. But conformance with
pre-1990 procedures should not be assumed to provide conformance with later
ones. Part D of this Guide contains advice which is considered safe but which
may not meet the requirements of later national codes. It is strongly
recommended that advice be sought before work on pressure systems is started.
17 INTRODUCTION
It is the policy of select European chemical companies to ensure that all of
their plants are constructed and operated in as safe a manner as is
reasonably practicable. To this end the Companies supports statute law
with Codes of Practice and Company Regulations. Each manufacturing
Division is responsible for implementing the requirements in a way
appropriate to its own business and technology. This is achieved by the
issue of Division and Works Instructions and by reference to Engineering
Procedures and Design Guides. (See Section 5).
The purpose of this Section is to guide the user through the principles
accepted by select European chemical companies as good engineering
practice for design, installation and operation of systems for the protection
of plant and equipment against excessive pressure.
The term 'mandatory' is interpreted as meaning that the instruction is
obligatory in any situation in which it is relevant. Instructions based on
statute law including local authority regulations and essential safety policy
are mandatory throughout the Company. This Guide is concerned with the
prevention of hazard to persons and damage to equipment or property
due to overpressure (or underpressure) in closed equipment. However,
the requirements directly relating to pressure relief that are unequivocally
set out in a Company document are sparse and generalized. They are
contained in the GBHE - EDPs which is primarily concerned with
procedures to ensure maintenance of the "fitness of the plant for its
purpose".
Though important and far-reaching in effect, GBHE - EDPs provides no
advice on identification of pressure relief requirements, the selection and
sizing of relief devices or the design of relief systems. Certain divisional
documents state additional requirements which may or may not be
accepted by any other than the issuing Division;

some of these requirements are therefore stated in this Guide. GBHE -
EDPs does, however, provide a number of definitions (including that of
Pressure Vessels) which are used in this Guide in addition to the statutory
definitions given earlier (See Section 5).
Most of the instructions and requirements given in this Section may well
become mandatory in certain circumstances whether or not each
requirement is written down in Company instructions. Such circumstances
might be, for example, when the instructions represent the best available
practice for the particular situation.
There is at present no clear-cut distinction between "mandatory
requirements" and "recommended practice" which, if ignored, could lead
to an unacceptable proposal. Thus this Section includes summaries of
such recommendations (with an indication of their source) while rather
more details from the relevant external standards are given in the
Appendices. These Appendices are made up of both abstracts (verbatim)
and synopses.
A number of other mandatory procedures which can have a bearing on
pressure relief studies though more directly aimed at other aspects of
design, construction and operation are listed in Section 2.
Note:
This Section is written primarily for users in the U.K. Most of the technical
advice is relevant overseas but the observance of local Statutory
Procedures is required and appropriate national or other locally accepted
standards may often be used in circumstances where they would be more
appropriate (See Section 5).
18 GENERAL RULES
GBHE - EDPs gives only general guidance on how to implement the basic
requirements with respect to the identification of where pressure relief is
required and virtually no guidance on the design and installation of relief
systems. Some general rules have therefore been formulated which are
believed to be acceptable in principle throughout the Company.

18.1 Basic Requirements of GBHE - EDPs
Note:
The wording used here is not always identical with the EPI and some
notes of interpretation have been added. See also Appendix F.
(a) All statutory requirements for pressure relief systems for pressure
vessels and piping shall be met. It is recognized that either
exemption or some relaxation in the form of a Qualification for
certain classes of vessels may be desirable; some of these are
listed in the EPI. Such Exemptions can be given and Qualifications
approved by the Chief Inspector of Factories who issues
"Certificates of Exemption" (See Section 5).
Any application to the Inspector in a new case shall only be made
with the approval of the Chief Engineer or an appropriate Authority.
(See Appendix F).
(b) Any vessel or run of piping (unless "qualified" or "exempted" as
above) which can as a result of any foreseeable cause (examples
given in the EPI) become subjected to pressure greater than design
pressure shall be protected at all times by a suitable pressure relief
system.
Note:
"Suitable" means that it is capable of maintaining the pressure within the
limits set by the design for any possible working conditions.
"At all times" means that no system of temporary isolation that can restrict
a discharge is acceptable (See (c) and (d) below). Where the foreseeable
pressure rise and risks there from can be shown by hazard analysis to be
acceptably small, then a relief system incorporating isolating valves
upstream/downstream of the relief device can be used.
This is, however, only acceptable in conjunction with an approved process
control procedure registered as a "Qualified Case" (see examples). Briefly,
the bases for such qualification given in the EPI are:
(1) low probability of a coincidence of events;
(2) duplicated relief system being unduly complicated.

(c) Provision shall be made to verify the integrity of the pressure relief device
and for its replacement or repair as necessary without the protected
equipment. while in operation, being temporarily deprived of pressure
relief protection (unless other approved equivalent means of protection are
provided).
Note:
This implies that it is not acceptable practice to carry out repairs to a
pressure relief system, either, while the plant continues in operation or,
when the item is isolated only by means of valves. Physical disconnection
or blanking-off is needed.
(d) At the discretion of the Equipment Engineering Group Manager,
Technical Department, the setting of a relief device may be related to the
vessel test pressure rather than to the design pressure.
18.2 Accepted Practice from Other Sources
Generally speaking, small vessels (up to several liters capacity only) used
for research purposes can be exempted, provided that other protective
measures (such as the erection of blast walls or cubicle) are taken.
19 REQUIREMENTS OF GBHE - EDPs
Before commencing a study of pressure relief requirements for any plant
the engineers concerned should be familiar with the relevant parts of
EDPs noting particularly the two aspects which follow.
19.1 Registration of Protective Devices
GBHE - EDPs is mandatory with respect to registration of protective
devices connected to equipment that can be subjected to internal pressure
above or below atmospheric pressure.
Mechanical interlocks associated with isolating valves and vents where
closure would lead to a hazardous situation should be registered.
There are however a number of exceptions. the most relevant being given
in 2.5) of GBHE - EDPs: II Protective devices which indicate and/or
control some process function where failure to operate will not result in a
potentially hazardous situation".

Separate inventories are required for pressure vessels and protective
devices (including pressure relieving devices and instruments which act to
prevent or limit a pressure rise). Any device that could be easily modified
and thereby affect the required rate of flow through a pressure relief
device (e.g. control valve trim. bypass. restrictive orifice plate) should be
registered and its characteristics recorded together with the design data
for the relief device concerned (See Section 8).
The documentation required for registration purposes will include:
the data sheet for the device. (with any special features such as back
pressure considerations)
for safety/relief valves - a hydrostatic test certificate.
for bursting discs - a manufacturer's batch sample test
certificate.
The basis of the sizing calculations, stating the precise source(s) of
pressure generation. needs to be stated bearing in mind that HSE
inspectors may require this information - especially in the event of a
hazardous incident at any future date.
19.2 Inspection and Maintenance
Relief and blowdown teams should have a basic understanding of the
inspection requirements under GBHE - EDPs since details of design.
layout and selection of relief devices may be influenced by these
requirements - for example. when considering access to relief devices for
the purpose of inspection or when designing for ease of removal when
handling toxic materials.
Inspection of protective devices is covered by Section 1. Clause 3 of
GBHE - EDPs and guidance on examination is also given. All protective
devices are required to be examined regularly and no protective device
shall be taken into service until the interval between examination and the
nature of any necessary tests have both been specified and recorded. The
Design Authority should. in conjunction with the Inspection Engineer.
define these intervals after taking into consideration the process materials
contained in the system and the process operating conditions as well as
the legal requirements. In case of doubt. the shorter interval shall be
adopted at least until experience of operation has been gained.

Where Hazard and Operability Studies have shown that following an
operation of the relief device to discharge materials, there will be risk of
the relieving capacity being impaired (e.g. by partial choking of the lines).
that relief system shall be inspected after each operation of the relief
device. This may require the fitting of a monitoring device to indicate the
event.
All safety valves shall be tested in the condition as removed from the plant
subject to the need for decontamination.
Recommended practice for the inspection of safety valves (and in fact
whole relief systems) is given in Appendix G.
20 RECOMMENDED PRACTICE
20.1 Use of External Standards in the United Kingdom
Acceptable practice for the technical design of pressure relief systems in
the U.K. is based on the standard specifications of the:
(a) British Standards Institution (BSI).
(b) American National Standards Institute (ANSI).
(c) American Petroleum Institute (API).
(d) International Organization for Standardization (ISO).
(e) American Society of Mechanical Engineers (ASME).
Published British Standards specifically related to, or referring to, relief
systems include the following:
BS 759: Part 1 Valves for steam boilers and boiler installations.
BS 1123 Safety valves for compressed air installations.
BS 1113 Steam generating plant.
BS 2790 Shell boilers • construction
BS 5500 Design of pressure vessels
Appendix J gives pressure relief requirements.

BS 2915 Bursting discs and assemblies.
BS 4434 Refrigeration safety.
BS 853 Calorifiers and hot water storage.
BS 759, BS 1123 and BS 2915 include both general requirements for the
relief system and also mechanical design and construction requirements
for the valves or bursting disc.
The principal U.S.A. standards have wide acceptance worldwide and, in
fact, are mandatory in certain countries • useful guidance to U.S.A.
practice being contained in the ASME Code Sections UF 125·135 and
Appendix M. Recourse to appropriate U.S.A. standards has usually been
made whenever British standards and GBHE Engineering Procedures or
Specifications have been inadequate. Many of the requirements have
been adopted for Company use, especially in the sizing and specification
of safety valves. The standards of the International Organization for
Standardization (ISO) are being developed and extended and can be
expected to replace many national standards during the next few years.
Thus ISO 4126 covers the principal requirements for the design and
construction of safety valves, while forthcoming specifications will cover
bursting discs (approximating to BS 2915) and bursting disc • safety valve
combinations. The interrelationship of the principal documents relating to
various aspects of pressure relief is shown in Figure 1.
The recommendations which follow. based on externally published
standards (references given), are generally accepted as essential
minimum requirements whenever appropriate. For the convenience of the
user of this Guide, some extended summaries are given in the form of
Appendices but the user should always consult the original document if in
any doubt about all its implications.
20.2 Steam Boilers and Associated Equipment
Steam boilers include any vessel in which steam is generated whether
solely for the purpose of a steam supply or in association with a chemical
process. The requirements beyond the statutory ones contained in the
Factories Acts are covered in three British Standards: BS 1113. BS 2790
and BS 759.

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