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

Process Information Disclaimer

Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.



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.



The essential requirements of BS 759 are: (a) Two (or more) safety valves are required if the heating surface of a boiler exceeds 50 m2 or the evaporation rate exceeds 3700 kg/h (1 kg/s). Even for smaller boilers, two valves are recommended. (b) Two or more valves are required on a water heater of more than 2350 kW rating. (c) The total discharge capacity calculated by the formulae of BS 759 shall be at least as great as the evaporative capacity of the boiler or the maximum rating in the case of a hot water heater. (d) An additional safety valve is required in the outlet side of every superheater. (e) Reheaters and economizers shall be fitted with safety valves unless integral with the boiler. More detailed and specific requirements are given in Appendix A.

20.3 Air Receivers and Associated Equipment

This includes compressed air. instrument air and compressed nitrogen service systems. The requirements additional to those contained in the Factories Act 1961 are covered in BS 1123. the essential requirements of which are: (a) Each air receiver shall be fitted with a safety valve except where all are supplied from the same supply pipe; (b) The safety valves shall have sufficient capacity. determined in accordance with BS 1123, to prevent the pressure in the receivers from rising beyond 10% above the set pressure when the compressors are delivering their full capacity. More detailed and specific requirements are given in Appendix B.



FIGURE 1 : STATUTORY ACTS etc., CODES OF PRACTICE, GUIDES AND STANDARDS



20.4 Process Vessels and Plant Protection of pressure vessels is covered in BS 5500, which requires that a safety valve or other protective device shall be used to prevent the pressure rising more than 10% above design pressure. This requirement does not apply if the source of pressure is external to the vessel or pipe system and under such positive control that the pressure in the system cannot exceed the design pressure adjusted for the operating temperature. Appendix J to BS 5500 provides a set of rules for the design of relief systems which are generally applicable to process equipment. It does not adequately cover all aspects of design and refers only to safety valves conforming to BS 1123 and BS 759. Bursting discs are recognized as an alternative to safety valves (cf. steam boilers) but very little guidance on their application and specification use is included. Important requirements included in Appendix J are: (a) One relief device may be used for more than one process vessel if the vessels cannot be isolated from one another; (b) The total discharge capacity of the relief system shall be sufficient to discharge the maximum quantity of material generated or supplied without the pressure rising more than 10% above design pressure; (c) Isolating valves shall not be used except in special circumstances mentioned earlier (See Clause 18); (d) The design of the discharge pipe shall conform to specified requirements and the discharge shall be to a safe place. A shortened version of Appendix J of BS 5500 is given in an Appendix C.

20.5 Isolation of Relief Systems

Only in special circumstances, and subject to the following limitations, may relief systems be shut-off from the equipment they protect.



20.5.1 Steam Boilers (statutory)

The fitting of an isolating valve between the boiler and the safety valve(s) is not permitted.

20.5.2 Air Receivers

The fitting of an isolating valve between an air receiver and the safety valve is permitted only when: (a) The source of supply is also isolated by the same stop valve and also; (b) Provided that each receiver is fitted with a fusible plug to prevent excessive pressure rise due to fire.

20.5.3 Process Equipment (a) Basic rule: European chemical companies do not normally permit the installation of an isolation valve between a relief device and the protected system. This is in agreement with BS 5500. Certain exceptions to this rule are permitted under Appendix J of BS 5500 and may be invoked occasionally within the Company (See Clauses J.9.3.1 and J.9.3.2). (See also Appendix C). (b) Special Isolating arrangements - Interlocking: Isolating valves can be installed on a duplicated or triplicated relief system provided that they are interlocked in such a way that the system is at all times able to discharge at the required relief rate.

Thus it is permissible to employ three-way changeover valves provided that there is no position at which the net resistance to discharge is increased by the presence of the valves. (Such valves are usually designed so that the second flow passage opens before the first flow passage closes). (See Appendix C).

(c) Single relief device: The fitting of an isolating valve on a pressure relief system comprising a single relief device can be done on rare occasions subject to:



(1) agreement between the Design Authority and the Works; (2) the setting up of administrative procedures to ensure strict control of the operation of the isolating valve, the depressurizing (and decontamination) of the inter-space and removal of the relief device. (See Appendix F).

Cases where this arrangement have been permitted have usually been those where the sale function of the device was thermal relief of a pipeline system. See Part C, Section 8 of this Guide. Note: When the relief device discharges into a common relief header from which feedback could occur, an isolation valve may also be fitted on the outlet side of the device (See as 5500; Clause J9.3.2). Any such valve shall be interlocked with the inlet-side valve.

20.6 Relief Devices

20.6.1 Safety Valves The requirements for safety valves given in BS 759, BS 1123 and in the new draft standard for safety valves for process equipment (See BSI document 83/77090 (1982) relate to the design, construction, testing and certification of discharge capacity of valves for specific uses. These are supplemented by GBHE Specifications reviewed later in this Section (See Clause 21). It is mandatory to ensure that every safety valve is suitable for the service for which it is specified and that it conforms to the appropriate British Standard and/or GBHE Specification. Components (including safety valves and bursting disc assemblies) and materials should be purchased against GBHE Engineering specifications whenever appropriate but this is not always mandatory. The principal features that are specified in one or other of these documents are as follows: (a) Marking: to ensure that correct valves are always fitted.

(b) Construction details: minimum discharge areas and design of discharge passage, freedom from obstruction at all times, integrity of moving parts and specification of the spring design, material and tolerances, corrosion resistance, security of set pressure and in some cases provision of easing gear are covered.



(c) Classification: valves are classified according to the lift and whether or not the lifting action is assisted in any way. (d) Discharge capacity: values of discharge coefficient and associated flow formulae are specified.

Note: The usual formulae given are standard sonic flow equations arranged in a suitable form. Their derivation may, however, involve assumptions of coefficients that are not universally accepted. Use of any other form of calculation might have to be clearly justified in the case of an inquiry. The formulae given, however, are inadequate for many process applications, especially where two-phase flow is involved and more rigorous calculations are essential. This fact is recognized by the HSE Inspectorate and other external bodies. (See Part C, Section 5 of this Guide).

(e) Operating characteristics: limitations on overpressure and blowdown are included. (f) Testing and Certification: hydraulic and leak testing are specified; discharge capacity is normally tested on steam, air or water - appropriate conversion to rates for process fluids is necessary.

20.6.2 Bursting Discs BS 2915 and the GBHE Addendum cover bursting discs and bursting disc assemblies since the safe operation of a bursting disc depends so much upon the way it is installed into the relief discharge pipe; the whole assembly should therefore be regarded as the relief device. The use of bursting discs in conjunction with safety valves is permitted and often provides much benefit. See Part 0, Section 1 of this Guide. There are now a wide variety of bursting discs - simple domed, reverse buckling, supported or unsupported, metallic's, non-metallic's, etc. and it is obligatory to ensure that a suitable type of disc assembly is fitted. The selection of a suitable type demands more sophisticated knowledge and the British Standard recommends seeking a manufacturer's advice. To this end both the details that will be given to the supplier and those details required to be stated by the supplier shall be specified.



The use of discs confirming strictly to BS 2915 is not obligatory, but where it is not applied, or where it is inadequate, GBHE EDPs should be used. See also Appendix E. Though the Standard generally tends to be advisory rather than mandatory it gives clear warnings about such factors as: (a) Effect of operating temperature on bursting pressure. (b) Effects of corrosion on reliability. (c) Pulsating pressures and fatigue. (d) Use of discs for liquid relief. It includes the following requirements: (1) Discharge capacity: to be sufficient to discharge the maximum required rate of fluid whilst the pressure in the system does not exceed the design pressure by more than 10%. (2) Bursting pressure: the maximum bursting pressure (allowing for manufacturing tolerance) shall not exceed the design pressure by more than 10%. (3) Mounting: the requirements are designed to ensure protection of the disc, avoidance of blockage and correct replacement, noting that the risk of incorrect fitting is much greater than in the case of safety valves. (4) Testing: individual examination and sample testing is required - the proportion of manufactured items to be tested to bursting is specified. (5) Design of holders: several alternative types which are approved are described. Note: With a rapidly developing technology these requirements cannot at present be regarded as comprehensive or inviolable.



20.7 Installation This term embraces all aspects of relief systems other than the relief device itself and can be more widely interpreted as "design of the relief system". The requirements of the British Standards tend to be rather flexible, but again any total rejection of the principles could lead to an unacceptable proposal. They are covered in detail in Part 0, Section 4 of this Guide and therefore only briefly outlined here. Branches on vessels and connections on other equipment for relief devices are required to be dedicated and of adequate size. The sizing of pipelines on both sides of the relief device to limit the pressure drop to specified levels is of paramount importance. Mechanical requirements are specified in detail and this includes requirements for the design of piping supports adequate for the reaction forces imposed by rapid discharge through the system and detailed in Part 0 of this Guide. Provision for frequent inspection to ensure reliability at all times is included in the requirements. The point of discharge is also subject to hazard considerations. See also Part E of this Guide.

21 GBHE ENGINEERING SPECIFICATIONS

Two specifications are relevant: - Helical Compression Springs for Safety Valves. - Bursting Discs and Bursting Disc Devices - lei Addendum to BS 2915 : 1984. These specifications are purchasing specifications for relief devices and provide very little in the way of mandatory requirements or advice appropriate to other features of relief systems. Both documents provide useful background by drawing attention to some important aspects of these devices and their installation that can have a bearing on the choice of device and design of system. See also Part 0 of this Guide.



SECTION 7 : GBHE SYSTEMS AND PROCEDURES FOR REPORTING AND RECORDING PRESSURE RELIEF STREAM DESIGN 22 INTRODUCTION

It is a requirement in GBHE that al\ pressure relief streams are documented and the design verified. Design verification of a pressure relief stream involves a check that the requirements of the regulations, codes, standards etc. are satisfied and that those used are appropriate for the pressure system. The documentation is also checked for completeness but it is assumed that the relief philosophy and calculations have been approved prior to verification. Design verification is mandatory for all new and modified relief streams in chemical companies in the U.K. Outside the U.K. a similar process will be followed subject to local conditions. Design verification has to be completed before equipment is brought into service and is therefore an important part of any plant modification or new installation. It is more efficient to carry out the design and design documentation together. There are many GBHE Procedures, Guides and Specifications available to assist engineers in pressure relief design and in meeting design verification requirements. This Section provides an overall guide to the formal. GBHE documents. Guidance is also given on the most efficient way to carry out the design process in order to achieve Design Verification.

23 CODES, REGULATIONS, STANDARDS AND PROCEDURES

GBHE supports statutory requirements and relevant Standards with GBHE Engineering Procedures which relate both to new equipment and modifications to existing equipment. Section 5 discusses the statutory requirements for the design of pressure relief systems and Section 6 deals with GBHE mandatory requirements and recommended practice. Part F of this Guide considers specific equipment and gives advice on how statutory, code or good engineering practice requirements can be satisfied.



In GBHE certain Engineering Procedures are mandatory. They are listed in GBHE EDPs which includes Engineering Procedures relating to registration, design verification and inspection of relief streams. These cross reference further procedures which deal with documentation, design philosophy and project management. Outside GBHE there are no requirements to use the formal documentation described in this Section. However, GES 4 shall be satisfied and there may be national legislative conditions which demand external verification of designs. Where no such legislation exists, designs should be documented and subject to an appropriate approval system. A summary of the relevant GBHE Policies, Procedures, Guides and Specifications is given in Appendix H. The procedures require completion of standard data sheets as part of the design process. They also require all designs of new and modified pressure relief streams to be subject to design verification. Registration, Construction and Inspection activities associated with pressure relief are also covered by appropriate GBHE Engineering Procedures. Their requirements are outside the scope· of this Guide.

24 DESIGN AND DESIGN DOCUMENTATION 24.1 General

Documentation of the design is an important part of the design process and is partly achieved by completion of the appropriate data sheets recording process and project philosophy and design decisions. Different designs require different levels of detail being specified on the various data sheets and supporting documents. The main factors in this are: (a) The number of pressure relief streams involved. (b) The effects (if any) on existing installations. (c) The complexity of the design.



There are two alternative ways of treating the design and documenting of pressure relief streams. The broad groupings are: (1) Stand-alone Pressure Relief Streams of Uncomplicated Design: Generally, these are where pressure relief requirements can be simply described for new equipment. See 24.2. (2) Projects Involving Several Pressure Relief Streams or Interrelated Streams of Complex Design: These are where the design of one pressure relief stream needs to be considered in association with others. See 24.3. Appendix J shows how the two approaches fit into the design and documentation process.

24.2 Stand-alone Pressure Relief Streams of Uncomplicated Design The following steps are involved: (a) Initially all potential sources of over/underpressure should be identified and assessed and a decision made as to what equipment requires protection. This Guide and GBHE EDPs give detailed design guidance. This stage is documented as a design basis or relief philosophy. It focuses on what the protected equipment is being protected against and how. The "how" will include the pressure relief stream. It should also indicate the sources of over/underpressure which have been considered. Where significant, the required relief rates should be quantified and recorded. Reference should be made to relevant calculations. a form should be used and the relief philosophy should be incorporated and reviewed within the project hazard studies as appropriate. (b) A sketch of the pressure relief stream, or the system of which it is part, usually helps to explain and clarify the relief philosophy. Form should be used. (c) Protective device design: The type of protective device selected should be given; (see Part D of this Guide or consult the appropriate Piping Section). Data sheets should be completed for the pressure relief devices.



These specify both the process requirements for the device (Part 1). and the mechanical details of the chosen device (Part 2).

The supplier of the pressure relief device is usually expected to complete most of the information on Part 2 of the data sheet. The designer responsible for the mechanical engineering of the pressure relief stream should complete the sections of Part 2 of the data sheet which comprise GBHE requirements or details of the device specification previously agreed with the supplier.

(d) The pressure relief stream number should be shown on the engineering line diagram, and may also be shown on general arrangement drawings and detail drawings. 24.3 Projects Involving Several Pressure Relief Streams or Interrelated Streams of Complex Design

Managers of larger projects need to decide the overall pressure relief design policies at an early stage. GBHE EDPs gives guidance of what to cover and how to record the key aspects of pressure relief policy. GBHE Engineering projects should meet the requirements of the Design and Project Review procedures, which details the design reviews which may be required during a project and responsibilities for specific activities. This procedure reflects good practice, and is recommended for larger site-controlled projects. Individual pressure relief streams which protect several items of equipment or are of complex design should have decisions affecting all relief streams documented. Create a form to detail the decisions for individual relief streams, see 24.2.

24.4 Packaged Units and Proprietary Equipment Packaged units and proprietary equipment such as compressors, chillers, calorifiers etc. which are covered by the Pressure Systems and Transportable Gas Containers Regulations 1989 should have the data sheets completed by the supplier on the Standard Forms. Equipment outside these regulations e.g. refrigeration systems with a total installed power not exceeding 25 kW, positive displacement pumps for liquids, vehicle braking systems etc. may still require design verification.



Agreement on the level of documentation required should be reached with the supplier, the user and GBHE Inspection prior to purchase of the equipment. Where formal documents are required, completion of the data sheets should be made a condition of the order. GBHE specifications used to define the requirements will normally state this.

25 PRESSURE RELIEF STREAM DESIGN VERIFICATION A fully documented, checked and approved design (based on the standard data sheets outlined in 24.2 and 24.3) shall be submitted to an Authorized Person (Defined in GBHE EDPs) for Design Verification and issue of form. Verification ensures that: (a) The design of the pressure relief stream conforms with the specified requirements. (b) So far as practicable all relevant Regulations, Codes and Standards are complied with. (c) Good engineering practice is embodied within the pressure relief design. (d) The design has been fully documented Forms signed off. The Authorized Person for Design Verification applies judgment and experience in assessing the suitability of the design for the purpose as documented. The design basis and calculations are not checked at this stage. It is assumed that the design team has followed their agreed approval procedure prior to verification. Appendices J and K summarizes those aspects of pressure relief stream design which should be documented and which are reviewed in Design Verification. Appendix M shows some key areas which may need to be considered in design and verification.



26 SPECIFICATION REQUIREMENTS FOR PRESSURE RELIEF DEVICES Part D of this Guide gives guidance on the selection of pressure relief device types and their associated standards and code requirements. The following GBHE specifications should be applied, when appropriate, and be quoted on all orders: Safety valves: direct spring loaded Safety valve springs Bursting discs and bursting disc devices Orders should reference the appropriate GBHE Engineering specification. Variation from the Specifications should only be made with the agreement of the appropriate Piping Section. Outside the U.K. local national standards may apply. GBHE Specifications for bursting discs and safety valves should be regarded as additional specification requirements. Guidance on specific manufacturers' products can be obtained from the Piping Section pressure relief team.

27 ADVICE In addition to local site advice, GBHE Engineering is involved in Pressure Relief. 27.1 Integrated Safety and Environmental Technology

GBHE advises on the process engineering design aspects of pressure relief. These cover evaluating pressure relief requirements in general and include specialization in reactor relief and two-phase flow. The Section supports this Guide and some GBHE approved computer programs for sizing pressure relief stream piping.

27.2 Piping Section

GBHE Engineering specializes in the mechanical engineering aspects of pressure relief. There are dedicated Pressure Relief. This team is responsible for Design Verification of pressure relief streams and advise on pressure relief requirements. GBHE Engineering also offers a pressure relief stream mechanical design service.



27.3 In-Service Inspection Section Part of the Quality Assurance and Inspection efforts of GBHE Engineering - in-service inspection service. The duties and responsibilities are mainly in ensuring equipment is fit for its service duty. In addition, we prepare and maintain the Company procedures for registration and inspection of pressurized systems. Guidance and advice on all registration and inspection matters can be provided.

27.4 International Engineering Standards and Professionalism This Group is responsible for policy matters affecting pressure systems within GBHE. 28 SITE CONSIDERATIONS

Each pressure relief stream is required to have a unique number. Appendix L discusses how this can be done. Similarly, every protective device is required to be uniquely numbered. In addition, equipment such as restrictor orifices, control valves, slam shut valves, flame arresters etc. should be numbered and referenced in the documentation if they limit the required relief rate. Issue of these numbers is managed by the individual sites and should be obtained as soon as possible during design. Other factors which may affect the basis of the required relief rate such as fireproof lagging should be detailed in a form and will usually be referenced on the verification certificate. Spares requirements should be discussed at an early stage. Plant standardization can influence the selected device or supplier. All new safety valves should have their cold differential test pressures confirmed by an GBHE managed bench test prior to installation. The Project Manager (or equivalent) should ensure that the testing station receives the relevant test information in good time. All new and modified pressure relief streams should have an installation check prior to commissioning. This is needed to confirm that it has been installed exactly as designed. This confirmation should be recorded in the equipment file. All chemical sites in the UK are required to have a system for inspection of pressure relief streams.



This includes the holding of an equipment file for each relief stream. The file should contain inspection requirements for the stream and inspection records in addition to the design information recorded on forms referred to in this Section of the Guide.

29 RESPONSIBILITIES

The GBHE Engineering Procedures (see Appendix H) describe responsibilities for initiating and managing the design and related activities associated with pressure relief. Responsibilities for carrying out individual steps vary with organization, location and system. In general, process engineering skills are required for steps (a) and (b) in sub clause 24.2, with both process and mechanical engineering skills required for steps (c) and (d). Some sites and some GBHE Engineering project groups have laid down procedures which define who does what. The GBHE Mechanical Engineering Managers have overall Company responsibility for design verification.

30 AVAILABILITY OF DOCUMENTS AND FORMS

Copies of blank forms mentioned in Clause 24 are available from GBHE Engineering at (preferably quoting the "source" EDP). All documents referenced in Appendices H and K are available from GBHE Engineering.



APPENDIX A: BRITISH STANDARDS RELEVANT TO SAFETY VALVES AND RELIEF SYSTEMS FITTED TO STEAM BOILERS - BS 1113 AND BS759 A.1 INTRODUCTION

The requirements have been selected for quick guidance, from those which relate to pressure relief of steam boilers and associated equipment. Many of these requirements are applicable to process equipment and should be considered when defining relief systems for process plants. There is some overlap between these two specifications which are also referred to in relation to selection of relief devices. (See Part D, Section 1 of this Guide) CAUTION: ALWAYS REFER TO THE CURRENT VERSION OF THE STANDARD FOR FULL REQUIREMENTS

A.2 REQUIREMENTS OF BS 1113 (a) All boilers having a heating surface greater than 50 m2 shall be fitted with at least two safety valves. For boilers having a heating surface less than 50 m2 it is permitted to have only one safety valve, but it is recommended that two be provided. (b) Every superheater shall have at least one safety valve on the outlet side. Superheaters for design steam temperatures in excess of 425°C shall be fitted with safety valves having a capacity of not less than 20% of the maximum evaporation. Only when a boiler is fitted with an integral superheater without an intervening stop valve can the safety valves on the superheater be considered as part of the safety valve equipment of the boiler. (c) If an independently fired superheater is constructed for a maximum permissible working pressure lower than any of the boilers in which the steam it receives is generated, it shall be fitted with safety valves having a capacity not less than the maximum evaporation of such boilers.



(d) Every reheater shall have at least two safety valves with a capacity not less than the maximum steam flow for which the reheater is designed. At least one safety valve or valves having a capacity not less than 20% of the required total shall be located on the reheater outlet side. GBHE Note: Reheaters are used in turbine systems. (e) Economizers separated by a shut-off valve from the boiler shall be fitted with at least one safety valve. (f) Valves and fittings shall be constructed and tested in accordance with BS 759

A.3 REQUIREMENTS OF BS 759 A.3.1 General

(a) In no case shall a steam boiler with an evaporation rate of more than 3700 kg/h nor a hot water boiler with a rating of more than 2350 kW be fitted with less than two safety valves. (b) It is recommended ·that at least two safety valves be fitted to each boiler. (c) At least one safety valve shall be fitted on the outlet side of the superheater, where fitted, and may be considered a part of the safety valve complement of the boiler provided that there is no intervening stop valve between the boiler and the superheater. (d) Reheaters and superheaters separated from the boiler by an intervening stop valve shall be fitted with appropriate safety valves. These safety valves shall not be considered as part of the safety valve complement of the boiler. (e) The total rated discharge capacity of all the safety valves mounted on a boiler shall be calculated in accordance with the appropriate formulae given in BS 759 and be at least equal to the maximum evaporative capacity of the boiler in the case of steam boilers, or the maximum rating of the boiler in the case of hot water boilers.



GBHE Note: The "appropriate formulae" are simply convenient forms of basic sonic flow equations with numerical constants for steam; thus any equivalent form is acceptable.

(f) To prevent undesirable lifting of the safety valve(s) there should be a margin between the actual pressure at which the boiler generates and delivers steam and the set pressure of the safety valve(s).

GBHE Note: It is usually recommended that this margin should be at least 10% of the actual pressure at which the boiler generates steam.

(9) The base of the body seat of each safety valve connected directly to a steam boiler shall be not less than 20 mm diameter.

A.3.2 Economizer Safety Valves (a) Economizers, except those directly connected to the boiler without an intervening stop valve, shall be fitted with at least one safety valve. (b) Whenever practicable only 50 mm diameter safety valves shall be fitted. Several such safety valves shall be fitted to economizers containing more than 10 m3 of water, i.e. two for those containing over 10 m3 and up to 20 m3, three for those containing over 20 m3 and up to 30 m3 and pro rata. (c) Cast iron economizers to BS 1712 and BS 1713 arranged in groups or tiers having more than one pass of gases, and having circulatory piping between them shall have a separate safety valve mounted on each complete group or tier.

A.3.3 Mounting of Safety Valves

(a) Safety valves shall be mounted, without any intervening valve, on pads or branches used for no other purpose. The axis of the valve shall be vertical. The cross sectional area of the bore of each pad or branch shall be at least equal to the area of the bore at the inlet of the safety valve, or where two or more safety valves are mounted on the same pad or branch, at least equal to the sum of the areas of the inlet bores of all the safety valves.



(b) Branches shall be as short as possible so as not to impair the proper action of the safety valve or impose any undue stress on the branches at their point of attachment to the boiler. Nothing shall obstruct free flow to the safety valve. (c) Branches, particularly when full lift safety valves are mounted on them, should be radi used at the inlet.

A.3,4 Design of Safety Valve Discharge Piping

(a) Safety valve discharge piping shall be designed in accordance with SS 806. (b) The built-up back pressure shall not exceed 10% of the safety valve set pressure. In no case, however, shall the discharge piping be of less bore than the bore of the outlet flange of the safety valve. (c) For each safety valve fitted with discharge piping an individual unrestricted drain shall be provided. The drain pipe shall be laid with a continuous fall to a place where the discharge is visible and will not cause injury to any persons. (d) Discharge piping shall be so arranged that no excessive (mechanical) loads are imposed on the safety valve body under any conditions.

A.3.5 Easing Gear

(a) Safety valves connected to steam boilers associated with power generation plant shall be so arranged that, when they are under pressure, the mechanical load on the valve head can be eased by means of suitable gear. (b) Safety valves connected to steam distribution systems should not be fitted with easing gear. GBHE Note: Neither of these requirements are applied by lei to process plant since, in many cases, a greater hazard would be caused by making it easy to open the valve at any time.



A.3.6 Overpressure and Slowdown of Safety Valves

(a) The allowable overpressure is: 10% of the set pressure for ordinary lift and high lift safety valves. 5% of the set pressure for full lift safety valves (i.e. where the limiting area of discharge is 100%-80% of minimum area below the seat).

(b) Generally the blowdown shall be between 2.5% and 5% of the set pressure.



APPENDIX B: BRITISH STANDARD FOR SAFETY VALVES, GAUGES AND OTHER SAFETY FITTINGS FOR AIR RECEIVERS AND COMPRESSED AIR INSTALLATIONS· BS 1123 B.1 INTRODUCTION

The Standard covers, in addition to safety valves, pressure gauges and fusible plugs for compressed air equipment. Most of the requirements are applicable to nitrogen and inert gases also and have been selected for quick guidance. Detailed requirements of mechanical construction including springs and materials are specified together with testing procedures, and acceptable limits of overpressure and blowdown are specified. CAUTION: ALWAYS REFER TO THE CURRENT VERSION OF THE STANDARD FOR FULL REQUIREMENTS.

B.2 REQUIREMENTS OF BS 1123 B.2.1 Valves Needed and limits of Isolation

(a) Each air receiver used to contain air above atmospheric pressure shall be fitted with a suitable safety valve except that, where a set of air receivers is under uniform pressure supplied from a single supply pipe, the set may be treated as one receiver provided that the safety valve is fitted on the supply pipe. (b) When a stop valve or pressure reducing valve is installed between an air compressor and receiver(s), the pipe line on the compressor side of the valve shall be protected by a suitable safety valve so adjusted as to permit a discharge of air as soon as the design pressure of the piping or compressor is exceeded. This also applies to a non-return valve if it is of such a type that its failure could prevent the flow of air from the compressor. (c) It shall not be possible to isolate any safety valve from the air receiver(s) which it protects unless the source of supply is also isolated by the same stop valve.



(d) Where an air receiver(s) can be isolated from the associated safety valve, each receiver shall be fitted with a fusible plug to prevent excessive pressure rise due to fire. Such fusible plugs shall be fitted directly to the air receiver. (e) Where a pressure reducing valve is installed in a pipeline between the compressor and the receiver(s), the pipe line and receiver(s) on the low pressure side of the pressure reducing valve shall be protected by a suitable safety valve so adjusted as to permit air to discharge as soon as the design pressure of the low pressure system is exceeded. The discharge capacity of this safety valve shall be in excess of the maximum flow of the pressure reducing valve.

B.2.2 Discharge Capacity and Set Pressure of Safety Valves

(a) Safety valves shall be constructed, installed and adjusted so as to have sufficient capacity to permit the air to escape from the air receiver(s) without increasing the pressure beyond 10% above the set pressure when the air compressors are giving their full output and all outlets other than the safety valve(s) are closed. (b) To prevent undesirable lifting of the safety valve(s) there should be a margin between the maximum pressure at which the compressor delivers air and the set pressure of the safety valve(s). This margin should be 7-10% of the maximum delivery pressure of the compressor.

(c) The rated discharge capacity shall be determined by the formula given. (See NOTE under Appendix A, A.3.1 (e»).

B.2.3 Mounting of Safety Valves and Discharge Pipework

(a) Safety valves shall be mounted on pads or branches used for no other purpose (except for the fitting of a pressure gauge). (b) The axis of the safety valve shall be vertical. (c) The cross sectional area of the bore of each pad or branch shall be at least equal to the area of the bore at the inlet of the safety valve or, where two or more safety valves are mounted on the same pad or branch, at least equal to the sum of the areas of the bores of all safety valve inlets.



(d) Branches shall be as short as possible so as not to impair the proper action of the safety valves or impose any undue stress on the branches at their point of attachment. Nothing shall obstruct the free flow of air to, or from, the safety valve. (e) The outlet of the safety valve{s) shall be so arranged that discharging air will not cause injury to any persons. (f) Vent pipes from safety valves shall be as short as possible, but where the vent pipe is long it shall be sized so that the back pressure generated when the safety valve is discharging its maximum rated capacity does not impair the proper action of the safety valve.

B.2.4 Overpressure and Slowdown of Safety Valves The overpressure shall not be greater than 10% of the set pressure and the blowdown should be between 2.5 and 5% (10% for body seat less than 32 mm or set pressure less than 2 bar g).

B.2.5 Easing Gear Easing Gear shall be arranged so that the mechanical load on the valve head can be eased when they are under a pressure of not less than 75% set pressure. (See NOTE under Appendix A, A.3.5.)



APPENDIX C: BRITISH STANDARD FOR PRESSURE RELIEF PROTECTIVE DEVICES FOR PRESSURE VESSELS - BS 5500 APPENDIX J Note: The Appendix to the Standard is being substantially revised, therefore it may be necessary to issue a revision of this Appendix to the Guide in the near future. CAUTION: ALWAYS REFER TO THE CURRENT VERSION OF THE STANDARD FOR FULL REQUIREMENTS. C.1 INTRODUCTION

Appendix J states the requirements for the fitting of pressure relief devices and includes supplementary advice relating to types of valve and valve specifications. Though written mainly with safety valves in mind, most of the requirements apply either to safety valves or bursting discs. One section gives some advice about the use of bursting discs and of bursting discs in combination with safety valves. A section is devoted to the calculation of capacities for both vapor and liquid relief; the special requirements for marking both safety valves and bursting discs are given.

C.2 REQUIREMENTS OF BS 5500 APPENDIX J C.2.1 Pressure Relief Devices Needed and General Requirements

(a) Every pressure vessel shall be protected from excessive pressure or vacuum, by an appropriate protective device, except as provided for in (b). 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. (b) When the source of pressure is external to the vessel and is under such positive control that the pressure cannot exceed the design pressure, a pressure protective device need not be provided • for example: the generation of pressure by a compressor pump whose maximum output pressure cannot exceed the design pressure.



(c) Vessels connected together in a system by piping of adequate capacity which does not contain any valve that can isolate any vessel may be considered as a system of vessels for the application of pressure relief. (d) When a vessel is fitted with a heating coil or other element whose failure might increase the normal pressure of the fluid in the vessel, the designed relieving capacity of the protective device (Le. required capacity) shall be adequate to limit this increase to not more than 10% above the design pressure. (e) Consideration shall be given to the possibility of fire. It may be necessary to increase the size of the protective device or the capacity of the relief system to accommodate the extra discharge, and to make suitable arrangements to reduce the pressure to allow for the reduced strength of the hot vessel. (f) The design of safety valves and the choice of their materials of construction shall take into consideration the possible effect of differential expansion and contraction, and of gumming or deposits. (g) Heating of the safety valve and/or the inlet/discharge pipework should be provided where process fluids are prone to solidification or where ice is likely to be formed when the safety valve discharges.

GBHE Note: Do not forget to consider the hazards associated with failure of such a heating system.

(h) The total capacity of the pressure relief device or devices fitted to any vessel or system of vessels shall be sufficient to discharge the maximum quantity of fluid, liquid or gaseous, that can be generated or supplied without occurrence of a rise in vessel pressure of more than 10% above the design pressure.



C.2.2 Special Requirements

C.2.2.1 Drainage If the design of the safety valve is such that liquid can collect on the discharge side of the valve, an adequate drain shall be fitted at the lowest point where liquid can collect. Liquid may arise from normal operation of the safety valve, weepage or extraneous sources such as rainwater. Ensure that no hazard can arise from discharge into drain.

C.2.2.2 Safety Valve Location and Position The branch to which the protective device is connected should be dedicated to that use only. However, a vent or bleed valve, an excess flow valve, and/or a pressure tapping point may be connected to the safety valve inlet pipe.

C.2.2.3 Location in Gas and Liquid Space If a vessel contains both liquid and gas, and gas is to be relieved, a safety valve for use with gases shall be connected to the vessel in the gas space or to piping connected to this space and located in such a position so as to keep to a minimum the amount of entrained liquid when the safety valve discharges. Alternatively, if the liquid is to be relieved, a safety valve for use with liquid shall be connected to the vessel or piping below the liquid level at a point chosen to prevent ingress of gas. GBHE Note: In practice, with any system that flashes or "swells" when the pressure is reduced, gas and liquid are likely to be vented simultaneously, irrespective of where the vent outlet is located. See Part C, Section 5 of this Guide)

C.2.2.4 Proximity of Pressure Source The connection between the safety valve and the vessel shall be as short as possible and have a bore area at least equal to the area of the safety valve inlet. However, on installations where the pressure fluctuates and the maximum value is close to the set pressure of the safety valve, it may be advisable to relocate the safety valve or the connection in a region where the fluctuations are less severe.



In the case of connection to a high temperature system, it may be necessary to locate the relief device so that it is not affected by the high temperature. In the case of a safety valve a type fitted with a cooling spool should be fitted. In the case of a bursting disc a cooling section should be fitted. See Part D of this Guide.

C.2.3 Installation C.2.3.1 Accessibility for Maintenance

Easy access and sufficient work space and height shall be provided for the adjustment, servicing and removal of safety valves. To avoid difficult working conditions for maintenance (e.g. due to awkward location or high ambient temperature) the safety valve may be mounted further from the equipment it protects while ensuring that the inlet pressure drop is acceptable.

C.2.3.2 Mounting Position

Safety valves shall be mounted in a vertical position. Installing a safety valve in other than a vertical position adversely affects operation, because of induced misalignment and excessive friction between moving parts.

C.2.4 Isolation from Protected Equipment

Isolating (Stop) valves BS 5500 recognizes the very limited use of stop valves by the statement: "Intervening stop valves or cocks may be installed provided that they are so constructed and controlled by mechanical interlocks that a limited number only can be closed at any one time and that those stop valves or cocks which remain open are adequately sized to permit the unaffected pressure relieving devices to discharge at the required capacity for the vessel". GBHE Note: Full area stop valves or cocks may be occasionally installed in discharge systems serving a number of vessels provided the arrangement incorporates the principle given above and is agreed with the appropriate Authority (usually the Senior Inspection Engineer).



Arrangements shall, however, be made to ensure that any discharge from an operating vessel cannot flow into vessels out of use, or to safety valves undergoing maintenance.

C.2.5 Bursting Discs

(a) A bursting disc may be mounted in series with a relief valve provided that:

(1) the maximum pressure of the range for which the disc is designed to burst does not exceed the design pressure of the vessel. (2) the opening provided through the disc after breakage is sufficient to prevent interference with the proper functioning of the relief valve.

(b) Bursting discs fitted in place of, or in series with, relief valves shall be rated to burst at a pressure not exceeding the design pressure of the vessel adjusted for the operating temperature. When a bursting disc is fitted in series with a relief valve, the space between them shall be fitted with a device to indicate if and when leakage occurs. (c) Every bursting disc shall have a specified and certified bursting pressure at a specified temperature and shall be appropriately marked.

C.2.6 Set Pressure of Pressure Relieving Devices

(a) Relief valves shall be set to operate at a pressure not exceeding the design pressure of the vessel at the operating temperature, except as permitted in (b). (b) If the capacity is provided by more than one relief valve, only one of the valves need be set to operate as required by (a). The additional valve or valves may be set to discharge at a pressure not more than 5% in excess of the design pressure at the operating temperature provided the overall required relief rate is complied with. (c) The pressure at which a relief valve is set to operate shall include the effect of static head and maximum back pressure.



(d) In the case of bursting discs fitted in parallel with relief valves to protect a vessel against the consequences of explosion hazard, the bursting disc shall be rated at atmospheric temperature to burst at a pressure not greater than 1.3 times the design pressure. The relief valve shall be in accordance with (a).

GBHE Note: This is the wording of the specification quoted but the acceptance of this clause is questioned. This Guide does not cover explosion hazard as such and the clause is included because it refers to relief valves in parallel with bursting discs.

C.3 PIPING REQUIREMENTS C.3.1 Inlet Piping C.3.1.1 Flanges

Flanged connections in the pipeline between the safety valve and the vessel it protects (other than those flanges required for the vessel branch and the safety valve) are not recommended because of the possibility of inadvertent blanking-off or spading at the flanges and the consequent isolation of the safety valve from the pressure vessel.

C.3.1.2 Pressure Drop Limitation

The bore of the inlet piping shall in no circumstance be less than that of the safety valve inlet. The length of pipe between the protected equipment and the inlet of the safety valve shall be as short as possible and designed so that the total pressure drop to the safety valve inlet shall not exceed 3% of the set pressure of the safety valve. Greater pressure loss at the inlet of a safety valve may cause extremely rapid opening and closing of the safety valve, commonly known as "chattering", which may result in lowered capacity and damage to the safety valve seats and other components. The total pressure drop shall include any isolating valves (which should be of the full-ported type) and other line fittings and shall be calculated for the required relief rate.



The pressure drop can be reduced by rounding the entrance to the inlet piping and/or by increasing the bore of the inlet piping.

C.3.1.3 External Loads

It is essential to support the inlet piping adequately to ensure that no external mechanical loads are transmitted to the body of the safety valve. In addition, the inlet piping shall be sufficiently strong to withstand the effects of the reaction forces when the safety valve discharges.

C.3.1.4 Thermal Stresses

If the fluid to be relieved is at a temperature significantly different from ambient, provision shall be made to accommodate the thermal stresses induced in the inlet piping when the safety valve discharges.

C.3.2 Discharge Lines

(a) Wherever practicable, a pressure relieving device shall discharge to atmosphere through a vertical pipe, clear of adjacent equipment and any area normally accessible to personnel. Such a discharge pipe shall have at least the same bore as the relieving device outlet. Where discharge to atmosphere is not practicable, or where the nature of the contained fluid so requires, discharge shall be to a point where a hazard is not created. (b) Where discharge lines are long or where the outlets of two or more pressure relieving devices are connected into a common line, the effect of back pressure that may be developed shall be considered. (c) The size of a common discharge line serving two or more pressure relieving devices which may reasonably be expected to discharge simultaneously shall be based on the sum of their outlet areas, with due allowances for pressure drop in the downstream sections. (d) Discharge lines together with their supports and anchorages shall be so designed and constructed that reactions are resisted without excessive forces being transmitted to the pressure relieving devices.



APPENDIX D: DRAFT BRITISH STANDARD: BS DOCUMENT 82/70824 - SAFETY VALVES FOR USE IN THE CHEMICAL, PETROLEUM AND ALLIED INDUSTRIES This draft is a comprehensive document primarily concerned with mechanical design, construction and testing of safety valves for process equipment. It does, however, contain a considerable amount of background information data for calculation of discharge rates and guidance on installation. Clearly some of the requirements overlap with those of BS 759 and BS 1123. Recent information is that it may not be issued as a separate Standard but as one Part of the replacement for BS 759.

D.1 BASIC REQUIREMENTS

The proposed Standard quotes the BS 5500 requirements and lays down purchaser's and manufacturer's responsibilities with regard to adequacy of any safety valve to provide protection, thus: type of valve, discharge capacity, limit of accumulation, corrosion resistance.

D.2 VALVE TYPES Direct loaded and pilot operated safety valves are recognized and a range of types in each category listed. Basic features of acceptable designs are given with specifications for materials of construction for each part of the valve. Precautions against sticking and interference with the set pressure are included.

D.3 CERTIFIED CAPACITY Safety valves shall have a certified discharge capacity at an overpressure not greater than 10% for gas or 25% for liquid and the permissible blowdown is specified. Required operating and flow characteristics are quoted. Essential mechanical features are given with detailed requirements for the design and construction of the spring. Details of all mechanical and discharge tests are given noting that the latter may be done with any fluid of known characteristics which means that if there is any uncertainty about conversion from the usual steam, air or water tests, process fluids may be used for the tests. The coefficient of discharge is thus defined in relation to the quoted formulae for calculation of the discharge capacity for any fluid.



D.4 BACKGROUND INFORMATION

A considerable amount of general background, basic flow formulae and general safety requirements is included.

D.5 INSTALLATION REQUIREMENTS D.5.1 Valve Location and Connection

(a) The connection to a valve intended to relieve gas from a vessel containing liquid and gas shall be located so as to discharge gas and prevent entrainment.

GBHE Note: Clearly this is misleading and lacks an understanding of the effects of flashing and liquid "swell". Nevertheless, the recommendations given should be followed to minimize liquid carryover.

(b) The connection with the vessel shall be as short as possible and consideration given to the pressure drop through it. Where there are pressure pulsations and the set pressure is close to operating pressure, the connection should be located where pulsations are minimal. (c) Valves shall always be vertically mounted to minimize friction between the valve stem and the body. (d) Easy access for inspection and maintenance should be provided. (e) Welded connections shall meet the accepted standard for pipelines. (f) Generally no intervening stop valves are allowed though certain exceptions are admitted and these are in agreement with Part A, Section 6 of this Guide.

D.5.2 Inlet Piping (a) Flanged connections along the pipe should be avoided. (b) Any bursting disc in this line shall have a discharge area at least as large as the inlet pipe and safety provisions made for the inter- space.



(c) The bore shall not be less than that of the valve inlet. (d) The total pressure drop through the inlet pipe and fittings under maximum discharge shall be neither more than the blowdown pressure for the valve nor more than 3% of the set pressure. This pressure drop shall be calculated for the certified discharge capacity divided by 0.9. (e) Only a vent bleed valve or excess flow valve may be fitted to an inlet pipe. (f) Mechanical stress considerations shall ensure that:

(1) no mechanical loads are transmitted to the valve,

GBHE Note: For example connected pipework should be properly supported.

(2) thermal stresses are accommodated, (3) vibrations will not affect the valve.

D.5.3 Discharge Pipes

(a) They should be as short as possible and have a bore at least equal to that of the valve. (b) The use of a common discharge pipe should be limited and any superimposed back pressure should have no significant effect on any other valve. (c) The possibility of a liquid head developing should be avoided by provision of a drainage line. In the case of a closed disposal system, drainage should go to the disposal header. (d) Consideration shall be given to the possible effect of back pressure on both the set pressure and on the discharge capacity.

Back pressure shall be provided for by such actions as : (1) reduction of the set pressure, (2) use of a balanced bellows/piston valve.



D.6 BURSTING DISCS IN CONJUNCTION WITH SAFETY VALVES

A bursting disc may be used at the outlet subject to a number of conditions: (a) No effect on set pressure of the safety valve. (b) Flow area at least equal to valve outlet. (c) No risk of restriction by fragments. (d) Inter-space is vented and fitted with leakage detector.

GBHE Note: This requirement is under review. Current GBHE practice for some toxic fluids (e.g. CI2 is to rely on inter-space pressure monitoring.

(e) When used with balanced safety valves the sum of bursting pressure and the pressure on downstream side of the disc does not exceed stated values. (f) With conventional valves the sum of the bursting pressure and downstream pressure does not exceed the stated values.

D.7 MULTIPLE SAFETY VALVES When multiple safety valves are used in parallel to provide the required relief capacity, the set pressure of at least one shall be below design pressure. The total capacity shall be such that the permitted accumulation shall not be exceeded when discharging at the required relief rate; the effect of any reduced overpressure on the discharge capacity of each valve shall be considered and allowed for.

D.8 MISCELLANEOUS FACTORS (a) Open bonnet or exposed spring valves are only acceptable on process plants for steam and air duties. Advice is given on bonnet venting and on the discharge of a pilot valve. (b) Requirements of bolts, gaskets and joint surfaces are prescribed.



(c) Clean conditions and correct mode of storage of valves are specified. (d) Requirements for inspection and fitting are specified in order to prevent damage and incorrect fitting.

APPENDIX E: BRITISH AND ICI STANDARDS RELEVANT TO BURSTING DISCS AND BURSTING DISC ASSEMBLIES - BS 2915 AND GBHE ENGINEERING SPECIFICATION E.1 INTRODUCTION

The British Standard covers the application of bursting discs (alone or in conjunction with safety valves), installation, discharge capacity, some aspects of "preparation" (i.e. manufacture), holders and supports and testing procedures; it also provides information on various types of discs. It is a valuable guide to recommended practice but in view of the considerable progress made in recent years in the field of bursting disc technology, it cannot be regarded as fully authoritative in all aspects. In this field, manufacturers' advice is frequently accepted as good practice (and is recommended in the Standard) provided they conform to the basic requirements of the Standard. The GBHE Specification provides an addendum which " .... interprets and amplifies the requirements of BS 2915, particularly with regard to the correct specification of the disc and holders and the marking and identification requirements". CAUTION: ALWAYS REFER TO THE CURRENT VERSION OF THE STANDARD OR SPECIFICATION FOR FULL REQUIREMENTS

E.2 DEFINITIONS Definitions conforming to BS 2195 are given in the Glossary of terms, (See Part G, Section 1 of this Guide).



E.3 SELECTION Bursting disc materials should be chosen to suit the chemical and physical conditions on both sides and be designed for the temperature to which the disc will be exposed. Protection of the disc from corrosive materials, from pulsating pressures and the limitations of reverse buckling discs for liquid relief shall all be considered.

E.4 APPLICATION Subject to the provisions of the Factories Act, etc. a bursting disc may be used as sole safety device but its maximum possible bursting pressure shall not exceed 1.1 times the design pressure. For extremely high rates of pressure rise, the bursting pressure should be as close as practicable to the operating pressure and "much less than" the design pressure. A bursting disc fitted in series with a relief valve is subject to special requirements including: (a) Bursting pressure at operating temperature not exceeding design pressure. (b) Opening sufficient to prevent interference with functioning of relief valve. (c) Monitoring (and preferably venting) the space between the valve and the disc.

Note: This requirement is under review. Current GBHE practice for some toxic fluids (e.g. C12) is to rely on interspace pressure monitoring.

(d) If upstream of the valve ensuring that no particles resulting from the rupture can affect the operation of the valve. A bursting disc fitted in parallel with a relief valve (as an additional safeguard) is required to burst at the operating temperature at a pressure not more than 1.3 times the design pressure. (See Appendix C, C.2.6(d).



E.5 INSTALLATION The disc assembly should be as close as possible to the protected equipment, and discharge piping shall be of ample size. Discs should be easily accessible for inspection and replacement, and should be protected from all possible causes of damage: Precautions should be taken against any deposition on the pressure side that could affect the reliability. Provision shall be made for the following possibilities: (a) Mechanical effect of recoil when sudden discharge occurs. (b) Risk of ignition in discharge pipe.

E.6 DISCHARGE CAPACITY The venting capacity of a bursting disc used as a primary relief device shall be sufficient to discharge at the maximum required relief rate without the pressure exceeding 1.1 times the design pressure.

E.7 REQUIREMENTS OF DISCS AND HOLDERS These are set out for the three principal types: (A) Simple domed metallic discs. (B) Reverse domed (Le. reverse buckling) metallic discs. (C) Graphite (carbon) discs.

E.7.1 Types A and B For (A), (B) suitable metals, required properties and methods of testing are given - note especially that the pressure at which a test sample of foil bursts shall not vary from that specified by more than: 5% for bursting pressures more than 2 bar,

0.1 bar for bursting pressures less than 2 bar.



Other requirements are: (a) A specified pressure shall be used for doming. (b) Inspection for faults - use of a light box is needed for foil less than 0.2 mm thick. (c) A minimum number of discs, in relation to batch size, are to be tested. (d) Temperature of testing to be room temperature for Type A and at operating temperature for Type B. (e) The calculation of bursting pressure at room temperature (for Type A) from that required at operating temperature to be recorded.

For Types A and B details are given of a number of approved types of holder. These are not necessarily exclusive but give considerable guidance to types that are acceptable.

E.7.2 Type C

In this case, any failure of a test disc to burst within 10% (or 15% for membrane-covered discs) of its specified bursting pressure means that the whole batch fails to meet the Standard. Testing should normally be done at room temperature but no correction is made for temperature; the operating temperature should never exceed 180°C. Separate holders may be used as well as those which are integral with the disc (monobloc). For separate holders, the disc shall be firmly clamped without distortion - gaskets may be used. Discs for bursting pressures at or below 1.7 bar shall be supported for reverse pressure if it can occur.



E.7.3 All Types

Requirements are given for protection from corrosion under the following headings: (a) Coated metal foils. (b) Composite discs of plastics and metal foils. (c) Composite discs of multiple metal foils. (d) Graphite discs protected by membrane.

E.8 MARKING, IDENTIFICATION AND TEST CERTIFICATES

Owing to the greater risk of unsuitable bursting discs being fitted in error when compared with safety valves and the risk of wrongly fitting (e.g. facing wrong way), the marking of both discs and holders is specified. Again, since the disc itself cannot be tested, full details of batch sample tests according to the Standard shall be given on the Test Certificate. The Standard also specifies the information that shall be given by the purchaser to the bursting disc supplier. This requirement puts responsibility on both parties.

E.9 THE GBHE ADDENDUM - ENGINEERING SPECIFICATION

The Addendum was issued because the Company was not satisfied that BS 2915 covered all the requirements to ensure adequate safety with respect to the: (a) Specification of the bursting disc. (b) Specification of the holder supports, stiffening rings, etc. (c) Correct assembly. (d) Identification. Further authoritative advice is given on aspects which are not covered in the British Standard, for example:



(1) Surface finish of metal holders. (2) Testing of membranes and coatings. (3) Temperature adaptors for graphite discs. (4) Packaging to ensure that each disc is in perfect condition up to the time of fitting. (5) Positioning of bursting disc holders in pipeline. (6) Identification tags for disc and marking of holder .

APPENDIX G : RECOMMENDED PRACTICE FOR INSPECTION AND MAINTENANCE Relief systems. and equipment affecting the required relief rate. are registered and periodically examined; safety valves are periodically tested.



APPENDIX J : PRESSURE RELIEF STREAM DESIGN AND DOCUMENTATION



APPENDIX K : THE DESIGN OF PRESSURE RELIEF SYSTEMS - MAIN DESIGN STAGES AND USE OF DATA SHEETS K.1 PROJECTS INVOLVING MANY STREAMS

K.2 DESIGN OF EACH INDIVIDUAL RELIEF STREAM



APPENDIX L : NUMBERING OF PRESSURE RELIEF STREAMS L.1 PURPOSE

There is a requirement in GBHE that all relief streams are numbered and each number is unique within the operating site. The issue of numbers and support of a numbering system is a site responsibility. Similarly, all protective devices are required to have numbers unique within the site with control exercised in a way similar to relief streams. Before an equipment file for a relief stream can be set up, a relief stream number is needed. Design Verification of a relief stream cannot be completed without a relief stream number. It is therefore important that relief streams, pressure relief devices and associated items are identified by site reference numbers very early in a project. Application of the numbers has to consider: (a) The site system for numbering. (b) The connectivity of header systems. (c) Inspection requirements. It is important that relief stream numbers are attached to identified relief streams and relief header systems in a consistent manner. In particular, the extent of the relief stream, its start and end points should be defined on forms and any connectivity drawings and line diagrams. This Appendix considers some of the factors designers need to take into account and suggests how inspection requirements for relief streams can be incorporated into complex systems.

L.2 CONSIDERATIONS

(a) The final decision on numbering of relief streams normally rests with the site. Site policy should cover basic questions like: (1) should the relief stream number reflect the protected equipment number; (2) should the relief device number reflect the pressure relief stream number.



Circumstances which may need to be considered in this context are:

(i) relief streams protecting several items; (ii) regular replacement of relief devices.

(b) If more than one relief stream is required for protection of equipment, the numbering . system should consider if stream numbers need to be related. Figure 2 shows some options. In Figure 2(a) the duties of the streams are different but related. In Figure 2(b) the duties are the same but both streams are not required on line at the same time.

The consequences of using different numbering systems to those proposed is negligible in Figure 2(a). In Figure 2(b) a single stream number for the dual system, i.e. RS 1235, for both legs would mean that inspection of the stream would require the vessel to be shut down. This may be acceptable in some cases but defeats one of the most common reasons for installation of dual relief arrangements; viz. inspection whilst protected equipment is on line, removal of one relief device for maintenance.

(c) In Figure 2(b) inspection of the piping 1, 2, 3 can only take place when all the equipment is off line. The relief stream number RS 1235/1 may include pipes 1, 3, 2, 4 and RS 1235/2 may be 3, 5 so that there is a clear break point for inspection. It is not uncommon for piping 1, 2, 3 to be on an extended inspection interval consistent with the vessel inspection. (d) Figures 3(a) and 3(b) demonstrate two possible ways of numbering relief streams which serve vent header systems.

The principal considerations are: (1) the practicality of inspection; (2) flows through the header to enable verification to be accomplished without a repeat of the earlier hazard studies; (3) identification on site of the various streams.



It is not possible to give firm guidance on the best numbering methods since the weighting of factors (1), (2), (3) above and others not listed may vary from case to case. However, designers should aim for consistency throughout a project and if possible within a site.

FIGURE 2 : TYPICAL NUMBERING SCHEMES FOR SIMPLE RELIEF STREAMS

FIGURE 3 :TYPICAL NUMBERING SCHEMES FOR COMPLEX HEADER RELIEF SYSTEMS





