Explosion relief

This Technical Measures Document refers to the explosion relief measures that can be adopted in plant design to ensure safe operation.

The relevant Level 2 Criteria are:

• 5.2.1.3(29)b,g
• 5.2.1.6(38)e
• 5.2.1.10(53)

Related Technical Measures Documents include:

• Inerting
• Earthing
• Plant layout
• Design Codes - plant
• Design Codes - pipework
• Plant modification / change procedures
• Maintenance procedures

General principles

Operators should demonstrate that appropriate measures are in place either to prevent explosions from taking place, or to protect against/minimise the effects of explosions.

Explosion prevention is always preferable to explosion protection, particularly where an explosion is likely to result in emission of toxic material.

Operators need to draw a clear distinction between pressure relief designed to protect against an explosion, and relief designed to protect plant from some other cause of overpressure. Explosions are rapid events and consequently:

• Protective devices have to work very rapidly;
• Much larger sizes of vent panels or doors are needed in order to work successfully and
• Duct work downstream from the relief panel needs careful design to avoid throttling the flow and preventing the relief acting fast enough;

Historically, explosion relief has been used as a mitigation measure in three main types of plant: large-scale gas fired combustion plant, solvent evaporating processes/ovens and plants handling explosible dusts. It may also be provided on buildings, which contain plant that gives rise to risk of a rapid and substantial release of gas inside the building. Compressor houses are an example. The reports on the Manro and Chemstar incidents show the possibilities with distillation plant, but inadequate explosion relief was not the prime problem in this case.

The advent of improved gas control systems means that in many cases explosion relief is not now fitted to combustion plant. A European harmonised standard for solvent evaporating ovens prefers alternative precautions for many applications, but in the dust handling industries explosion relief remains a widely used mitigation measure. This is because it is often impossible to prevent the formation of dense dust clouds inside the process. The dust itself, or mechanical moving parts in contact with the dust often create ignition risks that cannot be eliminated completely.

The COMAH Regulations do not apply to a material if the only risk created is that of a dust explosion. However, many toxic materials are handled in fine powder form, and a serious dust explosion could cause a major accident. A dust explosion involving a non-toxic dust like polyethylene would not result in a major accident as defined in the regulations, unless it also led to loss of containment of a COMAH substance. A dust explosion could then be an initiator of a major accident. Measures to prevent major accidents should address all potential initiators.

• The material released from an explosion relief vent typically includes quantities of the original, unburnt substance in addition to combustion products. Alternative mitigation measures are available, including explosion suppression, or building the plant strong enough to withstand the anticipated explosion pressures. The adoption of measures such as these is likely to be necessary where the process of venting could itself lead to the release of sufficient toxic material to create a major accident. However, it is worth noting that even finely powdered solids are significantly less easily dispersed than are gases and volatile liquids - with shorter hazard ranges as a result.
• In assessing the risks from an explosion, Operators should consider flame propagation, pressure effects, recoil forces and the materials that would be released. An explosion of gas or vapour will release hot combustion products, but continuing burning is only likely if there is a continuing source of release, perhaps because nearby pipework has been damaged. A dust explosion, however, will usually release burning and unburnt dust, and may well disturb dust deposits in the area. A secondary explosion or continuing fire is much more likely. These factors will have a significant impact on plant layout, design of plant and supporting structures and explosion relief routes. Relief points from explosion protection devices should normally be located outside the building containing the plant, but certainly not in the vicinity of regularly occupied areas or plant that would be easily damaged.

The ACOPs and Guidance to DSEAR discuss the alternatives for explosion prevention and mitigation measures. In particular they provide a hierarchy of controls. This hierarchy relates to intentional or unintentional releases, but the precautions to prevent an explosion inside the process plant are not the same. Normally the following options should be considered to prevent an explosion inside the plant:

• Substitution of combustible materials;
• Control of concentration, e.g. by excluding air, or purging with air before start up of combustion plant
• Inerting, exclusion of oxygen by use of inert gases
• Monitoring and detection of smouldering particles with automatic quench systems (specific to dust explosions);
• Elimination of ignition sources;

Having implemented these precautions, risks may remain in the ALARP region. The ALARP demonstration should then include an assessment of the following options for mitigation:

• Containment (explosion-resistant construction);
• High speed isolation;
• Segregation (keeping catalysts or pyrophoric materials apart from other products);
• Explosion pressure relief (venting);
• Explosion suppression.

Explosion vent panels and doors are considered as autonomous protective systems within the meaning of the ATEX equipment regulations. As such, new panels or doors must comply fully with the essential health and safety requirements, and be tested and certified by a Notified Body. Existing panels do not need to be replaced, but they should be suitable for the purpose.

A harmonised standard for the design and testing of explosion vent panels and doors is under preparation within CEN TC 305, as EN 14491.

General principles that relate to sizing methods for events that are slower than an explosion are covered in the Technical Measures Document Relief Systems / Vent Systems.

Sizing of explosion panels

Explosion panels for gases and vapours

The size of vent area required for effective control depends upon a number of factors including:

• The explosion properties of the gas or vapour;
• The size of the vessel to be vented;
• The geometry of the hazardous region;
• The strength of the plant;
• The opening pressure of the vent panel/door and its inertia;
• Any initial or induced turbulence in the system;
• The location of the ignition source;
• The influence of any vent ducts needed.

In particular the possibility of pressure piling needs to be considered. If an explosion that starts at one location inside a plant causes the explosive mixture ahead of the flame front to be compressed, much higher ultimate pressures can result. In extreme cases, explosion flames can accelerate to detonation, generating very high pressures and explosion relief is unlikely to be a suitable method of protection in this case. See TD5/039, Detonations.

Recent work by HSL has extended our knowledge of the pressures likely to be developed by gas/vapour explosions in a variety of circumstances. This work is summarised in Paper 18 by Lunn and Pritchard from the IChemE's Hazards XVII conference held in March 2003. Operators are unlikely to present safety cases drawing on this work in the immediate future, but it can be used to check predictions, and if they vary sharply from the content of a safety report, Operators could be asked to reconsider the implications.

BS EN 1539 2000¹ recognises the need for explosion relief for some designs of ovens, and recommends using NFPA 68 as no European standard was available at the time the standard was approved. This remains the case.

Explosion vent relief sizing panels for dusts and powders

A considerable amount of experimental data has been used to develop empirical design equations. Many different equations have been published, each with a restricted range of applicability, and plants will be found which have used all these design equations. More recently, work within CEN has produced a rationalisation and simplification of the design equations recommended, and this standard should be adopted when it becomes available. Checking of calculations is best done by the computer expert system DUST EXPERT, developed by HSE, and available to relevant process safety specialists. Basic input data required is the strength of the process equipment, the equipment volume, KST and P max figures for the dust itself, and the opening pressure of the vent panel or door.

Some equipment suppliers use the German VDI 3673 guide as a basis for the design of explosion relief vents. This is acceptable, and much of the advice is contained in the other sources quoted.

Alternatively, the IChemE Dust Explosion book should be consulted.

Explosion relief from buildings

HSE's traditional advice for buildings storing flammable liquids has been to ask for a lightweight roof, which will act as explosion relief if a vapour cloud ignited within the building, and this has not changed as a result of DSEAR. Stores designed for storage of petroleum liquids under earlier legislation normally had heavy concrete structures, to provide maximum protection for the stored product from a fire in the vicinity. COMAH removed licensing for petroleum stores at COMAH sites, and DSEAR removed licensing for all drum stores. There is insufficient justification to seek structural alterations to buildings, if any petroleum-type stores are found on COMAH sites.

A vent sizing routine for buildings is given in the NFPA 68 code, and repeated in the IChemE Dust Explosion book, but without the explosibility constant for organic vapours; the missing value of C₁ = 0.045 is given in the NFPA code.

Dust explosions (especially in powder transfer and dryers)

A dust explosion can take place only if a number of conditions are simultaneously satisfied:

• The dust must be explosible (refer to table on dust explosion classes);
• The dust must have a particle size distribution that will allow the propagation of flame
• The atmosphere into which the dust is dispersed as a cloud or suspension must contain sufficient oxidant to support combustion;
• The dust cloud must have a concentration within the explosible range;
• The dust cloud must be in contact with an ignition source of sufficient energy to cause an ignition.

Dust may be grouped into dust explosion classes as determined using standard test apparatus. K_ST is defined as the maximum rate of pressure rise measured in a 1m3 vessel, it is usually calculated from measurements in smaller test apparatus. These groupings are as follows:

Grouping

• ST0     
  Not explosible
• ST1 KST less than 200  
  Slow explosion, but these can still be very destructive - 80% of explosible dusts fall in this group
• ST2 200<KST=<300      
  Medium speed explosion
• ST3 KST>300   
  Very high speed explosion, this group includes mainly metal dusts

Exothermic reactions

Overpressurisation of reactors is addressed in the Technical Measures Documents Relief Systems / Vent Systems and Reaction / Product Testing.

Hazards from exothermic reactions occur in the event of thermal runaway of the reaction mixture in which the rate of generation of heat is greater than the available cooling capacity of the system. Pressure relief needs to take into account the nature of the reaction mixture involved, (e.g. viscosity, boiling gas/liquid mixtures) and sizing routines for gas and dust explosions are not appropriate.

Various testing strategies and experimental methods are commonly available for determination of thermal decomposition hazards. The operator should have shown due consideration of these hazards and taken appropriate measures to provide pressure relief. Measures for provision of pressure relief in these cases are addressed in the Technical Measures Document on Relief Systems / Vent Systems.

Unstable substances

When unstable substances are in use, the operator should demonstrate that at the research stage of the product a systematic approach to the identification of hazards relating to the nature of the materials has been followed. These hazards should be identified and documented, with subsequent evidence of implementation of control measures. Hazards that merit consideration include:

• Explosibility;
• Thermal and pressure conditions;
• Flammability;
• Toxicity;
• Environmental problems.

If products show properties that indicate they can explode in the solid phase (ie the energy release comes from decomposition not combustion), explosion relief is an inappropriate mitigation measure.

The assessment is specifically concerned with the physical properties of the products, and possible by-products.

Guidance and Codes of Practice relating to explosion relief

• HS(G)103 Safe handling of combustible dusts: precautions against explosions, Health and Safety Executive, 2nd Edition, 2003.
  A guide to the prevention, control and mitigation of dust explosions, aimed at operators of such plant, not designers. Paragraphs 60 to 87 discuss explosion relief and plant design and layout.
• HS(G)51 Storage of Flammable Liquids in Containers, Health and Safety Executive, 1998.
  This asks for a lightweight roof for HFL stores.
• Approved Codes of Practice and Guidance to the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR)
• Discipline Information Note, DIN TD5 039 Detonations, Helen James, Oct 2001, HSE Intranet.
• Dust Explosion Prevention and Protection: A Practical Guide, IChemE, 2002 Edition, J Barton (Editor) ISBN 085295 410 7
  This should be the basic design reference for new plant.
• CIA Guidance for the location and design of occupied buildings on chemical manufacturing sites, (version 2) CIA/CISHEC, 2003.
• BS EN 1127-1: 1998¹: Explosive atmospheres. Explosion prevention and protection. Part 1. Basic concepts and methodology, British Standards Institution, 1998.
  Good general introduction to the subject, but not detailed enough for design purposes. Comments that 'explosion relief into the workrooms shall not be permitted unless there is proven evidence that persons cannot be endangered'.
• BS 5908: 1990¹: Code of practice for fire precautions in the chemical and allied industries, British Standards Institution.
  This provides many cross-references to other sources of information, but some of these are obsolete, and the whole document is about to be rewritten. Section 5, Paragraph 27.1 provides guidance on minimising explosion risks from processes by location of plant in the open air or in open-sided structures, excluding personnel while the plant is operating and the provision of explosion relief vents. Section 10, Paragraph 58.5 provides guidance on ignition energy of powder atmospheres and methods of handling powders to reduce risks.
• BS EN 1539 2000¹: Dryers and ovens in which flammable substances are released. Safety requirements, British Standards Institution.
• VDI 3673, Pressure relief of dust explosions, Verein Deutscher Ingenieure - Kommission Reinhaltung der Luft, Germany, 1995. Influential and widely used. Gives specific advice, but with a tendency to ignore all non German sources of research data and guidance.
• NFPA 68, Guide for venting of deflagrations, National Fire Protection Association, Quincy, USA, 1988.
  Useful while there is little European guidance on venting for gas explosions.
• Gas Explosion Handbook (in English), produced by the Norwegian research organisation, Gexcon.
  This requires you to sign in, but needs no password. It has much information about explosions presented simply with good graphs and diagrams. Useful for understanding the factors that influence an event. Also some brief video clips of explosion experiments.

Further reading material

• The Fire and Explosion at Chemstar Ltd September 1981, HMSO
• The Fire and Explosion at Manro Products Ltd December 1982, HMSO
• Dust Explosions in the Process Industries, 2^nd Edition 1997, R K Eckhoff ISBN 07506 3270 4
  A comprehensive survey of the topic, with a critical discussion of a large range of research work.
• Prevention of fires and explosions in dryers, Abbott, J. A. Institute of Chemical Engineers ISBN 0 85295 257 0, 1990
• Hazards from pressure: exothermic reactions, unstable substances, pressure relief and atmospheric discharge, European Federation of Chemical Engineering, EFCE Publication Series No. 59, Institution of Chemical Engineers, 1987.
• Flammable liquids and gases: Explosion hazards, FS6011, Fire Protection Agency.
  The section on explosion prevention provides guidance on the reduction of the likelihood of explosions of gas or vapour/air mixtures by:

• Provision of ventilation, where plant is within buildings, to reduce the concentration of gas or vapour to a safe value;
• Construction of plant so as to prevent gas or vapour escaping, forming explosive mixtures with air and reaching ignition sources;
• Removal or protection of ignition sources, especially electrical sources

References

1. For further information on the relevant British Standards, please access the British Standards web site and use the search facility. Back