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:
Related Technical Measures Documents include:
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:
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 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:
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:
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.
The size of vent area required for effective control depends upon a number of factors including:
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.
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.
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.
A dust explosion can take place only if a number of conditions are simultaneously satisfied:
Dust may be grouped into dust explosion classes as determined using standard test apparatus. KST 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:
|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|
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.
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:
If products show properties that indicate they can explode in the solid phase (i.e. 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.