Hazard and Risk 07
Hazard and Risk 07 : Types of Explosions: If the explosion is caused by a chemical reaction then there are two basic terms, which are used to describe the mechanism or type of explosion; namely a Deflagration or Detonation. A deflagration occurs when the reaction front advances at less than sonic velocity into the unrelated material. For a rapid deflagration of a flammable vapour-air mixture, the flame front moves at a velocity of a few tens to several hundred metres per second. A deflagration may have varying degrees of violence ranging from cases with negligible blast damage effects (cf flash fire) to cases in which a distinct blast wave with potential for serious damage is present. The conditions cannot be specified as yet, under which the flame front may accelerate sufficiently to create a deflagration with significant blast damage effects rather than a flash fire. A detonation, on the other hand, is where the chemical reaction is extremely rapid and the reaction front advances into the unrelated material at greater than sonic velocity. In the case of explosions of vapour or gas clouds, a number of terms are used to describe the circumstances of an explosion. The principal two are Confined Explosion which describes an explosion of flammable vapour-air mixture inside a closed system (e.g. vessel or building) and Unconfined Vapour Cloud Explosion (often shortened to UVCE), which relates to an explosion of a flammable vapour-air mixture in the open air. The latter term is very widely used, but is imprecise, as, in practice, an UVCE will nearly always be partially confined due to the presence of buildings, structures, trees, etc. It is included in this listing due to its widespread use, but the term Vapour Cloud Explosion is preferred. There are many other terms used which fall into the category of describing the characteristics of an explosion. Most of these are self-explanatory, e.g. dust explosion, mist or aerosol explosion. The term dense phase explosion relates to an explosion caused by the chemical reaction of a solid or liquid material, such as TNT (trinitrotoluene). A blast wave may be created by other means than evolution of gases from a chemical reaction. For example, the term Pressure Burst relates to the rupture of a pressurised system and subsequent formation of a blast wave. Similarly, when a material rapidly changes its state a blast wave may be formed. For example the sudden release of pressure and subsequent flashing of a liquefied gas may contribute to the blast wave created by the pressure burst. Where the change of state is as a result of a significant temperature difference between two or more substances as they come into contact, then the term Rapid Phase Transition is used usually to describe this event, which may produce a blast wave; for example, the instantaneous vaporisation of water to steam on contact with molten metal. In the case of these explosions, there is no combustion process, only a release of physical, rather than chemical, energy. The term BLEVE (Boiling Liquid Expanding Vapour Explosion) is similar to the extent that the limited blast involved arises only from physical energy. The acronym BLEVE is now used widely and is abused. It was introduced originally in the USA to describe a specific sequence of events commencing with the sudden rupture due to fire impingement, of a vessel/system under pressure containing liquefied flammable gas. The release of energy from the pressure burst and the flashing of the liquid to vapour (flash fraction) creates material. However, immediate ignition of the expanding fuel-air mixture leads to intense combustion creating a fireball, which rises away from the ground due to buoyancy. This is the principal hazard, together with the missile effects of the ruptured containment system. In times, attempts have been made to widen the usage of the term BLEVE to include any sudden failure of a system containing any liquefied gas under pressure. It if felt that to avoid confusion, the name BLEVE should be avoided wherever possible and term such as pressure burst, flashing and fireball should be used to describe the particular scenario. If the term BLEVE is to be used then it is recommended that it should only be used in its original sense as described above.
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