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ExplosionsThe definition of an explosion is difficult to formulate, as it is hard to differentiate between fast-burning fires and explosions. The basic difference between a fire and an explosion consists in the degree or rate of heat release. The definition of an explosion used in NFPA (The National Fire Protection Association) standard № 68 is the following “... the rapid development of energy due to burning a dust, vapour, or gas in a confined space". This release of energy develops forces or pressures capable of causing, in many cases, great physical damage and threat to life safety and a high property damage. Gas explosions occur when the gas-air mixture lies within the flammable range and is exposed to a source of ignition. The only difference between an explosion and the burning of a gas is the rate at which it occurs. Vapour explosions are caused by the vaporization of flammable liquids. The more volatile the liquid is, the more probable that it formed the vapour which exploded. Liquids in this case include petrol, alcohol, thinners, solvents and naphtha. These vapour mixtures are usually heavier than air and form a rich mixture close to the floor or the surface of the liquid. The rate of evaporation is a function of the surface area exposed to the atmosphere. Therefore, a spilled container, where the liquid has covered a floor surface with large quantity of a thin film of liquid, creates much more explosive atmosphere than an open container. But the longer a container remains open in an unventilated room, the more likely that the vapour will diffuse to the atmosphere and form a potential bomb. A dust explosion occurs when suspended dust particles are ignited. This phenomenon is similar to a vapour explosion in that1 the concentration of dust must exceed a lower explosive limit before an explosion can occur. Once the suspended particles have exceeded the lower explosive limit, they will explode by deflagration of the minute particles in the air. The violence of an explosion will cause heavier concentrations and result in further explosions generated from the first explosion. The ignition for subsequent explosions will be supplied by the initial explosion. Unlike vapours, the dust concentration has no upper explosive limit and deflagration will continue as long as material is available. There are also chemical explosions. A chemical explosion is usually a result of a chemical reaction which "got out of control". It commonly happens in processing plants2 or industrial installations where highly volatile chemical or flammable materials contact with each other, resulting in the generation of flammable gases or explosive reactions between the chemicals involved. Of the various types of explosions fire protection engineers are generally concerned with those which have an extremely high release of energy generated by rapid oxidation, termed deflagration. Energy release created by excessive pressure, such as a boiler explosion, rapid energy release created by rapid decomposition, such as usually occur in the use of high explosives, are known as detonation. The difference between explosion and detonation is based upon the rate and pressure produced. Notes 1. in that – тем, что 2. processing plant – нефтехимический завод
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