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5.4 Flammability limits
Prevention of unwanted fires and gas explosion disasters requires knowledge of flammability characteristics i.e. flammability limits, ignition requirements, and burning rates of pertinent combustible gases and vapors likely to be encountered under various conditions of use (or misuse). For a particular application the available data may not always be sufficient for use, as the data may have been obtained at temperature and pressure lower than is encountered in practice. The lower and upper flammability limits of a vapour/gas are the lowest and highest concentrations in air at normal pressure and temperature, at which the flame will propagate through mixture. They show the range of concentration will burn in air, if ignited. The flammability limit are determined experimentally. For example, the lower limit of flammability for hydrogen is 4.1 percent by volume and upper limit of 74.2 per cent by volume, whereas, for petrol the range is only 1.3 to 7.0 per cent. Flammability limits for various materials can be obtained from various sources (Sax's handbook, Lewis, 2004).
In some cases existence of flammable mixture in the space above liquid surface in a storage tank might occur. In such cases the vapour space above the liquid surface (highly flammable) is usually purged with inert gas or floating head tanks are used. The floating roof of the tank on the top of liquid eliminates the vapour space.
5.5 Explosions
The major difference between fire and explosions is the rate of energy release. In case of explosion there is a sudden release of energy (micro seconds) causing a pressure wave. While fires release energy slowly. An explosions can occur without fire such as failure through over pressure of a steam boiler or an receiver. Another simple example of how the energy releases rate affects the consequences of an accident is the case of a standard automobile tire. The compressed air in the tire contains energy. If the energy is released slowly through nozzle, the tire deflated without any harm. But if the tire ruptures suddenly and all the energy within the compressed tire releases rapidly, the result is a dangerous explosions.
While discussing the explosions of flammable mixtures it is necessary to differentiate between detonation and deflagration. In a deflagration the combustion process is same as in the normal boiling of a gas mixture and the combustion zone propagates at sub sonic velocity, if a mixture detonates the combustion is extremely rapid and the flame velocity is more than that of the velocity of sound (apporx. 30 m/s).
Mechanical explosions this explosions result due to the sudden failure of a vessel containing high pressure (non reactive gas)
Confined vapour cloud explosions (CVCE)
An explosion occurs within a vessel or a building due to release of a relatively small amount of flammable material (few kg).
Unconfined vapour cloud explosions (UCVCE)
Unconfined explosions results from the leakage of a considerable quantity of flammable gas, or vapour into atmosphere, and its subsequent ignition. In case of UCVCE the gas is dispersed and mixed with air until it comes in contact with an ignition source. Such explosion can caused extensive damage since large quantities of gas and large areas are frequently involved.
Boiling liquid expanding vapour explosions (BLEVE)
This type of explosions occur due to ruptures of a vessel which contains a liquid at a temperature above its atmospheric pressure boiling point, when an external fire heats the contents of a tank of volatile material. As the tank contents heat up, the vapour pressure of the liquid within tank increases and its integrity reduces due to excess heating. In such conditions if the tank ruptures the hot liquid volatilizes explosively.
Dust explosions: this type of explosions results from rapid combustion of finely divided solid particles. Metal such as iron and aluminium become very flammable when reduced to fine powder.