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Fire Extinguishment
A fire can be extinguished by destroying the fire tetrahedron. If the fuel, oxygen or heat is removed, the fire will die out. If the chain reaction is broken, the resulting reduction in vapour and heat production will extinguish the fire. Removing the fuel. One way to remove the fuel from the fire is to drag it physically away. In most instances, this is an impractical fire fighting technique. However, it is often possible to move nearby fuels away from the vicinity of a fire. Example of fuel removal is cutting a fire break in front of forest fire. Removing the oxygen. A fire can be extinguished by removing its oxygen or by reducing the oxygen level in the air to below 16%. Many extinguishing agents extinguish fire with a smothering action that deprives the fire of oxygen. This method of extinguishment is difficult (but not impossible) to use in an open area. Gaseous smothering agents like carbon dioxide would be blown away from an open area. In a confined space carbon dioxide enters the space and mixes with the atmosphere. The percentage of oxygen in the atmosphere is reduced below 16 % and extinguishment results. Removing the heat. The most commonly used method of extinguishing fire is to remove the heat. Cooling the material will stop the formation of combustible vapours, and the fire will be extinguished. The seat of fire is attacked with water to destroy the ability of the fire to sustain itself. Water is a very effective heat absorber. The production of vapour and radiant heat is reduced. Breaking the chain reaction. Once a chain reaction is broken, a fire can be extinguished rapidly. The extinguishing agents commonly used to attack the chain reaction and inhibit combustion are dry chemicals. These agents directly attack the molecular structure of compounds formed during the chain reaction. The attack is extremely rapid: in some automatic systems the fire is extinguished in 50 to 60 milliseconds. These agents do not cool a smouldering fire or a liquid, their container has been heated above the liquid's ignition temperature. It is best to use water for cooling along with extinguishing agents that attack the chain reaction. Extinguishment of paints. As liquid paints contain low flash solvents, water is not a suitable extinguishing agent and in this case foam must be used. Surrounding materials have to be cooled with water. Water is also the proper extinguishing agent for dry paint. Extinguishment of gas fire. Gas fire can be extinguished with dry chemicals. Carbon dioxide may extinguish certain gas fires. However, these fires, present a severe radiant hazard to firefighters. Additionally, there is the danger of gas escaping. Dry chemical and water spray offer good heat shields from the radiant heat of gas fire. Extinguishment of metals. Fires involving most metals present an extinguishment problem to firefighters. Frequently there is a violent reaction with water, which may result in spreading the fire or explosion. Sand, graphite, various other powder extinguishing agents and salts of different types are applied to metallic fires.
So, in the conclusion it should be underlined that three factors are essential to combustion, namely: 1) The presence of a fuel, or combustible substance. 2) The presence of oxygen or other supporter of combustion 3) The attainment and maintenance of a certain minimum temperature. Fire extinction, in principle, consists in the limitation of one or more of these factors, and methods of fire extinction may therefore be classified under the following headings: 1) Starvation, or limitation of fuel. 2) Smothering, or limitation of oxygen. 3) Cooling, or limitation of temperature.
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