Fire burns because three elements are present – heat, fuel and oxygen. In technical language, fire is a chemical reaction: It happens when a material unites with oxygen so rapidly that it produces flame. Think of fire as a triangle. If any one of three sides – heat, fuel or oxygen – is taken away, the fire goes out. This is the basis for fire extinguishment. Heat can be taken away by cooling, oxygen can be taken away by excluding air, fuel can be removed to a place where there is no flame, chemical reaction can be stopped by inhibiting the oxidation of the fuel.
Cooling a fire calls for the application of something which absorbs heat. Although there are others, water is the most common cooling agent. Water is commonly applied in the form of a solid stream, finely divided spray or incorporated in foam.
Often, taking the fuel away from a fire is difficult and dangerous, but there are exceptions. Flammable liquid storage tanks pan be arranged so their contents can be pumped to an isolated empty tank in case of fire. When flammable gases catch fire as they are flowing from a pipe, the fire will go out if the flow can be valved off.
Oxygen can be taken away from a fire by covering it with a wet blanket, throwing dirt on it or covering it with chemical or mechanical foam. Other gases which are heavier than air, such as carbon dioxide and vaporizing liquid, can be used to blanket the fire, preventing the oxygen from getting to the fire.
STOP THE REACTION
Studies made during recent years have indicated that the familiar statement, “Remove heat, remove fuel, or remove oxygen, to extinguish a fire” does not apply when dry chemical or halogenated hydrocarbons are used as the extinguishing agents. These agents inactivate intermediate products of the flame reaction resulting in a reduction of the combustion rate (the rate of heat evolution) and extinguishes the fire. A more detailed discussion of this action appeared in the April 1960 issue of the quarterly of the NFPA under the title of “The Chemical Aspects of Fire Extinguishment.”