Four ways to extinguish fire, and the extinguishing approach for each fire class.
The extinguishment of fire is based on an interruption of one or more of the essential elements in the combustion process. With flaming combustion, the fire may be extinguished by reducing temperature, eliminating fuel or oxygen, or by stopping the uninhibited chemical chain reaction. If a fire is in the smoldering mode of combustion, only three extinguishment options exist: reduction of temperature, elimination of fuel or oxygen.
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Extinguishment by Temperature Reduction
One of the most common methods of extinguishment is by cooling with water. The process of extinguishment by cooling is dependent on cooling the fuel to a point where it does not produce sufficient vapor to burn. If we look at fuel types and vapor production, we find that solid fuels and liquid fuels with high flash points can be extinguished by cooling. Low flash point liquids and flammable gases cannot be extinguished by cooling with water as vapor production cannot be sufficiently reduced. Reduction of temperature is dependent on the application of an adequate flow in proper form to establish a negative heat balance.
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Extinguishment by Fuel Removal
In some cases, a fire is effectively extinguished by removing the fuel source. This may be accomplished by stopping the flow of liquid or gaseous fuel or by removing solid fuel in the path of the fire. Another method of fuel removal is to allow the fire to burn until all fuel is consumed.
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Extinguishment by Oxygen Dilution
The method of extinguishment by oxygen dilution is the reduction of the oxygen concentration to the fire area. This can be accomplished by introducing an inert gas into the fire or by separating the oxygen from the fuel. This method of extinguishment will not work on self-oxidizing materials or on certain metals as they are oxidized by carbon dioxide or nitrogen, the two most common extinguishing agents.
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Extinguishment by Chemical Flame Inhibition
Some extinguishing agents, such as dry chemicals and halons, interrupt the flame producing chemical reaction, resulting in rapid extinguishment. This method of extinguishment is effective only on gas and liquid fuels as they cannot burn in the smoldering mode of combustion. If extinguishment of smoldering materials is desired, the addition of cooling capability is required.
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Classification of Fires & Extinguishment Methods
Class A Fires
Fires involving ordinary combustible materials, such as wood, cloth, paper, rubber and many plastics. Water is used in a cooling or quenching effect to reduce the temperature of the burning material below its ignition temperature.
Class B Fires
Fires involving flammable liquids, greases and gases. The smothering or blanketing effect of oxygen exclusion is most effective. Other extinguishing methods include removal of fuel and temperature reduction.
Class C Fires
Fires involving energized electrical equipment. This fire can sometimes be controlled by a non-conducting extinguishing agent. The safest procedure is always to attempt to de-energize high voltage circuits and treat as a Class A or B fire depending upon the fuel involved.
Class D Fires
Fires involving combustible metals, such as magnesium, titanium, zirconium, sodium and potassium. The extremely high temperature of some burning metals makes water and other common extinguishing agents ineffective. There is no agent available that will effectively control fires in all combustible metals. Special extinguishing agents are available for control of fire in each of the metals and are marked specifically for that metal.
Class K Fires
Class K is a classification of fire added in 1998 and involves fires in combustible cooking fuels such as vegetable or animal fats. Its fuels are similar to Class B fuels but involve high temperature cooking oils and therefore have special characteristics. Class K agents are usually wet chemicals, water-based solutions of potassium carbonate-based chemical, potassium acetate-based chemical, or potassium citrate-based chemical or a combination. These agents are usually used in fixed systems, but some extinguishers are available.
Source: Adapted from LBFD Training Division documentation.
