Elements of a Fire

FIRE
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, and chemical reaction can be stopped by inhibiting the oxidation of the fuel.

REMOVE HEAT
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

REMOVE FUEL
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

REMOVE OXYGEN
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.

 
CLASS A fires occur in ordinary combustible materials such as wood, cloth and paper. The most commonly used extinguishing agent is water which cools and quenches. Fires in these materials are also extinguished by special dry chemicals for use on Class A, B & C fires. These provide a rapid knock down of flame and form a fire retardant coating which prevents reflash

CLASS B fires occur in the vapor-air mixture over the surface of flammable liquids such as grease, gasoline and lubricating oils. A smothering or combustion inhibiting effect is necessary to extinguish Class B fires. Dry chemical, foam, vaporizing liquids, carbon dioxide and water fog all can be used as extinguishing agents depending on the circumstances of the fire.

CLASS C fires occur in electrical equipment where non-conducting extinguishing agents must be used. Dry chemical, carbon dioxide, and vaporizing liquids are suitable. Because foam, water (except as a spray), and water-type extinguishing agents conduct electricity, their use can kill or injure the person operating the extinguisher, and severe damage to electrical equipment can result

CLASS D fires occur in combustible metals such as magnesium, titanium, zirconium and sodium. Specialized techniques, extinguishing agents and extinguishing equipment have been developed to control and extinguish fires of this type.

Normal extinguishing agents generally should not be used on metal fires as there is danger in most cases of increasing the intensity of the fire because of a chemical reaction between some extinguishing agents and the burning metal

 The most commonly known flammable liquid is gasoline. It has a flash point of about -50° F (-65° C). The ignition temperature is about 495° E (232° C), a comparatively low figure. Burning gasoline has a temperature above 1500° E (945° C). Therefore, it can heat objects in the fire area above its ignition temperature. To prevent reignition after extinguishment, the agent should be applied for sufficient time to allow hot objects in the fire area to cool below the ignition temperature of the gasoline.

The flammable range of gasoline is only 1.3% to 6%. Gasoline vapors are heavier than air. They tend to flow downhill and downwind from liquid gasoline, making it possible for explosive mixtures to collect - in low points such as pipe trenches or terrain depressions.
If the amount of oxygen in a given atmosphere is reduced from its normal 21 per cent to 14 per cent, by diluting with carbon dioxide, most petroleum products cannot burn. As a result, a gasoline fire can be "suffocated" by diluting the atmosphere with an inert gas.

It is dangerous to use water in a solid stream on a gasoline fire because it may spatter the fuel or raise its level in a container so it overflows.