What is it
To describe what actually firefighting foam is, the National Fire Protection Association (NFPA), provides a full definition:
“…an aggregate of air-filled bubbles formed from aqueous solutions which are lower in density than flammable liquids. It is used principally to form a cohesive floating blanket on flammable and combustible liquids, and prevents or extinguishes fire by excluding air and cooling the fuel. It also prevents re-ignition by suppressing formation of flammable vapors. It has the property of adhering to surfaces, which provides a degree of exposure protection from adjacent fires.”
So basically the foam is a collection of small air filled bubbles made of water mixed with the foam concentrate. The way it extinguishes fire is by forming a sort of a bubble blanket over the fire, suppressing it by cutting the flames contact with air. In addition to that the fuels are cooled down and in some types of foams (AFFF) a film is formed over the burning substance, which prevents any further inflammation.
What is it used for
The usage of foams is preferred over other fire extinguishing agents due to several reasons, some of which include the ability to suppress vapors and prevent their ignition even before a fire has started, further more foam concentrates can provide protection from easily flammable liquids during critical operations of rescue personnel. Another important property of foams is that they can be used to cover oil (or any other type of fuel) spills with a bubble blanket and thus avoid any possible fire ignition.
Basically it is used to extinguish or prevent fires by:
Cooling the fuel surface with the water content of the foam
Emulsifying the fuel
Preventing the release of flammable vapors from the fuel surface
Excluding oxygen from the fuel vapors and thus preventing a flammable mixture.
However there are some situations where foam is not applicable, or at least not the best solution. Such cases are for example when electrical wiring has caught fire, as water can make the situation worse or bring further damage to the equipment or machinery; these are classified as Type C fires. Furthermore when a fire involving pressurized gasses needs to be extinguished, or fires on metals (Type D fires) or three dimensional fires as well, the usage of foam is ineffective. In such types of fires other types of extinguishing agents are to be used.
How is foam formed
To create foam which can be used for the extinguishing of fires by forming a blanket, four factors are required: Foam concentrate; Water; Air; Aeration. These four factors are combined when the foam concentrate is mixed with the water and aerated by being shot through a nozzle with a high speed. The devise which does this is a foam generator.