Introduction:-
Fire fighting foam is an aggregate of gas filled bubbles formed from aqueous solution of specially formulated concentrated liquid foaming agents, namely foam concentrates.
The gas used is normally air, but in certain applications it may be an inert gas. Since foam is lighter than the aqueous solution from which it is formed, and lighter than flammable liquids, it floats on all flammable or combustible liquids, producing an air excluding, continuous layer of vapour sealing material that halts or prevents combustion.
Foam is produced by mixing foam concentrate and water at the appropriate concentration, and then aerating and agitating the solution to form the bubble structure. Some foam is thick and viscous form a tough, heat resistant blanket over the burning liquid surface and vertical area; other foams are thinner and spread more rapidly. Some, foam is capable of producing a vapour sealing film of surface actives water solution on a liquid surface.
CLICK {"Major Tank Fire Extinguished by using foam. "}
Several properties of foam concentrates are of importance to he fireman, as follows;
i)Fluoro chemical or synthetic foam concentrates usually contains about more than 30 percent or organic solvent, such as anti-freeze and viscosity reducing agents. Contact of the concentrate with the skin should be avoided, as the combination of the surfactant, which acts as a detergent, and the solvent acting as a decreasing agent, may cause damage of the skin as a form of dermatitis.
ii) The surfactant in all foam concentrates are water pollutants and such foam concentrates should not be exercises where they could drain directly into the water courses and damage fish and plant life.
iii) Protein foam concentrates contains 5-10 percent of sodium chloride and therefore has the same effect as sea-water; equipments must be thoroughly washed with clean water after use to avoid corrosion.
Properties of Foam:-
It is already stated that foam is an aqueous solution of surfactant which has been converted into a mass of bubbles by vigorously mixing with air. When foam is made there are three factor which can be varied.
.a) The volume of air mixed with each volume of concentrate, i.e. "The Expansion".
b) The quantity of foam concentrate added, i.e., " The Concentration ".
c) The " Size of the bubbles ".
CLICK {Demonstration of AFFF FE(In hindi) }
CLICK {Demonstration in English}
•Classification by Expansion:
i)Low expansion (LX):
Expansion ratio is up to 50:1 but usually between 5:1 and 15:1.
ii) Medium Expansion (MX) :
Expansion ratio from 50:1 to 500:1.
iii) High Expansion (HX) :
Expansion ratio from 500:1 to 1500:1.
Classification by constituents:
I) Protein Foam Concentrate.
II) Fluoro Protein Foam Concentrate.
III) Fluoro-chemical Foam Concentrate.
IV) Synthetic Foam Concentrate.
V) Alcohol Resistant Foam Concentrate.
VI) Chemical Foam.
#Types of Foam:
A)Aqueous Film Forming Agents (AFFF) :
Aqueous film forming foam agents are composed of Synthetically produced materials that forms air foams similar to those produced by the protein based materials. In addition these foaming agents are capable of forming water solution films on the surface of flammable hydrocarbon liquids, hence the term "aqueous film forming foam" (AFFF) is used.
B) Fluro Protein Foaming Agent(F.P) :
The concentrates utilised for generating fluoro-protein foams are similar in composition to protein foam concentrates, but in addition to protein polymers, they contain fluorinated surface active agents that confer a "fuel shedding" Property to the foam generated. This makes them particularly effective for fire fighting conditions where the foam becomes coated with fuel, such as in the method of surface injector of foam for tank fighting, and nozzle or monitor foam application where the foam may often be plunged into the fuel.
C) Film Forming Fluoro-Protein Agent (FFFP) :
Film forming Fluoro-Protein agents are composed of protein together with film forming fluorinated surface active agents, which makes them capable of forming water solution films on the surface of flammable liquids, and of conferring a fuel shedding property to the foam generated.
D) Protein Foaming Agents(P) :
The concentrates contain high molecular weight natural proteinaceous polymers derived from a chemical digestion and hydrolysis of natural protein solids. The polymers give elasticity, mechanical strength, and water retention capability to foams generated from them. The concentrate also contains dissolved polyvalent metallic salts, which aid the protein polymers in their bubble strengthen capability when the foam is exposed to heat and flame.
E)Alcohol Resistant Foaming Agents(AR):
Air Foam generated from ordinary agents are subject to rapid breakdown and loss of effectiveness when they are used on fires that involves fuels which are water soluble, water miscible or of a "polar solvent" Type. Examples of this type of fuel are alcohol, ketones, Acetones, acetates, amines, etc. Even small amount of these substances mixed with common hydrocarbon fuel may cause rapid breakdown of ordinary fire fighting foams.
F) Medium and High Expansion Foaming Agents :
Medium and high expansion foam are agents for control and extinguishment of class 'A' and small class 'B' fires and are particularly suited as a flooding agents for use in confined space. The foam is an aggregation of bubbles mechanically generated by aspiration or a blower fan which forces air or other gas through a net or screen that is wetted by an aqueous solution of surface activity foaming agents. Under proper conditions, fire fighting foams of expansions from 20:1 to 1000:1 can be generated.
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