Automatic / Manual Fixed Foam System
An aqueous solution of a suitable foaming agent or concentrate. The gas used is normally air, but in certain applications, inert gases have been used. Foam for fire fighting purposes is essentially an aggregate of gas filled bubbles.
Foam is produced by mixing a foam concentrate with water to the appropriate concentration, and aerating and agitating the solution to form the bubble structure. The ratio of the volume of the made foam to that of the solution from which it is made is defined as the expansion. Other important foam properties are also defined. One of these is the critical shear stress, a measure of the stiffness of the foam and hence an inverse measure of its ability to flow readily over a fuel surface and around solid obstructions. Another is its drainage time, a measure of the stability of the foam, which relates both to its stiffness and ability to resist destruction by the radiant heat of the flames.
The three ranges are approximately:
(LX) Low expansion foam—expansion up to 20
Low expansion foams are generally used for the extinction of fires in flammable liquids by the formation of a blanket over the surface of the liquid, thereby reducing the rate of release of flammable vapors to the combustion zone below that which is necessary to maintain burning. The fires may include pool or spill fires, or fires in tanks and similar containers.
(MX) Medium expansion foam—expansion 20 to 200
(HX) High expansion foam—expansion 200 to 1000