Foam Injection Systems

Foams have been developed almost entirely from experimental work. While the technologies are rather mature no fundamental explanations of foam extinguishing performance have been developed based on first principles. As a result, foams are characterized by (1) fire tests for which there is no general international agreement and (2) physical and chemical properties witch may or may not correlate with empirical results. Firefighting foam consists of air-filled bubbles formed from aqueous solutions. The solutions are created by mixing foam concentrate with water in the appropriate proportions. (Typically 1, 3, or 6 percent concentrate to water) The solution is then aerated to form the bubble structure. Some foams, notably those that are protein based, form thick, viscous foam blankets on hydrocarbon fuel surfaces. Other foams, such as film formers, are much less viscous and spread rapidly on the fuel surface. The film formers are capable of producing a vapor sealing film of surface-active water solution on most of the hydrocarbon fuels of interest.
Since the foam is lighter than the aqueous solutions that drains from the bubble structure, and lighter than flammable or combustible liquids, it floats on the fuel surface. This produces an air-excluding layer of aqueous agent, witch suppresses and prevents combustion by halting fuel vaporization at the fuel surface. If the entire surface is covered with foam, the fuel vapor will be completely suppressed and the fire will be extinguished.
Low expansion foams (i.e. foam volume: solution volume of ≤ 10:1) are quite effective on two dimensional (pool) flammable and combustible liquid fires, but not particularly effective on three dimensional fuel fires. This is particularly true of three-dimensional fires involving a low flash point fuel. Typically, an auxiliary agent,such as dry chemical, is used with foam where a three dimensional fire (running fuel or pressurized spray) is anticipated.