Limitations on the Use of Foam
Foams are effective extinguishing agents when used properly. However, they do have some limitations,including the following:
- Because they are aqueous (water) solutions, foams are electrically conductive and should not be used on live electrical equipment.
- Like water, foams should not be used on combustible-metal fires.
- Many types of foam must not be used with dry chemical extinguishing agents. AFFF is an exception to this rule and may be used in a joint attack with dry chemical.
- Foams are not suitable for fires involving gases and cryogenic (extremely low temperature) liquids.
- If foam is placed on burning liquids (like asphalts) whose temperatures exceed 100°C (212°F), the water content of the foam may cause frothing, spattering or slopover. Slopover is different from boilover, although the terms are frequently confused. Boilover occurs when the heat from a fire in tank travels down to the bottom of the tank and causes water that is already there to boil and push part of the tank’s contents over the side. Certain oils with a high water content, such as crude oil, have a notorious reputation for boilover. Slop-over occurs when foam, introduced into a tank of hot oil [surface temperature over 100°C (212°F)] sheds its water content due to the high heat.The water forms an emulsion of steam, air and the foam itself. The forming of the emulsion is accompanied by a corresponding increase in volume. Since tanks are three dimensional, the only place for the emulsion to go is over the sides of open tanks or into the vents of enclosed tanks.
- Sufficient foam must be on hand to ensure that the entire surface of burning material can be covered.
In addition, there should be enough foam to replace foam that is burned off and to seal breaks in the foam surface.