Foam system should include provision to minimize the danger when foam is applied to the liquids above 100°C, energized electrical equipment or reactive materials. Since all foams are aqueous solutions, where liquid fuel temperatures exceed 100°C they may be ineffective and, particularly where the fuel depth is considerable (e.g., tanks) may be dangerous in use. The foam and drainage of the water from the foam can cool the flammable liquid but boiling of this water may cause frothing or slop-over of the burning liquid particularly crude oil. Boil-Over, which may occur even where foam is not applied, is a more severe and hazardous event. Large-scale expulsion of the burning contents of a tank is caused by the sudden and rapid boiling of water in the base of the tank or suspended in the fuel. It is caused by the eventual contact of the upper layer of liquid fuel in the tank, heated to above 100°C by the fire, with the water layer.
Particular care should be taken when applying foam to high viscosity liquids, such as burning asphalt or heavy oil, above 100°C. Because foams are made from aqueous solutions they may be dangerous to use on materials that react violently with water, such as sodium or potassium, and should not be used where they are present. A similar danger is presented by some other metals, such as zirconium or magnesium, but only when they are burning. Low-expansion foam is a conductor and should not be used on energized electrical equipment; in this situation, it would be a danger to personnel.