Foam Extinguishers: The Right Choice for Flammable Liquids

By Mark Conroy

Although foam fire extinguishers are rated for use on both Class A and Class B fires, they are normally installed where a spill or pool-type fire could occur. When foam is applied from an extinguisher, it will naturally spread across the burning liquid surface, effectively smothering the fire. If the flammable liquid vapor is prevented from contacting air, the fire is extinguished. Although re-ignition is a possibility with other extinguishing agent applications, it is nearly impossible due to the foam blanket preventing fuel vapors from contacting air.

Hydrocarbon Fuels

Foam extinguishing agent is a solution of foam concentrate mixed with water. Film-forming foam agents are either aqueous film-forming foam (AFFF) or film-forming fluoroprotein foam (FFFP). Foam extinguishers for hydrocarbon fuels (common flammable liquids) are available in hand portables with capacities of 1.6 gal (6 L) and 2.5 gal (9.5 L) and in wheeled extinguishers with 33 gal (125 L) of foam. These foam extinguishers have ratings of 2-A:10-B, 3-A:20-B, and 20-A:160-B.

Polar Solvents

Water-soluble flammable liquids (polar solvents) such as alcohol, acetone, esters, and ketones require the need for special foam extinguishers. Badger’s Universal Ultra Foam 2.5 gal (9.5 L) extinguisher contains an “alcohol resistant” AFFF which is formulated to work on both polar solvents and hydrocarbon fuels. The Badger foam extinguisher is rated 1-A:30-B.

Specific Applications

Foam extinguishers are often the first choice for dip tanks, quench tanks, and vats of flammable liquid. For those applications, foam extinguishers are permitted to be provided on the basis of 1-B of protection per square foot of hazard area. For example, an area of 30 sq ft would require a foam extinguisher with a rating of 30-B (P/N 23697B).

Areas where flammable liquids can spill and be ignited often include objects that can block the application of conventional dry chemical or clean agent extinguishers. For those areas, foam extinguishers are the solution. Foam from an extinguisher not only blankets and secures the visible surface area; it also floats and covers the surface on the back side of objects.

Foam extinguishers should be considered wherever a flammable liquid surface area is a potential fire hazard. Foam covers the surface area, smothers the fire, and prevents re-ignition.

Specific Applications