GUIDELINES FOR FIRE PROTECTION WITH FOAMS

The following general rules apply to the application and use of ordinary air foams:

1. The more gently the foam is applied, the more rapid the extinguishment and the lower the total amount of agent required.

2. Successful use of foam is also dependent on the rate at which it is applied. Application rates are described in terms of the amount by volume of foam solution reaching the fuel surface (in terms of total area) every minute. If the foam has an expansion rate of 8:1, an application rate of 0.1 GPM/ft2 (4.1 Lpm/ m2) will provide 0.8 gal/ft2 (32.8 L/m2) of finished foam every minute. Increasing the foam application rate over the recommended minimum will generally reduce the time required for extinguishment. However, little time advantage is gained if application rates are increased more than three times the minimum recommended. If application rates are less than the minimum, extinguishment time will be prolonged or may not be accomplished at all. If application rates are so low that the rate of foam loss by heat or fuel attack equals or exceeds the rate of foam application, the fire will not be controlled or extinguished.

3. The minimum recommended application rate is the rate found by test to be the most practical in terms of speed of control and amount of agent required. The general curve in Figure 17.4.1 illustrates the rate-time relationship for foam application to a hazard. The curve may be displaced right or left depending on fuel, method of application, and type of foam concentrate; hence, the a need for carefully engineered systems based on the appropriate standard and actual test information.

4. In general, air foams will be more stable when they are generated with water at ambient temperature. Preferred water temperatures range from 35 to 80°F (1.7 to 27°C). Either freshwater or seawater may be used. Water containing known foam contaminants, such as detergents, oil residues, or certain corrosion inhibitors, may adversely affect foam quality.

5. Foams are adversely affected by air containing certain combustion products. Although the effect is minor with ordinary air foam and ordinary hydrocarbon fuels, it is desirable to locate fixed foam-makers on the sides of, rather than directly over, the hazard.

6. Recommended pressure ranges should be observed for all foam-making devices. Foam quality will deteriorate if these limits (high and low) are exceeded.

7. Many air foams are adversely affected by contact with vaporizing liquid extinguishing agents, their vapors, and by some dry chemical agents.

*Extracted from FIRE PROTECTION HANDBOOK