Using Foam to Extinguish Flammable Liquid Fires
The Volunteers Corner
A successful use of foam to extinguish a flammable liquid fire depends as much on the planning done before the fire occurs as the tactics on the fireground.
Whether you use protein or high expansion foam, you should be aware of the area you can cover with the foam concentrate carried on your apparatus and how quickly you can cover this area with your foam nozzles or generators. These facts are important to successful extinguishment.
For example, flames will break down foam and there is always a certain loss of foam as it is applied to the surface of burning liquid. Therefore, unless the application rate is large enough, the fire will consume the foam before a foam blanket can be formed to completely cover the liquid surface.
When a tank, stationary or vehicle, is involved in a liquid fire—and usually there is a tank involved—your first concern must be to cool the tank with water. So now your foam will be broken down not only by the flames, but also by spray from hose streams cooling the tank.
Area and breakdown: This brings us to the problem of how much foam concentrate is needed to handle a fire. The first consideration is the size of the area that must be covered with foam and secondly, the extent of foam breakdown from flame, water and other factors must be evaluated. The amount of breakdown is a nebulous problem that defies any exact solution, but experience can provide reasonable estimates.
There is no such thing as a partially extinguished flammable liquid fire. If there is a lick of flame left after you have exhausted your supply of foam concentrate, then the fire will build up to its former intensity and go out only after consuming all the fuel.
It is generally best to delay a foam attack until you have enough foam concentrate on hand to complete extinguishment. Otherwise, you could exhaust the supply on the first alarm apparatus without extinguishing the fire, see the flames regain their former intensity and then find that the additional foam you called for still won’t do the job. On the other hand, if you delay your foam attack, a continuous application of foam from the supplies on the first alarm apparatus and those brought in later might be more than ample for extinguishment.
Estimating foam needs: The size-up will determine the approximate size of the area of flammable liquid that must be covered with foam. We like to aim for a 6-inch blanket of foam for security, but we know that countless flammable liquid fires are extinguished when there is no more than a 1 or 2 inches of foam covering the flammable liquid. This buys us time to get more foam on the flammable liquid to minimize the possibility of reignition until the spill has been disposed of by pumping into tanks, draining into a safe area or inerting with a chemical emulsifier, sand or earth.
Three gallons of 3 percent protein foam, at an 8-to-l expansion ratio, will make about 800 gallons of expanded foam. Although this foam is rated at a 10-to-l expansion ratio, I prefer the lower ratio as more realistic on the fireground to take care of metering adjustment, ordinary breakdown by the fire and some loss in application. The 800 gallons of foam make 106 cubic feet of foam, and this will provide a 6-inch blanket over a surface of about 200 square feet. From this, we can see that 1 gallon of 3 percent concentrate will provide one third of these figures, or about 265 gallons of expanded foam that will cover about 70 square feet of surface with a 6-inch blanket.
The 70 square feet are equivalent to an area of 7 X 10 feet, and this can be a useful measurement tool for estimating how many gallons of foam concentrate are needed for a specific fire area. If the area is about 20 X 35 feet, it is 10 times the size of the 7 X 10 area and you will need at least 10 gallons of 3 percent foam concentrate.
The square-footage rule of thumb is not precise, but it gives you a reasonable starting point for estimating foam needs. The extent and intensity of the fire, the configuration of the tank or tanker involved, breakdown from the spray of cooling lines, difficulties in operating foam lines, the possible need to protect an exposure with foam, and other fireground conditions make any estimate of foam needs just that—an estimate. However, this method of estimating foam requirements at a fire gives you a useful starting point.
Conversely, estimating the area that can be covered with a 6-inch foam blanket with the number of gallons of foam concentrate on your first-alarm apparatus can be an eyeopener. Some departments can expect to have no more than 20 gallons of foam concentrate on the first-alarm apparatus. Doing a little arithmetic will indicate that this amount won’t handle a fire of any sizable area.
Continued next month