You may recall that in the previous article (April 1997), we began a discussion of the fire service "voodoo science" of foam. We pledged to provide the fire service with some sound, generic information on foam operations. The intention was to clear up much of the confusion that surrounds this topic...
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You may recall that in the previous article (April 1997), we began a discussion of the fire service "voodoo science" of foam. We pledged to provide the fire service with some sound, generic information on foam operations. The intention was to clear up much of the confusion that surrounds this topic. In that article, we addressed the question of what type of nozzle to use with aqueous film-forming foam (AFFF) or film-forming fluoroprotein (FFFP) foam. We learned that both foam and fog nozzles have appropriate applications in certain situations.
Photo by Bill Bennett
Firefighters use foam to fight this fire at a power company substation in Valley Stream, NY, on Feb. 1, 1997.
In this article, we will tackle another of the most common questions I hear on the topic of foam: "Just how much fire can I expect to put out with the equipment and foam concentrate I have?" This is a reasonable question. If you have a foam system or eductor and foam concentrate on your engine company, you should have an accurate estimate of how much fire you can expect to handle.
My experience is that when you find out exactly what the capabilities of your equipment are, you tend to be shocked. And I don't mean "good" shocked. Many people order a 30-gallon foam concentrate tank and an around-the-pump proportioner or other onboard proportioning system on their pumper and think they are ready for a gasoline tanker to roll over. The rest of this article is bound to be sad news for those folks.
I used to work on an engine company that had a 95-gpm eductor in the compartment and two five-gallon buckets of foam concentrate stored above the pump panel, near the booster reels. Since my recent education on foam, I have deduced that it would have better use of that space to store a coffee pot and donuts.
To fully understand our capabilities, let's take two approaches to this question. First, we will look at a given fire scenario and determine how much foam concentrate and water we will need to extinguish it. Second, we will look at our foam system-equipped pumper and see how much fire we can reasonably expect it to put out.
Before getting too deeply into the calculations, let's establish ground rules for the rest of this article:
- We will use the flow rates and discharge durations that are established in NFPA 11, Standard on Low-Expansion Foam Systems. Various manufacturers may have slightly different figures for their products. We will assume the NFPA 11 figures to be generic.
- We are using 3% AFFF foam concentrate.
- We will be determining the capabilities required to extinguish hydrocarbon fuel fires. Vapor suppression at unignited spills or after extinguishing a fire is a different story. In fact, NFPA 11 does not even give a recommended flow rate for unignited spills. In real life, you will probably flow less than the recommended fire flow rate but you must be prepared to achieve the fire flow rate if ignition occurs.
Let's begin by looking at the typical kind of fire that may require foam application for extinguishment. In this case, a gasoline tanker has rolled over, split open and ignited on our city street. We assume no significant exposure problem and the fuel is not running anywhere (a best-case scenario).
Diagram Courtesy of IFSTA/Fire Protection Publications
In this scenario, a gasoline tanker has rolled over, split open and ignited on a city street.
Step 1: Estimate the size of the fire. Don't try to be too exact. Square off the sides and multiply them to get a square footage (see diagram). In this case the fire/spill area is 40 by 50 feet, for a total of 2,000 square feet.
Step 2: Determine the required flow rate for the fuel being faced. NFPA 11 lists the required flow rate for a hydrocarbon spill fire as 0.10 gpm/square foot. For this fire, the flow rate will be 2,000 square feet x 0.10 gpm/square foot, which equals 200 gpm of foam solution.