Foam: The Fire Service's Voodoo Science - Part 2

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.

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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:

  1. 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.
  2. We are using 3% AFFF foam concentrate.
  3. 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).

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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.

Step 3: Find the flow rate of concentrate required. Since we are using 3% foam, three gallons of concentrate are required for every 100 gallons of foam solution. Thus, 200 gpm x 0.03 = 6 gpm of foam concentrate required.

Step 4: Find the total amount of foam concentrate needed. NFPA 11 also specifies the amount of time you must be able to flow the given required flow rate. For hydrocarbon spill fires, it is 15 minutes, so 6 gpm of foam concentrate x 15 minutes = 90 gallons of concentrate needed to extinguish the fire.

Step 5: Find the flow rate of water needed. Since we established above that 3 gpm of foam concentrate was required per 100 gpm of foam solution, then the other 97 gpm must be water. Thus, 200 gpm x 0.97 = 194 gpm of water are needed to extinguish the fire.

Step 6: Find the total amount of water needed. Simple enough: 194 gpm x 15 minutes = 2,910 gallons of water needed.

Obviously, your pumper with 30 gallons of foam concentrate and a 1,000-gallon booster tank is not looking all that promising at this point. If you cannot achieve this flow rate or do not have this much concentrate and water on scene, you will not be able to successfully extinguish this fire. You will have to assume a defensive posture at this point. Protect any exposures you have and let the fire burn down to a manageable size or burn out all together. Keep in mind that these figures are just for an initial attack and knockdown. Additional concentrate and water will be needed to maintain a blanket over the fuel until it can be picked up. As well, if you have exposures, the amount of water required for the overall flow rate would also be increased.

Next, let's work backwards and take a look at the pumper in our station and see what it can do. I was shocked when I looked at my own pumper. My old department was probably much like yours. We had the seemingly standard 95-gpm around-the-pump proportioner, a 30-gallon concentrate tank and a 1,000-gallon booster tank. For ease of calculation, let's call it a 100-gpm proportioner.

Step 1: What's the biggest fire a 100-gpm proportioner/nozzle combination will put out? Divide the maximum capable flow rate by the NFPA 11 required fire flow rate. 100 gpm divided by 0.10 gpm/square foot = 1,000-square-foot fire.

Step 2: How much foam concentrate will we need for a 1,000-square-foot fire? If using 3% foam at 100 gpm, we will need 3 gpm of concentrate. Given the 15-minute flow requirement, we will need 3 gpm x 15 minutes or 45 gallons of foam concentrate.

Step 3: How much water will we need for a 1,000-square-foot fire? 97 gpm x 15 minutes = 1,455 gallons.

Do you see the problem here? We can never flow the rated capacity of the foam proportioning system for the required duration. There is not enough foam concentrate and water on board. You are going to require an external water supply. However, all but the newest around-the-pump proportioners are not designed to work properly with a pressurized water source feeding the pump. This is not a concern with newer injection types of foam proportioning systems.

So let's ask the question in a different way. How much fire will my pumper with 30 gallons of 3 percent concentrate and 1,000 gallons of water actually put out?

Step 1: Divide the 30 gallons of foam concentrate you are carrying by 15 minutes. This means we can only flow 2 gpm of concentrate.

Step 2: To get the total foam solution flow rate you're capable of, divide the 2 gpm of concentrate flow by 0.03 (for 3% foam concentrate) and get a flow rate of 67 gpm. That is 2 gpm of concentrate and 65 gpm of water.

Step 3: How much fire will 67 gpm of foam put out? 67 gpm divided by 0.10 gpm/square foot (the NFPA 11 fire flow rate) = 670 square feet. This is an area of about 22 by 30 feet.

Step 4: How much water will we need? 65 x 15 = 975 gallons.

I have developed this example only to give you a better understanding of how much fire your apparatus can reasonably be expected to handle. Don't be too disappointed by these figures or feel like you have a useless foam system. Most flammable liquids incidents involve unignited spills. The same foam system described for the pumper in our example will be capable of providing initial vapor suppression coverage on a much larger unignited spill than 670 square feet.

It was not the purpose of this article to belittle your apparatus or foam system. Rather, it's hoped that by reading this article and examining your own foam equipment and concentrate, you will have a better understanding of your capabilities. In future articles, we'll continue our efforts to lift the voodoo hex off of you!


Mike Wieder is a senior editor at IFSTA/Fire Protection Publications at Oklahoma State University. He holds several degrees in fire protection and adult education. He is a former member of the Pennsburg, PA, and Stillwater, OK, fire departments.

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