The High-Expansion Foam Triangle: Water Volume, Time and Expansion Rate
The Volunteers Corner
If you are going to understand the application of high-expansion foam in a fire attack, you must consider the volume of water required and the time necessary to generate enough foam for extinguishment.
The foam expansion rate is just one side of what you might call the triangle of foam application. The other two sides, as you can now guess, are water volume and time.
In my column last September, I discussed the use, expansion rates and generation volume rates for high-expansion foam liquids. For the sake of discussing our triangle, let’s presume that we are working with a foam liquid with a 1000 to 1 expansion rate and using a generator that will produce 2,500 cubic feet of foam per minute.
The foam liquid is educted into the water at a 1 percent rate. This means that every 100 gallons of the foam and water mixture that passes through the generator contains 1 gallon of foam liquid and 99 gallons of water.
Gallons available: Now this can be important if you are depending on booster tanks for water. If you have a 300-gallon tank, only 3 gallons of high-expansion foam liquid is needed to turn all that water into foam. With a 500-gallon tank, 5 gallons of foam liquid is all you need. Therefore, there isn’t any need to cany a quantity of foam liquid in excess of 1 percent of the available water.
How much concentrate?: Of course, if hydrants are available, then water will not be a limiting problem—the amount of foam liquid will. This brings us to the problem of how much foam will be needed. Once we determine that, then we can decide how much foam liquid should be available for response to an alarm.
If you could usually expect to hit a fire in a room no larger than 20 by 25 feet with a 10-foot ceiling, you would need to produce 5000 cubic feet of foam. This, of course, is purely theoretical and assumes a perfect expansion rate with no loss of foam through openings. I take a dim view of depending on everything going according to specifications when fighting a fire, so I would be prepared to generate at least twice as much foam.
So if we set 10,000 cubic feet of foam as our goal, at a 1000 to 1 expansion rate for the foam liquid-water mixture, this would mean that 75 gallons of water and 0.75 gallon of foam liquid would be required.
Here’s how we reach those figures. In round numbers, there are 7.5 gallons to the cubic foot; therefore, 10,000 cubic feet contain 75.000 gallons. At the 1000 to 1 expansion rate, 75 gallons of foam liquid and water mixture are necessary. Of the 75 gallons, 1 percent, or 0.75 gallon, of foam liquid is required.
Now let’s look at the picture the other way and see what 300 gallons of water and 3 gallons of foam liquid will do, theoretically, at the 1000 to 1 expansion rate. In even figures, assuming you get all the water out of your booster tank— which is dubious—you would generate 300,000 gallons of foam. Dividing by 7.5 (the rounded number of cubic feet in a gallon) you would have 40,000 cubic feet of high-expansion foain.
However, this does draw a picture of the potential of high-expansion foam’s ability to fill an enclosed area.
But don’t forget that one closed door may prevent you from filling an entire floor. Or an opening to the outside may let foam run out as fast as you can produce it.
The figures we have gone over can be used in determining how much of an area you could expect to cover fighting a spill fire in the open. When high-expansion foam falls on the ground, it will form at a greater height than protein foam. Where it takes sometime to build up a protein foam blanket to 6 inches, it takes no time at all to spew out a 6-to-12-inch blanket of high-expansion foam.
Therefore, I would suggest that you figure on a 1-foot-high blanket to determine the maximum area you could cover with high-expansion foam. This will give you the maximum area. A deeper blanket would mean a smaller area covered.
For a 300-gallon booster tank, 300,000 gallons of foam, or 40,000 cubic feet, means that you could cover 40,000 square feet with a 1-foot foam blanket. For a 40-foot-wide street, this means that your foam blanket would run from curb to curb and extend 1000 feet along the street.
Again, let me warn that this is a hopeful maximum figure. There is no allowance for irregular spill of the foam blanket nor for some blowing about by the wind.
The time factor: Referring back to our time side of the triangle, let’s consider our 2500-cfm foam generator again. A simple division into the total number of cubic feet needed will determine how many minutes it will take to produce the foam. Now your only problem is to determine whether one generator will do the job quickly enough, or whether you should have more generators.
But using these methods of computation, you can get some idea of how much foam concentrate and how many generators you need to produce enough foam to cover your own particular fire hazards.