The scenario is a fuel spill (a spill is considered less than one inch deep) from a fuel tank on a commercial vehicle. The spill takes up an area of about 30 feet by 30 feet and it has found an ignition source. The fire department incident commander (IC) has determined that the application of foam will be required. Using the data above, the IC considers the following elements: " The spill is a hydrocarbon and requires an application rate of 0.10 gpm/ft2 " The department uses a 3 percent x 6 percent AR-AFFF concentrate, which requires the 3 percent setting for hydrocarbon spills and fires " The area of the spill is 900 square feet " This requires a flow rate of foam solution of no less than 90 gpm " The logistics of such an application rate should be considered for 15 minutes " Based on these minimums, we see that the crews should be able to flow a foam solution through the eductor that consists of almost 3 gallons per minute of concentrate (actually 2.7 gallons), and just over 87 gallons of water per minute (In other words the eductor has its control set at 3 percent.) " For a 15 minute application, this requires about 41 gallons of foam concentrate and about 1,300 gallons of water to maintain the foam application
The exactness of tabletop calculations always goes out the window at the scene of an actually fire or emergency. Our goal is to error on the side of safety and to provide enough agent to safeguard us and the community we are charged with protecting. It pays to have extra resources available for a cushion of safety.
The benefit of the tabletop exercises is that it does give you an idea as to the theoretical details required. So long as you have a working knowledge of the concepts, you'll be able to apply the theories with some degree of accuracy even during the extreme stress of the emergency. Will it be perfect? Probably not, but if you error on the side of caution it may prove to be overkill and that's great!
Foam Expansion Ratios
So far in this series we have covered many elements of the foam delivery system. Now we'll bring our studies to the actual working end of the foam stream. The delivery of foam requires that each step in the process be understood and that certain key points be adhered to.
We have talked about the three primary ingredients that comprise finished foam: air, water, and concentrate. We know that water is delivered via a hoseline and foam concentrate is inducted into the hoseline via an eductor. This foam solution then makes its way to the end of the line where we find our nozzle. The proper nozzle is a key part in the delivery process.
As foam solution is discharged from the nozzle, the agitation of the stream with the air causes air to mix and become entrained with the stream of foam solution. In other words the foam/water solution now mixed with air is discharged as finished foam.
Finished foam has various expansion ratios. Expansion ratio is the amount of air that is mixed with the foam solution. For example, a 10:1 ratio simply means that 10 parts of air is mixed with 1 part of foam solution. If the nozzle is delivering a 125-gallon per minute foam solution stream and then is mixed at a 10:1 ratio with air, we ultimately have 1,250 gallons of finished foam. Obviously this large amount of foam is 90 percent air. Nevertheless, air is a key component of finished foam and helps to orient us towards a particular strategy.
Expansion ratios range from low expansion (up to 20 parts air to 1 part foam solution), to medium expansion (up to 200 parts of air to 1 part foam solution), to high expansion foam where up to 1,000 parts of air are delivered to 1 part foam solution.
Each expansion ratio has its own purpose and no one expansion ratio is better than another. Each is a tool in the toolbox and has its own purpose. For example, high expansion foam is delivered via a special delivery system that requires a fan similar in appearance to a smoke ejector to force air into a foam stream. The end result is a very thick blanket that can be used to smother a hidden fire in a hard to reach areas, such as a basement. The use of high expansion foam is out of the realm of this article, but it is another tool for the incident commander should he or she have access to such a resource. High expansion foam has been used successfully in the past to extinguish or at least, control, fires in very difficult to reach spaces such as a cellar or sub-cellar.
A fire in a cellar is primarily a Class A fire and the purpose of high expansion foam will be to smother the fire and reduce its intensity. High expansion foam must be delivered relatively close to an entry point. Because it consists of so much air that a good breeze could blow it away. Placing the foam delivery system near an opening, such as a Bilko door to the basement, also requires a vent point opposite this location. High expansion foam can be hindered from moving very easily because it is so fragile. A vent point opposite the delivery point will allow the foam to flow naturally and smother the main fire.