The application of foam requires the mixing of the three key ingredients – concentrate, water, and air – in the proper ratio. This finished foam is applied so that it blankets the fuel and excludes the vapors from mixing with the air. This smothering effect prevents the ignition of a spill, or should a fire occur, the finished foam spreads across the fuel and creates a film or barrier.
Water by itself can create additional problems if used on a spill. Fuels that are lighter in weight than water will rise to the top and continue to burn. Even worse, such an application could create a “running” fuel fire where the spill begins to meander downhill creating a river of flame. Polar solvents on the other hand, will mix readily with water.
Finished foam changes the properties of water to an extinguishing agent that is lighter than the flammable or combustible liquids. This change in physical properties gives us the advantage to attack a fire by creating that blanket or barrier on top of the fuel.
In the event a polar solvent fuel was to be attacked, the department would rely on an alcohol-resistant concentrate. This concentrate creates a protective “polymer-based” membrane that also prevents the mixing of fuel vapors with air. To our good fortune, the fire service has access to dual-purpose foams that can be used on both polar solvents and hydrocarbons, such as Alcohol-Resistant AFFF (AR-AFFF). Check with the manufacturer as to the proper ratio to which such foam concentrates should be applied.
For example, past AR-AFFF foams read 3% x 6% foam concentrates. This meant that for a hydrocarbon spill, the ratio of concentrate to water had to be 3%, while a spill involving polar solvents had to be 6%. Newer foams may have different percentages such as 3% x 3% (3% ratio with water for either hydrocarbon or polar solvents), while some foam concentrates rely on a 1% ratio.
In short, know your equipment before it has to be used. Read the directions on the 5-gallon pail of foam that’s on your engine, before it’s needed. The information that’s listed on these containers is of great value. It tells you the uses for and limitations of the concentrate.
We’ll talk later about the proper application of the finished foam blanket, but for our purposes now, let it be understood that foam should be applied in a manner that does not disturb the fuel, such as by banking the foam stream off an object and letting it cascade down on top of the fuel and spread out over top. We want to avoid plunging the stream into the spill as this creates a more dangerous situation.
One of the other advantages of some of today’s foams such as FFFP or AFFF is the ability to shed fuel. This is advantageous because the finished foam will rise to the top and spread out creating that barrier we talked of earlier.
The additional benefits of foam are the ability to create a cooling environment. Foam is mainly water (94% to 99%) and as such it has the advantages of water’s cooling properties. A spill that has ignited and is within a metal barrier such as a tank for example, can reap the benefits of foam’s cooling ability. Lowering the temperature of everything associated with the spill or fire pays large dividends for obvious reasons.
One other important note; as we said, foam is mainly water and therefore can create an electrocution hazard. A foam stream should not be used on energized electrical equipment. For fires involving Class C equipment, it is always a safe practice to de-energize the source before applying the stream.
Also, fuels that are too hot can react negatively to the application of foam and break the foam blanket down rapidly. For the typical spills and Class B fires that we respond to, this is not much of an issue, but does require knowledge, just to be safe.
In the next article in this series, we’ll discuss the proportioning of foam concentrate and introduce the typical in-line eductor. The next article in the series is going to build upon all we have covered up to this point. In the meantime, take a look at the foam concentrates you have on your apparatus or in fire station storage. Ask yourself these questions, or as part of a company drill, find out the answers to these: