In this series of articles, we’ll confine our studies to flammable and combustible spills and the ensuing fires that could develop. Also, we’ll concentrate on incidents involving spills. For our purposes, we’ll use the parameters from NFPA 11: Standard for Low-, Medium-, and High-Expansion Foam, 2010 Edition as it relates to spills of less than one inch in depth. Such incidents are more common to the first responder. The topics of incidents involving large industrial complexes, tanks, and infrastructure are covered in great depth in NFPA 11 and these topics are well outside the scope of this article series. For the typical spill that we may be called to, foam application can be of great value. Knowing the basics and being able to rapidly establish an effective foam stream can mitigate a potential disaster from even happening. Again, the key is training and understanding before the incident!
Foam is an effective tool in our toolbox. But if it is used improperly, all of our time and resources spent could be completely wasted. This is because not only is there the category of Class B liquids and gases to understand, but there are also different types of liquids that are not compatible with all types of foam.
Hydrocarbon fuels are typical of the responses we encounter on a day-to-day basis. It includes flammable liquids such as fuel oils and diesel fuel, kerosene, and gasoline (although newer blends of gasoline may be problematic and we’ll discuss that later). These fuels shed water and have a specific gravity of less than one, which means they will float on top of water. Hydrocarbons do not dissolve in water.
All members of the company must have an academic understanding of the theory as well. The truly valuable firefighter is the member that has both practical experience as well as an academic understanding. Knowing that the specific gravity of gasoline allows it to float on top of water allows a firefighter to take the necessary action to mitigate the threat.
Other types of fuels are referred to as polar solvents and these mix readily with water. Examples of polar solvents include flammable liquids such as alcohol, acetone, and even paint thinner. Both polar solvents and hydrocarbons can be a problem if we don’t know what we’re getting in to. The ability of a polar solvent to mix with water can be problematic. The danger of fire is still present up to a certain point. Beyond this point there is the potential to dilute the fuel to such an extent that it is incapable of burning, but other serious conditions could arise such as environmental damage or other types of unexpected contamination.
A key to our success is knowing the advantages and disadvantages of our equipment and abilities, and the hazards to which we are responding. A key disadvantage in the use of foam is that foam is not a cure-all for all types of incidents. Since foam is primarily water, it means that you cannot use it on an energized electrical source (a Class C fire) because of the danger of electrocution. Foams used on a Class D fires (combustible metals) can be another potential disaster. With foam being anywhere from 94 to 99 percent water, its application on energized electrical equipment and burning metals is unsafe.
Also, there are specialized types of foams that are geared towards hazardous materials incidents of a more technical nature. These foams concentrate on creating a seal over top of the chemical to prevent the release of vapors. See the different manufacturers in the endnotes for further details. There are a variety of specialty foams that are worth reading about and it is certainly an interesting topic, but it is also outside the scope of this series.
In addition, there are the advantages of Class A foam. Class A foam, like its name implies, is used on fires involving Class A combustibles and is not compatible with attacking a flammable liquids fire. This is another valuable tool in the toolbox. The surface tension of water is changed and this allows bailed materials such as bundled newspapers, for example, to be penetrated more effectively than plain water alone. This effective tool supplements traditional firefighting tactics, but doesn’t replace the need for the basics, such as the need for safe and effective overhaul tactics.
Even some types of Class B foams, such as Aqueous Film Forming Foam (AFFF), can be used to penetrate Class A combustibles. The principle of changing the surface tension with the various surfactants that are part of the foam concentrate allows for a reduction in overhaul time. This is not a primary use of Class B foams, but it goes to show that foam concentrates have great value.
As for the extinguishment aspect, the firefighter knows that for a fire to go out, they have to remove one side of the fire tetrahedron. This simply translates into one of the four methods below: