In previous editions of Firehouse® Magazine I began to unshroud some of the mystery that surrounds firefighting foams for many of us. In those articles we looked at nozzle selection and required flow rates for incidents involving Class B fuels (flammable and combustible liquids...
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In previous editions of Firehouse® Magazine I began to unshroud some of the mystery that surrounds firefighting foams for many of us. In those articles we looked at nozzle selection and required flow rates for incidents involving Class B fuels (flammable and combustible liquids).
Photo by Mike Wieder
Figure 5. A common self-educting master stream foam nozzle.
As we continue down the path to greater foam consciousness, it is important for us to understand the equipment we use to make foam. In order for us to be successful in developing a meaningful foam stream, we must understand how our equipment works and what limitations it has.
There are two basic manners in which foam can be generated: chemically and mechanically. The first foams that were introduced to the fire service more than 70 years ago were chemical foams. Chemical foams are those produced as a result of a reaction between two chemicals. Chemical foams are considered obsolete and are rarely, if ever, found in use today.
Foams in use today are of the mechanical type. Mechanical foams must be proportioned (mixed with water) and aerated (mixed with air) before they can be used. To produce quality firefighting foam, foam concentrate, water, air and mechanical aeration are needed. These elements must be present and blended in the correct ratios.
Diagram Courtesy of IFSTA/Fire Protection Publications
Figure 1. The foam tetrahedron.
The four elements need to create foam can thought of as a tetrahedron in a manner similar to the elements that are need for flaming combustion (Figure 1). Removing any element results either in no foam production or a poor quality foam, just as removing one of the combustion elements results in extinguishing a fire. Before discussing the foam-making process, it is important to understand the following terms:
Foam concentrate The raw foam liquid as it rests in its storage container or the apparatus tank before the introduction of water and air.
Foam proportioner The device that introduces foam concentrate into the water stream to make the foam solution.
Foam solution The mixture of foam concentrate and water before the introduction of air. In basic terms, this is the stuff in the hose between the proportioner and the nozzle unless you are dealing with a compressed air foam system (CAFS).
Foam The completed product after air is introduced into the foam solution (also known as finished foam). In basic terms, this is the agent after it is discharged from the nozzle.
The term "foam proportioning" is used to describe the mixing of water with foam concentrate to form a foam solution. Most firefighting foam concentrates are intended to be mixed with 94% to 99.9% water. For example, when utilizing 3% foam concentrate, 97 parts water mixed with three parts foam concentrate equals 100 parts foam solution. For 6% foam concentrate, 94 parts water mixed with six parts foam concentrate equals 100 percent foam solution.
The four basic methods by which foam may be proportioned are induction, injection, batch mixing and premixing.
Photo by Mike Wieder
Figure 3. A typical in-line foam eductor.
Photo by Mike Wieder
Figure 6. A jet ratio controller that is connected into a foam supply operation.
Induction. The induction, sometimes referred to as eduction, method of proportioning foam uses the pressure energy in the stream of water to induct (draft) foam concentrate into the fire stream. This is achieved by passing the stream of water through a device called an eductor that has a restricted diameter (Figure 2).
Within the restricted area is a separate orifice that is attached via a hose (called a pick-up tube) to the foam concentrate container. The pressure differential created by the water going through the restricted area and over the orifice creates a suction that draws the foam concentrate into the fire stream.