Producing Class B Foam Solutions
Your engine responded to a report of a vehicle fire on an interstate route. Upon arrival, you found a fully involved passenger car with fire fed by spilled gasoline. You attempted to use the Class B foam concentrate carried on the engine to extinguish the fire, however your crew was unable to do so because the foam blanket was ineffective.
During a critique of the incident, the pump operator indicated that the pump discharge pressure used with the inline foam eductor was 150 psi, the same as always used on a pre-connected hose line and that the adjustable gallonage fog nozzle was at the lowest setting to conserve foam. The manufacturer of the foam equipment states that your pump discharge pressure and the type of nozzle used were incorrect and likely resulted in the poor foam blanket.
Class B foam is a material that allows water to be used on fires that would otherwise not respond well to water application. To use foam, a concentrate is first mixed with water to create a foam solution. This solution is then expanded by mixing it with air at the nozzle in order to produce a lightweight foam blanket with air bubbles. Class B foam works on the surface of a burning flammable liquid in several ways:
In addition to extinguishing burning flammable liquids, Class B foams can be used to prevent spills of these materials from igniting.
Equipment used to produce foam solutions can be built into fire apparatus along with a storage tank for the foam concentrate. Foam solutions can also be produced using various handheld appliances. The appliance most critical to the production of foam solutions is typically referred to as an inline eductor. This device includes both female and male thread connections, a siphon tube, and a metering valve that allows for variable dilution ratios, usually in the range of 1%, 3%, or 6%. The siphon tube is designed to be inserted into a container of foam concentrate and moved rapidly in succession to fresh containers as each is emptied of its contents.
The discharge or male thread of an inline eductor is normally 11/2 in. in diameter, while the intake or female coupling may be 21/2 in. or 11/2 in. in diameter, depending upon the requirements of the user. The female coupling of the inline eductor is usually attached to a discharge outlet on fire apparatus, and a hose line is then attached to the male coupling.
There are two important points for first responders to remember when selecting the discharge outlet to which the inline eductor will be attached. The first is to ensure that the siphon tube from the eductor will reach to the bottom of foam concentrate containers used (usually these containers are placed on the ground).
The second is that someone must be available to switch the siphon tube as each container of foam concentrate is emptied. This switch needs to be quick, and since extra personnel on the fire ground are rare, this task is often assigned to a pump operator.
If this is the case, the discharge outlet selected to attach the inline eductor is probably best located in proximity to the pump control panel. First responders should remember that apparatus with top-mounted pump panels may require two people to perform foam operations successfully. The alternative is that the pump operator must abandon the pump panel in order to stand on the ground and switch the siphon tube of the inline eductor.
When using handheld foam appliances, the foam concentrate is usually stored in 5-gallon containers. Whenever attempting to remove the lid from one of these containers, it is always helpful to have available a wrench specifically designed for this purpose. Using this type of wrench can prevent delays when first responders encounter a stubborn lid during the heat of battle. Any delay in switching the siphon tube to a fresh container of foam concentrate can result in a decreased concentration of foam solution at the nozzle.
Two characteristics of inline eductors are often overlooked by first responders. Most of these appliances have a required minimum inlet pressure as well as a maximum length of hose that can be used. The decision for maximum hose length takes into consideration friction loss, so this length will change based upon the diameter of the hose used.
The maximum length for 13/4-in. diameter hose is generally greater than that of a 11/2-in. diameter hose. For some inline eductors, this maximum length may range from 500 to 700 ft when using 13/4-in. diameter hose. The manufacturer of this appliance can provide first responders with maximum hose lengths as well as the required minimum inlet pressure.
Another appliance important to the production of foam is the nozzle. The nozzle not only provides for an effective pattern but is also responsible for mixing the foam solution with air to produce the desired expansion. Depending upon the type of foam concentrate, use of a standard fog nozzle may work, while other types of concentrates may require use of a special foam nozzle. As a general rule, the flow rate of the nozzle must match the flow rate of the inline eductor used.
Therefore, when selecting foam producing equipment, whether foam concentrate, inline eductors, or nozzles, check with the manufacturers of these items to ensure their compatibility. This should be done before any equipment is purchased because incompatible equipment may not produce desired results. When producing Class B foam solutions, be smart, be safe and remember everyone goes home!
1
