As a largely misunderstood firefighting tactic, the use of foam has at times been confusing to the fire service. The result is that many municipal brigades/departments have avoided the use of foam, especially Class A foam, and have transitioned to using emulsifiers or wetting agents.
While the end objective is to stretch valuable and at times limited water resources, it is more important to fully understand the fit for purpose of each of these technologies. This assures these valuable tools provide maximum benefit. Foam is a very effective firefighting tool for flame knockdown, fire control, extinguishment, and burn-back resistance (Class B foam). Control, extinguishing time, and burn-back resistance are paramount to the safety of firefighters everywhere. So where do we start?
Firefighting foams have been in commercial use since the early 1900’s. The National Fire Protection Association in (NFPA) 11 – Standard for Low, Medium and High Expansion Foam, Section 3.1.10, defines foam as “a stable aggregation of bubbles of lower density than oil or water.” Foam is made by three component parts: foam concentrate + water + energy. Energy can take the form of air or mechanical agitation and, when added to foam solution (foam concentrate mixed with the appropriate amount of water), finished foam is produced through means of a discharge device. The finished foam is very fluid and readily flows over liquid surfaces to extinguish fire in four ways:
- Excludes Oxygen (separates fuel from vapor)
- Cools Fuel Surface(water content of foam)
- Prevents Release of Vapors(flammable fuel)
- Separates Flame from Fuel Surface
Class “A” Foam
Developed in the mid-1980’s, Class A foams initially were used for wildland fires but as their popularity grew throughout the 1990’s the use of Class A foams was expanded for use on structural fires.
Class A fires consist of ordinary combustible materials such as paper, cloth, wood, and some plastics. These type fuels require the heat-absorbing effects of water (cooling) or water solutions. Class A fires may consist of two types of combustion processes. The first type is flaming combustion. This involves burning gases that are liberated from the thermal decomposition of the fuel. The second type is deep-seated or glowing combustion. This type represents combustion within the mass of the fuel and has a slow rate of heat loss and a slow rate of reaction between oxygen and the fuel.
As a synthetic based foam concentrate, Class A foam is applied at low mix ratios ranging from 0.1% to 1.0% (see proportioning rates below). Cooling and wetting are the primary extinguishing mechanisms. The use of Class A foam makes “water wetter” on average increasing the effectiveness of water tenfold.
Typical Class “A” Foam Proportioning Rates
- Exposure Protection: 1.0%
- Fire Brake: 0.75%
- Initial Suppression or Fire Lines: 0.3 – 0.5%
- Overhaul: 0.20 – 0.30%
These proportioning rates make the use of Class A foam a cost effective means of combating fires because relatively small amounts of foam concentrate can be used to make effective foam. Class A foam is biodegradable and non-toxic, so it is environmentally sustainable. Class A foam is deployed through a variety of portable and fixed appliance devices ranging from firefighters’ backpacks, brush and fire apparatus, to rotary and fixed wing aircraft.
Using Foam Increases Operational Efficiency
Many wildland firefighters, including rural and small urban fire brigades, already have embraced the use of Class A foam as part of their everyday operational tactics. Their use of Class A foam, quite simply, makes good sense for them. For any wildland or structural brigade that has to bring its water to the scene, and has to establish water tanker shuttles, using Class A foam can easily increase operational efficiency of fighting the fire. The reason is that, Class A foam, when properly deployed, allows the fire to be extinguished more quickly and with far less water than would be required if foam was not used. On average, the use of Class A foam increases water’s operational capability tenfold. In more simplified terms, “making water wetter and stretching it further.” In addition, the amount of time required post extinguishment during overhaul or mop up is greatly reduced. The result, crews get less callbacks due to re-ignitions or flare-ups.
Class A Foam Applications and Tactics
Class A Foam is used extensively in structural and wildland applications. There are no hard and fast rules when using Class A foam. Each fire scenario requires choices to be made at the fire scene to determine how to best deploy manpower and foam resources. Additionally, different countries or different geographical regions within countries deploy Class A foam differently. Aerial applications include fixed wing aircraft as well as helicopters through the use of belly tanks or “bambi” buckets. Ground applications run the gamut of everything from pump packs to high pressure units or CAFS (Compressed Air Foam Systems) on brush trucks to foam being delivered from front line fire service apparatus (pumpers and aerial trucks). The type of foam generated for ground applications depends on the intended use. The type of foam (its consistency or appearance) is often times referred to as being either “wet” or “dry.” These two terms have less to do with the actual water content in the foam and more to do with how they visually appear when discharged. A “wet” foam looks runny and does not hold much of its shape. Wet foams are used most often for direct attack on the fire and for overhaul or mop up operations. A “dry” foam holds its shape and has a thicker looking consistency. Dry foams are often times used for exposure protection or to build a firebreak. Wet foams drain out quickly (i.e. loose their water content) while “dry” foams hold their water content. Manipulating the foam consistency (dry vs wet) is accomplished through choice of proportioning rates (mix ratios), choice of discharge equipment (aspirated vs non-aspirated or CAFS) and operating pressure for the system.
The use of Class A foam precludes the use or need to change established firefighting tactics, application rates or equipment when responding to a structural or wildland fire. What is required, is the ability to inject the foam concentrate into a water stream at the selected proportioning rate that results in decreased extinguishing times. Using either non-air aspirated or air aspirated discharge nozzles is acceptable; although using an air aspirated nozzle can increase the expansion of the foam resulting in an increased surface area to adsorb heat.
The use of Class A foam in this application takes the form of cutting a “firebreak” or coating a physical structure (roof, wall) allowing the fire to progress to the point of the treated area or structure. The area/structure treated thus becomes soaked by the water draining from the foam allowing the fire to be controlled.
Often it is necessary to protect a structure(s) from the ravages of fire. Blanketing the structure with Class A foam will protect the structure in the following ways: acts as a physical barrier/insulating blanket; water draining from the foam blanket will retard further combustion; and the foam blanket itself is white and will reflect radiant heat away from the exposed structure
Because water has a high surface tension, it tends to bead up and runs off Class A fuels (ordinary combustibles). The result is large quantities of water are required to assure complete extinguishment of a fire. The use of Class A foam makes “water wetter” on average increasing the effectiveness of water tenfold by allowing the water to soak into the treated fuel.
The use of firefighting foam by structural and wildland firefighters is not a single extinguishing solution, rather a tool (amongst many tools) that when combined with tactics, creates a more efficient operational scenario.
The use of foam, like other resources available to the structural and wildland firefighters, is a force multiplier that when employed with traditional tactics stabilizes the fire hazard.
While there are efficiencies associated with the use of foam be it advancements in system hardware technology or the foam concentrate itself, the use of Class A foam is an asset fire service (structural, wildland) should not overlook.
One area frequently overlooked in the wildland urban interface for the use of Class A foam is zero lot line properties or multi-family dwellings where the likelihood of multiple exposure fire scenarios is high. The ability to use foam for both internal and external exposure protection and simultaneous active fire suppression is an extremely valuable resource for those in the fire service. Another advantage in using Class A foam is that lower nozzle flow rates can be used. Fire flow rates can easily be reduced by half or more when Class A foam is deployed.
Bottom line, the use of Class A foam just makes smart sense. It enhances the firefighters ability (regardless of service type) to suppress fires more rapidly. It improves water’s wetting capability thereby providing faster penetration and greater fire control; increases protection of personnel and maximizes operational efficiency through the use of variable proportioning rates, thus minimizing post fire clean up time, and conserving valuable water resources.
For more information, go to www.solbergfoam.com