This article is the second of four excerpts from a new book, Class A FoamBest Practice for Structure Firefighters by Dominic J. Colletti. Larry Davis is the technical editor of this 240-page educational textbook (© 1998 by Lyon's Publishing, Royersford, PA). Ordering information is available on Lyon's Publishing's Web page at www.classafoam.com or by telephone at 610-792-3115.
Class A foam concentrates are special formulations intended for use on those ordinary combustible fuels usually encountered in structural firefighting. The first Class A foam concentrate was developed by George Cowan and Eddie Cundsawmy in Canada in 1983, and was a blend of chemicals which created a foam that had excellent wetting and penetrating ability when applied on wood fuels. It was originally used for wildland firefighting. Originally called wildland foam, its use in structural firefighting came about when wildland firefighters found themselves faced with increasing numbers of structure fires within wildland/urban interface areas.
Class A foam is a mechanical foam. To utilize it, Class A foam concentrate, as received from the manufacturer, usually in a five- or 55-gallon container, must first be diluted with water to form a foam solution. The foam solution must then be mechanically agitated with air to create a mass of bubbles known as foam (sometimes called finished foam). The finished-foam is then applied onto the fire.
Various foam generating systems can be used to generate Class A foam. This includes proportioning devices such as eductors and direct-injection, discharge-side proportioning systems, and foam bubble generating devices such as fog-nozzles, air-aspirating nozzles and compressed air foam systems (CAFS).
The net effect of turning our water supplies into Class A foam streams is that the same water supplies are then more effective for structural fire control. Simply, Class A foam "makes water work better," resulting in less time to make a fire stop.
The Pros And Cons Of Water
Before Class A foam, water was regarded as the ideal firefighting agent. Over the last decade, Class A foam application in structure firefighting has changed the industry's perceptions of the efficiency of water in achieving life and property savings.
Why do we want to make water work better for fighting structure fires? Simply to increase the effectiveness of our firefighting resources. One of the benefits of Class A foam is higher efficiency from our resources in stopping fire. This results in, among other things, improved customer service through reduced damage at structure fires.
Nevertheless, in the past, water has served us well for a number of reasons:
- Wherever we find people, we find some sort of water supply. People cannot survive without water. Con-sequently, wherever there are people and property to protect from fire, water supplies in some form must exist.
- Compared to other extinguishing agents, water is inexpensive.
- Water, once dispensed from fire apparatus, remains a useful fire fighting agent because its vaporization point (212 degrees Fahrenheit) is above normal atmospheric air temperatures. This prevents water from vaporizing prior to application onto burning materials.
- Water's vaporization point is well below the temperature range in which rapid pyrolysis of ordinary combustibles occurs. Therefore, water extinguishes fire even if it vaporizes and only cools fuels to 212F.
Simply stated, when water is applied onto burning ordinary combustibles, it vaporizes at 212F. Once water turns to vapor, it leaves the fuel surface and provides little benefit in removing additional heat. Surrounded by steam, a fuel's temperature cools to 212F. While a fuel at 212F is "hot" to the human touch, this temperature is "cool" enough to provide fire extinguishment. This is since fuel surfaces cooled to 212F are well below the temperature of 450F required for fuel solid-to-vapor production, a needed element of combustion.
- Water has a high heat transfer value when vaporized. Vaporizing a mass of water takes a higher volume of heat away from fuels, compared to vaporizing the same mass of other liquids.
- When water is used for compartment fire extinguishment, the steam generated displaces combustible gases aiding flame knockdown.
- Water's properties allow application from points up to several hundred feet away from the fire.
While water is a good extinguishing agent, in today's fire operations water also has disadvantages for the following reasons:
- The volume (gallons) of water and/or the delivery rate (gpm) available are not always adequate to meet those required to achieve knockdown and extinguishment.
And even when the volume and delivery rate are adequate:
- Water's efficiency in suppressing fire can be poor because it runs off vertical surfaces. This can lead to excessive property damage from the fire that also hurts the environment by releasing toxic gas and particles into the atmosphere.
- Gravity causes water to run off burning fuel surfaces, wasting this precious resource and damaging unburned property.
- Application rates (gpm) of water less than those required are often no better than doing nothing at all and letting the fire continue to burn. Inadequate application rates happen when the number of personnel needed to operate fire pumps, advance hoselines or perform support functions to apply the required delivery rate is insufficient or not available in a timely manner.
- Water consumed during large-scale firefighting operations can cause financial hardship for some communities by depleting limited supplies of this precious and in high-demand resource.
- Water runoff from firefighting operations can carry contaminants away from the fire causing soil and aquifer contamination.
- The additional weight of water (250 gpm = 1 ton/minute) applied to a structure fire can lead to structural collapse. This is especially the case in structures weakened by fire or in which lightweight construction materials have been used.
- The costs of equipment required - pumps, hose, tankers, etc. - to move water for fire extinguishment can be beyond the means of a community.
- Water conducts electricity and can react violently with certain materials.
Class A Foam Is Not A Chemical Fire Retardant
Some firefighters confuse Class A foams with chemical fire retardants. Class A foams do not contain chemical fire retardant compounds.
Chemical fire retardant compounds are popular in forestry applications and are usually dropped by fixed wing aircraft during wildland fires. These fire retardant chemicals are not intended for structure fire attack. (Class A foam's origin was in forestry firefighting in the mid-1980s. Nevertheless, today, fire retardant chemicals are regarded by most forestry officials as the agent of choice when making wildland air drops in advance of oncoming flame fronts.)
Fire retardants are gum-thickened powders that are mixed with water, then dropped by aircraft onto tree canopies and plant growth above the forest floor. Fire retardant chemicals are easily distinguished from Class A foam or water drops because of their bright red color that's also a telltale sign of their application point.
The definition of a chemical fire retardant is: any substance (except water) that by chemical action reduces the combustibility of fuels or slows their rate of combustion. Class A foam concentrate's components do not classify as chemical fire retardants. Class A foam concentrates classify as "fire suppressants." As a suppressant, Class A foam concentrate is mixed with water and directly applied onto burning fuels to provide blanketing and wetting actions. Class A foam does not chemically inhibit fuel ignition.
Class A Foam Wets Fuels
There is a subtlety of Class A foam that confuses firefighters into thinking that it is a chemical retardant. The subtlety is that it delays ignition by the efficient wetting of fuels that absorb moisture, such as a dwelling with unsealed wood siding, that is a threatened fire exposure.
When performing Class A foam solution applications, do not expect the product to act as if it were a fire-retardant compound. Fire retardants are applied to fuel surfaces before flames approach. And they continue to retard fuel ignition even after the water mixed with the chemicals evaporates from fuel surfaces. This retardant action continues to occur for a specific time period after application.
A common question is: When Class A foam is used for direct flame knockdown, will fuels ignite after the foam evaporates? Once the foam evaporates, fuel ignition occurs. Class A foam solution has no post-evaporation fire-stopping effectiveness because, after evaporation, it leaves no fire retardant compounds on fuel surfaces to chemically inhibit ignition.
Another common question: Is a single application of Class A foam for structure exposure protection sufficient? A single Class A foam application for exposure protection eventually evaporates from surfaces. This happens quickly if the exposure is adjacent to a high-intensity fire. After a foam application evaporates off a wood exposure, the wood continues to absorb heat, boils-off internal moisture, and erupts in flames after reaching its ignition temperature. This is why periodic foam replenishment is required. One coat of Class A foam is not a "cure-all" during most exposure protection operations.
Class A Foam Concentrate
If Class A foam does not chemically retard fuel ignition, just how does it work better than water? Class A foam wets and cools fuels better than plain water.
Class A foam concentrate serves two primary functions in improving water for structure fire attack, overhaul and exposure protection:
- Class A foam concentrate reduces water's surface tension so it can penetrate Class A fuel surfaces. Class A foam also emulsifies wax and oil off fuel surfaces (in the same fashion that dish detergents remove grease from dirty dishes). This further enhances moisture penetration into fuels having those types of coatings.
- The foam solution is aspirated to create wet finished foam that's ideal for flame knockdown and fuel cooling during fire attack. The major advantage of this finished foam is that its bubble structure adheres to vertical surfaces. The finished foam bubbles have clinging power allowing the majority of the finished-foam (which is about 99.5% water) to evaporate on the fuel, cooling it.
Class A foam gives firefighters two fundamental advantages in structure fire attack:
- Reduced knockdown time
- Reduced overhaul time
The primary advantage in using aspirated Class A foam for structure fire attack is its ability to adhere to fuel surfaces including walls and ceilings. This keeps the water contained in the finished-foam where it's needed - on the fuel. The foam solution then either penetrates (wetting the fuel) or evaporates (cooling it). Aspirated foam's clinging quality reduces the run off and wasting of water that normally occurs with non-aspirated foam or water streams. Since aspirated foam clings to fuel surfaces instead of rolling off, the water within the foam stays in contact with fuels for a longer time. This results in more efficient cooling.
Bubble Structure
Before we examine how to produce and apply finished-foam at the fire scene, it is helpful to review finished-foam chemistry.
Foam bubbles are the colloidal suspension of air within a liquid (Class A foam solution). If we agitate air in water, foam bubbles do not form. However, if we agitate air into Class A foam solution, foam bubbles develop because the concentrate's surfactants orient themselves at air/water interfaces. If we continuously agitate foam solution, more of these interfaces develop and the finished-foam becomes more stable.
Class A foam solution's surfactants have two ends: a water-loving end and an oil-loving end. The surfactants align at the foam solution/air interface so that the water-loving end is in the water and the oil-loving end is exposed. This minute film at the interface is what allows a very small quantity of Class A foam concentrate to profoundly affect the properties of water. If we reduce the surface tension of water by 60%, the wetting action can increase by 1,000 times.
A simple experiment demonstrates water's ability to "foam." If we take a glass of water and blow air into it using a common soda straw, we can make two observations. First, the bubbles are very large. Furthermore, the bubbles consolidate, rise to the surface and burst. However, if we add a single drop of Class A foam concentrate to the glass of water and repeat the experiment, the bubbles are much smaller, remain stable and show no tendency to consolidate.
Class A foam surfactants stabilize liquid films (foam bubbles) in three ways. The surfactants:
- Give the bubble film a viscosity that minimizes drainage.
- Give the bubble film the ability to self-heal to minimize rupture.
- Give the bubble film flexibility to minimize rupture.
What Is Class A Foam?
Quite simply, Class A finished foam is an aggregate of bubbles that has a lower density than water. Class A foam concentrates modify water to make it a more effective fire suppression agent. Applying Class A finished-foam for structure fire attack does the following:
- Adheres water to fuels. The major component of Class A foam solution that we apply during structure fire operations is water. Mixing Class A foam solution with air creates a finished-foam blanket that clings to surfaces. On direct structure attack, this foam blanket holds water where we want it, directly on vertical surfaces. By using various foam bubble generating methods (e.g., nozzle aspirating foam systems or CAFS), diverse foam consistencies from wet to dry are produced for different tactical uses.
Finished-foam adheres to fuel surfaces and then acts to release water contained within the bubbles at a specific rate (measured by 25% drain time). This gives water time to do two things: penetrate the fuel to raise its moisture content; and evaporate, cooling the fuel. Both actions occur until all the finished-foam is gone. The net effect is that water is used more effectively to eliminate the heat side of the fire tetrahedron. This also raises the moisture content of Class A fuels, reducing the likelihood of rekindle.
- Vapor sealing. In the same way that Class B foams work to suppress the vapors of burning flammable liquids, Class A foam seals Class A fuels. The Class A foam adheres to ceilings, walls and furnishings to reduce combustible vapor release, and prevents oxygen in the atmosphere from contacting those vapors. This vapor barrier, for the short time it remains, attacks the oxidizer and fuel sides of the fire tetrahedron to accomplish extinguishment. This action continues until the foam either evaporates or drains out into foam solution.
- Insulating qualities. Class A foam blankets consist of millions of foam bubbles. These bubbles are essentially dead-air spaces that provide an R-value or insulation factor. Applying Class A finished-foam onto fuels exposed to radiant heat or hot fire gases can insulate them to prevent or delay their ignition.
- Increased surface area of coverage. Each of the bubbles within a foam blanket is a thin foam-solution film. This means more surface area of contact with hot fuels or hot fire gases and efficient heat transfer for faster foam solution vaporization, versus applying water.
- Reflective capacity. Because the finished-foam blanket is white and not transparent like water, the foam can actually reflect some of the heat away from fuels.
- Fire tetrahedron. Finished-foam application can be effective at attacking the fuel, heat, and oxidizer sides of the fire tetrahedron.
NAFS And CAFS
Two types of foam bubble generating systems produce Class A foam. These are nozzle aspirated foam systems (NAFS) and compressed air foam systems (CAFS).
In Part 3, we'll review NAFS and CAFS including how they mechanically operate, the foam quality types that they produce and their firefighting utilization.
Dominic J. Colletti is the national OEM accounts manager at Hale Products Inc. in Conshohocken, PA. He is a volunteer firefighter with the Royersford, PA, Fire Department and has served with Engine Company 3 of the Coram, NY, Fire Department. Colletti has over a decade of experience in the research and development of Class A foam and CAFS application for structure firefighting, His e-mail address is: [email protected]
