|In this experiment, peat moss (left) is placed in a bowl of water with 10 drops of Class A foam concentrate. At first the peat moss floats, but then the surfactant properties of the foam solution allow the peat moss, a Class A fuel, to absorb water (center). In 10 minutes, the peat moss sinks (right) and is rendered nonflammable. Peat moss placed in an identical bowl of plain water at the beginning of the experiment was still floating after 30 minutes.|
|A drop of water, using a dime for scale, is placed on cardboard (left). When a single drop of foam concentrate is added to the water droplet, the surface tension is broken, and the water is absorbed into the cardboard within two seconds (right). Notice the increased surface area.|
What’s driving the movement toward Class A foam? In my 32-year career, I’ve only used water to extinguish fires. None of the fires I’ve responded to are still burning out of control. Plain old water worked just fine. The few times I did use Class A foam was during overhaul. In two cases, we had a rekindle. (More on that in Part 2)
I realize the wheels of change in the fire service turn slowly, but sooner or later, technological advances take hold and the fire service is forced to change with the times. I didn’t want to be the one still advocating for horses and steamers out of ignorance, so I did some research on the obvious transition to Class A foam. Proponents will say, “It’s been around for years.” But now it’s here to stay.
Back in 1987, Ron Rochna and Paul Schiobohm, two of the most experienced experts on wildland firefighting and Class A foam at the time, declared, “Foam will replace all current water applications and present new suppression opportunities to the fire management communities.”
For my department, that day has arrived. Approximately 30 of our 33 first-line engines now have built-in 10- gallon tanks for Class A foam in addition to six 5-gallon containers of foam carried on each apparatus.
To understand the movement and its history, I did some reading. (References are listed below.)
In the 1970s, the Texas Forest Service experimented with techniques and products to improve rural and wildland firefighting. At the same time, operators of paper mills were looking for ways to dispose of a concentrated residue called “soap-skim,” a dark brown, sticky material resembling axle grease that was a byproduct of paper manufacturing.
It was discovered that when water was added to the soap-skim in an 8-9 percent concentration, it produced a wetting agent that penetrated and soaked through brush, wood and charred surfaces. If compressed air was injected into hose lines, the wetting agent would foam up and discharge from the nozzle as snow-like foam.
Though initial results were encouraging, there were some logistical drawbacks to efficiently delivering the product for a sustained fire attack. One of the drawbacks with compressed air foam systems (CAFS) was that large capacity air compressors were needed to support structural firefighting. Fire apparatus without air compressors had to return to quarters to recharge their systems, similar to recharging a pressurized fire extinguisher. Unfortunately, fire departments had a hard time justifying the expense for this equipment when water was readily available to extinguish fires the way it had been done for years.
Foam manufacturers experimented with mixtures through the 1980s, substituting detergents for soap and increasing water’s ability to penetrate through and be absorbed by Class A fuels.
Foam solutions were also developed that were not dependent on compressed air to make the foam. Air-aspirated venturi nozzles and eductors could also agitate foam concentrations to make thick, effective blankets of foam. The new solutions were biodegradable, non-toxic and safe for the environment.
The difference between Class A and Class B foam is the way they extinguish fires.
The Foam Blanket
In Class B mixtures, water is needed for the proper foam solution ratio, and water is the vehicle that delivers the product into the fire. For example, with aqueous film forming foam, AFFF, as the foam solution is applied to a flammable liquid like gasoline, a film is formed over the surface of the burning gasoline to prevent further release of flammable vapors. The fire goes out as long as the film barrier remains intact. With protein foam and high expansion foam, the foam blanket is the actual barrier sealing the vapors. Water does have some cooling effect on the flammable fuel.
With Class A foam, water is the dominant extinguishing agent, not the foam. The foam enhances the effectiveness of water by creating bubbles, increasing the heat-absorbing surface area of the water droplets. The water is fortified with detergent wetting agents called surfactants, which reduce the surface tension of water, allowing it to penetrate fuels and inhibit combustion.
Without foam, surface tension – a natural characteristic of water – holds a fire stream together in relatively large droplets, limiting the water’s heat absorption capabilities. A small percentage of the droplet (the outer 10 percent) actually removes heat while the majority (the inner 90 percent) slides off the fuel source as water run off.
When a Class A concentrate is added to water, the surfactant molecules spread the water molecules apart. As air is introduced into the process, foaming agents in the mixture allow the combination of the water molecules and surfactant molecules to form bubbles, increasing the surface area of the water available to absorb the heat. The foam covering contains water to cool the fire; a surfactant to allow the water to penetrate burning and unburned material; and air which makes the mass lighter in weight and provides an effective heat insulating cover.
Research by the National Institute of Standards and Technology (NIST) determined that small droplets were more efficient in absorbing heat than larger, heavier droplets. An example would be cooling a glass of water. Crushed ice would cool the water faster than an ice cube of the same volume.
Tests conducted by the Department of the Interior, the U.S. Forest Service and NIST have proven that the use of Class A foam can increase the effectiveness of water by 3 to 15 times. Researchers believe that Class A suppression agents provide the avenue to deliver a more efficient droplet size to the flame and fuel surface area without having the droplets evaporate or run off during the application, as would be the case with plain water.
Emulsifiers are an important chemical component to Class A foams. An emulsifying agent is one that is capable of rendering a fuel nonflammable by encapsulating the hydrocarbon molecules. When added to surfactants, these agents emulsify grease, petroleum hydrocarbons, paints, and other barriers to water penetration.
Class B foams do not contain emulsifiers. If they did, instead of floating on the surface, the foam blanket would sink to the bottom of the flammable liquid. There are a couple of simple experiments you can try at the fire station; they really helped me understand how Class A foam works.
For the first experiment, take two small pieces of cardboard and lay them side-by-side. Place a drop of water on one piece. Notice how the surface tension holds the drop of water in the shape of a dome. The water does not immediately soak through the cardboard. In fact, in my experiment, the drop sat there for 30 minutes unchanged.
Next, place a drop of water on the other piece and quickly add a drop of Class A concentrate. The water is immediately absorbed by the cardboard, saturating it.
For the second experiment, fill two large clear glass bowls to their midlines with water. Add 5 to 10 drops of Class A foam concentrate to one of them and stir. Fill the two bowls with equal amounts dry peat moss without stirring and let them sit. The peat moss floats on the water. In the foam bowl, as the foam breaks down the surface tension of the water, it starts to penetrate and is absorbed by the peat moss (a Class A fuel).
In my experiment, the foam solution took 10 minutes to saturate all the peat moss. In the bowl with plain water, the peat moss floated on the surface for 30 minutes unchanged.
Using Class A foam (or not using it) has been compared to washing dishes. We don’t wash dishes in plain, cold water. We add soap, a surfactant detergent, which reduces the surface tension of water to break down grease and cut through the dirt, thus making the water more effective in getting the dishes clean. (Since truckmen use paper plates, this concept doesn’t make sense to them.)
There are other advantages to Class A foam. The bubbles make its mass lighter in weight, allowing the solution to resist gravity and stick to three-dimensional surfaces for an extended period of time. This allows time for the foam to penetrate and cool the surfaces of the fuels.
The foam also provides a thick barrier of air and water to help protect exposures from radiant heat. Since each bubble has a hollow center, it absorbs heat faster than a water droplet with a solid center. The combination of thick foam, water, and surfactant form a barrier at the flame/fuel interface that suppresses the release of flammable vapors being emitted from burning material.
With this combination of barrier formation and the ability to “stick” to three-dimensional surfaces, Class A foam has established its domain in prepping and protecting threatened structures in wildland/urban interface fires and within the structural wildland interzone. These terms refer to a wildland fire moving out of the wildland vegetation into a developed and inhabited area, usually residential.
Making the transition from wildland/ urban interface firefighting to big- city structural firefighting with regard to using Class A foam during the initial offensive attack has been slow in coming. Large fire departments steeped in tradition will resist change unless they see the need for it. Water has always worked in the past.
Unless the foam can be pre-plumbed into the apparatus for a hose line to be stretched as quickly as a regular preconnected attack line, it’s not happening. A company officer is not going to take the time to set up an eductor with 5-gallon containers just so the crew can use Class A foam on a house fire when seconds count.
Well, the days of pre-plumbed apparatus are here. Newer apparatus have systems for Class A and B foam. Combined with the benefits to firefighter safety, which we haven’t even touched on, big city fire departments are taking a closer look at Class A foam for initial company strategy and tactics.
- Fire Stream Management Handbook by David P. Fornell; Fire Engineering/ PennWell, 1991.
- Seattle Fire Department Training Guide #1-1: Firefighting Foam Operations, Apparatus, and Equipment by Lt. Kurt Plunkett
- Our Department Has Class A Foam; Now What? by Jeff Cotner; Fire Engineering Magazine, July 2007.
Editor’s Note: Raul A. Angulo, a veteran of the Seattle Fire Department and captain of Ladder Company 6, has more than 30 years in the fire service. He is on the Board of Directors for the Fellowship of Christian Firefighters. He lectures on fire service leadership, company officer development and fireground strategy and accountability throughout the U.S., Canada and Mexico.