Using Class A Foam for Fire Attack

One of several ways we can improve the efficiency of our suppression operations is by using Class A foam during fire attack, not just after the fire is out and overhaul has begun. The knowledge, development, delivery equipment, techniques, and basic understanding of the chemistry of Class A foam are by no means “new.” I am merely advocating that we look to foam to aid us in the battle against the fire behavior increases and refresh our knowledge, skills, and abilities in the proper application of this great enhancer.

The fire service uses two types of foams: Class A and Class B. The two products have very specific applications and tactical advantages and have a primary difference in how they react with the carbon molecule: Class A foam attracts carbon, and Class B repels it. For this article, I will focus on the properties, advantages, and delivery mechanisms of Class A foam solutions (mixture of water and foam concentrate) during suppression operations.

1 Photos by author.

2 Photos by author.

Photos by author.

Scientific research conducted at the National Institute of Standards and Technology (NIST) and Underwriters Laboratories has now proven what all firefighters across our nation have been witnessing and directly experiencing at fires in modern times: Class A fires in our structures and the wildland have been growing in intensity and complexity as a direct result of changes in fuel contents and quantities. The modern residential single-family dwelling is a compressed fuel source with its contents and construction materials capable of liberating large amounts of heat rapidly. The synthetic products we surround ourselves with daily are convenient, relatively easy to get, and cheap, but they hold a hidden danger of a substantial volatility when exposed to fire conditions. There is no dispute that the time/temperature curve has become more compressed in modern times when compared to historic fire behavior. This compression exposes firefighters to more dangerous conditions in a shorter time and requires us to be critical thinkers and modify our tactics, personal protective equipment, training, and decision making in response to these conditions. Water is still the most prevalent and affordable suppression agent, and the physical structure and molecular properties make it such a powerful agent in cooling combusting Class A fuels.

The use of Class A foam solution in our hose streams during the attack phase adds a demonstrated advantage to plain water alone because of its chemical formula and properties. Now more than ever in structural and wildland suppression operations, we as a fire service need to be knowledgeable, train, and equip ourselves with every advantage possible to combat this compressed curve and protect our communities and each other.

The Class A Chemistry Advantage

It is imperative that we continually learn about new information, and that also includes a refresher on the Class A foam concentrate advantages provided by the chemical properties of the formula. Any refresher on Class A foam should start with understanding the water molecule and many of its wondrous physical properties. As we learned in our fire academies, water is a powerful tool for controlling fires for two primary reasons: (1) It has a very high specific heat (calorie), defined as the amount of heat needed to raise 1 g of substance 1°C; and (2) water can expand by significant volume when changing from liquid to vapor by a factor of about 1,600 at 100°C at atmospheric pressure.

Another property of the water molecule that makes it such an amazing element is the adhesion and cohesion forces of the molecule caused by the polarity of the H2O molecule and the powerful hydrogen bonds formed when water is surrounded by other water molecules. Recall what you learned in high school physics, specifically the observable proof of this strong affinity for other H2O molecules, better known as the surface tension of water. The chemical ingredients of Class A foam concentrate are specifically designed to target and modify this property. When the concentrate is mixed or proportioned into our hose streams, the wetting effectiveness of the water is increased, giving it the ability to penetrate and permeate into the Class A fuel particle.

In studies conducted by NIST, it has been estimated that Class A foam used during suppression can wet a Class A fuel up to 20 times more rapidly than water alone. The more penetration of the solution into the heated fuel, the faster heat is absorbed and the faster the combustion process is stopped.

The ingredients of foam solution operate in two primary actions to provide the suppression advantage: One way is interrupting or “breaking” the surface tension of water, allowing it to spread and soak faster, called the surfactant properties. The other is capturing the carbon-based fuel particles being ripped from the fuel source by the heating process in a blanket, called the emulsifying properties.

Another powerful property of a foam solution is how the concentrate creates a foam in the water stream. These bubbles increase the ability of the suppression agent to cling to vertical surfaces longer than water alone. This ability aids in a more rapid absorption of heat because of the longer contact the solution has on vertical fuel. This longer contact period allows the surfactant properties of the solution to increase cooling. The bubbles also aid in heat absorption by creating a larger surface area with the water molecules for better absorption and less runoff.

Tactical Applications

Direct fire attack. Foam solutions are an economical way to enhance suppression operations, regardless of the incident operational mode of direct, indirect, or defensive exposure protection because of the low cost of concentrate and the low mixing percentages. There is no need to change your agency’s current tactics when operating in these modes, just the ability to inject the concentrate into the hose stream at the right proportion.

Using this economical advantage with your normal equipment will increase the performance of the stream and decrease fatigue on firefighters, resulting in faster extinguishment. In many cases, direct attack with an aspirating nozzle is used in offensive attack mode. An aspirating nozzle creates better expansion of the foam, resulting in a larger bubble size with a better heat-absorbing surface.

If your agency desires a more diverse combination of foam solutions, investing in a compressed-air foam system (CAFS) for your apparatus may be indicated. A CAFS provides a wide range of expansion ratios and is dependent on different nozzle types and proportioners, creating a wide variety of foam solution properties.

In addition to the CAFS’ flexibility, the suppression stream is consistent and mixed at the fire apparatus; thus, it is not influenced by the hydraulics of the apparatus plumbing and equipment. The solution is pushed into the hose at the source, and some of the expansion has already occurred but can be further expanded by the type of nozzle selected.

Contrast this advantage with the proportioner type that is at the nozzle and uses the venturi effect to pull the concentrate into the stream. Not only can this type make offensive attack a bit more challenging because of the limited maneuverability of the hose team, but the correct proportion at the discharge point is subject to the hydraulics of the hoseline and can fluctuate during fireground operations.

Indirect fire attack. With the increase in the fire’s intensity and size as a result of the modern fuel environment, more often incident tactics involve an indirect attack or a transitional attack to take some of the energy out of the fire before placing crews in harm’s way. Some agencies have the staffing levels to operate in an almost exclusively offensive tactical mode, but a large percentage of agencies do not. Many departments have limited staffing and long response times for additional resources to arrive at the scene, demanding that indirect fire attack be used. Even in this mode of attack, a foam solution provides an enhanced stream that can blanket fuels, absorb heat, and reduce vapor release. This application of suppression streams “softens the target” before crews advance for full extinguishment. Tactically, this makes sense.

A military force facing a larger, more dangerous force doesn’t rush in for a frontal assault, surely to be decimated. That smaller, more intelligent force uses techniques and all advantages available to weaken the larger opponent or create a window of opportunity to maximize the effectiveness of the limited resources. Similarly, foam solution delivered through whatever means gives firefighters with limited incident resources this advantage.

Although much of this article focuses on structural applications, the use of Class A foam in the wildland is also prudent and prevalent. With the increased fuel loads in our wildlands and the addition of structures as a compressed fuel source placed in and around the natural vegetation, the stage is set for aggressive and explosive fire growth. After aggressive initial attack tactics are proven ineffective, firefighters must quickly transition to indirect fire attack. Firefighters must practice and be proficient in making this rapid transition.

One example of the use of foam solutions that I encourage firefighters to study is in firing operations. Small handlines hardened by high-expansion foam lines can be placed to ignite from. The application of foam on the green side of the line reduces by encapsulation the receptivity of the fuels on the other side of the line and increases the speed of construction because less width of the line cut is necessary. In fact, under certain conditions and in certain fuel types, handlines may not be necessary. Firing can commence directly from the “wet line,” facilitating more rapid containment.

Exposure Protection

For the reasons listed above, application of foam to protect an exposure provides numerous advantages. By the very nature of the white color of the expanded foam solution, more radiant heat is reflected from the treated unburned surface. The blanket of foam solution at high-expansion ratios adheres to the exposed surface and creates a physical barrier to heat absorption. In the wildland urban interface, this foam blanket intercepts the ember shower that is largely responsible for burning the structures. As the foam sits on the surface of the Class A fuels, some of the water drains and soaks into the fuel bed, reducing susceptibility to heat exposure.

Foams are a very economical way to treat exposures and act as a force multiplier with limited suppression personnel. It is important to train with the type of foam delivery systems that you have at your agency. This gives personnel a better understanding of the capabilities of the foam solution and the residency life of the foam on the exposure surface, which will vary based on the type of application system.

Mix Ratios

The use of Class A foam is a proven tool to increase effectiveness and efficiency of any water stream. From structure to wildland suppression, using foam during operations is a cost-effective enhancement and another tool in the toolbox. Foam delivery can be highly complex or relatively simple and is a scalable tool for fire agencies of any size or operating budget.

The following table lists approximate percentages of foam concentrate in a water stream regardless of whether you are using a standard, agitating, or air-aspirating nozzle:

Direct Attack        0.4-0.6 percent

Exposure Protection   0.5-1.0 percent

Indirect Attack     0.5-1.0 percent

Mop-Up/Overhaul   0.2-0.4 percent

These are approximate ranges, and every combination of equipment and apparatus will have unique performance characteristics that personnel need to be familiar with; that means TRAINING! The best mix ratios are obtained by constant practice with the equipment and concentrate to figure drain times, expansion rate, and consistency of the generated foam. Foam will break down faster on a hot day as opposed to a cold day. Practice prior to fireground operations. Whether you have access to a CAFS unit or simple inline induction, it is of paramount importance that all firefighters undergo training and familiarization with Class A foam dispensing methods.

Enhancing Effectiveness

Use of Class A foam in suppression streams enhances effectiveness by the following means:

  • Makes water wetter through the surfactant properties, breaking down surface tension and allowing for better penetration into the fuel bed.
  • Better cooling from the increased surface area of the foam bubbles over plain water droplets.
  • Vapor suppression by way of coating the fuel surface.
  • Radiant heat reflection.
  • Adhesion to vertical surfaces and increased insulation of fuels from heat absorption.
  • Prolonged durability of effective foam blanket with certain application systems.
  • Ability to isolate and insulate carbon-based molecules, breaking the chain of combustion by the emulsifying properties.
  • Inexpensive advantage because of low mixing proportions and long shelf life of concentrate when stored properly.
  • Water-saving ability by making the water more efficient with a corresponding reduction of water damage.
  • Speed of application for rapid advantages of time-sensitive wildland firing operations.
  • Reduced exposure to danger and physical stress of fire personnel through faster extinguishment and shorter overhaul/mop-up in structural and wildland fire applications.
  • Environmentally friendly and biodegrades rapidly when used properly and if kept out of waterways.
  • Uses less water, which can be a big advantage in drought-sensitive areas.

Remember that water treated with a foam concentrate will become three to five times more efficient than water alone, providing a significant tactical advantage when combating the intensity experienced in the modern structural and wildland fire environments.

TODD McNEAL has 27 years of experience in the fire service and is chief of Twain Harte Fire in Tuolumne County, California. He is a California certified chief fire officer, a division/group supervisor on a federal incident management team, and a California certified fire instructor. He has a bachelor’s degree in natural resource management.

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