Foam Fundamentals

April 1, 2019
Todd Bane outlines the best uses of Class B foam while Anthony Tricarico addresses Class A foam and wetting agents.

Every firefighter knows when you put water on the fire, things are going to get better—usually. Water, as efficient as it is, obviously is not the perfect agent for all situations. For example, foam is sometimes the best option. There are many different types of foam for firefighting: Class A foam is used for combustibles, structural fires and wildfires; Class B foam is used for ignitable liquids, like gasoline and diesel; and polar solvent foams help extinguish alcohol-based liquids and alcohol-type fuels. So, how do you choose the best and most efficient option for the job?

There are eight different types of foams on the market to choose from, and all will do various jobs. Here we will discuss Class B AFFF (Aqueous Film Forming Foam) and AR-AFFF (Alcohol-Resistant Aqueous Film Forming Foam), which seem to be the most widely used throughout the industry. Captain Anthony Tricarico covers Class A foam and wetting agents in the sidebar below.

Class B basics

From an early age, we are all taught not to put water on a grease or oil fire because the grease (oil) is physically lighter than water and will therefore be carried and the fire will spread. Applying water to a fuel fire is the same principle. So how do you solve that problem?

The U.S. Navy spent a lot of time and money developing Class B foam, or as it used to be called in some cases, “Light Water,” to help fight shipboard fires involving aircraft and other fuels. This involved the creation of a concentrate of surfactants that allowed the water to become lighter than the fuel in the form of bubbles. As the foam bubbles break down, the surfactants create a barrier on top of the fuel. This in turn has a smothering effect, suppressing vapors and cooling the fuel. 

These foams have since been adopted by other branches of the military and the civilian world. Specifically, AFFF use is extensive in the aviation, petroleum and firefighting world. It is efficient and has strong knock down on large fires, but like any other tool, it must be used and applied correctly. 

AR-AFFF is used with alcohol-based liquids/products. It can be used on both alcohol and non-alcohol-based fires, but keep in mind that AFFF and AR-AFFF are very different products. AR-ARFF should be used at 3 percent on non-alcohol fires and 6 percent on ethanol fires.

AFFF foam will not work well on fires involving alcohol because water is miscible in alcohol meaning it absorbs it. With regular AFFF foam, it can be metered at 1 percent, 3 percent or 6 percent. For every 100 gallons of water, there is one, three or six gallons of foam concentrate mixed in it. Alcohols will destroy the foam blanket quickly. Gasoline in use today has about a 10 percent ethanol to fuel mixture rating. AFFF can be used to knock the fire down but will quickly dissipate, leaving little to no vapor suppression and increase burn back time. With E-85 (85 percent ethanol to gasoline), AFFF is a poor choice. 

AFFF and AR-AFFF are also not compatible as a finished product. AR-AFFF contains a polymer that is activated by the ethanol and is the barrier that provides vapor suppression. There are many videos on the internet that show how poorly AFFF reacts with ethanol and how AR-AFFF works so well with it. Never mix AFFF and AR-AFFF. The AR-AFFF seems to solidify in the AFFF and may clog eductor lines. 

Application checklist

  • Application of foam can be accomplished with several different appliances, but all present challenges and, if not set up and discharged correctly, can have negative consequences on the fireground. The following Class B checklist may help:
  • Proper delivery: If an eductor is going to be used, in many cases, unless your equipment is set up for mass application, your flow rate may only be about 95 gpm. This will of course depend on the type of eductor. An on-board system may allow for greater mass application. Try to match the nozzle to flow rate of the eductor for best results.
  • Proper amount of concentrate: The longer a fire has been burning, the more foam concentrate will be needed to extinguish the fire. Heat will dissipate the foam until it starts to cool what is burning. You will also need to figure your area burning for the proper application rate.
  • Proper aeration of foam solution: It takes three things to make finished foam. Water, foam concentrate and air. Water and concentrate make a foam solution, discharging and aerating it makes a finished foam product. Any nozzle can be used, but adding a nozzle that provides mechanical agitation and air intake will increase the volume of the finished foam product being produced. This will provide greater vapor suppression for a longer period of time.
  • Proper application of finished foam: Application streams must be controlled and foam must be applied so that fuels are not splashed and the foam blanket integrity is maintained.
  • Re-application of foam: Once the fire has been extinguished and there is still pooled fuel present, it may be necessary to reapply foam to keep vapors suppressed.

Let’s review the above in greater detail. 

Proper delivery

One of the most common ways to deliver foam with structural fire departments is the eductor and bucket method. Foam concentrate, usually in 5-gallon containers, is drawn using a Venturi Effect and introducing it into a hose stream at a proportioned or metered rate. Pressure should be between 180 and 200 psi at the educator and hose should not exceed 150 to 200 feet in length beyond the eductor. Always try to make sure the hose is running downhill. Remember, with a 95-gpm discharge rate coming from the eductor, anything you can do to increase, not decrease flow will be helpful. Matching a nozzle with the flow rate can be very helpful with delivery efficiency. 

Proper amount of concentrate

Having enough foam concentrate on scene to fight the fire is as important as proper setup for delivery. If there is not enough foam concentrate, it may be better to set up for environmental protection and, depending what the product is, let it burn off. An area of 2,000 square feet burning can require 90 gallons of foam concentrate applied for 15 minutes. Gather all containers of foam and make sure that they can be easily moved into the eductor area and then moved out to keep it clear of trip hazards. As soon as the fire is knocked down and it looks like extinguishment has been accomplished, shut down the foam line. Make sure the foam blanket is covering the area extinguished. Do not waste product if you have a limited amount on hand.

Proper aeration of foam solution

The better the foam blanket, the better vapor suppression you will have with Class B foam. It is important that the foam solution is properly agitated to get that good coverage. That being said, increasing the amount being proportioned simply to get a better foam blanket is not a prudent choice either. For instance, fighting a fire involving diesel fuel would require a 3 percent selection on an eductor. By increasing it to 6 percent, you will increase the volume of the foam blanket but use twice the foam concentrate. 

Proper application of finished foam

There are several techniques for application of finished foam:

  • The Rain Down Technique: This is accomplished by spraying the hose stream into the air and allowing the finished foam to gently cover the surface of the burning product.
  • The Build-On Technique: This technique uses the ground surface at the base of the burning product to agitate the finished foam and it builds up and moves across the burning product.
  • Bounce-Off Technique: This is much like the build-on technique but uses vertical or elevated surfaces to allow the foam to build onto the burning product surface. It is important to keep in mind that any appliance that is used to introduce air into the stream will reduce effective range about in half. See attached photos. 

The above techniques are important to avoid “digging up” and breaking the foam blanket, which exposes fuel and in turn can cause vapor release. Do not operate foam and water lines on burning product! The water will dilute the foam, making it ineffective. Even spray from a master stream can weaken the foam blanket.

Re-application of foam

Re-application of the foam is necessary to maintain an intact foam blanket, especially with large pools of burning liquids once they are extinguished. As stated above, the foam blanket will break down over time. Many factors will be involved in how long the foam blanket stays intact, including wind weather, terrain and product. It is important to try to control run off to minimize environmental impact if possible. All devices that were used to educt or pump foam should be thoroughly cleaned and flushed at the completion of their use. Failure to do so may result in a non-operational device at a later time. 

In sum

Foam has definitely come a long way since its inception as a firefighting tool. That is exactly what it is—a tool to assist you in your job. Foam needs to be practiced with and understood as to how it can be used effectively on the fireground. Incident commanders need to know where supplies are and how much is available. Airports with aircraft rescue fire fighting (ARFF) services are excellent resources with which to train. Be smart, be safe.

Sidebar: Class A Foam vs. Wetting Agents

By Anthony Tricarico

Foams and wetting agents are both considered water additives as defined by the NFPA: “An agent that, when added to water in proper quantities, suppresses, cools, mitigates fire and/or vapors, and/or provides insulating properties for fuels exposed to radiant heat or direct flame impingement.” Let’s review some of their benefits and applications. 

Class A foams use a formula that helps the water penetrate the burning material (fuel) by breaking down the surface tension of the water to make extinguishment quick and reduce the possibility of a rekindle. 

The true difference between a Class A foam and a Class B foam is that the Class B foam repels the carbons forming a film over the liquid, resulting in the suppression of the vapors, which is what is actually burning, while the Class A foam is penetrating the fuel. So basically, foams attack the fire triangle by blanketing the fuel, reducing the fuel’s capacity to seek out a source of oxygen and suppressing the vapors.

Class A foams were developed in mid-1980s for fighting wildland fires. The favorable experiences in the wildland arena, coupled with the foam’s ability to penetrate the duff due to its ability to break the surface tension of the water, has led to its acceptance for fighting other types of Class A fires, including structure fires.

A compressed-air foam system (CAFS), another Class A foam deployment system, is defined as a standard water pumping system that has an entry point where compressed air can be added to a foam solution to generate foam. The air compressor also provides energy that propels compressed air foam. The CAFS solution will adhere to vertical surfaces, aiding in rapid reduction of the heat.

Summarizing some of the benefits of Class A foam: It reflects radiant heat, it is visible, it will absorb heat, it can adhere to vertical surfaces, it reduces the surface tension of water permitting penetration, and it has a variable proportioning rate. This foam will also isolate the fuel, suppress the flammable vapor and omit oxygen.

Turning to wetting agents, Underwriters Laboratories (UL) defines wetting agents as: “liquid concentrates, which, when added to plain water in proper quantities, materially reduce the surface tension of plain water and increases its penetration and spreading ability.” 

The goal of these alternative agents is similar to foam agents in that they aim to reduce the surface tension of water, altering the properties of the water to allow it to flow more rapidly and permit deeper penetration in to the fuel. Wetting agents simply improve the efficiency of water in extinguishing Class A fuel fires. Water to which a wetting agent has been added is sometimes referred to as “wet water” because of its increased ability to wet surfaces to which it is applied. 

William W. Parker wrote a report in the New England Association of Chemistry Teachers Journal, saying, in part: “Fundamentally, wetting and surface-active agents as employed to alter the liquid properties of water and enable it to flow more rapidly and efficiently and penetrate into combustible materials. Water so treated can flow more readily around surfaces to protect exposures and combustibles more rapidly to prevent or delay ignition. A 1 percent aqueous solution of a good wetting agent will penetrate loose cotton batting 300 times as fast as untreated water.” 

While it is true that treated water will flow more readily with some wetting agents, there are other wetting agents, that, when they are added to water, have a higher viscosity, like a gel, which will, in turn, reduce the amount of water damage inside of a structure.

Not all wetting agents are liquid, there are powders on the market as well. The liquids, powders and foams all have their inherent advantages/disadvantages. Some must be shaken every few months, plus freezing weather and clogging eductors are possible when using these products. Powders usually require their own eductors to prevent clogging. It should also be noted that all equipment—pumps, nozzles and hoses, powders, liquids and foams—without exception, require cleaning after use.

Summarizing some of the benefits of a wetting agent: It reflects radiant heat, it is usually visible, it will isolate the fuel, omit oxygen and suppress the flammable vapors, all while adhering to a vertical surface. It absorbs water to repel high amounts of BTUs, it reduces the surface tension of water permitting penetration, and it has a variable proportioning rate. 

There is always the question arising on the comparison of foams to wetting agents. Both are firefighting agents and each one has its “best use.” Therefore, your selection should be based on the same process you use when selecting a tool for a job. 

Captain Tony Tricarico has been a member of the fire service since 1977 and began service with the FDNY in 1981. In 2002, he was assigned to the Special Operations Command, where he served as the captain of Squad 252 until his retirement in 2008. Tricarico is a national- and New York State-certified fire instructor, and instructs and lectures throughout the country on engine and truck operations, RIT/ firefighter survival, and special operations tactics. He is an active member of the Mount Sinai Volunteer Fire Department on Long Island. 

Voice Your Opinion!

To join the conversation, and become an exclusive member of Firehouse, create an account today!