One of the most promising technological advances to occur within the fire service over the last 25 years was the technology associated with Class A foam and compressed air foam systems (CAFS). This technology, which primarily had its beginnings in wildland fire operations, represents a revolutionary breakthrough today for use in structural firefighting.
In the more than two decades that I have been involved in fire service training and education, I have seen a lot of innovations that held promise. Some worked and were adopted by the fire service; some worked and were not adopted; and others just didn't work. But few innovations have come along that represent such a significant step forward in our capability to control structure fires.
The intent of this three-part series is to share the basic concepts of Class A foams and CAFS and their benefits to the structural fire service, even though virtually all fire departments that must fight fires in other types of ordinary combustible fuel could reap the same benefits. For a much more comprehensive text about Class A foam and CAFS technology, obtain a copy of The Compressed Air Foam Systems Handbook at cafsinstitute.org.
Finished foam is a foam solution that has been "aspirated" to make the solution bubble. Children blowing bubbles dip a ring in a form of foam solution and then aspirate the bubble when they blow on the film of solution suspended on the plastic ring. In making firefighting foam, aspiration is achieved by mechanically agitating the foam solution to add air, which creates a bubbly mass of finished foam. It is the bubbles in the aspirated or finished foam that let the agent cling to the vertical surfaces of fuels and hold the water-based liquid in place until it has absorbed enough heat to evaporate.
You can perform a simple demonstration of the difference between foam solution and finished foam. Take a soda bottle and fill it about halfway with water, then add a couple of teaspoons of liquid detergent to the water. The water and detergent are essentially a foam solution. Cover the top of the bottle and shake it vigorously. You will see that the solution is bubbly — the solution has expanded into finished foam. If you really want to see a high-quality foam bubble structure, pour the solution into a blender. Since the blender creates much more agitation than shaking the soda bottle, the bubble mass will be much denser.
Finished-foam bubbles also provide a temporary "vapor seal" for Class A fuels that assists in fire extinguishment by affecting both the fuel and oxygen components of the fire tetrahedron. Foam bubbles create dead air spaces that "insulate" the fuel from heat and flames, thereby slowing heat transfer to the insulated fuel and flame spread.
The most important fact to remember about Class A foam is that regardless of the type of foam-generating system used, it is the water within the finished foam that extinguishes the fire. All the foam concentrate does is make the water work better. Under ideal conditions, 100% of the finished foam will cool the fuel or penetrate the fuel to which it is applied with no runoff. However, achieving 100% efficiency on the fireground is unlikely. This is because the efficiency of a foam application is affected by variables such as foam production methods, application methods, application rates, as well as the fire situation itself.
While a number of foam proportioning devices are available, we will look at three basic methods. These are tank or batch mixing; eduction; and direct injection.
• Tank or batch mixing — One of the easiest ways to mix a Class A foam solution is to pour foam concentrate directly into the water tank of a pumper. This is known as "tank" or "batch" mixing. Many departments that want to experiment with Class A foam use this method, since all that is needed is a container of foam concentrate. Mixing 2½ gallons of foam concentrate with 497½ gallons of water in a 500-gallon booster tank produces 500 gallons of foam solution with a 0.5% concentration.
While tank mixing works, it can have some disadvantages:
- Since the foam solution is in the water tank and must pass through the pump and discharge piping, the foam concentrate's "degreasing" action can attack lubricants and packings over time.
- The discharge of Class A foam ends when the apparatus water tank is empty.
- Class A foam cannot be produced when the pumper with the batch-mixed tank is drafting or fed by a pressurized supply.
- Once the foam solution is tank-mixed, the ratio of foam concentrate to water cannot be easily changed for specific fire conditions.
- When refilling the water tank, foam solution residue can become agitated and expand in the water tank and overflow before the tank is full of water. The expanded foam inside the tank can also become lodged in the fire pump causing extensive pump priming times.
• Eduction — Another method of mixing foam solutions is to use some form of foam eductor. An eductor is a mechanical device that makes use of a venturi and atmospheric pressure to force foam concentrate from a container into a stream of water.
Most foam eductors are generally designed for flows from 60 to 95 gpm, and have adjustment settings of 0.5%, 1%, 3% and 6% to regulate or proportion the proper quantity of foam concentrate to the amount of water passing through the eductor. In general, however, Class A foams are usually proportioned at 0.5%.
Of critical importance in using foam eductors are the following:
- The rated flow of the nozzle used must match that of the eductor. If the eductor to be used is rated at 95 gpm, then the nozzle used with it must have a 95-gpm rating.
- The eductor manufacturers' recommendations for pump discharge pressure and maximum hoseline length must be closely followed.
- The nozzle must be in the fully opened position during use, maintaining the rated flow, or a loss of foam concentrate in the fire stream will result.
• Direct-injection discharge-side proportioning systems — Today, this method of foam proportioning is king. That's because it eliminates the logistics and inherent limitations associated with the tank-mix and foam-eduction methods. The direct-injection method uses a foam pump supplied by a foam concentrate tank to deliver the foam concentrate into the water stream on the discharge side of the fire pump to create foam solution. These systems make the mixing of foam solution very easy and accurate. Direct-injection devices provide the capability to adjust the percentage of foam concentrate injected to compensate for changing fire conditions. These direct-injection or discharge-side proportioning systems work throughout a wide variety of flows and pressures and keep foam concentrate out of the fire pump and water tank.
Foam-injection systems provide a precise method of proportioning the foam concentrate to water, so foam concentrate is not wasted and cost-effectiveness is enhanced. When Class A foam concentrate is proportioned at 0.5%, the cost per gallon of foam solution created is typically from seven to nine cents.
Next: Generating Class A foam, conventional nozzles, air aspirating nozzles and compressed air foam systems
DOMINIC COLLETTI is the foam systems product manager for Hale Products and the author of the books The Compressed Air Foam Systems Handbook and Class A Foam — Best Practice for Structure Firefighters. Colletti is a former assistant fire chief and serves on the technical committee of the National Fire Protection Association (NFPA) 1500 Fire Department Occupation Safety and Health Program. He is a fire instructor with over 20 years of CAFS tactical firefighting experience. Colletti may be reached at email@example.com.