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...
To access the remainder of this piece of premium content, you must be registered with Firehouse. Already have an account? Login
Register in seconds by connecting with your preferred Social Network.
Complete the registration form.
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 firstname.lastname@example.org.