Suppression: Firefighting Foams

Firefighting foams delivered through fixed-pipe systems have been used for decades, especially in locations where there are petroleum storage tanks and aircraft hangars. However, the current systems, which incorporate aspirating-type nozzles and blower-type foam generators, have several potential limitations, including poor foam quality due to the use of fire-contaminated air to generate the foam. Current systems are also unable to provide foams with a high injection velocity, which is an especially important requirement in the case of high-ceiling storage warehouses and hangars, where the injected foam needs to travel relatively long distances and penetrate fire plumes before reaching the seat of the fire.

If compressed air is used for foam generation, the resulting foam is superior quality and has substantial injection velocity. It also requires a much smaller quantity of water and foam concentrates, resulting in reduced cost.

Until now, attempts to adapt the compressed-air-foam (CAF) approach to fixed installations have failed because of two fundamental technical problems. First, traditional sprinkler-type nozzles cannot distribute compressed air foam without collapsing it. Second, the foam itself degenerates in fixed piping.

The National Research Council of Canada has overcome these difficulties with an improved technical understanding of foam behaviour, and by engineering sophisticated air injection hardware with a special nozzle. The result is a compressed-air-foam system whose performance has been proven in full-scale tests.

Nozzle prototype

If a fire occurs in an aircraft hangar, protecting the aircraft from possible fuel spill fires is the first priority, while protecting the building itself is the second. National Fire Protection Association (NFPA) standard 409 for aircraft hangars specifies 90% fire control within 30 seconds, and fire extinguishment within 60 seconds. These criteria are based on the approximate time a severe fire (flammable liquid fuel) is expected to take to melt the aluminum skin of an aircraft and gain entry to the interior.

Recently, NRC, in collaboration with National Defence Canada and with the assistance of Ken Richardson Fire Technologies of Ottawa, investigated the feasibility of using a compressed-air-foam system to provide fire protection for aircraft hangars. Using the NFPA criteria, NRC developed a prototype compressed-air-foam system for a 55-metre by 37-metre hangar using specially designed ceiling-level and floor-level nozzles to rapidly cover the hangar floor with foam. The overhead nozzles provide coverage of approximately 100 square metres in a circular pattern with a diameter of 11.3 metres. The floor-level nozzles provide coverage of approximately 66 square metres in a circular pattern with a diameter of 9.1 metres.

To determine the extinguishing capability and effectiveness of the prototype system, a series of full-scale fire tests were conducted using a circular pan, 2.44 metres in diameter, with a lip height of 127 mm. The total quantity of fuel (gasoline) in the large pan was 40 litres, floating above a base of 100 mm of water. The pre-burn time for the gasoline fire was 20 to 30 seconds. The first test involved a ceiling-level nozzle alone, the second involved a floor-level nozzle alone, and the third involved both nozzles together. For each test, two kinds of foam concentrate were used: 0.3% class A foam, which is normally used in foam systems at 1% concentration for solid fuel fires, and 2% aqueous film-forming foam (AFFF), which is normally used in other foam systems at 6% concentration for liquid fuel fires.

The test results showed that the overhead nozzle system with 2% aqueous film-forming foam controlled the gasoline fire in 62 seconds and extinguished it in 118 seconds. The floor-level nozzle system with 0.3% class A foam controlled the gasoline fire in 120 seconds and extinguished it in 220 seconds. The floor-level nozzle system with 2% aqueous film-foaming foam (as shown in the photos) controlled the fire in 25 seconds and extinguished it in 39 seconds.

The results indicated that the newly developed compressed-air-foam system with its overhead and floor-level nozzles performed well in extinguishing the test fires. Using 2% aqueous film-forming foam achieved better performance than using 0.3% class A foam, in both control and extinguishment. The overhead nozzle alone had difficulty meeting the NFPA control and extinguishment criteria for hangar protection. However, the floor-level nozzle alone or together with the overhead nozzle met the control and extinguishment criteria in all fire scenarios.

Compressed-air-foam technology is a new development with good potential for fire suppression not only for aircraft hangars, but also for many other special applications such as warehouses because of its good suppression capability, low water requirement and easy clean up afterwards.

This fixed-pipe compressed-air-foam “CAF” technology has been licensed to a Canadian company, Fire Flex Systems of Montreal, for commercialization. Because of its advantages, the commercial CAF system is being developed for wider application beyond hangars and warehouses.

Dr. Andrew Kim, Ph.D. is a senior researcher in the Fire Risk Management Program of the National Research Council’s Institute for Research in Construction. Tel. (613) 993-9555, e-mail andrew.kim@nrc.gc.ca.