By Andrew Kim
No. 83, October 2014
Construction Technology Update No. 75, Advances in Fire Suppression Systems, reviewed the characteristics and performance of several fire suppression systems including compressed-air foam (CAF). This new Update, focusing exclusively on CAF, reviews the performance of CAF systems in further detail, including tests done to improve the fixed-pipe foam systems, which expand the fire suppression applications for foam technology.
CAF is generated by combining air, chemical agents and water to produce foam with a high nozzle velocity. It uses much less water and foam concentrate than traditional foam systems. It suppresses fire by forming a blanket that blocks the radiation from the flames and reduces the evolution of gaseous fuel. The foam blanket also constitutes a slowly draining source of water confined in the foam bubbles, which cools the fuel.
Fixed-pipe systems using CAF technology are most suitable for suppressing fires in such spaces as liquid fuel storage areas, machinery spaces and any compartments that have limited water supply (Figure 1).
Early research reported that the ignition-delaying capability and fire suppression effectiveness of CAF was much better than that of water. However, simple injection of the foam solution and air into steel piping caused instability in the discharge pressure, and resulted in the generation of a pulsating, soapy water stream that is a poor-quality fire suppressant.
Subsequent research focused on three innovations that led to the successful production of good quality CAF in a fixed-piping system:
- Air injection and mixing zone: Air was injected through a small orifice into a large-diameter pipe carrying a stream of foam solution. This eliminated the pulsations that occurred in a large-diameter pipe when air and water pressures were not balanced.
- Foam development zone: As a mixture of foam solution and air travel through a length of smooth, flexible pipe, the friction on the tube walls as well as the flexibility of the tubing generate uniform-sized, small bubbles. For the successful CAF system, a 10 m long, 25 mm diameter section of polypropylene tubing was used for the development zone. This allowed the production of uniform foam with expansion ratios ranging from 1:4 to 1:20.
- Piping geometry and discharge zone: It was determined that it is important for the piping to be free of sharp bends or impact points such as sprinkler deflectors so that the foam does not collapse. In addition, the geometry of the discharge orifice affects foam quality. A special nozzle was designed to permit the smooth flow of foam from the delivery pipe to form a circular discharge pattern.
Foam for the fixed-pipe system tests was made by mixing water with foam concentrate in a tank. This foam solution was then pressurized to 690 kPa (100 psi). Compressed air was injected into the flowing foam solution, and the mixture evolved into foam as it flowed through the piping system. All tests were made using a solution of 0.3% Class A foam or 2% Class B foam — less than half the concentration needed for an air-aspirated system.
Early experiments included CAF tests in both open spaces and enclosed compartments for heptane, diesel fuel, and wood-crib fires. The research resulted in the development of a fixed-pipe CAF system that provides effective fire suppression for Class A and B fires. CAF fixed-pipe systems were able to extinguish fires more efficiently and effectively than standard sprinklers or water mist, in both open spaces and enclosed compartments. The tests demonstrated the importance of uniform foam distribution, meaning that nozzle placement in multiple-nozzle systems is important. It was also noted that the positive effect that an enclosure has with water-mist systems is not a factor with CAF, which is equally effective with or without an enclosing compartment. Finally, CAF adhered well to the enclosure walls and other vertical surfaces, providing an effective ignition-retarding barrier for the surfaces.
Researchers identified and verified several applications where CAF could perform better than current fire protection systems.
Flammable Liquid Storage Fires
CAF was tested for extinguishing flaming liquids typical of both spill and shelf fires that might occur with paints or solvents in hardware or building supply stores, and in storage rooms found in a number of occupancies.
Four suppression methods were tested: no suppression (free burn); standard sprinkler suppression; water-mist nozzle suppression; and CAF nozzle suppression. These experiments showed the CAF system was able to suppress the fire, whereas the standard sprinkler and water-mist systems were able to cool the space but did not achieve suppression until the fuel was completely burned. Figure 2 shows the ability of CAF to lower fire temperature faster than water deluge.
The tests were repeated with the walls of the test space enclosed with 26-gauge sheet steel, except for a doorway opening. Again, only the CAF system was able to extinguish the fires. The sprinkler and water-mist systems were able to cool the enclosure but not extinguish the flames. As well, CAF effectively adhered to the enclosure walls and shelves.
These experiments demonstrated the capabilities of fixed-pipe CAF systems to penetrate the plumes of fires with heat release rates in excess of 2 MW and to extinguish the fire either by direct contact from above or by foam flow to the fire source. The CAF system performed well in both the open-space and enclosed-space scenarios, achieving fire control and suppression in approximately 1 min 30 s, even in situations where the fire was shielded from direct CAF spray.
Another benefit of CAF is that firefighters are able to see objects in a room or space through the spray while it is discharging. This enables responders to more quickly identify the base of a fire, which is sometimes difficult to do with discharging sprinkler and water-mist systems.
Aircraft Hangar Fires
Foam-water sprinkler systems with overhead applicators are commonly used for aircraft hangar protection. NRC used a full-scale prototype CAF fire suppression system for an aircraft hangar to determine whether it could match or surpass conventional fire suppression systems.
A systematic comparison of the fire suppression performance of the CAF system and foam-water sprinkler system was made using procedures similar to that prescribed in UL162, UL Standard for Safety for Foam Equipment and Liquid Concentrates.
The tests showed that the CAF system performed much better than the foam-water sprinkler system, using Class B foam concentrates. The CAF system used 40% of the sprinkler water flow rate and extinguished the fire in less than half the time of the sprinkler system. The CAF system used a concentration of only 2% foam concentrate while the sprinkler system used 3%. Burn-back time (the length of time it takes for flames to fan up again) of the CAF system was also much longer than the sprinkler system, typically 20 minutes for the CAF system and approximately 10 minutes for the sprinkler system.
The height of the CAF nozzles above the fire had little effect and clearly showed that the CAF system can provide much better fire protection in aircraft hangars compared to the currently used foam-water sprinkler systems.
Power Transformer Fires
Power transformers contain hazardous materials, and it is costly to provide the infrastructure to contain run-off water from fire suppression activities. Sprinkler fire protection systems for power transformers require large quantities of water, which may affect their electrical function, and cause water damage and environmental impacts.
NRC evaluated the potential of CAF to extinguish power transformer fires by constructing a full-scale representation of the front half of the transformer of Hydro Quebec’s Berri Station in Montreal (Figure 3). The fire scenario tested was an explosion in the main transformer body due to internal arcing. This caused a high voltage bushing to blow through the top of the transformer or its oil reservoir to rupture, leaking oil onto the top of the transformer.
Several configurations of CAF distribution systems were developed to determine the most efficient way to distribute foam around the vertical and horizontal obstacles of the transformer. The final CAF system selected incorporated two types of nozzles: a large-flow, gear-driven rotary (GDR) nozzle and a small-flow, turbine-action rotary (TAR) nozzle. A parallel test was conducted using a water deluge system similar to the one at the Berri Station.
The CAF systems performed much better than the water deluge system. The CAF system with three TAR nozzles using 0.3% Class A foam concentrate extinguished the test fire in 4 min 2 s, almost the same time as the water deluge system. However, it used less than 8% of the water.
The CAF system with two GDR nozzles using 2% Class B foam concentrate achieved extinguishment in 1 min 58 s, about half the extinguishment time of the water deluge system. Water usage was less than 18% of the total flow rate of the water deluge system.
The CAF system with eight TAR nozzles using 2% Class B foam concentrate extinguished the test fire in 1 min 29 s, using far less water than the water deluge system.
Residential Fires in Remote Northern Areas
Residential fires in remote northern areas result in heavy losses owing to the lack of nearby fire services. As well, replacement costs are much higher than in urban areas. Although conventional sprinkler systems are feasible for such areas, water supply is often limited and the cost of installing them is high. An option is the use of CAF technology.
In cooperation with Canada Mortgage and Housing Corporation (CMHC), NRC developed a prototype CAF system for protecting housing in remote areas. It was tested in a vacant house in Yellowknife, Northwest Territories, in realistic residential fire scenarios: a kitchen fire involving cooking oil and a living room fire involving upholstered furniture.
The tests showed that the CAF system was very effective in extinguishing the kitchen cooking oil fire and had a shorter extinguishment time than a residential sprinkler system. For the living room scenario involving a large fire load and challenging geometric configuration, the CAF system provided good control and prevented the fire from spreading from the sofa to other combustibles in the room, thus preventing flashover.
The CAF system produced a deep layer of foam over all open surfaces of the combustibles in the room and on the floor, thus preventing any remaining small flames from spreading to other combustibles and flaring up again after the termination of the foam discharge. These small flames eventually self-extinguished when all combustibles in the sofa were burned off. In comparison, the residential sprinkler system failed to prevent flames from spreading to other combustibles; as a result, flashover occurred. Twice the water was used compared to the CAF system.
NRC research demonstrated that compressed-air foam (CAF) technology is effective for suppressing fires involving flammable liquid storage spaces, aircraft hangars, power transformers, and housing in remote areas. Although the benefits vary depending on fire type, the use of CAF generally results in reduced suppression time, lesser tendency of fires to reignite, lower temperatures, reduced water use, shorter clean-up time and cost, and safety advantages for firefighters.
CAF technology has been licenced to a company that provides engineering services and equipment sales. Its integrated compressed air foam (ICAF) system has received Factory Mutual (FM) approval for Class B hydrocarbon spill fires, pool fires, and cascading fires. CAF systems have been recognized by the National Fire Protection Association in NFPA 11, 850 and 851.
- Madrzykowski, D., Study of the Ignition Inhibiting Properties of Compressed Air Foam, NISTIR 88-3880, National Institute of Standards and Technology, Gaithersburg, MD, 1988.
- Madrzykowski, D. and Stroup, D., ed., Demonstration of Biodegradable, Environmentally Safe, Non-Toxic Fire Suppression Liquids, NISTIR 6191, National Institute of Standards and Technology, Gaithersburg, MD, 1998.
- Kim, A.K. and Dlugogorski, B.Z., An Effective Fixed Foam System Using Compressed Air, Proceedings of the International Conference on Fire Research and Engineering, Orlando, FL, 1995.
- Kim, A.K. and Dlugogorski, B.Z., Multipurpose Overhead Compressed-Air Foam System and its Fire Suppression Performance, Journal of Fire Protection Engineering, Vol. 8, No. 3, 1997.
- UL 162 – UL Standard for Safety for Foam Equipment and Liquid Concentrates, seventh edition, Underwriters Laboratories Inc., Northbrook, IL, 1994.
For information about engineering and system specification, contact FireFlex Systems Inc.
Construction Technology Updates: a series of technical articles containing practical information distilled from recent construction research.
Andrew Kim, Ph.D.
Senior Research Officer, NRC Construction