History and Background
Compressed-air foam (CAF) is a fire-suppression medium created by injecting compressed air into a foam solution.1 CAF fire suppression systems are high-energy, foam generation systems which produce small-bubbled, stable, uniform foam in a high-momentum jet. 2, 3 While fire-fighting foams have been around for over 100 years, CAF fire suppression, using hose streams, first appeared in 1941 as a means of combating fires on floating bridges. 2
CAF fixed-pipe fire-suppression systems became a reality in the late 1990s with the development at the National Research Council of Canada (NRCC) of means to reliably generate and transport CAF through a fixed-pipe network and to distribute it effectively on a hazard.1 Until this CAF system development emerged, fixed-pipe foam fire suppression systems utilized aspirating nozzles, blowers and sprinklers.
The first applications of CAF fixed-pipe technology were for the suppression of flammable liquids spill fires and shelf storage fires.1 Using Class A and Class B foams, that research demonstrated the superior fire suppression performance of CAF systems on those hazards compared to regular sprinkler and water mist technology. It also demonstrated the lower water and agent concentration flow rates with CAF technology, and the improved visibility in the fire area. Even greater advances in evaluating and advancing CAF technology have occurred since then.
Understanding CAF Fire Supression
Research on CAF systems has resulted in advances in understanding the scientific basis for CAF fire-suppression performance. Figure 1 is a schematic of a fixed-pipe CAF generating system. Effectively, water, compressed air and foam concentrate are brought together in a mixing chamber, and the resulting CAF is pushed through a piping network to the nozzles. The resulting CAF, distributed by rotary nozzles, has been compared to a snowfall of foam.
Studies have shown that the CAF mode of foam generation leads to the production of a uniform bubble size, which has a positive bearing on the stability of the foam.3, 4, 5 This CAF foam blanket establishes its fire suppression characteristics sooner and retains them longer than a foam with larger or nonuniform bubble distribution. In further studies on the deformation and flow of foam through pipes and nozzles, researchers found that the stability and flow of foam are strongly influenced by the foam's bubble size distribution and gas-liquid fraction.6,7 The smaller bubbles result in a decrease in drainage; the more uniform bubble size distribution and high initial gas-volume fraction result in greater stability. The researchers point out that the ratio of air and foam solution in CAF can be varied for almost any application - low expansion for wetting and direct application to a fire, and higher expansion for adherence to materials and vertical surfaces to act as a barrier to thermal radiation.
Other research shows how CAF exhibits better coarsening characteristics (meaning the growth of the average bubble diameter which leads to bubbles breaking) and better disproportionation characteristics, which is the widening of bubble size distribution resulting in lesser performance.3,7 The Australian researchers have also provided a better understanding of the complex flow properties of CAF through piping.4 As well, a Canadian company has developed a computer program for the hydro-pneumatic calculation of CAF flow through piping.8 CAF flow involves a mixture of both hydraulic and pneumatic elements that must be addressed together to preserve the CAF bubble structure until discharge on a hazard. As an example of the differences between CAF and water flow that must be accommodated by new calculation techniques, the pressure loss of CAF due to elevation difference is approximately one-tenth that of water.
Comparisons with Air-Aspirated Foam and Unexpanded Foam-Water Solution
Scientists at NRCC compared the fire-suppression characteristics of CAF with foam generated using air-aspiration means. They also addressed the differences in fire suppression using Class B foam-water solution and CAF. The scientists chose CAN/ULC-S560 - Standard for Category 3 Aqueous Film-Forming Foam9 for these comparative fire tests. In these tests, heat flux meters were installed to view the entire pan to assess the extent of fire suppression over time. A comparison of the performance of CAF with air-aspirated foam,10 based on the heat flux meter readings, is shown in Figure 2.A similar comparison with foam-water solution is shown in Figure 3.
This research10 showed that using three percent Class B foam on a heptane fire and flowing CAF through a slot-type nozzle provided fire suppression in approximately 70 percent of the time required for air-aspirated foam with almost identical burnback times. This was accomplished using 30 percent less foam solution. Using three percent Class B foam on gasoline fires, CAF, on the average, extinguished the fire in 33 percent of the time and used only 35 percent of the solution required for air-aspirated nozzles. In most experiments using three percent Class B unexpanded foam-water solution, the fire was not extinguished as shown graphically in Figure 3. CAF, using 0.6 percent Class A concentrate, was also shown to provide comparable fire-suppression performance to three percent Class B air-aspirated foam.
Impact of Varying Parameters
Research was undertaken to determine the effects on fire-extinguishment performance of varying CAF parameters, such as air pressure, water pressure and foam concentrate, above and below the design level.11 The Class A fires used wood cribs, designed to burn with a heat output [Compressed-Air Foam Fixed-Pipe Fire Suppression Systems] of 450 KW. The Class B tests utilized both shielded (50 percent) and unshielded heptane fires measuring 1m x 1m x 0.15m deep. Tests were also conducted to determine the possible degradation of CAF fire-extinguishment performance with simultaneous sprinkler operation.
The results of this research11 were that CAF is stable and can withstand normal variations in air and water pressure and foam concentrate levels while still offering good fire-suppression performance. For Class A fires, there were positive effects of simultaneous sprinkler/foam operation with more rapid wood crib fire extinguishment than with foam or water independently. For Class B fires, the extinguishment times were the same for simultaneous operation.
Comparisons with Foam-Water Sprinklers
Fire tests were conducted12 to investigate the comparative performance of CAF and foam-water sprinkler systems using the test method in UL 162 - Standard for Safety for Foam Equipment and Liquid Concentrates.13 An illustration of this test arrangement during operation is shown in Figure 4. This test involved four sprinklers or nozzles located above a 4.65 m2 pan fire using heptane fuel. Since CAF systems are not designed to flow water alone following foam discharge, the provisions of UL 162 for polar foams (that permit no sprinkler discharge following foam discharge, but require extended foam sealing time in the pan) were employed.
For Class B foam, a foam-water sprinkler system (three percent AFFF) and a CAF system (two percent AFFF) were evaluated. The results of two identical tests are shown in Table 1 for sprinklers and nozzles located 4.42m above the floor.12
In these tests, the CAF system extinguished the fire in 33 percent of the time of the foam-water sprinklers, and the burn back time was 2.6 times longer. The solution flow rate for CAF was 60 percent less and used one-third less foam concentrate. Changing the height of the sprinklers and nozzles to 7.62m provided similar results as shown in Table 2.
CAF System Applications
Two applications for CAF systems were initially examined: flammable liquids hazards and electrical transformers. Research1 demonstrated that CAF can be used where flammable or combustible liquids are stored, handled or processed, either on exposed or shielded Class B hydrocarbon fires. Research was also conducted to determine the potential to use CAF systems, instead of water spray systems, to protect large electric transformers. This full-scaletesting14 demonstrated that CAF systems can provide protection against three-dimensional fires in transformers, up to the 12 MW fire size tested, with superior fire suppression performance and lesser solution flows. Figure 5 shows the fire tests at different times during comparable water spray and CAF discharge. Table 3 shows two comparable transformer tests using CAF and water spray systems.
Enhanced FM Approval - Miscible Liquids and Maximum Nozzle Height for Hydrocarbons
In September 2006, a new series of fire tests was completed on polar solvent fires, which led to an enhancement of the FM Approval to include miscible liquid applications (Alcohols & Ketones). The full-scale fire tests demonstrated full extinguishment with times shorter or equal to the foam-water sprinkler system. During the burnback test, the fire self-extinguished, which further demonstrated CAF's ability to provide extended fire protection once a discharge cycle is complete. This result is important to further illustrate CAF's ability to resist against reignition because of its inherent stability and long drain time. The CAF system extinguished the pan fire at densities of 0.06 GPM/ft2 (2.4 L/min/m2) using six percent AR-AFFF for both solvent types compared with densities of 0.24 (9.8 L/min/m2) and 0.26 GPM/ft2 (11 L/min/m2), respectively, of three percent AR-AFFF using foam-water sprinklers. Water consumption was maintained at a level that is four times less than the foam-water sprinklers.
Also, the new fire lab at NRCC allows for high-bay fire testing. This facilitated new nozzle heights to be tested which led to a revised maximum installed nozzle height of 46 feet (10 m) for hydrocarbon fuels. The revised nozzle height has been incorporated in the enhanced FM Approval.
In recognition of this new fire-suppression technology, in March 2006, the NFPA Standards Council issued a Tentative Interim Amendment to NFPA 11- Standard for Low-, Medium- and High-Expansion Foam.15 The TIA 05-1, which is referenced as a new chapter in NFPA 11, covers the design, installation,operation and testing for CAF systems for fire protection.
J.P. Asselin is with Fire Flex Systems Inc. G.P. Crampton and A.K. Kim are with the National Research Council of Canada. J.K. Richardson is with Ken Richardson Fire Technologies Inc.
1 Crampton, G.P., Kim, A.K., and Richardson, J.K., A New Fire Suppression Technology, NFPA Journal, July/August, 1999 (pp 47-51).
2 Rochna, R., Compressed-Air Foam: What is it!, American Fire Journal, August Edition, Fire Publications Inc., Bellflower, CA, 1991 (pp 28-34).
3 Magrabi, S.A., Dlugogorski, B.Z., and Jameson, G.J., A Comparative Study of Drainage Characteristics in AFFF and FFFP Compressed-Air Fire-Fighting Foams, Fire Safety Journal, Vol. 37, 2002 (pp 21-52).
4 Gardiner, B.S., Dlugogorski, B.Z., and Jameson, G.J.,Prediction of Pressure Losses in Pipe Flow of Aqueous Foams, Industrial & Engineering Chemistry Research, Vol. 38, No. 3, 1999 (pp 1099-1106).
5 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.
6 Gardiner, B.S., Dlugogorski, B.Z., and Jameson, G.J.,Rheology of Fire-Fighting Foams, Fire Safety Journal, Vol. 31, 1998 (pp 61-75).
7 Magrabi, S.A., Dlugogorski, B.Z., and Jameson, G.J.,Bubble Size Distribution and Coarsening of Aqueous Foams, Chemical Engineering Science, Vol. 54, 1999(pp 4007-4022).
8 Integrated Compressed-Air Foam Fire-Extinguishing System for Fixed-Piping Network, FM Approvals Project3019601, FM Global, Norwood, MA, 2004
9 CAN/ULC-S560 " Standard for Category 3 Aqueous Film-Forming Foam (AFFF) Liquid Concentrates, Underwriters' Laboratories of Canada, Scarborough, ON,1998.
10 Crampton, G.P., and Kim, A.K., A Comparison of the Fire-Suppression Performance of Compressed-Air Foam with Air-Aspirated and Unexpanded Foam-Water Solution, Proceedings of the FPRF Conference on Fire Suppression, Orlando, FL, 2004.
11 Crampton, G.P., and Kim, A.K., The Effects of Varying Conditions and Cycling on the Performance of Compressed-Air Foam Systems, Report B-4135.1,Institute for Research in Construction, National Research Council of Canada, Ottawa, ON, 2002.
12 Kim, A.K., Crampton, G.P., and Asselin, J.P., A Comparison of the Fire-Suppression Performance of Compressed-Air Foam and Foam-Water Sprinkler Systems for Class B Hazards, IRC Research Report No.146, Institute for Research in Construction, National Research Council of Canada, Ottawa, ON, 2004.
13 UL 162 " Standard for Safety for Foam Equipment and Liquid Concentrates, Underwriters Laboratories Inc., Northbrook, IL, 1999.
14 Kim, A.K., and Crampton, G.P., Use of Compressed-Air Foam Technology to Provide Fire Protection for Power Transformers, Report B-4142.1, Institute for Research in Construction, National Research Council of Canada, Ottawa, ON, 2004.
15 NFPA 11 - Standard for Low-, Medium- and High-Expansion Foam, National Fire Protection Association,Quincy, MA, 2005