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Application of a newly developed compressed-air-foam fire
suppression system
Kim, A. K.; Crampton, G. P.
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Application of a newly-developed compressed-air-foam fire suppression system
Kim, A.K.; Crampton, G.P.
NRCC-44514
A version of this document is published in / Une version de ce document se trouve dans
Interflam 2001, 9th International Fire Science and Engineering Conference, Edinburgh, UK, Sept. 17-19, 2001, pp. 1219-1224
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APPLICATION OF A NEWLY-DEVELOPED
COMPRESSED-AIR-FOAM FIRE SUPPRESSION
SYSTEM
Andrew K. Kim and George P. Crampton Fire Risk Management Program Institute for Research in Construction
National Research Council Canada Ottawa, Ontario, CANADA, K1A 0R6
SUMMARY
National Research Council (NRC), in collaboration with National Defence Canada (DND), has initiated a project to investigate the feasibility of using a newly-developed compressed-air-foam (CAF) fire suppression system to provide fire safety protection to aircrafts and hangar structures. Previous research has indicated that such an objective can be met if the fire suppression system achieves 90% control of the fire in 30 s and extinguishment in 60 s. As a part of this project, a prototype CAF protection system was developed and evaluated to determine whether it meets the project objective. Fire suppression tests of the prototype CAF system with three simulated aircraft hangar fire scenarios, provide technical evidence that the CAF system can meet the fire protection objective using a combination of overhead CAF nozzles and portable low-level nozzles, which could be located near each aircraft to suppress any fire that would be concealed from the overhead nozzles.
INTRODUCTION
Current foam 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 for foam generation. Also, current foam systems are unable to provide foams with high injection velocity, and this is especially important in the case of high-ceiling storage warehouses and hangars, where the injected foam needs to penetrate fire plumes before reaching the seat of a fire. However, if compressed air is used for foam generation, the resulting foam possesses superior quality and substantial injection velocity, as well as requiring much lower quantity of water and foam concentrates, with resulting reduction in cost.
Until now, attempts to adapt this approach, known as compressed-air-foam (CAF), to fixed installations have failed due to two fundamental technical difficulties: first, traditional sprinkler-type nozzles cannot distribute compressed air foam without collapsing it, and secondly, the foam itself degenerates in fixed piping. National Research Council (NRC) has overcome these difficulties, and developed a CAF system whose performance was proven in full-scale tests1, 2. Recently, NRC, in collaboration with National Defence Canada (DND), has initiated a project to investigate the feasibility of using a newly-developed CAF fire
suppression system to provide fire protection equivalent to a conventional fire system for aircraft hangars. As a part of this project, a prototype CAF protection system was developed and evaluated to determine whether it meets the project objective of providing fire safety protection to aircrafts and hangar structures.
This paper will describe the newly-developed CAF system, and also will discuss the study that was carried out to develop the CAF system for installation in an aircraft hangar.
COMPRESSED-AIR-FOAM SYSTEM
Compressed air foam (CAF) is generated by injecting air under pressure into a foam solution stream. The process of moving the solution and air mixture through the hose or piping, if done correctly, forms compressed air foam. The energy for the CAF comes from the combined momentum of the foam solution and air injection streams in the hose or piping. One significant advantage of such CAF systems is the increased momentum of the foam, enabling it to penetrate fire plumes and reach the seat of the fire, Another advantage of CAF is that it possess better stability with respect to drainage than aspirated foams, since CAF is characterized by a narrow distribution of bubble sizes. Also, fixed CAF systems would be more flexible than air-aspirated installations since the foam expansion ratios could be easily changed by adjusting the flow rates of foam solution and compressed air.
The basic elements of CAF fixed pipe systems are shown in Figure 1. It consists of three zones:
1. Air injection zone: Air is injected into a stream of water, ensuring that the air pressure is balanced with the water pressure. This eliminates the pulsations occurring in a pipe if air and water pressures are not balanced.
2. Development zone: After injection of air into the stream of foam solution in the mixer, foam flows through a segment of flexible tubing, which acts as a foam improver. After passing through the tubing, the foam is directed to the distribution piping. Abrupt bends in the piping as well as flow contractions and manifolds promote redistribution of foam into separate gas and liquid phases. The present system can produce uniform foams with expansion ratios ranging from 1:4 to 1:20.
3. Discharge zone: A special nozzle was designed to permit the smooth discharge of foam. The CAF nozzles have no sharp bends and contain no impact points, which are normally present in sprinklers and in fixed aspirated nozzles.
A series of performance evaluation tests were conducted3. Full-scale fire tests with various fire scenarios were carried out with a prototype CAF system. The fire scenarios used were 0.9 m diameter heptane and diesel pool fires and 0.6 m x 0.6 m x 0.3 m wood crib fire. The foam concentrates used were 0.3% Class A and 1 % and 3% Class B foam concentrates. The tests3 showed a superior performance of CAF system in extinguishing both heptane and diesel fires as well as the wood crib fire with far less water requirement, compared to other fire suppression systems such as sprinkler and water mist system.
APPLICATION OF CAF FOR AIRCRAFT HANGAR PROTECTION
A project was undertaken to investigate the feasibility of using CAF to provide fire suppression in Group II aircraft hangars. The challenge for the project was to determine
whether a prototype system using CAF fixed pipe technology can meet the fire suppression and economic objectives for Group II aircraft hangars.
The hangar being considered for this study was approximately 55 m by 37 m with a hangar door height of approximately 7.6 m. The roof of the hangar was a built-up steel deck supported by steel trusses. The trusses run parallel to the door with their bottom chords approximately 7.6 m above the floor.
The major concern was a potential fire, primarily at night, with the hangar full of helicopters or other aircraft. The primary focus for fire suppression was to protect the helicopters, and the protection of the hangar itself was a secondary focus. It was estimated that total involvement of the helicopters was probable within 10 min of ignition without some form of fire suppression.
The primary fire source of concern is a fuel spill from an aircraft tank. The capacity of the helicopter fuel tanks that could create a spill on the hangar floor is 2,373 L. Drip pans, to collect leaking hydraulic fluids and fuel, are located on the floor beneath each helicopter in the hangar. These pans are approximately 1.25 m by 1.0 m with an approximately 100 mm lip height above the floor.
The primary reference document for fire safety in commercial aircraft hangars is National Fire Protection Association [NFPA] standard 4094. The current flow/area requirements in NFPA 409, for those systems which protect aircraft as well as the hangar itself, are based on a 90% fire control with foam (for a spill fire on the hangar floor) after 30 s from discharge and total extinguishment after 60 s5. This is based on research conducted in the early 1970s by Factory Mutual Research Corp. These criteria5 are based on the approximate time for a severe fire (flammable liquid) to melt the aluminium aircraft skin and gain entry to the interior of the aircraft6.
Since protection of the aircraft is a priority, the criteria of 90% control in 30 s and extinguishment in 60 s, then, become the basis for the design of the CAF system for the prototype hangar. These criteria usually mean that low-level protection will be required in addition to overhead protection, especially if there are shaded areas on the hangar floor, as would occur beneath aircraft wings or fuselages.
To meet the objective of 30 s control/60 s extinguishment, the CAF system must be able to cover the hangar floor with foam rapidly. This will normally require overhead nozzles and low-level nozzles (or monitors).
CAF SYSTEM FOR HANGAR APPLICATION CAF Nozzles
NRC has developed prototype nozzles to provide the required distribution of CAF both at ceiling level and at floor level. These nozzles are currently being patented so most details cannot be provided in this paper.
The overhead nozzles were able to provide a coverage of approximately 100 m2 in a circular pattern with a diameter of 11.3 m. The low-level (FoamPods) nozzles were able to provide a
coverage of approximately 66 m2, also in a circular pattern, with a diameter of 9.1 m. These prototype nozzles were investigated to ensure that they can control and extinguish a spill (or pan) fire within the stated limits of 30 and 60 s, respectively.
CAF Generation
To ensure that the appropriate quality of CAF is delivered to the nozzles, NRC has developed foam generation equipment and a foam generation methodology that provides the required air/water/foam mixture for fire control and extinguishment.
The experimental system consisted of a water supply capable of delivering 100 L/min at 100 psi, an air compressor with a capacity of 1,100 L/min at 100 psi, a pressurized concentrate tank, and a mixing chamber (NRC design) to correctly proportion the elements to form CAF. All flow rates and pressures were monitored to ensure that the appropriate parameters were being met. From the mixing chamber, foam was delivered to the nozzle through 30 m of nominal 38 mm diameter polyethylene tubing.
Fire Tests
Preliminary testing had demonstrated that, to control and extinguish spill and pan fires, a density (on the floor) of 1.6 L/min/m2 for the low-level nozzle, and 1.0 L/min/m2 for the overhead nozzle, was required for 2% AFFF or 0.3% Class A foam, at an expansion ratio of 10 to 1. These densities were used in all fire tests with both the overhead and low-level nozzles.
To determine extinguishing capability and effectiveness, the following tests were conducted: Small Pan Tests – In these tests, 8 L of heptane was placed over 80 mm of water in the 1.22 m by 1.22 m pan, and the fuel ignited and permitted to burn for 30 s before commencing fire suppression. The nozzle (either overhead or low-level) was activated manually and allowed to operate until the fire in the pan was extinguished.
Fire test results showed that overhead nozzle CAF system with 0.3% Class A foam controlled (90% fire size reduction) the small pan fire in less than 35 s and extinguished the fire in less than 141 s. The same system with 2% AFFF controlled the fire in less than 55 s and extinguished it in 70 s. Low-level nozzle CAF system with 0.3% Class A foam controlled the small pan fire in less than 28 s and extinguished the fire in less than 56 s. The same system with 2% AFFF controlled the fire in 20 s and extinguished it in less than 53 s.
Large Pan Tests – Fire tests were also conducted using a circular pan, 2.44 m in diameter, with a 127 mm lip height. The total quantity of fuel in the large pan (either gasoline or JP5) was 40 L floating above a base of 100 mm of water. The pre-burn times for the gasoline fires were 20 to 30 s while, for the JP5 fires, the pre-burns were from 15 to 20 s from the time the flames reached the walls of the pan.
Fire test results showed that overhead nozzle CAF system with 0.3% Class A foam controlled the large JP5 pan fire in 180 s and extinguished the fire in 247 s. The same system with 2% AFFF controlled the JP5 fire in 48 s and gasoline fire in 62 s, and extinguished the JP5 fire in 65 s and gasoline fire in 118 s. Low-level nozzle CAF system with 0.3% Class A foam controlled the large JP5 pan fire in 35 s and gasoline fire in 120 s, and extinguished the
JP5 fire in 137 s and gasoline fire in 220 s. The same system with 2% AFFF controlled the JP5 fire in 18 s and gasoline fire in 25 s, and extinguished the JP5 fire in 29 s and gasoline fire in 39 s.
Spill Fire Tests – For the spill fire tests, 0.9 L/min of heptane was fed through a 12.7 mm diameter pipe and allowed to flow across a sheet of 12.7 mm thick gypsum wallboard placed on the floor. The heptane was ignited and permitted to burn for 15 s before commencing fire suppression.
Fire test results showed that the overhead nozzle CAF system with 0.3% Class A foam controlled the spill fire in less than 40 s and extinguished the fire in less than 132 s. Using 2% AFFF, the fire was controlled in 25 s and extinguished in less than 48 s. Low-level nozzle CAF system with 0.3% Class A foam controlled the spill fire in less than 20 s and extinguished the fire in less than 56 s. The same system with 2% AFFF controlled the fire in less than 20 s and extinguished it in less than 53 s.
The overall test results show that the newly-developed CAF system with its overhead and low-level nozzles performed well in extinguishing the test fires, that simulate possible fire scenarios in an aircraft hangar. Using 2% AFFF in the CAF system performed better than using 0.3% Class A foam, in both the control and extinguishment of the three test fires. Overhead nozzle alone had difficulty in meeting the control and extinguishment criteria (30 s control and 60 s extinguishment) of the hangar protection, however, low-level nozzle performed much better and met the control and extinguishment criteria in most of the fire scenarios.
CONCLUSIONS
The technical feasibility of using CAF fire suppression for Group II aircraft hangars is predicated on the system being able to meet the design parameters for suppression implied in NFPA 409 with the added caveat of aircraft protection. This means that the prototype system must achieve fire control in 30 s and extinguishment in 60 s.
As can be seen from the fire test results, the prototype CAF system with overhead and low-level nozzles together was capable of meeting the required objectives with 2% AFFF foam for the large pan test and was close to meeting the objective with Class A foam. Both Class A and AFFF foam met the objective in the small pan and spill fire tests.
With these results, it can be concluded that it is technically feasible to use CAF fire suppression with AFFF to meet the design parameters for fire suppression in Group II aircraft hangars.
ACKNOWLEDGMENTS
The financial support of National Defence Canada for this project is gratefully acknowledged. The contribution of Mr. J.K. Richardson of the Ken Richardson Fire Technologies Inc. in carrying out this project is also acknowledged.
REFERENCES
1. 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, Society of Fire Protection Engineers, Orlando, FL, 1995.
2. Crampton, G.P., et al, A New Fire Suppression Technology, NFPA Journal, Vol. 93, No. 4, National Fire Protection Association, Quincy, MA 1999.
3. 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.
4. NFPA 409 – Standard on Aircraft Hangars, National Fire Protection Association, Quincy, MA, 1995.
5. DiNenno, P. (ed.), SFPE Handbook of Fire Protection Engineering, National Fire Protection Association, Quincy, MA, 1995.
6. Breen, D.E., Hangar Fire Protection with Automatic AFFF Systems, Fire Technology, Vol. 9, No. 2, National Fire Protection Association, Quincy, MA, 1973.
