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Extinguishment of liquid fuel fires by local application of water mist
Liu, Z. G.; Kim, A. K.; Carpenter, D. W.; Yen, P-L.
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application of water mist
Liu, Z.G.; Kim, A.K.; Carpenter, D.W.; Yen,
P-L.
A version of this paper is published in / Une version de ce document se trouve dans : The Combustion Institute/Canada Section, 2001 Spring Technical Meeting, 13-16 May
2001, pp. 21.1 - 21.4
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Extinguishment of Liquid Fuel Fires by Local Application of Water Mist
Zhigang Liu, Andrew K. Kim and Don Carpenter
National Research Council of Canada, Ottawa, Canada, K1A 0R6, Ping-Li Yen
Fountain Fire Protection Inc., San Gabriel, CA 91776, USA
INTRODUCTION
Statistics show that almost 80 percent of fires are extinguished without the intervention of the fire service and within that 80 percent, local or portable fire extinguishers play a crucial part in limiting fire losses [1]. However, some currently available extinguishers create clean-up and environmental problems and often their use is limited to specific types of fires. In addition, they may also generate toxic or corrosive combustion by-products during fire suppression, causing danger to people and damage to facilities [2], and thus are not suitable for protecting medical centres, telecommunication facilities, industrial ‘clean rooms’ or food processing plants.
The National Research Council of Canada, with Fountain Fire Protection Inc., has carried out a series of full-scale fire tests to investigate the capability and limitation of the local application of water mist in suppressing liquid fuel fires. The types of fires used in the tests included cooking oil, n-heptane, and diesel fuel fires. The impact of key water mist characteristics, such as flow rate, spray angle and droplet size, as well as the nozzle type and its location on fire suppression was studied.
WATER MIST SYSTEM CHARACTERISTICS
Two commercially available water mist nozzles were used in the tests. They were both single-fluid nozzles, with discharge pressures varying from 60 psi to 400 psi. These nozzles produced droplet sizes varying from medium to coarse (400< Dv0.9<1,000
microns), depending on discharge pressures. The flow rate of Nozzle #1 varied from 0.7 to 4.4 gpm and its spray angle was 60 degree. The flow rate of Nozzle #2 varied from 0.71 to 6.30 gpm. Its spray angle was 120 degree under 100 psi discharge pressure, however, the spray angle was reduced with increase in discharge pressure. During the tests, the discharge pressure of the water mist varied from 125 to 225 psi. The cylinder used in the tests had a capacity of 9.4 L of water.
TEST RESULTS AND DISCUSSION
The capability and limitation of water mist fire extinguishers in use against liquid fuel fires were studied with two types of nozzles, using different discharge pressures, flow rates, spray angles and the nozzle distance from the fire. Three types of fires were used in the tests: vegetable cooking oil, n-heptane, and diesel fuel fires.
Cooking Oil Fires
Seven full-scale tests involving cooking oil fires were conducted. A commercial propane-fired deep fat fryer with a frying area of 0.457 m x 0.457 m, and a 0.153 m drip board was used in the tests. The depth of the fryer was 0.457 m. During the tests, the fryer contained 42 L of vegetable cooking oil (0.228 m of depth in the fryer). The cooking oil (a mix of canola and soyabean oils) in the fryer was heated continuously at a rate of 8 – 12oC/min and it auto-ignited at a temperature of 368oC. After auto-ignition, the fire was left to burn freely with the heating source remaining on for 60 s, which further increased the oil temperature to 396oC.
Water mist was discharged to extinguish the fire after a 60 s free burning period. The heating source to the fryer remained on during the discharge. Test results showed that it was difficult to extinguish cooking oil fires, because the hot cooking oil had a strong propensity for re-ignition due to its high burning temperature (approximately 396oC) compared to its auto-ignition temperature (approximately 330oC). To effectively extinguish the cooking oil fires, the water mist discharge must be able to reach the entire fuel surface, cool the fire plume and the bulk of the hot oil, and displace oxygen available for combustion by quick vaporisation of the fine water mist. During Tests K-1 and K-2, the nozzle was located 0.22 m away from the edge of the fryer and 0.47 m above the oil surface. In the tests, discharged water mist could not reach oil fires behind the integral drip board, because the nozzle was located too far from the fryer. The local application of water mist, using both Nozzles #1 (discharge pressure of 155 psi) and #2 (discharge pressure of 170 psi), could not extinguish the fires after 1 min of the discharge.
During Test K-3 with Nozzle #2 and 175 psi discharge pressure, the operator stood 1.1 m away from the fryer with the nozzle located near the edge of the fryer, and activated a full water mist discharge. In Test K-3, water mist was able to reach the entire oil surface. The fire was extinguished at 3 s after water mist discharge. No fire ball and no splashed burning oil were observed in the test. A large amount of steam was produced after the fire was extinguished. The cooking oil was cooled below to 315oC after 25 s of water mist discharge.
Test results also showed that in the same test conditions, the extinguishing time was substantially increased by using lower discharge pressure, because water spray had lower flow rate and less discharge momentum with lower discharge pressure, and was not able to fully penetrate the fire plume. In Test K-4 with Nozzle #2, the discharge pressure was reduced to 125 psi and the fire was extinguished at 19 s after the start of water mist discharge. The extinguishing time in Test K-4 was much longer than that in Test K-3 due to its low discharge pressure.
In Test K-5, the nozzle was changed from Nozzle #2 to #1 and the other test conditions were the same as in Test K-3. The oil fire was extinguished at 8 s after discharge and the extinguishing time was longer than that using Nozzle #2, because
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the discharged water flow rate by Nozzle #1 was lower than that by Nozzle #2 at the same discharge pressure.
Heptane and Diesel Fires
Previous research [3] showed that it was very difficult for water mist to
extinguish flammable liquid fuel fires by local application, because for liquid fuels with a flash point below normal ambient temperature, water mist cannot cool the fuel
temperature low enough to reduce the vapour/air mixture above the surface of the fuel to below its lean flammability limit. To study the capability and limitation of local water mist application for suppressing liquid fuel fires, a total of 4 full-scale fire tests were conducted. A 0.47 x 0.47 x 0.3 m high steel pan was used in the tests. The test fuel consisted of not less than a 0.05 m deep layer of liquid fuel. The surface of the liquid fuel layer was located 0.15 m below the top edge of the pan, so that the fire could not be blown away by water mist. Two selected nozzles, Nozzles #1 and #2, that were used in the tests involving cooking oil fires, were again used in these tests.
After ignition, the fuel was allowed to burn for 60 s before attacking the fire with the water mist. The attack was made from one side only and the operator was not
allowed to extend any part of his body past the edge of the test pan while fighting the fire. During Tests B-1 and B-2 involving n-heptane fires, Nozzle #1 was used with discharge pressures of 175 and 225 psi, respectively. During the tests, the operator stood 1.1 m away from the pan and the nozzle was located near the edge of the pan. Water sprays discharged from Nozzle #1 were able to cover most of the liquid fuel surface but could not effectively cover much beyond the steel pan. The fires in both Tests 1 and B-2 were quickly controlled by the water mist but they could not be extinguished after 1 min of water mist discharge.
During Test B-3, Nozzle #2 was used with discharge pressures of 175 psi while keeping other test conditions the same as in Tests B-1 and B-2. The large water spray generated from Nozzle #2 was able to cover the entire liquid fuel surface as well as the whole steel pan. Discharged fine water droplets effectively controlled the fire and cooled the fire plume and the pan. At the same time, the large and dense water mist spray discharged from Nozzle #2 blocked fresh air entraining into the fire plume and reduced the oxygen available for the combustion. As a result, the n-heptane fire was extinguished at 4 s after the start of discharge and no auto-re-ignition was observed. Test results also showed that at the initial suppressing stage of Test B-3, a large momentary fire flare-up was observed, because initial water mist discharge increased turbulence and stirring around the fire and brought more fresh air into the fire plume, resulting in an increase in fire size and heat release rate. With the increase in nozzle distance from the fire, the initial momentary fire flare-up was further increased due to more fresh air entraining into the fire plume.
During Test B-4, n-heptane fuel was replaced by diesel fuel while keeping the other test conditions the same as in Test B-3. The diesel fuel fire was extinguished at 2 s after discharge and a small momentary fire flare-up was observed during suppression. The total discharge duration was 5 s and no auto-re-ignition was observed. The total water quantity used in the test was 1.21 Litres. Compared to n-heptane fires, it was much easier to extinguish diesel fires with water mist. Diesel fuel has a flash point (FP = 60oC) above normal ambient temperature, and the water mist was able to cool the diesel fuel to below its flash point and to extinguish the diesel fuel fires.
CONCLUSION
The study showed that liquid fuel fires, including cooking oil, n-heptane and diesel fires, can be extinguished by local application of water mist under optimum
conditions. In this study, local water mist system with Nozzle #2 effectively extinguished n-heptane fires (C7H16, FP = -4oC) by cooling the fire plume and the metal pan, blocking fresh air entraining into the fire plume and reducing the oxygen available for the
combustion, while local water mist system with Nozzle #1 which had a lower water flow rate and a small spray angle, could not extinguish n-heptane fires. Water mist momentum and coverage area, water flow rate and nozzle location, are important parameters in determining the effectiveness of local application of water mist against liquid fuel fires.
REFERENCES
1. Willingham, J., “Survey demonstrates vital fire-fighting role for portable extinguishers,” Fire Safety Engineering, August 2000
2. Su, J. Z., Kim, A. K., Crampton, G. P. and Kanabus-Kaminska, M., “Exposure Simulation Dring CF3I Streaming Fire Tests,” Halon Options Technical Working Conference, Proceedings, Albuquerque, USA, 2000
3. Mawhinney, J. R., Dlugogorski, B. Z., and Kim, A. K., “A Closer Look at the Fire Extinguishing Properties of Water Mist,” Fire Safety Science – Proceedings of the Fourth International Symposium, 1994, pp. 47-60.
