Water mist does have total flooding limitations, particularly for the nozzles tested by Mawhinney, as do all types of total flooding fire suppression systems. For the purposes of the Canadian Navy, the maximum compartment size is set at 200 m3 to meet economic and space and weight restrictions for storage.

Mawhinney concluded that water mist is potentially an effective fire suppressant for hydrocarbon liquid pool and spray fires depending on the geometry of the compartment.

Sintef (Norway)

A fire research organization in Norway (Sintef) has conducted extensive tests on water mist in flammable and combustible liquid fires. The tests were performed on two different scales. The first was a 30-m 3 test enclosure used to develop the characteristics for the full-scale 70-m3 enclosure. The purpose of the phase I work was to identify the extinguishing characteristics of various BP Sunbury Research Center nobles and determine the efficiency of Ginge-Kerr Offshore's total fire suppression system. The suppression system had a dual fluid nozzle design using air and water at 5 bar. The nozzles produced a high-velocity, small-droplet water mist.

Phase II tested and evaluated the efficiency of fine water spray nozzles in fighting various turbine hood fires in a full-scale test enclosure, consisting of an engine mock-up used to simulate the hot engine surfaces, insulation mats, and piping that would be found in a real engine hood. Diesel pool and spray fires, and diesel-soaked insulation mat fires, were fought under differing conditions of air flow and nozzle position and flow.

The test results ran the full range of possibilities. Large underventilated gas, pool, and oil spray fires were extinguished with the addition of small amounts of water. This was due to near self-extinguishment caused by lack of oxygen being introduced into the hood.

Large well-ventilated gas, pool, and oil spray fires, and fires from oil spray hitting hot metal surfaces, produced varying results. The fires were extinguished in the cases where the mist was able to reach the base of the fire, but not when the droplets could not do so. The oil spray fire on hot metal surfaces was extinguished consistently when the water spray system covered the full area at which the oil spray hit the metal surface, even in the cases when the metal surface temperature remained high.

It was found that 1-m2 (medium) well-ventilated pool fires, small pool fires (<< 1 m2), and fires in oil-soaked insulation mats were very difficult to extinguish. The droplets were not able to penetrate the fire to effectively evaporate the water in the flame zone, nor could they reach the base of the fire.

In the final condition, oil-soaked insulation mats with hot metal surfaces below the mat, the fires were extinguished successfully but had a tendency to reignite. Reignition could be curbed with sustained addition of the water mist to both displace oxygen and cool the metal surface.

The effectiveness of water in the form of a fine water spray as an extinguishant has been demonstrated recently in full-scale testing conducted at Sintef laboratories in Trondheim, Norway. A full-scale mock-up of an enclosed ABB Stal GT-35 gas turbine was used for the purpose of these tests.

In conclusion, the ability of a fine water spray to extinguish fires in gas turbines has met the initial performance requirements with substantial safety margins built in. In installations equipped with 200 liters of water, only 10 liters were required to extinguish a large fire, leaving ample amounts for additional discharges. The concerns of thermal shock were resolved.

Current Status of Water Mist Systems

The efficacy of water mist fire suppression systems as an alternative to halon 1301 or other total flooding gases in naval and marine flammable and combustible liquid hazard areas, including machinery spaces, has been demonstrated. Water mist has a particular advantage over gases due to the substantial environmental and hot-surface cooling that occurs. While water mist systems may also have advantages with respect to reduced space and weight requirements and lower cost relative to total flooding gas systems, these parameters vary widely for systems and specific applications.



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