Appendix D
Water Mist Fire Suppression Technology

Water mist fire suppression systems have been studied for at least 50 years. While no practical or commercially demonstrated systems have been developed until recently, the basis for use of fine liquid water droplets for gas-phase fire suppression is relatively old. Recent interest in water mist technology has been driven by two events. The need for low-weight-impact replacement sprinkler systems on commercial ships driven by International Maritime Organization (IMO) regulations requiting retrofit of most commercial marine vessels gave immediate impetus to the development of low-water-demand, high-efficiency mist systems to replace sprinkler systems. The second driving force was the phase-out of halons and the search for alternative technologies that preserve all or most of the benefits of a clean total flooding agent without having a negative environmental impact.

The state of technology is such that water mist systems for replacing low-pressure water sprinkler systems aboard ships are relatively well developed and have been commercialized. The use of water mist as a replacement for halon 1301 as a total flooding agent is in its infancy but has been demonstrated for certain naval applications.

Fine water mist has been an active area of research and development, and many commercial systems are available or in development. Fine water mist relies on relatively small (<200 µm) droplet sprays to extinguish fires. In theory, the small drops allow the mist to move around obstructions and extinguish fires, mimicking characteristics of a total flooding gas. The mechanisms of extinguishment include flame cooling by droplet heating and evaporation, oxygen depletion by steam expansion, wetting of surfaces, and oxygen depletion due to combustion products.

Water mist systems may have a number of advantages, including possible low cost, absence of toxicity or adverse environmental effects, efficacy in suppression of flammable liquid pool and spray fires, and potential efficacy as inerting or explosion suppression systems. The potential efficacy of water mist fire suppression systems has been demonstrated in numerous studies and in a wide range of applications, including Class B spray and pool fires1,2,3,4 and fires m aircraft cabins,5,6 shipboard machinery and engine room spaces,7,8,9,10,11,12 shipboard accommodation spaces,13 and computer and electronics applications.14,15

To summarize, these experimental efforts have shown that the efficacy of a particular water mist system is strongly dependent on the ability not only to generate sufficiently small droplet sizes but also to distribute a critical concentration of droplets throughout a compartment. 16,17,18 There IS some evidence that the droplets must interact with the flame sheet with sufficient momentum to penetrate the flame. Factors that affect the distribution of this critical concentration of water mist include droplet size, velocity, the spray pattern geometry and the momentum and mixing characteristics of the spray jet, and the geometry and other characteristics of the protected area. While it is relatively easy to generate a dense aerosol of small droplets, it is more difficult to provide sufficient momentum to distribute the spray throughout the space, around obstacles, and so on. Hence, water mist must be evaluated in the context of a system, and not just as an extinguishing agent.

It is apparent that when water mist systems are being evaluated for fire extinction capability as opposed to fire suppression (an easier task), their sensitivity to the details of the area being protected must be considered. It is therefore essential to develop worst-ease fire scenarios and hazard geometries to evaluate the fire extinguishing capabilities of water mist systems.



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--> Appendix D Water Mist Fire Suppression Technology Water mist fire suppression systems have been studied for at least 50 years. While no practical or commercially demonstrated systems have been developed until recently, the basis for use of fine liquid water droplets for gas-phase fire suppression is relatively old. Recent interest in water mist technology has been driven by two events. The need for low-weight-impact replacement sprinkler systems on commercial ships driven by International Maritime Organization (IMO) regulations requiting retrofit of most commercial marine vessels gave immediate impetus to the development of low-water-demand, high-efficiency mist systems to replace sprinkler systems. The second driving force was the phase-out of halons and the search for alternative technologies that preserve all or most of the benefits of a clean total flooding agent without having a negative environmental impact. The state of technology is such that water mist systems for replacing low-pressure water sprinkler systems aboard ships are relatively well developed and have been commercialized. The use of water mist as a replacement for halon 1301 as a total flooding agent is in its infancy but has been demonstrated for certain naval applications. Fine water mist has been an active area of research and development, and many commercial systems are available or in development. Fine water mist relies on relatively small (<200 µm) droplet sprays to extinguish fires. In theory, the small drops allow the mist to move around obstructions and extinguish fires, mimicking characteristics of a total flooding gas. The mechanisms of extinguishment include flame cooling by droplet heating and evaporation, oxygen depletion by steam expansion, wetting of surfaces, and oxygen depletion due to combustion products. Water mist systems may have a number of advantages, including possible low cost, absence of toxicity or adverse environmental effects, efficacy in suppression of flammable liquid pool and spray fires, and potential efficacy as inerting or explosion suppression systems. The potential efficacy of water mist fire suppression systems has been demonstrated in numerous studies and in a wide range of applications, including Class B spray and pool fires1,2,3,4 and fires m aircraft cabins,5,6 shipboard machinery and engine room spaces,7,8,9,10,11,12 shipboard accommodation spaces,13 and computer and electronics applications.14,15 To summarize, these experimental efforts have shown that the efficacy of a particular water mist system is strongly dependent on the ability not only to generate sufficiently small droplet sizes but also to distribute a critical concentration of droplets throughout a compartment. 16,17,18 There IS some evidence that the droplets must interact with the flame sheet with sufficient momentum to penetrate the flame. Factors that affect the distribution of this critical concentration of water mist include droplet size, velocity, the spray pattern geometry and the momentum and mixing characteristics of the spray jet, and the geometry and other characteristics of the protected area. While it is relatively easy to generate a dense aerosol of small droplets, it is more difficult to provide sufficient momentum to distribute the spray throughout the space, around obstacles, and so on. Hence, water mist must be evaluated in the context of a system, and not just as an extinguishing agent. It is apparent that when water mist systems are being evaluated for fire extinction capability as opposed to fire suppression (an easier task), their sensitivity to the details of the area being protected must be considered. It is therefore essential to develop worst-ease fire scenarios and hazard geometries to evaluate the fire extinguishing capabilities of water mist systems.

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--> Theoretical and Design Considerations The major difficulties with water mist systems are those associated with design and engineering. These problems arise from the need to generate, distribute, and maintain an adequate concentration of properly sized drops throughout a compartment while gravity and agent deposition losses on surfaces deplete or reduce the concentration. There is no current theoretical basis for predicting optimal drop size and velocity distribution, spray momentum, distribution pattern, and other important water mist system parameters. This is of course analogous to the lack of a theoretical basis for nozzle design for total flooding gaseous systems, and/or even conventional sprinkler and water spray systems. Extensive experimental and theoretical work aimed at predicting critical adiabatic flame temperatures appears to indicate a range between 1600 to 1900 K, depending on the fuel. According to Holmstedt,19 there are two possible methods by which water spray may extinguish a fire: by extinction of the flame or by cooling of the fuel. Holmstedt states that fuel cooling is performed by larger drop sizes and hence is only relevant to suppression of solid combustible fires. It is possible, however, to use the momentum of large droplets (> 200 µm) to drag or entrain smaller droplets, thus providing a mechanism for mixing and distribution. The potential is present for water mist to act as a true flooding agent if the mass median drop size is below 20 microns. At this level, its suppressing efficiency is twice that of halon 1301 per unit weight. Before this becomes possible, methods of controlling the droplet transport must be developed. The amount of water required to lower the flame temperature to the range of limiting values is between 0.15 to 0.25 L/m3 (1.0 to 1.8 gal/1000 ft3). The actual concentration required may be less than this due to the oversimplification discussed previously. The predominant variables contributing to the production of this concentration are drop size and flow rate. Drop size plays an important role in estimation of the required flow rate as well as in the production of a critical concentration of drops. Drops under 50 microns begin to exhibit characteristics of a gas in the increase in fall time and decrease in terminal velocity. Conversely, larger drops fall faster, resulting in greater fallout losses. Water flux densities (flow rate per unit area) vary significantly across experimental test programs, ranging from 1.5 Lpm/m2 to as high as 10 Lpm/m2.20,21 The significantly higher water flux densities recommended by Gameiro and Mawhinney may be a function of inefficient production of critical concentration (i.e., greater losses due to a larger drop size, poor mixing, and so on). The primary loss mechanism, plate loss caused by gravity and spray impact on walls and obstructions, presents a very difficult technical challenge. From a design standpoint, the loss is overcome by using larger water flow rates and continuous discharge of water. In this sense, current water mist systems greatly exceed the theoretical minimum water concentrations described previously. Vent loss rates are a function of the size of the vent, the size of the fire (which drives the flow through the vent), any other pressure induced across a compartment boundary, and the concentration of drops in the compartment. Evaporation losses are significantly more difficult to calculate or estimate. The evaporation of a drop is a function of drop size, initial temperature, velocity with respect to the surrounding gas, gas temperature, and so on. It is worth noting, assuming all things are constant, that the life of a drop is usually proportional to the square of its diameter. Recent Technology Advances Misting/Atomization Technology During the past decade or so, there has been an expansion of the science and technology of the transportation of bulk liquids into fine sprays (atomization). The primary contributors to this technology have been the combustion industry (fuel spray atomization), the chemical industry (spray drying), and the power industry (evaporative cooling). Significant information relevant to water mist applications for fire suppression can be extracted from this knowledge base.

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--> Atomized sprays may be produced in various ways. Basically, all that is needed is a high relative velocity between the liquid to be atomized and the surrounding air. Some atomizers accomplish this by discharging the liquid at high velocity into a relatively slow-moving stream of air. Notable examples include the various forms of pressure atomizers. An alternative approach is to expose the relatively slow-moving liquid to a high-velocity airstream. The latter method is generally known as dual-fluid, air-assist, or air-blast atomization. Two atomization technologies are incorporated in water mist suppression systems under development and/or consideration: single- and dual-fluid systems. Single-fluid systems (pressure atomizers) utilize water stored or pumped at high pressure (40 to 200 bar) and spray nozzles with relatively small orifice sizes. Dual-fluid systems use air, nitrogen, or other gases to atomize water at a nozzle. Both types of systems have been shown to be effective fire suppression systems. U.S. Navy Water Mist Technology The U.S. Navy has developed a machinery space water mist system that utilizes a modified high-pressure spray nozzle. The nozzle design is described in several Naval Research Laboratory (NRL) reports.22,23,24 The basic design approach is to produce high volumes of 100-µm droplet (mean diameter) sprays with very high spray momentum to achieve rapid suppression of large hydrocarbon pool or spray fires. These nozzles emit 2 gpm at 1000 psi and are spaced approximately 8 ft apart on a uniform grid mounted in the overhead and at the intervening deck level in the machinery space. This system has been tested extensively on the ex-USS Shadwell, the NRL fire test vessel in Mobile Bay, Alabama. It is capable of suppressing fires in seconds. The Navy's water mist system is not particularly effective on highly obstructed small fires, although it provides substantial cooling and limits the fire size, thereby enabling relatively safe manual fire fighting. Compared to some commercially available technology, the Navy system uses relatively high water flow application rates (approximately 0.06 to 0.07 gpm/ft2, which is on the order of three to four times the rate of the best available systems). The relatively high water flow rate requires significant pumping and electrical power capacity. For the LPD-17's largest machinery space, a 250-hp motor is required for a 200-gpm pump. On new-design naval vessels, electrical power and water supply are not particularly difficult constraints, and so the relative efficiency of the system is not an issue. For any retrofit application, however, the current Navy design would be problematic, and the water flow rates of this system would make the use of stored pressure cylinders (vs. pumps) quite difficult. This would substantially limit the application of this system for small individual compartments such as flammable liquid storage rooms. One important component of the Navy system is that it was designed, developed, and tested under significant time constraints and is scheduled for installation on the LPD-17. Optimization of the system or evaluation of alternative designs can and should be pursued if additional applications, particularly retrofit or protection of small enclosures, are envisioned. Commercial Water Mist Fire Suppression Systems At least 11 water mist system technologies are currently available or under development using either dual-fluid (N2/air and water) or single-fluid high-pressure systems. Table D.1 summarizes the commercially available water mist systems that can be used to protect against flammable and combustible liquid hazards. While the performance of these systems varies widely, development of this technology has just begun, and improvements in the effectiveness and efficiency of water mist systems can be expected.

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--> Table D.1 Commercially Available Water Mist Systems Manufacturer Atomization Method Pressure Source IMO Machinery Space Approvala Other Flammable/Combustible Liquid Applicationsb ADA Technologies/Fike Dual fluid atomization Cylinders, low pressure No Yes Reliable/Baumac High pressure, single fluid Pumps No Yes Kidde International Low pressure, single fluid Pump No Yes Grinnell Low pressure, single fluid Pump No Yes Unifog High pressure, single fluid Pump/cylinders Yes (<500 m3) Yes Marioff High pressure, single fluid Pump or cylinders Yes (< 3000 m3) Yes Securiplex Dual fluid, air assisted Cylinders, low pressure No Yes Wormold/Total Walther Low pressure, single fluid Pumps No Yes a International Maritime Organization (IMO) approval based on successful completion of machinery space testing in accordance with MSC Circular 668 (1995) is noted along with limitations on machinery space volume for those tests. b as indicated by testing performed by the U.S. Navy, U.S. Coast Guard, National Research Council of Canada, or a similar national laboratory. Experimental Evaluation of Water Mist Systems The efficacy of water mist fire suppression systems recently has been demonstrated through experimental programs for a range of applications, including the following: Class B spray and pool fires;25,26,27,28 Aircraft cabins;29,30,31 Shipboard machinery and engine room spaces;32,33,34,35,36,37 Shipboard accommodation spaces;38 and Computer and electronics applications.39,40 In addition, Factory Mutual Research Corporation has developed a performance-based approval standard for water mist applications for turbine generator enclosures and machinery spaces. The following sections partially summarize water mist system testing in applications similar to those for the U.S. Navy requiring replacement of halon 1301. Naval Research Laboratory, Small Compartment Testing Over 500 water mist system tests have been conducted by the NRL. Many of these tests were part of an ongoing investigation into the use of water mist as an alternative for halon in machinery space applications for the U.S. Navy.41,42,43 These tests have included both generic systems utilizing modified

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--> industrial spray nozzles and commercially available fire protection misting hardware. The systems tested cover the spectrum of available technologies, including dual-fluid fixed orifice, dual-fluid sheet/slit orifice, single-fluid, high-pressure multiple-orifice heads, and single-fluid, high-pressure grid/matrix-type systems. It was not the intent of this set of NRL investigations to judge one system in terms of another, but rather to determine the capabilities and weaknesses of water mist technology. Each system was evaluated in a variety of configurations to achieve optimal results. The fire-fighting capabilities of these optimized systems varied only slightly for a given flux density. The results were driven primarily by the similarity in drop size distribution between the systems, with the mass mean diameter of drops measured as Dv0.5 ~ 75 Tm ± 25 Tm. (The mass mean diameter, Dv0.5, is defined as the diameter of a drop such that 50% of the total liquid volume/mass is in drops of a smaller diameter.) Some general observations from this effort to assess the fire-fighting performance of water mist systems are as follows: All of the systems evaluated were able to extinguish unobstructed fires on the floor of the compartment with spray flux densities on the order of 1.0 Lpm/m2; Many fires located at higher elevations in the compartment were extinguished, and the remaining fires were reduced dramatically in size; Large fires are easier to extinguish than small fires owing to the displacement of oxygen by the expansion of the water mist to steam as well as to higher plume entrainment rates associated with larger fires; The fire-fighting capabilities of the two-fluid systems were found to increase when nitrogen and other inert gases were substituted for air as the second fluid; and Obstructed fires become more difficult to extinguish with increased horizontal drop travel distance (i.e., horizontal distance from the higher flux density region near the spray pattern to the fire source). Many fires were extinguished at distances on the order of 0.3 m (1 ft) but were not extinguished from greater distances. It is worth noting that many of the highly obstructed fires, although not extinguished, were greatly reduced in size by the presence of the water mist. The MicroMist system by Baumac and the Marioff system represent the extremes of design philosophy for single-fluid, high-pressure water mist systems. One relies on spray momentum for distribution and mixing of drops; the other utilizes many nozzles that produce small droplets with virtually no spray momentum. The major feature of the Marioff nozzle is its droplet size distribution. The flow pattern comprises both large (~100 µm) and small (<50 µm) drops. The large droplets provide spray momentum, which assists in penetration and mixing. Typical spacing is 120 to 150 ft2 per head. Utilizing 1000-psi water supplied by a pump, the Baumac International MicroMist system produces a large amount of very small droplets with almost no momentum in the spray. This is the system that most closely approximated a ''total flooding'' system. It was capable of effectively extinguishing a majority of the unobstructed fires and demonstrated superior fire-fighting capabilities (compared to the other systems tested) in the obstructed pan and comer fire scenarios. In a broader context, these extinguishment efficiencies are still dramatically lower than those of a gaseous agent and would be viewed as inadequate for a total flooding system. Dual-fluid systems (air atomized) use air at 30 to 100 psi to atomize water supplied at 25 to 100 psi. The droplet size distribution can be varied across a wide range by changing the relative water and air flow rates, air pressure, and nozzle orifice design. Several of these types of systems are commercially available. They have been shown to be very effective against localized flammable-liquid hazards.

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--> Table D.2 Performance of Two Navy Water Mist System Types   Extinguishment Time (min:s)   Grinnell AquaMist Spraying Systems Company SYSTEM CHARACTERISTICS         Nozzle pressure 18 bar (250 psi) 18 bar (250 psi) 105 bar (1500 psi) 105 bar (1500 psi) System flow rate 310 Lpm (82 gpm) 310 Lpm (82 gpm) 166 Lpm (44 gpm) 166 Lpm (44 gpm) Application rate 1.86 Lpm/m2 (0.046 gpm/ft2) 3.69 Lpm/m2 (0.091 gpm/ft2) 1.01 Lpm/m2 (0.025 gpm/ft2) 2.03 Lpm/m2 (0.050 gpm/ft2) Nozzle location Single level Bilevel Single level Bilevel SCENARIO         Scenario 2 (4.5 MW)*         Fire #1 3:30 1:18 1:45 0:20 Fire #2 4:00 0:41 1:27 0:20 Fire #3 0:50 No 0:40 0:20 Fire #5 2:05 No 1:30 0:20 Scenario 4 (7.5 MW)*         Fire #1 1:40 2:05 1:20 0:20 Fire #2 2:25 1:02 1:25 0:15 Fire #3 0:20 1:12 0:50 0:20 Fire #5 0:55 No 1:15 0:25 Scenario 5 (7.5 MW)**         Fire #1 3:41 No 2:30 0:40 Fire #2 No No No No Fire #3 0:55 No 0:20 0:40 Fire #5 No 2:40 1:15 0:50 * Ventilation (exhaust and supply) was secured during mist system activation. ** Ventilation (exhaust and supply) remained operating during this test. Naval Research Laboratory, Machinery Space Testing The NRL conducted several series of full-scale tests aboard the ex -USS Shadwell, a damage control and fire-fighting test vessel located in Mobile Bay, Alabama. Testing was performed in a simulated machinery space with a volume of 926 m3 (36,000 ft3). Five water mist nozzle types were evaluated. The tests, which were conducted m 1994 and 1995,44,45,46,47 included diesel and heptane pressurized spray and pool fires and heptane pool fires ranging in size from 3.5 to 7.5 MW. Table D.2 summarizes the performance of two systems, including the Spraying Systems Company's modified nozzle for a two-level nozzle grid installation. For all fires, the extinction times with the high-pressure system were very short. The only failure to extinguish occurred in Scenario 5, where the ventilation system was not left operating throughout the tests. Scenario 4 was identical to Scenario 5 except that ventilation was secured. In an actual installation, the ventilation would be secured, as it is for halon systems, during the system discharge.

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--> U.S. Coast Guard, Machinery Space Testing The U.S. Coast Guard conducted a series of full-scale tests in a simulated 560-m3 machinery space. The tests were designed to evaluate the performance of five different water mist nozzles, including the performance of the Spraying Systems Company/U.S. Navy nozzle as measured by the IMO test procedure for merchant vessel machinery space fire protection systems. The IMO test procedure includes heptane and diesel fuel spray and pool fires ranging in size from 1.0 to 6.0 MW. Overall, these tests demonstrated the ability of water mist systems to control or extinguish a range of liquid pool and spray fires. The tests underscored the difficulty of extinguishing small or obstructed fires and the importance of compartment size in evaluating mist systems (due to oxygen depletion caused by the fire). The tests also demonstrated the generally superior performance of high-pressure (>60 bar) nozzles, including the Navy nozzle, particularly in extinguishing small or obstructed fires. Factory Mutual Gas Turbine Enclosure Testing Factory Mutual Research Corporation (FMRC) has developed a fire test procedure for evaluating water mist-based fire protection systems for combustion turbine enclosures. At least three commercial water mist systems have successfully completed the testing regime, but only one system (Securiplex) has completed all product certification requirements. The FMRC test procedure is used to evaluate water mist systems for compartments up to 260 m3. It is designed to demonstrate system performance on relatively small (1 MW), highly obstructed, flammable and combustible liquid pool and spray fires. An additional important feature of the testing is evaluation of the heat transfer rates between the mist spray and turbine casings. Tests are conducted and heat transfer modeling performed to ensure that, under worst-case conditions, the turbine casing does not deflect excessively. While the FMRC tests are conducted in compartments that are relatively small compared to Navy machinery spaces, the results are of interest for naval gas turbine enclosures and also are indicative oft he performance of current commercial water mist hardware. These systems are significantly more efficient than the Navy mist system owing to several factors. Since the use of pumped systems has significant cost and complexity penalties for commercial applications, stored-pressure systems are preferred. As a result, very space- and weight-efficient, completely self-contained water mist systems have been developed. For example, one system utilizes three 50-L cylinders and four nozzles to provide protection for 15 minutes in an enclosure with a volume of 260 m3, a capability that may be of interest for the protection of Navy flammable storage spaces and turbine enclosures. Another feature of these systems is that the water flow is cycled on and off, a technique that not only minimizes water use but also has been shown to improve fire extinguishing performance. The FMRC approval testing has demonstrated the efficiency of water mist in this application. The systems developed are very cost effective and have performance advantages over halon 1301 and other total flooding gas systems. National Research Council of Canada Mawhinney48 developed engineering design criteria for machinery space water mist fire suppression systems based on Canadian Navy experiments conducted at the National Fire Laboratory in Canada. Key characteristics of such systems include drop size distribution, spray flux, and spray momentum, among others. If the spray is mixed with additives, this, too, is an important characteristic that can affect performance. Test results are based on the use of a particular set of nozzles for a particular set of conditions. Committing to one particular nozzle or system design for all applications would not be appropriate until all engineering constraints have been analyzed. System design must be based on fire suppression objectives and overall system economics in making the decisions on whether to use low-pressure, intermediate-pressure, high-pressure, or twin fluid nozzles.

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--> 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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--> The Navy-developed nozzle, a modification of a commercially available high-pressure nozzle, has demonstrated good performance over a range of fire scenarios and may represent substantial improvements in water mist technology relative to the other designs currently contemplated for use by the Navy. Such improvements may provide opportunities to broaden the application of water mist systems through improvements in space and weight impacts and retrofit potential. However, the need for substantial full-scale testing to improve the currently inadequate fundamental understanding of water mist fire suppression and extinguishment mechanisms represents a substantial cost, timing, and optimization barrier to additional development. References 1. P.G. Papavergos, "Fine Water Sprays for Fire Protection," Proceedings of the Halon Alternatives Technical Conference, Albuquerque, N. Mex., May (1991). 2. J.R. Butz and R. Carey, "Application of Fine Water Mists to Fire Suppression," Proceedings of the Halon Alternatives Technical Conference , Albuquerque, N. Mex., May (1991). 3. R. Wighus, "Fine Water Spray Against Hydrocarbon Fires," SINTEF—Norwegian Fire Research Laboratory, Trondheim, Norway (1993); R. Wighus, "Fine Water Spray System—Extinguishing Tests in Medium and Full Scale Turbine Hood," Norwegian Fire Research Laboratory (1993). 4. C.S. Cousin, "Recent Work on Fire Control Using Fine Water Sprays at the Fire Research Station," Proceedings of the First International Conference on Fire Suppression Research, Stockholm and Bors, Sweden, May 5-8 (1992). 5. R.G. Hill, C.P. Sarkos, and T.R. Marker, "Development and Evaluation of an On-Board Aircraft Water Spray System for Postcrash Fire Protection," SAE Technical Paper 912224, Aerospace Technology Conference and Exposition, Long Beach, California, September 23-26 (1991). 6. R.T. Whitfield, Q.A. Whitfield, and J. Steel, "Aircraft Cabin Fire Suppression by Means of an Interior Water Spray System," CAA Paper 88014, Civil Aviation Authority, July (1988). 7. J.R. Mawhinney, "Fine Water Spray Fire Suppression Project," Proceedings of the First International Conference on Fire Suppression Research , Stockholm and Bors, Sweden, May 5-8 (1992). 8. A.R.F. Turner, "Water Mist in Marine Applications," presented at the Water Mist Fire Suppression Workshop, National Institute of Standards and Technology, Gaithersburg, Maryland, March 1 (1993). 9. M. Arvindson and A. Ryderman, "Tests in Simulated Ship's Engine Rooms with Hi-fog Fire Protection System," 91 R30189, Swedish National Testing and Research Institute, Bors, Sweden, July 28 (1992). 10. M. Tuomissari, "Fire Suppression Tests in Simulated Ship's Engine Room with a Hi-fog Fire Protection System," PAL 2210/92, VTT Fire Technology Laboratory, Helsinki, Finland, November 16 (1992); M. Tuomissari, "Enclosed Space Fire Suppression Tests," PAL 2206/92, October 23 (1992); M. Tuomissari, ''Extinguishing Tests of Simulated Computer Room Fires by a Hi-fog Sprinkler System," PAL 2196/92, August 11 (1992); and M. Tuomissari "Withstand Voltage of Switch Gears in the Presence of Operating Hi-fog Fire Protection System," 9AFX92-98, ABB Strömberg Research Centre, Vassa, Finland, August 3 (1992). 11. E. Soja, "DGME Waterfog Trials," YARD Report No. 4175-NM0609, British Ministry of Defense - Navy, Bath, England (1990). 12. V. Gameiro, "Fine Water Spray Technology Water Mist Fire Suppression Systems," presented at the Halon Alternatives Technical Working Conference, Albuquerque, N. Mex., May 11-13 (1993); V. Gameiro, "Fine Water Spray Fire Suppression Alternative to Halon 1301 in Machinery Spaces," The 1993 International CFC and Halon Alternatives Conference Proceedings , Washington, D.C., October 20-22 (1993). 13. M. Arvindson and A. Ryderman, "Tests in Simulated Ship's Engine Rooms with Hi-fog Fire Protection System," 91 R30189, Swedish National Testing and Research Institute, Bors, Sweden, July 28 (1992). 14. A.T. Hills, T. Simpson, and D.P. Smith, "Water Mist Fire Protection Systems for Telecommunications Switch Gear and Other Electronic Facilities," presented at the Water Mist Fire Suppression Workshop, National Institute of Standards and Technology, Gaithersburg, Maryland, March 1 (1993). 15. M. Tuomissari, "Fire Suppression Tests in Simulated Ship's Engine Room with a Hi-fog Fire Protection System," PAL 2210/92, VTT Fire Technology Laboratory, Helsinki, Finland, November 16 (1992); M. Tuomissari, "Enclosed Space Fire Suppression Tests," PAL 2206/92, October 23 (1992); M. Tuomissari, "Extinguishing Tests of Simulated Computer Room Fires by a Hi-fog Sprinkler System," PAL 2196/92, August 11 (1992); and M. Tuomissari "Withstand Voltage of Switch Gears in the Presence of Operating Hi-fog Fire Protection System," 9AFX92-98, ABB Strömberg Research Centre, Vassa, Finland, August 3 (1992).

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--> 16. J.R. Mawhinney, "Design of Water Mist Fire Suppression Systems for Shipboard Enclosures," Water Mist Instead of Halon?—International Conference on Water Mist Fire Suppression Systems, Borås, Sweden, November 4-5 (1993); J.R. Mawhinney, "Waterfog Fire Suppression System Project: Full Scale Fire Tests Summary Report," ND Project No. DNASE40291, National Research Council Canada, Ottawa (1993); J.R. Mawhinney, "Engineering Criteria for Water Mist Fire Suppression Systems," presented at the Water Mist Fire Suppression Workshop, National Institute of Standards and Technology, Gaithersburg, Maryland, March 1 (1993); and J.R. Mawhinney, "Characteristics of Water Mists for Fire Suppression in Enclosures,'' Halon Alternatives Technical Working Conference 1993, New Mexico Engineering Research Institute, Albuquerque, N. Mex. (1993). 17. C.S. Cousin, "Recent Work on Fire Control Using Fine Water Sprays at the Fire Research Station," Proceedings of the First International Conference on Fire Suppression Research, Stockholm and Bors, Sweden, May 5-8 (1992). 18. L.A. Jackman, "Mathematical Model of the Interaction of Sprinkler Spray Drops with Fire Gases," Proceedings of the First International Conference on Fire Suppression Research, Stockholm and Bors, Sweden, May 5-8 (1992). 19. G. Holmstedt, "Extinction Mechanisms of Water Mist," Water Mist Instead of Halon?—International Conference on Water Mist Fire Suppression Systems, Borås, Sweden, November 4-5 (1993). 20. V. Gameiro, "Fine Water Spray Technology Water Mist Fire Suppression Systems," presented at the Halon Alternatives Technical Working Conference, Albuquerque, N. Mex., May 11-13 (1993); and V. Gameiro, "Fine Water Spray Fire Suppression Alternative to Halon 1301 in Machinery Spaces," The 1993 International CFC and Halon Alternatives Conference Proceedings , Washington, D.C., October 20-22 (1993). 21. J.R. Mawhinney, "Design of Water Mist Fire Suppression Systems for Shipboard Enclosures," Water Mist Instead of Halon?—International Conference on Water Mist Fire Suppression Systems, Borås, Sweden, November 4-5 (1993); and J.R. Mawhinney, "Waterfog Fire Suppression System Project: Full Scale Fire Tests Summary Report," ND Project No. DNASE40291, National Research Council Canada, Ottawa (1993); J.R. Mawhinney, "Engineering Criteria for Water Mist Fire Suppression Systems," presented at the Water Mist Fire Suppression Workshop, National Institute of Standards and Technology, Gaithersburg, Maryland, March 1 (1993); and J.R. Mawhinney, "Characteristics of Water Mists for Fire Suppression in Enclosures," Halon Alternatives Technical Working Conference 1993, New Mexico Engineering Research Institute, Albuquerque, N. Mex. (1993). 22. J.T. Leonard et al., "Full-scale Machinery Space Water Mist Tests: Phase I—Unobstructed Space," NRL Ltr. Rpt. Ser. 6180/0713.1, Naval Research Laboratory, Washington, D.C., October (1994). 23. J.T. Leonard et al., "Full Scale Machinery Space Water Mist Tests: Phase II—Simulated Machinery Spaces," NRL Ltr. Rpt. Ser. 6180/0868.2, Naval Research Laboratory, Washington, D.C., December (1994). 24. J.T. Leonard, G.G. Back, P.J. DiNenno, and R.L. Darwin, "Full-scale Tests of Water Mist Fire Suppression Systems for Navy Shipboard Machinery Spaces: Part II—Obstructed Spaces," NRL/MR/6180-96-7831, Naval Research Laboratory, Washington, D.C., March 8 (1996). 25. P.G. Papavergos, "Fine Water Sprays for Fire Protection," Proceedings of the Halon Alternatives Technical Conference, Albuquerque, N. Mex., May (1991). 26. J.R. Butz and R. Carey, "Application of Fine Water Mists to Fire Suppression," Proceedings of the Halon Alternatives Technical Conference , Albuquerque, N. Mex., May (1991). 27. R. Wighus, "Extinguishment of Enclosed Gas Fires with Water Sprays," Fire Safety Science—Proceedings of the Third International Symposium , Edinburgh, (Elsevier, ISBN 1-85166-719-9) (1991); and R. Wighus, "Active Fire Protection—Extinguishment of Enclosed Gas Fires with Water Sprays," SINTEF Report STF25 A91028, Trondheim, Norway (1991). 28. C.S. Cousin, "Recent Work on Fire Control Using Fine Water Sprays at the Fire Research Station," Proceedings of the First International Conference on Fire Suppression Research, Stockholm and Bors, Sweden, May 5-8 (1992). 29. R.G. Hill, C.P. Sarkos, and T.R. Marker, "Development and Evaluation of an On-Board Aircraft Water Spray System for Postcrash Fire Protection," SAE Technical Paper 912224, Aerospace Technology Conference and Exposition, Long Beach, California, September 23-26 (1991). 30. R.G. Hill, T.R. Marker, and C.P. Sarkos, "Evaluation of an On-Board Water Spray Fire Suppression System in Aircraft," presented at the Water Mist Fire Suppression Workshop, National Institute of Standards and Technology, Gaithersburg, Maryland, March 1 (1993). 31. R.T. Whitfield, Q.A. Whitfield, and J. Steel, "Aircraft Cabin Fire Suppression by Means of an Interior Water Spray System," CAA Paper 88014, Civil Aviation Authority, July (1988).

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--> 32. J.R. Mawhinney, "Fine Water Spray Fire Suppression Project," Proceedings of the First International Conference on Fire Suppression Research, Stockholm and Bors, Sweden, May 5-8 (1992). 33. E. Soja, "DGME Waterfog Trials," YARD Report No. 4175-NM0609, British Ministry of Defense - Navy, Bath, England (1990). 34. V. Gameiro, "Fine Water Spray Technology Water Mist Fire Suppression Systems," presented at the Halon Alternatives Technical Working Conference, Albuquerque, N. Mex., May 11-13 (1993); V. Gameiro, "Fine Water Spray Fire Suppression Alternative to Halon 1301 in Machinery Spaces," The 1993 International CFC and Halon Alternatives Conference Proceedings , Washington, D.C., October 20-22 (1993). 35. A.R.F. Turner, "Water Mist in Marine Applications," presented at the Water Mist Fire Suppression Workshop , National Institute of Standards and Technology, Gaithersburg, Maryland, March 1 (1993). 36. M. Arvindson and A. Ryderman, "Tests in Simulated Ship's Engine Rooms with Hi-fog Fire Protection System," 91 R30189, Swedish National Testing and Research Institute, Bors, Sweden, July 28 (1992). 37. M. Tuomissari, "Fire Suppression Tests in Simulated Ship's Engine Room with a Hi-fog Fire Protection System," PAL 2210/92, VTT Fire Technology Laboratory, Helsinki, Finland, November 16 (1992); M. Tuomissari, "Enclosed Space Fire Suppression Tests," PAL 2206/92, October 23 (1992); M. Tuomissari, "Extinguishing Tests of Simulated Computer Room Fires by a Hi-fog Sprinkler System," PAL 2196/92, August 11 (1992); and M. Tuomissari "Withstand Voltage of Switch Gears in the Presence of Operating Hi-fog Fire Protection System," 9AFX92-98, ABB Strömberg Research Centre, Vassa, Finland, August 3 (1992). 38. M. Arvindson and A. Ryderman, "Tests in Simulated Ship's Engine Rooms with Hi-fog Fire Protection System," 91 R30189, Swedish National Testing and Research Institute, Bors, Sweden, July 28 (1992). 39. R.G. Hill, T.R. Marker, and C.P. Sarkos, "Evaluation of an On-Board Water Spray Fire Suppression System in Aircraft," presented at the Water Mist Fire Suppression Workshop, National Institute of Standards and Technology, Gaithersburg, Maryland, March 1 (1993). 40. M. Tuomissari, "Fire Suppression Tests in Simulated Ship's Engine Room with a Hi-fog Fire Protection System," PAL 2210/92, VTT Fire Technology Laboratory, Helsinki, Finland, November 16 (1992); M. Tuomissari, "Enclosed Space Fire Suppression Tests," PAL 2206/92, October 23 (1992); M. Tuomissari, "Extinguishing Tests of Simulated Computer Room Fires by a Hi-fog Sprinkler System," PAL 2196/92, August 11 (1992); and M. Tuomissari "Withstand Voltage of Switch Gears in the Presence of Operating Hi-fog Fire Protection System," 9AFX92-98, ABB Strömberg Research Centre, Vassa, Finland, August 3 (1992). 41. J.T. Leonard et al., "Full-scale Machinery Space Water Mist Tests: Phase I—Unobstructed Space," NRL Ltr. Rpt. Ser. 6180/0713.1, Naval Research Laboratory, Washington, D.C., October (1994). 42. J.T. Leonard et al., "Full Scale Machinery Space Water Mist Tests: Phase II—Simulated Machinery Spaces," NRL Ltr. Rpt. Ser. 6180/0868.2, Naval Research Laboratory, Washington, D.C., December (1994). 43. J.T. Leonard, G.G. Back, P.J. DiNenno, and R.L. Darwin, "Full-scale Tests of Water Mist Fire Suppression Systems for Navy Shipboard Machinery Spaces: Part II—Obstructed Spaces," NRL/MR/6180-96-7831, Naval Research Laboratory, Washington, D.C., March 8 (1996). 44. J.T. Leonard et al., "Full-scale Machinery Space Water Mist Tests: Phase I—Unobstructed Space," NRL Ltr. Rpt. Ser. 6180/0713.1, Naval Research Laboratory, Washington, D.C., October (1994). 45. J.T. Leonard et al., "Full Scale Machinery Space Water Mist Tests: Phase II—Simulated Machinery Spaces," NRL Ltr. Rpt. Ser. 6180/0868.2, Naval Research Laboratory, Washington, D.C., December (1994). 46. J.T. Leonard, G.G. Back, P.J. DiNenno, and R.L. Darwin, "Full-scale Tests of Water Mist Fire Suppression Systems for Navy Shipboard Machinery Spaces: Part II—Obstructed Spaces," NRL/MR/6180-96-7831, Naval Research Laboratory, Washington, D.C., March 8 (1996). 47. G.G. Back et al., "Full-scale Testing of Water Mist Fire Suppression Systems in Machinery Spaces," U.S. Department of Transportation, U.S. Coast Guard, Groton, Conn., November (1995); M. Tuomissari, "Suppression of Compartment Fires with a Small Amount of Water," Water Mist Instead of Halon?—International Conference on Water Mist Fire Suppression Systems, Borås, Sweden, November 4-5 (1993). 48. J.R. Mawhinney, "Fine Water Spray Fire Suppression Project," Proceedings of the First International Conference on Fire Suppression Research, Stockholm and Bors, Sweden, May 5-8 (1992).