Water Mist Fire Suppression Systems
Theory and Applications

by

Robert M. Gagnon, P.E.

Last month's edition included discussion relative to the theory of water mist systems, research efforts, shipboard applications, and aircraft applications. This article concludes with several additional applications for water mist, including possible alternatives for existing halon systems, and applications where water mist may serve as an alternative to traditional sprinkler systems in areas with severely limited water supplies.

Total Flooding - Electronic/Telecommunications Applications

Applications mentioned previously primarily were concerned with class "A" and "B" fires. A general reluctance to provide water protection of class "C" fires exists because of fears of conductivity. Kidde-Fenwal, in cooperation with GTE and the Fire and Safety International Research Facility in Great Britain, has tested water mist systems on telecommunications switchgear equipment, consisting primarily of vertically-mounted circuit boards.1,2 Halon systems or Carbon Dioxide systems provide the majority of current protection schemes for such equipment. Carbon Dioxide, applied in concentrations of 34% or greater, is dangerous in areas occupied by personnel, while halon systems are marked for replacement by the Montreal Protocol.3

The test was designed to provide water mist protection within the switchgear modules, as opposed to ceiling-mounted area fire protection. Ignition was initiated by a ribbon placed on a board in the center of a switchgear module.

With no water mist protection, temperatures ranged in the 600-1000zC range. Fire spread was rapid vertically, then horizontal spread was eventually accomplished at peak temperature, with flames extended up to four meters above the module, and smoke reaching 100% in the test room in 20 minutes. Damage to the switchgear was considerable.

Tests involving water mist protection reduced maximum temperatures to the 350-500zC range, and smoke obstruction was reduced after system actuation, with nozzles under high pressure providing effective results. Nozzles in the switchgear modules were significantly more effective than nozzles mounted at the ceiling. It was noticed that the water mist was able to negotiate some obstructions within the switchgear, depending upon placement of the nozzle. Extinguishment was accomplished within two seconds, using less than one liter of water. The water mist proved to be less conductive than smoke encountered in the unsuppressed tests, and did not damage any electrical equipment within the switchgear module.

The tests were encouraging enough to prompt further testing on other types of electrical equipment, with the intent of providing a water mist system that could be standardized among varying types of electronic equipment.

A study by the National Institute of Standards and Technology for the U.S. Navy4 showed that fresh water was not found to be a cause of the shorting of electrical equipment. Salt water, or water heavily laden with rust, was significantly more likely to be a potential problem. Fresh water in stainless steel piping, when used in conjunction with a water mist system, appears to be a promising fire protection scheme for electrical and switchgear equipment.

Total Flooding -- Electronic/Telecommunications References

1. Hills, A.T., Simpson, T, and Smith, D.P.: Water Mist Fire Protection System For Telecommunications Switchgear and Other Electronic Facilities. Water Mist Fire Suppression Workshop, March 1-2, 1993: Proceedings, National Institute of Standards and Technology, Gaithersburg, MD.
2. Spring, D.J., Simpson, T., Smith, D.P., and Ball, D.N.: New Applications of Aqueous Agents for Fire Suppression: Proceedings-Halon Alternatives Technical Working Conference, May 11-13, 1993, Albuquerque, N.M.
3. The Montreal Protocol on Substances That Deplete the Ozone Layer, Final Act, United Nations Environment Program, HMSO, CM977, September 1987.
4. Levine, R.S., Navy Safety Center Data on the Effects of Fire Protection Systems on Electrical Equipment, 1991, National Institute of Standards and Technology.

Total Flooding -- Gas Turbine

Applications

Another application that has been directly affected by the phaseout of halon is the fire protection of gas turbines. A series of tests to determine the suitability of water mist systems in gas turbines was performed at the SINTEF Laboratories in Trondheim, Norway.1 A full-scale replica of a gas turbine was built, with a modular water mist system and self-contained water supply.

A special nozzle, developed and patented by British Petroleum Research,2 mixing air and water in the correct proportions at pressures of 60 to 90 PSI, was used to create a fine mist with very small droplet sizes. The air is stored at a pressure of 250 PSI, with a reducing valve capable of attaining the 75 PSI nozzle pressure. A strainer is required for each nozzle, and it was found that the use of salt water or water containing moderate amounts of rust or debris was possible.

The types of fires anticipated by the tests included pool fires, fuel spray fires, combination pool/spray fires, or smoldering insulation fires. The objective of the tests was to determine proper nozzle choice, to use the most appropriate nozzle on a full scale turbine model, then to validate design methods used in nozzle selections and system design. The turbine manufacturer was particularly interested in the effect of water spray on the turbine unit. Twenty tests were conducted, with turbine on or off, with open and closed doors, and with ventilation on and off.

The system provided discharge for 10 seconds, with immediate extinguishment observed in most cases. In some tests, a fuel soaked mat became dislodged by the water mist, was extinguished, then re-ignited. A second discharge of 10 seconds was necessary in these cases. It was decided that a cycling feature would be added to the water mist system, and that the mats would be treated with fire retardant. Door position and ventilation status determined that conditions that were survivable for humans were maintained in the turbine enclosure throughout the test period, providing safe conditions for personnel that might become trapped within the enclosure.

The tests served to reinforce water mist systems as an alternative to halon in the protection of gas turbine enclosures. The concerns with regard to the application of water within these enclosures was effectively dispelled.

Concerns related to total flooding water mist systems are maintenance and reliability.3 Many of the nozzles and nozzle selection criteria, piping and pipe configurations, and storage and pumping components are relatively new in their specific application to water mist technology. Maintenance procedures are unknown for an application that is still in the development process. Further research is being conducted to address these concerns.

Total Flooding -- Gas Turbine Applications

1. Gameiro, V.M., Fine Water Spray Fire Suppression Alternative to Halon 1301 in Gas Turbine Enclosures, Proceedings - Halon Alternatives Technical Working Conference, May 11-13, 1993, Albuquerque, N.M.
2. Papavergos, P.G., Fine Water Sprays for Fire Protection - A Halon Replacement Option 1990., British Petroleum Ventures, BP Research, Sunbury Research Center.
3. Alpert, R.L., Incentives for Use of Misting Sprays As a Fire Suppression Flooding Agent, Proceedings - Water Mist Fire Suppression Workshop, March 1-2, 1993. National Institute of Standards and Technology, Gaithersburg, MD.

Gas-Well Blowout Fires and Fuel Explosion Suppression

In 1984, The National Institute of Standards and Technology undertook a batter of large scale simulation tests in Oklahoma City to determine the effectiveness of water spray with respect to gas-well fires.1,2 An objective was to determine the minimum quantity of water spray application that would be required for effective automatic extinguishment. Such a technique could eliminate the hazards to personnel who would be required to approach these flames and manually extinguish them.

Variables tested included water spray injection internal or external to the flame, varying flame heat release rates from 144 to 222 megawatts, and varying water flow rates. 15z solid spray cone nuzzles were employed, delivering water pressurized at 100 PSI by a diesel engine-powered pump designed to create very small droplets. Two scenarios were used, one involving a nozzle centered in the gas discharge, and another involving four nozzles placed around the circumference of the gas discharge. Differing spray angle geometries were tested and evaluated.

Testing demonstrated that nozzles spraying vertically and parallel to the flame were most efficient, and it became clear that four nozzles, as opposed to two, surrounded the flame more completely and extinguished it most effectively. Testing continued with decreasing gallonage delivery rates, until a rate of 129 GPM was capable of extinguishing the flame in 5 seconds using only 10 gallons of water. Rates significantly below this rate served to separated the flame form the outlet nozzle and decrease the heat release rate, but did not completely extinguish the flame. Flame radiation was decreased by 50% in the tests where total extinguishment was not accomplished.

It was determined that very small amounts of water are require for extinguishment of a 200 megawatt methane jet flame when the nozzles are strategically located, lessening or eliminating danger to personnel involved in fire fighting efforts.

Process industries using flammable solvents must guard against the possibility of explosion. Large-Scale tests were performed that simulated the effectiveness of water mist suppression to prevent explosions.3,4 Diesel oil was sprayed into a test vessel at high pressure, and was ignited, with pressure measurements taken to determine explosion severity.

Halon 1301 was used as a basis of comparison to water spray with and without water additives. It was determined that water with inorganic additives performed with an efficiency similar to Halon 1301. An issue of concern was the possibility of freezing and the measures that would need to be taken to address this concern.

Further studies have been performed relative to the water additives that would be required to prevent freezing or to maximize extinguishment efficiency.5 Water additives, such as light water, a commercially-available product manufactured by 3M, 6 decreases surface tension and serves to smother a pool fire better than tap water. Tests involving water mist extinguishment of hydrogen deflagrations,7 resulted in the finding that 1 GPM, distributed by two water mist nozzles for a period of 90 seconds, will successfully extinguish a hydrogen deflagration. Temperatures with mist protection were reduced by about 200zF to about 150zF. Standard sprinklers had no effect upon the hydrogen deflagration.

Gas Well Blowout Fires and Fuel Explosion Suppression References

1. Pfenning, D., and Evans, D.: Suppression of Gas Well Blowout Fires Using Water Sprays - Large and Small Scale Studies. Proceedings of Production Session, September 11-13, 1984, American Petroleum Institute, San Antonio, TX.
2. Evans, D., and Pfenning, D.: Water Sprays Suppress Gas - Well Blowout Fires, Oil and Gas Journal, April 29, 1985.
3. Spring, D.J., Simpson, T., Smith, D.P., and Ball, D.N.: New Applications of Gaseous Agents for Fire Suppression.. Proceedings - Halon Alternatives Technical Working Conference, May 11-13, 1993, Albuquerque, N..M.
4. Ewing, C.T., Hughes, J.T., and Carhart, H.W.: The Extinction of Hydrocarbon Flames Based on the Heat-Absorption Processes that Occur in Theory. Fire and Materials, 1984, Vol. 8, No. 3.
5. Rosandser, M., and Giselsson, K.: Making the Best use of Water for Fire Extinguishing Purposes. Fire, October 1984, pp. 43-46.
6. Light Water Products and Systems Engineering Manual. 3M Fire Protection Systems, 1993.
7. Butz, J.R., Application of Fine Mists to Hydrogen Deflagrations. Proceedings-Halon Alternatives Technical Working Conference, May 11-13, 1993, Albuquerque, N.M.

Exposure Protection Applications

Waterspray protection of vessels containing flammable liquids is a well-known technology, but Shell Research Limited undertook studies to determine the mechanics of the cooling phenomena of water spray on such vessels.1 The objective was to determine proper water applications rates and define system design techniques necessary to improve exposure protection, for tanks of varying insulation thickness.

It is determined that the best exposure protection must involve a water spray that completely envelops the vessel and eliminates dry spots. Without water spray, steel temperatures quickly reach failure ranges, while the addition of a very thin layer of water spray maintains a shell temperature of 100zC.

A significant finding of this research dealt with water supply limitation relative to intermittent application. It was found that intermittent application, with very low flow rates and strategically-placed nozzles, will provide adequate hot surface protection with a small expenditure of water spray. Additional research is being performed to further understand the phenomenon of intermittent discharge at low flows.

Exposure Protection Applications References

1. Lev, Y., Cooling Sprays for Hot Surfaces. Fire Prevention, 1989, No. 222, p. 42-47.

Life Safety Applications -- Living Quarters

Water spray efficiency in living quarters dates back to the earliest uses of sprinklers as a life and property saving tool. An awareness of drop size occurred in the early 50s with the development of the standard spray sprinkler as a replacement to the "old style" sprinkler. It was noted during testing that the smaller droplets provided a greater available surface area for cooling and heat absorption. It was decided to make the "teeth" on the sprinkler deflector more closely spaced than the old style head, thus breaking the water spray into finer drops.

Subsequent tests, related to sprinklers installed in corridors outside of a living unit, were conducted by the National Institute of Standards and Technology in 1977.1 Full scale tests, bench scale test, and mathematical modeling were employed to determine the best nozzle placement, nozzle diameter and water droplet size.

It was determined that large droplets were an inefficient method of distributing water to a fire hazard in a living occupancy. The findings clearly showed that the smaller droplets, as predicted by intuition and mathematical modeling, were significantly more efficient in absorbing heat, evaporation and in cooling of the fire plume. Additional studies indicated that velocity and droplet trajectory were also significant factors in the rate of evaporation.

Full scale studies confirmed the bench studies, and resulted in the conclusion that smaller orifice nozzles, and their associated smaller droplet sizes, achieved a greater plume temperature reduction, resulting from more efficient evaporation.

Testing is now being performed at the Maryland Fire and Rescue Institute of the University of Maryland to test water mist systems for sleeping quarters. 2,3 A new 24 foot by 48 foot building was constructed that includes a control center for data collection, a 12 foot by 12 foot room with a table and ladder, and a 12 foot by 12 foot room with an underfloor area. One room contains a residential system installed in accordance with NFPA 13R, the standard for installation of sprinkler systems in one and two family dwellings and mobile homes. The other room is equipped with a water mist system of varying piping configurations and with nozzle sizes of varying orifice diameters.

Tests that were viewed featured a test room with a table in the center and a ladder in the corner of the room. Four containers of Heptane were situated at the quarter points of the room, with one additional container under the table, and three containers at varying elevations on the ladder. Two primary piping configurations were tested -- the cornice protection system, with nozzles surrounding the room at the ceiling perimeter, and the ceiling configuration, with two lines of nozzles installed at the ceiling. Nozzle outlets were about 6 inches on center, installed into pre-tapped outlets on stainless steel pipe.

Other tests are being performed to test the effectiveness of water mist on underfloor applications. Halon has been the protection method of choice for this application in the past.

Thorough tests, over the period of a year, will be conducted to test both Heptane fires and wood crib fires with a wide variety of nozzle orifices, piping configurations, and design applications.

Life Safety Applications References

1. Liu, S.T., Analytical and Experimental Study of Evaporative Cooling and Room Fire Suppression by Corridor Sprinkler System, 1977. National Institute for Standards and Technology, Gaithersburg, MD
2. Marlatt, F.P., Maryland Fire and Rescue Institute Hosts High-Pressure Sprinkler Testing Program. MFRI Bulletin, Vol. 24, No. 5, November 1993.
3. Fleming, R.P.: New Interest in Water Mist. NFSA Sprinkler Quarterly, Fall 1993, No. 84.

Conclusion

The recent flurry of activity surrounding water mist systems and the many successful research efforts with water mist are cause for encouragement and excitement. Successful application of water mist for several passenger vessels in Europe and approvals obtained by several maritime approving authorities demonstrates a portion of the future of water mist. In the United State, water mist primarily remains a research effort with exciting possibilities. In the interest of environmental protection, we should welcome water mist as a promising potential substitute for Halon. In the interest of public passenger safety, we should welcome water mist as a potential answer to the vexing problem of fire protection on aircraft and ships. Ongoing testing may demonstrate that under certain conditions, water mist could serve as a potential option to traditional sprinkler systems in areas where extremely limited water supplies exist. Fire protection professionals should adopt the view of water mist as an exciting opportunity, not as a potential threat.

Reprinted from FireWatch!