Water-Mist Fire Suppression Systems

Water-Mist Fire Suppression Systems

The fire protection community has renewed its interest in fine-spray water application, which long has been recognized as an effective fire suppression medium. The impending phaseout of halon has focused more attention on viable alternatives, water mist among them.

Another factor in the rapid development of water-mist technology is its use as an alternative to automatic fire sprinkler systems on passenger ships. Following the fire aboard the Scandinavian Star in 1990, the International Maritime Organization (IMO) required automatic fire sprinkler protection for new and existing passenger vessels carrying more than 36 passengers in international waters. Recent provisions added tt the original rule have determined that systems equivalent to automatic sprinkler systems are acceptable Vi ater-mist systems are being installed under this provision.

MECHANISMS OF EXTINGUISHMENT

Water mist extinguishes fire in three primary ways: cooling, oxygen displacement. and radiant heat blocking. In some of the higher-pressure water-mist systems, flame displacement or “blowout” also plays a minor role in fire extinguishment.

Cooling. Fires are extinguished by cooling the combustible or the gaseous combustion zone. When water is used to cool the combustion zone, it acts to stop the production of vapors, resulting in an unstable flame that no longer can support combustion. This is the mechanism by which conventional automatic sprinklers and manual firefighting extinguish fire.

Water mist, on the other hand, cools the gaseous combustion zone or flame. Combustion requires high temperatures to sustain itself. The combustion reaction must produce enough heat to compensate for heat losses and yet maintain the necessary high temperatures in the reaction zone. Flame temperature is critical because the oxidation reaction is very temperature-sensitive. A small reduction in the flame temperature causes a large decrease in the reaction rate. When water mist is introduced into the gaseous combustion zone, it upsets this relatively finebalance and extinguishes the fire.

This method of extinguishment is extremely effective. The relative effectiveness of water mist as an extinguishing agent has been tested in laboratory experiments. When compared with dry chemical, large water droplets, Halon 1211, and Halon 1301. water mist comprised of droplets in the 20-micron range exhibited a very high extinguishing efficiency, almost twice that of Halon 1301.

Combustion-zone oxygen displacement. The combustion reaction requires that oxygen be present. When water mist enters the combustion zone (of the flame itself), it rapidly absorbs energy and is converted to steam. During this transformation to steam, the water expands to nearly 1,700 times its original volume. This expansion displaces oxygen in the combustion zone and prevents additional oxygen from entering. Oxygen is not displaced from the room.

Radiant beat blocking. Fire spreads through a process known as “radiant feedback.” The radiant energy from the combustion zone/flame is directed back onto the burning combustible and to nearby objects. These objects in turn begin to heat and drive off combustible vapors. If this process is allowed to continue, eventually all the preheated combustibles within a space give off sufficient vapors to become ignited: this is commonly referred to as “flashover.” When water mist is introduced into the enclosure, the small drops reduce the rate of radiant heat transfer from the flame to the nearbyobjects. slow fire growth, and act to prevent flashover.

Blowout. Water-mist fire suppression systems vary in operating pressure from conventional pressures of approximately 60 psi (414 KPa) to very high pressures, in excess of 4,000 psi (27.600 KPa). The higher-pressure water-mist systems mayuse the pressure and resulting spray velocity to aid in extinguishment through “blowout.” This extinguishment technique. which involves distortion of the combustion zone or flame, reducing its thickness so that the vapors have a much shorter time to react, is commonly used to control oil-well fires. If the combustion zone becomes too thin, combustion will be incomplete, the flame will be cooled below a point at which the reaction can be self-sustaining, and the fire will be extinguished.

APPLICATIONS

Life safety. Water mist is presently being used to protect passenger spaces on marine vessels; successful extinguishments have been reported. To be accepted as an alternative to automatic sprinklers, water-mist systems must pass standardized tests developed to demonstrate a proposed system’s ability to control fires involving various fire scenarios on ships. The Swedish National Testing and Research Institute (SP) and Factory Mutual Research Corporation (F.MRC) have been at the forefront of this testing. These testing anti research facilities have developed standardized tests that include typical shipboard passenger cabins and publicspaces. In addition to these standardized tests developed following surveys of several typical passenger vessels, tests using the Underwriters Laboratories (UL) residential fuel package are also performed. It is conceivable that water-mist systems that pass the UI. tests could be used successfully in a variety of residential occupancies, especially where limited water supplies exist

Protection of electronic equipment. Research tests to evaluate the use of water mist as a viable option to halon for the protection of electronic equipment have been conducted on simulated computer cabinets and electronic telephone switchgear.” They demonstrated that water mist can be used successfully to protect electronic equipment without the water causing unnecessary damage. In these tests, the water-mist protection systems were installed within the cabinets, which limited discharge to the equipment of fire origin. The best results were achieved by using some form of early warning fire detection system, such as smoke detectors. to activate the water-mist system. Testing to evaluate the use of water mist for the protection of the space beneath a raised computer floor is planned. This is another area in which halon has been used extensively in the past.

Hazardous-materials storage atid handling areas. While very little testing has been done in the area of using w’ater mist to protect hazardous materials, it seems that it could be used successfully. Water mist is particularly effective for controlling liquid fuel fires. Many model codes require local containment for hazardousmaterials areas, based in part on the anticipated or design flow’ of any installed automatic fire protection system for a period of up to 20 minutes. Conventional automatic sprinkler systems designed to protect flammable and combustible liquid storage and handling areas deliver fairly high densities of water, up to 0.60 gpm/ft’. After a few quick calculations, it becomes evident that large containment areas are required. Water mist, on the other hand, has demonstrated its ability to control or extinguish flammable liquid fires with a very limited amount of water for a very short duration.

STANDARDS DEVELOPMENT

The National Fire Protection Association has appointed a technical committee to develop a standard for the design. installation, maintenance, and testing of water-mist fire suppression systems. The committee is currently preparing a draft document; it is expected that the document will be presented to the association for adoption at the Fall 1995 meeting in Chicago.

Water-mist fire suppression systems appear to be a viable alternative to halon in certain fire protection applications. It is important to understand that they should not be viewed as a universal replacement for halon but as another option for protecting from tire certain hazards previously protected by halon. Research in the application of water mist is continuing and undoubtedly will demonstrate its viability as a suitable fire protection tool for a number of applications.

References

  1. Ilolmstedt, G., Extinguishing Mechanisms of Water Mist. Lund University, Department of Fire Safety Engineering, Sweden.
  2. Cousin, C, The Potential of Pine Water Sprays as Halon Replacements for Fires in Enclosures. Fire Research Station (FRS), U K.
  3. Simpson, T., A Fully Integrated Water Mist Fire Suppression System for Telecommunications and Other Electronics Cabinets. Fire and Safety International (FSI), U.K.

Paul Dansbach and Glenn Corbett

Buildings That Front on Two Streets

Paul Dansbach and Glenn Corbett consider buildings that front on two streets, share basements, or might otherwise challenge firefighters.

Fire Burns Through PA Aerospace Supplier

A large fire broke out Monday night at SPS Technologies, an aerospace supplier in Abington Township.