On Jan. 1,1996, the U.S. Government banned all domestic production and importing of Halon 1301. The EPA has released a preliminary list of acceptable Halon alternatives; but at present, as all are denser and have lower fire-quenching capacities, their incorporation requires redesign of existing Halon-based systems. In particular, reservoirs and supply piping usually must be larger to offer the same required coverage.
Three general kinds of Halon-alternative fire suppressants are presently acceptable, as described below:
Streaming agents. These nonwater suppressants usually discharge from a hand-held fire extinguisher nozzle that produces a spray, mist, fog, or jet that covers the fire. They generally protect a particular process or piece of equipment and work best when the fire is small.
Total flooding agents. When these nonwater suppressants are discharged they vaporize into a gas whose temperature drops greatly. They are best for deep-seated fires such as overheated circuits in computer equipment and beneath access floors, in which the gas invades every cranny to smother and cool the fire. These suppressants are stored under pressure in a reservoir connected to process piping that usually terminates at ceiling-mounted nozzles; thus they are pre-engineered systems.
Water mist. This is a metricated pre-engineered deluge sprinkler system whose atomizing nozzles produce a fine mist of water that is used to extinguish flammable liquids whose metal storage tanks do not exceed 500 gallons. The mist cools the flame and absorbs radiant heat around it, which turns the water droplets into steam, which expands greatly and displaces the air that feeds the fire. The system works rapidly, is highly effective, and uses less water than standard sprinklers; but its coverage is limited, as follows:
Maximum nozzle coverage = 4.0 m2 (43 sf).
Maximum nozzle distance from walls =1.0 m (39.4 in)
Maximum ceiling height of coverage area = 5.0 m (16.4 ft)
Maximum volume of coverage area = 500 m3 (17,657 sf)
Minimum water reserve = 30 min.
Water mist systems have the following drawbacks: (1) breezes and ventilation drafts can disperse the mist, (2) they work only on small incipient fires when the nozzles are aimed directly at the fire's base (which may be hard to arrange in advance of a fire and requires a sensitive and swiftly acting alarm), (3) their high operating pressures (175-250 psi) usually require air compressors with separate piping, (4) the nozzles' tiny orifices require fine strainers, and (5) the systems are expensive. In summary, water mists may be used to protect valuable or irreplacable equipment where standard sprinkler and gaseous systems are unacceptable.
For any Halon-alternative fire suppressant to be effective, the fire hazard area must contain the suppressant for a considerable time. Thus enclosing walls and ceilings should have few or no openings, every entry door should automatically close after being opened, and the area must be fully ventilatable after the fire is out.
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