Dry powder extinguishers

A colourless gas, b.p. — 59-C/740 mm. Used as a relatively non-toxic propellant gas in fire fighting apparatus, e.g. dry-powder extinguishers. Made by the bromination of fluoro-form, CHF3. 

Dry powders are effeetive on flammable liquid or eleetrieal fires. Speeial powders are available for use on metals. Dry powder extinguishers may be used on Class C fires, ineluding gases and liquefied gases in the form of a liquid spillage or a liquid or gas leak. This must be aeeompanied by other aetions, e.g. stopping the leak this is neeessary to avoid aeeumulation of an unburned flammable gas-air mixture whieh eould subsequently result in an explosion. Aetivation may be automatie by a deteetion system, or manual. 

Leave space for possible hose runs for a fire in any stack and provide dry powder extinguishers around the area. 

Impact damage occurred to one of 48 10 kg polypropylene tubs of calcium hypochlorite, each closed with a polythene lid, all supported on a wooden pallet and stretch-wrapped round the base with polyethylene film. The spilled material was swept up and discarded, and the pallet was moved to another part of the warehouse, and some 30 mins, later flames were seen at the base of the pallet [1]. Ignition was attributed to contact between residual solid hypochlorite on the pallet and lubricant drips on the concrete floor from fork lift traffic. Application of a dry powder extinguisher had little effect on the fire, but use of a 40 1 water extinguisher led to rapid escalation of the fire as contact between hypochlorite... 

Surface contamination of 2 kg of the dry salt with as little as 2 flakes of moist potassium hydroxide caused a vigorous self-sustaining fire, which was extinguished with water, but not by carbon dioxide or dry powder extinguishers. 

Fires involving alkylaluminium compounds are difficult to control and must be treated appropriately to particular circumstances [1,5,6], usually with dry-powder extinguishers. Halocarbon fire extinguishants (carbon tetrachloride, chloro-bromomethane, etc.), water or water-based foam must not be applied to alkylaluminium fires. Carbon dioxide is ineffective unless dilute solutions are involved [5,6], Suitable handling and disposal procedures have been detailed for both laboratory [1,2,5,6,7] and manufacturing [5,6] scales of operation. 

Caution. Trimethylaluminum and dimethylzinc burn spontaneously in air and react very violently with water. This reaction should be carried out in a good hood, and personnel should wear adequate protective clothing. A container of vermiculite or other suitable dry powder extinguisher should be immediately... 

General caution and safety. The metal alkyls trimethylaluminum, trimethylgallium, trimethylindium, triethylindium, dimethylzinc, and diethyl-zinc all spontaneously inflame in air (pyrophoric) and are violently reactive toward water and carbon dioxide. Dimethylcadmium fumes upon exposure to air but does not inflame, and decomposes, sluggishly, in water. As with cadmium dichloride, and in common with all cadmium compounds, dimethylcadmium is extremely toxic. A dry powder extinguisher, sand bucket, and fire blanket should be at hand at all times. 

Fires involving sodium metal are very hot and localised and are best dealt with by smothering with sand or by using a dry powder extinguisher, not a carbon tetrachloride or carbon dioxide extinguisher. 

All teaching and research laboratories are provided with a color-coded safety island, a highly visible, easily accessible area in which are located the emergency water sprayer, one or more CO2 extinguishers, a bucket of sand and a wall-mounted fire blanket. Dry powder extinguishers were avoided because of the destructive effect of the airborne powder on electronic instruments. 

To fight fires, the flow of gas should be stopped and dry powder extinguishers should be used. 

Dry powder extinguishers are the most effective for inexperienced Are Aghters and are suitable for both chemical and electrical Ares. They are, however, of limited use in a wind. 

Dioxane vapor forms explosive mixtures with air at concentrations of 2 to 22% (by volume). Fires involving dioxane should be extinguished with carbon dioxide or dry powder extinguishers. 

Class B For fires involving flammable liquids and similar materials, use type BC or ABC dry powder extinguishers. Carbon dioxide (COj) extinguishers may also be used. 

Dry powder extinguishers. There are several types of chemicals used in these extinguishers. Sand is the most inexpensive version of a Class D extinguishing agent. Commercial Class D extinguishers may contain a variety of agents such as sodium chloride and thermoplastic additives, copper powder, graphite, or sodium carbonate. 

Fire is propagated by means of a free radical chain reaction and so anything which will interrupt this chain will cause the fire to die. The halons family of chemicals, which are chloro-, bromo-, fluoro-hydrocarbons, are very efficient at this. Spreading the heat over large surface areas, as in flame traps or with the powder of a dry powder extinguisher, also effectively removes the free radicals and hence results in a suppression of the fire. 

Figure 25,17 Section through a dry powder extinguisher. (Courtesy Thorn Security Ltd)...

Fires involving metals, e g. magnesium and aluminium. These fires are extinguished using special types of dry powder. Extinguishment is achieved by smothering. 

Class D - fires which involve metals such as aluminium or magnesium. Special dry powder extinguishers are required to extinguish these fires, which may contain powdered graphite or talc. 

Fig. 1 a Auto extinguisher with pressure gauge, filled with 2 kg of dry powder extinguishing agent, for fire classes A, B and C... 

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