fire extinguisher(redirected from Wet Chemical)
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fire extinguisher:see fire fightingfire fighting,
the use of strategy, personnel, and apparatus to extinguish, to confine, or to escape from fire. Fire-Fighting Strategy
Fire fighting strategy involves the following basic procedures: arriving at the scene of the fire as rapidly as possible;
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a device for stopping fires by means of fire-extinguishing substances. It is carried to the location where it is to be used and is operated manually.
A fire extinguisher consists of a cylindrical vessel with a volume of 1–100 cubic decimeters, a release valve, and a nozzle for forming a stream of the fire-extinguishing substance, which is usually forced out of the vessel by excess pressure within the vessel. The pressure can be maintained continuously (in fire extinguishers of the pressurized type), or it can be generated when the extinguisher is operated. Pressurized fire extinguishers are filled either with the fire-extinguishing substance alone or with a propellant (such as air or nitrogen) as well. In the second type the pressure may be generated by a propellant contained in a flask or as a result of chemical reaction of substances that are components of the fire-extinguishing material.
Among the substances used as fire-extinguishing materials are carbonic acid (carbon dioxide), chemical foams and mechanically produced aerated foams, halogenated hydrocarbons (ethyl bromide or Freon), powders, and water. The carbon dioxide in a fire extinguisher is in the liquid state; it leaves the extinguisher through a diffusing nozzle as a jet that consists of a gaseous phase and a solid phase (snow). Chemical foams are formed in a fire extinguisher as the result of a reaction between an alkaline solution (based on NaHCO3) and an acid solution (based on H2SO4) that takes place when the solutions are mixed before entering the nozzle. The foam ratio—that is, the ratio of the volume of foam to the volume of solution—is 4–6. Mechanically produced aerated foams are obtained by passing a 5–6-percent aqueous solution of a surface-active agent through the nozzle. The solution is broken down into small droplets in the atomizer of the nozzle. The stream of droplets is mixed with the air that is ejected into the nozzle, thus forming a foam with a foam ration of 6–8. In nozzles of the mesh type, foam is produced from bubbles that are generated on the mesh by blowing; the foam ratio in this case is 50–70. The jet of foam is 3–6 m long. Upon passing through the nozzle, the halogenated hydrocarbons form a stream of the aerosol type consisting of finely dispersed drops; powders produce a cloud-forming stream.
The uses of fire extinguishers depend on such properties as extinguishing capacity, the corrosive activity, toxicity, and electric conductivity of the fire-extinguishing substance, capacity, and ability to withstand vibrational loads.
The design of a fire extinguisher depends on the type of the fire-extinguishing substance and the means used to expel the substance. The steel gas cylinders of carbon dioxide fire extinguishers are built for operating pressures of 15 meganewtons per sq m (MN/m2), or 150 kilograms-force per sq cm (kgf/cm2). In all other fire extinguishers the pressure does not exceed 2 MN/m2. The vessels of small fire extinguishers that use powders can be made of plastic.
Fire extinguishers are checked throughout their period of use (beginning with the time of charging) for strength of the vessel and operability. Inspection intervals and procedures are determined in the specifications.
O. M. KURBATSKII