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Chemical Aspects of Fire Extinguishment Concepts of fire a n d fire extinguishment must be revamped be cause recent findings a d d a fourth dimension to the fire triangle by A. B. Guise, Ansul Chemical Co.
f\s A RESULT of fairly recent and not too widely publicized research, our understanding of how fire ex tinguishing agents work has increased greatly. This may lead to more effective provision for the control and extinguishment of fires. The old familiar fire triangle concept which postulates that fire can exist only when heat, fuel, and oxygen are present, is an oversim plification. I n the light of new findings it must be refined to a square. A new factor, a chemical chain reaction in the flame itself, is a fourth essential. Consequently, in fire extinguishment a new factor has been added: "interrupt the flame chain reaction." This action may be extremely important as an explanation of the effectiveness of certain extinguishing agents. Water. T h e most common— oldest, cheapest, and most plentiful of all—is water, applied to a fire as a liquid stream or spray. With one exception, the extinguishing ac tion of water and water solutions is purely physical. Steam acts as a smothering agent by displacement of oxygen, and the mechanical and chemical foams act as blanketing agents to separate the fuel from the air. Wetting agents and viscosityincreasing agents increase the effec tiveness of the cooling action of water by retarding runoff. The first indication of true chemi cal action in extinguishment where water is concerned is the "loaded stream," a solution of potassium carbonate in water with other addi tives to reduce the freezing point to —40° F., used in hand portable and wheeled extinguishers. It is more effective than plain water on fires in ordinary combustibles such as wood, cloth, or paper and is ap proved for use on flammable liquid
fires where water would not be effec tive even as a spray. Early work in this area was described by Thomas and Hochwalt in 1928. They concluded that water solutions of metallic salts based on elements in Group I of the Periodic Table (lithium, sodium, potassium, rubidium, or cesium) were more effective as the atomic weights of the cations increased. Alkali metal salts with anions containing oxygen and halogens were most effective and they ascribed the action to a "negative catalytic" effect on the combustion reaction. It is probable that the effectiveness of these solu tions is due to a chemical chainbreaking action. This is also in dicated by practical experience with the loaded stream extinguisher, which is most effective on flammable liquid fires when the solution is applied to produce a spray. Carbon Dioxide. Carbon dioxide is essentially as simple in its action as water. When introduced into the combustion zone, it dilutes the reactants—oxygen and fuel vapors—below the concentration necessary to support combustion.
Explosion buret flammability peak data (Η. Ε. Moran,Jr., U.S. Naval Civil Engineer ing Research and Evaluation Labora tory) Fixed nozzle labora tory fire tests Extinguishment of large fires with ex tinguishers (Under writers' Laboratories)
Carbon dioxide may be used on any type of fire, regardless of the type of fuel involved or the presence of electrical hazards. However, con finement is required in fires involving ordinary combustibles to assure ef fective exclusion of oxygen long enough to cool to below rcignition temperatures. Halogenated Hydrocarbons. Much research has been carried out on the extinguishing effective ness of a large variety of halogenated hydrocarbons. At one time it was thought that their action was due to cooling by vaporization and smothering by vapor dilution of air and fuel vapors. It is now believed that the major extinguishing mechanism is a chemical chainbreaking action and that the halo gens inhibit continuation of the combustion reaction by combining with hydrogen atoms and removing them. Carbon tetrachloride is an inex pensive extinguishant, but its ap plication is declining because of its toxicity and the toxicity of the products of thermal decomposition. Chlorobromome thane, developed
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1.
C o m p a r a t i v e effectiveness o f extinguishing agents on w e i g h t basis VOL. 5 1 , NO. I I
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NOVEMBER 1959
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I/EC
G u i d e b o o k for Technical
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AGENT PARTICLE
Figure 2.
Branched-chain combustion reaction of hydrogen and oxygen
by the Germans during World War II, has slightly less toxicity and is a more effective extinguishing agent. Methyl bromide gained wide acceptance in foreign countries but has never been widely used in the United States. Its use abroad has also declined, because of toxicity. New halogenatcd agents of considerable effectiveness are fluorobromo compounds. Bromotrifluoromethane (Halon 1301) is receiving concentrated attention from the military, but its cost, about $4 per pound, has delayed commercial application despite excellent extinguishing effectiveness and low toxicity. Laboratory evaluation of halogenatcd compounds must be care-
fully weighed in the light of field application. There is considerable discrepancy between practical extinguishing results and laboratory evaluation, especially by the explosion buret method. Although the explosion buret method is a convenient way of carrying out preliminary evaluations in the laboratory, it does not represent actual conditions of fire extinguishment, because the values are obtained under conditions where combustion is not already proceeding. Laboratory evaluations on a larger scale by the use of fixed nozzles may be converted to practical applications, where properly engineered piped systems with fixed nozzles
Figure 3. Effect application rate
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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are used. Where streams from extinguishers or hose lines must be used, the physical characteristics of the halogenated compounds may not allow the theoretical effectiveness of the extinguishing agent to be obtained. This is shown by Figure 1. Dry Chemical. Dry chemical' consists of finely divided sodium bicarbonate with additives used primarily to enhance flowing action and water repellcncy. Potassium bicarbonate is under consideration, but is not yet commercially available. Both laboratory and practical evaluations indicate greater extinguishing effectiveness than sodium bicarbonate dry chemical. The finely divided chemical is expelled from its container by dry gas (air, carbon dioxide, or nitrogen) under pressure through a nozzle which controls the pattern of application much as water is controlled. Dry chemical is believed to extinguish primarily by chemical chainreaction interruption. Cooling or dilution of reactants is believed to be a relatively minor factor. Even chemicals not readily decomposed by heat, such as sodium chloride, are effective extinguishing agents when applied as a powder. The chain reaction involves the presence of free radicals in the flame zone. These interact with fuel and oxygen to produce a continuing or increasing supply of free radicals to continue the flame reaction (Figure 2). A simple hydrogen flame reaction illustrates this point. The free radicals are self-propagating unless captured by condensation on or interaction with some inerting substance. The fine particles introduced to the flame area are believed to capture sufficient free radicals to interrupt the chain reaction, and flame suppression is nearly instantaneous. A cloud of dry chemical therefore is similar to a flame arrester, in that a flame cannot pass through a cloud of particles, when in proper concentration ; conversely, when the cloud of particles is produced where the combustion reaction is proceeding, extinguishment takes place. It takes about 200,000,000 particles of dry chemical per cubic
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Figure 4 . Relation of particle size to extinguishing effectiveness o f dry chemicals
foot of flame to extinguish a gaso line fire a n d a p p r o x i m a t e l y 2,000,000,000 particles to extinguish a h y d r o g e n fire. All extinguishing agents m a y be generally c o m p a r e d for effectiveness in terms of the q u a n t i t y required for extinguishing a given fire. E a c h c u r v e in F i g u r e 3 is derived from extinguishing d a t a a n d all illustrate a n i m p o r t a n t factor in the effective ness of a n y type of a g e n t — r a t e of application. I n the case of d r y chemical cx T tinguishing one m o r e factor is vital— particle size (Figure 4 ) . A t t a c h m e n t of free radicals to a surface is a p p a r e n t l y a basic factor in chain-reaction interruption. The d r y chemical particles in the flame zone provide such a surface a n d the finer the p o w d e r the g r e a t e r the effective surface for a given q u a n t i t y of agent a n d the shorter the distance of diffusion of a free radical.
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If
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99-5
Division of Industrial and Engineering Chemistry, 136th Meeting, ACS, Atlantic City, N. J., September 1959.
OF HYDROGEN
PURE
Illustration shows plant recently installed in Spain and consists of 256 18,000 amoere cells producing 2,100 cu. metres per hour.
Knowles Electrolytic plant produces hydrogen of a constant high standard of purity, together with oxygen 99.8% pure. The plant is uniquely simple and safe and requires no highly skilled labour. Plants of all sizes up to a single installation absorbing 25,000 kW are in operation in all parts of the world. Our authors like to hear from readers. If you have questions or comments, or both, send them via The Editor, l/EC, 1155 16th Street N.W., Washington 6, D.C. Letters will be forwarded and answered promptly.
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VOL. 51 NO. 11 ·
NOVEMBER 1959
75 A
