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Main functions of foam extinguishing agent and its decomposition steps


2014-12-04


The main functions of foam extinguishing agent include:
(1) The foam covering layer formed by extinguishing foam on the surface of combustor can isolate the surface of combustor from air.
(2) The foam layer seals the surface of the combustor, which can block the thermal radiation of the flame on the combustor, prevent the evaporation or pyrolysis volatilization of the combustor, and make it difficult for combustible gases to enter the combustion zone.
(3) The liquid released from foam can cool the combustion surface.
(4) The water vapor generated by the evaporation of foam by heating has the effect of diluting the oxygen concentration in the combustion zone.
The interaction between foam and combustible liquid is a combination of phenomena from the time foam extinguishing agent is sprayed on the combustion surface to the time when dense foam layer is formed. To simplify the considerable complexity of these phenomena, they can be broken down into several time periods:
① When putting out a fire, the formation of foam insulation layer on the surface of combustible liquid depends on the speed ratio of two opposite processes: one is the speed of foam damage on the surface of combustible liquid; The other is the spraying intensity of foam extinguishing agent. If the strength of the sprayed foam exceeds its breaking speed, a foam insulation layer will be immediately formed on the surface. As the combustible liquid is cooled by the foam, the foam breaking speed decreases with time, while the foam layer thickness increases, and the foam dispersion speed on the surface of the combustible liquid also increases. If the foam supply strength is lower than the foam damage speed, the foam insulation layer will not be formed immediately, but will take a period of time, that is, when the temperature of combustible liquid decreases to the point where the foam supply strength begins to exceed the foam damage speed.
The heating layer of combustible liquid is cooled by foam, thus reducing the evaporation rate of liquid, thus reducing the concentration of combustible materials in the combustion zone, the rate of chemical reaction, the rate of heat release, and finally the combustion temperature.
② Once the foam insulating layer on the surface of the combustible liquid is formed, it can prevent the flame radiation heat flow from flowing onto the surface of the combustible liquid, and can cool the surface heating layer of the liquid. This will lead to a decrease in the temperature of the flammable liquid heating layer, which will reduce the amount of evaporation entering the combustion zone, reduce the oxidation reaction speed, reduce heat release, and decrease the combustion temperature.
③ When the foam layer on the liquid surface reaches a certain thickness, the foam layer can prevent the flammable liquid vapor from entering the combustion area, so the foam separates the flammable liquid (its vapor, to be exact) from the combustion area, and combustion stops.
Water vapor cools the combustion zone. The water vapor entering the combustion zone not only reduces the concentration of combustibles in the space within the zone, but also cools the zone. This will increase the heat loss in the reaction zone, thereby reducing the combustion temperature. Therefore, when the thickness of the foam layer is supplied to the surface of the combustible liquid so that the combustible liquid vapor cannot pass through it and enter the combustion area, the purpose of complete fire extinguishing can be achieved. Therefore, the air mechanical foam belongs to the isolation extinguishing agent. The insulation capacity of foam is related to such parameters as air impermeability, dynamic viscosity coefficient and shear stress, which are first determined by foam multiple, fineness, bubble film thickness, etc.