泡沫消防系統(tǒng)外文翻譯(文件)
【正文】 ee ponents is with a “ foam maker” . Water under pressure via pipeline or hose is proportioned by an internal orifice. The water moving at high velocities draws the foam concentrate into the water stream resulting in a soluting of 3%10% foam. This solution also moving at high velocities creates a vacuum and draws air into the solution. The foam solution now expands 8to10 times. The foam itself is a tough position entrained with bubbles. The effective use of foam requires its placement on the burning liquid surface as a stable blanker, and it must therefore possess certain desirable physicl properties for this object to be achieved. 1. Fluidity An important property of foam is that of being able to flow easily over the surface of oil and around any obsturctions which may be in its path and so exclude air. An index of fluidity is given by a torsional vane type of viseter which measures the shear stress of the foam (in bynes per square centimeter). A “stiff” foam would have a shear stress of around 1000 dyness/ sq cm, whereas a foam produced by the normal foammaking branch pipe would have a shear stress within the range of 150 to 250 dynes/sq. cm. Mechancical aeration of the aqueous foam solution expands the solution into a foam which is less dense than oil, so that the foam can float on the oil surface. Expansion ratios of 6:1 to 12:1 represent the normal range obtained from conventional foammaking equipment, depending upon the appliance and foam solution used. The expansion ratio is measured by paring the volume of foam produced with the volume of solution required to produce it. Thus assuming an expansion ratio of 10, one unit volume of solution will expand on aeration to ten volumes of foam. 3. Drainage rate The heatresistance of a foam blanket is dependent upon the stability of the bubble formation and the retention of water in the foam. Liquid drains away in time from the foam as a result of bubble breakdowm and gravitation between the bubble walls. The time taken in minutes for 25 per cent of the total liquid content to drain away under standard conditions of test is known as the 25 per cent drainage rate. Since it is not possible to produce a foam which has all the desired properties, the experienced firefighter will utilize his foammaking appliances to the best effect according to the cirumstances. For example, varying the amount of foam pound introduced into the water stream will affect the expansion ratio, shear atress, and draina
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