泡沫消防系統(tǒng)外文翻譯-展示頁
2025-05-26 22:21本頁面
【正文】 . 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 drainage rate, and formulations of foam pounds can be selected to give low (6 to 7 ), intermediate (8 to 12 ) or high expansion (12 to 16) foams. There are three main ytpes of foam extinguishment systems. The fixed system consists of storage tanks of the foam concentrate, foam pumps and proportioning system, as well as pipelines to various equipment or structures, and foam discharge devices. It is a centralized foam solution system, that will produce foam only at specified locatoins. The fixed systems have been used where large storage tanks are in use such as large marine terminals or bulk storage plants. The advantages of a fixed system are the following. (1)It is fast to operate. (2)It requires low manpower. (3)It makes efficient use of foam. The disadvantages include the following: (1)low flexibility
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