Entering the table,, we find that for 10 feet velocity in a 20 inch pipe, the friction head per 100 feet of pipe is 1.87 feet. per second as the proper figure for 4000 feet of line and an elevation head of 15 feet. Ordinarily the velocity should not be less than 12 feet per second, but as this length of pipe is the ultimate condition and as the assumption of a high velocity here would result in an excessively high velocity for short lines even with a smaller impeller in the pump, we will fix 10 ft. The next step is to choose the desired velocity of flow through the 4000 feet of line. It is generally accepted that a 20 or 22 inch pump is the most advantageous for these conditions. Let us assume that it is desired to design a hydraulic dredge capable of raising average hydraulic material 15 feet above the pump through 4000 feet of pipe line and at the same time be suitable for general all round work involving also short lines and low heads.
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