Operation Of Pumps As Turbines

Centrifugal pumps may be used as hydraulic turbines in some cases where low first cost is paramount. Because the pump has no speed-regulating mechanism, considerable speed variation must be expected unless the head and load remain very nearly constant. Some speed control could be obtained by throttling the discharge automatically, but this would increase the cost, and the power lost in the throttle valve would lower the overall efficiency.

Pump Selection After the head, speed, and power output of the turbine have been specified, it is necessary to select a pump that, when used as a turbine, will satisfy the requirements. Assuming that performance curves for a series of pumps are available*, a typical set of such curves should be normalized using the head, power, and flow rate of the best efficiency point as normal values. These curves will correspond to the right part of Section 1 of either Figure 52 or 53. In normalizing the power P,let p = P/Pn and the curve of p/n3 will be identical with the curve of m/n2 in Figure 52 or 53. The normalized curves may be compared with the curves in Sections 1 of Figures 52 and 53 to determine which curves best represent the characteristics of the proposed pump. When a choice has been made, the approximate turbine performance can be obtained from the corresponding figure of Figures 60 to 63.

example Assume that the turbine specifications are HT = 20 ft, PT = 12.75 hp and n = 580 rpm, and that the characteristics of the DeLaval L1O/8 pump are representative of a series of pumps from which a selection can be made. The turbine discharge QT in gallons per minute after substituting the above values is

HjhT hT

;

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