Page 264 - Chemical process engineering design and economics

244 Chapter 5

cause of this characteristic of positive-displacement pumps, if a valve, located on
the discharge side of the pump, is used to vary the flow rate, then, the discharged
pressure must be controlled. A variable-speed drive could also be used to vary the
flow rate. Generally, positive-displacement pumps are employed where it is re-
quired to deliver low flow rates at high pressures. If high flow rates at high pres-
sures are required, then the pumps are installed in parallel. To develop high pres-
sures requires close clearances between the moving parts to minimize leakage, but
close clearances means that the pump must move at slower speeds to avoid ex-
cessive wear. Thus, pumps designed to develop high pressures are forced to de-
liver low flow rates. On the other hand, pumps designed to deliver high flow rates
usually cannot develop high pressures.
Positive-displacement pumps are self priming, which is the ability of a
pump to lift liquids from a level below the center line of the pump. This charac-
teristic of positive-displacement pumps is attributed to the tight seal between the
discharge and suction sides of the pump. Thus, at startup air is compressed and

Figure 5.24 Pump-classification chart.

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