21 - Pumps

Abstract

This chapter focuses on different types of pumps. Centrifugal pumps consist of a stationary pump casing and an impeller mounted on a rotating shaft. The impeller contains rotating vanes that impart a radial and rotary motion to the liquid. The volute collects the liquid discharged from the impeller at high velocity and gradually causes a reduction in fluid velocity by increasing the flow area, converting the velocity head to a static head. A diffuser increases the efficiency of a centrifugal pump. There are three indications that a centrifugal pump cavitates—noise, fluctuating discharge pressure and flow, and fluctuating pump motor current. Shutoff head is the maximum head that can be developed by a centrifugal pump operating at a set speed. Pump run-out is the maximum flow that can be developed by a centrifugal pump without damaging the pump. A positive displacement pump is one in which a definite volume of liquid is delivered for each cycle of pump operation. Positive displacement pumps may be classified as reciprocating piston pump, gear-type rotary pump, lobe-type rotary pump, screw-type rotary pump, moving vane pump, and diaphragm pump. As viscosity increases, the maximum flow rate through the pump also decreases. The process of the formation and subsequent collapse of vapor bubbles in a pump is called cavitations. Centrifugal pumps are especially sensitive to variations in liquid condition, suction variations, and variations in demand. Cavitation in the centrifugal pump, has a significant negative effect on the performance, which is a common failure mode.