Centrifugal and Positive Displacement Pumps

A pump is a machine that provides energy to a fluid in a hydraulic system. It assists to increase the pressure energy or kinetic energy, or both, in the fluid by converting the mechanical energy. The basic difference between a turbine and the pump, from a hydrodynamic point of view, is that in the former flow takes place from the high-pressure side to the low-pressure side, whereas in pump flow takes place from the low pressure forwards the higher pressure. Thus in a turbine, there is accelerated flow while in a pump the flow is decelerated. Accelerated flow throughout the hydraulic turbines is less subjected to turbulence therefore the runner passages are relatively short and high efficiency is available for this machine due to reduced values for the friction losses. Decelerated flow throughout the centrifugal pumps is sensitive to separation and vortices therefore impeller passages are relatively long and gradually increased in cross-section area for lowering the friction losses – "centrifugal pumps" efficiency is normally lower comparing to the turbines.

At the beginning of this chapter, one presents a classification of centrifugal pumps, reciprocating pumps – (Fig. 5.1) and pump turbines. In addition, basic centrifugal pump theory and a brief analysis of the net positive suction head (NPSH) that are very useful for the design and selection of the pumps are detailed. In the next sections similarity laws, specific speed, cavitation and selection of the pumps are available. All these items are illustrated by solved problems.

Chapters on "similarity law, specific speed and cavitation and pumps section" acquiring great efficiency in using the tool of mathematics and at the solved problems are available.

Keywords: Cavitation in pump, Centrifugal pumps, Efficiencies, Force and Power, Head of the pump, Negative suction lift, NPSH required, NPSH, Positive Displacement Pumps, Positive suction lift, Pump Turbine, Reciprocating pump.