Volumetric pumps for fluids

The simplest and most widespread means of conveying liquids is the centrifugal pump, which is also the most economical. However, there are at least four types of application for which the centrifugal pump is no longer effective or operational:

• pumping viscous products: above a certain degree of viscosity, to be determined by the user, the use of a centrifugal pump would require the use of an oversized machine that delivers beyond its optimum characteristics, with very poor efficiency and, consequently, very high power consumption;

• problems of precise instantaneous dosing, for which the use of a centrifugal pump would oblige the plant designer to use a flowmeter to control the speed of the centrifugal pump, with the risk of using the pump outside its optimum characteristics;

• pumping of liquids considered "sensitive", i.e. fragile, which do not cope well with the internal turbulence of a centrifugal pump (milk, wine, beer, volatile liquids, etc.);

• the generation of low flows at very high pressure (jet cleaning machines, water jet cutting machines, etc.).

These four types of application call for the use of a positive-displacement pump. The choice of design technology will essentially depend on the characteristics of the product conveyed: viscosity, temperature, density, chemical composition, susceptibility, etc.; the type of application: transfer, mixing, dosing; and operating conditions: flow rate, suction and discharge pressure, etc.

The purpose of this article is not to enable the design of a pump, but to give the engineer in charge of studying an installation the necessary notions to choose the most appropriate type of pump.