3 - Pump hydraulics

Abstract

Centrifugal Pumps are hydraulic machines; therefore, one needs to get familiar with the hydraulic terms associated with centrifugal pumps and of the liquids that they handle. Materials with a specific gravity of less than 1 are less dense than water, and will therefore float on it. Substances with a specific gravity of more than 1 are denser than water and will sink. Viscosity describes the internal friction of a moving fluid. For a pure liquid, vapor pressure depends only on the temperature. The first and most important point to consider is that centrifugal pumps are volumetric machines. The liquid pumped is measured in terms of the volume flow rate. The pressure of the liquid can be stated in terms of meters (feet) of head of the liquid column (mlc). As in the case of volumetric flows rate, the head generated by the pump in mlc for a single point of operation is the same for any liquid. The required head comprises two components: a static component and a friction head loss component. The pump curves are generated while testing the pump using cold water as the liquid. The curve is fixed for a particular speed, impeller diameter, and water. It is not necessary that the pumps' actual operation throughout its life will be for the same speed or impeller diameter and service. When any of these change, the pump flow and head generated will differ.