Centrifugal pumps can vary in type from radial-vane to axial flow as shown in Figure 1. Specific speed, which is the correlating parameter for the pump profiles in Figure 1 is usually expressed as:
The specific speed is defined as the RPM at which a geometrically similar impeller would run if it were of such a size as to discharge one gpm at a head of one foot.
Specific speed is indicative of the shape and characteristics of an impeller. Specific speed is useful to the designer in predicting proportions required and to the applications engineer in checking suction limitations of pumps.
Impeller form and proportions vary with specific speed as shown in Figure 1. We will be only discussing radial flow centrifugal pumps since these are the most generally used pumps in the refineries.