Centrifugal pumps should be selected and normally operated at or near the manufacturer’s design rated conditions of head and flow. This is usually at the best efficiency point (BEP).

Pump impeller vane angles and the size and shape of the internal liquid flow passages are fixed and can only be designed for one point of optimum operation. For any other flow conditions, these angles and liquid channels are either too large or too small.

Any pump operated at excess capacity — i.e., at a flow significantly greater than BEP and at a lower head — will surge and vibrate, creating potential bearing and shaft seal problems as well as requiring excessive power.

When operation is at reduced capacity — i.e., at a flow significantly less than BEP — and at a higher head, the fixed vane angles will now cause eddy flows within the impeller, casing and between the wear rings. The radial thrust on the rotor will increase, causing higher shaft stresses, increased shaft deflection, and potential bearing and mechanical seal problems, while radial vibration and shaft axial movement will also increase. Continued operation in this mode will result in the accelerated deterioration of the mechanical and hydraulic performance and may ultimately result in the failure of the pump.