Discharge recirculation occurs when a pump is operating significantly away from its best efficiency point (BEP).
The BEP is the operating point at which the angle of the flow most closely matches the impeller and diffuser/volute vane geometry. The close correspondence of the fluid angle and fixed vane angle at this point produces the least possible losses incurred throughout the pump operating range, resulting in maximum pump efficiency. The farther that the pump operates from the BEP, the greater the mismatch between the flow angle and the fixed geometry vane angles. This mismatch results not only in increasing shock losses that reduce pump efficiency, but also many other detrimental effects on pump health; these effects are all caused by an increasing susceptibility of the flow to internal recirculation.
Discharge recirculation occurs at the outlet of the impeller and is related to the angle of the exiting fluid. In this condition, there is a mismatch between the angle of the flow exiting the impeller and the inlet angle of either the volute cutwater or diffuser vanes. Recirculation becomes more severe when the divergence is more significant.