For centrifugal pumps whose mechanical design does not clearly separate the pump shaft from the drive shaft, such as is the case with close-coupled pumps and submersible motor pumps, the efficiency of the pump set (ηGr) is specified in place of pump efficiency (see DIN 24 260) (Gr stands for group). This figure describes the ratio of pump power output (PQ) to the power consumed by the driver (see Drive), which is measured at an agreed position (e.g. at the terminals of the motor or where an underwater cable starts).
Achievable pump efficiency is very much a function of specific speed as well as the size and type of the pump and increases as these two variables increase. Reference values for achievable efficiency of modern centrifugal pump types are based on statistical analyses of the values for existing
pumps. See Figs. 1 and 2 Pump efficiency