Each stage of a multistage centrifugal pump consists of a combination of an impeller with a diffuser element such as a diffuser, a circular casing or volute casing.
The characteristic feature of a centrifugal pump's stage is the transmission of power from the pump shaft (see Power input) to the fluid handled (see Pump power output). These two factors allow a value for each stage's efficiency to be specified which represents the targeted highest attainable efficiency constant across all stages in multistage pumps, unless adverse inlet conditions or NPSHa values have to be taken into account.
The number of stages (i) of a multistage centrifugal pump should be selected in such a way that hydraulically favourable conditions prevail at the individual stages depending on the type of impeller. In this context, the determining factor for the individual impeller is not the specific speed of the pump (ns.P), but the specific speed related to the stage (ns.St).