Impeller types can be radial, mixed flow, axial and peripheral and are selected on the basis of the pump design and the application. The number of vanes will affect the efficiency; in general more vanes are more efficient.
Open impellers
Vanes are attached to the central hub, without any form, sidewall, or shroud, and are mounted directly onto a shaft. Open impellers are structurally weak and require higher NPSHR values. The impeller to volute or back plate clearance must be adjusted when the pump is at operating temperature and all axial thermal growth has occurred.