An impeller is a rotating iron or steel disc with vanes in a centrifugal pump. Impellers transfer energy from the motor that drives the pump to the fluid being pumped by accelerating the fluid radially outwards from the centre of rotation.
The velocity achieved by the water pump impeller transfers into pressure when the outward movement of the fluid is confined by the pump casing. Impellers are available as vortex or channel impellers.
The dimension of the free passage usually refers to the largest spherical object that may pass through the impeller.
For small and medium-sized sewage pumps, a free passage of 80 mm is normally sufficient. Free passage for large pumps (above 100 l/s) should normally be at least 100 mm. The bigger the free passage, the lower the risk of clogging.
Grundfos impeller pumps are built using a range of technologies, for example with large free passage through the water pump impeller, where this is required.