Abstract: A centrifugal pump has a rotating shaft (1) with a centrifugal impeller (3) mounted on it. The shaft (1) and impeller (3) are mounted within a stationary casing (7) and a mechanical seal (8) is provided where the shaft (1) passes out through the casing (7). In order to cool rubbing, sealing faces (9, 11) of the seal, fluid is caused to flow along a path from a tip (6) of the impeller, through the casing (7), into a seal chamber (14), through a clearance (15) between a hub (2) of the impeller and the casing (7) and returning through a port (16) in the impeller. In order to act as a vent for and to clear debris from the seal chamber (14), the clearance is located radially outwardly of and remote from the shaft (1). Especially those parts which are moving and are subject to wear can be replaceable.