Self priming centrifugal pumps are designed with an external casing that floods the inner pump or volute. The casing is filled with liquid, then the pump is ready for operation. A low pressure area is formed at the eye of the impeller when the impeller rotates in the casing. Water is pushed up the suction pipe by atmospheric pressure, as the pressure at the eye of the impeller is lower than atmospheric pressure. The self priming pump mixes the air with the recirculating water in the casing. The air separates from the liquid and is discharged from the casing.
Self priming centrifugal pumps are used in water, fuels, effluent/grey water, industrial wastewater and sewage sludge applications.