Self-priming centrifugal pumps

Description self-priming centrifugal pump:
A standard centrifugal pump is not capable to pump air and is not self-priming.
The self-priming centrifugal pump is a centrifugal pump with special features which enables it to process air and water/air mixture. Generally semi-open impellers are used for these pumps. The casing of the pump is shaped in such way that the impeller is always below water level when the casing is filled with water. 

For emptying pits/ wells or when repeated priming is required.