Calpeda Self-priming pumps.
A self-priming pump is a type of centrifugal pump, used in the civil environment and for pressurisation in domestic environments.
Centrifugal pumps can be installed with pump suction below or above the liquid level or inflow.
In the first case, pump suction is at a lower level than the liquid, therefore the liquid has no difficulty in entering the pump suction.
In the second case, pump suction is at a higher level than that of the liquid to be pumped, therefore suitable priming is needed to operate correctly.
This priming is obtained by filling the pump body and the section of pipe for suction with liquid. Centrifugal pumps, if correctly primed, can generally suck with differences in height of up to 5 – 6 metres.
Self-priming is the ability to suction the air in the inflow channel as the pump is being started.
After installing the pump and before starting it, its body must be filled with water; once started, the rotary movement of the impeller creates a flow in the liquid present inside the pump body.
The physical principle of self-priming therefore consists in the strong depression generated at inflow. The principle is known as the Nozzle-Venturi system: the impeller suctions the water which, passing through the nozzle, increases in speed; after this, the speed decreases as the water passes through the Venturi pipe, this fast variation generates a depression inside the suction pipe. This depression allow to remove the air in the suction pipe ant the raises of the liquid level inside the inflow pipes.
Self-priming pumps can create a depression of 0.8 bar. Some pumps can raise liquid from a depth of 9 metres in just 4 minutes.