A self-priming pump is a type of centrifugal pump, used mainly in the civil environment, for irrigation and 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.
What makes self-priming pumps different from the other centrifugal pumps installed at inflow?
• the higher level of suction, up to 8 m of difference in height;
• self-priming, in other words operation without needing to fill the inflow channel.
The most widespread self-priming pumps are of the jet type.
[accordion] HOW JET SELF-PRIMING PUMPS WORK
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.
[accordion] Some observations
- Other types of self-priming pumps with lower self-priming performances operate with a different physical principle than the Nozzle-Venturi system used by jet pumps. The principle, in this case, is water recirculation between inflow and outflow.
- These types of pump are useful when the suction depth is not so high and when it must be guaranteed that the pump will not become un-primed during operations.
[accordion] CONSTRUCTION DETAILS
To make the device more compact, the ejector, namely the nozzle-Venturi pipe, is incorporated into the pump. For Calpeda self-priming pumps, a special optimised ejector + diffuser system was studied to strengthen the self-priming capability.
It is always advisable to install a one-way valve in self-priming pumps to maintain a higher quantity of liquid in the pump body and ensure that the machine operates in a better way.