Self priming is a special impeller enabling a vertical shaft centrifugal pump to work without liquid for a well-defined period of time if it is coupled with the main radial impeller.

When there is a liquid decrease, de-priming will occur at the main radial impeller inlet. The fluid that has been pumps with immediately stop. On the contrary, the self-priming system will make sure that the impeller will be supplier with the fluid even if operation occurs with a lower capacity. A liquid/air mixture is actually pumped in order to increase the air value progressively so as to empty the hidraulic circuit completely. Afterwards, the reverse process of the air/liquid mixture will start and the self-priming system will start supplying the main radial impeller with a higher and higher liquid quantity until no air flows in any longer.

The application of this system (covered by a petent) can be summed up as follows:

• possibility of emptying a tank since the main radial impeller will not de-prime and the pumps can also work in the presence of air.
• possibility of operating the pumps, letting the fluid flow in again and resume the normal operation cycle after having emptied a tank

These operations can be performed without stopping the pumps. Simple actions on the valves will enable the operator to achieve a considerable time saving.