Surface pumps

Centrifugal and self-priming: this is how surface electric pumps work.

Surface pumps differ from submerged ones in that they do not work submerged in liquid. The latter is instead aspirated through a duct and then moved through the centrifugal force imparted by the movement of the impeller.

The surface pumps can be installed under the head or above the head: in the first case they are placed at a lower level than the liquid to be withdrawn and handled, therefore the force of gravity can be exploited to make it convey inside the pump; in the second case, since the pump is at a higher level than the liquid, it must be capable of "self-priming".

Before starting, the pump body is filled with water; in this way, once put into operation, the movement of the impeller generates a strong turbulence in the internal liquid and a depression in the suction duct which causes the air to flow inside the pump. The sucked air mixes with the moving liquid contained in the pump body and being lighter it separates and is expelled from the delivery pipe. At this point the pump is primed and functions as a centrifugal pump.

In self-priming surface electric pumps the use of a non-return valve prevents emptying when the pump stops and therefore it is not necessary to fill the pump body before each start.