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Friday, August 6, 2010

Self Priming Pump

The basic requirement for a self priming centrifugal pump is that the pumped liquid must be able to entrain air in the form of bubbles so the air will be removed from the suction side of the pump. This air must be allowed to separate from the liquid after the mixture of the two has been discharged by the impeller, and the separated air must be allowed to escape or to be swept out through the pump discharge. Such a self priming pump therefore requires, on its discharge side, an air separator, which is a relatively large stilling chamber, or reservoir, either attached to or built into the pump casing. Alternatively, a small air bleed line can be installed from the discharge pipe between the pump and the discharge check valve back to the suction source.

There are two basic variation of the manner in which the liquid from the discharge reservoir makes the pump self priming.
  1. recirculation from the reservoir back to the suction and
  2. recirculation within the discharge and the impeller itself
Recirculation to Suction
In such a pump, a recalculating port is provided in the discharge reservoir, communicating with the suction side of the impeller. Before the first handles whatever liquid comes to it through the recalculating port plus a certain reservoir, where the two elements are separated, the air passing out of the pump discharge and the liquid returning to the suction of the impeller through the recirculation port. This operation continuous until all the air has been exhausted from the suction line.

Recirculation at Discharge
This form of priming is distinguished from the preceding method by the fact that the priming liquid is not returned to the suction of the pump but mixes with the air either in the impeller or at its periphery. The principal advantage of this method, therefore, is that it eliminates the complexity of internal valve mechanisms.

Regenerative Turbine Pumps
Because these pumps can handle relatively large amounts of gas, they are inherently self-priming as long as sufficient liquid remains in the pump to seal the clearance between the suction and discharge passages. This condition is usually met by building a trap in the pump suction.