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During priming of a pump, its impeller is fully submerged in the liquid (say water) to remove any air trap (any kind of cavity that can lead to pressure drop), vapors, etc.

This is important because lower density fluid produces a head rise proportional to the ratio of its density to the liquid to be pumped. If the pump is placed above the liquid's upper surface, then suction line has a cavity that does not allow proper suction. To avoid this, priming is necessary. 

This is for centrifugal pumps and not for positive displacement pumps.

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Centrifugal pumps are made to pump liquids not gas. When gas cut-in the liquid into the centrifugal pump volute through the suction/leaking side on the pump, the pump will fail to pump at its design rate and pressure due to excessive stress of breaking the expanded gas bubble in the pump volute. This effect is called CAVITATION.  This could result to failure or breaking of the pump impellers or cracking of the pump volute as well as other components if not corrected immediately.

In other words,  we prime pumps to avoid cavitation and its effects. 

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On 22/01/2016 at 7:41 PM, Satish Waskar said:

Priming is process of removing air from from suction pipe by filling water in suction.


to give precise touch for the above answer I will say ' Priming is the process of removing Vapour from pumping liquid.'

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Priming is the process to remove all the entrapped air from the suction line and the pump by filling water through a connection close to the discharge nozzle either manually or through an self-regulating system especially when there is first start up

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