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When a centrifugal pump dry start, the vacuum at the suction will draw air into the system. When there is air in the system pumping efficiency will drop. Priming is the process to get rid of the air drawn in during dry startup.

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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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