Priming of a centrifugal pump is defined as the operation in which the suction pipe, casing of the pump and a portion of the delivery pipe upto the delivery valve is completely filled up from outside source with the liquid to be raised by the pump before starting the pump. Thus the air from these parts of the pump is removed and these parts are filled with the liquid to be pumped.
The work done by the impeller per unit weight of liquid per sec is known as the head generated by the pump. Head generated by the pump as $\dfrac 1g Vw_2 U_2$ metre. This equation is independent of the density of the liquid, This means that when pump is running in air the head generated is in terms of metre of air. if the pump is primed with water, the head generated is same metre of water. But as the density of air is very low, the generated head of air in terms of equivalent metre of water head is negligible and hence the water may not be sucked from the pump. To avoid this difficulty priming is necessary.