centrifugal pumps are uniquq because they can provide high or very high flow rate (much higher than most positive displacement pumps)
if no water is present instially, the negative pressure developed by the rotating air,at the eye will be negligibly small to suck fresh steam of water. as a result the imeller will rotate without sucking and discharging any water content. sothe pump should we instially filled with water before starting it ,this process is known as priming

use of casing of centrifugal pump it has an increasing area along the flow direction such increasing area will help to accommodate new added water stream and will allow also help to reduce the the exit flow velocity reduction in the flow velocity will result in increase in the static pressure which is required to overcome the resistance of pumping system.


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