It can be defined as " The operation in which the suction pipe, casing of pipe, pump, and a portion of delivery pipe up to the delivery value is completely filled up with liquid from outside source which is to be raised by the pump before starting the pump".

Thus, air from these parts of the pump is removed and these parts are filled with the liquid to be pumped.

Characteristic curves of centri fugal pump.

These curves are plotted from the results of tests performed on centri fugal pumps. These curves show the behavior and performance of the pump when the pump is working under different flow rate, head and speed.

1] Main characteristic curves.

The main characteristic curves of centri fugal pump consists of variation of Head, Power, discharge w.r. to speed.

2] Operating characteristic curves.

If the speed is kept constant, the variation of manometric head, power and efficiency with respect to discharge gives the operating characteristics of pump.

Input curve should not pass through origin as even at zero discharge some power is needed.

It starts from Q = o, N = o

($\because$ $n = \frac{output}{input}$)

3] Constant efficiency curve.