Use
Rotodynamic pumps - Operating

A pump can only be started under suitable conditions if the impeller is filled with water or immersed in the fluid it is to pump.

In fact, when the pump is empty of liquid, it delivers a pressure difference Δp proportional to the density of the fluid surrounding it, i.e. 800 times less in air than in water.

For example, a pump supplying a head of 80 m with the valve closed will only supply air with a Δp of 100 mm of water, which is generally insufficient to bring the free surface of the...