When there is air in the pump casing, less centrifugal force is generated because the air density is much lower than the liquid density. Therefore, the pressure difference between the liquid level above the sump and the pumping inlet is insufficient to press the liquid in the sump into the pump. That is, the centrifugal pump has no self-priming capability, so that the centrifugal pump cannot transport the liquid. This phenomenon is called "gas binding. phenomenon".
In order to fill the pump with liquid, a bottom valve with a filter is usually installed at the bottom of the suction pipe. The bottom valve is against the valve. The role of the filter is to prevent solid matter from entering the pump and damaging the impeller or preventing the pump from operating normally.