If a pump draws in air while in operation the noise is produced and the amount of sea water discharged decreases, significantly lowering the pump's operating efficiency. Also, the amount of sea water being discharged will drop to zero in an instant once a pump is drawing a large amount of air. Why? When a small amount of air is drawn into the pump, the air passes out of the pump together with the sea water.
But, if a large amount of air has entered a pump, it gathers and gradually spreads to the pump periphery, decreasing the centrifugal force the sea water receives from the impeller and disabling the pump from discharging. If the discharge amount is extremely small, there are cases where unnoticeably small amounts of air gather within the pump and build up to a level that could disable the pump. If left as it is, sea water in the pump will run out resulting in burn.
There are several ways to prevent air drawing. Purge air thoroughly before starting the pump. Decrease the suction flow from the bell mouth by reducing the discharge valve opening to decrease the discharge amount. Note, beforehand, the water level at which pump begins to draw air and stand on the alert as the water approaches the critical level. You may use other tanks with a suitable water level to continue discharging. With some ships, cargo planning is based on the hull with trim at the stern. It is the suitable condition to discharge all the remaining ballast.