The de-ballast system design should be able to switch to the stripping pump(s) at a ballast tank level that is higher than 150 mm.
As the level in a ballast tank gets very low, it becomes difficult to prevent air from entering the suction line to the pump. This is primarily due to two factors:
1) stiffeners in the floor of the tank prevent water from traveling efficiently to the suction bellmouth
2) vortexing at the suction bellmouth introduces air into the line
Air can be prevented from entering the suction line by throttling a globe valve in the de-ballast pump’s discharge line in order to reduce the flow rate. However, there is a limit to how much the flow rate can be reduced before reaching the pump’s allowable minimum continuous flow rate. Thus, it may not be practical to achieve a flow rate low enough to pump a tank down to 150 mm using the de-ballast pumps. That is the reason to have the de-ballast pump(s) to keep the tank level higher than 150 mm.