Now also in centrifugal because of high rpm as it is directly coupled to electric motor the pump creates partial vacuum which means the liquid will flow through vacuum hence the liquid will become recieved at the centrifugal pumps suction and as the Centrifugal Vacuum Pump impeller rotates the fluid will be displaced aside forward with a particular energy.
A vacuum pump has the inlet hooked up to one or both valve covers, sometimes the valley pan. It SUCKS the atmosphere from the engine, thus reducing the atmosphere pressure build-up created by blow due to combustion gases going at night piston rings into the pan.