Today also in centrifugal due to high rpm since it is directly coupled to electric motor the pump creates partial vacuum which means the liquid will certainly flow through vacuum hence the liquid will end up being Centrifugal Vacuum Pump recieved in the centrifugal 
pumps suction and since the impeller rotates the fluid will be displaced aside forward with a specific energy.
A vacuum pump has the inlet hooked up to 1 or both valve covers, sometimes the valley pan. It SUCKS the atmosphere from the engine, therefore reducing the air pressure build up created by blow due to combustion gases going past the piston rings into the pan.
