The centrifugal pump pumps the difference between the suction and the discharge heads. The height we are pumping to, or the height to the discharge piping outlet that is filling the tank from the top. Pressure head . System or dynamic head . Caused by friction in the pipes, fittings, and system components.
Suction head is measured the same way.
If the liquid level is above the pump center line, that level is a positive suction head. If the pump is lifting a liquid level from below its center line, it is a negative suction head. If the pump is pumping liquid from a pressurized vessel, you must convert this pressure to a positive suction head. A vacuum in the tank would be converted to a negative suction head. Friction in the pipes, fittings, and associated hardware is a negative suction head.