The total dynamic head is the sum of total suction lift plus total discharge head. The sum of the gauge pressure head and the velocity head at the discharge flange, minus the sum of the corresponding heads at the suction flange, equals the energy (in foot-pounds) added per pound of liquid pumped. This is called the "total head" developed by the pump and is expressed as:

The discharge head of a centrifugal pump is determined by the impeller diameter and its rotating speed, not by the horsepower of the motor.

If the discharge line of a centrifugal pump is closed, the horsepower input decreases and the pressure (head) increases only slightly. As the valve on the discharge is gradually opened, the horsepower input increases rapidly and pressure (head) decreases slightly.

Pump horsepower. Hydraulic horsepower (hhp) is expressed as follows:

where Q = Flow rate, gpm H = Head, ft P = Pressure drop, psi

The actual brake horsepower of a pump is greater than the theoretical hydraulic horsepower by the amount of losses incurred in the pump through friction, leakage, and so forth. The efficiency of a pump is therefore:

. Hydraulic horsepower

Brake horsepower and

Horsepower, hp = (Flow rate, gpm) (Total head, ftHSpGr) K r 3960 (Pump efficiency)