The purpose of a pump is to overcome elevation differences and head losses due to pipe friction and fittings. The amount of head the pump must add to overcome elevation differences is dependent on system characteristics and topology (and independent of the pump discharge rate), and is referred to as static head. Friction and minor losses, however, are highly dependent on the rate of discharge through the pump. When these losses are added to the static head for a series of discharge rates, the resulting plot is called a system head curve.

Pumps are designed to lift water from one elevation to another, while overcoming the friction and minor losses associated with the piping system. To correctly size a pump, one must understand the static head (elevation differences) and dynamic head (friction and minor losses) conditions under which the pump is expected to operate. The static head will vary due to changes in reservoir or tank elevations on both the suction and discharge sides of the pump, and the dynamic head is dependent on the rate of discharge through the pump.

System head curves are a useful tool for visualizing the static and dynamic head for varying rates of discharge and various static head conditions. The system head curve is a graph of head vs. flow that shows the head required to move a given flow rate through the pump and into the distribution system.

Flat system head curves generally indicate a great deal of capacity in the piping system. Steep curves indicate limited capacity. For variable speed pumps, system head curves tend to be steep because flow is restricted by outflow orifices.

Flat system head curves generally indicate a great deal of capacity in the piping system. Steep curves indicate limited capacity. For variable speed pumps, system head curves tend to be steep because flow is restricted by outflow orifices.