A pump is a device used to move fluids. Industrial pumps mostly fall into three categories; centrifugal, positive displacement or metering.

A centrifugal pump uses a spinning "impeller" to add kinetic energy to the fluid by increasing the fluids velocity in the pump casing. The higher energy fluid is discharged into the piping system at which point the flow and pressure is governed by the downstream system characteristics. If the discharge is blocked, energy continues to be added to the fluid and eventually it begins to boil (cavitation). This is a very damaging event and most industrial centrifugal pump applications include protetions to ensure that this does not happen.

A positive displacement pump causes a fluid to move by trapping a fixed volume of fluid and forcing it into the discharge side of the pump. If the discharge is blocked, a positive displacement pump will build very high pressure into the downstream system. This pressure must be vented or result in a failure. Most industrial positive displacement pump applications include a relief valve on the discharge side that flows back to the source of the fluid or to the suction of the pump.

A metering pump may be centrifugal or positive displacement. However, most of the time it will be a positive displacement pump. A metering pump is used when the discharge flow must be accurately controlled.