Charge Pump: A pump used to boost the pressure of a fluid before it enters the main pump(s) of a system.
A charge pump is used to boost the pressure of a fluid before it enters the inlet of the main pump. The use of a charge provides enough pressure to overcome the required net positive suction head (NPSH) of the main pump. Providing adequate pressure is necessary to prevent cavitation in larger pumping equipment.
The most common charge pumps are single stage centrifugal pumps. ANSI pumps will often fill the requirements of a charge pump, but the composition of the fluid should be taken into consideration. Depending on abrasive materials, specific types of centrifugal pump designs may be needed.
Centrifugal pumps provide a desired flow and pressure relationship. When a centrifugal pump is used, they are able to operate across a range of flow rates, while still providing an adequate pressure. Pumps other than a centrifugal may be used, but it is rare. Use of other types of pumps is not recommended unless the system is specifically designed for the type of pump selected. Using other types of pumps may cause premature failure of either the charge pumps or the main pumps unless they are designed to be used together.
Charge pump systems are common on positive displacement pumps. Depending on their speed, the positive displacement pump may pull the fluids at the inlet of the pump well below vapor pressure causing cavitation, A charge pump can help to reduce sudden pressure drops below vapor pressure within the suction stream and eliminate cavitation.
Charge pumps may also be used in other pump type applications. Multistage pumps often are not designed to pull fluids from a reservoir located below the pump, such as a river or a lake. Careful attention is given to these pump designs to provide adequate suction pressure. A smaller pump designed for lower NPSH is often placed near the fluid reservoir to provide the adequate pressure needed at the inlet of the main pump.
Charge pump systems are designed for the main pump(s). Experience with the pump, pump curves, or the manufacturers manual may provide the information necessary to select an adequate charge pump. When a wide range of flows is desired, the charge pump may be bypassed at low flows. Since the NPSH increases with speed, it may only be necessary to provide a charged pressure at higher pump speeds. Careful calculation should be completed to know when a charge pump should be used.
Note: A charge pump in positive displacement applications can reduce vibration and pressure pulsations in the suction piping but does not eliminate it. A pulsation dampener should still be used to reduce pulsation in the suction and discharge lines in order to prevent vibration and pulsation damage.
