Pump Types

Pumps fall into two principal classifications, defined by their fundamental mode of operation. A displacement pump moves fluid by trapping a fixed volume and forcing it into the discharge pipe. A dynamic pump adds kinetic energy to the fluid by increasing its flow velocity; that energy converts into pressure as the velocity is reduced at the pump outlet. Less familiar types with more limited usage — ejectors, pulsators, gas-lift pumps, hydraulic elevators — sit outside these two families, and their sub-classification is of more academic than practical interest.

Classification charts: dynamic pumps · displacement pumps

Dynamic pumps: the centrifugal family

In a centrifugal pump, most of the energy conversion is due to the outward force that curved impeller blades impart on the fluid. Some of the energy also pushes the fluid into a circular motion, which conveys additional energy and increases the pressure at the outlet.

Displacement pumps

In pumps of this type, energy is periodically added by applying force to one or more movable boundaries of fluid-containing volumes, directly increasing pressure up to the value required to move the fluid into the discharge line. The family divides into rotary and reciprocating machines.

Rotary pumps

Rotary pumps deliver a continuous flow, without the pulsing action of reciprocating machines. The key element is a rotor designed as an impeller, vane or screw. Internal leakage back into the pumped liquid rises with pressure, which makes rotary pumps less suitable than reciprocating ones for high-pressure duties. They handle a wide range of fluids and viscosities, but the narrow clearances between rotor and casing make them unsuitable for liquids containing solids.

Reciprocating pumps

Reciprocating pumps deliver liquid against a pressure determined by the system they serve, and their high efficiency suits them to high-pressure duties. The choice between centrifugal and reciprocating designs depends on system pressure, physical size, relative cost of pump and ancillary equipment, operating cost, erosion, and the impact of the reciprocating pump's pulsating, uneven flow — even when pulsation dampers are fitted.