Less familiar types with more limited usage include ejectors, pulsators, gas lift pumps, hydraulic elevators and various miscellaneous types difficult to classify.
Note that both positive displacement (PD) and centrifugal (CP) pumps are broad descriptions.
Positive displacement pumps break down into reciprocating and rotary pumps, each type available in a range of configurations. For rotary pumps, the key element is a rotor which may be designed as an impeller, vane or screw. Displacement pumps include many other types apart from reciprocating and rotary units e.g., bucket, scoop, wheel, ejectors, pulsators etc. Note, however, that some types are neither displacement nor turbo machines in characteristics, in which case sub-classification is more of academic than practical interest.
A displacement pump causes a fluid to move by trapping a fixed amount of it and then forcing (displacing) that trapped volume into the discharge pipe. In a dynamic pump by contrast, kinetic energy is added to the fluid by increasing the flow velocity. This increase in energy is converted to a gain in potential energy (pressure) when the velocity is reduced before or as the flow exits the pump into the discharge pipe. Accordingly, pumps can be divided into dynamic or displacement categories.
In a centrifugal pump, most of the energy conversion is due to the outward force that curved impeller blades impart on the fluid. Invariably, some of the energy also pushes the fluid into a circular motion, and this circular motion can also convey some energy and increase the pressure at the outlet. Subclassifications of centrifugal pumps are shown here.
In pumps of this type, energy is periodically added by application of force to one or more movable boundaries of any number of fluid-containing volumes resulting in a direct increase in pressure up to the value required to move the fluid through the valves or ports into the discharge line. The two types of displacement pump are rotary and reciprocating pumps. A more detailed breakdown is set out here.
Rotary pumps are positive displacement machines but, in contrast to reciprocating machines, the flow of pumpage is continuous, lacking the pulsing action of the latter. However, internal losses are generally higher because of internal leakage back into the liquid being pumped. Leak back increases with increasing pressure, so rotary pumps are less suitable compared to reciprocating pumps where high pressures are required. Rotary pumps are used for handling a wide range of fluids across a wide range of viscosities. The narrow clearances between impeller and casing mean that they are unsuitable for handling solids containing liquids.
Reciprocating pumps deliver liquid against a pressure determined by the system to which the pump is connected. With their high efficiencies, they are used for high pressure duties. The choice between centrifugal and reciprocating pumps is largely dependent upon the pressure of the system, the physical size of the units, relative costs of both the pump and the ancillary/supporting equipment along with operating costs, erosion, the likely impact of the pulsation of the pump (even with pulsation dampers), the uneven flow of the reciprocating pump and relative efficiencies.