Cost

Pressure ripple and noise
Suction performance
Contaminant sensitivity
Speed
Weight
Fixed or variablea displacement
Maximum pressure and flow, or power
Fluid type.
3. Types of pumps and motors

The mechanical principle that is chosen in the design of high-pressure positive displacement pumps and motors, which includes those using pistons, vanes and various gear arrangements depends on a number of factors. These include the operating speed and pressure, the type of fluid and the requirement for providing variable displacement control.

Pumps normally operate at constant speed (e.g. driven by electric motor) although in some situations (e.g. those driven by an internal combustion engine as found typically in mobile applications) the speed will vary over a small range. However, for motors it is normally required to operate at varying speeds including starting from rest (e.g. winch drives) and this aspect is reflected in the design of some available types.

Positive displacement machines are quite distinct from those using rotodynamic principles, which are often used for the transfers of fluid at relatively high flow rate and low pressure. Positive displacement units operate relatively low flow rates and high-pressure and normally can only be used with fluids having good lubricating properties. However, there are machines that can be used with fire resistant fluids and pure water.