Machines that add energy to a liquid to transfer it between process points — overcoming elevation, pressure and friction losses. In the most common form, energy travels through a 3-stage chain: (1) the driver converts an external energy source — electrical, thermal, hydraulic — into rotation at its own shaft; (2) the shaft transmits that mechanical energy to the pump element (rotor, piston, gear); (3) that element transfers the energy to the liquid as hydraulic energy (pressure + velocity). The most common industrial case is a three-phase electric motor driving a rotor — covered in detail in the Centrifugal Pump note. In special-effect devices this mechanical link simply does not exist: energy reaches the liquid directly from a motive fluid/gas or an electromagnetic field, with no shaft or rotating element.
How energy is supplied to the liquid
What distinguishes the equipment that moves liquids is not whether it adds energy — all of them do — but where that energy comes from and by which mechanism it is transferred:
| Equipment | Energy source | Transfer mechanism |
|---|---|---|
| Pump (rotor, piston, gears) | motor → shaft → mechanical element | the mechanical element accelerates or displaces the liquid directly |
| Ejector / eductor (jet pump) | high-pressure motive fluid | a motive jet entrains and pressurizes the drawn liquid (Venturi effect) |
| Air lift (gas-lift pump) | compressed air (compressor) | injected air lowers the column density, which then rises |
| Hydraulic ram | kinetic energy of the water itself | water hammer lifts a fraction of the flow to great height |
| Hydraulic intensifier | another hydraulic circuit | converts the pressure of one fluid into higher pressure in another |
An important standards point: under the Hydraulic Institute classification, the three moving-part-free, fluid-driven devices — ejector, air lift and hydraulic ram — are not separate categories but special-effect pumps, a subfamily of kinetic pumps (alongside the rotodynamic ones). The intensifier is the only one that departs: it is a pressure multiplier, not a fluid transporter — handled separately.
Working principles
The family splits into fundamental principles — the choice between them defines the physics, the characteristic curve and the dominant failure modes:
- Rotodynamic — energy is transferred continuously by a rotating impeller (Euler's equation); e.g. centrifugal, axial. Flow varies with system pressure.
- Positive displacement — discrete volumes are captured and displaced (gear, piston, screw). Flow nearly independent of pressure; requires a relief valve.
- Special effect — no moving parts, driven by a motive fluid or gas (ejector, air lift, hydraulic ram). Robust and cheap, with low efficiency.
More than 80% of the rotating assets in a typical process plant are pumps — dominated by the centrifugal pump.