Water-Lifting Devices – Pumps - Practices of Irrigation and On-farm Water Management - page 441

series). Each separate section is called a stage, and the greater the number of stages,
the greater the pressure or lift that is created.

Displacement Pumps

Displacement pumps have limited capacities and are not suitable for pumping large
amounts of water required for irrigation or drainage. They are used mainly for
chemical injection in agricultural irrigation systems.

12.2.4.5 Classification Based on the Means by Which the Energy Is Added
According to this approach, all pumps fall into two general categories:

• rotodynamic (centrifugal, mixed flow and axial flow), and

• positive displacement.

A rotodynamic pump converts kinetic energy to potential or pressure energy. The
pumping units have three major parts: (i) the driver (that turns the rotating element),
(ii) the impeller and shaft (the rotating element), and (iii) the stationary diffusing
element.
Positive displacement (PD) pumps use gears, pistons, or helical rotors with tight
tolerance to the casing so that pressure can build up beyond normal rating.
PD pumps can be classified into two main groups: rotary and reciprocating.
Rotary pumps transfer liquid from suction to discharge through the action of rotating
screws, gears, rollers, etc., which operate within a rigid casing. It typically works at
low pressure (up to 25 bars or 360 lb per square inch (psi)). In case of reciprocat-
ing pumps, the rotary motion of the driver (such as diesel engine or electric motor)
is converted to reciprocating motion by a crankshaft, camshaft, or swash-plate. It
typically works at high pressure (up to 500 bar).

Other Types of Pumps

Other pump types less common in irrigation include:

Non-positive displacement pumps have an impeller which spins to create pres-
sure but does not have close tolerance to the casing. This type of pump will
not build pressure beyond its normal rating.
Helical rotor pumps force water through with an auger type action.
Booster pumps are in-line pumps (series connection) used to increase the
operating pressure of the system.
DC powered pumps use direct current from motor, battery, or solar power to
move liquids such as acids, chemicals, lubricants and oil, as well as water,
wastewater, and potable water.
Hydraulic pumps deliver high-pressure fluid flow to the pump outlet. Hydraulic
pumps are powered by mechanical energy sources to pressurize fluid.