Positive displacement pumps

With these types of pumps the same quantity of water is delivered for each revolution of the pump shaft, and priming is usually not necessary.

The two main types are piston/reciprocating pumps and double acting piston pumps.

Piston or Reciprocating Pumps

Reciprocating pumps are used most commonly on farms using wind-power. These pumps are the simplest type of pump to construct.

• They have a piston arrangement with a valve or valves set on a piston.
• This piston is then set on a long shaft from the windmill to below the water level.
• As the piston is forced down into the water, the valve allows water to flow into the piston.
• When the action is reversed and the piston is raised, it lifts the water vertically.
• The water then overflows into a storage tank or dam.
• The cycle is repeated.

This type of pump is most commonly use for lifting water from wells and bores into a header tank. Although these pumps work with low volume of water, they are extremely efficient for lifting water from great depths. The water can then be gravity fed or pumped with high pressure pumps to irrigation systems from the header tanks.

Most piston pumps have a surging action, that is, each time the pump travels through one cycle the water surges out and then stops. Double acting and piston pumps help to alleviate this problem. When connected to an air chamber and flexible diaphragm, this surging action can be minimised.

Double acting piston pump

As the piston travels from left to right, water is drawn through the inlet pipe and the inlet valve (C) behind the piston opens allowing the water to fill the chamber behind the piston. The water in front of the piston is driven up to the outlet valve (B) and forced into the outlet pipe.

As the piston reverses, the outlet valve (B) closes and inlet valve (D) opens. The water is then pumped out of outlet valve (A).

These types of pumps produce a pulsating delivery of water. The air chamber assists to cushion the effects of the stop start of the piston as it reverses its travel.

A more modern design of these types of pumps is the rotary positive displacement pump. These pumps deliver larger quantities of water than the reciprocating piston pump, but at less pressure.

The following flash movie shows an animated cross-section of a positive displacement pump.