How does it work?

Initially, the waste valve (4) is open and the delivery valve (5) is closed. The water in the drive pipe (1) starts to flow under the force of gravity and picks up speed and kinetic energy until it forces the waste valve (4) closed. The momentum of the water flow in the drive pipe (1) against the now closed waste valve (4) causes a water hammer, raises the pressure in the pump and opens the delivery valve (5), so that some water flows into the delivery pipe (3). Since this water is being forced uphill through the delivery pipe farther than it is falling downhill from the source, the flow slows down and when it reverses the delivery valve (5) closes and the waste valve (4) reopens, allowing the process to begin again.

A pressure vessel (6) containing air cushions the hydraulic pressure shock when the waste valve (4) closes, and it also improves the pumping efficiency by allowing a more constant flow through the delivery pipe (3). Although, in theory, the pump could work without it, the efficiency would drop drastically and the pump would be subject to extraordinary stresses which would shorten its life considerably. This air is under pressure and is gradually dissolved in the water, so to maintain this cushion a small inlet below the delivery valve (5) automatically draws in a bubble of air with each pump cycle. (Source; Wikipedia).