The force pump
For raising water to a height of more than 10 m, the force pump is used (Fig. 11.3). It consists of a pump with a solid plunger and foot valve B, connected by a pipe to a chamber C through a valve A.
The upstroke (Fig. 11.3 (a))
On the upstroke valve A closes and the atmospheric pressure pushes water up into the pump through valve B. The downstroke (Fig. 11.3 (b))
On the down stroke, valve B closes and water is forced into the chamber C through valve A by the pressure due ‘to the mechanical force exerted on the plunger. The exit pipe P projects into the chamber C so that some air becomes trapped at the top of the chamber. This is compressed and acts as a cushion, thus preventing a sudden jolt to the pump when the water column in P falls slightly and sharply closes valve A at the beginning of the upstroke. C also helps to expel water on the upstroke.
The maximum height to which water may be raised by this means depends on: (a) The force which is exerted on the plunger during the down stroke.
(b) The ability of the pump and its working parts to withstand the pressure of the long column of water in the exit pipe P.