The Force Pump
A force pump can be used to raise water by a height of more than 10m, the maximum height allowed by atmospheric pressure using a common lift pump. It consists of a pump with a solid plunger and valve B, connected by a pipe to a chamber C through a valve A.
On the upstroke valve A closes and atmospheric pressure pushes water up into the pump through valve B.
On the downstroke valve B closes and the water is force into the chamber C through valve A. 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, preventing a sudden jolt to the pump when the water column in P falls slightly and valve A closes at the beginning of the upstroke.
The maximum height to which water may be raised depends on
The force exerted on the plunger during the downstroke.
The ability of the pump to withstand the pressure due to thepipe P.
Force pumps were used in manual fire pumps. These consisted of a pair of force pumps connected to a long handle, worked by a team of four men. Both pumps fed alternately in a chamber with a compressed air space as in the diagram above. At the moment of changeover from one pump to another, the compressed air expanded, maintaining a steady flow of water to the hoses.