The displacement of a plunger pump is less than the above, by reason of there being a certain amount of clearance or space between the circumference of the plunger and that of the cylinder bore.
It is desirable to keep the clearance space in all pumps as small as the conditions will allow, especially if the pump is liable to lose its water.
Losing the water means the falling of the suction water back into the source of supply, which may occur when the engine has to stop temporarily, and there is a leak in the suction valves.
Fig. 3322.
Rotary pumps are those in which the piston revolves, an example of the most successful form of rotary pump being shown in [Fig. 3322], which is that used by the Silsby fire engine.
The advantage possessed by a rotary pump is that it keeps the water passing through the suction in a continuous and uniform stream, as it has no valves.
It may therefore be run at a high velocity or attached direct to the engine shaft.
If a rotary pump leaks, the efficiency is not impaired so much as in a piston or plunger pump, all that is necessary being to run the pump at a high speed.