. They are used to convey fluid from one place to another by mechanical
means. They can be classified into two main groups, namely, centrifugal
and positive displacement pumps.
Centrifugal pumps make use of centrifugal force to work. The
liquid to be pumped is usually water. The construction of the
housing is such that water is piped in at the centre of the impeller.
The impeller is then rotated at a high speed so that the water
is thrown outwards with high velocity. The housing of the impeller
is usually constructed in the shape of a volute such that the
volume is gradually expanded until the placed the water is led
out as the discharge. The gradual expansion of the volume has
the effect of converting the velocity of the water into pressure.
Some centrifugal pumps make use of diffuser fins at the housing
to achieve the same change of state.
Positive Displacement Pump
Positive displacement pumps make use of a mechanical object to
displace the liquid. The liquid to be pumped can be water, oil,
or other thick liquid. The mechanical object can move in a reciprocating
manner or a rotary manner. Examples of the mechanical object are:
pistons, gears, screws, lobe, vanes. The liquid is literally pushed
aside by the object. Some pumps need to have inlet and outlet
non-return valves fitted to work. Examples of this are piston
pumps. Others like gears, screws, lobe, and vanes rotating at
high speed need not employ valves to work. One characteristic
of these pumps is the fact that the pressure can build up in time
if it is not released. Usually a pressure relief valve is fitted
at the discharge side of the pump to prevent over pressure.