Centrifugal pumps are the most preferred pumping devices
in the hydraulic world.
In this video we will have a conceptual overview of
working of centrifugal pumps.
At the heart of the system lies the impeller.
It has got a series of curved vanes,
fitted inside shroud plates.
The impeller is always immersed in water.
When the impeller is made to rotate,
it makes the fluid surrounding it also rotate.
This imparts centrifugal force to the water particles
and the water moves radially out.
Since rotational mechanical energy is
transferred to the fluid,
at discharge side of the impeller,
both pressure and kinetic energy water will rise.
At the suction side water is getting displaced,
so a negative pressure will be induced at the eye.
Such a low pressure helps in
sucking freshwater stream into the system again.
and this process continues.
This is the reason why priming is important for centrifugal pumps.
If no water is present initially.
the negative pressure developed by the rotating air
at the Eye of impeller
will be negligibly small to suck fresh stream of water.
Impeller is fitted inside a casing.
So the water moving out will be collected inside it,
and will move in the same direction of rotation of impeller,
to the discharge nozzle.
Here you can note
one speciality of casing.
It has got increasing area along the flow direction.
Such increasing area will help in accommodating,
newly added water stream
and will also help in reducing exit flow velocity.
Reduction in flow velocity will result in
increase in static pressure,
which is required to overcome resistance of pumping system
Here you can see more details of veins inside impeller.
They are backward curved vanes
with state-of-the-art Eye configuration
This vane is extracted from a KIRLOSKAR pump model.
If pressure at suction side impeller goes below vapor pressure of water,
a dangerous phenomenon could happen.
Water will start to boil forming vapor bubbles
and spoil impeller materials over time
This phenomenon is known as cavitation.
More the section head, lesser should be the pressure at the suction side,
to lift the water.
This fact puts a limit to maximum suction head a pump can have.
Careful pump selection is required
to avoid the problem of cavitation.
The current impeller type is enclosed,
semi-open and open impellers are also in use
depending upon the application.
If the working fluid is cloggy in nature,
it is preferred to use open kind of impeller.
But they are slightly less efficient.
Mechanical design of centrifugal pump
is always challenging.
A shaft is used to connect between the impeller and motor
Since water pressure inside the casing is huge,
a proper sealing arrangement
is imperative in arresting water leakage through the shaft-casing clearance.
Mechanical seal or stuffing box based mechanism is used for this purpose.
Impeller is mounted on bearings,
but at the suction side impeller,
it is not advisable to fit a bearing,
since it will block the flow.
So bearings have to be fitted at the other end.
This means impeller is mounted like a cantilever.
For high flow rate pumps,
a bearing housing with cooling oil is necessary
for improving the life for bearings.
We thank KIRLOSKAR bothers
for their technical support, in creation of this video.