We were
discussing the pumps and
basic pumping system, total
head developed by the centrifugal pump, parts
of centrifugal pump and their function, heads
and efficiencies of a centrifugal pump, work
done by the centrifugal pump on water, expression
for minimum starting speed of a centrifugal pump, multistage
centrifugal pumps, cavitation in hydraulic machine and specific
speed of a centrifugal pump in our previous post.

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Now we
will find out here the cavitation and its effects in centrifugal pump with the
help of this post. First of all we will see here the basic fundamentals of
cavitation in a centrifugal pump and further we will see here cavitation and
its effects in centrifugal pump.

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**Cavitation in hydraulic machine**** **

Let us
consider that we have one pipe line and water is flowing through this pipe
line. As we know that, vaporization of liquid will be started if pressure at
any point in flowing liquid becomes equal to or less than the vapour pressure
of flowing liquid.

Let us
consider one low pressure region in pipe line through which water is flowing.
Water will be converted in to vapour in this low pressure region and these
vapour bubbles will be carried by this flowing liquid.

These
vapour bubbles will be collapsed and will develop a tremendous rise in
pressure, once they will reach in high pressure region. Due to tremendous rise
in pressure, material from the wall of pipe line will be eroded and there will
be formation of cavities over the surface of pipeline. Such a phenomenon will
be termed as cavitation.

Cavitation
could be defined as the phenomenon of formation of vapour bubbles in flowing
liquid in a region where the pressure of liquid falls below or equivalent to
the vapour pressure of flowing liquid and sudden collapsing of these vapour
bubbles in a high pressure region.

Sudden
collapsing of these vapour bubbles will develop tremendous rise in pressure and
the metallic surface, over which liquid is flowing, will be subjected with this
tremendous rise in pressure.

Therefore
metallic surface, over which liquid is flowing, will be subjected with the pitting
action or in simple there will be developed small-small holes over the metallic
surface.

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**Cavitation
in centrifugal pump **

In case of centrifugal pumps, Cavitation will occur
at the inlet of the impeller of the pump or at the suction side of the pumps.
Pressure at the suction side of the pump will be reduced considerably and hence
chances of cavitation will be high at the suction side of the centrifugal pump.

Now, let us think the case in which cavitation will
occur in centrifugal pump.

Cavitation will occur, if the pressure at the
suction side of the centrifugal pump will be dropped below than the vapour
pressure of liquid flowing through pump.

We can come to know about cavitation in centrifugal
pump by observing the efficiency of pump and head developed by the pump. If
there is sudden drop in the efficiency and head developed by the pump, it
indicates that there will be cavitation phenomenona in the centrifugal pump.

In order to determine whether cavitation will occur
in any portion of the suction side of the pump, critical value of Thoma’s
cavitation factor i.e.

**σ**wil be calculated.

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**Thoma’s
cavitation factor (σ) for centrifugal pumps **

Thoma’s
cavitation factor will be given by following expression as mentioned here.

Where,

H

_{atm}= Atmospheric pressure head in m of water or absolute pressure head at the liquid surface in pump
H

_{V}= Vapour pressure head in m of water
H

_{S}= Suction pressure head in m of water
h

_{LS}= h_{fS }= Head lost due to friction in suction pipe
H = Head developed by the pump

We will determine the value of Thoma’s cavitation
factor (σ)
and we will compare this value with the value of critical cavitation factor (σ

Empirical relationship which is basically used for determination of critical cavitation factor (σ

_{C}) for given centrifugal pump. Critical cavitation factor (σ_{C}) will be secured by the tables or empirical relationship.Empirical relationship which is basically used for determination of critical cavitation factor (σ

_{C}) will be given by following equation.
If the value of Thoma’s cavitation factor (σ) is more
than the critical cavitation factor (σ

If the value of Thoma’s cavitation factor (σ) is less than the critical cavitation factor (σ

_{C}), cavitation will not occur in the centrifugal pump.If the value of Thoma’s cavitation factor (σ) is less than the critical cavitation factor (σ

_{C}), cavitation will occur in the centrifugal pump.
So, we have seen here the cavitation and its effects in
centrifugal pump with the help of this post. We
have also introduced here the Thoma’s cavitation factor (σ) and critical
cavitation factor (σ

_{C}). We have discussed here the case when cavitation will occur in centrifugal pump and when cavitation will not occur in centrifugal pump.
Do you have any suggestions?
Please write in comment box.

Further we will find out, in our next
post,
effect of cavitation in hydraulic turbines.

### Reference:

Fluid mechanics, By R. K. Bansal

Image courtesy: Google

Image courtesy: Google

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