Now we will find out here the effect
of cavitation in hydraulic turbines with the help of this post. First of
all we will see here the basics of cavitation in a hydraulic machine i.e.
turbine and further we will see here effect of cavitation in hydraulic turbines.

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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.

###
**Effect of cavitation in
hydraulic turbines **

In case of hydraulic turbines, only
reaction turbines will be subjected to cavitation. If we talk about effect of
cavitation in reaction turbines, cavitation will occur at the outlet of the
runner of the turbine or at the inlet of the draft tube where pressure will be
reduced considerably. Pressure at the outlet of the runner of the turnbine or
at the inlet of the draft tube will be reduced considerably and hence chances
of cavitation will be high.

Now, let us think the case in which
cavitation will occur in hydraulic turbines.

Cavitation will occur, if the
pressure at the outlet of the runner of the turnbine or at the inlet of the
draft tube will be dropped below than the vapour pressure of liquid flowing
through hydraulic turbine i.e. reaction turbine.

We can come to know about cavitation
in hydraulic turbine by observing the efficiency of turbine. If there is sudden
drop in the efficiency, it indicates that there will be cavitation phenomenon
in the turbine.

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 of runner
vanes and draft tube.

In order to determine whether
cavitation will occur in any portion of a reaction turbine, critical value of
Thoma’s cavitation factor i.e.

**σ**will be calculated.

###
**Thoma’s cavitation factor
(σ) for turbine**

Thoma’s cavitation factor wil be
given by folowing expression as mentioned here.

Where,

H

_{atm}= Atmospheric pressure head in m of water
H

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

_{b}= Barometric pressure head in m of water
H

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

_{LS}= Head lost due to friction in suction pipe
H = Net head on the turbine

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

_{C}) for given reaction turbine. 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

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

_{C}), cavitation will occur in the reaction turbine.
So, we have seen here
the cavitation and its effects in a hydraulic turbine i.e. reaction
turbine 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 also discussed here the case when cavitation will occur in a hydraulic turbine i.e. reaction turbine and when cavitation will not occur in reaction turbine.
Do you have any
suggestions? Please write in comment box.

###
**Reference: **

Fluid mechanics, By R. K.
Bansal

Image courtesy: Google

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