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Monday, 9 September 2019

EFFECT OF CAVITATION IN HYDRAULIC TURBINES

We were discussing the pumps and basic pumping systemtotal head developed by the centrifugal pumpparts of centrifugal pump and their functionheads and efficiencies of a centrifugal pumpwork done by the centrifugal pump on waterexpression for minimum starting speed of a centrifugal pumpmultistage centrifugal pumpscavitation in hydraulic machine and specific speed of a centrifugal pump in our previous post. 

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. 

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, 
Hatm = Atmospheric pressure head in m of water 
HV = Vapour pressure head in m of water
Hb = Barometric pressure head in m of water
HS = Suction pressure head in m of water
hLS = 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 (σC) for given reaction turbine. Critical cavitation factor (σC) will be secured by the tables or empirical relationship. 

Empirical relationship which is basically used for determination of critical cavitation factor (σC) will be given by following equation. 

If the value of Thoma’s cavitation factor (σ) is more than the critical cavitation factor (σC), cavitation will not occur in the reaction turbine. 

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

Further we will find out, in our next post, maximum suction lift of centrifugal pump

Reference: 

Fluid mechanics, By R. K. Bansal 
Image courtesy: Google 

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