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Sunday, 8 September 2019

CAVITATION AND ITS EFFECTS IN CENTRIFUGAL PUMP

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 pumps, cavitation in hydraulic machine and specific speed of a centrifugal pump in our previous post. 

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. 

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. 

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. 

Thoma’s cavitation factor (σ) for centrifugal pumps 

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


Where, 

Hatm = Atmospheric pressure head in m of water or absolute pressure head at the liquid surface in pump
HV = Vapour pressure head in m of water
HS = Suction pressure head in m of water
hLS = hfS = 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 (σC) for given centrifugal pump. 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 centrifugal pump. 

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

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