CAVITATION IN HYDRAULIC MACHINE

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 and specific speed of a centrifugal pump in our previous post. 

Now we will find out here the cavitation in hydraulic machine with the help of this post. First of all we will see here the basic fundamentals of cavitation in a hydraulic machine. 

Further we will try to find out the causes of cavitation, precaution against cavitation and finally we will summarize with the effects of cavitation on hydraulic machines.  

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. 

What causes cavitation ?

There are following causes of cavitation in a hydraulic system as mentioned here.

1. Hydraulic machines handle liquid
2. Performance of the hydraulic machine will be affected when vapour phase will be formed 

Precautions against cavitation

In order to avoid the cavitation in hydraulic machines, we need to consider the following points as mentiond here. 

The pressure of the liquid, which is under operation, in any part of the hydraulic system must not fall below the vapour pressure of the liquid flowing. If we are considering water as the flowing liquid here, then the absolute pressure head must not be below than 2.5 m of water. , 

Special materials or coating materials such as aluminium-bronze or stainless steel must be used as they are cavitation resistnat materials. 

Effect of cavitation 

We can see here the following points which will provide the summary of the effect of cavitation in hydraulic machine. 

1. Metallic surface will be damaged and cavities will be forned on the surface
2. Efficiency of hydraulic machine will be dropped due to cavitation 
3. Noise level will be increased because of sudden collapsing of vapour bubble
4. Vibration level will be increased because of sudden collapsing of vapour bubble
5. Performance of hydraulic machine will be dropped 
6. Maintenance time or failure duration of hydraulic machine will be increased 
7. Reliability of hydraulic machine will be reduced 
8. Cavitation may cause serious damages too in internal components of hydraulic machine

So, we have seen here the basics of cavitation in hydraulic machine with the help of this post, we have also discussed here causes of cavitation, precaution against cavitation and effect of cavitation. 

Do  you have any suggestions? Please write in comment box.  

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

Reference: 

Fluid mechanics, By R. K. Bansal 

Image courtesy: Google   

Also read 

Basics of shear stress in turbulent flow

Minor head losses in pipe flow, 

Hydraulic gradient and total energy line

Working of syphon

Flow through pipes in series

Flow  through pipes in parallel

Flow through branched pipes

Power transmission through pipes  Flow through nozzle