NPSH and Cavitation

Damaging cavitation can lead to pits on pump components especially the impeller and lead to catastrophic pump failure.

 


Hence, understanding the Net Positive Suction Head and its impact helps us to operate the pump without major implications.

 

Net Positive Suction head as the name goes by, is the Net Suction head that you need to have in the pump system to avoid the damaging cavitation.

 

The truth of the matter is, there will always be cavitation or the presence of vapor bubbles.

 

Hence, our responsibility is to make sure that the Net Suction Head is abundant which reduces the vapor bubble volume and hence avoids the damaging cavitation.

 

NPSHA is the Net Suction Head of the system   minus the Vapour Pressure Head

 

 Suction Head of the system has components such as  Surface Pressure Head at suction (Hpa) + Static Suction Head (Hss)  - Friction Loss Head at suction (Hfs ).

 

All of them derived from the absolute pressure values.

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You can refer to the Pressure Section of this series to know more abt the absolute pressure

 

NPSHr is an empirical value which means it is the average of infinite tests done on the same pump design

 


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Common NPSHr terms is NPSH3  which implies the NPSH value at  3% differential head drop delivered by the pump.

Then we also have NPSH0 - That can be done only with advanced instruments or through analysis programs. It is a suction pressure point where the liquid turns into vapor.

 

As a Thumb Rule, difference between NPSHA - NPSHr > 0.6m to ensure a reliable pump operation.


If you would like to listen to this segment, the above link is for you to understand the concept a bit better. 



In the next segment, we are going to see about the specific speed, suction specific speed, some facts and some mythbusters, stay tuned. 




 Image Credits:

  • Shutter stock ( Damaged impeller)
  • All other photos are licensed under CC by NC 2.0

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