Specific Speed and Suction Specific Speed of a Pump

Is the Max Suction specific speed value is  11000 US Units to ensure wider operating range. Let's look at this section. First let's talk about Specific speed.

 

Specific Speed

Specific Speed is a Dimensionless Number indicates the impeller geometry

 

It has components of speed, Flow at BEP , Differential Head at BEP all for the max impeller condition.

 



High Ns refers to Axial flow or propeller impellers ( > 9000 Units) and is responsible for larger flow rates.

 

Low Ns  refers to radial flow/Centrifugal impellers (< 4000 US Units) which provides higher heads.

 

Medium Ns values refers to Mixed flow Impellers ( 4000- 9000 US Units) which provides a bit of both.


 Suction Specific Speed 

Suction Specific Speed is in reference to impeller eye  design and is an indicator of suction performance. It has components of Speed in RPM, Flow/ Impeller eye at Best Efficiency Point, NPSHR at Best Efficiency Point all of them derived from the maximum impeller condition.

 

Traditionally

 

Low NSS means - More operating range of NPSH values but higher value of NPSHr.

 

High NSS - less operating range of NPSH values but lower value of NPSHr

 

Since Vibration have been directly proportional to the NSS of older impeller designs.

 

Hence 11000 US units used to be the upper limit of impeller designs.

 

But studies have shown that with a variation in vane leading edges keeping NSS constant, the vibration can be brought down significantly 



& stable operating window looks like this with parabola profile being the best performer and blunt profile being the worst performer.

 


Hence 11000 US Units of NSS is no longer applicable for newer impeller designs  


as they can produce wide operating range without compromising on NPSHr.

 

So if you have any lower NPSHr requirement, do look out for pump vendors and get their suggestions on a better design and don't get bogged down by the higher NSS values clause as it not applicable in the current technology .


Conclusion

That’s all I have for today.

Hope you all learnt something today about the pump hydraulics.

Key Take aways

I would like to know from you what are your key take aways for today’s session 





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




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