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What is suction specific speed and how can it be used in centrifugal pump selection?

by Engineered Software, Inc.

Suction specific speed is an index that describes the characteristics of the suction side of the impeller. It is calculated using the NPSHr in feet, speed (N) in rpm, and flow rate (Q) in gpm at the pump’s best efficiency point and maximum impeller diameter. The equation for suction specific speed (designated Nss or S) is:

\[ Nss = \frac{N \sqrt{Q}}{NPSH^{0.75}_r}\]

 

From the equation we can see that the lower the NPSHr for a pump the higher the Nss. Nss values for many standard impellers typically range from 7000 to 9000, but some designs may have an Nss as high as 18,000 to 20,000.

For the pump user, it is important to consider that increasing the Nss of a pump has been shown to shift the onset of suction or discharge recirculation closer to the best efficiency point (BEP) flow of the pump. This effectively decreases the window of stable operation for the pump. Suction recirculation is a reversal of flow in the impeller eye that can lead to increased noise, surges, and cavitation-like damage on the pressure side of the impeller vane.  Discharge recirculation is a similar reversal of flow occurring at the discharge of the impeller vane.   All centrifugal pumps will exhibit recirculation if the flow rate is sufficiently reduced. Many studies on recirculation have produced graphs and equations that indicate the minimum continuous flow or stable operating region (as a percentage of BEP flow) based on the Nss of a pump. There are a number of researchers, companies, and organizations that specify maximum Nss recommendations for pump selections. For instance, an article by Karassik recommended an Nss of no more than 9,000 for water and 11,000 for hydrocarbons. A number of resources also suggest that suction and discharge recirculation is not a concern for lower horsepower (<100 hp) pumps.

Overall, there is not a consensus on Nss recommendations for pumps and there is some debate over when the onset of recirculation occurs and what factors predict it.

References

Further reading on the subject is encouraged in order to develop your own Nss guidelines for pump selection. Below are a number of additional resources on the topic.

  1. Henshaw, T., “Suction Specific Speed Part 1, 2, and 3,” Pumps and Systems Magazine, 2009.
  2. Hirschberger, M. and James, I., “A Review of Nss Limitations-New Opportunities,” 25th International Pump Users Symposium Proceedings, 2009.
  3. Karassik, I.J., “Centrifugal Pump Operation at Off-Design Conditions,” Chemical Processing Magazine, 1987.
  4. Karassik, I.J., “Setting the Minimum Flows for Centrifugal Pumps,” Pumps and Systems Magazine, March, 1994. 
  5. Volk, M., Pump Characteristics and Applications, 2nd Edition, CRC Press, 2005.

 

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