Suction Specific Speed (Nss)

[mfakharmechanica]

Dear Sir/Madam,

Would you please explain the physical meaning and effect of the Nss?
I have found that it is related to pump performance and calculated for BEF of the maximum impeller diameter...
Who it connect duty point to the BEP of maximum impeller diameter?
Different reference present different maximum limitation (11,000, 13,000… in US). Would you introduce a perfect reference for me.

Thanks

 

[BigInch]

Have a look here,

http://www.mcnallyinstitute.com/07-html/7-03.html

As you can see there the Ns relates directly to the equation which is a function of Flow, Head and Speed and can be indirectly related to the optimum shape of the impeller, power requirements and subsequent pump efficiency.

You can calculate Ns for any combination of Flow, Head and Speed, to get an idea of how well the design of a pump is suited for any combination of variables within its range, but it must be calculated at a given pump's BEP conditions to be used as a meaningful reference point for comparison of the advantages and disadvantages between various pump designs.

Also useful is the related calculation of Nss, suction specific speed, which correlates the suction head requirements to the same pump design attributes. I have found calculation of Nss at various flowrates over a pump's range to be indicative of NPSH Required at flowrates other than BEP flow. Very helpful, if the manufacturer didn't furnish a NPSHR curve.

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