Theoretical NPSHr or Theoretical Nss calculations

[curtis26]

Does anyone have a equation for calculating the Theoretical Suction Specific Speed or NPSHr of an impeller based on its geometry?

 

[ione]

Nss = N*Q^0.5/NPSHr^0.75

Nss = suction specific speed
N = rotational speed
Q = flow rate
NPSHr = Net Positve Suction Head Required


Take a look at page 286

http://books.google.it/books?id=9xA...age&q=NPSH experimental determination&f=false

 

[curtis26]

I don't have a test value of NPSHr. I am looking for an way to calculate theoretical NPSHr based on geometry alone. I of course can do that with the equation above if I can find an imperical formula that estimtes Nss in terms of inlet and exit flow area and impeller angles.

 

[ione]

NPSHr is determined experimentally by the pump manufacture on a test rig

http://www.eng-tips.com/viewthread.cfm?qid=9873

 

[BigInch]

Centrifugal?
For estimating, take the volume of the casing * RPM as the BEP flowrate and then some estimated, or the desired value of NPSHr, initially try a 23 feet NPSHr at the BEP flowrate and calculate Nss, or use a target value for NPSHr, if that makes more sense to you. You can find typical pump impeller geometry charts vs Nss to give you an idea of what you should expect for certain shapes.

You might be able to zero in on a close range for both by massaging those values against eachother.

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

 

[curtis26]

Thanks BigInch. I have seen reference to empirical equation but don't have any reference materials that covers advanced impeller design. I was hoping that someone here might know how the typical impeller geometry charts were created and be able to help me get a little more accuracy than the calibrated eyeball. Still I think that the massaging method might get me what I need if none else has a better method.

 

[GPRnD]

Try Johann Guelich's book Centrifugal Pumps. There is a general method in there for calculating NPSHr based on pump geometry.

I'll warn you it is fairly math intensive.

Obviously pump companies have their own internal methods (mine included), but these are not public domain.

 

[vpl]

There appears to be a description of how to do this in the handout from an ASME short course: Centrifugal Pump Design and Application Seminar, Chapter 4.




Patricia Lougheed

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[PumpSmart]

curtis26,
Do you want to make reverse engineering, new impeller design or improve old impeller design?
Nss and NPSHr will be theorically calculate by some nomograph like pusblished by Lobanov, Karasik, etc. See one of them attached published by Lobanov.
Other designers use for first calculation impeller design:
NSS= - A B1 + C, where B1 is the impeller inlet angle and
A, C are varibles that depends of type of impeller mechanical detail.
NPSH r = 1.1 cm1 (square)/ (2 * g) + Cb x u1 (square) / ( 2 * g)
cm1 is the corrected inlet absolute meridional velocity
Cb is the inlet vane tip coefficient
u1 is the suction pheripheral velocity.
g: gravity.
But take in mind, that are rough approximation.
Today pump manufactures uses CFD analyzes to improve and validaty old pump design theory.
I hope it helps.