Theoretical NPSHr or Theoretical Nss calculations
Theoretical NPSHr or Theoretical Nss calculations
(OP)
Does anyone have a equation for calculating the Theoretical Suction Specific Speed or NPSHr of an impeller based on its geometry?
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Theoretical NPSHr or Theoretical Nss calculations
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RE: Theoretical NPSHr or Theoretical Nss calculations
Nss = suction specific speed
N = rotational speed
Q = flow rate
NPSHr = Net Positve Suction Head Required
Take a look at page 286
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RE: Theoretical NPSHr or Theoretical Nss calculations
RE: Theoretical NPSHr or Theoretical Nss calculations
http://www.eng-tips.com/viewthread.cfm?qid=9873
RE: Theoretical NPSHr or Theoretical Nss calculations
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
RE: Theoretical NPSHr or Theoretical Nss calculations
RE: Theoretical NPSHr or Theoretical Nss calculations
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.
RE: Theoretical NPSHr or Theoretical Nss calculations
Patricia Lougheed
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RE: Theoretical NPSHr or Theoretical Nss calculations
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.