NPSH Margin Reliability Curve

[feo1966]

Hi

Several studies and authors have noted that peak impeller damage can happen around 1.5 to 2 X the NPSHR(3%) line. Mr Budris even managed to get that language into the 1998 version of ANSI 9.6.1.

Peak erosion, peak noise, peak pulsations, all happen in this area. Once air starts coming out of solution as you approach a margin ratio of 1.0, noise drops, erosion drops and pulsations drop.

So I am struggling with the figure of NPSH Margin Ratio vs Reliability curve that Mr Budris has published many times. Why doesn't the reliability take a large drop near 1.5 to 2 X, and them come back up as you approach a ratio of 1.0.

It seems like the reliability curve should be an upside down version of the erosion curve, or the pulsation curve.

 

[georgeverghese]

What does Mr. Budris mean by "NPSH margin ratio" ? Is it (NPSHa - NPSHr) / NPSHr ?
Agreed, the last reliability graph doesnt seem to reflect the bump seen in Fig 3 in reverse.

Fig 3 seems to imply that we need to keep this ratio at above 1.8 or so to keep a pump out of trouble, which is a lot more than what is generally provided, assuming my guess at what what this author means by "NPSH margin ratio " is correct.

Typically, for a pump with NPSHr = 8ft say, we would attempt to provide a 3ft minimum margin. Which means this ratio is 3/8 <<1.8 in current general practice. While this graph implies NPSH margin should be at least 1.8 x 8 = 14.4ft??

 

[rotw]

Maybe this helps

https://www.ksb.com/centrifugal-pump-lexicon/npsh/191284/

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