Changing the Capacity and its affect to NPSHA 1

[virata]

Hi there,

If the pump (centrifugal) flow rate is changed, will the NPSHA (also the suction prerssure?) be also affected? Does this has to do with the affinity law?

Let me know your thoughts.

Thanks,
V.

 

[rotw]

It is the NPSH required that will change and you should get this info from manufacturer with usually a curve NPSHr vs. flow for a given design/selection of pump.

"If you want to acquire a knowledge or skill, read a book and practice the skill".

 

[Artisi]

Yes, NPSHa will change due to the inlet losses (friction loss) in the approach pipe work etc. Increased flow will account for higher inlet losses whereas lower flow will account for less losses, which are measured at the impeller eye.
NPSHr is not really a function of the affinity laws although they are often used for an approximation, true NPSHr figures should come from the manufacturer or tested pump performance curves.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[rotw]

Basically if flow increases/decreases through the pump it means that there are less/more system resistance then you need to check the NPSH_a vs. actual NPSH_r at the actual operating point.

So you may end up in an operating case where system resistance is lower, NPSHa is higher but margin to NPSHr has at the end possibly reduced because the operating point shifted to the right of the pump NPSH/ performance curve.

"If you want to acquire a knowledge or skill, read a book and practice the skill".

 

[GPRnD]

It is pretty normal for NPSHa to reduce at higher flowrates due to the reasons discussed by Artisi and rotw.

Thus it is important to check NPSHr vs. NPSHa at the worst case condition, this typically being runout flow of the pump. Keep in mind that a margin between NPSHr and NPSHa is recommended. Hydraulic Institute standard 9.6.1 (2012) can help guide you as to the appropriate margin to use.

You can also refer to the webinar link below which covers the topics of NPSH margin, HI 9.6.1 and cavitation in some detail.

http://www.gouldspumps.com/Tools-and-Resources/Webinars/Demystifying-NPSH-Webinar-Preview/

I hope this helps.