NPSH,HEAD and EFFICIENCY relation

[MrProjectEng]

Hello,
It can be said generally; when NPSH decrease, HEAD increase and EFFICIENCY increase or that case could be changed accordingly

Thank you

 

[LittleInch]

First two yes there is a link as in general this equates to lower flow.

Last one it depends on where you are on the pump curve and where your duty point is with respect to efficiency.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Cecorros]

Normally with a centrifugal pump efficiency tends to drop towards part-load (lower flows of BEP) and also at overload (higher flows of BEP).

image_source:

http://petrowiki.org/File:Vol3_Page_243_Image_0001.png

 

[Artisi]

Of course the leading question is "what is the use if this information"?
Any pump curve will generally answer all relevant questions.

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.)

 

[JohnGP]

As Artisi points out, the information is on the pump curve. Just remember though that the information on the curve relates to the NPSH required by the pump (NPSHr) to avoid cavitation (or very nearly so). It is used to ensure that a suitable pump can be selected, or existing pump confirmed as satisfactory, by confirming that the available NPSH (NPSHa) exceeds that required by the pump.

 

[LittleInch]

johnGP - Too many people seem to think that NPSHR curve is all they need to get to, maybe with 1m spare.

NPSHR is a number obtained by throttling the inlet valve until the differential head reduces by 3%. Cavitation can start some distance in head above that, hence why I reproduced the curve I did to show the difference between NPSHR and onset of cavitation.

The 3% figure is IMHO, somewhat arbitrary, but vendors like it because it is something they can test quite easily and accurately, whereas onset of cavitation is very difficult to test for.

If you're not at or close to the pump BEP, then be cautious about using the bare NPSHR figure

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JohnGP]

Yes, I was responding in a general way to a general question which I struggled to make sense of. Mostly one can get by with a margin on NPSHr, but your chart dramatically illustrates the importance of designing near the BEP.

 

[Artisi]

You don't need to design near to BEP for a successful installation- you just need to understand what you are doing and why.

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.)

 

[JohnGP]

Artisi, I acknowledge each installation is designed with different constraints, and one can't always attain the ideal.

 

[Artisi]

JohnGP: in a perfect pump world we could - but unfortunately ....

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.)