Impeller ratio in VSD application 5

[virata]

Hello guys,

Our specification specified that the ratio of rated impeller to its max impeller size is to be less than of equal to 98%. Our vendor has offered with 98.4% impeller ratio. Also its pump motor are VSDs and thus the speed can be increased to mitigate this as Vendor stated. Is this acceptable? What is its relation to VSD application?

Regards,
V

 

[BigInch]

Very interesting. Thanks very much for sharing!

you must get smarter than the software you're using.

 

[1gibson]

To be clear, you are saying that the maximum efficiency (at BEP) for a pump is reduced by 1-2% for every 5% reduction in diameter? Or is this something to do with maintaining the same head or flow after a trim?

The 1% seems right for volute, but I've seen a lot of VS1/VS6, multistage vertical diffuser pump tests, and 2% seems high, closer to 1% there is what I'd expect. "Middle of the road" pumps, 2500-3000 specific speed.

Not trying to disagree with you, I don't have any curves in front of me, but I am curious about the details.

 

[GPRnD]

I wouldn't necessarily disagree with you either. Experience being reality trumps rules of thumb. I will say the basis for that particular rule was mostly from lower specific speed horizontal pumps.

I went a pulled a few random VS1 curves just for fun:

9RAHC 1564 Ns, efficiency loss per 5% trim = 1.0%
13ALC 1682 Ns, efficiency loss per 5% trim = 1.25%
12JLO 2620 Ns, efficiency loss per 5% trim = 0.65%
10JMC 2700 Ns, efficiency loss per 5% trim = 1.63%
14JHC 2713 Ns, efficiency loss per 5% trim = 0.8%

So plenty of scatter, but perhaps for those pumps I'd hedge closer to 1% to 1.5%.

 

[1gibson]

Interesting. To go further off on a tangent, for those low Nss horizontals, did you trim only the vanes, or the vanes and the shroud? Potential to recover some efficiency if you leave the shroud full diameter, but trim just the vanes? If losing more than 2% efficiency for 5% diameter, it seems like something could be done to mitigate.

 

[GPRnD]

I took a look at a current BB4 horizontal line that I have confidence in the test data for.

4x5-10B 1074 Ns, efficiency loss per 5% trim = 1.8%
5x6-11B 1333 Ns, efficiency loss per 5% trim = 1.9%

Those pumps, like all current BB4 and BB5 models have their shrouds left at full diameter and only the vanes are trimmed. The reason for this is to maintain the A gap and reduce axial thrust variation.

So this data at least fits the rule of thumb pretty well.

 

[1gibson]

Huh, good to know. Apparently the bigger reduction is just a penalty from the disc friction of the full diameter shrouds, which is intuitive.

P.S. I obviously meant Ns, not Nss, in my previous post.