Minimally trimmed impellers

[tronical]

Hi everybody,

First of all you guys form a great community.

My questions is: does anybody know the harms in having minimally trimmed impellers for vertical turbine pumps?

Some vendor is offering 6 stage pumps with impellers in stages2-6 to be trimmed to the minimum diameter possible.

It feels like an undersirable design because of the raw river water supply (30000 ppm solids), but I can't technically explain why. I think its reasonable to have mid-trim or below mid-trim design, but not to the point of having the minimum impeller diameter possible.

Can anyone comment?

Thanks,

Tronical

 

[JJPellin]

I haven't come across this problem with vertical turbine pumps. In most of our pumps, I think the allowable impeller diameter is a pretty small range. Since you have the option of adding and removing stages, I don't know why you would want to drastically trim the diameter. Can you simply remove a stage? Since these impellers are sometimes mixes flow and sometimes radial flow, I don't know if I can generalize. Based on the configuration, There might be some increased chance of erosion damage in the diffuser behind the impellers because of the turbulence and recirculation. If it was radial flow, you could leave the shroud full diameter and cut back the vanes only. The remaining shroud acts to channel the flow into the diffuser. I don't know if this is possible with a mixed flow configuration. We have rarely used the option of full diameter shrouds, but I think it is a good choice. Bloch and Budris suggest that this should reduce internal recirculation in "Pump User's Handbook"; 2004.

 

[EdStainless]

The question is how much trim? A bit doesn't hurt, in some cases pumping dirty material it might even help.

Why the trim at all? How far off of BEP are you?

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[Artisi]

First up I would be asking the vendor - WHY? - and if you can't get a technically sound explanation I would be looking elsewhere.

Addressing your question, presumably, the first of the 6 stages is full diamter, although this is not stated in your posting and without a set of performance curves for the pump it is hard to say what their intention is, however, if the remaining impellers are trimmed to their minimum diameter, I would suggest that the pump is probably not a good hydraulic fit for the duty -- but as noted, without a set of curves etc it is hard to give you any definite advice at this point.

Naresuan University
Phitsanulok
Thailand

 

[hydrae]

Get a hold of the performace map, I have seen maps that have the best efficiency point just off of the smallest trim and in other cases the bep is at max trim. Playing with the trim vs stages you can pinpoint the flow, head, and hp required to match the need.

As for not trimming the first impellor, a guess may be to assist in NPSH, the curve for that may be best if the impellor is at full trim, a chart with NPSH vs trim are not very common, may be it was only tested at full trim.

Hydrae

 

[tronical]

FYI

VTP's do have much smaller clearances. Although his impeller was minimally trimmed, pumps were operating to the right of BEP! just a bit though.

But with VTPs, between the minimum and maximum diameter possible is usually a few percent (<10), so its that uncommon to see minimally trimmed impellers since the clearance is so tight (at least relative to slurry pumps).

Anyways we met with vendors and the minimum impeller vendor was rejected. The guy can't even remember why he offers minimally trimmed impellers.

He was also the highest bidder, more than twice the next highest bidder.


Thanks for your input guys

 

[Artisi]

He can't remember why !!!! seems like you made a wise choice.

Naresuan University
Phitsanulok
Thailand

 

[escheir]

Hi Everybody,

What can you tell about a pump house intake design with two vertical turbine pumps positioned in-line, one behind the other, in a narrow channel?
The first pump is normally not running, (stand-by pump) the second pump has a capacity of 5100 m3/h at 25 m WC

Any comment?

Erik

 

[Artisi]

escheir My suggestion is to put this question into its own post.

Naresuan University
Phitsanulok
Thailand

 

[ChickenSoop]

the guy with the minimum trim and over priced - he offered larger pumps, and had to trim to minimum trim to decrease the pump capacity to meet the target flow.

This is just a pump sizing issue - the larger the pump, the more it costs. The smaller trimmed impellers are needed to meet the flow requirements. Sometimes you just cannot get a good fit.

Performance-wise, there is no problem with minimum trimmed impellers - just don't trim below manufacturers minimum recommended trim or you will get recirculation in the pump - which results in erosion, vibration - and the pump curve is no longer accurate.