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Curran919

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Hey everyone,

I've got some 2500hp cartridge centrifugal pump packages that have failed FAT vibration testing.

Client has recently made some modifications in lightening up their skid, and as a result we have a 60.1 Hz natural frequency (Pump - Horizontal; Ends Out of Phase). Clear resonance. End customer not happy, etc. Pedestal modifications are possible and probably effective, but would be a enormous cost hit due to the contract. Even re-adding some more cross-beams to the skid would probably be useful.

The kicker is, the whole skid (6000x2700x350mm) is getting filled with grout on site.

I've had a few projects in a past job on plunger pumps where we have done this as a shotgun measure with incredibly good results. Concrete can be a great band-aid. We fully expect that this project will solve itself once the grout is poured, but for how long and how can we convince the customer?

As I have no feedback of past troubleshooting projects about the long term health, what can be expected of the long term effects of filling the skid with grout? At some point, I suspect the effect to diminish through delamination and cracking, but is this significant assuming the grout is kept in good condition? Should we ever expect the grout to let the natural frequency drop back into resonance?Anyone have some good resources on this that I can use to help convince the end user?
ivibr8

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Curran
This may be difficult to answer without more information.

It seems you are convinced it is due to structural resonance in the horizontal direction (60.1 Hz <--wow...down to nearest 0.1 Hz?); but without more data, I think it would be difficult for anyone to answer with certainty that filling with grout will fix the problem.

Grouting will likely solidify the base and likely make it "SOLID" and rigid, but may not help if the remaining structure is relatively flexible and is the root cause problem of the 60 Hz resonance.  I am not aware of any "long term" adverse effects of grouting the skid to the base.

I've used MeScope to help me visualize and quantify issues such as this. This program (and others like it) can also be used to model the structure and perform "what if..." calculations (i.e. what if I add a cross-stiffener, weld a plate at the ends, etc.) to make changes to the structure.

Hope this helps

Jim P 


Curran919

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Thanks Jim.

We have this guy modeled in ANSYS. It estimates the expected mode at about 75 Hz, which is far off, but at least shows that the flexibility in the skid is a significant contribution to the mode, not just the pedestal. With that, I'm fairly confident the concrete will work at first and expect the mode to get up to about 70hz if I'd have to guess from the model. I just don't know if it will stick there or will gradually sink back to 60hz.

Used mescope years ago for mode shape visualization. Had no idea it had any sort of mechanical modelling capability.

My guy on the ground can't take a half decent bump test, so to confirm the mode, they ran the package on a slow ramp from 58-62 Hz. That's why the precise frequency. We were going to originally try steel modifications. Wanted to confirm we should add steel and not remove, as we don't have much room for stiffening.

CURRAN
ivibr8

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OK....so sounds like you already have it modeled.

I would think you could alter the boundary conditions on the structure so that the portion of the skid that is bounded by the grouting is anchored and see what that does to the modal analysis. Yes? No?

JP

BTW- During the slow ramp testing, could you at least take some amp AND PHASE readings to confirm that the mode shape is as expected in the ANSYS model?
OLi

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Normally a good grout would fix the problem if it attach to the surface and don't shrink etc. In 10-20 years when it crack you may regrout or fill up with epoxy injection..... Maybe you can add some expansion bolts or similar to simulate that when testing?
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Curran919

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Quote:
Originally Posted by ivibr8
I would think you could alter the boundary conditions on the structure so that the portion of the skid that is bounded by the grouting is anchored and see what that does to the modal analysis. Yes? No?
model?


Yes, this was done. The model is not accurate, for my taste, but we know the upper bound of how useful this modification can be.

Quote:
Originally Posted by ivibr8
BTW- During the slow ramp testing, could you at least take some amp AND PHASE readings to confirm that the mode shape is as expected in the ANSYS model?


Yeah, the phase shows that each of the bearing ends are out of phase, so we have picked the corresponding modeled eigenmode as a comparison point.
Curran919

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Quote:
Originally Posted by OLi
Normally a good grout would fix the problem if it attach to the surface and don't shrink etc. In 10-20 years when it crack you may regrout or fill up with epoxy injection..... Maybe you can add some expansion bolts or similar to simulate that when testing?


It was originally intended to just use cementitious grout, but I think it may be worth to upgrade to epoxy grout to get the more robust bond to the steel surfaces. Either way, they should both work at the beginning, but I am indeed looking for estimates on how long I can expect it to last. 10-20 years may satisfy the customer, but nobody is going to want (be able?) to jackhammer cracked grout out of a skid.

Can you clarify what you mean by adding expansion bolts to simulate when testing? simulate cracking once the grout is poured?
OLi

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Reply with quote  #8 
No during the FAT test to make more holding points. I did grout a frame/skid 40 years ago but it was special so I put it on a rubber mat, just wanted the internal stability and weight to reduce coupling and vibration of the standby pump of a parallel pump pair, the plant will be shut down in 2-4 years so I guess the filling did it purpose and I believe it is still there it was some 15 years ago when I saw it last time. I was not getting expanding stuff at the time so we injected epoxy directly..... Maybe you can do the same in the FAT, casting grout with a plastic sheet under and moving will be a difficulty.
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Dan Timberlake

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Reply with quote  #9 
So, before the " lightening"it passed OK ?  Please post drawings and pictures of the before and after design.  With care and especially understanding, lighter can often be stiffer as well.


Could you post some pictures of the equipment? And how it is mounted for Factory acceptance testing ?

Also a screenshot or an animation of the FEA results showing the 75 Hz mode?  And the assumptions about boundary conditions?

Also a map of the 1X vibration and phase horizontally from the bearing centerlines, down to the skid lower surface, catching the action above and below every interface, and above and every structural connection?
Also a map of the 1X vibration and phase vertically at all four corners?
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