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George D

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We removed a 5400 rpm pump element from a boiler feed pump due to increasing vibration.  The vibration indications were increasing 1X in all directions radial.  Once removed, we observed evidence that the passive magnetite layer had, in some locations, started flaking away from the impeller... which validated, from my perspective, an emerging imbalance condition.  An additional discovery was that the bearing and journal surface at the inboard 5-pad tilt-pad bearing was scored.  It is, from my perspective, unique that the journal is scored... as, I believe, most failure modes of a bearing will wipe the babbitt, but leave the journal undamaged.  Suspect hard particle abrasion... but still considering possibilities?

I would like to put the element in the balance stand and check the balance condition to try to evaluate whether the suspected imbalance is due to the disposition of the magnetite layer, or possibly that the center of rotation at the IB journal may have become displaced due to journal surface scoring... creating an imbalance.  I proposed that we check the balance condition in our soft-bearing machine by spinning on the scored journal surface; then, again, at the undamaged shaft surface just adjacent to the scored area of the journal... comparing the results.

I'm getting pushback to spinning on the scored journal surface.  We estimate that the journal surface is either 63 RMS or 125 RMS... depending on who is making the comparisone against our surface sample guide.  I admit there are uncertainties in checking the balance condition by spinning on a scored surface, but can find no guidance in either ISO standards, or our IRD B150 balance machine manual, of what is an acceptable/unacceptable surface condition to spin a rotor on.

I don't want to walk away from this without understanding what happened here... that's why I'm proposing this, despite uncertainties in the method.  Anybody have insight on acceptable surface conditions to conduct soft-bearing machine balancing... or, in general, my strategy?

Thanks in advance...
George

John from PA

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Reply with quote  #2 
Quote:
Originally Posted by George D
I'm getting pushback to spinning on the scored journal surface.  We estimate that the journal surface is either 63 RMS or 125 RMS... depending on who is making the comparisone against our surface sample guide.  I admit there are uncertainties in checking the balance condition by spinning on a scored surface, but can find no guidance in either ISO standards, or our IRD B150 balance machine manual, of what is an acceptable/unacceptable surface condition to spin a rotor on.


API generally recommends a 0.8 μm (32 microinch) finish for journals.  As a repair procedure, I have seen chrome used but you have to be very careful about the probe target areas or you can end up with a serious electrical run-out issue from the chrome to steel boundary area. 
George D

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Reply with quote  #3 
Thank you John.  Didn't think to look in an API standard.  Gives me a good benchmark to discuss our next step.
I think this is my first post since Rusty started this board.  I was pretty active on the previous board... unfortunately I got distracted and otherwise occupied for a seperate reason.  Glad to be back.  Appreciate everybody's participation and contribution...
George
electricpete

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Reply with quote  #4 

I admire your drive to get to the bottom of the question. In similar situation (component already removed), I've hard a hard time convicing our plant people to spend time / effort to narrow down the defect.... after all (they say) why bother if the repair shop is going to fix both conditions anyway.

The concern I guess is whether you might further damage the shaft journal, right? (or are they worried about the balance machine). 

To my thinking there is a slight potential of damage to a shaft journal any time you spin the rotor on the journal. We actually seem to have caused motor journal damage (circumferential marks that couldn't be polished out) simply by spinning the shaft BY HAND for TIR with the journals on hard rollers. I can't explain exactly how or why, but that's what seems to have happened.  I'm sure there are others with a lot more experience then me.... I'm curious if anyone else has heard of shaft journal damage when rotating shaft either manually on rollers or in balance. machine. 

So, what is the harm of doing your test? Yes, maybe it's more if the rotor journal is already undergoing some unknown degradation which might create debris that dents the journal surface. But in the end you are going to clean up the journal surface anyway, right? In that case a little more damage isn't going to create more work as long as you don't do so much damage that the shaft ends up undersized and needs a sleeve or shaft buildup.

Also, to my thinking a soft bearing balance machine does not generally transmit as much dynamic force through the journal to the rollers as a hard balance machine does. Maybe that means the chance of damage is less, I'm not sure.

Got any pictures?

Dan Timberlake

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Reply with quote  #5 
"I proposed that we check the balance condition in our soft-bearing machine by spinning on the scored journal surface; then, again, at the undamaged shaft surface just adjacent to the scored area of the journal."

Please post some nice pictures of your balance machine's V-rollers.

Recent Experiences with a couple of US manufacturers of big fans made it clear that their standard practice is to balance their fan rotors while supported on the shaft diameters adjacent to the actual bearing journal.  Their explanation is to avoid marking up the gorgeous bearing journals. One fan used adapter mounted spherical roller bearings, and I forgot what type bearings the other fans were.  I understand their reluctance to damage, even superficially and strictly visually, the bearing journals by supporting them on the rollers typical of many balance machines.  If I had known this was their plan, I would have "requested" measuring and recording runout of each and every bearing journal as part of the balance report.   The reason that runout is important to me is my familiarity with the mechanics behind ISO 1940/1, as is well presented on document page 3 here - - http://www.irdbalancing.com/assets/balance_quality_requirements_of_rigid_rotors.pdf   .   Note that for 5400 rpm service a change in centering of a few "ten-thousandths" of an inch clobber the possibility of reaching even lowly G 6.3.

I think your questions might be well answered by first supporting the pump in your balance machine adjacent to the bearing journals, with the stanchions locked out, and indicating the shaft bearing journals, and adjacent regions being supported as well ( to check ovality and other departures from good cylindricity. )
George D

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Reply with quote  #6 
Pete and Dan,
Thanks for your interest and patience.  I had trouble responding earlier... in the middle of a 6-week outage.
Following your questions, I should probably explain our situation a little better.
Our Main Feed pump showed gradually increasing IB 1X vibration in all directions radial over about 18 months.  It went from about 1.8 mils p-p to, eventually, 4.8 mils p-p.  We, initially, suspected opening clearances in the 5-pad tilt-pad bearings.  Performing a mid-cycle inspection, the clearances did not change that much... went from a lift check of 0.008 to 0.010 in.  We noticed, however, scoring at the IB journal surface, and damage that looked like electrical discharge at the babbitt surface.  We hand dressed the damaged journal, replaced the bearing, and put it back in service.  We, subsequently, survived another 2 months to our scheduled outage.
Once disassembled, we saw further evidence of electrical discharge and that chunks of the passive layer had broken off the impeller (we call it magnetite).  So, now, I'm thinking "imbalance".  My assumption is that the imbalance is due to the chunks of passive layer breaking off... but don't want to discount that journal scoring may have changed the rotational centerline of the shaft, thereby causing imbalance.  That's why I asked to check balance at both the damaged journal and adjacent undamaged area to eliminate this as the cause of imbalance.  I agree with Dan... it doesn't take much eccentricity to cancel a G2.5 balance threshold.  I did a napkin estimate, and believe a quarter-mil of eccentricity will cancel a G2.5 balance threshold at 5400 rpm.
I recommended that we check balance at both the damaged and adjacent journal to confirm my thoughts.  It was met with dismissive remarks... that running on an area of damaged journal does not replicate the rotor condition in service.  I agree with that... but believe it will allow us to narrow the cause.
Hence my initial question.  The journal is not totally trashed... see pics.
I'm toying with the possibility that electrical discharge may be causing two things... 1. damage to the journal and 2. somehow discharge of the passive layer?  Electrical discharge is an easy guess to damage the journal.  Never heard of it causing a passive to break away?
Still in the middle of diagnosing this.  We'll see how it rolls up following our overhaul.
Thanks for your interest... see images attached...  
  FWP1-1 As-Found IB Shaft (2).jpg  FWP1-1 Shaft - hand dressed.jpg  DSCN9912.jpg  Balance Rollers.jpg  Balance Stand.jpg 

John from PA

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Reply with quote  #7 
Quote:
Originally Posted by George D
I'm toying with the possibility that electrical discharge may be causing two things... 1. damage to the journal and 2. somehow discharge of the passive layer?  Electrical discharge is an easy guess to damage the journal.  Never heard of it causing a passive to break away?


I've seen plenty of ESD incidents in steam turbines and the path to ground is often through the bearing or bearings affording the shortest path.  This is one reason the actual damage is often seen in gearbox fluid film bearings even though the build up of charge occurs in the steam turbine.  

Since you feel ESD may be a culprit, have you done any measurements to confirm the issue?  That could included measurements to ground while in operation or very careful examination of the time domain and orbit data for spiking.  Is anything being done to mitigate the potential problem?
electricpete

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Reply with quote  #8 

my rambling thoughts fwiw

The impeller seems like an obvious smoking gun for the 1x vibration (even if cause of impeller flaking is not known).   And the journal seems like a longshot for causing 1x because whatever change occurred would have to be uneven around the circumference (take of 0.2 mil on one side but only 0.1 mil at the point 180 opposite).  But who knows, maybe there are some mechanisms that can do that.  A visual inspection of the journal mighht support the scenario of eccentric journal "wear" if  the journal appearance changes around the circumference, but probably not conclusive if it looks uniform.  Your experiment is the gold standard way to take the guesswork out of it, I don't see the harm in it.   You probably want to know what caused that shaft journal pattern anyway (regardless of understanding the 1x), and knowing whether there is some associated eccentricity could be a useful datapoint along the way to answering that question.

I'm sure you're aware, static (dc) voltages/currents can be generated by steam turbines, and dc (vs ac) current is the type that could contribute to electrochemical  / galvanic action. 

My impression fwiw is whatever caused the shaft journal scoring is not electrical damage. 

http://gaussbusters.com/resources/articles/damage-photos/

The link above purports to describe various types of shaft current damage, specifically frosting, spark tracks, pitting, welding.  Yours seems characterized by tiny circumferential grooves (what you might imagine from a few stationary abrasive particles).  Spark tracks seem to match the closest, but still doesn't match the photo to my eye.

I'm curious whether the bearing babbit showed any patterns. 

I'm pretty sure there's not much I said that you haven't already thought about.  It looks like the beginnings of a good case study.

George D

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Reply with quote  #9 
Looked real close at the time-orbit real close early in our investigation.  This based on a rather well-known GE published case history in which ESD was foretold by this type of random spiking.  There was no evidence of this.  We are, of course, going to investigate once in service to see if there is potential to ground using a grounding brush.  Will know more then.  Nothing, at this point, to mitigate it.  Three similar components have not, to date, evidenced what we're seeing here.  Will wait to see if potential to ground confirms that course of action...
George D

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Reply with quote  #10 

Thanks, all, for your insight.  We're in the throes of putting this back together and rolling back up.  Really not equiped to install permanent shaft ground detection at this point... but will certainly investigate whether we have potential to ground once back in service.
Pete... Wonderful link to the summary of electrical damage!  Thanks so much!  Will be useful to help us figure this out...
George
George D

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Reply with quote  #11 
Sample of bearing babbitt at the mid-cycle inspection... appears to be the shoe most severely affected...
George
FWP1-1 IB Brg Upper Half (3).jpg 

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