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JKozlo

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Hello all,

I'm trying to troubleshoot some vibration alarms I'm getting at the inboard bearing on one of the steam driven generators at my site.

The machine in question is a 50 MW steam turbine driven generator, originally supplied by GE about 40 years ago.

Using an Enpac 2500, we're seeing a nearly pure 1x signal with peak magnitude of ~0.446 ips. For comparisons sake, the sister generators run around 0.04 to 0.14 ips at the same location. There is continuous vibration monitoring on the machine train via a Bently Nevada 3500 rack. In the orbit for the inboard bearing (see below), the signature looks oddly squared off to me. In your experiences, what might be causing this sort of pattern?

Thanks for your time.

John Kozlowski
 Generator IB Brng Orbit.jpg


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John from PA

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Have you tried any form of runout compensation on the signals?

[Edit} I deleted my previous question wondering if it was a 2-pole or 4-pole generator. Running 3597 RPM it obviously is 2-pole. That raises the question is the data taken before synch?

fburgos

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for me it looks like an unfiltered orbit, some scratches/lumps on the surface of the journal normal, high 1x looks like unbalance.
John from PA

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Quote:
Originally Posted by fburgos
for me it looks like an unfiltered orbit, some scratches/lumps on the surface of the journal normal, high 1x looks like unbalance.


It is unfiltered and also uncompensated.  Hopefully the OP has a coast down data where he can apply compensation.
George D

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We recently observed something similar on our Main Feed Pump.  We asked our Bently rep to apply waveform compensation to the orbit... which cleaned it up pretty well.  See attached.
I believe this supports the notion of some type of square-shaped runout on the shaft.  Maybe from how they chucked it up when initially machining the shaft?

 
Attached Files
docx Msg .docx (51.12 KB, 15 views)

John from PA

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Another thing that might cause this is a design characteristic of the two machines, the one by the OP and the data presented by "George D".  I notice that in both instances that the direction of rotation is CCW.  I'd further speculate that the bearing is possibly a 4-pad tilt pad.  This type bearing tends to create a somewhat rectangular shaped orbit with the large dimension along the "soft" side of the bearing (7-8 o'clock to 1-2 o'clock) and the smaller dimension along the "hard" side (10-11 o'clock and 4-5 o'clock).
George D

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John,
Thanks for weighing it.  Makes me feel better about considering your diagnosis of a 4-pad bearing myself when I first saw it... and wishing that was the case?  In our case, it is a 5-pad, LOP, tilt-pad bearing. 
John from PA

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Perhaps of some interest...https://www.maintenance.org/topic/square-orbit-plot-what-would-possibly
ivibr8

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The first concern I have is your statement that this level of vibration is 5 to 10 times higher than other similar machines. To me, that is a red flag needing explanation.
What I can't tell from your post is past history.
Do you have previous data to compare with?  Is this increasing?  or steady?    <---TREND INFO

Assuming you have your probes mounted in typical 45 deg from vertical, I agree with most others that there appears to be a sharp loading at the top of the plot.    Why?
An unbalance condition could be the cause or perhaps the bearing(s) are not in proper alignment (i.e. inboard bearing sitting low?).
Another possibility is thermal growth at this particular bearing.  
Again, past history could hep support one or the other possible cause or at least plot data as it gets up to speed and temperature

I recall reading a paper many years ago (no details - in a section of my brain no longer accessible) about multi-pad bearing that had excessive wear between the pads and the pivot surface.  Though I believe this condition showed more of a "squared" waveform than what you show here.

Regards
Jim P
 


OLi

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Reply with quote  #10 
Did you check that the upper part where the sensors are mounted are bolted down firmly and in good shape? Just in case of Murphy.
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John from PA

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Quote:
Originally Posted by ivibr8
Assuming you have your probes mounted in typical 45 deg from vertical, I agree with most others that there appears to be a sharp loading at the top of the plot.    Why?


The probes are configured to be at 45° left and right of TDC as indicated by the small black squares (labeled "X" and "Y") at the top left and top right on the plot.

What gives you the impression of "sharp loading at the top of the plot"?  Having asked that I wonder if the OP can overlay the orbit onto a properly scaled shaft centerline plot to judge the dynamic motion within the clearance. 
JKozlo

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Thanks for the feedback folks.

Quote:
Originally Posted by John from PA


It is unfiltered and also uncompensated.  Hopefully the OP has a coast down data where he can apply compensation.


Unfortunately, coast down data is something we don't currently have and the generator isn't scheduled to come down for at least another year. The Technical/Reliability department has been trying to capture that data for a while but Operations seems to take an almost childlike delight in shutting things down in the middle of the night or on weekends when no one from Technical is around.

Digging into the trend, the vibes have been wandering around 3+ mils at the X probe going back to ~2015, though there does seem to be some degradation over time; I'm seeing more 3.5 to 4 mil readings on the Bently in the last two or three months and the handheld data is doing much the same.

I'll have to check the maintenance records to see if anyone's been out to look at the instrumentation connections.

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John from PA

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Quote:
Originally Posted by JKozlo

Unfortunately, coast down data is something we don't currently have and the generator isn't scheduled to come down for at least another year. The Technical/Reliability department has been trying to capture that data for a while but Operations seems to take an almost childlike delight in shutting things down in the middle of the night or on weekends when no one from Technical is around.


You don't have to have a group of people involved to capture good shutdown data; simply configure System 1 to capture the shutdown.  Then it doesn't matter when that shutdown occurs, it will be captured. 
ivibr8

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John
It just appears to me that from ~ 2 to the 10 o'clock position (i.e. top half) there appears to be a relatively "flattened" profile.  For problems like this, I prefer to look at the raw data as shown.

It would also be nice to see the "slow roll" plot

The best paper I can recall that covers severity of the vibration for turbomachinery was authored by Dr. James McHugh 

https://oaktrust.library.tamu.edu/bitstream/handle/1969.1/163584/T18127-136.pdf?sequence=1&isAllowed=y

So, in my view, it would be helpful to know bearing to shaft clearance tolerances (in addition to the history of the machine).

Thanks for confirming the X and Y probes......I just missed seeing it in the plot

JP
John from PA

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Quote:
Originally Posted by ivibr8
The best paper I can recall that covers severity of the vibration for turbomachinery was authored by Dr. James McHugh


Jim McHugh would be honored to be called Dr., a title to my knowledge he didn't include among his many accolades.  I knew him well as we both served on a ISO/TC108 committee which met in the BN offices in Broomall PA.

Sadly, Jim passed away at the age of 80 in 2007, while living in Tucson AZ.

There is a sister paper titled Estimating The Severity of Sub-Synchronous Shaft Vibrations Within Fluid Film Journal Bearings to the one you mention.  There are "web" references but most cost money.
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