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Noknroll

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We had a step change in 1X vibration at the outboard bearing of a center hung axial flow fan.
Fan recently had dust in hub cleaned out and the outboard bearing was humming along nicely at 2.5mm/sec then in an instant jumped to 5.8mm/sec meanwhile the inboard dropped slightly from 2 to 1.7 mm/sec. A thrown balance weight is what first comes to mind
Outboard overall vibration is slightly unstable and tends to react to load. 
Load is via variable inlet or guide vanes.
Screen shot shows a re start after fan was stopped to affix some reflective tape. 
Note "outboard mm/sec" unstable prior to guide vane opening and then climbing with "vane % open"
not worried about the big spikes in red "inboard mm/sec" but note how it is very sable with vane % opening
I would have thought if we were looking at an imbalance problem the outboard line would be very steady and unchanged with load.
I'm waiting for a phototac to turn up and then get some phase readings at various loads.
If out of balance I expect to see stable phase throughout the vane % opening range.
Is this a fair comment?

fan vibration.PNG

Edwin

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Reply with quote  #2 
I suppose there is no possibility to change rpm? An unbalance will result in vibration proportional to rpm.
Maybe there is a damage of a fan blade? This will both result in imbalance as in an unsteady flow, changing with vane opening. Do you see any of that in spectral data? (low freq noise and/or sub harmonic components)

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GaryVibe

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Reply with quote  #3 
Hello Noknroll. What type of bearings on the fan shaft? Sleeve or rolling element? Direct driven or belt driven with the motor outside of the housing?
This looks similar to one that I had, belt driven with motor outside of housing. We found that the outer race was loose in the bearing housing and would move under slight load or at startup, but not loaded......it took several days to find, it was not obvious with a visual inspection. 
Noknroll

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Edwin
no chance of changing speed, something going through the fan and damaging a blade has also been considered, I guess we wont know until a physical inspection is done
Gary
This one is a direct drive off a 9 MW motor with Rolling element bearings.
I got some more route data today but haven't had a chance to review it yet
Edwin

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Reply with quote  #5 
And what about spectral? Do you see disturbed air flow?
Motor temp has also gone up so I think there is loss of efficiency.

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Shoveldr

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Reply with quote  #6 
Are the blades fixed on the hub or are they adjustable?  Could one of the blades shifted?
GaryVibe

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Reply with quote  #7 

Typically with adjustable pitch blade assemblies if one blade is set different it shows up as a "Vane Pass Anomaly", have seen this several times where they forgot to properly tighten the blade mounts.  Another possibility (seen from experience) dirt was dislodged within the blade hub (bowl) assembly and is now the unbalance force. But the comment is that the outboard is unstable, more likely something has come loose, or a piece of dirt/debris is moving around in the fan hub. 
Working at Howden, I have seen this several times, fan was cleaned and returned to service, but they did not remove the pieces of debris left inside the Bowl that supports the fan blades. Every time the fan restarted the phase angle changed, sometimes 10 degrees, sometimes 60 degrees- very random. 

Noknroll - this is a 9 mw motor with rolling element bearings? is this an "on the fly" variable pitch fan with the motor outside of the inlet housing? Howden Variax?

Noknroll

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Reply with quote  #8 
some answers in no particular order
9MW motor has plain bearings, fan has rolling element
Not Variax
It is a howden with fixed blades on the impeller. load is via variable pitch static inlet vanes
Motor is outside, as is the drive end bearing then a long tunnel (6metre/20ft) to the enclosed NDE bearing and impeller
No turbulence in the spectrum attached, the tiny little peak at 19 orders is 19 vanes 
the little step up in trend is Sunday night at 350 MW, and Wednesday morning at 660 MW. Capture.PNG 

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