Go Figure...

Problem
Large induced draft fan had high axial vibration (@15 mils at 1X) on Friday. Horizontal vibs were @6-7mils. Unit was shut down so we wanted to have the axial vibration back to acceptable levels by Monday.


Evaluation
Machine was out of alignment. High off set in the vertical direction (.030 on a 700 rpm machine). After alignment I double checked the numbers from rotoralign to ensure the smiley faces were present.

Broken bolt was found at bearing pedestal and repaired.

Took an oil sample at the inboard fan bearing
- no metal was present

Washed fan with a steam cleaner (twice). Lots of hard stuff (looked like concrete - has been there for years - result of large duct leaks at the suction) on the blades. Weird ash loading on the inboard side (removed)

Test run
Inboard axial vibration increased to 44 mils

Evaluation part 2
Checked alignment again - numbers had changed (not by much - we had lost our happy faces) so a different team aligned it again. We ensured the team had a lead that is considered the best alignment guy we have.

Inspected the inboard fan bearing - looked OK, a little babbet had moved around. But measurements were within tolerance and the bearing was re-used.

Took oil sample at the outboard bearing - no metal in the oil.

Inspected coupling - it wasn't locked up and the grease looked good.

Pressure washed the fan to remove all the hard stuff we could - took 4 hours to get it all & still some of the stuff refused to fall off. Must have removed 10 lbs.

Repaired bolt was loose again - tightened back up.

Test run part 2
Inboard axial vibration decreased to 20 mils

Inservice Evaluation part 3
Repaired bolt had came loose again. Soft foot was present on 1 side of pedestal. Tried to tighten bolt with fan running and monitoring vibration and the vibration increased to 24 mils axial while the horizontal went down (from 6.5mils to 6)

Attempted to place come alongs on the pedestal and pull the bearing in different directions. There are 2 I-beams nearby. Each direction caused the axial vibs to increase.

Left bolt loose and put the comealongs away.

Balancing time - Monday @0100
I figured that if the soft foot was amplifying the vibration that the fan had, if we could reduce the horizontal vibration energy by 75% and maybe the axial movement would drop by 75% as well.

Results? Glad you asked!
26oz weight 130D from the high spot reduced the horizontal vibration by 40% (what was 6mils is now 4) The phase angle was unchanged. Good - makes TW2 plot easier for me! If 1 is needed...

Axial Vibs did what???
Dropped from 22 mils down to 3.

Conclusion
Need to fix the pedestal. Looks like it's been sinking (wearing down?) for the last 50 years. For now the work around will be balancing. I know balancing doesn't fix every thing.

But it sure is fun!

Yes, that's an interesting one all right. Who would have thunk that single plane balancing would have resolved that high axial vib on a between bearings machine. With the benefit of retrospect, I guess we conclude that the support structure was transferring that radial unbalance force into axial movement due to the mechanical problems of the pedestal?

By the way, did you ever do an axial bump test?

Thanks for sharing that.

Guess what? There are no shims under the inboard side of the motor. Any direction down may have been throwing it out of alignment.

"Did you ever do an axial bump test?"

Nope. Never done one before. What would it have told me? Natural freq's?

Well, sometimes fans on mining, cement or fertilizers industries can be a nightmare. Be sure your maintenance or operations guys don´t "clean up" rotors based on scheduled maintenance (could be dirty but balanced). Instead is better to implement "cleaning based condition" and tell them when is good to clean rotors (when unbalanced, not before). My two cents.

Yes - the only reason I suggested axial bump test was to check for axial resonance at running speed since that might help explain why axial vib is sensitive to unbalance (although the pedestal problem might be the whole explanation).

Dear pete
first, thank you for your intelligently answers.
second, the phase data before balance didn't show symptoms of unbalance (neither static nor couple nor dynamic unbalance). So whether we must revise the rules for detecting unbalance via phase analysis or we must say in this example, doing balance componsates other causes?
Regards

This message has been edited. Last edited by: Farahani,