Hello,
I have some questions about direct time waveform signals which could be truncation and/or even a partial rub.
During transient taken data from a turbine analyses we see 0,5X component (27microns). The 1X (5microns) is at that time de-creasing due stiffness increasing (probarly). At base load condition we see 0,25X in de the full spectrum plot. (forward and backward visible). Also data taken with a portabel data collector we see similar truncation of the time signal. The same cut off (truncation) we see with the portable on-line measurement Bently system.
As we looking towards the avarage centerline position plots, the rotor is crawling up the non driven end bearing. The direct orbit is flat at baseload conditions. (also the 1X component, compensated). pre load is high!! (misalignment check?)
The main questions are:
How dus truncation relate towards (partial) rub effect??
When do we see a rub @ 0,25X??
How does truncation relates toward the stiffness of the rotor(system)??
What can we do about truncation??
How and when can a investigate a cocked bearing.
(several documents were read from this forum, thank you guys for that !!! Where can i learn more about the truncation info?

That is a bunch of questions...

Would it be possible to post some of the data here? That would help a lot.

Truncated wave forms are an indication of a rub. Rubs typically increase stiffness of the rotor system.

Rubs in turbines typically are either in the interstage seals (where clearance is tight) or axial. I would be looking to make sure there was not a differential expansion issue.

I also had a very preloaded and truncated orbit in a turbine where the diaphragm side clearance had not been set properly, resulting in a packing rub. The solution was to overhaul the turbine again.

1/2X rubs are not too uncommon, I don't believe I have ever seen a 1/4X rub. Bently Nevada maintains that in order to have a 1/2X rub the operating speed must be more than twice the critical speed. And so for a 1/4X rub the operating speed must be more than 4 times the critical speed. How fast is this machine spinning? What are the rated criticals?

I have experienced an exact 1/4 on a generator. Bently Nevada training materials also has content about a normal loose situation as distinguished from the classical rub situation which is normal tight.

In the case in which I encountered the 1/4X the most recent work accomplished on the machine had been an alignment between a turbine and generator. A torque wrench was applied to the hold down bolts of the generator bearing support (while running) and the 1/4X immediately disappeared. It was later determined that the bolts had originally been tightened with a simple adjustable wrench that was about two feet long resulting in very low torque relative to the bolt size.

John from PA

Monitor,

If the machine speed is at least 4-times more than the first rotor resonant speed (critical), it is possible you are getting a re-excitation of the first resonance from a rub. Look toward the seals for answers and problems. The truncation is a by product of rubbing, which essentially provides a broad band excitation of all available frequencies.

Regarding a cocked bearing, best to disassemble to take careful clearance measurements of bearing to journal in horizontal and vertical directions (feeler guages & lead wire or plasti-gauge respectively). Then get bearing to cap clearance / pinch measurements in both directions. Put the entire enchilada together and see how the bearing is sitting.

If your are dealing with a generator, beware of hydrogen seals, especially after outages! They are typically much tighter than the oil bores. If of floating design, make sure they have not hung up.

Also sounds like you have a misaligment condition at running speed based on the compensated orbital data. You should perform a thermal growth study to get the cold rotor offsets properly established.



Steve, I see you're in KL. I just got back from there. New permanent position??? Check out the Beach Club downtown when you get a chance....

What took you to KL Stan?

I am not quite there yet. Going back and forth, and planning to finalize the move in September.

I'll be officed in the Petronas towers, and if I remember the Beach club is not far from there.

Regarding cocked bearing. Take phase reading around the bearing . Change in phase will be equal to change in accelerometer angular position .

Steve - I've had the pleasure of sailing in Langkawi - beautiful sandy beaches and a short flight from KL. Enjoy your stay and be safe.

I have some data taken with accelerometers on the casing of a turbine bearing. The data clearly shows a light impact from a laby seal rub. Every 4th revolution (1/4x) and every other revolution (1/2x) the impact is higher in amplitude in the waveform and the spectrum is full of 1/4 and 1/2. Unfortunately, no prox probes installed.

Beach club is just off jalan sultan ismail, right around the corner from Hard Rock Cafe and Petronas towers. Langkawi is nice, but check out Terengannu on the east coast for some real beauty!