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We had performed preventive maintenance of one of our barrel type compressors. This activity included journal bearing inspection, thrust bearing inspection and alignment check alongwith shim pack coupling inspection.

Basically it is Carbon Dioxide gas train at Urea plant. It is typically a two speed machine train, i.e. low pressure axially split low speed compressor ( ~ 7000rpm ) direct driven by a turbine and high speed high (13500 rpm) pressure barrel compressor driven by the same turbine through a speed increasing gear box at a ratio typicall ranging 1.9~2.

Since the recent maintenance we are having a heavyly eliptical orbit. I mean a high value of left side 45 degree vibration probe -- BN proximity probes (80 micron) and very low value at the right side vibration probe i.e. 29 micron. Shaft centre line at the Coupling end is at the top, middle and at hight clearance circle.

Alignment was checked and was as per recommended i.e gear box is kept left radially for about 0.30mm and its thermal growth is expected in the tight direction.


Is it an alignment issue caused by more than expected rightward and upward thermal growth?

Some things we need that would be extremely helpful. First of all type of bearing, and if a tilt pad let us know if it is load on pad or load between pad.

Also, since you have obviously viewed the orbit and the shaft centerline plots (since you mention them) put them into some form of a document (Word, pdf, etc) and post them here so we can see things rather than work from a verbal description. Some spectrum plots would also be useful as well. Make sure we also know the direction of rotation. The direction of the canted orbit you describe might be normal, then again depending on the direction of rotation it could be abnormal, hence why we need to know.

John from PA

Which type of shaft seals used for this high speed compressor ? If bushing type oil seal is used, just check oil seal leakage rates also. Vibration value of 80 u for 13500 rpm sounds, it is running near danger/trip limit. Alignment was disturbed from original values before overhaul ?

If it is an alignment issue, there should be some significant change in the operating bearing metal temperatures from prior to the maintenance. It would be better as John mentions to see the data. I would suggest presenting direct orbits and snaft positions.

As John suggested, a picture is worth a thousand words. Plots from before and after the maintenance activities could be useful. Did bearing and coupling inspections reveal anything, and were any parts replaced?