During rubbing, there's a high rotor lateral excursion from the rubbing spot and, therefore, more impacting; unsteady transient motion occurs. Within a worn (or oversize) bearing, the journal remains close to the bearing surface, even when the contact is broken. The consecutive contacts don't produce relatively high power impacts. There are far fewer transient components and weaker high harmonics in the vibration spectrum of a worn bearing compared to the rub cases.
Let's now look at three process-plant case studies that show the value of vibration data in diagnosing malfunctions.
One site relies on a large, ≈14-MW, ≈3,600-rpm compressor. Vibration values (peak-to-peak) of 36 µ, 5 µ and 12 µ were observed at 1×, 1.5×, and 2×, respectively. There also was considerable ½× sub-synchronous vibration (31µ, peak-to-peak). Detailed study of the vibration data showed the strong ½× sub-synchronous vibration came from light rubbing of the rotating assembly of this compressor train. Inspection of the machine confirmed this rubbing.
Another plant was concerned about a compressor train with an electric-motor-driven rotor supported by oil-lubricated bearings that ran at ≈6,000 rpm. Observed main vibration elements (peak-to-peak) were ≈41 µ at ≈0.4× of the rotating speed and ≈34 µ at 1×. The high ≈0.4× value indicated a bearing whirl problem. So, the plant installed modern tilting-pad bearings in the compressor train to solve the problem.
The third facility operates a medium-size, 11-MW, ≈4,000-rpm compressor train. Recorded main vibration amplitude components (peak-to-peak) were 41µ at ⅓×, 47µ at ½×, 52µ at 1×, 15 µ at 1.5×, 16µ at 1× and 13 µ at 2×. The ½× and ⅓× sub-synchronous vibrations, plus the rich spectrum of higher harmonics (particularly 2×), pointed to a loose pedestal connection. A careful site inspection showed loose foundation bolts. The plant repaired the foundation and regrouted the train.
AMIN ALMASI is lead rotating equipment engineer at WorleyParsons Services Pty. Ltd., Brisbane, Australia. E-mail him at email@example.com.