Finding the Root Cause
Put into operation in 2000, each compressor train has two casings: a low-pressure (LP) casing and a high-pressure (HP) casing. The vibration issues occurred in the first, LP casing only.
The OEM bearings were 5-pad load-on-pad point-contact-pivot TPJ bearings, with a bearing outer diameter smaller than the machine casing bore to provide a squeeze film damper function. O-rings at both ends of the bearings were installed to provide stiffness and control damper leakage.
After each direct OEM bearing replacement, rotor vibrations would be reduced for a time. As months wore on, however, vibrations would increase again – requiring repeat maintenance and another bearing replacement.
Inspection of the removed OEM bearings showed severe pivot wear on the tilt pads and bearing shell bore (Figure 2). This wear increased the bearing clearance by 63 microns or more in a five-month period, causing alarming vibrations. The inability of the O-rings to provide centering capability under static deflection, together with the increased vibrations, resulted in bearings bottoming out in the casing (Figure 3) and loss of squeeze film damper performance. These factors converged to increase vibrations over time in the LP casings of the compressors.