Process Compressor DE bearing damage

Baker Hughes Bently Nevada has an active Support Service Agreement (SSA) with a Gas Processing Unit in MENAT region providing regular proactive machinery diagnostics services for all critical machines connected to S1. The success story captured here demonstrates how the customer could leverage the SSA to utilize the expertise of MDS services.

• A Process compressor train comprises a multistage centrifugal compressor driven by a gas turbine through a step-up gear box in a gas processing plant located in middle east. 
• Customer requested Bently Nevada Engineer to diagnose the machine’s vibration behavior as the compressor DE bearing vibration showed increasing trend and it crossed trip limit on 26-Sept-2020. The high vibration protection logic uses 2 out of 2 voting and the max vibration reached on DE bearing was 127 µm pp, other orthogonal probe read 66 µm pp, while the alarm and trip limits are 80 µm pp & 119 µm pp respectively. 
• The machine had major outage during which turbine and compressor rotors were replaced. Compressor NDE bearing replaced with new bearing and DE bearing replaced with refurbished bearing due to new bearing unavailability. The machine was started on 9th Sep 2019 after the maintenance. 
   

• Review of System-1 historical data showed a gradual rise in compressor DE vibration over 4 days from 30 µm to 120 µm on X-probe where as the vibration on Y-probe increased from 30 µm to 60 µm. The rotor showed dynamic change in behavior with the direct orbit changed from circular to elliptical shape. No vibration changes noticed on GB HSS DE and Comp NDE bearing.
• Review of bearing metal temperature during  start-up on 9-Sept showed a sudden rise and drop crossing 100°C which indicates a potential issue in the bearing probably a slight rub. 
• Based on the observations, BN MDS engineer recommended customer to inspect the compressor DE bearing and the inspection revealed severely damaged bearing.

The vibration levels on Compressor DE bearing showed increasing trend from 25-Sept and the vibration increase is 1X dominant. The vibration amplitude on X-probe crossed trip limit where as on Y-probe it stayed below alarm, the unit continue to run as the vibration protection is based on 2 out of 2 logic.

The compensated orbit shape changed from near circular during low vibration period to highly elliptical when the vibration crossed trip level.

Bearing metal temperature during start-up on 9th Sep crossed 100°C with a sharp rise and drop trend indicating potential Babbitt issue. The steady state bearing metal temperature stayed at 80 °C.

Review of GB HSS and Compressor NDE bearings showed stable vibration trends when the Compressor DE bearing showed increase in vibration, this indicates a localized issue and could be related to DE bearing.

Inspection Results & Vibration amplitudes after bearing replacement

• Upon MDS engineer recommendation, site inspected the Compressor DE bearing and found severe damage as shown in the photos here.
• Site team replaced the bearings and the follow up start-up did not show any high vibration or high bearing temperatures. The max vibration on compressor bearing was less than 40 µm pp and bearing temperatures were less than 80°C during steady state condition.
• Root cause analysis by customer reported lack of bonding between Babbitt to base metal on the refurbished bearing as the cause for Compressor DE bearing failure.

DE bearing lower half and upper half as found condition

DE bearing vibration and metal temperature trend post bearing replacement
 

• Accurate & firm analysis helped customer to pinpoint the right location for inspection, avoided potential journal hardware damage.
• Immediate corrective action of bearing inspection and replacement helped customer to restore the machine back to normal operation. The unit was restarted within a couple of days, the bearing was replaced with new and the vibration levels were found within acceptable limits.