Predicting Problems on Critical Assets through Vibration Monitoring
Case Study #1 Evaporation Compressor
A steam driven compressor operating through a speed reducing gearbox to an overhung rotor. The speed can range up to 5500 RPM. During peak operations, the equipment is typically reduced to speeds around 1790 RPM. This critical compressor functions to take the cooled vapors from the Evaporator column, compress the vapors to reheat them and send back to the Evaporator column to heat the incoming product.
In addition to the routine vibration monitoring conducted as part of the predictive maintenance program, this critical compressor’s utilizes an online protection system to monitor the overall vibration levels and report those levels to the Operator’s controls.
During a check on this asset, our PdM Technician recorded overall levels of vibration that were higher than normal baseline or recent trends. The vibration levels were measured in velocity (in/sec pk) and were noted at 0.237 in/sec. The normal operating levels for the compressor bearings are between 0.04 and 0.07 in/sec. This immediately prompted a detailed analysis of all the collected data. The velocity and demodulation spectrums both indicated multiple harmonics of the shaft running speed. Additionally, the analysis noted strong levels of impacting in both the high frequency and demodulation waveforms.
Based upon previous data and the monthly trends, this condition was considered abnormal. The analysis also showed that the high levels were not present on the drive end bearing and were strictly isolated to the opposite drive end bearing. Upon further review, it was determined that the levels increased immediately after a planned maintenance activities including a pressure wash of the impeller. There was speculation that the impeller may not have been cleaned properly and the result was causing rotor unbalance.
The equipment was diagnosed as wear or clearance problem on the opposite drive end bearing. The analysis was not totally agreed with plant personnel, as there was no indication of a rise in temperature for that bearing. A decision was made to continue operation of the asset but at a lower RPM. Data was collected again at the adjusted operating parameters and there were no signs of harmonics or impacting.
After operating the machine for approximately a month at the lower RPM settings, an inspection was performed of the bearing. Upon opening the opposite drive end bearing it was found to have extensive damage. The damage was on the bottom half of the bearing.
The bearing was removed, replaced and the housing cleaned of all Babbitt material. Upon returning the compressor to service, vibration data was collected and analyze exhibiting no sign of vibration problems at the normal operation speed.
A process is underway to evaluate the vibration levels at different operating parameters to ensure optimal performance without creating mechanical issues within the machine. Vibration monitoring and analysis is a key component of this evaluation.