Airborne and structure-borne ultrasound have become major players in bearing condition monitoring rather than being viewed as just a leak detector. More maintenance and reliability professionals are beginning to realize the many benefits associated with using ultrasound for condition monitoring applications (FIG. 1). The P-F curve shows that ultrasound is the first method used to detect defects that may lead to asset failure. Changes in the operating condition of equipment usually appear first in the ultrasound frequency range. Other condition monitoring techniques generally begin detecting the problem only after the equipment has deteriorated further, and the fault becomes more severe.
Observations. A bearing fault was initially detected using ultrasound technology on a cooling tower fan motor of a fertilizer and petrochemical plant in West Africa that was commissioned in 2022. A week later, the issue was also identified through vibration analysis. During a routine ultrasound survey, high noise levels and a crest factor increase were observed at the motor's non-drive end (MNDE). As a precaution, greasing was recommended, and the motor was kept under observation. At the time of the ultrasound alert, no bearing fault frequencies or impacts were detected in the vibration spectrum or time waveform. The initial vibration envelope survey showed no signs of bearing impact, with a maximum value of 0.8 gE (acceleration enveloping). However, one week later, the vibration envelope increased to 2.6 gE. After greasing, the vibration envelope continued to rise, reaching 6.7 gE (FIG. 2).
Actions taken. It was recommended to replace both motor bearings and inspect the housing dimensions. Upon disassembly, the old grease was found to be caked and degraded, preventing fresh grease from properly entering the bearing, causing it to channel out into the housing. Further inspection revealed scuffing marks and shifting of raceways on the MNDE inner race, and skidding marks on all balls of the MDE bearing. NDE bearing housing dimensions was out of tolerance.
To resolve these issues (FIG. 3), new bearings were installed, and the NDE bearing housing was sleeved to restore proper tolerance.
Takeaway. The motor failure was primarily due to inadequate bearing lubrication. The old grease had deteriorated and hardened, obstructing proper lubrication and causing bearing damage. Shifting of raceways on the MNDE inner race was unclear since all other parameters were within acceptable range. To prevent future occurrences, improving motor preservation during transportation and storage was recommended. This includes safeguarding bearings from grease contamination, ensuring proper sealing and following best practices for lubrication maintenance before commissioning. HP
LITERATURE CITED
Ultrasound, “Ultrasound…Your first line of defense for condition monitoring,” online: https://sdtultrasound.com/using-ultrasound/
Sanjay Kumar is a condition monitoring specialist with more than 14 yrs of experience in refineries, combined-cycle power plants, thermal power plants and liquefied natural gas (LNG) process plants, focusing on the condition monitoring and asset reliability of rotating equipment. He now works at a fertilizer and petrochemical plant in Indorama, West Africa. Kumar’s expertise includes vibration analysis, ultrasound and root cause analysis of rotating equipment, aiming to optimize performance and reduce downtime. Passionate about industrial innovation, he actively share insights on best practices in maintenance and reliability engineering. Kumar has recently completed his Vibration Analysis category-3 certification from the Vibration Institute USA.