This case study is about troubleshooting and diagnosing elevated vibration on the inboard bearing to a Fitchburg feed pump steam turbine at a paper mill. Since its overhaul in Spring 2018, the turbine had been experiencing elevated vibration of 1.5 in/s during operation at or near 5200rpm, resulting in multiple outages to attempt to correct it. At each outage, excessive journal runout was found on the 2.5-inch diameter shaft at this bearing. Analysis with temporarily-mounted velocity probes indicated that the unit appeared to be operating on a structural resonance due to a lack of structural support at the inboard bearing. Changing the rotor did not correct the vibration issue, suggesting it was not due to an unbalance condition. Subsequent troubleshooting involved fixing a mass to the bearing structure, which shifted this resonance down by 300rpm. A stiffening bracket was then manufactured to support the bearing bracket, which shifted the resonance above operating speed. Velocity probe readings indicated vibration levels of 0.15 in/s or less across both bearings at all speeds. A possible explanation for the change in bearing bracket stiffness was the replacement of hard bearing oil drain piping with flexible piping to assist in machine alignment.

Takeaways

1. Recognizing and diagnosing a structural vibration issue on a feed pump turbine;
2. Implementing short-term corrective actions to allow continued operation until long-term corrections can be implemented;
3. Understanding the impact changes to structures can have on the turbine’s operation.

From Pittsburgh, PA and currently in Atlanta, GA. Jamie has been a Steam Turbine Product Service Engineer for GE Power in Atlanta since 2018 and with GE since 2015. Her educational background is in mechanical and nuclear engineering. She obtained her Masters in Mechanical Engineering from the Georgia Institute of Technology in 2018. Prior to joining Product Service, Jamie completed GE’s Edison Engineering Development Program with assignments in steam turbine design and testing, performance, and reliability. She is presently specializing in steam turbine vibration diagnostics and issue troubleshooting.