This is a machine train consists of gas turbine driving an air-cooled synchronous generator (3600 rpm and 80 MW rated power), which showed increasing vibration levels on the generator bearings after each shutdown/startup event starting October 2020.
A multichannel analyzer was hooked to collect the data during cold startup. Generator bearings showed high casing vibration levels close to alarm setpoints. The review of data during the transient modes (startup/shutdown) and at steady state indicated a clear symptom of couple-unbalance of the generator rotor without significant effect on the turbine bearings.
Onsite balancing activity showed non-linear synchronous rotor response (different influence coefficient between the trial and correction runs), also a repeatability test (two similar runs without changing the balancing weights) on the generator rotor showed non repeatable synchronous response.
Based on the above observations it was recommended to perform a borescope on the generator rotor. The investigation revealed that the root cause of the non-repeatable behavior was the sand ingress from the makeup generator air breather due to lack of proper maintenance. Dry cleaning of the rotor carried out and sand was removed by vacuum, then filters/gaskets replaced with new and healthy ones. Later, machine started with very acceptable vibration levels well below alarm setpoints.
This case study will outline how to identify the nonlinear synchronous rotor response based on the balancing influence vector calculations using the transient polar plots data.