A GE A-10 steam turbine returning from a major outage had vibration amplitudes within ISO 20816-2 Zone B, at all but the number one bearing. The turbine owner requested balancing for this bearing, which was reading just over 4 mils (76.2 microns) overall. However, while calculating a position for the balance weights the balancing engineer noticed a subsynchronous vibration frequency that was 1 mil (25.4 microns) in amplitude at the number one bearing. He did not expect oil whirl on this 16-inch 4-pad double tilt pad load on pad bearing. Nevertheless, he recommended performing a number of inspections instead of balancing the rotor. The team at site completed his list of inspections in the LP of the steam turbine, including opening the number one bearing, measuring bearing load, checking for piping leaks, and measuring the oil supply orifice in the piping. The oil supply orifice was found to be smaller than recommended by GE design engineering for this type of steam turbine, and was therefore enlarged to meet the design requirement. Bearing loading was also found below its expected value, but was not changed since a large change in elevation was needed to achieve bearing loading at this location. When the turbine restarted, subsynchronous vibration was gone and vibration amplitudes were below 3 mils and acceptable for continued operation. The cause was found to be oil wedge instability on the lower pads of the tilt pad bearing due to insufficient oil flow. Although the turbine had operated 14 years with an oil orifice below the recommended diameter without problems, following changes and repairs made during a major outage that included removal and re-installation of the rotor, instability appeared on the tilt pad bearing and was rectified with a simple fix.
1. Tilt pad bearings, typically portrayed as being immune to instability, can go unstable when insufficient oil is supplied to develop a wedge.
2. Bearing instability is not always the result of insufficient loading.
3. Just because a machine has run well for a number of years, you should not assume that the configuration is correct (e.g. bearing size, orifice size, alignment, etc.)
Bio - Edward Michel
Raised in New Canaan, Connecticut. 20 years working for GE Power in Atlanta after graduating as a Mechanical Engineer from Georgia Tech. Currently balancing turbines and generators, resolving technical issues, and leading root cause analyses in Steam Turbine Product Service. My first four years at GE were in the Monitoring and Diagnostics Center, monitoring the GE fleet and diagnosing trips, and writing monthly customer reports. The next four years I handled compressor and vibration issues in the fleet as part of Gas Turbine Product Service. Then I spent two years as a Six Sigma Black Belt, improving processes for updating drawings, resolving customer issues, and enhancing technical depth. And before my current position I traveled the Southeast US for two years as a Machinery Diagnostics Engineer at Bently Nevada, a GE Company.