We are experiencing high vibration problem at gearbox of 10 MW steam turbine (GE made) during start-up condition. Spectrum is showing sub-synchronous peaks (0.2 X, 0.4 X, 0.6X and 0.8 X) during start-up only which disappear when machine gains full speed. Amplitude of these peaks are up to 28 microns and is directly related to rpms of machine. Never seen anything like this before.
Expert opinion is required please.