This case is related to a gearbox that consists of a bullgear and three rotors. Each rotor consists of a pinion with overhung impellers mounted at each end. So, this Integrally Gear Compressor is running in normal operation within acceptable and stable vibration levels but suddenly appears some sub synchronous vibration (1- 2 Hz) along with another sub synchronous activity that produce some vibration peaks that is hitting the alarm setpoint everyday. Peaks occur only at the 1 stg.
All instrumentaion system and loop related to the sensor of 1 stg was already replaced, inlcudin the probe. The compressor has only one sensor per stage, and the another probe at the end of the shaft is not vibrating.
Is it possible any instability that could be related to this problem at a very low frequency (1- 2 Hz)??????????????