Reported in this paper are the field rotor instability and vibration elimination experienced recently on a compressor train installed on an offshore platform for natural gas service. The compressor train consists of a low-pressure compressor and a high-pressure compressor, which is driven by a gas turbine through a gearbox. The compressor train is rated at a maximum continuous speed of 12,054 rpm. During its first commissioning, a high subsynchronous vibration showed up on the high-pressure compressor when it was put on load at full speed. The high-pressure compressor has nine stages, which are arranged back-to-back in two sections. The high-pressure compressor is rated at a discharge pressure of 176.5 bar (2,560 psia). Field vibration data were analyzed and compared to the rotordynamic results from the lateral vibration model of the rotor. The root of the subsynchronous vibration was identified to be the destabilizing aerodynamic excitation generated mainly by the intermediate seal and the impellers. To eliminate the subsynchronous vibration, a gas pocket damper seal was designed specially to replace the existing intermediate labyrinth seal. Meanwhile, the existing tilt-pad bearings were replaced with the deflection pivot bearings to further improve the rotordynamic performance. The compressor was tested with the new center seal and journal bearings at full load, full pressure, and full speed. The subsynchronous vibration was eliminated. Now the compressor train operates smoothly at its design conditions and the vibration readings remain low and stable.

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