Effects Of High Pressure Oil Seals On The Rotordynamics Response Of Centrifugal Compressores.

Abstract

The influence of high pressure bushing-type oil seals on the rotordynamic characteristics of a centrifugal gas compressor, which compresses hydrogen from 1700 psi to 1800 psi is discussed. The compressor rotor is driven through a gear coupling by a 10000 hp steam turbine, which has a governor speed range of 7900 cpm to 8850 cpm. This machine has had a long history of exhibiting high synchronous vibration under certain operating conditions. A computer simulation identified that this rotor system can operate as two entirely different systems, which correlates well with field observations. The first system corresponds to the "normal" operating condition, where the bearings support the rotor and the oil seal bushings operate near the "centered" position. The second system corresponds to the condition where the seals carry the rotor weight and unload the bearings. This allows a level of flexibility to be introduced under the bearings, which reduces the effective damping. These rotor support changes shift the second critical speed into the operating speed range, which produces the high synchronous vibrations. In addition, the analysis indicates that the rotor is unstable in the condition where the seals support the rotor and where flexibility is introduced under the unloaded bearings.