Unexpected Rotordynamic Instability In A "Proven" FCC Wet Gas Compressor.
Abstract
The cause and effects of a large subsynchronous vibration (3.5
mils at 3490 cpm) in an FCC wet gas compressor are examined.
The compressor is driven at 7850 rpm by a 6000 hp electric motor,
through a speed-increasing gearbox, and compresses hydrocarbon
gases from 20 psig to 235 psig. A rotordynarnic analysis of the
compressor determined that even though the compressor had a long
history (approximately 25 years) of smooth operation (overall
amplitudes less than 0.5 mils), it had a very low margin of stability.
This lack of stability was caused by a high bearing to shaft stiffness
ratio as well as destabilizing forces in the balance piston and shaft
seals. The pressure dam bearing design is not destabilizing to the
rotor system because the journal is very heavily loaded; however,
the high direct stiffness that is produced reduces the effective
damping of the system. Labyrinth and oil seals are usually not
considered to have large effects on a compressor with such low
pressures. However, the atmospheric side of the oil seal was
determined to contribute the largest destabilizing effect. The
effective length to diameter (LID) ratio of the oil seal was reduced
by 50 percent to lower the amount of cross-coupled stiffness the
seal produces. This modification reduced the subsynchronous
vibration from 3.5 mils to 0.1 mils. The rotordynarnic analysis also
identified other modifications to the bearing and balance piston
design that should be made to increase the rotor's margin of
stability. Both steady-state and transient spectral vibration data are
provided before and after the oil seal modification to validate the
rotordynarnic model of the compressor.
Description
LecturePg. 41-50
Subject
TurbomachinesCollections
Citation
Wilcox, Ed (1999). Unexpected Rotordynamic Instability In A "Proven" FCC Wet Gas Compressor.. Texas A&M University. Turbomachinery Laboratories. Available electronically from https : / /hdl .handle .net /1969 .1 /163396.