Page 22 - Compression Machinery for Oil and Gas

Oil and Gas Compressor Basics Chapter 1 11

compressors since they are not often a concern for reciprocating units due to
their low operating speed and resulting frequency separation between lateral
natural frequencies and excitation frequencies.
Regarding lateral rotordynamics, there are two types of vibration: forced
response (typically from unbalance) and self-excited (usually referred to as sta-
bility). Rotordynamic stability in centrifugal compressors depends on forces in
the bearings, seals, and secondary flow passages that can be stabilizing or desta-
bilizing. The stabilizing forces include the bearing and seal damping which
works to dissipate energy. The destabilizing forces arise from tangential forces
that increase rotor vibration amplitudes. Assessing rotordynamic stability is an
accounting exercise, quantifying the rotordynamic forces of each component on
the stability of the overall rotor system. Destabilizing forces can be represented
by cross-coupled stiffnesses, which will be described in more detail in a later
chapter.
Destabilizing forces in compressors extract energy from the process gas and
rotation to excite a rotor vibration mode (usually the first forward whirling
mode). If these forces grow large enough and overcome the damping forces
in the rotor system, the vibration amplitude of that mode will grow unbounded
until a rotor-stator rub occurs. Vibration monitoring systems will shutdown the
unit after an instability, but usually the vibration amplitude grows so quickly
that seal rubbing will typically result. Further aggravating the situation is that
many operators will restart the compressor and repeat the exercise many times
before they realize they have a serious problem. The damage can go beyond just
replacing the internal labyrinth seals and more serious damage to shafts, impel-
lers, bearings, and diaphragms can result.

References

[1] B.E. Poling, J.M. Prausnitz, J.P. O’Connell, The Properties of Gases and Liquids, McGraw-Hill,
2001.
[2] D. Beinecke, K. Luedtke, Die Auslegung von Turboverdichtern unter Beruecksichtigung des
realen Gasverhaltens, VDI Berichte (1983) 487.
[3] S. Kumar, R. Kurz, J.P. O’Connell, Equations of State for Compressor Design and Testing,
(1999) ASME Paper 99-GT-12.