Retrofit Of Gas Lubricated Face Seals In A Centrifugal Compressor.
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There are significant advantages in using gas lubricated face seals (dry gas seals) in centrifugal compressor service. Foremost among these are the elimination of the seal oil system resulting in lower maintenance, increased safety, and higher operating availability. For these reasons, one of four identical compressors at an installation having severe problems with seal oil contamination was selected for trial conversion to dry gas seals. The rotordynamic engineering portion of this job was done by the compressor manufacturer in conjunction with the gas seal supplier. The seal assembly was designed so that it would be essentially a drop-in conversion from a mechanical standpoint. Rotordynamic studies indicated that the conversion would result in a "better" machine. Unfortunately, sustained operations were not possible, due to excessive vibration levels at startup with the new seals. The shaft vibration exceeded 0.007 in, peak-to-peak, at a subsynchronous frequency of 4900 rpm (the machine rated speed is in excess of 10,000 rpm). These levels were sufficiently high to cause extensive damage to all internal labyrinths. The midspan labyrinths were wiped open in excess of 0.060 in, radial. Analysis of tape recorded data indicated that the vibration was due to a rotor/bearing system dynamic instability. Additional computer simulations of the compressor rotordynamics revealed that the oil seals had provided sufficient damping to the system to bound the instability. This extra damping was not being provided by the gas seals. Bearing redesign to increase stability and realignment of the rotor within the bundle to remove suspected excitation appear to have eliminated the problem.
Kocur, John A.; Platt, John P.; Shabi, Larry G. (1987). Retrofit Of Gas Lubricated Face Seals In A Centrifugal Compressor.. Texas A&M University. Turbomachinery Laboratories. Available electronically from