Determining The Root Causes Of Subsynchronous Instability Problems In Two Centrifugal Compressors.
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This paper discusses the rotordynamic instability problems experienced with two separate centrifugal compressors. While the root causes of the instabilities are very different, the analysis methodology of reconciling the rotordynamic model with measured vibration data was the same. The first problem occurred with a propylene compressor in a Gulf Coast chemical plant that had experienced high journal bearing temperatures for several years. A modified bearing was installed to alleviate the temperature problem; however, a large subsynchronous vibration appeared after the new bearings were installed. A lateral stability analysis shows that the compressor with the modified bearings was very stable with the aerodynamic destabilizing effect predicted by the Alford and/or Wachel equation. A comparison analysis was made of the stability predicted with the original bearings (which were stable) as well as the modified bearings (which were not). This allowed the user to determine the magnitude of the destabilizing forces present in the compressor and design a new bearing that was both stable and would operate at an acceptable temperature. The new bearing was installed and the compressor has operated without the subsynchronous vibration for the past year and a half. The second problem occurred with a very similar ethylene compressor in a Midwest ethylene plant. The compressor had operated for over two years after an overhaul with low vibration. Then a subsynchronous vibration appeared that was very erratic, but was slowly increasing in amplitude over time. To solve the problem, a rotordynamic analysis was performed that suggested that replacing the bearing would solve the stability problem However, comparison between the measure field vibration and the rotordynamic model did not agree on all points. A more indepth look at the compressor revealed that the increase in subsynchronous vibration was tracking very closely with the balance line differential pressure. This fact, along with the characteristics of the balance piston seal, suggested that a bearing change alone may not completely address the problem A new balance piston seal was designed to reduce its destabilizing effects on the rotor. The compressor was inspected during the next scheduled downtime to determine the cause of the high vibration and install the new balance piston seal and bearings. Examination of the internals revealed a large seal rub in the compressor, but at the dry gas seals, not the balance piston. The rub was addressed and the subsynchronous vibration was eliminated. While the exact source of the destabilizing force was not correctly “guessed” before disassembly, the indepth rotordynamic analysis did reveal that there was a large destabilizing force in the compressor, and a bearing change alone would not eliminate the vibration. The compressor has been operating without the subsynchronous problem for the past year since the modification.
Wilcox, Ed; O'Brien, Dave P. (2003). Determining The Root Causes Of Subsynchronous Instability Problems In Two Centrifugal Compressors.. Texas A&M University. Turbomachinery Laboratories. Available electronically from