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Centrifugal Compressor Seal and Bearing Rubbing Due to a Destructive Repeatable Reverse Rotation


 This case study is focused on the diagnostics of an abnormal event observed during each compressor train shutdown and led to a seal failure later through a detailed analysis of historical transient data collected using an online vibration online monitoring software during shutdown mode to identify the root cause of this abnormal behavior.
 The unit consists of a centrifugal gas compressor (12440 RPM) driven by an induction electrical motor (3100 KW and 1790 RPM) through an increasing single stage double helical gearbox.
 Machine train was running smoothly during steady state condition. Compressor bearings showed high vibration levels during each shutdown event close to alarm setpoints during last year, however no one took care as it was happening only during shutdown. The unit was compressing a hazardous gas, and any failure or leak could lead to a major safety incident.
 Steady state data review showed good compressor rotor dynamics with acceptable vibration levels well below alarm set points, however during shutdown showed high vibration levels reaching alarm setpoints during shutdown. Review of the vibration and speed data through the online condition monitoring software showed symptoms of reverse rotation event repeating each shutdown after reaching zero speed.
 After 3 month of operation and several shutdown events encountered, a gas leak took place from the compressor. During the internal inspection, severe rubbing marks found in the compressor seals and the rotor found damaged at these areas. Also, the compressor inboard bearing found totally damaged. Major overhaul took place changing the rotor and overhauling the compressor parts. The non-return valve installed on the discharge was dismantled and overhauled.
 Compressor started up later and during the next shutdown the compressor bearings showed normal vibration behavior, and no reverse rotation events observed.