Abstract

The integrity of compressors and pumps is of paramount importance for the gas and oil industry. Failures may result in serious production losses that are in no proportion with the cost of the equipment involved. Besides, the equipment may be inaccessible for maintenance for a long period of time due to unfavourable weather conditions.

The requirements of compressors with respect to pressures, capacity, power and operating range, are increasing. The answer to this is innovative compressor concepts, advanced materials and manufacturing techniques. Unexpected phenomena, however, for instance due to fluid structure interactions, will occur that can lead to serious damage of the machine. In this paper such a phenomenon is discussed by presenting a case study.

On the NAM L9 platform, a centrifugal compressor was operated for the transport of gas to shore. The compressor showed serious vibrations at certain conditions that finally led to failure. An analysis of the vibration measurements showed that extremely large vibrations could occur at the blade passing frequency. By means of modeling, an analysis could be made of the acoustic resonances that could possibly occur in the compressor. It was found that a likely cause of the failure was a resonance mode in the shroud cavity of the 3rd and 4th stage. The acoustic resonance modes could be excited by pulsation sources present in the machine. Especially, the sources at the low solidity vanes were suspected to contribute to the excitation of the resonance. Vortex shedding at the impeller edge was also suspected to contribute. The resonance in the cavity may exert large dynamic forces near the trailing edge of the impeller shroud. Due to a structural vibration modes of the shroud, a fatigue failure occurred.

The paper describes the root cause analysis consisting of an analysis of the acoustic and structural vibration modes. The consequences for the design of similar compressors are subsequently reviewed. A future outlook is presented concerning dense gas compression systems for CO2 and high pressure reinjection.

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