Fatigue Assessment of a Damaged Anti-Surge Valve – Root Cause Analysis and Vibration Mitigation Measures

Abstract At an Equinor operated offshore production platform the Anti Surge Valve (ASV) of the gas export compressor suffered damage to the internals of the valve. One of the main issues was a crack in the weld between the plug and stem, which could be caused by excessive vibrations during startup of the compressor or when running in recycle mode. Following the observed damage a root cause analysis (RCA) was initiated. This RCA started with a systematic measurement campaign, consisting of pulsation and vibration measurements during the startup cycle of the compressor, i.e. for an ASV that closes from 100 to 0% opening. From these measurements it was concluded that very high vibration levels were present at the valve stem, for specific opening ASV positions. This could be the reason for the damage to the valve internals. The frequency of these vibrations matched well with peaks in the measured pulsations, suggesting that there is an acoustic and/or mechanical resonance that could cause the vibration amplification. Hammer impact testing under both atmospheric and pressurized conditions confirmed that indeed a mechanical resonance was present that matched with the dominant frequency of the vibrations. This led to a preliminary conclusion that a mechanical resonance is excited by an acoustic source in the ASV and/or flow line, causing the high vibration levels at specific opening positions of the ASV. As a next step a 1D acoustic model of the compressor suction, discharge and recycle line was made. It showed that in the critical frequency range multiple acoustic sources and resonances present in the lines connected to the ASV, but the sources are generally broadband in nature and the resonances weak in overall amplitude. Therefore a more detailed 3D acoustic model was made of the valve internals and directly connected recycle piping. It showed that for the critical opening positions a clear acoustic resonance was present in the valve and directly connected recycle piping, which causes high dynamic forces at the plug. In parallel to the acoustic assessment, a mechanical model of the valve internals was also made. The measured vibration levels at the stem were used to scale the modeled results. This resulted in fatigue stress levels that could indeed cause damage of the welded connection. Based on this RCA the valve was modified, such that no acoustic and mechanical resonance was present anymore in operating range of the valve. Moreover, the acoustic and mechanical resonance were separated from each other with sufficient margin. Verification measurements after these modifications showed that the vibrations indeed reduced significantly, and no further damage occurred on the plug-stem connection.