Low frequency piping vibration was experienced during the startup of a large natural gas compression facility. Extensive laboratory testing, field measurements, and analysis have shown that the cause is a novel form of acousto-hydraulic excitation. The anti-surge valve was the source of the pulsation that was amplified by the piping system. The frequency of the pulsation was determined by piping acoustic modes. The strength of the hydraulic instability was found to be highly dependent on the valve’s stroke.

Traditionally the turbulence generated downstream of throttling valves is considered to be a broadband source of pulsation. Special noise-abatement trims are often used to control the amplitude and spectral content of this excitation. For this phenomenon, the hydraulic instability occurs upstream of the valve’s primary restriction and the upstream piping acoustics determine the pulsation frequencies and amplification. In this particular field system, high vibrational response results from a mechanical resonance of the piping structure coincident with the acoustic pulsation frequency. Ultimately a valve plug design change eliminated the formation of the hydraulic instability and piping vibration.

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