Acoustically induced vibration (AIV) is a high-frequency vibration phenomenon that can occur downstream of pressure-reducing devices such as control valves, restriction orifices, and pressure relief or safety valves in compressor piping systems. These vibrations can lead to high cycle fatigue failures of downstream piping at side branches or welded supports. Existing methods for screening and analyzing acoustically induced vibration are not well-grounded in the underlying physics and thus do not provide a methodology for evaluating a variety of mitigation strategies. Modeling of acoustically induced vibration is computationally challenging, as it requires the interaction between tens or hundreds of higher-order acoustic modes with a similar number of piping shell modes.
In order to obtain better insight into the underlying physics of AIV and to characterize the effectiveness of several mitigation methods, full-scale blow-down testing was performed at Southwest Research Institute. Tests were performed using 20 MPa nitrogen gas vented at 28 kg/s through a 3×4” pressure safety valve and multiple header pipe sizes ranging from 12” to 36”. Test configurations included baseline piping geometry at each size and several AIV mitigations including stiffening rings, viscous damping wrap, and internal acoustic mode disruptors. Test results from strain gauges, accelerometers, and dynamic pressure transducers show a broadband multimodal response with dynamic stresses up to 3 kHz near the safety valve tailpipe connection to the test header, and various mitigations reduced dynamic stresses by 8–52% depending on the piping and type of mitigation.
Acoustic and structural finite element models were analyzed in order to determine the coincident modes that match in both axial/circumferential shape and natural frequency and compare coincident frequencies with measure stresses. The results show that observed peak stress frequencies do not generally correlate well with predicted coincident modes, and that flow-induced turbulence excites frequencies below piping shell modes that can also result in significant stresses that combine with AIV.