Identifying and Understanding the Risk of Acoustic-Induced Vibration Failures

Introduction

One source suggests that 10-15% of piping failures are a result of fatigue from vibration.^[1] Piping vibration can be caused by multiple different mechanisms such as flow-induced turbulence, slug flow, mechanical excitation from machinery, and acoustic pulsation. These mechanisms are often observed by plant operators since they present themselves during normal operation. Acoustic-induced vibration (AIV) on the other hand, in pressure relief lines and downstream of control valves used for blow-down (BDV), is difficult to identify since the vibration is not readily observable unless the valves are opened.

AIV occurs in gas systems when acoustic waves generated at flow restrictions, often downstream of a pressure-reducing device (PRD), excite natural modes of the piping, thus leading to vibration. AIV is different from most types of piping vibration because it generally occurs at much higher frequencies (300-1,500 Hz) and is the result of shell mode excitation. For this reason, AIV is generally not visible to the human eye but is noticeable by touch and transmits high-frequency audible noise outside the piping to the surrounding areas. The occasionality, combined with the high-frequency, low-amplitude vibration, allows AIV to usually go unnoticed until the vibration is heard, the pipe is touched, or a failure occurs.  

One common location for AIV is downstream of pressure safety valves (PSVs) or BDVs. Most flare header piping designs are not robustly designed to handle AIV. Flare piping is usually plagued with thin wall piping and fabricated branch connections given the low design pressures. The authors have observed multiple failures in flare system branch connections at under-filled weldolet connections. 

AIV failure is predicted to occur if the calculated internal sound power level of the acoustic noise (PWL) downstream of a PRD exceeds a screening limit derived from comparisons to previous industry failures. The risk of failure in systems that are prone to AIV can be mitigated using operational, design, and monitoring controls. However, in most piping systems, the use of operational and monitoring controls is not practical since the high-frequency/high-cycle nature of AIV often results in rapid failure.

Risk can be mitigated by reviewing the design of flare systems and performing visual inspection to identify such things as under-filled weldolet connections or thin-walled fabricated branch connections. However, the scope of such a project can be a substantial undertaking, especially in existing plants. This article provides insights into the AIV damage mechanism, reviews various approaches to calculate the probability of AIV failure in piping, provides basic design practices to prevent AIV failure, and introduces a risk-based inspection (RBI) approach to evaluate risk of AIV failures for existing flare systems or during the design phase of new projects.