Acoustic-Induced Vibration Analysis

AIV refers to structural vibration excited by intense acoustic pressure in a piping system with vapor flow. The acoustic pressure is usually created from pressure-reducing devices due to high pressure drops and mass flows of vapor services. These acoustic energies excite the pipe wall circumferentially due to high-frequency sound waves in the range of 500–2,500 Hz, where most of the energy is captured. The circumferential mode of vibration causes the pipe to displace radially, and this leads to fatigue failures where stress concentrations occur downstream, such as at pipe fittings and welded pipe supports.
Common Problem Areas
The AIV study is typically performed for:
⦁ Relief valves
⦁ Blowdown valves
⦁ Restrictive orifice plates
⦁ Pressure reducing valves
⦁ Recycling valves
⦁ Control valves
⦁ High flow rate piping
⦁ Concern over external noise levels

Methodology
At the initial design stage of a new plant or when modifying an existing one, a 2-step preventive approach to resolve such vibration issues is offered. This approach involves a qualitative assessment and prioritisation followed by a qualitative assessment which results in practical recommendations.
Step 1 – Qualitative assessment & prioritisation
Using a rigorous methodology, we carry out a qualitative assessment of all the main lines in a process system to identify the potential excitation mechanisms. We then produce a report prioritising and identifying the potential excitation mechanisms that need to be quantitatively analysed.
Step 2 – Quantitative assessment & Solutions
Main lines: For each excitation mechanism that has been identified as potentially at risk, a quantitative assessment is carried out to determine the likelihood of vibration-induced pipe failure.
The potential excitation mechanisms that can be addressed are:
⦁ Flow induced turbulence
⦁ Mechanical excitation
⦁ Pulsation:
⦁ Reciprocating/Positive displacement pumps and compressors
⦁ Rotating stall
⦁ Flow induced excitation
⦁ High frequency acoustic excitation
⦁ Surge/Momentum change due to valve operation
⦁ Cavitation and Flashing

Results
The analysis helps in assessing pipe vibration proactively, early in the design phase, so that mitigation measures can be developed and implemented before vibrations can cause issues after start-up.