What is Acoustic-Induced Vibration or AIV?
Acoustic-Induced Vibration or AIV is a severe high-frequency vibration that the piping systems carrying vapor or gases may experience near the high pressure reducing devices. Due to high-pressure drops of vapor/gas services in this pressure-reducing device like a relief valve, orifice plate, control valve, depressuring valves, Choke Valve, Blow Down valve, etc a high-frequency sound wave in the range of 500–2,000 Hz is generated. This wave energy induces vibration (and stress) and excites the pipe wall in the circumferential direction causing radial pipe displacement and eventually acoustic fatigue failure within a very short period of time (Few minutes or hours). Piping components with high-stress concentration zones like pipe fittings, small-bore connections, Fabricated tee, welded pipe supports, etc are prone to such failures.
Since the high viscosity of liquid or two-phase fluid dampens the circumferential pipe displacements, AIV is not a concern for such systems.
Cause of Acoustic Induced Vibration
So, the main cause of AIV is
- high-pressure drops and
- high flow rates in vapor/gas services
Mechanism of AIV
In AIV, the high-velocity fluid impingement on the piping wall, turbulent mixing, and shockwaves downstream of the flow restriction give rise to a high level of noise. This noise level is a function of pressure drop across the pressure reducing device and gas/vapor mass flow rate. The noise is transmitted downstream of the flow restriction losing energy to friction, work is done by vibrating the pipe, and heat lost to surroundings. A noise or sound level of 155 dB is considered a safe level when the circumferential vibration is no longer a concern. Industry standards and experience show acoustic energy attenuates 3 dB for every 50D of piping from the source. The response caused by high-frequency acoustic excitation affects the piping downstream of the source to the first major vessel, i.e, Separator, KO drum, etc.
Screening for Acoustic Induced Vibration
Design Engineering Practice (DEP) by Shell Global Inc provides rules for screening the systems for AIV. To study the effect of AIV on the piping system it is required to calculate sound power level at the concerned pressure-reducing device based on the following equation:
- P1 is upstream pressure (bara)
- P2 is downstream pressure (bara)
- W is flow rate (kg/s)
- T is the upstream temperature (K)
- Mw is molecular weight (grams/mol)
- SFF is a correction factor to account for multiple occurrences of sonic flow in a line. If consecutive sonic conditions exist, then SFF=6; otherwise SFF = 0.
All the above-mentioned data can be received from the Process engineering team. If the calculated Sound power level is less than (or equal to) 155 dB, then there is no concern from AIV viewpoint. However, if the calculated Sound power Level is more than 155 dB, then the LOF (Likelihood Of Failure) value needs to be calculated following steps provided by Energy Institute guidelines for the avoidance of vibration induced fatigue failure in process pipework.
Mitigation of AIV
Considerations for AIV mitigation must be done during the design stage of the project as failures in AIV can happen within minutes of operation. There are various options for AIV mitigation. However, the following options are the most common:
- Using a higher pipe schedule or lowering the D/t ratio.
- Decreasing flow velocity by increasing pipe diameter.
- Using smoother pipe fittings which ensures a smooth transition from branch to the main header.
- Using Clamp-on supports and stiffening rings.
- Using a full wrap-around pad on welded pipe supports. Full wrap-around is more commonly accepted in industry standards than partial reinforcing.
- Using Multi-Stage Pressure Drop Internal Trim (Low noise trim) to reduce noise levels at the valve.
- Using Acoustic Silencer.
- Increasing line length between AIV source and high-risk locations
Comparison of FIV and AIV
Some more Resources for You…
References and Further Study
- Energy Institute guidelines for the avoidance of vibration induced fatigue failure in process pipework
- DEP 31.38.01.26 V43