Acoustic Induced Vibration (AIV) Fatigue

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An oil & gas processing and production facilities may consist ofseveral level of pressure let down station. Typical examples arechoke valve between christmas tree and flowline/production header,slugcatcher control valve, onshore pressure reduction station, steamcontrol valve, desuperheating station, etc. Above valves are normallyin continuous operation. Besides, there are other valves such ascompressor surge / capacity control valve, overpressure dumpingcontrol valve, blowdown valve with restriction orifice, pressure reliefvalve, etc will experience large pressure drop and they are operatedintermittently.

There is pressure drop with mass passing through the valve,internal acoustic energy is generated and transmitted to downstream piping andpotentially lead to severe piping excitation, vibration and stresses on downstream piping,in particular at discontinuity section i.e fabricated Tee, small bore connection, welded pipeand pipe support, etc. If the downstream piping system is not properly designed, effect ofthis acoustic excitation would lead to fatigue failure. Above acoustic excitation phenomena isalso known as Acoustic Induced Vibration Fatigue. Generally high frequency (morethan 1000Hz) acoustic energy contribute to AIV fatigue and once piping expose to AIV,piping can fail in very short period (possibly in several minutes to hours).

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Fluid PhaseGenerally Acoustic Induced Vibration (AIV) fatigue occur in gaseous system and normally AIVdoes not occur in liquid system. For two phase gas liquid flow system with high gas flow (i.e.more than 50% gas flow), AIV may starts to be problem and need to be investigated. As twophase flow is complicated in estimating the Acoustic energy, it is always conservative toconsider 100% gas flow.

Short & Long Term OperationAIV fatigue failure of piping downstream of piping is subject to operation time i.e. number offatigue cycle. A piping in continuous (long term) operation, a design fatigue limit of 80MPa isnormally used based on ASME fatigue design limits with suitable safety factors for long termsevice life. A fatigue limit of 185MPa (10^7 cycles) has been used for short-term operationbased on published fatigue life data for carbon steel and stainless steel. This fatigue limitrepresents the maximum acceptable level of stress for 10^7 cycles (12-24 hours ofoperation) without any safety margin. Those pressure reduction devices has totalaccumulated service hour within the plant life less than 12 hours, one may considerAIV fatigue may not occur. Pressure relief valve is one of those devices potentially drop inthis category.

Experiences has shown that there is high frequency of operation of pressure relief deviceduring plant black start-up and restart-up, AIV fatigue may occur in very short period(minutes to hours), PRV and other pressure reducing device may connect on samedownstream pipe, etc, in many event AIV studies for PRV downstream piping to beconducted. Those PRV relief to ATM has very high potential drop in the short term category.

Sound Power Level (PWL)Sound Power Level (PWL) is the acoustic energy generated by a pressure reduction device.There are several ways to assess the adequacy of the piping to resist AIV fatigue. One ofthe way to ensure piping downstream of pressure reducing device sufficiently storng to resistAIV fatigue is to ensure the PWL allowable limit of downstream piping higher than the PWLgenerated by the pressure reducing device. This will be disucssed in coming post.

Piping Excitation When Expose to Acoustic EnergySound Power Level (PWL) Prediction from AIV AspectSeveral Criteria and Constraints for Flare Network - PipingCheck Valve Types and SelectionPotential Problem associate with Double NRV in Series within a LineFlow Element (FE) Upstream or Downstream of Control Valve (CV) ?Why a globe valve is located downstream of manual block valve on drain line?

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Labels: AIV, Flare, Piping

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Acoustic Induced Vibration (AIV) Fatigue