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An oil & gas processing and production facilities may consist of several level of pressure let down station. Typical examples are choke valve between christmas tree and flowline/production header, slugcatcher control valve, onshore pressure reduction station, steam control valve, desuperheating station, etc. Above valves are normally in continuous operation. Besides, there are other valves such as compressor surge / capacity control valve, overpressure dumping control valve, blowdown valve with restriction orifice, pressure relief valve, etc will experience large pressure drop and they are operated intermittently.

There is pressure drop with mass passing through the valve, internal acoustic energy is generated and transmitted to downstream piping and potentially lead to severe piping excitation, vibration and stresses on downstream piping, in particular at discontinuity section i.e fabricated Tee, small bore connection, welded pipe and pipe support, etc. If the downstream piping system is not properly designed, effect of this acoustic excitation would lead to fatigue failure. Above acoustic excitation phenomena is also known as Acoustic Induced Vibration Fatigue. Generally high frequency (more than 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).


Fluid Phase
Generally Acoustic Induced Vibration (AIV) fatigue occur in gaseous system and normally AIV does 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 two phase flow is complicated in estimating the Acoustic energy, it is always conservative to consider 100% gas flow.

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

Experiences has shown that there is high frequency of operation of pressure relief device during 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 same downstream pipe, etc, in many event AIV studies for PRV downstream piping to be conducted. 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 of the way to ensure piping downstream of pressure reducing device sufficiently storng to resist AIV fatigue is to ensure the PWL allowable limit of downstream piping higher than the PWL generated by the pressure reducing device. This will be disucssed in coming post.

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posted by Webworm, 6:22 AM

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