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Tuesday, April 21, 2009

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High frequency acoustic excitation downstream of pressure reducing device and potential of downstream piping failure on Acoustic Induced Vibration (AIV) has raised concern in many plant design. Recent incident such as Insalleh Gas plant shown that the piping failure caused by Vibration (acoustically induced, or flow) is typically occur in flare piping with thin wall, high sound power level and high velocity (Closed or equal to Mach 1). It is also indicated that acoustically or flow induced may occur in INTERMITTENT service. A quick analysis on the previous failure cases conducted and following discusses the common points among all the failure cases.

Typical Failure Location
Earlier discussion "Piping Excitation When Expose to Acoustic Energy" has briefly discussed piping segment may be excited in circumferential and longitudinal mode and images showing piping circumferential and longitudinal excitation patterns in different nodal arrangement. Although entire piping system including pipe, welded part, support,tee, etc, previous experiences have shown that typically (not necessarily only) failure occur at the following locations :
  • Small bore connection (SBC) i.e. instrument tap, bypass line, etc
  • Fabricated Tee
  • Welded Pipe support
  • Internals i.e. attemporator & inline silencer
Above has based on data available in Carucci & Mueller (1982) studies and some private communications. Special attention shall be paid in above locations when carry out critical line analysis. Above area typically a location within a piping system with DISCONTINUITY. Those area with discontinuity shall pay extra attention. Having said that, welded piping may still remain a concerns.

Typical Failure System
Based on analysis of data available in Carucci & Mueller (1982) studies and some private communications, following are typical system exprienced failure :
  • Compressore anti-surge recycle or Capacity control
  • Main Gas Letdown station
  • Gas letdown to Flare
  • Steam desuperheater
  • Small bore connection at large pipe
Thus, special attention shall be paid in above systems when carry out critical line analysis.

High Velocity (Mach no)
Another common syntom among failures is piping experiencing sonic flow or choked flow. Whenever sonic flow or choked flow occured, it is expected that flow in the piping has reached it limit. Extremely high turbulence level impacting the piping and results irregular movement of piping. This potentially lead to high frequency of peak-to-peak stress.

Sonic flow with flow limitation will results pressure upstream of choking location increase. As pressure increases, it generate higher density and lead to breakage of sonic flow at choking location. Higher flow is passing the choking location. As the flow is increases, sonic flow form immediately and flow is decreases. Above will results in a "pulsating flow" in the choking location. This "pulsating flow" will occur in very short period, induce additional stress to the piping and increase likelihood of failure. Read more advices in piping related to Mach no.

Although High mach no is one of the main common point among all failure, previous failure cases includes piping with low mach no (Mach no less than 0.3)). Avoiding high velocity is one of the recommendation however it is not ultimate factor in determining failure of piping due to AIV.


Failure Time & Period
One of the common points is the failure occured during start-up and the failure to occur period is range from 5-10 hours to 2-3 months subject to the sound power level and piping condition. Thus, special attention shall be paid during start-up period.

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posted by Webworm, 6:36 PM

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