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Aiv Calculation - Application Of Sff

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#1 Reliefman


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Posted 22 May 2020 - 09:55 AM

Hi everyone,


I'm hoping to get some advice about AIV assessment as described in the Energy Institute - Guidelines for the Avoidance of Vibration Induced Fatigue Failure in Process Pipework, specifically about the application of the Sonic Flow Factor (SFF) of 6 dB. The description is somewhat vague in the guidelines and I’ve seen different companies apply this factor in different ways, which has a big impact on results.


Is anyone able to clarify the following:

  • If there are multiple choke points in outlet piping, should 6 dB be added for each choke? This is what I believe is done in Aspen Flare System Analyser (Flarenet).
  • Should the SFF only be applied where there is sonic flow at the discontinuity for which LOF is being assessed, or is it applied if there is sonic flow anywhere in the downstream piping? For example, if you are assessing the LOF at a small bore connection and there is no choke at this connection but there is sonic flow upstream and/or downstream of the connection, should the 6 dB be added.

We have some tailpipes where AIV has been found to be an issue and I've been tasked with redesigning them so that there is no longer sonic flow. This would mean the SFF does not need to be applied and the AIV sound power level is decreased to acceptable levels. However, this is proving difficult as there appears to be a choke at the PSV outlet flange, which will not improve if the tailpipe size is increased as planned.


If anyone could provide guidance it would be a great help.

#2 TreyB


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Posted 02 June 2023 - 01:02 PM

Per the Energy Institute feedback, the 6 dB SFF is a local penalty that only applies 10 pipe diameters upstream and downstream of the choke point.

#3 TreyB


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Posted 02 June 2023 - 01:31 PM

Also note that choked relief valves, letdown valves, and restriction orifices do not get the 6 dB SFF, per the Energy Institute feedback.

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