Hello everybody!

I wonder if it is necessary to model a small branch connections like a vent in pipe stress analysis ?

For a static analysis you should consider adding the attachment type just to get an SIF applied in the correct place.

For a dynamic run it might be necessary to include the valve for cyclic loads and proper eigenvector extraction.

Thank you for your reply MoverZ

For vents and drains, you do not need to consider the SIF at its branch connections because these branches are free to move together with the parent pipe, thus not creating any bending stress at branch point. But if this a closed drain system or small condensate line from steam header drip leg near the loop location, you may need to simulate the small line to know the bending leg requirement that will not overstress the branch/olet point.

Fkx: The SIF applies to the header and the branch. Read App. D of ASME B31.3.

The small branch connection may not generate a bending moment but it may affect the strength of the run pipe. Therefore, as MoverZ states, you shouldn't simply ignore the unloaded connection.

Thanks MoverZ and Dave, for pointing out that App.D SIF's should be both applied to the branch and run pipes. I almost forgot because recently I'm working on nozzlepro SIF's where there is different SIF's for branch and run pipes. Just like for 2" branch, I am getting a SIF value of less than 1.0 on run pipe. Is App.D SIF's too conservative for the run pipe?

And yes it's a good practice for beginners to consider all SIF's in a system, unless he can identify and narrow down where the high stress points are before the start of analysis.