Dead right John.!!!!
If you search, you will find that most of the questions to this have been asked by myself!!!!!!!
I default to the worst case i can possibly project onto the pipe and work from there.
But for a closed system, i insist the branch to header ratio is high, so that the "slug" discharges into an effectivly "open vessel"
I then treat the relief event as firing a shell out of a cannon.
Lots of actions and reactions, canceling out or not as the case may be.
It will pay to:
A) have help from somebody who is competant
B) Have the philosophy agreed by client and your design team
I don't believe there is a right answer, but there is a wrong one!!
(and i'm not being smart)
Dynamic analysis might help if things get pretty wild,
Tim
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Best Regards