Hello MAHDIKHANI,
I hope the following helps, althoug I realize it is not much:

> I registered recently in COADE FORUM,and i was searching the topics of
> this forum.in your answer you mentioned youself as an stress analyst who
I am not a stress engineer, I am a pulsation engineer.

> doing dynamic analysis for 25 years.honestly, most of stress analysts
> who i've met have no enough experience in dynamic analysis,and because
> of that they try to avoid dynamic analysis,but i am so interested to
> learn more about dynamic analysis and to apply it on some cases which
> must be considered in dynamic conditions.

> a few days ago we recieved a letter from process department that wanted
> the stress section to consider dynamic effects(vibration) on some lines.
>
> I'll explain the conditions of that line and I hope to recieve your
> favour to me.

> there is a steam header with a branch.a valve is located near to
> intersection of branch and header.when the valve become open the hot
> steam comes from header into the branch and because of much temperature
> differntial between header and branch,there is some condenses at
> branch.the process department want to be considered the effect of this
> condense on dynamic behaviour of piping.
I have little experience with that. I can only think of two things: sound velocity will change which should be considered. And, when there is sufficient liquid, 2-phase flow patterns may occur, resulting in a different acoustic behaviour as well.
This really needs attention from a specialist. On the other hand, is there really a problem? Or do they expect a problem? One can perform many types of calculations but they are all waste of time when there is no problem at all.
> I read and search about this but I couldn't find enough answers.I though
> I should consider acoustic analysis about that.and also existance of
> condense in steam will produce a condition like to 2 phase fluid.please
> help me about this problem.