Thanks umair

It's more clear now.
I think the first time i read that thread i was very much sleeping.

Thanks RK,
I had read nonmendatory app.ASME 31.1, it doesn't say much about closed systems, does it? though it was very informative, for sure.

..also there is a doubt I couldn't clarify
why is:
p3=p1(A1/A3) ......(II-2.2.1)
v3=v1 ......(II-2.2.1)