Thanks Samhuff for your reply,
Yes, that is my mistake, Ai is inner area of pipe, I will change it,
I had a situation is to model victaulic flexible coupling piping system, pipe directional anchor with 3 mmm gap each side, when flexible coupling fully opened by pressur up, the coupling will hold pipe axial forece to resist expansion,
if I can make sure, flexible coupling had been fully opened before directional anchore is working, and flexible couple and groove of pipe had took pressure thrust forece,
Does nozzle is only taken force F=P.Ai-v.P.Di/2t x As (from autopipe wiki) without considering pressure eara?
for expansion joint case, I was using Caesar II, it has option to define tie- rod type or others, when Tie-rod type had been selected, pressure area will automaticly set as 0.