Hi Uzair

If I understand you correctly, you try to model tied expansion bellow with nut only outside of tie rod not inside of tie rod. You are expecting it will take care thrust force as well as thermal contraction of bellow.
However it will not work like that way.

Say bellow contract due to thermal expansion then tie rod will be ineffective & you will find pressure thrust in both end of bellow.

If you are using untied expansion bellow you need to consider only annular portion* pressure to calculate pressure thrust on nozzle, rest portion of pressure thrust will directly go inside equipment.

Regards

Habib