Habib,

In my opinion to simplify modelling an in-line pressure balanced expansion joint,if you have all the data for expansion joint , model it as zero length expansion joint , put effective ID equal to zero ( this will make the pressure thrust zero).

In line or the elbow type pressure balanced bellow takes care of the axial pressure thrust load ( like a tied bellow ), but do not offer the high axial rigidity ( comparable with the pipe axial stiffness ) of the tied bellow.

So here you need use the true axial stiffness of the bellow and not the rigid stiffness of the tied bellow.

Regards