You should specify the effective ID. However, in the absence of tie-rods, the high axial load you're seeing at the nozzle in CAESAR II isn't entirely applied to the nozzle in reality. Only a small faction of it is applied to the nozzle, while the rest is actually applied to the equipment anchor and your pipe main anchor. The force on the nozzle due to pressure is
Fnozzle=P(Ae - Ai),
where Ae is the effective area of the bellows and Ai is the inside area of the pipe.

For your reference, EJMA handbook has an example on this subject.