Hi,

First of all, the tie rods are connected to the expansion joints flanges not to the pump Nozzle, that's how you have to model your tie rods because the pressure thrust force is transferred to the expansion joint flange before passing to the pump nozzle.

I think that the pressure thrust forces applied on the expansion joints flanges is greater then the nozzle and pipe forces applied on the same flanges thats why when you use tie rods, the rods absorbs the pressure thrust effect until this forces becomes equilibrated.

This is my personnel analysis, may be someone else have a better explication of this issue.

Regards,