One thing to keep in mind - your travel stops need to be in place while the flange is disconnected. It is true that the operating load of your spring needs to be sized for the operating state, but you don't want to try to design a spring to carry a piping load with no movement when one of the end points of the system is missing.

You should be able to run a case, replacing your spring with a hard support (or actually just defining it as rigid stiffness), with the flange disconnected and see what your displacments are. If your system is properly supported, you should have very little displacement at the flange.

While it's true that it is impossible to get "zero" load at a flange in any condition, the piping system should be well supported so that the flange isn't sagging or listing wildly while the millwright's are trying to get the system lined up.