I think it best to calculate a state of stress with a set of forces and moments that work together. I am not certain that calculating stress using a group of maximum load components from several states is valid. The interaction of terms will complicate this. For example, start with a nozzle with a moment load. That moment will cause a local stress on the vessel wall at the nozzle intersection. Now consider the shear load on the nozzle flange. Depending on its direction, that shear load may add to or reduce that local stress due to bending.
If moment arms are significant, signs are important. And combining maxima ignores the significance of signs.
That said, with occasional loads it may be possible to think up additional local load combinations for your nozzle/attachment.
I would work out extreme sets of occasional loads (Fx,y,x & Mx,y,z)and keep these numbers together when calculating local stress.