r600gtr-
Use UG-27 for the thickness required for internal pressure. As pointed out in UG-22, however, other loadings may, and typically do, exist. These additional loadings need to be considered also. You'll develop a feel for what will be a significant load in your industry. In my case, smaller nozzles will most frequently be governed by UG-45(b)(4). It would take some pretty hefty piping loads to start to govern the nozzle neck wall thickness (local loads at the shell may be a different story).
Take a close look at Appendix L-7.
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