Hello.

A vessel is subjected to external pressure only. How do we calculate the nozzle minimum thickness? Where are the rules mentioned?

If the nozzle requires reinforcement, then the rules are mentioned in UG-37 (c), which says that the reinforcement required for openings in single-walled vessels should be 50% that required for internal pressure.

Thanks a lot.

Sharjeel

Treat the nozzle neck as a cylindrical shell and find the required thickness per rules of UG-28.

As you indicate, UG-37(d) specifies that the area required for external pressure condition is 1/2 of that as given in the formula in UG-37(c). But note that the required thickness of the shell, "tr", to be used for the calculation is that based on the external pressure, thus the actual area required may be more than you may expect. I have seen some people incorrectly determine "tr" based on formula for internal pressure, not external pressure.

what about UG 45b(2)?