There are three methods for calculating the strength of reinforcement required for openings in pressure vessels:
1. Area replacement rules per UG-36(b).
2. Analysis per Appendix 1-7. a. 2/3 area replacement rule. b. Membrane-bending stress analysis.
3. FEA. The Code defines when and where these methods apply.
Reinforcement for large-diameter openings has been in the Code for a long time. The previous rule was simply to move the majority of the area replacement closer to the nozzle neck, also called the 2/3 rule. Unfortunately, there were a few cases of flange leakage where the flange was located close to the shell. It was discovered that as the opening opened up, the flange was distorted. It was actually bending. In addition, the 2/3 rule did not allow for an accurate way to determine MAWP for the vessel without proof testing. This issue was addressed in 1979 by McBride and Jacobs. The principle was to calculate stresses in two distinct areas, membrane and bending. Membrane stresses are based on pressure area times metal area. Bending is based on AISC beam formulas. The neck-and-shell section (and sometimes the flange as well) is assumed as bent on the hard axis. This is not a beam-on-elasticfoundation calculation. It is more of a brute-force approach.