U-2(g) and Appendix 46 Address ASME Div 1 Gaps

What is the impact of U-2(g) and Appendix 46 on ASME Div 1 Design Gaps?

The 2019 code includes several changes within U-2(g) and the addition of Appendix 46. Appendix 46 allows Div 1 vessels to use Div 2 design methods, with some caveats.

For design by rule, this boils down to using the Div 2 equations with the Div 1 allowable stresses per UG-23; for compressive strength though, the Div 2 values are used instead.

U-2(g) gives preference to Div 2 followed by other pressure vessel standards, like those from Europe, to fill in the gaps for Div 1. This means that when Div 1 rules do not explicitly cover all the aspects of the vessel design, U-2(g) encourages the use of Div 2, per Appendix 46, and then use of other recognized and generally accepted standards like EN 13445 and PD 5500, with allowable stress limitations like those mentioned above.

This effectively eliminates the need for code case 2695.

Related Article: 2020 Pressure Vessel & Heat Exchanger Design Guidelines and Resources

What Design Options in ASME Div 2 are not in Div 1

These options include shallow cones, as well as external loads on cylinders, cones, and spheres. There’s also a wider range of Do/t values for compressive loads and external pressure, and external loads on flanges.

Overall, use of the new Appendix 46 will lead to more favorable designs in many cases.

What are the changes in 2019 Section VIII, Div 1?

Other changes made in Section VIII, Div 1 are worth noting.

Besides the MDMT curve change for SA-105 to a less favorable curve, there is also a curve change for SA/GB 713 Gr Q34R. In this case, it became a more favorable curve.

There’s also new materials added to Table UHA-23, as well as UNF-23.2 and UHT-23. UG-44 has now incorporated code case 2901 with an explicit method for considering external loads on weld neck flanges.

You can compare these changes with the parallel changes in Section II, Part D. There have been additions and deletions to the stress/yield/and ultimate tables, which is a change that happens every two years. Expansions have been implemented to the TE and TM tables, mainly for 7Ni materials and copper alloys. In the Table PRD, there were additions for copper alloys.

If you’re building heat exchangers, there has been a change to the tubesheet shear stress allowable. It now includes a hard limit on yield strength, Sy: Min(0.8S, 0.533Sy). This requirement may require a thicker tube sheet. If you’re still governed by allowable stress, you’ll see no change, but if you’re now governed by yield, there’s a good chance your tube sheet thickness will need to increase.

The result is that you have less options. It’s important to remember the tubesheet thickness is the main variable that affects your tubesheet shear stress.