An Overview of ASME Section VIII, Division 1, Appendix 46
Division 1 of Section VIII of the ASME Boiler & Pressure Vessel Code does not contain rules to cover all details of design and construction of pressure vessels. Where design rules do not exist, U-2(g) permits several courses of action, one of which is to use Mandatory Appendix 46. Appendix 46 provides the requirements for using Section VIII, Division 2 (Alternate Rules) in the design of Division 1 pressure vessels. This article provides a general overview of Appendix 46 from the 2019 Edition of the ASME Boiler & Pressure Vessel Code. It does not address all of the requirements of Appendix 46. Appendix 46 of the applicable Code edition should be consulted for complete information.
Mandatory Appendix 46 in Section VIII, Division 1 permits the use of Division 2 to determine thickness and other design details of a component for a Section VIII, Division 1 pressure vessel, but with certain restrictions and conditions. This is largely because the Alternate Rules of Section VIII, Division 2 have their own material, design, and fabrication requirements. Since the Division 1 vessel will not meet all of these Division 2 requirements, there are restrictions in using Division 2 in the design of Division 1 pressure vessels.
There are two design methods in ASME Section VIII, Division 2: Design by Rule, and Design by Analysis. The Design by Rule section, Part 4 of Division 2, provides formulae for designing pressure vessel components. Design by Analysis, Part 5 of Division 2, provides requirements for designing pressure vessels using numerical analysis such as finite element analysis (FEA).
The restrictions and conditions in using Division 2 in the design of Division 1 pressure vessels can be separated into five categories:
Allowable Stresses
Design Thickness and Weld Joint Efficiency
Design Loads and Load Case Combinations
Weld Joint Details
Fabrication Tolerances
Allowable Stresses
As stated above, Division 2 has its own material, design, and fabrication requirements. This includes material allowable stress values. Sometimes the same material will have different allowable stress limits depending on whether it is designed and fabricated in accordance with Division 1 or Division 2. The use of Appendix 46 in the design of Division 1 pressure vessels requires that while Division 2 is being used for design, the allowable stress limits are based on those from UG-23 in Section VIII, Division 1. Two notable exceptions to this are the allowable compressive stress and the allowable stress for wind and seismic loading. When using Design by Rule, the allowable compressive stress shall be determined using Part 4.4.12 in Division 2 in lieu of UG-23 in Division 1. Additionally, the 1.2 factor increase in allowable stress for wind and seismic loading in UG-23 is not permitted.
Design Thickness and Weld Joint Efficiency
The design thickness of pressure vessel components may be calculated using Division 1, or Division 2, Part 4 (Design by Rule). The design thickness, however, cannot be reduced using Division 2, Part 5 (Design by Analysis).
If Division 2, Part 4.3 or 4.4 is used to determine the design thickness of a shell or formed head, nozzles penetrating these components, and their reinforcement, shall be designed in accordance with Division 2, Part 4.5. Additionally, for conical transitions, each component comprising the cylinder-to-cone junctions must be designed in accordance with Division 2, Parts 4.3 or 4.4.
The weld joint efficiency used in Division 2, Design by Rule (Part 4) shall be established in accordance with Section VIII, Division 1, UW-11 and UW-12. When using Division 2 Design by Analysis, the weld joint efficiency is established using the full radiography requirements of UW-11 and UW-12.
Design Loads and Load Case Combinations
When Division 2, Part 4 (Design by Rule) is used in the design of pressure vessel components, the full set of load case combinations listed in Division 2, 4.1.5.3 are not required except when necessary to satisfy Section VIII, Division 1, UG-22. The fatigue screening criterion of Division 2, 4.1.1.4 is not required. However, it may be used when evaluation of cyclic loading is required by UG-22.
When Division 2, Design by Analysis is used, all of the failure modes listed in Division 2, Part 5 shall be considered. All of the load case combinations of Division 2, Part 5 must also be considered. In evaluating for Protection Against Collapse from Buckling, the design margin of the Division 2 assessment procedure shall be used, and when evaluating for Protection Against Failure from Cyclic Loading, the design margin from Division 2 shall be used except the allowable stress shall be from Division 1, UG-23.
Weld Joint Details
Section VIII, Division 1 provides acceptable weld joint details. Likewise, Section VIII, Division 2 also provides acceptable weld joint details. When using Division 2 Design by Rule for the design of Division 1 pressure vessels, weld joint details shall be in accordance with Division 2, Part 4.2, with the exception of Category E welds.
Division 2, Design by Analysis allows for exemptions of evaluating for Protection Against Local Failure if weld details are one of those shown in Division 2, Part 4. This exemption does not apply to weld details from Division 1, only to those details shown in Division 2, Part 4.
Fabrication Tolerances
Fabrication tolerances shall be in accordance with Division 2, 4.3.2 and 4.4.4, as applicable. Provisions provided in Division 2, 4.14, however, are not permitted.
In closing, when design rules do not exist in Section VIII, Division 1, Mandatory Appendix 46 provides the requirements for using Section VIII, Division 2 to design Division 1 pressure vessel components. While this article provides an overview of these requirements, Appendix 46 and the applicable Code edition should be consulted for complete details.