ASME B&PV Design by Analysis - When Can It Be Used?
Historically, pressure vessels designed in accordance with the ASME Boiler & Pressure Vessel Code have been largely done so using formulae provided within Section VIII, Division 1. However, with the increasing prevalence of finite element analysis (FEA) as a design tool, and its ability to produce more accurate results than traditional formulae, an engineer may be inclined to want to design a pressure vessel using FEA rather than formulae. But when is the use of finite element analysis permitted by the ASME Code in pressure vessel design? And what are the limitations imposed by the Code on its use?
ASME Section VIII Divisions
The rules for the design and construction of pressure vessels are provided in Section VIII of the ASME Boiler & Pressure Vessel Code. Section VIII contains three divisions. Each of these divisions contain requirements pertaining to materials, design, fabrication, inspection, and testing of pressure vessels. In general, Division 1 pertains to pressure vessels designed using formulae, Division 2 contains requirements for designing pressure vessels using numerical analysis, and Division 3 is used in the design of vessels subjected to high pressure (10,000+ psig).
However, to state that Division 1 is “Design by Rule” and Division 2 is “Design by Analysis” is not entirely correct. Division 2 is divided in several parts, one of which (Part 5) contains the requirements for pressure vessel “Design by Analysis”, and another (Part 4) that provides formulae for “Design by Rule”.
While there are many identical requirements between Section VIII, Division 2, Part 4 (Design by Rule) and Section VIII, Division 1, there are differences. Hence, Division 2 is given the title of “Alternative Rules”. Furthermore, as stated earlier, each of the three divisions contain fabrication requirements, some of which are identical, others which are not. As such, the differences between Division 1, 2, and 3 go beyond merely design methods and requirements.
While Division 3 permits numerical methods, this article only addresses the requirements of Section VIII, Divisions 1 and 2 since most pressure vessels are designed to these two divisions of the Code.
When Can Design by Analysis Be Used and What are the Limitations of Its Use?
While the different divisions of Section VIII permit the use of numerical methods (design by analysis), there are restrictions.
Division 1 Vessels
Section VIII, Division 1 of the ASME Code recognizes that it does not contain rules to cover all details for the construction of pressure vessels. In U-2(g)(1) of Section VIII, Division 1 of the 2021 Edition of the Code, it is stated that where design rules do not exist in Division 1, one of three methods shall be used:
Mandatory Appendix 46
Proof test in accordance with UG-101
Other recognized and generally accepted methods, such as those found in other ASME, EN, ISO, national, and industry standards or codes.
Appendix 46 of Section VIII, Division 1 provides rules for the use of Section VIII, Division 2 in the design of Division 1 pressure vessels. It permits the use of both Part 4 (Design by Rule) and Part 5 (Design by Analysis) of Section VIII, Division 2. However, Appendix 46 places restrictions on using these parts of Section VIII, Division 2. These restrictions mostly pertain to allowable stress limits and which parts of Division 2 must be met.
Probably the most important requirement is the use of Design by Analysis (Part 5 of Division 2) for Division 1 pressure vessels is only permitted where design rules do not exist in Division 1. If there is a design requirement or formula in Division 1 that is applicable for the component under consideration, you must meet it. You cannot use Design by Analysis (Part 5 of Division 2) to supersede the requirements of Division 1.
A more detailed overview of the requirements in Appendix 46 can be found here.
Division 2 Class 1 Vessels
Pressure vessels designed and constructed in accordance with ASME Section VIII, Division 2 are denoted as being either Class 1 or Class 2 pressure vessels. In a nutshell, Class 1 pressure vessels have to meet the Design by Rule (Part 4) requirements of Division 2.
More information on the differences between Class 1 and Class 2 pressure vessels can be found here.
Similar to Appendix 46 of Section VIII, Division 1, the Design by Analysis methods in Section VIII, Division 2 (Part 5) can only be used for Division 2 Class 1 pressure vessels when the rules/formulae of Part 4 are not applicable. Likewise, Design by Analysis cannot be used to supersede the requirements of Part 4 (Design by Rule) for Division 2 Class 1 pressure vessels.
Division 2 Class 2 Vessels
Unlike Division 1 pressure vessels and Division 2 Class 1 pressure vessels, Division 2 Class 2 pressure vessels are not required to meet the design requirements (i.e., use the design formulae) in Part 4 of Section VIII, Division 2. In other words, Division 2, Part 5 (Design by Analysis) can be used in lieu of the design thickness requirements of Division 2, Part 4 (Design by Rule).
Summary
Division 1 pressure vessel components can be designed using numerical methods, such as finite element analysis, provided the existing requirements and/or formulae in Division 1 are not applicable for the component being evaluated. Limitations on the use of numerical methods for Division 1 vessels are provided in Mandatory Appendix 46.
Likewise, Division 2 Class 1 pressure vessel components can also be designed using numerical methods provided the existing requirements and/or formulae in Part 4 (Design by Rule) are not applicable.
Division 2 Class 2 pressure vessels do not have to meet the Design by Rule requirements (e.g., design thickness formulae) of Part 4 in Division 2. All of the components of Division 2 Class 2 pressure vessels can be designed using numerical methods.