While there are currently no explicit rules on how to perform a finite element analysis (FEA) if you are doing so in support of an ASME Section VIII, Division 1 vessel, there is good practice. Regarding rules, all you have is from Article U-2(g), which says:
This Division of Section VIII does not contain rules to cover all details of design and construction. Where complete details are not given, it is intended that the Manufacturer, subject to the acceptance of the Inspector, shall provide details of design and construction which will be as safe as those provided by the rules of this Division.
So, how exactly does an engineer perform an FEA, for which Section VIII, Division 1 has absolutely no rules, that is “as safe as” the rules otherwise provided in Section VIII, Division 1? Luckily, I’m not the first person to think about this. In fact, there is some pretty decent guidance in API 579/ASME FFS-1. Also, the ASME Section VIII Code Committee has formed a Task Group on U-2(g). I have attended (as a visitor) meetings of that Task Group, and can report that they are looking at far more than this topic. However, they have drafted something which is exactly what I would recommend as good practice.
So, here’s my take on what constitutes good practice:
- The allowable stress for all product forms except bolting needs to be from Section II, Part D, Table 1 and Table 1A (i.e. the allowable stress for Section VIII Division 1 construction).
- For bolting materials the allowable stress needs to be determined from Section II, Part D, Table 3.
- Limiting values that are not calculated using the allowable stress, S, such as fatigue and compressive allowable stresses, can be determined from the current rules in Section VIII Division 2.
- The weld joint efficiency needs to be established in accordance with UW-11 and UW-12 of Section VIII Division 1. (more on this item in another post)
- The Design By Analysis Rules in ASME Section VIII, Division 2, Part 5 should be followed. This is only possible for temperatures not in the creep regime. If your vessel has operating temperatures into the creep regime (indicated by the allowable stress being in italics in Table 1 or Table 1A), I would suggest that you retain the services of an expert, because there are NO rules right now for that situation.
- ALL of the load case combinations of the applicable Division 2 assessment procedure need to be considered in addition to any other combinations defined by the User. In evaluating load cases involving the pressure term, P, the effects of the pressure being equal to zero needs to be considered (this ought to be standard practice of anyone using Part 5, but it is worth stating, again).
- All of failure mechanisms in 5.2, 5.3, 5.4, and 5.5 of Part 5 need to be evaluated. In 5.3, a component is exempt from the Local Failure Criteria evaluation if the component design is in accordance with the standard details of Part 4. That said, there are many details permitted in Division 1 that are not permitted in Division 2. If the component being evaluated is not covered by a standard detail from Part 4, then an evaluation per 5.3 is required.
- When elastic-plastic analysis is performed, the required load case combinations from ASME FFS-1/API-579 Table B1.4 Note 6 need to be used.
- Evaluation of the test condition per paragraph 22.214.171.124 of Section VIII, Division 2 is not mandatory, but consideration of the test condition per UG-22(j) of Section VIII, Division 1 is mandatory.
All other requirements for construction need to comply with Section VIII, Division 1.
It is my opinion that if a designer chooses to apply U-2(g) and then uses a Design-By-Analysis approach, then Appendix KK ought to become Mandatory. However, it is unlikely that will be required by the Codes. Therefore, it is good practice for the engineer performing the FEA to ask for as much detail as they will need to perform the analysis – especially Protection Against Failure From Cyclic Loading.
Remember that an FEA cannot be used to supersede existing rules in the Code. This is true not just for basic design calculations such as wall thickness, but also for weld details, PWHT requirements, NDE requirements, etc.
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