Clarification for ASME Sec. VIII-1 U-2 (g) 2

[Paulettea]

Dear all,

Occasionally I am asked to perform calculations for equipment as per ASME Sec.VIII Div-1 and they want me to perform calculations using U-2 (g). Till now I automatically went to some other codes or Div.2 DBR or DBA and followed the instructions provided by these codes. I just started to think about the exact meaning of U-2 (g)

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.

Unfortunately, I cannot understand the meaning of "as safe as" in this paragraph. How can I be sure that the design is as safe as Div.1 rules? Is it about allowable stresses of materials of construction?
How can I understand the construction is as safe as Div.1

I think this part of the code needs much more clarification than just saying the design has to be as safe as this division.

Warm Regards.

 

[TGS4]

You are certainly correct that this paragraph could use some work. In fact, the ASME Code Committees had created a task group specifically on U-2(g). It has been working for about 7 years. There is a current item before the main Section VIII Committee to add substantial clarification to this paragraph.

For starters, where analysis methods use the allowable stress, you should use the allowable stress from Div 1. Where there is no comparable comparison, use the allowable values from Div 2 (ratcheting, fatigue, etc). I wrote a blog post on this exact topic. There's some very good guidance in it.

https://becht.com/blog/performing-a...ision-1-to-qualify-an-article-u-2-g-component

 

[Paulettea]

Thank you TGS4

The points in the blog are very helpful.

Warm Regards

 

[MrPDes]

ASME PTB-4 "Div 1 examples manual" has two pages of discussion related directly to your question. A couple of relevant passages include:

"The requirement “be as safe as” is difficult to quantify because the actual design margin for a particular failure mode is not provided in VIII-1."

"....Therefore if the VIII-1 explicit margins on the strength parameters, i.e. the allowable stresses from VIII-1, are used in conjunction with the design-by-rule or design-by-analysis procedures in VIII-2, then vessels constructed in accordance with this practice are judged to be “be as safe as” those constructed to VIII-1. To support this notion, VIII-1 Code Case 2695 permits the use of VIII-2 design rules with VIII-1 allowable stresses with some limitations."

Div 1 Allowable stresses should be used. Div 2 rules are the simplest to adapt to Div 1 requirements, however there is nothing that prevents the use of other codes such as PD 5500 Appendix C for fatigue assessment.