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#176 - 09/06/00 12:55 AM Flange calculations
Danny Bekaert Offline
Member

Registered: 12/15/99
Posts: 1
Loc: ZELZATE, Belgium
I would like to apply the ASME VIII div 2 par. 4-141 statement by doubling the bolt allowable in a flange calculation according ASME B31.3 (ASME VIII div 1 appendix 2 calculation with ASME B31.3 allowables for the flange and bolt materials)
Does anyone think this is founded or is this not allowed (because of mixing design Codes)?

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#177 - 09/06/00 10:08 AM Re: Flange calculations
John Breen Offline
Member

Registered: 03/09/00
Posts: 482
Loc: Pittsburgh, PA (& Texas)
Hi Danny,

Within a Code Section, be it a B&PV Code Section or a B31 Code Section, allowable stresses are established with consideration being given to many related issues. These issues include the relative degree of rigor required by the design (analysis) rules, the magnitude of the energy typically contained by the systems being designed, the severity of service, the potential for exposing the public to the process, the need of the public for uninterrupted service, etc. The rules of a Code Section (and its maximum allowable stresses) may also be more restrictive in the use of materials and other considerations beyond design. Allowable stresses established by the Code Section Committees include a design margin (i.e., factor of safety) that is consistent with the overall design philosophy of the SPECIFIC Code document. It would never be judged prudent to capriciously employ the design rules of one Code Section with the allowable stress limits of another Code Section. With due respect to the question (and questioner), it would in fact be unconscionably presumptuous for a designer to assume that he/she could take such liberties without having established a complete (and may I suggest, documented) rationale for the design. This assumption would clearly be in conflict with the intent of the Codes and would be a sound basis for action by the local jurisdiction. It would certainly, upon review, get their attention.

It is important to note that virtually every Code Section explicitly permits the designer to apply greater rigor in the design than is specifically required by the Code’s design rules. The is NO intentional limit on conservatism. Quoting from ASME B31.1, Code for Power Piping: “It is intended that a designer capable of applying a more complete and rigorous analysis to special or unusual problems shall have latitude in the development of such designs and the evaluation of complex or combined stresses. In such cases, the designer is responsible for demonstrating the validity of the approach.” It is important to recognize that alternate approaches may also carry with them the responsibility to call for increased rigor in fabrication and perhaps nondestructive examination and testing. All this is a LOT of responsibility.

Quoting from the ASME B&PV Code, Section VIII, Division 2: “In relation to the rules of Division 1 of Section VIII, these rules of Division 2 are more restrictive in the choice of materials which may be used but permit higher design stress intensity values to be employed in the range of temperatures over which the design stress intensity value is controlled by the ultimate strength or the yield strength; more precise design procedures are required and some common design details are prohibited; permissible fabrication procedures are specifically delineated and more complete examination, testing and inspection are required” Also, I would suggest reading the Division 2 Forward regarding the designer’s responsibilities when using computer programs.

The more we understand about the response to loading of the system (structure) under design the less design “factor of safety” (or, as some call it, “index of ignorance”) is needed. However, when the designer “crosses” from one Code Section to another the “big picture” must be kept in view. Are the stresses being calculated combined using the same theory of failure (Tresca, Von Mises, Rankin, ...) in both Code Sections? Is the basis for the establishment of the maximum allowable stress the same? Are we considering stresses, stress intensities, intensified stresses, or...? Are we about to compare apples to oranges?

In conclusion, unless you are VERY familiar with all the nuances of the Code Sections involved, do not try to mix Codes. Even if you are VERY familiar with the philosophies and requirements of these Codes, be very careful. Keep in mind that beyond design, there may be additional requirements attached to the maximum allowable stresses presented in a Code Section. Personally, I would take another approach. There is something to be said for staying within one Code Section and benefitting from the certainty of time proven results. When well done, this path will not draw the attention of the jurisdiction or the insurance underwriter - and for good reason. All the above is just my opinion.

Best regards, John.
_________________________
John Breen

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