If a component is not in accordance with a listed standard and/or the design rules provided in para. 304 are not applicable, para. 304.7.2 is applicable. This paragraph requires that some calculations be done in accordance with the design criteria provided by the Code and be substantiated by one of several methods. The meat of this paragraph is considered to be the substantiation; the aforementioned calculations are not generally given much consideration. The methods to substantiate the calculations, and thereby the design, include the following:
Extensive, successful service experience under comparable conditions with similarly proportioned components of the same or like material.
Experimental stress analysis, such as described in ASME BPVC Section VIII, Division 2, Annex 5.F.
Proof test in accordance with either ASME B 16.9, MSS SP-97, or ASME BPVC Section VIII, Division I, para. UG-101. Note that of these standards, those of B16.9 and MSS SP-97 are more applicable to piping components and have a margin of safety consistent with other components in the Piping Code (factor of three on burst rather than the factor of four in the Pressure Vessel Code).
Detailed stress analysis (e.g., finite-element method) with results evaluated as described in ASME BPVC Section VIII, Division 2, Part 5. These are the design-by-analysis rules in the Pressure Vessel Code. Note that the allowable stress from ASME B31.3 is used in the assessment.
Of the above, the methods normally used to qualify new unlisted components are proof testing and detailed stress analysis. It should be noted that the Code permits interpolation between sizes, wall thicknesses, and pressure classes, and also permits analogies among related materials. Extrapolation is not permitted. The issue of how to determine that the above has been done in a satisfactory manner has been discussed in detail in B31.3 Section Committee meetings. Earlier editions of the Code required only that proof testing be approved by the Inspector. However, this created concerns that it may be interpreted that the Inspector must witness the proof test, which is not practical when the manufacturer performs proof tests to qualify a line of piping components. Obviously, all the potential future owner’s Inspectors could not be gathered for this event. Furthermore, the other methods are of at least equal concern, and their review may be more appropriately done by an engineer rather than an Inspector. As a result of these concerns, the requirement was added that documentation showing compliance with the above must be available for the owner’s approval. The review would be by an Inspector or some other qualified individual for the owner. Although MSS SP-97 and ASME B16.9 provide a clear approach for determining that the rating of a component is equivalent to or better than matching straight pipe, they do not provide clear procedures for determining a rating for a component that may have a unique rating which may differ from matching straight pipe. The procedure generally used here is to establish a pressure temperature rating by multiplying the proof pressure by the ratio of the allowable stress for the test specimen to the actual tensile strength of the test specimen. An example of this approach is provided by Biersteker et al (1991). In the proposed B31H Standard, this would be reduced by a testing factor depending on the number of tests. A new standard is under development by ASME that will eventually add to or replace the existing proof test alternatives in para. 304.7.2. This is B31H, Standard Method to Establish Maximum Allowable Design Pressure for Piping Components. This Standard provides procedures to determine that a component has a pressure capacity at least as great as a matching straight pipe, or to determine a pressure-temperature rating for a component.
The above post is just illustration and interpretation for the ASME B31.3, it is recommended to read the ASME B31.3 code at https://www.asme.org/