If the flange is B16.5 standard, it is required to be marked as such. Likewise, the size and class is a required marking. From there, the B16.5 standard will give you dimensions.

I assume that, because you looked for the material in IID, that you want the rating at temperature as well. That might be a bit tricker. I can't think of a way to legitimately correlate the material types to something in the B16.5 standard. Perhaps someone else will have some input to that end.

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Fitter, happier, more productive

One path forward would be to model the flange as a custom appendix 2 flange using the properties from ASME Div II Part D, and label the part as having the 'dimensions of a 16.5 flange'. This unfortunately requires knowing the specific gasket being used, and would likely result in different pressure ratings for different flange sizes.

I would be interested in learning if there is a better way forward though.