I am doing flange calculations for the flanges selected as per ASME B16.5. Though the flanges are standard, calculations are required to check against external piping loads. When I input the standard dimensions of flanges and gaskets, flanges are failing in gasket seating condition and gives the message as "insufficient bolting area". Though I use any of the bolting material, same message appears again. i am wondering that how a standard flange can fail in gasket seating condition. In past I had come across the cases where the flange thickness was insufficient.
Any suggestions for designing such flanges? Or should I go for higher rating?

I'm not sure if I understand you correctly. Flange calculation (what are you describing here) is done when you have girth flanges...and I don't know that you can apply pipe load to them...

Did you choose correct item? If you have piping load applied to your nozzle you have under nozzle window tab called "Local Stress Analysis" and there you can enter pipe loads...

In this case you won't have flange calc.

I think that you have mixed up here two different things, but as I said, maybe I don't understand you good.

Cheers

Dear Mr. Kunal
You are right. It happens. It normally happens with low rating flanges such as 150 #, Where bolt get fail in Gasket setting condition.

I made my way by decreasing width of gasket. Probably you can also try this.

Its been guaranteed that ASME B16.5/B16.47 can take such working pressure as per Table 2-1 ~ 2-3 (ASME 16.5). Been noted if we re-calculate with appendix 2 it will be fail on certain rating & sizes.
This mean the originality these design is not according to ASME VIII- Appendix 2. But in ASME VIII UG-44 ensuring us to use these flanges without re-calculate the design.

As for the C.A., this query as raised before, whatever case that will impact the original design is not covered by ASME B16.5 nor ASME B16.47.

However.. i give example for CA impact i.e (6mm) on raise face blind 150#,
the logic is the c.a will corrode the flange thick though the raise face. But somebody telling me that the blind is replacable so it is not be the big issue... (replacing the blind would not have any additional NDT or whatsoever)

Conclusion..
it is up to engineers / designers to evaluate the scenario.. the best thing is recalculate it with appendix 2 and change the design as necessary for avoid future problems..