hplogger,
1. Code permits using of standard flanges acc. to press.-temp. ratings specified in the appropriate standard: UG-11(a)(2) + UG-44.
2. Be aware of MDMT requirements for selecting flange material.
3. Pressure to be considered for your flange is design press. / MAWP at top of the vessel + static head over flange location. Be aware of the possibility that operating media density is lower than testing media. If so, you should consider testing media static head.
4. Dimensions of flange (your flange is ASME B16.5 Tab. 18) are not applicable to corroded conditions (Interpretation 2-27 to ASME B16.5 1996: "The applicability of press.-temp. ratings for material that is corroded or otherwise deteriorated is the responsibility of the user").
5. For corroded conditions you may rerate your flange acc. to the rules of app. 2 for the ring flange + UG-34 (sketch j or k) for the blind. Please note that the blind flange does not stay alone, as you should consider gasket material, contact facing and bolting.
I suggest you look at former discussion on this topic: thread292-179367