Run Compress with the 300# flange and check the flange report impact exemptions. Could be that with a #300 flange the stress ratio meets the requirement of UHA-51(g). Whereas the stress ratio of the #150 flange or the #300 flange with external loads at the same design conditions does not meet...
In the 2021 ed. of ASME Sec II Part D there are lines for SA335, P91 type 1 and 2. Looks like this is a revison from the 2019 ed of Sec II part D which was similar to what is listed in B31.3, 2020 ed.
API 560 Defines "setting" as "Heater casing, brickwork, refractory, and insulation, including the tie-backs." Some of the references to setting in Sect. I also imply that it is the external walls or enclosure of a boiler.
