The ASME explanations for the intended applications of and differences between formulas PG27.2.1 and PG27.2.2 are seen in Interpretations 1-83-47 and 1-86-25.
Regarding official languages in ASME to allow pipe materials to be used in heated services, I haven't found any myself. You might have to contact ASME committee for answers but, as I mentioned, it is a very common practice to use pipes inside a furnace or flue gas path. Examples are evaporator wall headers and economizer headers plus many components of special dimensions that are not available with tube materials. Nevertheless, I haven't seen using large quantities of pipes to substitute tubes but the main reason is because that if you need a lot of tubes you can always get a special order from the manufacturers without having to find off-shelf items. For your special situation of, I assume, unable to find the exact tube material without high cost and can't get an AI or QC to approve using an existing available pipe material as substitute, see if you could contact the supplier to check the material test report and if they also qualify for the specs of the tube that you are looking for, the supplier might be able to re-certify these pipes as tubes. It is very common to see dual-certifying materials on the markets. For the dimension and tolerance differences, either you can change your design documents (after calculations and necessary drawing changes, of course) of the tube specs to match the pipe's or the supplier might be willing to machine, at a small cost, the OD's and ID's of the pipes to meet the requirements of your tubes. This (machining to meet requirements) was heard done quite often before.
