I am not so sure that there is/are any federal, state, or local regulations in my area that requires a compressed air piping system to be designed and installed per any ASME code. Whereas it may be prudent to use ASME B31.3 for the system, check with the controlling authority to see what if any code is applicable. Some municipalities may require compliance with the Uniform Plumbing Code. The owner’s Insurance Company may have a say in this as well, so you may want to look at FM Global’s requirements. If it is for a large company, they may have their own standards that you may have to comply with. If I was involved in designing the system, I would use ASME B31.3 to help defer the potential of any future legal liabilities, provided I had no other mandated code requirement to follow.