The Owner is responsible for determining which section of the code is applicable.
Now let's agree that the owner is the least likely entity to know what code is applicable. From there you can look into the building code to see what piping code is specified for the application. Assuming that is no help if you are working in an area that doesn't have a building code, then ask the contractor that is responsible for the installation.
I encountered a similar situation several years ago. I happened to be on-site for an unrelated problem when the TPI bounded into the office trailer and announced he had just rejected 90% of the pipe welds on the project. Knowing the inspector, I asked him what piping code was he inspecting to?
"Its just pipe." he responded.
"I know it is just pipe. It is process piping, so it should be B31.3, but what is the service category?" I asked.
"Its just pipe. How do I know what the service category is?" He snorted.
So, I turn to the Project Manager and ask him the same question.
"Its just pipe." was his response.
Since the contractor installing the pipe was one of my clients, I ventured on to the job site to hunt down the supervisor. When I found him I told him, "The inspector just rejected 90% of your pipe welds. What is the piping code and what is the service category?" I asked.
"Its just pipe Al." was the response.
"B31.3 is the piping code that is most likely specified, but the service category is going to make a major difference in the acceptance criteria." I told him.
"The project specification is in the desk over there." he said.
I rifled the desk and found the project specifications. It listed B31.3, category normal service. Only two of the 100 or so welded joint rejected by the inspector did not meet the VT acceptance criteria. However, the integrally reinforced branch fittings he did accept; none of them were welded right. In the middle of a (almost) stainless steel system, i.e., tanks, pipes, catwalks, ladders, etc., was a cast iron fitting. A cast iron fitting that was well rusted in the middle of an all stainless steel system didn't seem to bother the inspector in the least.
Gotta go do something useful.
Best regards - Al
Compressed gases piping falls sometimes under other codes than ASME B31. Chlorine, for example, follows the rules of The Chlorine Institute.
It's possible, but I'm not sure, that compressed air piping is under CAGI (Compressed Air and Gas Institute). I'm going to check and come back to you.
Giovanni S. Crisi
Sao Paulo - Brazil