Perhaps the real question is this: when does a piece of pressure piping become a pressure vessel?
Imagine a 16" NPS pipe connecting two pressure vessels. It's bigger than 1.5 cubic feet and relieved above 15 psig, but this line is a piece of pressure piping per a piping code like ASME B31.3.
Imagine that the line is now swaged down to 2" at each end, but still connected to two pressure vessels. Still a pipe.
Imagine now that you delete one or both of the pressure vessels- voila, now it's an ASME VIII vessel.
The codes as far as I've read them, do not teach you this lesson. The first pipe meets all the requirements to fall within the scope of ASME VIII, but it is a minor volume in the system and hence despite this, is still subject to the piping code.
However, without the vessels, it is a vessel in its own right. It is now the major source of stored pressure energy in its own vicinity, and is promoted from being mere pipe to being a pressure vessel It now needs its own nameplate with stamp, its own overpressure protection etc.