Coded Pressure Vessels and ASME Code Requirements

You are correct in that some locales may have alternate rules regarding volumes, take for example Alberta:

R E G U L A T I O N A N D N O T E S S E C T I O N 5
A B - 5 1 6 R E V . 1 : 2 0 0 6 - 0 4 - 1 8 19
Section 5: Pressure Vessels
5(1) The following types of pressure vessels, regardless of volume, must meet all the
requirements for pressure vessels in this Regulation:
(a) steam jacketed sterilizers;
(b) autoclaves;
(c) steam jacketed kettles;
(d) air cooled heat exchanger headers;
(e) compressor bottles;
(f) hairpin type heat exchangers.
The pressure vessels listed here must be designed and
constructed as pressure vessels, and must meet all regulatory and
code requirements for pressure vessels. They cannot be registered
or constructed as fittings, even if their volume is less than 42.5
litres.
Generally, these items have a more complex design and require
more detailed design work than other small pressure vessels,
which are registered as fittings.
(2) A type of pressure vessel not listed in subsection (1) that has a volume not
exceeding 42.5 litres shall not be manufactured or operated unless it is registered by
the Administrator and it may be registered and manufactured as a fitting.
(3) If there is a dispute as to whether an item of pressure equipment may be
registered by the Administrator and constructed or manufactured as a fitting or as a
pressure vessel, the dispute may be referred to the Administrator for a ruling.
In Alberta, Figure 1(b) in CSA B51 is used as a guide to determine
if a small pressure vessel can be treated as a category H fitting.
However, the Administrator has final say over whether an item may
be registered as a fitting or must be registered as a pressure
vessel.

See: Figure 1(b) in CSA B51

(It would not be registered as an ASME "Pressure Vessel" but rather a "fitting")

And that is why I suggest you check with the Arizona officials