pressure vessel code VIII 1

Maybe. It depends on the local authority sometimes. I have worked in plants where as long as the 'vessel' was constructed out of piping components (pipe, end caps, etc) and was less than 1.5 ft3 internal volume, that we did not always fabricate and stamp it as a code vessel (note, we also did not limit ourselves to the 15 psig rule).

Condensate pots from exchangers were a frequent example of this (it was always a bit of a grey area). The usual arguement was 'if it's just a wide spot in the line and made of piping components, we don't need to call it a vessel'. For a condensate pot, where you are just holding a level, that's sort of true though I bet there are other opinions on it. The main reason for doing this was to avoid having to put a PSV on something this small as well as the associated paperwork with another legal pressure vessel.

As soon as you starting using the 'vessel' to separate two streams or it became more than just a wide spot in the line, we'd call it a pressure vessel and design it accordingly.

TD2K is correct in regards to the jurisdictional authority. However, ASME Section VIII, Division 1 states in U-1(c)(2)(i) that vessels having an inside diameter, width, height, or cross section diagonal not exceeding 6 in. with no limitation on length of vessel or pressure are excluded from the scope of the book.

U-1(j)(3) states that vessels not exceeding 1-1/2 cu.ft. AND 600 psi design pressure are exempt from inspection by AI's as defined in UG-91 provided they do not require to be fully radiographed.

U-1(c)(2)(h) exempts vessels with an operating pressure of 15 psi or less from the scope of the Code. There is no limitation on size.

A vessel that does not serve to mix, separate, snub, distribute, meter, etc. is still a vessel even if it's only 1.5 cu.ft, and greater than 6 in. diameter and greater than 15 psi MAWP. Be careful that you are not in compliance with the laws of your state on vessels. For instance, the State of Illinois exempts vessels up to a certain volume, irregardless of pressure; but larger than that, they must be registered.
The other reason that may process folks and capital project managers do not want to build to Section VIII is they feel the inspection requirements are too onerous. Nothing is further from the truth. Detail inspection that comes from building to Section VIII will provide a better vessel everytime (provided of course the inspector is well qualified).
And finally, whether or not it meets the Code scope, you can still put a stamp on it and have a U1 form as long as you build it to all the requirements of Section VIII.

If you are still in doubt about what meets the Code and what doesn't, send an Inquiry to the Code Committee as directed in Appendix 16 of Section VIII, Division 1.

Dear Babyyoda,
The vessels with the conditions you describe do not fall under the purview of ASME SECTION VIII Div 1. But still they are pressure vessels. If you have to design such low pressure vessels as per ASME Code then you must adopt the design pressure with in the purview of the code, irrespective of its working conditions.

Regards.
V.Dwarakakrishna.