There are vessels which fall outside the scope of the ASME code. If you refer to the Scope in ASME Section VIII, Div 1. para's U-1(c)(2) to U-1(j). These sections will tell you what is outside the scope and therefore if the vessel you are building is a "pressure vessel" or not. Hope this helps.
As far as I know, if the design pressure is below 15 psig, it is not possible to use section VIII "U" stamp"
Also, above a certain diameter ratio ( which relates to design pressure), the code recommends you use special rules or use an alternate code for ultra thick vessels. This upper limit may come into play for diameter ratios greater than 1.6:1
For very thick vessels, it may be wiser to not use a monolithic wall and instead use the special rules for ribbon-wound multi layer pressure vessels ( common in ammonia reforming work)
As the prior message indicates, it is best to read the preamble to each code to confirm its scope.
It is always possible to use a Section VIII-1 "U" stamp. Simply having a design pressure (less than 15 psig) which is out of the specified scope of VIII-1 as defined in part U-1 doesn't exclude the possibility of stamping the vessel. In fact, U-1(c)2 explicitly states that "Based on the Committee's consideration, the following classes of vessels are not included in the scope of this Division; however, any pressure vessel which meets all the applicable requirements of this Division may be stamped with the Code U Symbol:"
The treatment of a tank and similar items as pressure vessel should be checked against local or national codes in your area. It is not worth the time an effort to classify a tank that operates at say 15 psig just because ASME say so. Some local ordinance refer to ASME others dont. You may want to convert a tank to a pressure vessel (for calculation purposes??) but will it serve the purpose? If the tank is used for non-hazardous service, it may just stay as tank rather than a PV. Again local ordinance should be checked for this requirement, not the ASME code.
