You will need to do several calculations to be ASME Section VIII compliant, do you have the book?
These will be based on the diameter, material type, length of shell, type of head, pressure (int/ext, MAWP, hydrostatic), temperature (MDMT, max), supports (legs, saddle, lugs, skirt...), wind load, seismic... based on your question it seems obvious you are not ready to calculate a ASME Section VIII vessel. If you are using API 650 the pressure is limited to just over atmospheric so this code may not apply to your situation.