As per ASME VIII div 1, UG-46 :
"All vessels 18 in. (450 mm) to 36 in. (900 mm), inclusive,
I.D. shall have a manhole or at least two handholes or two plugged, threaded inspection openings of not less than NPS 2 (DN 50)."
"(7) Flanged and/or threaded connections from which piping, instruments...