This falls under the definition of a pressure vessel, per ASME Section VIII, Div 1.
I would suggest that you re-rate this vessel under the rules of the National Board Inspection Code, because this vessel was probably registered with the National Board.
Get a copy of the NBIC and review the requirements for re-rating under Alterations. You will need to obtain design calcs to support your re-rate (if you are altering the pressure retaining capacity or lowering the design metal temperature).