When unfired pressure vessels are replaced or new vessels are installed in either existing or new buildings, clearances shall be provided to allow access for safe operation, inspection, maintenance, and/or repair. Passageways around all sides of pressure vessels shall have an unobstructed width of not less than 18 inches. Exception: Unfired pressure vessels may be installed with a side clearance of less than 18 inches if the lesser clearance does not violate the manufacturer's installation instructions or inhibit inspection, maintenance, and/or repair. Any exception shall be notated in object comments in the jurisdictional database. All unfired pressure vessels having a manhole shall have five foot clearance at the manhole opening to allow an individual to have safe access to the inside of the vessel to perform inspection and/or maintenance.