Okla. Admin. Code § 380:25-7-14
When boilers are replaced or new boilers are installed in either existing or new buildings, a minimum of at least three (3) feet shall be provided between all sides of the boiler and adjacent walls or other structures. Boilers and pressure vessels having manholes shall have five (5) feet clearance from the manhole opening and any wall, ceiling or piping that will prevent a person from entering the boiler or vessel. Marine type boilers shall have a clearance from the floor level to the bottom of the shell plate of not less than eighteen (18) inches. All boilers and pressure vessels shall be so located that adequate space will be provided for the proper operation of the boilers and pressure vessels and their appurtenances, for the inspection of all surfaces, tubes, waterwalls, economizers, piping, valves and other equipment, and for their necessary maintenance and repair and replacement of tubes.
Okla. Admin. Code § 380:25-7-14