17. The maximums determined in section 16 are reduced when an inspection proves that the condition of a pressure vessel or of a pressure vessel installation so require. They are raised if the circumstances limiting them happen to be changed or corrected. In this case, the pressure vessel or the pressure vessel installation shall undergo and successfully pass a new inspection. A new periodic inspection certificate, as provided in section 21 of the Act, is issued when the working conditions are changed, and the last certificate shall be destroyed.