After a long period of continuous use, not to exceed 20 years, physical tests shall be made upon specimens cut from the thin material of the crown plating, or after a period of continuous service, not to exceed 25 years, the tank shall be de-watered and a thorough interior inspection made of the condition of the shell, steel work, the internal rollers, and the permanent frame. The holder shall not be restored to service until the necessary preliminary precautions and the repairs as indicated by the inspection have been made.