Mo. Rev. Stat. § 650.270: Operation without inspection certificate, penalty.

It shall be unlawful for any person, firm, partnership or corporation to operate in this state a boiler or pressure vessel, except a pressure vessel covered by owner or user inspection service as provided for in section 650.265, without a valid inspection certificate. The operation of a boiler or pressure vessel without an inspection certificate, or at a pressure exceeding that specified in the inspection certificate, is a class A misdemeanor. Each day of unlawful operation is a separate offense.