Municipalities of over fifty thousand inhabitants, through their local councils, may regulate the use of stationary and portable steam boilers, and may appoint a board of examiners of stationary and portable steam boilers for the carrying out of this purpose, and fix its powers and responsibilities.  The board of examiners may be authorized to fix certain fees for its maintenance, to conduct examinations, to have and use a seal, to issue graded certificates of authority and year renewals to qualified engineers passing satisfactory examinations before the board, to suspend or revoke the certificates or order the discharge of engineers in cases where the public safety requires it, to conduct trials, subpoena witnesses and administer oaths, which oaths shall have the same sanctity as oaths administered in courts of justice.  Municipalities may require persons operating steam boilers to have in their possession and posted in conspicuous places in their engine rooms, certificates of authority, and proper renewals thereof.  Municipalities may require owners of plants or establishments, who employ engineers to operate boilers, to have none but certified engineers, having proper certificates and renewals thereof, as aforesaid, in charge of operating the boilers.  Municipalities may enforce ordinances, passed under the provisions hereof, by imposing both fine and imprisonment.  This Section shall not apply to locomotive engineers engaged in operating locomotive engines on railroad tracks, nor to engineers of steam vessels coming within the jurisdiction of the United States Board of Supervising Inspectors.