The Engineer is hereby authorized to issue "Drain layer's certificates" to such persons as shall furnish him satisfactory evidence of their responsibility and skill to carry on the work of a pipe layer. Which certificate shall be issued free of expense to the applicant, and shall be revocable at the discretion of the Engineer for any violation of the provisions of this Code relating to sewers and drains or for any violation of the rules and regulations of the department of public works relating to that subject.