The director shall enforce the provisions of this act and the rules promulgated by the board pursuant to this act and where, owning to special conditions, a literal enforcement of the provisions of these rules will result in unnecessary hardship or involve practical difficulties, the director, upon application in specific cases, may authorize variations or modifications of the terms of these rules which will not be contrary to the public interest and so the spirit of these rules shall be observed, public safety secured, and substantial justice done.