Where a number of boilers deliver to a common steam main, they shall be equipped with a shutoff or throttle valve for each boiler to take it out of service for repairs and inspection necessitating the entry therein of workmen. A metal shield shall be constructed covering the hand wheel of the valve hinging in the center and containing hasp and lock. The shield shall be painted red and marked with the words Man in boiler. The workman shall be allowed to retain key in his possession while in such boiler.