When two or more boilers which are allowed different pressures, are connected to a common steam main, and all safety valves are not set at a pressure not exceeding the lowest pressure allowed, the boiler or boilers allowed the lower pressure shall each be protected by a safety valve or valves placed on the connecting pipe to the steam main; the area or combined area of the safety valve or valves placed on the connecting pipe to the steam main shall not be less than the area of the connecting pipe, except when the steam main is smaller than the connecting pipe, when the area or combined area of safety valve or valves placed in the connecting pipe shall not be less than the area of the steam main. Each safety valve placed on the connecting pipe shall be set at a pressure not exceeding the pressure allowed on the boiler it protects.

When two or more boilers are allowed to operate at different steam pressures and are connected to a common steam main and their respective safety valves are not set to blow off at the lowest pressure allowed, the headers or outlets from the lower pressure boilers shall be equipped with safety valves, the aggregate capacity of which shall be sufficient to pass the total amount of steam that can be generated in the higher pressure boilers.

When one or more boilers are used for reserve purposes only and such boilers are not operated in common with the other boilers, nonreturn valves of an approved type may be used in lieu of required safety valves. The nonreturn valves shall be located as close as practicable to the boiler with ample drain between valves.