A dual-valve is used for the root valve.

The Code for Design of Layout of Metal Piping in Petrochemical Industries stipulates that double valves shall be installed for the venting or draining of high-pressure pipelines； If a single valve is used, a blind flange and a flange cover must be added. Furthermore, in devices that require semi-fixed nitrogen purging, nitrogen connectors and double valves should be provided at the hose station.

The requirement to equip high-pressure steam pipelines with two root valves may be specified in the \"Regulations on Safety Inspection of Boilers\" or in the \"Design Codes\" for certain industries; to find the relevant provisions, it is necessary to consult these documents. .

I found an answer on Zhihu, but it didn’t mention high-pressure steam; could it be that I’m remembering incorrectly? 1: Article 5 of the \"Guiding Opinions of the State Administration for Work Safety on Strengthening the Management of Leaks in Chemical Enterprises\" (An Jian Zong Guan San [2014] No. 94): When designing discharge valves such as those at the outlets and sampling points of equipment and pipelines, measures such as installing blind flanges, thread plugs, pipe caps, and double valves should be employed to reduce the likelihood of leaks. For processes involving highly toxic substances, a closed sampling system should be used, and safe pressure relief systems for toxic and flammable gases must also be designed in a closed manner. 2: Article 5.3.7 of the \"Design Code for Tank Areas in Petrochemical Storage and Transportation Systems\" SH/T 3007-2014: A main isolation valve should be installed near the tank base at the inlet and outlet pipes of the storage tank, and an operating valve should also be installed on each of these inlet and outlet pipes. The drain pipe of the storage tank should be equipped with a double valve. 3: Article 7.1.5.6 of the \"Design Code for Occupational Safety and Health in Petrochemical Enterprises\" SH/T 3047-2021: Discharge valves for highly corrosive liquids should be equipped with dual valves. Article 8.1.7: For the venting or drainage of pipelines carrying extremely hazardous media, double valves shall be installed, and the flow shall be directed into a closed recovery system. 4: Article 8.1.10 of the \"Code for Design of Layout of Metal Piping in Petrochemical Industries\" SH/T 3012-2011: Double valves shall be installed for the venting or draining of high-pressure pipelines; when a single valve is used, a blind flange and a flange cover shall be added. Article 9.4.1: When semi-fixed purging nitrogen is required in the device, a nitrogen connection should be provided at the hose station, along with a double valve. 5: Article 6.0.7 of the \"Design Code for Instrument Piping and Wiring\" HG/T 20512-2014: When the pressure class is greater than or equal to PN160 and the root valve is a double-valve type, the instrument drain valve should be designed as a double valve or as a single valve accompanied by a pipe cap. 6: Article 10.2.4 of the \"Code for Environmental Protection Design in Petrochemical Industries\" SH/T 3024-2017: For pipelines carrying highly hazardous media, double valves shall be used for discharge, and the flow shall be directed into a closed recovery system ； The discharge of other toxic media can use a single valve with a flange cover. For the discharge of high-pressure fluid medium pipelines, a dual-valve system or a single valve with a flange cover should be used; for the discharge of other fluid medium pipelines, a single valve with a flange cover should be employed. Article 10.2.10: Centrifugal pumps or rotary pumps used to transport toxic and hazardous media shall be equipped with bottom drain valves, which shall be designed as a dual-valve system. 7: Article 7.2.8 of the \"Code for Fire Protection Design of Petrochemical Enterprises\" GB50160-2008 (2018 edition): Two drain valves shall be installed at the lowest point of combustible gas pipelines operating continuously, and the liquid discharged therefrom shall be sent to a closed system ； For drain valves used only during startup and shutdown, a single valve can be installed along with a plug, pipe cap, blind flange, or flange cover.