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\"Code for Fire Protection Design of Petrochemical Enterprises\" GB 50160-2008 (2018 edition), 7.2.7: When utility pipelines are connected to pipelines or equipment carrying flammable gases, liquefied hydrocarbons, and flammable liquids, the following requirements shall be met: 2. Check valves and one isolation valve, or two isolation valves, shall be installed on utility pipelines that are used intermittently; an inspection valve shall be placed between the two isolation valves. 7.2.8: Two drain valves shall be installed at the lowest point of continuously operated flammable gas pipelines, with the drained liquid being discharged into a closed system; for drain valves that are used only during startup and shutdown, one valve may suffice, provided that it is fitted with a plug, pipe cap, blind flange, or flange cover. Explanation of Clause 7.2.8: Two drain valves are installed at the lowest point of combustible gas pipelines that are operated continuously. The first valve (located closer to the pipeline) is a normally open valve, while the second valve is a valve that is operated frequently. When a leak was detected in the second valve, the first valve was closed and the second valve was replaced. Code for Design of Layout of Metal Piping in Petrochemical Industries SH 3012-2011: 8.1.7 For pipelines carrying extremely hazardous media, dual valves shall be installed for discharge or emptying, and the flow shall be directed into a closed recovery system. 8.1.8 Except for extremely hazardous media, the exhaust outlets for toxic gases shall meet environmental protection requirements, and toxic liquids shall not be discharged into the sewer system. 8.1.9 For pipelines carrying highly hazardous media, dual valves should be installed for venting or draining; when a single valve is used, a blind flange and a flange cover shall be added. 8.1.10 Double valves shall be provided for the venting or draining of high-pressure pipelines; when a single valve is used, a blind flange and a flange cover shall be installed. 8.1.11 For the drain at the lowest point of combustible gas pipelines that are in continuous operation, a double valve should be installed, and the liquid discharged should be sent to a sealed system; for the drain on combustible gas pipelines that is used only during startup and shutdown, a single valve may suffice, along with a plug, pipe cap, or flange cover. When a shut-off valve is installed for draining on pipelines carrying flammable liquids and vapors, a plug, pipe cap, or flange cover should be placed at the end. 9.4.1 When semi-fixed purging nitrogen is required in the unit, a nitrogen connector shall be provided at the hose station, along with a double valve. \"Code for Design of Tank Areas in Petrochemical Storage and Transportation Systems\" SH/T 3007-2014 5.3.7: 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. 6.4.4 Anti-freezing measures shall be taken for the bottom piping of liquefied hydrocarbon storage tanks in cold regions. A double valve should be installed on the dehydration pipeline of the liquefied hydrocarbon tank. According to the \"Technical Specifications for the Design of Process System Engineering\" HG/T 20570.18-1995, 2.0.3.1: For storage tanks used to hold liquefied petroleum gas, other flammable, toxic, or valuable liquids, highly corrosive substances such as concentrated sulfuric acid and caustic soda, as well as substances with special requirements (such as those with unpleasant odors that cause severe environmental pollution), two valves in series should be installed on the pipes leading from the bottom of the tank to other equipment. Regardless of whether there are valves near those other devices, these two valves must be installed; one of them should be positioned right next to the tank’s connection port. When the tank has a large capacity or is located at a significant distance, it is preferable that this valve be a remotely controlled one. To reduce the number of valves, where operation permits, several pipes are combined and connected to a single outlet. The drain valve of the container containing the aforementioned medium should also be a dual-valve design. The sampling valve and drain valve on the aforementioned medium pipeline should have dual valves installed or not, depending on the frequency of operation and other conditions. 2.0.3.2 For equipment that needs to be shut down during operation of the unit for maintenance, cleaning, or regeneration, a double valve arrangement should be used, with an inspection valve installed between the two valves. When the device is disconnected from the system, both valves close and the check valve opens. 2.0.3.3 Public utility pipelines should, as much as possible, not be fixedly connected to process material pipelines; instead, they should be connected via hose stations using quick connectors. When direct connection is required for operation, a double-valve connection should be used, with a check valve installed in between; this check valve opens when feeding is stopped, or it can be sealed with a lead seal. Add a check valve in situations where pressure may fluctuate. 2.0.3.5 Double valves shall be installed upstream of the connection points between hydrocarbons, toxic and harmful chemicals, and other process materials, as well as on the vent and drain pipes. \"Code for Safe Design of Spherical Storage Tanks for Liquefied Hydrocarbons\" SH/T 3136-2025 7.4.4 The water injection pipeline shall be equipped with double isolation valves, check valves, and inspection valves. . SH 3009-2013 \"Code for Design of Flammable Gas Emission Systems in Petrochemical Industries\" 5.5.5: Subject to the requirement of safe emission of flammable gases, switching operations can be carried out between multiple flares ； The connecting pipe for switching between torches should be installed in front of the water seal tank, and it should be equipped with double isolation valves and blind flanges. \"Code for Pressure Piping – Industrial Piping, Part 3: Design and Calculation\" GB/T 20801.3-2020: For piping carrying substances of acute toxicity Category 1 and Category 2, dual valves shall be installed for venting or draining, and the discharge shall be directed into a closed recovery system. B.5.1.8 For pipelines carrying acutely toxic substances of Category 1 and Category 2, double valves should be installed for venting or draining; when a single valve is used, a blind flange or flange cover should be applied. B.5.1.9 Double valves should be provided for the venting or draining of high-pressure pipelines; when a single valve is used, a blind flange or flange cover should be installed. B.5.1.10 For the drainage at the lowest point of combustible gas pipelines in continuous operation, a double valve should be installed. The liquid discharged should be sent to a closed system. Combustible gas pipelines should be drained only during startup and shutdown; in such cases, a single valve suffices, along with a plug, pipe cap, or flange cover. GB/T 20801.6-2020, Code for Pressure Piping – Industrial Piping – Part 6: Safety Protection, 7.4.11: When utility pipes such as steam, air, and nitrogen are connected to GC1-class or GC2-class pipes, the following requirements shall be met: a) Check valves shall be installed on the utility pipes that are in continuous use, with isolation valves placed at their bases ； b) Two isolation valves shall be installed on the utility pipes used intermittently, with a check valve placed between the two valves. Code for Fire Protection Design of Buildings GB 50016-2018 5.4.15 For boilers and diesel generators installed within buildings, their fuel supply pipelines shall comply with the following requirements: 1. Automatic and manual shut-off valves shall be installed on the pipelines both before they enter the building and inside the equipment room. Code for Environmental Protection Design in Petrochemical Industries SH/T 3024-2017 10.2.4 For pipelines carrying highly hazardous substances, dual valves shall be used for discharge, and the waste fluid 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. 10.2.10 Centrifugal pumps and rotary pumps used for transporting process materials should preferably be equipped with mechanical seals. For centrifugal pumps and rotary pumps used to transport toxic media, sealless pumps should be chosen, and the sealing grade should be increased. Centrifugal pumps or rotary pumps used to transport toxic and hazardous media should be equipped with bottom drain valves, which should be designed as a dual-valve system. \"Code for Design of Instrument Piping and Wiring\" HG/T 20512-2014 6.0.7 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 with a pipe cap. Code for Design of Industrial Metal Piping GB 50316-2000, 2008 edition, 14.2.8: For piping carrying extremely hazardous media (such as highly toxic or highly corrosive substances), venting or drainage pipes must be equipped with double valves and connected to a closed recovery system. GB 11984-2024 \"Code for Safety Management of Liquid Chlorine Storage Tanks\" 6.1.6 The gas/liquid process connections of liquid chlorine storage tanks must be equipped with two valves, and their sealing performance must be checked regularly. \"Code for Occupational Safety and Health Design in Petrochemical Enterprises\" SH/T 3047-2021 7.1.4.2 Reliable sealing methods shall be employed at those parts of equipment, pumps, pipelines, fittings, etc. where material leakage is likely to occur. At the discharge ports and sampling ports of equipment and pipelines, measures such as installing blind flanges and dual valves are advisable. 7.1.5.6 Double valves should be provided for the discharge valves of highly corrosive liquids. \"Guidelines for the Implementation of Safety Standardization in Ammonia Synthesis Enterprises\" AQ/T 3017-2008 5.5.4.6 Double isolation valves shall be installed on the inlet and outlet pipelines of liquid ammonia storage tanks, one of which shall be an emergency isolation valve. \"Technical Specifications for the Production of Anhydrous Hydrogen Fluoride\" GBT28603-2012: 9.12 The low-point drain valves in liquid pipelines must ensure tightness; two such valves can be installed, and the discharged liquid should enter a sealed system. Article 5 of the **Guiding Opinions of the State Administration of Work Safety on Strengthening Leak Management in Chemical Enterprises** (An Jian Zong Guan San [2014] No. 94) stipulates that when designing discharge valves such as those at the outlets and sampling points of equipment and pipelines, measures such as the use of blind flanges, thread plugs, pipe caps, and double valves should be employed to reduce the likelihood of leaks. For processes involving highly toxic or extremely toxic substances, a closed sampling system should be designed, and closed systems must be used for the safe pressure relief of toxic and flammable gases. Chapter 5 of the \"Guidelines for the Identification and Rectification of Safety Risks and Hazards in Hazardous Chemical Enterprises\" (Emergency [2019] No. 78) stipulates that at the discharge points such as outlets and sampling ports of equipment and pipelines involving flammable, explosive, or toxic substances, measures such as the installation of blind flanges, plug valves, pipe caps, and double valves should be taken to reduce the likelihood of leaks. In summary: 1. Based on the hazard level of the medium, for extremely/highly hazardous media (such as highly toxic or highly corrosive substances), double valves must or should be installed at the venting, drainage, sampling points of their pipelines, as well as on the pipes at the base of storage tanks; the emissions must be directed into a closed recycling system. This is the highest level of leakage prevention requirement. Flammable liquids/gases: Continuous-operating flammable gas pipelines should be equipped with a double-valve system at their lowest point for draining into a closed system. Dehydration/water separation pipelines for liquefied hydrocarbon (such as liquefied petroleum gas) storage tanks should be equipped with dual valves. Highly corrosive, valuable, or specially polluting media: Dual valves should be used for the pipes at the bottom of the storage tank as well as for the drain valves. 2.2. Selection is based on operational characteristics and reliability requirements: Continuous operation vs. intermittent use: For pipelines that are in continuous operation and require regular drainage (such as at the low point of flammable gases), a dual-valve system should be installed to provide an additional sealing mechanism during operation. For the drain port used only during startup and shutdown, a valve can be installed, but it must be equipped with a plug, pipe cap, blind flange, or flange cover. Equipment that needs to be isolated for maintenance: Equipment that must be shut down while in operation for maintenance, cleaning, or regeneration requires double valves in the pipes connecting it to the system, with a check valve (relief valve) installed between the two valves to create a reliable isolation section. 3.3. According to pipeline category and parameter settings: For high-pressure pipelines, it is advisable to install double valves at the vent or drain ports, or use a single valve along with a blind flange/flange cover. For GC1/GC2 grade process pipelines: When connected to utility systems (steam, water, nitrogen, etc.), the side of the utility system that is used intermittently shall be equipped with two isolation valves along with check valves ； Check valves and root cut-off valves should be installed for continuous use. Prevent process media from flowing back into the utility system. 4.4. Installation of storage tanks and containers at key locations: For storage tanks used to hold hazardous materials such as liquefied hydrocarbons, toxic substances, and highly corrosive media, a main shut-off valve should be installed near the base of the tank at the inlet and outlet pipes for these materials; in addition, an operating valve should also be installed on each inlet and outlet pipe. Drain pipes and dewatering pipes should be equipped with double valves. Emission ports and sampling ports: For the emission ports and sampling ports of all equipment and pipelines involving flammable, explosive, or toxic substances, measures such as the installation of blind flanges, thread plugs, pipe caps, or double valves should be taken to reduce the risk of leaks. Isolation and switching points between systems: For piping used for switching between multiple flares, as well as the connection points between process materials and utility materials, double isolation valves and blind plates (or inspection valves) should be installed to ensure effective isolation and prevent cross-contamination. 5.5 Principle of Alternative Solutions: When the specification states that “a dual-valve setup is recommended,” it is permissible to use a “single valve + blind flange/flange cover” as an alternative solution. However, if it is stated that “two valves should be installed,” then two valves must be provided. “Operation sequence of the “double valve”: Usually, the first valve located closer to the material side is kept open (as a backup seal valve), while the second valve is the one that is used for regular operation. In the event of a leak, close the first valve first, then repair or replace the second valve.