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1. The original intention behind installing two valves is that, according to the interpretation of Article 7.2.8 in the Petrochemical Code GB 50160, two drain valves should be installed at the lowest point of combustible gas pipelines that are in continuous operation. The first valve (the one located closer to the pipeline) is a normally open valve, while the second valve is a valve that is operated regularly. When a leak was detected in the second valve, the first valve was closed and the second valve was replaced. ” 2. Usage after installing two valves: According to the explanatory provisions of the Petrochemical Code GB 50160, when two valves are installed, not both valves are closed during operation; only one valve remains closed while the other is kept open as a backup in case the first valve leaks. In actual production at present, we can require one of the two valves to be open and the other closed, or we can require both valves to be in a closed state. This is because, first, the valve itself has insufficient manufacturing precision, and the imperfect sealing surfaces pose an inherent risk of leakage ； Secondly, the frequent use of valves can also lead to internal leakage as a result of wear on the sealing surfaces ； Third, the presence of crystals (scale) and other deposits in the equipment and pipelines causes the valves to not close properly, leading to leaks ； Fourth, improper operation by the staff may result in the valve not being closed properly, leading to leaks. For exhaust outlets that lead directly to the atmosphere, it is still required that both valves be closed. 3 Locations where dual valves should be installed: Given that the primary purpose of installing dual valves is to prevent leaks and avoid accidents resulting from the leakage of materials, dual valves should be used to enhance leak prevention at the following discharge and sampling ports that handle flammable, explosive, toxic, harmful, or corrosive substances: (1) Discharge ports at the low points of equipment and pipelines where liquid leakage may occur. This is because the liquid medium can easily accumulate in front of the outlet valve at the lower points due to static pressure, increasing the likelihood of leakage. Although a single valve with a blind flange can be preferentially used for these discharge ports, for discharge valves that need to be operated frequently, a dual-valve design can still be adopted to avoid frequent removal of the blind flange. (2) Manually operated liquid and gas sampling ports. This is due to the frequent operation of these valves, which may result in them not closing properly. 4 Locations where dual valves should not be installed: Dual valves should not be installed on the outlets used for gas release. This is because the relief pipeline is designed for emergency release in case of system overpressure; installing two valves actually reduces the speed at which emergency responses can be carried out, and it may even lead to accidents due to delayed release. 5 The following locations do not require the installation of dual valves: (1) Exhaust outlets installed at the top of pipes or equipment used for transporting liquids. This is because system exhaust mainly occurs in situations such as startup/shutdown or abnormal production when operating conditions need to be adjusted; in these cases, these valves are operated infrequently. Moreover, there is little gas accumulated in the pipes of the equipment used for transporting liquids, the air pressure is low, and the likelihood of gas leakage is minimal. Therefore, it is not necessary to install dual valves. (2) Pipeline discharge outlets related to utility systems (water, steam, gas). Although these pipe openings are also prone to medium leakage, from a risk-based perspective, it is necessary to analyze the severity of the potential consequences resulting from leaks of water, steam, and gas. Especially when companies carry out operations that involve increased use of dual valves and that entail higher risks such as production shutdowns, halts in operations, and hot work, adding dual valves proves to be counterproductive. (3) For sampling points involving flammable, explosive, toxic, and harmful materials, it is preferable to start with closed-system sampling in order to permanently resolve leakage issues, rather than simply requiring the installation of double valves. For sampling ports that already use sealed sampling, it is not necessary to install a double valve. Selection requirements for 6-way valves: For locations where dual valves are required, the valve type should be selected scientifically. Generally, flanged valves are preferred over welded valves; ball valves and gate valves are advisable, while globe valves should be used as little as possible. This is mainly to facilitate replacement and maintenance in the future, as well as to make it easier to clear blockages in the pipes. When emission control is required, a ball valve can be considered for the front control valve, while a gate valve or needle valve is recommended for the rear control valve. Once it is detected that the rear hand valve does not close properly, the front hand valve can be closed quickly, and the ball valve is not prone to being clogged by solid impurities. The selection of drain valves for on-site level gauges should be determined based on the specific conditions. For the drainage valves installed at the lowest point of the on-site level gauge, they need to be chosen according to the properties of the material being measured. At the low points of the liquid guiding pipelines that are prone to crystallization-induced blockages, ball valves, gate valves, or cock valves should be installed ； To measure the low point of a level gauge for materials that are difficult to crystallize and clog pipelines, a valve + plug configuration can be used.