Valve is an essential equipment for safe operation, maintenance, transformation and development of gas transmission and distribution and storage system. If the type selection of gas valve is improper or the quality is poor, it may cause accidents such as leakage and shutdown. Once an accident occurs, it will affect the normal life and production of the society, or bring heavy losses to the life and property of the state and people. Therefore, the selection of gas valves must be careful.

　　1. Application status of gas valve

　　With the development of gas industry, the demand for special gas valves is increasing. Shanghai Ba valve manufacturers continue to launch new products to meet the needs of market competition. At present, China's buried gas valves mainly include gate valve, ball valve and butterfly valve. The transmission modes mainly include manual, worm gear transmission, electric, pneumatic, pneumatic-hydraulic linkage, etc. The installation methods include gate shaft construction and direct burial. According to relevant data, manual gate valve is the most widely used in urban coal to gas transmission and distribution system. Taking Tianjin as an example, there are more than 120 km of medium pressure pipelines and more than 300 valves in Hebei, Hongqiao and Beichen districts, of which more than 80% are gate valves, followed by butterfly valves and ball valves. Before the implementation of the gas source conversion project, artificial gas was operated in the gas pipelines in the above three areas. Artificial gas contains more impurities, especially the "gas glue" formed by the mixing of tar, aromatic hydrocarbon and dust often affects the valve seal and even "bites" the valve stem. Therefore, in the selection of valves, we mainly choose those valves that can solve this problem from the structural characteristics, so as to ensure that the valve can be opened and closed flexibly and without leakage. After years of practice and data analysis, we found that gate valves (including parallel double gate valves and elastic seal single gate valves) are more applicable. However, with the end of the natural gas instead of coal project, the nature of gas has changed. Natural gas is cleaner and drier than coal to gas, but it contains sand dust and higher pressure than coal to gas. Under the action of high pressure, sand dust will form a strong scouring effect on the inner cavity of the valve, and natural gas contains highly corrosive hydrogen sulfide. Therefore, how to select natural gas valves on urban underground pipelines is a new topic in front of us.

　　2. Selection and analysis of natural gas valves

　　2.1 requirements for buried natural gas valves

　　Natural gas is flammable, explosive and highly corrosive, so the natural gas valve installed on the underground pipe network shall meet the following requirements:

　　2.1. 1. Corrosion resistance of materials

　　The natural gas transported by the pipeline contains a large amount of hydrogen sulfide before desulfurization (this is a toxic and highly corrosive gas, which reacts with iron to form iron sulfide, flaking and corroding mechanical equipment). Even if the natural gas is treated by desulfurization and other processes, there is still residual hydrogen sulfide. Therefore, sulfur resistant and corrosion-resistant materials shall be selected for pipeline valves.

　　2.1. 2. Reasonable structure

　　The buried gas valve shall be of full bore design to reduce the flow resistance, facilitate the passage through the pipeline cleaner or pipeline detector, and save the operation cost; Reduce the structure height as much as possible in order to save the installation cost; A fully enclosed opening and closing indicator shall be installed on the top of the valve to facilitate the operator to see the state of the valve at any time to avoid misoperation.

　　2.1. 3. Good sealing performance

　　The leakage requirements of natural gas valves are very strict. Cj3055-95 test and inspection of urban gas valves stipulates that soft seal valves are not allowed to have any perceived internal leakage under 1.1 times the nominal rated service pressure, and the allowable internal leakage of hard seal valves under 1.1 times the nominal rated service pressure is less than 0.3dnmm3/s. As for leakage, it is absolutely not allowed. Generally, buried and more important valves adopt full welding structure of valve body. In order to ensure the sealing performance of pipeline valves, the sealing pair is required to have excellent corrosion resistance, wear resistance, self lubrication and elasticity.

　　2.1. 4 convenient operation

　　The vast majority of underground pipeline valves are opened and closed manually, so it is required that the opening and closing torque of the valve is small and the number of turns in the whole process can not be too many, so that the gas source can be cut off as soon as possible after the accident.

　　2.1. 5 simple maintenance

　　The design of valve parts shall consider the use of less maintenance and maintenance free structure, so as to reduce the workload of inspection and maintenance as much as possible, and reduce the closure of roads due to valve inspection and maintenance and the impact on traffic