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Requirements of underground pipe network valves Gas valves installed in the underground pipe network should be able to withstand the maximum pressure of gas transmission and distribution within the pipeline, more importantly, must have sufficient rigidity, that is, subject to underground pipe network subject to various external forces Can maintain the valve does not occur deformation, reliable guarantee valve sealing performance. 1) As much as possible to reduce the height of the valve structure (referring to the center of the valve flow cross-section to the height of the valve top), in order to reduce the depth of the buried pipe. 2) The top of the valve should be equipped with fully enclosed opening and closing indicator, easy to check the status of the valve is an important condition for safe operation. 3) full-bore design, reduce the flow resistance, easy to pass through the pipe cleaner or pipe detector, but also to meet the non-stop closed hole drilling machine in the pipe openings. 4) Reliable sealing performance, soft seal valves are not allowed to have any noticeable internal leakage at 111 times the nominal rated service pressure. Hard seal valves allow less than 13 DN mm3 / s internal leakage at 111 times nominal service pressure. This requirement is more stringent, for example: 500 mm nominal diameter gas valve internal leakage can not be greater than 9 ml / min, the external leakage is absolutely not allowed. 5) The gas valve used in underground pipe network should be corrosion-resistant housing, it is recommended according to the pipeline transmission medium and the use of pressure using different shell material. 6) Valve parts design should consider the use of less maintenance or maintenance-free structure, as much as possible to reduce the workload of inspection and maintenance, reduce the maintenance and repair of the valve closed roads affect traffic. 7) The vast majority of underground pipeline valves are manually opened and closed, requiring the valve opening and closing torque is small, the entire number of turns reasonable, the accident can cut off the gas source as soon as possible. 8) Under the urban roads in our country, all kinds of pipelines are crowded and intricate. It is not suitable to excavate cellar wells, so it is recommended to use buried valves. Valve shell material selection according to the use of gas pipeline pressure, the rational use of valve shell material, both to meet the safe operation of the pipeline can achieve the purpose of reducing costs, make the best use. 1) As for the transmission and distribution pipelines of Medium Pressure Class B (12 MPa), it is recommended to use gray cast iron valve. Its biggest advantage is good anti-corrosion performance and low price, which is very suitable for underground pipelines. 2) Below the high-pressure class B (18 MPa), and even below the medium-pressure class A (014 MPa), it is recommended to use nodular cast iron or cast steel valves, focusing on ductile iron. For example: ductile iron grade QT400218 and cast steel grade WCB compared to the former mechanical properties of not less than the latter, and corrosion resistance and casting process are superior to cast steel, the price is lower than the cast steel (usually cast steel 70 %about). Therefore, the overall performance and price of ductile iron valves are superior to cast steel valves, ductile iron valves as far as possible in the above pressure range, but with particular attention to the quality of ductile iron material monitoring. 3) At high pressure A level (116 MPa) or higher pressure level, it is recommended to use cast steel valves to make valves below 4MPa pressure level. Generally, WCB grades are used. 4) In recent years, gas companies around the world have started to use PE pipes, which are currently used below the pressure level of 014 MPa. PE ball valve has also come out, the series of ball valves can be buried, sealing performance is better. However, the current price of PE ball valve, especially larger than 100mm diameter ball valve.