Valves are essential equipment for gas transmission and distribution, safe operation of storage systems and overhauls, alterations and developments. If the gas valve improper selection or poor quality, it may lead to leakage, stop production and other accidents. In the event of an accident, it will affect the normal life and production of the society, while causing heavy losses to the country and people's lives and property. Therefore, the selection of gas valves must be cautious. 1 gas valve application status With the development of gas industry, the demand for gas-specific valves is growing, Shanghai Ba valve manufacturers continue to introduce new products to meet the needs of market competition. At present, the buried gas valves in our country mainly include gate valves, ball valves and butterfly valves. Transmission methods are mainly manual, worm gear, electric, pneumatic, gas - liquid linkage and so on. There are two ways to install the gate and burial. According to the relevant information, the most widely used type of manual gate valve in the coal gas transmission and distribution system in town and town is. Taking Tianjin as an example, there are more than 120 medium-pressure pipelines and over 300 valves in the three districts of Hebei, Hongqiao and Beichen, of which over 80% are gate valves, followed by butterfly valves and ball valves. Prior to the implementation of the gas conversion project, artificial gas was operated in the gas pipelines of the three districts mentioned above. Artificial gas contains more impurities, especially tar, aromatic and dust mixed to form the "gas glue" often affect the valve seal or even "kill" the valve stem. Therefore, the choice of the valve we mainly choose from the structural characteristics of the valve to solve this problem, so as to ensure the valve opening and closing flexibility, no leakage. After many years of practical exploration and data analysis, we found that gate valves (including parallel double gate gate valves and flexible gate seals) are more suitable. However, with the end of the Dayai Coal Project, the nature of gas has changed. Natural gas is cleaner and drier than coal gas, but contains grit dust. The pressure is also higher than that of coal gas. Under high pressure, sand dust will form a strong scour to the valve cavity and the natural gas contains extremely aggressive hydrogen sulfide , So how to choose the natural gas valve on the urban underground pipeline is a new issue before us. 2 natural gas valve selection analysis 2.1 Buried natural gas valve should meet the requirements Natural gas is flammable and explosive corrosive and so on, so installed in the underground pipe network of natural gas valves should meet the following requirements: 2.1.1 material corrosion-resistant pipe conveying Natural gas contains a large amount of hydrogen sulfide (a highly toxic and highly corrosive gas) that reacts with iron to form iron sulfide, flaking and corrosion of machinery before desulfurization. Even after natural gas desulfurization process, there is still residual hydrogen sulfide. Therefore, the selection of pipeline valves should be selected anti-sulfur corrosion-resistant materials. 2.1.2 reasonable structure Buried gas valve should be full-bore design, reducing the flow resistance, easy to pass through the pipe cleaner or pipe detector, while saving operating costs; as much as possible to reduce the structural height in order to save installation costs; valve should be installed at the top Fully closed opening and closing indicator, easy to see the operator at any time the valve state, in order to avoid misuse. 2.1.3 tightness Natural gas leakage of the valve requirements are very strict, CJ3055-95 "town gas valve experiment and inspection" standard: soft seal valve at 1.1 times the nominal rated pressure does not allow any perceived internal leakage, Hard seal valve 1.1 times the rated nominal pressure allowed within the leakage of less than 0.3DNmm3 / s. As for the external leakage is absolutely not allowed. Usually buried and more important valves are all welded body structure. In order to ensure the sealing performance of pipeline valves, sealing pair with excellent corrosion resistance, wear resistance, self-lubrication and flexibility. 2.1.4 easy operation Underground pipeline valve for the vast majority of human opening and closing, requiring the valve opening and closing torque is small, the entire number of revolutions can not be too much to facilitate the accident as soon as possible after cutting off the gas source. 2.1.5 Maintenance of simple valve parts design should consider the use of less maintenance, maintenance-free structure, as much as possible to reduce the workload of maintenance and repair, reduce the valve overhaul and closed roads, affecting traffic conditions. 2.2 Comparison of several commonly used valves At present, China's natural gas industry gas valves from the structure of the main points are three categories, namely, valve, ball valve, butterfly valve. Here are three aspects of these types of valves for analysis and comparison: 2.2.1 Comparison of working principle and structural characteristics Gate valve through the gate up and down movement, to open and close the valve, in order to achieve a site on the pipeline system needs "full , Full close "control, and to meet the medium through only a slight pressure drop requirements. Gate valve is usually applied to do not often need to open and close, and keep the gate fully open or fully closed conditions. Not suitable for use as a regulator or throttling. Gate valve is generally full-bore design, flow resistance is small, by cleaning the ball and pipe detectors. High gate valve structure (usually 3-5 times the diameter), suitable for deeper pipeline depth situation. Ball valve is to rotate the valve to open and close the valve (open, closed only 90 ° rotation). Ball valve switch light, small size, can be made of large diameter, reliable sealing, sealing surface and the spherical surface often closed state, easy to be media erosion, in various industries have been widely used. Its simple structure, easy maintenance, full-bore design, flow resistance is small, by cleaning the ball and pipe detectors. The butterfly valve is designed according to the principle of the tube baffle. The flow control element is an inclined plate fixed on the mandrel. The rotating mandrel controls the opening and closing, and the valve seat is fixed on the valve body wall. The body is a pancake-type, suitable for regulating the media flow. Butterfly valve structure, small size, light weight, easy to operate, but the circulation resistance and can not pass through the cleaning ball and pipe detectors. From the above structural characteristics and working principle to analyze, gate valve and ball valve is more suitable for natural gas pipelines. 2.2.2 Economic comparison We compare the cost of installing a valve with a rated pressure of 4 kg and a nominal diameter of DN200. The cost of using a ball valve is the highest, which is about three times the cost of a gate valve and a butterfly valve. Gate valve although the price is much higher than the butterfly valve (about 4 times the butterfly valve), but because of this type of valve can be buried, so save a lot of installation costs, so that the overall cost of gate valve and butterfly valve similar. And from the results of many years of use of performance and service life of the valve is far superior to the butterfly valve. So from this point of view, gate valve should be the preferred valve. 2.2.3 Comparison of safety With the continuous improvement of technical level, the safety of various gate valves has been continuously improved. Parallel double gate valve installed inside the valve stem protection sleeve, the valve stem from erosion of the medium; shell with a special design of the "squirrel cage frame stiffeners" to reduce the overall weight of the valve to enhance the shell strength and stiffness ; Elastic sealing valve with flexible hard seal, the valve is fully open or fully closed, the seal completely separated from the media chamber with the valve so that the valve has a fire-resistant, high temperature, corrosion-resistant features. Gate valve with a fully enclosed opening and closing indicator, the operator clearly understand the state valve. Ball valve also has fire resistance, high temperature characteristics. After the fire burns the PTFE material on the sealing seat, the metal sealing seat and each sealing part can form a metal-to-metal sealing structure to prevent the diffusion of the gas medium and prevent the disaster from continuing to expand. In addition, it also has an anti-static structure, Ball valve static electricity formed during opening and closing into the ground to avoid the accumulation of static electricity ignite the media to ensure the safety of the device; ball valve with a limited locking mechanism to prevent operator misuse or illegal operation. Butterfly valve sealing deputy isolation width is too narrow, likely to cause valve flap closing too far or not in place, affecting the seal; In addition, the seal deputy - part of the rubber or PTFE, easy to damage or fall off gas flushing, and In the event of high temperature easily damaged, the service life is short. From the above analysis results we can see, no matter from which point the butterfly valve is not suitable for buried gas pipelines. However, due to space constraints, only the choice of butterfly valve. Select the butterfly valve should be used when more eccentric eccentric butterfly valve, sealing material selection of PTFE or hard seal, debugging must be accurately adjusted to the closed position. Gate valve and ball valve from the structural characteristics and safety are more suitable for buried natural gas pipelines. But they also have their own shortcomings. Ball valve from design to manufacturing require a higher level of technology, so its cost is higher. Gate valve opening and closing need to rotate a lot of laps, opening and closing time is longer. Therefore, when we use the valve should be integrated in all aspects of factors, to ensure safe and reliable, try to save costs, from this perspective, we should be based on gas pressure and the rational use of the pressure of the valve selection, both to meet the pipeline Safe operation can achieve the purpose of reducing cost and making the best use of it. Conclusion With the continuous development of China's gas industry and science and technology, there will be more new technologies, new materials used in the manufacture of gas valves. Therefore, the selection of gas valves should also be continuously improved standards to meet the needs of gas users.
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