Valve is gas transmission and distribution, storage systems, safe operation and maintenance, transformation, essential for the development of important equipment. If the gas valve selection of inappropriate or poor quality, it may lead to leakage, stop and other accidents. When the accident occurred, ranging from the normal life of society and production, while in the state, significant loss of life and property. Therefore, the selection of the gas valve must be careful.

      1, Application of gas valve

      With the development of gas, gas increasing demand for special valves, the Hai Ba valve valve manufacturers continue to introduce new products to meet market competition. At present, gas valves buried with the main points from the structural form of gate valve, ball valve, butterfly valve. Transmission mainly manual, worm gear drive, electric, pneumatic, gas – liquid interaction and so on. There is a need GATE installation of two wells and buried. According to statistics, in cities and towns in the coal gas transmission and distribution system is the most widely used manual valve. A Case Study, the city of Hebei, Hongqiao, North Star in the three districts have more than 120 km pipeline pressure, valve 300, of which 80% are gate valve, followed by butterfly valve, ball valve. Gas source in the implementation of the conversion works, these three areas is a gas pipeline running artificial gas. Artificial gas containing more impurities, especially tar, aromatic hydrocarbons and dust mixed to form the “gas plastic” often affect the valve seal or even “killed” stem. Therefore, the selection of the valve, we select those from the major structural characteristics of the valve to solve this problem, thus ensuring flexible and open and close the valve, no leakage. After years of practice and exploration and data analysis, we found that valve (including the parallel elastic sealing Gate Valve and a single gate valve) is more appropriate. But with the days of the end of the project on behalf of coal, gas nature of the change. Natural gas than coal gas clean, dry, but with the quality of sand dust, pressure is also higher than the coal gas at high pressure valves under the sand dust will form a strong flushing of the cavity and the gas contains corrosive sulfide hydrogen, so how in the urban underground natural gas pipeline valve is used in front of us a new topic.

      2, the gas valve selection of

2.1 Underground Gas valves shall meet the requirements of

      Corrosive gas with a flammable and explosive characteristics, so the installation of natural gas in underground pipe-line valves shall meet the following requirements:

      2.1.1 Corrosion-resistant materials

      Pipeline transportation of natural gas desulfurization ago contains a lot of hydrogen sulfide (which is a highly toxic and corrosive gases, it is the reaction of iron sulfide and iron, peeling off in sheets, corrosion, mechanical equipment). Even after the desulfurization of natural gas, there are still remnants of hydrogen sulphide. Therefore, selection must be selected pipeline valve sulfur corrosion resistant materials.

      2.1.2 structured

      Buried gas valve should be full bore design, flow resistance, easy cleaning device through a pipe or pipe detectors, while saving operating costs; minimize the structural height in order to save installation costs; top of the valve should be fitted with closed open and close indicator, easy to see the valve operator in which the state at any time, in order to avoid misuse.

      2.1.3 tightness

      Valve leakage of natural gas is very strict, CJ3055-95 “town gas valve experiment and test” standards: soft sealing valve rated at 1.1 times the nominal pressure is not allowed to use any detection of internal leakage, seal valves door rated at 1.1 times the nominal operating pressure within the allowable leakage rate is less than 0.3DNmm3 / s. For the external leak is absolutely not allowed. Usually buried and have adopted the more important of the valve body welded structure. In order to ensure the sealing performance of pipeline valves require valve with excellent corrosion resistance, wear resistance, self lubrication and flexibility.

      2.1.4 easy to operate

      The vast majority of human underground pipeline valves open and close, thus requiring the valve opening and closing torque, full revolutions can not be too easy to cut off as soon as possible after the incident gas source.

2.1.5 maintenance is simple

      Design of the valve components should be considered less maintenance, maintenance-free structure, repair and maintenance to minimize the workload, reduce maintenance and road closure valve repair, the impact on traffic to happen.

      2.2 Comparative analysis of several commonly used valves

      At present, China uses the gas valve and gas industry from the structural form of points there are three main categories, namely, gate valves, ball valves, butterfly valves and so on. The following three aspects were analyzed and compared for these types of valves:
 
      2.2.1 The principle and structure characteristics of the comparison

      Valve is moving up and down through the gate to open and close valves in order to achieve a certain part of the pipeline system needs to “fully open, fully closed” control, and produced only small to meet the media through pressure drop requirements. Valve is usually applied to less frequent opening and closing, and maintain the gate fully open or fully closed condition. Does not apply to a regulator or throttle. Generally full bore valve design, flow resistance, by sweeping the ball and pipe detector. Gate structural height is higher (typically 3-5 times the diameter), the situation for deeper pipeline depth.
 
      By a rotating ball valve is to open and close the valve (open, closed only rotate 90 °). Valve switch light, small size, can be made into a large-caliber, reliable sealing, sealing surface and the spherical often in the closed state, the media can not easily be eroded, in the various sectors has been widely used. Its simple structure, easy maintenance, full bore design, flow resistance, can clean the ball and pipe detector.

      Pull the tube block valve is based on the principles of design, the flow control element is a disk inclination, disk fixed mandrel, and rotating spindle to control the opening and closing the valve seat fixed in the body wall. The body is pizza type, suitable for regulating media flow. Butterfly structure, small size, light weight, easy to operate, but the flow resistance is large and can not clean the ball and pipe detector.

      From the above structural characteristics and working principles to analyze, gate and ball valve fits the natural gas pipeline.

 2.2.2 Economic Comparison

      We install a rated pressure of 4 kg, nominal diameter DN200 valve economic cost comparison drawn, using the highest cost ball valves, gate valves and butterfly valves cost about three times. Although the butterfly valve is much higher than the price (about 4 times the butterfly), but because such a valve can be buried, so saving a lot of installation costs, so that the valve and butterfly valve similar to the overall cost. The view from the many years of using the results of valve performance and life is much better than the butterfly. Therefore, from this part of view, gate valves should be preferred.

      2.2.3 Comparison of the safety

      As technology continues to improve, the safety of all valve also been improved. Parallel Gate Valve with stem within the protective cover, so that stem from the erosion of media; Enclosure is designed to “frame rib cage” and reduce the overall weight of the valve, increased shell strength and stiffness; flexible elastic sealing valve seal, valve fully open or fully closed, sealing the valve chamber completely isolated from the same medium to make the gate with a fire-resistant, high temperature, corrosion-resistant characteristics. Closed with the opening and closing valve indicator, so that the operator a clear understanding of the state in which the valve.

      Ball also has a fire resistance, high temperature properties. The fire burned hot seat PTFE seal material, the metal sealing seat and the various sealing parts can form metal to metal seal structure to prevent gas diffusion media to prevent the disaster continue to expand; addition it also has anti-static structure, so that ball in the opening and closing process into the formation of static electricity underground, to avoid accumulation of static electricity igniting the media, to ensure the safety of equipment; limited bit lock body valve to prevent operator misuse or illegal operation.

      Vice isolation valve seal width is too narrow, making it easy during the valve closed too far or not in place, affecting sealing; addition, since the sealing of – part of rubber or Teflon, in the gas are easily damaged or washed off, and easily damaged in a fire when the temperature, the use of life is short.
 
      From the above analysis we can see, no matter from which point of view, are not very suitable for butterfly valves buried gas pipeline. But because of space constraints, only selected valve. More eccentric butterfly valve should be selected high-quality selection of butterfly valves, PTFE seal or seal material selection, commissioning must be accurately adjusted to the closed position.

      Valve and ball valve from the structure and safety features are more suitable for underground gas pipeline. But they also have their own drawbacks. Ball needs from design to manufacture a high level of technology, so its higher cost. Required to rotate a lot of valve opening and closing circles, open and close with a longer time. So we should be integrated in the selection of valves of various factors, to ensure safe and reliable as far as cost savings, from this perspective, we should use the gas pipeline pressure characteristics and rational use of the valve, can meet the safe operation of pipelines and can to reduce costs, make the best use purposes.