Abstract: This paper mainly discusses the design of ultra-low temperature valve for LNG, and introduces the structural design, industrial control characteristics, manufacturing materials and manufacturing process of ultra-low temperature valve. On this basis, the closing torque and sealing structure of ultra-low temperature valve are expounded. As well as material properties, I hope to provide reference value for ultra-low temperature valve designers.
Key words: liquefied natural gas; cryogenic treatment; ultra-low temperature valve; design and research CLC number: TH134 Document code: A
1 Valve material selection 1.1 Austrian stainless steel ultra-low temperature valve material needs to have strong stability and toughness to ensure that it will not deform due to phase change in low temperature or ultra-low temperature conditions, which will affect the sealing performance of the valve. In general, the body-centered cubic structure has low-temperature brittleness, and the low-temperature toughness and plasticity of the face-centered cubic structure is better. The face-centered cubic austenitic stainless steel is used as the valve flap, valve seat and valve body material of the valve, because the above materials are not available. Low temperature cold and brittle critical temperature, so it can exhibit high toughness and plasticity under low temperature conditions. At 26 ° C ~ -265 ° C, the yield strength and tensile strength of the austenitic stainless steel material will increase with the decrease of temperature.
1.2 PCTFE
This material is a chlorotrifluoroethylene polymer and its material properties are thermoplastic resins. Since the C-CL bond is introduced into the molecular structure of PCTFE, it is relatively low in chemical inertness, heat resistance, rigidity, and creep resistance as compared with materials such as tetrafluoroethylene. Among liquefied natural gas and liquid oxygen, PCTFE does not exhibit creep and brittle fracture. Among most non-metallic materials, the water vapor permeability is low, does not penetrate into the gas, has no combustion-supporting properties, and is a sealed polymer. Since the low temperature resistance of the substance is remarkable, it can reach -270 ° C under certain conditions.
2 Cryogenic treatment Cryogenic treatment is the use of refrigerant as a cooling medium, the program is programmed to cryogenic treatment (-196 ° C) and low temperature tempering, so as to improve and strengthen the performance of metal materials. Cryogenic treatment is the latest in the world to improve and strengthen the performance of metal materials. It is the most effective and economical technology. During the cryogenic treatment, a large amount of retained austenite in the metal is transformed into martensite, especially the supersaturated metastable martensite will reduce the saturation during the treatment, precipitate and disperse, and maintain ultra-fine carbonization in a coherent relationship with the matrix. The material can reduce the martensite lattice distortion and reduce the microscopic stress, while the finely dispersed carbide can hinder the dislocation motion when the material is plastically deformed.
3 Structural design 3.1 Long-neck valve cover LNG ultra-low temperature valve The long-neck valve cover is mainly used to avoid the external heat transfer to the device; the packing part can be kept away from the LNG in the valve body to ensure the filling temperature is above 0 °C, avoiding The stem and bonnet components are frozen due to the low packing temperature, ensuring stable packing operation.
3.2 Drip board This part can effectively prevent the valve body temperature from being transmitted to the upper end of the valve stem and the packing, and fully ensure that the temperature of the upper part of the valve stem and the packing part is above 0 °C. Fig. 1 and Fig. 2 show the temperature field simulation diagram of the valve body without drip plate and drip plate. Comparing and analyzing it, it can be seen that the temperature at the upper end of the valve cover with the drip plate is significantly higher than that without the drip plate. The temperature can be effectively lowered after the upper portion of the bonnet is extended. Generally, the valve is exposed to the air, and when it encounters a low temperature, it is liquefied into water droplets. The diameter of the drip plate is larger than the diameter of the middle flange, so as to prevent the low temperature liquefied water drop from falling on the middle flange bolt and preventing the bolt from rusting.
3.3 Pressure-relieving parts Liquefied natural gas will expand rapidly after gasification, and there is a problem of abnormal pressure rise in the middle cavity. The liquefied natural gas remaining in the valve body cavity after closing the valve will absorb heat from the surrounding environment and rapidly vaporize, and the pressure in the valve body cavity will rise rapidly. When the pressure in the valve body cavity exceeds the limit of use of the valve body material, the valve will Will explode. Therefore, the ultra-low temperature valve needs to provide the design of the pressure relief structure, and the pressure is released to the front of the valve or the back of the valve according to the actual requirements of the process pipeline to avoid abnormal pressure rise of the cavity.
3.4 Anti-static structure Due to the flammable and explosive characteristics of liquefied natural gas medium, it is necessary to comprehensively analyze anti-static measures when designing liquefied natural gas ultra-low temperature valves. Especially for the PCTEF material valve seat, there is a hidden static electricity hazard, and the electrostatic friction generates sparks to cause the LNG to explode. A conduction device is required between the valve body and the valve stem, and between the closing member and the valve stem, so that static electricity can be taken out to avoid a safety accident. For metal-sealed ultra-low temperature valves, the conduction device may not be provided, but the resistance values ​​of the closing member, the valve stem and the valve body need to meet the standard specifications.
4 Valve sealing 4.1 Stem seal The valve stem leakage is mainly divided into external leakage and internal leakage. LNG is flammable and explosive, so the leakage is dangerous. Valve stem seals are the main cause of leaks throughout the leak. Valve stem seals for ultra-low temperature valves are typically made using flexible accumulator seals, low temperature seals and packing. In order to fully ensure the low-temperature sealing performance, the sealing structure of the valve stem adopts a combined sealing structure in which a plurality of sealing members are combined, and an additional elastic composite device, such as a butterfly spring gasket, can be used to enable the filling of the filler under low temperature conditions. The pre-tightening force is compensated to ensure the sealing effect of the packing.
4.2 The medium flange seal has good resilience and mechanical strength under low temperature conditions, and the coefficient of linear expansion is small. The medium flange seals for ultra-low temperature valves are typically combined seal structures using flexible graphite wound gaskets and elastomeric accumulator seals. The gasket seal will have a relatively reduced specific pressure at low temperatures, which may cause media leakage. Therefore, it is necessary to use a disc spring washer to compensate the middle flange fastening bolt joint.
In conclusion, with the rapid development of LNG receiving stations, factory gasification stations and transport vessels, the application range of ultra-low temperature valves has been gradually expanded. Therefore, in the design and manufacture of LNG ultra-low temperature valves, it is necessary to comprehensively analyze and test the valve structure and the influence of materials on the products, comprehensively study related topics, and verify the design theory using test methods, so as to ensure the design structure of the LNG ultra-low temperature valve. Effectiveness, fundamentally improve the reliability and safety of the valve.
References [1] Zhang Dachuan. Design of low temperature pipeline material for LNG receiving station[J].Refining Technology & Engineering, 2016, 46(6): 35-38.
[2] Tao Guoqing, Song Zhongrong, Yu Hongbing, et al. Design and research of ultra-low temperature valve for LNG engineering[J]. Fluid Machinery, 2015, 43(10): 47-51.
[3]Luo Qifei. Transient heat transfer and sealing of large diameter LNG ultra-low temperature ball valve[J].Shandong Industrial Technology,2015(11):11.
[4] Luo Qifei. Design of LNG ultra-low temperature valve and study on low temperature physical properties of materials [J]. Shandong Industrial Technology, 2015 (11): 57.
[5] Huang Yu, Song Kun, Chen Haiping. Domestic status and engineering application analysis of low temperature valves for LNG[J].Petrochemical Equipment, 2015,44(2):89-94.
Key words: liquefied natural gas; cryogenic treatment; ultra-low temperature valve; design and research CLC number: TH134 Document code: A
1 Valve material selection 1.1 Austrian stainless steel ultra-low temperature valve material needs to have strong stability and toughness to ensure that it will not deform due to phase change in low temperature or ultra-low temperature conditions, which will affect the sealing performance of the valve. In general, the body-centered cubic structure has low-temperature brittleness, and the low-temperature toughness and plasticity of the face-centered cubic structure is better. The face-centered cubic austenitic stainless steel is used as the valve flap, valve seat and valve body material of the valve, because the above materials are not available. Low temperature cold and brittle critical temperature, so it can exhibit high toughness and plasticity under low temperature conditions. At 26 ° C ~ -265 ° C, the yield strength and tensile strength of the austenitic stainless steel material will increase with the decrease of temperature.
1.2 PCTFE
This material is a chlorotrifluoroethylene polymer and its material properties are thermoplastic resins. Since the C-CL bond is introduced into the molecular structure of PCTFE, it is relatively low in chemical inertness, heat resistance, rigidity, and creep resistance as compared with materials such as tetrafluoroethylene. Among liquefied natural gas and liquid oxygen, PCTFE does not exhibit creep and brittle fracture. Among most non-metallic materials, the water vapor permeability is low, does not penetrate into the gas, has no combustion-supporting properties, and is a sealed polymer. Since the low temperature resistance of the substance is remarkable, it can reach -270 ° C under certain conditions.
2 Cryogenic treatment Cryogenic treatment is the use of refrigerant as a cooling medium, the program is programmed to cryogenic treatment (-196 ° C) and low temperature tempering, so as to improve and strengthen the performance of metal materials. Cryogenic treatment is the latest in the world to improve and strengthen the performance of metal materials. It is the most effective and economical technology. During the cryogenic treatment, a large amount of retained austenite in the metal is transformed into martensite, especially the supersaturated metastable martensite will reduce the saturation during the treatment, precipitate and disperse, and maintain ultra-fine carbonization in a coherent relationship with the matrix. The material can reduce the martensite lattice distortion and reduce the microscopic stress, while the finely dispersed carbide can hinder the dislocation motion when the material is plastically deformed.
3 Structural design 3.1 Long-neck valve cover LNG ultra-low temperature valve The long-neck valve cover is mainly used to avoid the external heat transfer to the device; the packing part can be kept away from the LNG in the valve body to ensure the filling temperature is above 0 °C, avoiding The stem and bonnet components are frozen due to the low packing temperature, ensuring stable packing operation.
3.2 Drip board This part can effectively prevent the valve body temperature from being transmitted to the upper end of the valve stem and the packing, and fully ensure that the temperature of the upper part of the valve stem and the packing part is above 0 °C. Fig. 1 and Fig. 2 show the temperature field simulation diagram of the valve body without drip plate and drip plate. Comparing and analyzing it, it can be seen that the temperature at the upper end of the valve cover with the drip plate is significantly higher than that without the drip plate. The temperature can be effectively lowered after the upper portion of the bonnet is extended. Generally, the valve is exposed to the air, and when it encounters a low temperature, it is liquefied into water droplets. The diameter of the drip plate is larger than the diameter of the middle flange, so as to prevent the low temperature liquefied water drop from falling on the middle flange bolt and preventing the bolt from rusting.
3.3 Pressure-relieving parts Liquefied natural gas will expand rapidly after gasification, and there is a problem of abnormal pressure rise in the middle cavity. The liquefied natural gas remaining in the valve body cavity after closing the valve will absorb heat from the surrounding environment and rapidly vaporize, and the pressure in the valve body cavity will rise rapidly. When the pressure in the valve body cavity exceeds the limit of use of the valve body material, the valve will Will explode. Therefore, the ultra-low temperature valve needs to provide the design of the pressure relief structure, and the pressure is released to the front of the valve or the back of the valve according to the actual requirements of the process pipeline to avoid abnormal pressure rise of the cavity.
3.4 Anti-static structure Due to the flammable and explosive characteristics of liquefied natural gas medium, it is necessary to comprehensively analyze anti-static measures when designing liquefied natural gas ultra-low temperature valves. Especially for the PCTEF material valve seat, there is a hidden static electricity hazard, and the electrostatic friction generates sparks to cause the LNG to explode. A conduction device is required between the valve body and the valve stem, and between the closing member and the valve stem, so that static electricity can be taken out to avoid a safety accident. For metal-sealed ultra-low temperature valves, the conduction device may not be provided, but the resistance values ​​of the closing member, the valve stem and the valve body need to meet the standard specifications.
4 Valve sealing 4.1 Stem seal The valve stem leakage is mainly divided into external leakage and internal leakage. LNG is flammable and explosive, so the leakage is dangerous. Valve stem seals are the main cause of leaks throughout the leak. Valve stem seals for ultra-low temperature valves are typically made using flexible accumulator seals, low temperature seals and packing. In order to fully ensure the low-temperature sealing performance, the sealing structure of the valve stem adopts a combined sealing structure in which a plurality of sealing members are combined, and an additional elastic composite device, such as a butterfly spring gasket, can be used to enable the filling of the filler under low temperature conditions. The pre-tightening force is compensated to ensure the sealing effect of the packing.
4.2 The medium flange seal has good resilience and mechanical strength under low temperature conditions, and the coefficient of linear expansion is small. The medium flange seals for ultra-low temperature valves are typically combined seal structures using flexible graphite wound gaskets and elastomeric accumulator seals. The gasket seal will have a relatively reduced specific pressure at low temperatures, which may cause media leakage. Therefore, it is necessary to use a disc spring washer to compensate the middle flange fastening bolt joint.
In conclusion, with the rapid development of LNG receiving stations, factory gasification stations and transport vessels, the application range of ultra-low temperature valves has been gradually expanded. Therefore, in the design and manufacture of LNG ultra-low temperature valves, it is necessary to comprehensively analyze and test the valve structure and the influence of materials on the products, comprehensively study related topics, and verify the design theory using test methods, so as to ensure the design structure of the LNG ultra-low temperature valve. Effectiveness, fundamentally improve the reliability and safety of the valve.
References [1] Zhang Dachuan. Design of low temperature pipeline material for LNG receiving station[J].Refining Technology & Engineering, 2016, 46(6): 35-38.
[2] Tao Guoqing, Song Zhongrong, Yu Hongbing, et al. Design and research of ultra-low temperature valve for LNG engineering[J]. Fluid Machinery, 2015, 43(10): 47-51.
[3]Luo Qifei. Transient heat transfer and sealing of large diameter LNG ultra-low temperature ball valve[J].Shandong Industrial Technology,2015(11):11.
[4] Luo Qifei. Design of LNG ultra-low temperature valve and study on low temperature physical properties of materials [J]. Shandong Industrial Technology, 2015 (11): 57.
[5] Huang Yu, Song Kun, Chen Haiping. Domestic status and engineering application analysis of low temperature valves for LNG[J].Petrochemical Equipment, 2015,44(2):89-94.
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