With the progress and development of society, medium, temperature, pressure and so on encountered in the engineering construction engineering design are becoming more and more extensive, and the application of the pipeline is extremely important as the construction project Components, more need to make the appropriate selection of materials design. The selection and design of cryogenic pipes and valves are the most important part of the pipeline design. Only the operational characteristics of the pipeline, temperature and pressure, and the environmental factors of the project construction site are comprehensively considered. The selection of the pipe material Analysis, the selection of the valve to be studied, can we really promote the reliability and high quality of the pipeline, to be able to ensure the safety of the operation of the pipeline.
A low-temperature environment, the choice of pipe material First, in the low-temperature environment, in order to effectively reduce or avoid the occurrence of fracture conditions, pipe materials should have good toughness and embrittlement resistance, the general steel at low temperatures will appear low temperature Fragile, metallic materials in the low temperature environment before brittle fracture without any signs can be found, a sudden fracture, which often results in very serious consequences.
Second, at low temperatures, careful consideration should be given to the shrinkage of long pipelines to ensure that the problem of cold shrinkage of the pipelines is resolved. Since pipelines are installed at normal temperatures and used at low temperatures, when the pipe's design temperature is very different from room temperature , The metal's thermal expansion and contraction effect will make the pipeline installed at room temperature produce obvious internal stress, so the choice of materials, the linear expansion coefficient of the material should be as small as possible.
Third, the pipework and welding should be more convenient, in order to prevent the occurrence of low temperature brittle fracture, butt welding of low temperature materials, should be carried out Charpy (V notch) impact test; low temperature pipe materials, should be based strictly on Relevant specifications of the requirements to determine whether to do low temperature impact test, the test requirements should be consistent with standard specifications.
Fourthly, the easy availability of pipeline materials is an important basis for material selection of pipeline materials. Under the premise of meeting the requirements of working conditions, the design should give priority to selecting low-cost materials to save investment costs for the construction of the project. In general, the economy of carbon steel is easy Have higher strength, but poor corrosion resistance and temperature resistance, good toughness of stainless steel, good corrosion resistance, excellent plasticity and welding performance, welding more convenient, both high temperature conditions, or low temperature environment , The effect is also better; According to the standard ASTM A403 / A403M provides to meet the chemical composition and mechanical properties specified in two or more grades required material can be marked with multiple grades, so the two grades of stainless steel (such as 304 / 304L And 316 / 316L) are currently used more commonly in foreign countries, which can simultaneously exert the higher strength of ordinary stainless steel and the good corrosion resistance of L-class stainless steel, save materials and have higher safety and reliability.
2 low temperature valve material selection 2.1 Low temperature working valve material selection First of all should consider the minimum temperature of the material to ensure that the valve performance under low temperature conditions, mainly to ensure its impact strength; low temperature valve body material domestic brands usually There are LCB, LF2, LC3, LF3, CF8, F304 and so on.
2.2 The principle of low temperature valve body parts selection of materials When the temperature is higher than -100 ℃ ferritic steel selection; when the temperature is lower than -100 ℃, choose austenitic steel; the temperature is higher than -100 ℃, the stem and bolt material Ni ,, Cr-Mo and other alloy steel; temperature below -100 ℃, the use of austenitic stainless steel.
① in the design of cryogenic valves, on the one hand by the structural design to ensure that the filler is close to ambient temperature work, on the other hand in the choice of filler is to consider the low temperature characteristics of the filler; cryogenic valve can also be used without filler bellows seal structure; Cryogenic valve gasket must be at room temperature, low temperature and temperature changes with reliable sealing and recovery.
② on the structure of low-temperature valve, and ordinary valve is basically the same, about its type, including ball valves , check valves , gate valves , butterfly valves and globe valves and so on. To avoid the ingress of heat, a long-necked bonnet construction is often selected for valve bonnet applications on cryogenic piping to provide good protection of the stuffing box's function and to avoid knots at stuffing and stem locations The occurrence of ice, to ensure that the valve stem at normal temperatures can be in the normal work. Once the valve is cut off, the cryogenic liquid absorbs the heat in the air, causing the gasification to take place, which in turn affects the valve's function.
③ In the process of installation of the low temperature valve with pressure relief hole, the position of the pressure relief hole should be closely watched. The pressure relief hole should be located on the side of the closed pipeline to ensure the direction of installation of the valve is guaranteed. In order to ensure the correct installation of low-temperature valve work.
Cryogenic valve cryogenic treatment is the Valve Parts immersed in low temperature liquid nitrogen tank insulation for a certain period of time, in order to reduce the temperature and the microstructure changes caused by the deformation, thereby enhancing the valve at a low temperature sealing performance of a treatment.
⑤ low temperature valve test parts at room temperature and low temperature test two parts, room temperature test should be consistent with the provisions of GB / T13927, JB / T9092, API598 and API 6D; low temperature valve low temperature test should be carried out after normal temperature test.
3 Conclusion Low temperature pipe and valve selection and design for the entire pipeline design, direction, application has a very important impact, in order to achieve better design purpose and effect, you need to conduct an in-depth study of materials to understand the characteristics of low-temperature media , Do a good job of medium transmission conditions and the layout of low-temperature pipes, so that the entire pipeline can be applied to enhance the performance of the pipeline at low temperatures can still play its effectiveness.
references:
Li Huwen, Ge Xinrui, Zhang Jian, Zhang Yongjian.LNG low temperature pipe design principle [J] .Guangdong Chemical Industry, 2016,43 (14): 198-202.
[2] Zhang Ship. Design of low temperature pipeline materials for LNG receiving station [J / OL]. Refining Technology and Engineering, 2016,46 (06): 35-38.
[3] Meng Bo, Liu Long, Wang Qi.LNG cryogenic valve safety service performance research [J]. Chemical Machinery, 2015,42 (05): 701-705.
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