The pressure-bearing parts of the valve should be subjected to strength analysis and strength design. These parts mainly include valve bodies, bonnets, wedges and bonnet bolts. For the globe valve, sometimes, it is necessary to carry out the verification of strength on the valve seat part. As a non-pressure-bearing part, the valve stem is also an important part that must be designed for strength. The so-called design for strength should include two parts, that is, strength and rigidity.
The minimum wall thickness (diameters) of the main parts of the valve is specified in some major foreign valve specifications such as ANSI B16.34 and API 600. Some domestic manufacturers directly adopt these specifications or bigger sizes as the design dimensions, and the design for strength is not conducted. This is not strict, because first, the internal structure of the valve is different, and the structural size of the upper cavity of the valve is different. Therefore, the calculated wall thickness is also different. The appearance and structure of the valve, especially the treatment of sudden changes in shapes, are different, and the calculated stress values, especially the level of stress concentration, are also different, which may eventually lead to different calculated wall thicknesses. Second, most valve bodies are castings, and valve factories with poor smelting conditions and complex sources of raw materials have relatively great differences in the properties of casting materials. The performance of the casting material is related to its own casting defects, such as segregation, dendritic structure, inclusions, gas holes, porosity and cracks, and the fluctuations are great, which results in the great difference in the basic data of the calculation of strength. Third, different application environments have different corrosion conditions and different corrosion allowances should be considered. For high-pressure hydrogenation valves, it is necessary to calculate the strength and rigidity of related parts due to the influence of these factors, because a slight error may bring serious consequences.
At present, most domestic valve factories use mathematical analysis to calculate the strength and rigidity of the valve. This method is laborious and time-consuming, and the calculation accuracy is relatively poor, especially for the sudden change of the shape of the parts. It can not accurately calculate the level of stress. Most valve factories abroad have adopted the finite element analysis method, which is fast and accurate. For high-pressure and demanding valves, it is necessary to use the finite element method to analyze the strength and rigidity of parts.
Materials
The operating conditions of the high-pressure hydrogenation unit not only have high requirements on the reliability of the material, but also the media such as hydrogen and hydrogen sulfide have high requirements on the material's properties, that is, the medium is more sensitive to the defects of the material itself. If there are non-metallic inclusions, slag inclusions, gas holes, cracks and other discontinuous defects in the material, it is easy to cause the accumulation of hydrogen. At room temperatures, it will cause hydrogen deformation due to the partial high pressure formed by it, and even induce micro cracks. It will also make the material brittle deterioration (hydrogen embrittlement). At high temperatures, these defects are more conducive to the progress of internal decarburization of hydrogen-induced, thereby accelerating the process of hydrogen corrosion and cracking of materials. The medium of hydrogen sulfide is more sensitive to the external discontinuous defects of the material, especially in the wet hydrogen sulfide environment. The external discontinuous defects often become the inducement of stress corrosion cracking. Therefore, reducing or limiting the defects in the pressure-bearing parts of the valve is one of the key factors to ensure its reliability and prolong its service life.
There are two kinds of manufacturing methods for pressure-bearing parts of valves, that is, casting and forging. There are no defects such as gas holes, porosity, big circular inclusions, columnar structure and dendritic structure in the forged parts. The metal is dense. The comprehensive mechanical properties are good, and the reliability is good. Therefore, forging is an ideal method for manufacturing high-pressure hydrogenation pressure-bearing parts. However, considering that most of the pressure-bearing parts have complex shapes and many of them exceed the size of general die forging, most valve factories at home and abroad still use castings for the main pressure-bearing parts of valves with DN greater than 50 mm. In order to ensure the quality of castings, quality control should be carried out from three main aspects, including smelting, casting processes and welding repair. The influence of smelting on the quality of the material is the most basic influencing factor. Different smelting methods have relatively great differences in the quality of materials obtained. At present, domestic valve factories generally use electric furnace smelting, while most foreign valve factories use VOD or AOD smelting methods. Compared with electric furnace smelting, VOD or AOD has less burning of beneficial alloying elements, and the material composition is easier to approach the ideal state; the degassing performance is good, and the harmful impurity elements are less. Therefore, the quality of the obtained material is relatively good. The casting process is a key factor that affects the performance of the material. It involves the selection of the casting film material, the wooden outer mold, control of the casting temperature, and selection of the casting method. In short, casting processes that are conducive to improving the quality of castings, such as precision casting, pressure casting and vacuum casting should be the future development direction of valve manufacturers.
Repair welding is a remedial measure to deal with casting defects. Most castings need to be processed by repair welding. If the defect exceeds the standard, the product will be scrapped, which will increase the production cost of the valve. However, the number of welds, area of repair welding and number of repair welding for each valve should be limited, because the metal in the repair weld area is different from the casting metal. The more amount of the repair welding and the larger the repair welding area are, the more nonuniform the cast metal becomes, resulting in a decrease in the overall performance of the material. Each repair welding is equivalent to heating the casting once, and heating the casting multiple times will bring a series of adverse effects to it. Therefore, the number of repair welding of the valve should also be limited. The specifications in the ASTM put forward certain requirements for the repair welding of casting materials, but its requirements are relatively low. Most foreign valve manufacturers have stricter control for the repair welding of castings than that specified by ASTM. In fact, the control of repair welding of castings also reflects the balance between the quality of casting materials and production costs. Therefore, the key is to improve the casting quality of castings and minimize casting defects.
The minimum wall thickness (diameters) of the main parts of the valve is specified in some major foreign valve specifications such as ANSI B16.34 and API 600. Some domestic manufacturers directly adopt these specifications or bigger sizes as the design dimensions, and the design for strength is not conducted. This is not strict, because first, the internal structure of the valve is different, and the structural size of the upper cavity of the valve is different. Therefore, the calculated wall thickness is also different. The appearance and structure of the valve, especially the treatment of sudden changes in shapes, are different, and the calculated stress values, especially the level of stress concentration, are also different, which may eventually lead to different calculated wall thicknesses. Second, most valve bodies are castings, and valve factories with poor smelting conditions and complex sources of raw materials have relatively great differences in the properties of casting materials. The performance of the casting material is related to its own casting defects, such as segregation, dendritic structure, inclusions, gas holes, porosity and cracks, and the fluctuations are great, which results in the great difference in the basic data of the calculation of strength. Third, different application environments have different corrosion conditions and different corrosion allowances should be considered. For high-pressure hydrogenation valves, it is necessary to calculate the strength and rigidity of related parts due to the influence of these factors, because a slight error may bring serious consequences.
At present, most domestic valve factories use mathematical analysis to calculate the strength and rigidity of the valve. This method is laborious and time-consuming, and the calculation accuracy is relatively poor, especially for the sudden change of the shape of the parts. It can not accurately calculate the level of stress. Most valve factories abroad have adopted the finite element analysis method, which is fast and accurate. For high-pressure and demanding valves, it is necessary to use the finite element method to analyze the strength and rigidity of parts.
Materials
The operating conditions of the high-pressure hydrogenation unit not only have high requirements on the reliability of the material, but also the media such as hydrogen and hydrogen sulfide have high requirements on the material's properties, that is, the medium is more sensitive to the defects of the material itself. If there are non-metallic inclusions, slag inclusions, gas holes, cracks and other discontinuous defects in the material, it is easy to cause the accumulation of hydrogen. At room temperatures, it will cause hydrogen deformation due to the partial high pressure formed by it, and even induce micro cracks. It will also make the material brittle deterioration (hydrogen embrittlement). At high temperatures, these defects are more conducive to the progress of internal decarburization of hydrogen-induced, thereby accelerating the process of hydrogen corrosion and cracking of materials. The medium of hydrogen sulfide is more sensitive to the external discontinuous defects of the material, especially in the wet hydrogen sulfide environment. The external discontinuous defects often become the inducement of stress corrosion cracking. Therefore, reducing or limiting the defects in the pressure-bearing parts of the valve is one of the key factors to ensure its reliability and prolong its service life.
There are two kinds of manufacturing methods for pressure-bearing parts of valves, that is, casting and forging. There are no defects such as gas holes, porosity, big circular inclusions, columnar structure and dendritic structure in the forged parts. The metal is dense. The comprehensive mechanical properties are good, and the reliability is good. Therefore, forging is an ideal method for manufacturing high-pressure hydrogenation pressure-bearing parts. However, considering that most of the pressure-bearing parts have complex shapes and many of them exceed the size of general die forging, most valve factories at home and abroad still use castings for the main pressure-bearing parts of valves with DN greater than 50 mm. In order to ensure the quality of castings, quality control should be carried out from three main aspects, including smelting, casting processes and welding repair. The influence of smelting on the quality of the material is the most basic influencing factor. Different smelting methods have relatively great differences in the quality of materials obtained. At present, domestic valve factories generally use electric furnace smelting, while most foreign valve factories use VOD or AOD smelting methods. Compared with electric furnace smelting, VOD or AOD has less burning of beneficial alloying elements, and the material composition is easier to approach the ideal state; the degassing performance is good, and the harmful impurity elements are less. Therefore, the quality of the obtained material is relatively good. The casting process is a key factor that affects the performance of the material. It involves the selection of the casting film material, the wooden outer mold, control of the casting temperature, and selection of the casting method. In short, casting processes that are conducive to improving the quality of castings, such as precision casting, pressure casting and vacuum casting should be the future development direction of valve manufacturers.
Repair welding is a remedial measure to deal with casting defects. Most castings need to be processed by repair welding. If the defect exceeds the standard, the product will be scrapped, which will increase the production cost of the valve. However, the number of welds, area of repair welding and number of repair welding for each valve should be limited, because the metal in the repair weld area is different from the casting metal. The more amount of the repair welding and the larger the repair welding area are, the more nonuniform the cast metal becomes, resulting in a decrease in the overall performance of the material. Each repair welding is equivalent to heating the casting once, and heating the casting multiple times will bring a series of adverse effects to it. Therefore, the number of repair welding of the valve should also be limited. The specifications in the ASTM put forward certain requirements for the repair welding of casting materials, but its requirements are relatively low. Most foreign valve manufacturers have stricter control for the repair welding of castings than that specified by ASTM. In fact, the control of repair welding of castings also reflects the balance between the quality of casting materials and production costs. Therefore, the key is to improve the casting quality of castings and minimize casting defects.
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