2.1.2 Three-dimensional motion simulation
In order to verify the reliability of the design and calculation of the eccentric distance of the sphere and the inclination angle of the valve seat, the SolidWorks software was used to simulate the process of assembling the sphere. Fixed the valve seat, and made the sphere move vertically up and down. As shown in Figure 7, in the process of simulating the installation of the sphere, even if the dimensions of the sphere and valve seat met the requirement of formula (4), the disassembly and assembly was not smooth, and interference occurred. The interference occurred in the upper part of the sphere and valve seat, and the interference change of the ball's install process can be seen from Figure 8. The maximum interference size could be seen at the largest diameter on the horizontal plane of the sphere. Interference occurred because only the coordination between points on a vertical symmetry plane passing through the center of the valve seat was analyzed in the design process. It was only a two-dimensional analysis, and the relationship between the sphere and valve seat was not considered.
In the design process, in order to ensure the sealing effect of the ball valve, the elastic energy storage element had a certain amount of compression. Therefore, as long as the size of the interference didn't exceed the compression limit of the elastic energy storage element, it could be disassembled and assembled theoretically without causing a great influence. In order to reduce the resistance of the ball in the process of disassembly and assembly, the elastic energy storage element that needed to be selected should have sufficient compression, which should be at least greater than the interference size. Moreover, the pre-tightening force generated after compression could meet the requirements for sealing, and it should not be too great so as not to increase the opening and closing torque of the ball valve.
Therefore, the disc spring could generate a great pre-tightening force with only a small amount of compression, which was not applicable here. Several sets of springs could be selected as the required elastic energy storage elements, which not only had enough compression, but also could produce the required pre-tightening force.
Figure 7 The simulation for the assembly
Figure 8 Interference data
3. The design of the middle seal
Middle seals generally use metal wound gaskets. Metal wound gaskets are widely used in the field of ultra-low temperature valves. They are made by the alternate winding of metal and various fillers, which can withstand ultra-low temperature, high-pressure and other environments. By changing the material combination of the gasket, corrosion of the gasket caused by media can be solved. Graphite metal wound gaskets are generally used in LNG ultra-low temperature valves. Gas and liquid are difficult to penetrate graphite, which makes graphite used more. When the medium pressure is not high, the sealing structure shown in Figure 9 (a) and (b) can be used. The structure is simple and the processing method is convenient. When the pressure is high, the sealing structure shown in Figure 9(c) and (d) can be used, and using the octagonal ring gasket or tongue and groove surface sealing can make the gasket uniformly pressed. However, the processing is complicated.
When the medium doesn't have high pressure, and is non-toxic and non-flammable, the requirement can be met just by one winding gasket. When the medium has high pressure, and is non-toxic and non-flammable, if the sealing fails due to only one sealing and an accident, which is easy to cause danger. Therefore, in this case, two seals are generally adopted, and a lip seal is adopted for the other seal. The sealing of the lip seal is to make the lip deformed by the pressure of the medium, and the lip is tightly attached to the sealing surface to achieve the sealing effect. The higher the pressure of the medium is, the tighter the lip and the sealing surface and the better the sealing effect become. There is a metal spring in the lip seal, which can provide initial sealing force, meet the requirements for the initial sealing specific pressure and make up for the wear loss. In the LNG industry, the use of two seals in the middle of the ultra-low temperature valve is more safe and reliable.
Figure 9 The sealing structure of the middle channel
4. The sealing structure of the stem
In ultra-low temperature valves, the stem's sealing packing is generally PTFE or graphite. However, PTFE has a great expansion coefficient and a serious cold flow. It is rarely used in ultra-low temperature valves. Gas and liquid are difficult to penetrate graphite and it is widely used in the ultra-low temperature field.
In the liquefied natural gas industry, two sealing structures are also used for safe use, and a lip seal is provided before the packing seal to improve the sealing performance. The matching position of the valve stem and the packing can be nitrided to improve the surface hardness and wear resistance of the valve stem, and to increase the service life.
Figure 10 The sealing structure of the stem
5. Conclusion
In order to meet the needs of the rapid development of the LNG industry, the structure of ultra-low temperature valves needs to be continuously optimized to provide convenience for the development of the industry.
Compared with some common floating ball valves, the structure of this new type of ultra-low temperature floating ball valve can not only reliably achieve the sealing effect, but also can be easily disassembled and assembled without tooling, which is convenient for online maintenance. Moreover, it has simple parts and easy processing.
The difficulty of this new type of ultra-low temperature floating ball valve lies in the design of the eccentric distance of the ball and inclination angle of the valve seat. It needs to be calculated according to a certain formula, and combined with the requirements for the initial sealing force before it can be determined; the SolidWorks software can be used to carry out assembly simulation and check its reliability.
There are different choices for the sealing for the middle channel and the valve stem according to the chemical properties and pressure of the medium. In the transportation of LNG, the middle channel generally uses two seals; one is a lip seal, and the other seal is a metal wound gasket. The valve stem's sealing also adopts two sealing structures. One is lip sealing, and the other is graphite packing.
In order to verify the reliability of the design and calculation of the eccentric distance of the sphere and the inclination angle of the valve seat, the SolidWorks software was used to simulate the process of assembling the sphere. Fixed the valve seat, and made the sphere move vertically up and down. As shown in Figure 7, in the process of simulating the installation of the sphere, even if the dimensions of the sphere and valve seat met the requirement of formula (4), the disassembly and assembly was not smooth, and interference occurred. The interference occurred in the upper part of the sphere and valve seat, and the interference change of the ball's install process can be seen from Figure 8. The maximum interference size could be seen at the largest diameter on the horizontal plane of the sphere. Interference occurred because only the coordination between points on a vertical symmetry plane passing through the center of the valve seat was analyzed in the design process. It was only a two-dimensional analysis, and the relationship between the sphere and valve seat was not considered.
In the design process, in order to ensure the sealing effect of the ball valve, the elastic energy storage element had a certain amount of compression. Therefore, as long as the size of the interference didn't exceed the compression limit of the elastic energy storage element, it could be disassembled and assembled theoretically without causing a great influence. In order to reduce the resistance of the ball in the process of disassembly and assembly, the elastic energy storage element that needed to be selected should have sufficient compression, which should be at least greater than the interference size. Moreover, the pre-tightening force generated after compression could meet the requirements for sealing, and it should not be too great so as not to increase the opening and closing torque of the ball valve.
Therefore, the disc spring could generate a great pre-tightening force with only a small amount of compression, which was not applicable here. Several sets of springs could be selected as the required elastic energy storage elements, which not only had enough compression, but also could produce the required pre-tightening force.
Figure 7 The simulation for the assembly
Figure 8 Interference data
3. The design of the middle seal
Middle seals generally use metal wound gaskets. Metal wound gaskets are widely used in the field of ultra-low temperature valves. They are made by the alternate winding of metal and various fillers, which can withstand ultra-low temperature, high-pressure and other environments. By changing the material combination of the gasket, corrosion of the gasket caused by media can be solved. Graphite metal wound gaskets are generally used in LNG ultra-low temperature valves. Gas and liquid are difficult to penetrate graphite, which makes graphite used more. When the medium pressure is not high, the sealing structure shown in Figure 9 (a) and (b) can be used. The structure is simple and the processing method is convenient. When the pressure is high, the sealing structure shown in Figure 9(c) and (d) can be used, and using the octagonal ring gasket or tongue and groove surface sealing can make the gasket uniformly pressed. However, the processing is complicated.
When the medium doesn't have high pressure, and is non-toxic and non-flammable, the requirement can be met just by one winding gasket. When the medium has high pressure, and is non-toxic and non-flammable, if the sealing fails due to only one sealing and an accident, which is easy to cause danger. Therefore, in this case, two seals are generally adopted, and a lip seal is adopted for the other seal. The sealing of the lip seal is to make the lip deformed by the pressure of the medium, and the lip is tightly attached to the sealing surface to achieve the sealing effect. The higher the pressure of the medium is, the tighter the lip and the sealing surface and the better the sealing effect become. There is a metal spring in the lip seal, which can provide initial sealing force, meet the requirements for the initial sealing specific pressure and make up for the wear loss. In the LNG industry, the use of two seals in the middle of the ultra-low temperature valve is more safe and reliable.
Figure 9 The sealing structure of the middle channel
4. The sealing structure of the stem
In ultra-low temperature valves, the stem's sealing packing is generally PTFE or graphite. However, PTFE has a great expansion coefficient and a serious cold flow. It is rarely used in ultra-low temperature valves. Gas and liquid are difficult to penetrate graphite and it is widely used in the ultra-low temperature field.
In the liquefied natural gas industry, two sealing structures are also used for safe use, and a lip seal is provided before the packing seal to improve the sealing performance. The matching position of the valve stem and the packing can be nitrided to improve the surface hardness and wear resistance of the valve stem, and to increase the service life.
Figure 10 The sealing structure of the stem
5. Conclusion
In order to meet the needs of the rapid development of the LNG industry, the structure of ultra-low temperature valves needs to be continuously optimized to provide convenience for the development of the industry.
Compared with some common floating ball valves, the structure of this new type of ultra-low temperature floating ball valve can not only reliably achieve the sealing effect, but also can be easily disassembled and assembled without tooling, which is convenient for online maintenance. Moreover, it has simple parts and easy processing.
The difficulty of this new type of ultra-low temperature floating ball valve lies in the design of the eccentric distance of the ball and inclination angle of the valve seat. It needs to be calculated according to a certain formula, and combined with the requirements for the initial sealing force before it can be determined; the SolidWorks software can be used to carry out assembly simulation and check its reliability.
There are different choices for the sealing for the middle channel and the valve stem according to the chemical properties and pressure of the medium. In the transportation of LNG, the middle channel generally uses two seals; one is a lip seal, and the other seal is a metal wound gasket. The valve stem's sealing also adopts two sealing structures. One is lip sealing, and the other is graphite packing.
Next: Various Corrosion and Measures of Valves
Previous: The Failures of the Valve & Its Cause Analysis (Part Two)
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