3. Design of equal width of sealing surfaces of valve bodies and plugs
The main sealing surface of the pressure-balanced plug valve is a cone surface. The angle of the cone surface affects the consistency of the sealing surface width, thereby affecting the sealing of the entire valve. The sealing cone surface and other vertex design scheme is that the cone angle of the sealing surface of the plug and the valve body is the same as the plug flow channel window angle vertex (shared vertex O1). At this time, B/D on any cross section of the plug is equal (B = H × tgα, D = H × tgβ, B/D = tgα/tgβ = constant), B/D being a constant is beneficial to the strength design of the plug. At this time, the sealing surface width b of the plug at any position is not equal. The error of the sealing surface width b1, b2, and b3 on the small end, middle position, and large end sections reaches about 15% of large-diameter plug valves (based on b2). The great difference in valve sealing surface width is not conducive to valve sealing. The design of the high-pressure and large-diameter plug valve has been optimized. The taper angle of the plug sealing surface and the angle vertex of the plug flow channel window have been adjusted. After the improved design, the width b of the sealing surface on any section is approximately equal, and the width error of the upper sealing surface of the small end, middle position, and large end sections are b1, b2, b3 is about 2% (based on b2); the sealing of the valve is greatly improved. The design of the sealing cone surface with equal vertices is shown in Figure 2, and the design of the sealing cone surface with approximately equal width is shown in Figure 3.
Figure 2 Design of equal vertices of sealing cones
4. Low torque of the plug valve
The working principle of the plug valve is that the valve stem drives the plug to rotate to open and close the valve. The plug and the sealing surface of the valve body are rotated by metal contact, and grease lubrication plays a certain auxiliary role. The opening and closing friction torque of high-pressure and large-diameter plug valves is small; the diameter changes greatly, and it is difficult to open and close due to large diameters and high pressure. It is necessary to carry out special friction reduction treatment on the sealing surface between the plug and the valve body, that is, the sealing surfaces of the valve body and the plug are sprayed with polytetrafluoroethylene. There is a certain process from when the plug starts to rotate to when the valve releases pressure. The rotational friction before pressure relief is relatively great. The quality of the PTFE coating becomes the core of ensuring the low torque of the valve. The following points need to be noted for the quality of PTFE coatings:
1) The valve body and plug are coated.
2) The fillet at the joint part of the valve body, plug sealing surface and flow channel (the fillet at the window) must have a smooth transition. If the chamfering quality is not up to standard, the anti-friction coating will be cut off like a sharp knife, and the PTFE coating will fail.
3) The oil dirt on the valve body and plug must be cleaned as required before spraying.
Parts generally come into contact with oil during processing, for example, applying anti-rust oil. If the oil is not cleaned properly, the adhesion of the coating will be reduced.
4) Choose the appropriate wear-resistant and anti-friction spraying process. There are generally two PTFE spraying processes. One is suitable for highly anti-corrosion working conditions, and the other is suitable for wear-resistant and friction-reducing working conditions. After choosing a wear-resistant and friction-reducing process, you must pay attention to the specific heat treatment temperature under this process. The temperature should be kept as low as possible while ensuring the spraying quality. High temperatures will cause great deformation of the valve body and plug sealing cone surface, which is not conducive to valve sealing.
Figure 3 Approximately equal-width of the sealing cone surface
Figure 4 Valve body chamfers
5. Long-term sealing and balance
With the improvement of manufacturing technology of plug valves, the valve body and plug sealing cone surface have been processed separately (Figure 4 and Figure 5). The sealing cone surface of the parts no longer needs to be matched, and can be independently ground in batches. The valve body and plug can be selected at will during assembly. Because of the harsh working conditions of the pressure-balanced plug valve, its main sealing surface is all-metal large-area contact. At the same time, in order to reduce large-area metal friction, oil storage tanks are generally designed at the bottom of the valve body and plug so that grease can be evenly applied to the sealing surface, reducing friction and the risk of damage to the sealing surface. At the same time, in order to ensure the sealing effect of the plug valve after long-term use, the bottom of the valve body is generally designed with an adjustment set screw so that the plug cone surface can effectively close to the valve body cone surface to achieve sealing after the valve is opened and closed for a long time and is worn. To realize the above adjustment, the following points should be paid attention to in the structural design of the plug:
1) The cone surface of the small end of the plug should be higher than the sealing cone surface of the valve body.
2) The position of the oil groove C2 at the bottom of the valve body and the position of the oil groove C1 at the bottom of the plug should be positively staggered by the h1 and h2 positions. If the above h1, h2 and h3 positions are negative values, after the contact part of the sealing surface of the plug valve body is worn, but the valve body is not worn (the sealing surface not being in contact with the plug), it will be difficult for the plug to achieve the sealing surface again through the bottom set screw and long-term use of the sealing effect will be difficult to guarantee.
Figure 5 Plug chamfers
Figure 6 Valve bodies and plug sealing
7. Conclusion
In summary, the equal deformation of the valve body sealing surface, the approximately equal width of the valve body and the main sealing surface of the plug, the influencing factors of the low torque of the valve, and the lasting balance of the seal should be paid attention to when designing and manufacturing high-pressure and large-diameter pressure-balanced plug valves, guaranteeing measures and adjustments to the pressure test sequence and ensuring good sealing and reliability during the valve's service life.
The main sealing surface of the pressure-balanced plug valve is a cone surface. The angle of the cone surface affects the consistency of the sealing surface width, thereby affecting the sealing of the entire valve. The sealing cone surface and other vertex design scheme is that the cone angle of the sealing surface of the plug and the valve body is the same as the plug flow channel window angle vertex (shared vertex O1). At this time, B/D on any cross section of the plug is equal (B = H × tgα, D = H × tgβ, B/D = tgα/tgβ = constant), B/D being a constant is beneficial to the strength design of the plug. At this time, the sealing surface width b of the plug at any position is not equal. The error of the sealing surface width b1, b2, and b3 on the small end, middle position, and large end sections reaches about 15% of large-diameter plug valves (based on b2). The great difference in valve sealing surface width is not conducive to valve sealing. The design of the high-pressure and large-diameter plug valve has been optimized. The taper angle of the plug sealing surface and the angle vertex of the plug flow channel window have been adjusted. After the improved design, the width b of the sealing surface on any section is approximately equal, and the width error of the upper sealing surface of the small end, middle position, and large end sections are b1, b2, b3 is about 2% (based on b2); the sealing of the valve is greatly improved. The design of the sealing cone surface with equal vertices is shown in Figure 2, and the design of the sealing cone surface with approximately equal width is shown in Figure 3.
Figure 2 Design of equal vertices of sealing cones
4. Low torque of the plug valve
The working principle of the plug valve is that the valve stem drives the plug to rotate to open and close the valve. The plug and the sealing surface of the valve body are rotated by metal contact, and grease lubrication plays a certain auxiliary role. The opening and closing friction torque of high-pressure and large-diameter plug valves is small; the diameter changes greatly, and it is difficult to open and close due to large diameters and high pressure. It is necessary to carry out special friction reduction treatment on the sealing surface between the plug and the valve body, that is, the sealing surfaces of the valve body and the plug are sprayed with polytetrafluoroethylene. There is a certain process from when the plug starts to rotate to when the valve releases pressure. The rotational friction before pressure relief is relatively great. The quality of the PTFE coating becomes the core of ensuring the low torque of the valve. The following points need to be noted for the quality of PTFE coatings:
1) The valve body and plug are coated.
2) The fillet at the joint part of the valve body, plug sealing surface and flow channel (the fillet at the window) must have a smooth transition. If the chamfering quality is not up to standard, the anti-friction coating will be cut off like a sharp knife, and the PTFE coating will fail.
3) The oil dirt on the valve body and plug must be cleaned as required before spraying.
Parts generally come into contact with oil during processing, for example, applying anti-rust oil. If the oil is not cleaned properly, the adhesion of the coating will be reduced.
4) Choose the appropriate wear-resistant and anti-friction spraying process. There are generally two PTFE spraying processes. One is suitable for highly anti-corrosion working conditions, and the other is suitable for wear-resistant and friction-reducing working conditions. After choosing a wear-resistant and friction-reducing process, you must pay attention to the specific heat treatment temperature under this process. The temperature should be kept as low as possible while ensuring the spraying quality. High temperatures will cause great deformation of the valve body and plug sealing cone surface, which is not conducive to valve sealing.
Figure 3 Approximately equal-width of the sealing cone surface
Figure 4 Valve body chamfers
5. Long-term sealing and balance
With the improvement of manufacturing technology of plug valves, the valve body and plug sealing cone surface have been processed separately (Figure 4 and Figure 5). The sealing cone surface of the parts no longer needs to be matched, and can be independently ground in batches. The valve body and plug can be selected at will during assembly. Because of the harsh working conditions of the pressure-balanced plug valve, its main sealing surface is all-metal large-area contact. At the same time, in order to reduce large-area metal friction, oil storage tanks are generally designed at the bottom of the valve body and plug so that grease can be evenly applied to the sealing surface, reducing friction and the risk of damage to the sealing surface. At the same time, in order to ensure the sealing effect of the plug valve after long-term use, the bottom of the valve body is generally designed with an adjustment set screw so that the plug cone surface can effectively close to the valve body cone surface to achieve sealing after the valve is opened and closed for a long time and is worn. To realize the above adjustment, the following points should be paid attention to in the structural design of the plug:
1) The cone surface of the small end of the plug should be higher than the sealing cone surface of the valve body.
2) The position of the oil groove C2 at the bottom of the valve body and the position of the oil groove C1 at the bottom of the plug should be positively staggered by the h1 and h2 positions. If the above h1, h2 and h3 positions are negative values, after the contact part of the sealing surface of the plug valve body is worn, but the valve body is not worn (the sealing surface not being in contact with the plug), it will be difficult for the plug to achieve the sealing surface again through the bottom set screw and long-term use of the sealing effect will be difficult to guarantee.
Figure 5 Plug chamfers
Figure 6 Valve bodies and plug sealing
7. Conclusion
In summary, the equal deformation of the valve body sealing surface, the approximately equal width of the valve body and the main sealing surface of the plug, the influencing factors of the low torque of the valve, and the lasting balance of the seal should be paid attention to when designing and manufacturing high-pressure and large-diameter pressure-balanced plug valves, guaranteeing measures and adjustments to the pressure test sequence and ensuring good sealing and reliability during the valve's service life.
Next: Measures for High-Temperature and High-Pressure Valve Failures
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