1. Overview
The triple eccentric hard seal butterfly valve has developed rapidly in recent years.
2. structural features
The sealing ring of triple eccentric butterfly valve (Figure 1) generally adopts a multi-layer structure (Figure 2 with or without interlayer), and its single layer thickness is only 2-3mm, which has a certain degree of flexibility, but this structure also has defects. i. It is not easy to achieve close fit between multiple layers. ii. Since hard-sealed butterfly valves require a larger torque to achieve bubble-level sealing, and the thickness of a single layer is thin, although it is easy to achieve sealing, it is easy to warp between layers, iii. Although it uses a multi-layer structure, only one layer is the real seal. Therefore, for some demanding conditions, an integral sealing ring structure is adopted. Although the overall sealing ring reduces the elasticity to a certain extent, it improves the service life of the valve and can still maintain better sealing performance even in the case of thousands of cycles.
3. Force analysis
From the force analysis of the sealing ring (Figure 3), it can be seen that whether it is a multi-layer sealing ring or an integral sealing ring, there are problems of uneven force and uneven deformation around the sealing ring, which affects the sealing performance. Because the length of the periphery of the sealing ring relative to the supporting edge is different (similar to the unequal length of the cantilever), that is, X1≠X2. In addition, the influence of the radial eccentricity B of the rod axis, under the action of the stem torque, the sealing around the sealing ring The force and the generated stress are different, there are high-pressure areas and low-pressure areas, resulting in inconsistent deformation around the sealing ring. This inconsistent deformation along the periphery affects the sealing effect of the valve. In order to make a low pressure plus area produce sufficient sealing ratio Pressure, greater torque is required to seal the valve, but increasing the torque will cause the specific pressure of the high-pressure area to exceed the allowable specific pressure of the sealing surface, resulting in a gradual decrease in the sealing performance of the valve.
4. equal-strength seal ring design
Aiming at the situation that the uneven force of the triple eccentric butterfly valve sealing ring affects the sealing performance, and the shortcomings of the lack of elasticity of the overall sealing ring, a new high-elastic and equal-strength integral sealing ring (Figure 4) is designed, so that x1=X2 and corresponding Change the shape of the butterfly plate and the pressure plate so that the distance from the edge of the sealing ring is basically equal, and minimize the radial eccentricity of the stem shaft. B. The thickness of the sealing ring and the valve seat for sealing is still 2-3mm, so While ensuring the high elasticity of the sealing ring, uniform stress and deformation are formed around the sealing ring to achieve a better sealing effect.
5. Material selection and processing
The sealing ring of triple eccentric hard sealing butterfly valve is generally 304 or 304L, austenitic stainless steel, which has lower strength. When the thickness of the sealing ring is thin (the thickness of the sealing lip of the equal-strength sealing ring is generally 2~3mm), it is easy to produce elasticity, but it is also easy to produce plastic deformation and warpage, resulting in a decrease in sealing performance. Therefore, in order to make the equal-strength sealing ring To obtain better sealing performance, choose 17-4PH or 17-7PH precipitation hardening steel, the hardness after aging is 32-36HRC, the yield strength>860Mpa, and the cobalt-based cemented carbide is surfacing on the sealing surface of the valve seat, and the hardness is 38-43HRC. There is a hardness difference of 6~10HRC between the two sealing surfaces. The sealing ring has high elasticity and is not prone to warping. These measures provide a guarantee for obtaining good long-term sealing performance. Equivalent strength sealing ring The structure is special. To evenly remove the metal of the X size part through machining (Figure 5), a milling machine needs to be processed layer by layer to obtain an equal-strength seal ring that approximates a curved surface, or the curved surface can be processed through a 5-axis machining center. At the same time, parts similar to the length of the cantilever beam in contact with the sealing ring, such as the shape design and machining of the butterfly plate and the pressure plate, are also the key to ensuring equal strength of the sealing ring.
6. Tooling design
In order to ensure the sealing performance of the equal-strength sealing structure, a processing device (Figure 6 and Figure 7) is designed to ensure the accuracy of the valve seat sealing surface and the butterfly plate sealing surface (i.e. sealing ring). The same bottom inclined plate is used for processing to ensure that the angles of the two workpieces are consistent, and the same tooling is used for grinding to further improve the surface accuracy of the sealing surface.
7. Sealing surface processing
The special tooling can ensure that the two sealing surfaces obtain the same angle and obtain a certain surface accuracy. Since the sealing surface is a beveled part of a cone, when turning 360° in lathes, grinders, etc., most of the stroke The tool is idling, and there is no contact with the workpiece to cut the metal. Only a small part of the stroke is in contact with the workpiece to cut the metal, forming an intermittent machining process. The workpiece will be subject to instantaneous impact at the moment when it leaves the cutting and gradually enters the cutting. These factors lead to irreparable small dimensional deviations and small irregular deformations in the processed surface.
Since the sealing surface is a chamfered part of a cone, the sealing surface of the valve seat and the sealing surface of the butterfly plate cannot be ground. Therefore, the small size deviation and microscopic unevenness of the sealing surface can only be eliminated by the deformation of the sealing ring itself. This requires increasing the torque of the valve stem, but after the torque exceeds the limit, the sealing performance of the valve will be reduced. It is for this reason that in the sealing test of the triple eccentric hard seal butterfly valve, when the forward test pressure rises to a critical value, it will cause leakage (theoretically, the forward pressure is to help the valve seal, and the higher the pressure, the easier it is to seal). The pressure is related to the microscopic unevenness and dimensional deviation of the sealing surface. After analysis, the sealing ring blank (Figure 8) adopts a structure that covers the size of the original oblique cone, so that when the sealing ring is processed, the cutting tools are in the cutting state, avoiding discontinuous cutting, and can be well ensured The dimensional accuracy of the sealing surface of the sealing ring is required.
If the sealing surface of the valve seat is also in a continuous cutting state during the machining process, part of the size of the valve body needs to be adjusted, but this may result in an increase in the quality of the valve body (Figure 9). The hard alloy is surfacing on the entire widened valve seat sealing surface. In the turning and grinding process, although there is still discontinuous cutting at the beginning and the end, it is in the entire effective sealing area that cooperates with the sealing ring. The inner part is continuous cutting. Because the widened sealing surface blank is used in the machining process, it is necessary to improve the tooling of the valve body and the sealing ring to ensure smooth machining and ensure the requirements of machining accuracy. After turning and grinding, the sealing surface of the valve seat and the sealing ring is ground to eliminate the microscopic surface unevenness and obtain a surface accuracy similar to the mirror surface, thereby ensuring excellent sealing performance. In further processing according to the drawing size, remove Before the excess metal caused by this process, a sealing test is performed. This process has a higher cost, such as the increase in the amount of surfacing welding of cobalt-based cemented carbide.
8. Conclusion
The high elasticity and strength sealing ring of the triple eccentric hard sealing butterfly valve improve the force distribution of the sealing ring so that the surrounding of the sealing ring has basically uniform stress and deformation under the action of the stem torque. Aiming at the problem of discontinuous cutting and inability to grind during processing, a method of widening the sealing surface of the valve seat and the sealing ring in the manufacturing process is proposed to make the sealing surface a continuous cutting state during the processing and improve the cutting processing conditions. Improve the force of the tool and the workpiece, obtain better surface dimensional accuracy, and realize the grinding of the sealing surface of the valve seat and the sealing surface of the butterfly plate. Through these measures, excellent sealing performance and long-term service life of the triple-eccentric hard-seal butterfly valve can be obtained.
Author: Junye Li
