The non-technical failures are all caused by human factors and come from subjective reasons. If the operator is cautious enough or experienced enough, they can be completely avoided. The cause of the failure has nothing to do with the internal structure of the valve. Technical failures, as the name suggests, it is caused by the fact that the current technology of metal-seated ball valves is not perfect enough. In many cases, we can only try to reduce its impact as much as possible, but it is difficult to eliminate it. Of course, if a metal seated ball valve with a service life of 3 months can extend its service life to half a year through troubleshooting or prevention, then it can be considered a success even if the fault cannot be eliminated.
1.1 Leakages caused by damage to sealing surfaces
For ball valves, even for valves, the sealing surface is the most important part that affects the operation of the entire valve. Judging from the working principle of the ball valve, the ball valve is an opening and closing part, and its operation process is the process of opening or closing the steel ball on the valve seat. If the sealing surface is damaged in the opening or closing process, the sealing surface may be damaged. Internal leakage of the valve happens, and the valve will fail.
1.1.1 Reasonable selection of valve seat springs
The biggest difference between metal seated ball valves and soft-seated ball valves is that the steel ball and valve seat are both metal. Unlike the soft-seated steel ball, which is metal, the valve seat is non-metallic. Therefore, the sealing mechanism is also different. The sealing principle of soft-seated ball valves is that the sealing force between the steel ball and the valve seat causes the non-metallic valve seat to deform, converting the uneven line contact between the valve seat and the steel ball into contact. At the same time, after the soft seated valve seat is extruded and deformed, it further fits the surface of the steel ball to compensate for the unevenness of the surface of the steel ball. If the extrusion deformation of the steel ball and the valve seat is larger or smaller, it will have little impact on the sealing performance. The sealing principle of metal seated ball valves is that the sealing force between the steel ball and the valve seat makes it difficult to deform the metal valve seat. Therefore, a greater sealing force is required to make the valve seat’s sealing surface deform to achieve the fit between the steel ball and the valve seat, thereby achieving sealing. However, this deformation is only subtle. If the extrusion force of the steel ball seat changes slightly, the sealing may not be possible. The two conclusions can be come: first, the extrusion force between the steel ball and the valve seat of the metal seated ball valve is greater; second, the sealing adjustment of the metal seated ball valve is weaker than that of the soft-sealed ball valve.
Since the extrusion force between the steel ball and the valve seat of the metal seated ball valve is greater, when the ball valve is opened and closed, sliding friction occurs between the metal steel ball and the metal valve seat. According to material mechanics, it can be known that sliding of metals with similar hardness is likely to cause damage to the sealing surface, creating scratches on the sealing surface. In metal seated ball valves, a spring (a disc spring or column spring) is generally provided behind the valve seat to slightly adjust the pressure. In this way, the sealing can be achieved within a certain range.
1.1.2 The adjustment of the valve seat pressure ring
The previous article analyzed the adjustment of the spring. The elasticity of the spring is related to the amount of compression in a linear or quadratic curve function, and the effective value range is very short, so it can only be adjusted within a very small range. When the valve leaks and the pressure is higher than the standard or lower than the standard valve position, it needs to be adjusted by the thickness of the valve seat pressure ring installed behind the valve seat.
1.1.3 Leakages of sealing surfaces caused by working conditions
In fact, the biggest trouble caused by the function of metal seated ball valves is not technical problems. Even if the design is precise enough, there will still be some problems during actual use. Such problems generally occur in the coal chemical industry and polysilicon industry. The common feature of these working conditions is that the medium is not composed of a single gas or liquid, but contains a large number of solid particles. These solid particles cause great scouring on the flow channel opening of the steel ball and valve seat, and even pass through the flow channel opening and enter the sealing surface, which causes fatal damage to the sealing surface. Valve repairs caused by this situation account for more than 20% of the repair. The requirements for the sealing surface of the steel ball and valve seat of metal sealed ball valves are very high. Once the scratch exceeds 0.1mm, there is no possibility of re-grinding and reuse. Once this happens during use, the ball seat is often scrapped at the same time. Therefore, we should try to reduce this situation to a minimum.
Two steps are built on both sides of the sealing surface of the valve seat, which is a double scraper. The steps on both sides are like two scrapers. In the opening and closing process of the steel ball, they can effectively scrape solid particles adsorbed on the steel ball and prevent them from entering the sealing surface. Of course, this kind of valve seat is generally only used in working conditions with a lot of solid particles. For water and steam, the scraper reduces the strength of the valve seat and increases the processing cost, which is unnecessary.
1.2 Great torque value
According to statistics, leakages of sealing surfaces account for 20% of the total failures, and excessive torque accounts for at least 40% of the failures. The torque of metal seated ball valves is 3 to 4 times greater than that of soft-seated ball valves of the same specification due to its special sealing mechanism. Under the conditions of media with strong adsorption capacity such as mud and dust, the solid-liquid mixture is adsorbed on the outside of the valve seat sealing surface of the steel ball and valve, causing the steel ball to have great torque when opening and closing, and even stuck. If it is opened and closed forcibly, the valve stem will even be twisted and damaged. At present, no perfect solution has been found. In most cases, it involves disassembly and repair and removing the mud. In terms of prevention, you can start from the following aspects:
① The double scraper can scrape mud from the sealing surface.
② The shape of the flow channel opening should be made as streamlined as possible, as sediment is more likely to accumulate at the edges and corners.
③ An automatic driver should be adopted. First, it speeds up the opening and closing speed and reduces the time for scouring of mud and adhesion to the sealing surface. Second, because the torque of the driver has an upper limit. When the steel ball gets stuck, it will not forcefully twist the valve stem or cut the sealing surface.
1.1 Leakages caused by damage to sealing surfaces
For ball valves, even for valves, the sealing surface is the most important part that affects the operation of the entire valve. Judging from the working principle of the ball valve, the ball valve is an opening and closing part, and its operation process is the process of opening or closing the steel ball on the valve seat. If the sealing surface is damaged in the opening or closing process, the sealing surface may be damaged. Internal leakage of the valve happens, and the valve will fail.
1.1.1 Reasonable selection of valve seat springs
The biggest difference between metal seated ball valves and soft-seated ball valves is that the steel ball and valve seat are both metal. Unlike the soft-seated steel ball, which is metal, the valve seat is non-metallic. Therefore, the sealing mechanism is also different. The sealing principle of soft-seated ball valves is that the sealing force between the steel ball and the valve seat causes the non-metallic valve seat to deform, converting the uneven line contact between the valve seat and the steel ball into contact. At the same time, after the soft seated valve seat is extruded and deformed, it further fits the surface of the steel ball to compensate for the unevenness of the surface of the steel ball. If the extrusion deformation of the steel ball and the valve seat is larger or smaller, it will have little impact on the sealing performance. The sealing principle of metal seated ball valves is that the sealing force between the steel ball and the valve seat makes it difficult to deform the metal valve seat. Therefore, a greater sealing force is required to make the valve seat’s sealing surface deform to achieve the fit between the steel ball and the valve seat, thereby achieving sealing. However, this deformation is only subtle. If the extrusion force of the steel ball seat changes slightly, the sealing may not be possible. The two conclusions can be come: first, the extrusion force between the steel ball and the valve seat of the metal seated ball valve is greater; second, the sealing adjustment of the metal seated ball valve is weaker than that of the soft-sealed ball valve.
Since the extrusion force between the steel ball and the valve seat of the metal seated ball valve is greater, when the ball valve is opened and closed, sliding friction occurs between the metal steel ball and the metal valve seat. According to material mechanics, it can be known that sliding of metals with similar hardness is likely to cause damage to the sealing surface, creating scratches on the sealing surface. In metal seated ball valves, a spring (a disc spring or column spring) is generally provided behind the valve seat to slightly adjust the pressure. In this way, the sealing can be achieved within a certain range.
1.1.2 The adjustment of the valve seat pressure ring
The previous article analyzed the adjustment of the spring. The elasticity of the spring is related to the amount of compression in a linear or quadratic curve function, and the effective value range is very short, so it can only be adjusted within a very small range. When the valve leaks and the pressure is higher than the standard or lower than the standard valve position, it needs to be adjusted by the thickness of the valve seat pressure ring installed behind the valve seat.
1.1.3 Leakages of sealing surfaces caused by working conditions
In fact, the biggest trouble caused by the function of metal seated ball valves is not technical problems. Even if the design is precise enough, there will still be some problems during actual use. Such problems generally occur in the coal chemical industry and polysilicon industry. The common feature of these working conditions is that the medium is not composed of a single gas or liquid, but contains a large number of solid particles. These solid particles cause great scouring on the flow channel opening of the steel ball and valve seat, and even pass through the flow channel opening and enter the sealing surface, which causes fatal damage to the sealing surface. Valve repairs caused by this situation account for more than 20% of the repair. The requirements for the sealing surface of the steel ball and valve seat of metal sealed ball valves are very high. Once the scratch exceeds 0.1mm, there is no possibility of re-grinding and reuse. Once this happens during use, the ball seat is often scrapped at the same time. Therefore, we should try to reduce this situation to a minimum.
Two steps are built on both sides of the sealing surface of the valve seat, which is a double scraper. The steps on both sides are like two scrapers. In the opening and closing process of the steel ball, they can effectively scrape solid particles adsorbed on the steel ball and prevent them from entering the sealing surface. Of course, this kind of valve seat is generally only used in working conditions with a lot of solid particles. For water and steam, the scraper reduces the strength of the valve seat and increases the processing cost, which is unnecessary.
1.2 Great torque value
According to statistics, leakages of sealing surfaces account for 20% of the total failures, and excessive torque accounts for at least 40% of the failures. The torque of metal seated ball valves is 3 to 4 times greater than that of soft-seated ball valves of the same specification due to its special sealing mechanism. Under the conditions of media with strong adsorption capacity such as mud and dust, the solid-liquid mixture is adsorbed on the outside of the valve seat sealing surface of the steel ball and valve, causing the steel ball to have great torque when opening and closing, and even stuck. If it is opened and closed forcibly, the valve stem will even be twisted and damaged. At present, no perfect solution has been found. In most cases, it involves disassembly and repair and removing the mud. In terms of prevention, you can start from the following aspects:
① The double scraper can scrape mud from the sealing surface.
② The shape of the flow channel opening should be made as streamlined as possible, as sediment is more likely to accumulate at the edges and corners.
③ An automatic driver should be adopted. First, it speeds up the opening and closing speed and reduces the time for scouring of mud and adhesion to the sealing surface. Second, because the torque of the driver has an upper limit. When the steel ball gets stuck, it will not forcefully twist the valve stem or cut the sealing surface.
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Previous: Selection of Metal-seated Ball Valve Accessories
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