The operating conditions of metal seated ball valves are much more stringent than that of soft-seated ball valves. High temperature, strong corrosion, and media such as dust, particles, and mud have caused considerable obstacles to the continued use of metal seated ball valves. During the use of metal seated ball valves, valves often malfunction due to working conditions or human factors and are forced to be repaired. Understanding the causes of valve failures and finding ways to prevent them are necessary and effective for workers and product users in the industry.
The metal seated ball valve is analyzed in this article. From the problems discovered by customers during use, the design of the metal seated ball valve and its parts are discussed, and improvement is made. Classified from the perspective of product use, failures of metal seated ball valves can be divided into the following two categories: non-technical failures and technical failures.
1. Non-technical failures
This type of failure has nothing to do with technology, that is, it is not a problem with the internal structure of the valve or a failure in the operation of internal parts. Therefore, the cause of its failure is generally caused by external conditions.
1.1 The wrong selection of valves
When some customers choose a valve model, due to a lack of understanding of the product's operating conditions or cost, the valve they choose cannot meet the requirements of working temperature, working pressure, medium corrosiveness, and erosion under working conditions, resulting in once the valve is put into service, it quickly fails. Therefore, in the selection process, the customer or design institute must give top priority to the reliability of the valve. Choosing a valve that is too low-end due to cost may cause failures and damage, resulting in the loss outweighing the gain. This is particularly important in the selection of metal seated ball valves with harsh working conditions.
1.1.1 The wrong material selection of carbide surfaces of ball seat sealing surfaces
The key to the performance of metal seated ball valves lies in the surface hardening of the steel ball and valve seat. To ensure metal-to-metal sealing, the sealing ratio pressure between metal seated ball seats is much greater than that of ordinary ball valves, which makes metal materials prone to strain, which will directly cause the ball valve to fail to function normally. At present, the most widely used and best surface hardening method for ball seats is spraying cemented carbide technology (mainly nickel-based alloys and chromium-based alloys). The spray coating layer of steel balls and valve seats of metal seated ball valves greatly determines the performance and service life of the seat sealing surface. Therefore, in actual complex working conditions, the cemented carbide layer should be selected according to the actual situation.
The cemented carbide layer should be selected according to the two aspects: The first is to ensure the hardness. High-hardness alloy layer has good wear resistance and erosion resistance. It is often used in working conditions where small and hard solid particles exist such as the coal chemical industry and polycrystalline silicon industry. The second is to pursue corrosion resistance. The corrosion-resistant alloy layer can effectively protect the internal base material of the ball seat, corroded by some highly corrosive media, thus extending the service life. It is often used in highly corrosive media conditions such as petrochemical and papermaking industries. In the material selection of metal seated ball valves, the material for spraying the cemented carbide layer on the ball seat sealing surface is the top priority. Carbide that does not match the working conditions and medium conditions must not be selected.
1.1.2 The material selection of some internal parts of the valve not matching the working temperature
For metal seated ball valves, the selection of some internal parts is closely related to the operating temperature or medium temperature, such as packing, sealing gaskets and valve stem sleeves. There are generally two reasons for choosing a metal seated ball valve. One is that the working temperature is high and the soft-seated valve seat cannot withstand it. The other is that the medium has strong abrasive and erosion capabilities and the soft-seated valve seat is not hard enough. It can be seen that the metal seated ball valve is not equal to the high-temperature ball valve. In some working conditions, its operating temperature is not necessarily very high.
When the medium temperature is lower than 200°C, synthetic plastics can be used for packing, sealing gaskets, including body bonnet gaskets, lower stem gaskets and middle flange gaskets sealing gaskets. This configuration is similar to that of related accessories in soft-seated ball valves.
When the medium temperature is higher than 200°C, graphite needs to be used as the filler. Graphite has good temperature resistance and no change in performance at high temperatures. The sealing gasket is generally wrapped with metal wire around the graphite ring. Graphite is used to seal and the metal wire is shaped. The valve stem sleeve should also be replaced with metals. Therefore, in the selection process of metal seated ball valves, the operating temperature or medium temperature must be one of the necessary factors to determine the direction of selection. The wrong selection of internal parts can result in a waste of costs at least, or cause the valve to fail and become unusable.
The metal seated ball valve is analyzed in this article. From the problems discovered by customers during use, the design of the metal seated ball valve and its parts are discussed, and improvement is made. Classified from the perspective of product use, failures of metal seated ball valves can be divided into the following two categories: non-technical failures and technical failures.
1. Non-technical failures
This type of failure has nothing to do with technology, that is, it is not a problem with the internal structure of the valve or a failure in the operation of internal parts. Therefore, the cause of its failure is generally caused by external conditions.
1.1 The wrong selection of valves
When some customers choose a valve model, due to a lack of understanding of the product's operating conditions or cost, the valve they choose cannot meet the requirements of working temperature, working pressure, medium corrosiveness, and erosion under working conditions, resulting in once the valve is put into service, it quickly fails. Therefore, in the selection process, the customer or design institute must give top priority to the reliability of the valve. Choosing a valve that is too low-end due to cost may cause failures and damage, resulting in the loss outweighing the gain. This is particularly important in the selection of metal seated ball valves with harsh working conditions.
1.1.1 The wrong material selection of carbide surfaces of ball seat sealing surfaces
The key to the performance of metal seated ball valves lies in the surface hardening of the steel ball and valve seat. To ensure metal-to-metal sealing, the sealing ratio pressure between metal seated ball seats is much greater than that of ordinary ball valves, which makes metal materials prone to strain, which will directly cause the ball valve to fail to function normally. At present, the most widely used and best surface hardening method for ball seats is spraying cemented carbide technology (mainly nickel-based alloys and chromium-based alloys). The spray coating layer of steel balls and valve seats of metal seated ball valves greatly determines the performance and service life of the seat sealing surface. Therefore, in actual complex working conditions, the cemented carbide layer should be selected according to the actual situation.
The cemented carbide layer should be selected according to the two aspects: The first is to ensure the hardness. High-hardness alloy layer has good wear resistance and erosion resistance. It is often used in working conditions where small and hard solid particles exist such as the coal chemical industry and polycrystalline silicon industry. The second is to pursue corrosion resistance. The corrosion-resistant alloy layer can effectively protect the internal base material of the ball seat, corroded by some highly corrosive media, thus extending the service life. It is often used in highly corrosive media conditions such as petrochemical and papermaking industries. In the material selection of metal seated ball valves, the material for spraying the cemented carbide layer on the ball seat sealing surface is the top priority. Carbide that does not match the working conditions and medium conditions must not be selected.
1.1.2 The material selection of some internal parts of the valve not matching the working temperature
For metal seated ball valves, the selection of some internal parts is closely related to the operating temperature or medium temperature, such as packing, sealing gaskets and valve stem sleeves. There are generally two reasons for choosing a metal seated ball valve. One is that the working temperature is high and the soft-seated valve seat cannot withstand it. The other is that the medium has strong abrasive and erosion capabilities and the soft-seated valve seat is not hard enough. It can be seen that the metal seated ball valve is not equal to the high-temperature ball valve. In some working conditions, its operating temperature is not necessarily very high.
When the medium temperature is lower than 200°C, synthetic plastics can be used for packing, sealing gaskets, including body bonnet gaskets, lower stem gaskets and middle flange gaskets sealing gaskets. This configuration is similar to that of related accessories in soft-seated ball valves.
When the medium temperature is higher than 200°C, graphite needs to be used as the filler. Graphite has good temperature resistance and no change in performance at high temperatures. The sealing gasket is generally wrapped with metal wire around the graphite ring. Graphite is used to seal and the metal wire is shaped. The valve stem sleeve should also be replaced with metals. Therefore, in the selection process of metal seated ball valves, the operating temperature or medium temperature must be one of the necessary factors to determine the direction of selection. The wrong selection of internal parts can result in a waste of costs at least, or cause the valve to fail and become unusable.
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Previous: Notes for Quick Installation of Ball Valves
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