I. The phenomenon of valve stem breakage 

 In industrial production, valve stem fracture is a problem that cannot be ignored. Valve stem fracture generally occurs at the root of the upper and lower threads, where the cross-sectional area is the smallest, and it is prone to stress concentration and exceeding the standard. Especially when the working conditions deviate significantly from the design parameters, for example, a power plant has repeatedly broken the valve stem trapezoid thread return groove after the DN150 electric gate valve is opened. Another type of valve stem breakage accident occurs at the moment of opening. It is manifested that the gate plate has not been separated from the valve seat, and the valve stem is broken at the upper or lower thread root. The reason is usually considered to be that the gate is stuck, which is actually only a partial or secondary reason. An important reason is the abnormal pressure increase after the middle cavity of the valve body is closed, that is, after the valve is closed, the fluid pressure of the middle cavity closed between the sealing surfaces on both sides of the upstream and downstream is much higher than the upstream pressure. 

 There are two reasons for this phenomenon. One of the middle cavity fluids is heated and expanded by the upstream fluid, resulting in a sharp increase in pressure. When the second gate plate is closed, the fluid is closed in the middle cavity and cannot flow out. The middle cavity space is further squeezed by the valve stem. Because the compressibility of the liquid is very limited, the pressure will also increase sharply. This phenomenon is especially easy to occur on the Z962 main supply gate valve of power plants. At the same time, its abnormal pressure increase will generally increase in geometric series, far exceeding the design limit of valve stem strength. 

 There are two reasons for this phenomenon. One of the middle cavity fluids is heated and expanded by the upstream fluid, resulting in a sharp increase in pressure. When the second gate plate is closed, the fluid is closed in the middle cavity and cannot flow out. The middle cavity space is further squeezed by the valve stem. Because the compressibility of the liquid is very limited, the pressure will also increase sharply. This phenomenon is especially easy to occur on the Z962 main supply gate valve of power plants. At the same time, its abnormal pressure increase will generally increase in geometric series, far exceeding the design limit of valve stem strength. 

 II. Analysis of common fracture causes 

 (I) Design and manufacturing factors 

 1. Thread root design problems 

 The cross-sectional area of the upper and lower thread roots of the valve stem is small, which makes it prone to stress concentration and exceeding the standard during use. Especially when the working conditions deviate significantly from the design parameters, this problem is more prominent. For example, a power plant has repeatedly had an accident in which the DN150 electric gate valve was broken at the ladder-shaped thread turning groove after the valve was opened. This was mainly because the cross-sectional area of the thread root was small, which could not withstand the stress in actual work. 

 2. Surface treatment problems 

 Nitriding treatment is a common process in valve manufacturing, but if it is not handled properly, it will have an adverse effect on the valve stem. Nitriding treatment may increase the brittleness of the valve stem, and it is more likely to break during use. For example, in some cases, the nitrided valve stem fractures after a period of work. After analysis, it was found that the increase in brittleness caused by nitriding treatment was an important reason. 

 (II) Installation and debugging factors 

 1. Over-debugging of electrical installation 

 Take a power plant as an example, the installation stroke was over-debugged, and the protective torque was too large, which caused an accident of valve stem breakage. The investigation found that the pre-tightening nut of the valve cover was loose, the valve cover was moved up, and the stem nut was stuck. In this case, the electric installation stroke was overde-debugging, resulting in the protective torque exceeding the tolerance range of the stem, thus causing the stem to break. 

 2. Valve stroke assembly problem 

 If the valve stroke assembly is too long or too short, it will affect the valve stem. If the valve stroke assembly is too long, the valve stem sealing slope and the valve core pin nail pressure will increase, resulting in the valve core and the valve seat ring being sealed too tightly, and the valve stem force increases and fatigue is easy. For example, in the valve failure of the natural gas treatment station, the 6-inch valve stroke is too long, resulting in increased force on the valve stem and easy fatigue, resulting in fracture accidents. If the valve stroke assembly is too short, the valve stem sealing slope and the valve core pin nail pressure are not enough, resulting in the valve core and the valve seat ring are not tightly sealed, and the valve is easy to leak. 

 (III) Working environment factors 

 1. Abnormal pressure increase in the middle cavity 

 The abnormal pressure increase phenomenon after the middle cavity of the valve body is closed will cause damage to the valve stem. There are two reasons for this phenomenon: first, the fluid in the middle cavity is heated and expanded by the upstream fluid, resulting in a sharp increase in pressure; second, when the gate is closed, the fluid is closed in the middle cavity and cannot flow out, and the middle cavity space is further squeezed by the valve stem. Because the compressibility of the liquid is very limited, the pressure will also increase sharply. This phenomenon is especially easy to occur on the Z962 main water supply gate valve of power plants. At the same time, its abnormal pressure increase will generally increase in a geometric series, far exceeding the design limit of the valve stem strength. 

 2. Temperature difference and pressure fluctuation 

 Temperature and pressure fluctuations can also affect the valve stem. In the case of a large temperature difference, the valve stem is prone to thermal expansion and cold contraction, which leads to stress concentration and increases the risk of valve stem fracture. For example, when the valve is closed in the cold state and then opened in the hot state, due to the thermal expansion and elongation of the valve stem, the gate plate is further tightened, which increases the closing torque of the gate plate, causing the gate plate to stop and increasing the possibility of valve stem breaking. At the same time, pressure fluctuations will also make the valve stem bear alternating stress, which is easy to cause fatigue fracture. 

III. Measures to prevent valve stem breakage 

 (I) Reasonable design and material selection 

 1. Reduce stress concentration 

 When designing the valve, the force of the valve stem should be fully considered to avoid stress concentration in parts with small cross-sectional areas such as the threaded root. The degree of stress concentration can be reduced by optimizing the structural design, such as increasing the transition rounding angle and reducing the sharp angle of the thread root. At the same time, changes in working conditions should be fully considered in the design process to ensure that the valve can operate safely and reliably under various working conditions. 

 2. Choose suitable materials 

 Choosing the appropriate valve stem material is an important measure to prevent breakage. Materials with sufficient strength, toughness and corrosion resistance should be selected according to the working pressure, temperature, medium and other factors of the valve. For example, for high-temperature and high-pressure valves, high-strength alloy steel materials such as 15CrMo can be selected to improve the bearing capacity of the valve stem. In addition, the surface treatment of the material can also be carried out. For example, the process parameters should be strictly controlled during nitriding treatment to avoid the increase in the brittleness of the valve stem due to improper treatment. 

 3. Appropriate surface treatment 

 When surface treatment, the appropriate treatment method should be selected according to the characteristics of the material and the working environment. For example, for valve stems that need to improve wear resistance, treatment methods such as hard chrome plating can be used, but attention should be paid to controlling the hardness after treatment to avoid excessive hardness leading to increased brittleness. At the same time, the quality of surface treatment should be ensured to avoid defects and unevenness. 

 (II) Correct installation and debugging 

 1. Operate strictly according to the specifications 

 During the installation of the valve, it should be operated in strict accordance with the installation specifications. Make sure that the valve is installed in the correct position, firmly connected and well sealed. During the debugging process, pay attention to the debugging of the electric installation stroke to avoid excessive protection moment. Take a power plant as an example, the installation stroke was over-debugged, and the protective torque was too large, which caused an accident of valve stem breakage. Therefore, during the commissioning process, the parameters of the valve should be carefully checked to ensure that it meets the design requirements. 

 2. Avoid installation problems 

 During the installation process, pay attention to the assembly of the valve stroke and avoid too long or too short. If the valve stroke assembly is too long, it will lead to an increase in the pressure of the valve stem sealing slope and the valve core pin, so that the valve core and the valve seat ring will be sealed too tightly, and the force of the valve stem will increase and it will be easy to fatigue; if the stroke assembly is too short, it will cause the seal to be loose and the valve will be easy to leak. For example, in the valve failure of the natural gas processing station, the 6-inch valve stroke is too long, resulting in an increase in the force of the valve stem and easy fatigue, resulting in a fracture accident. 

 3. Regular overhaul and maintenance 

 Regular overhaul and maintenance of the valve is a key measure to prevent valve stem breakage. Through regular inspection, potential problems of the valve, such as wear and cracks on the surface of the valve stem, can be found in time, and corresponding measures can be taken to repair or replace them to avoid further deterioration of the problem. At the same time, regular maintenance can also ensure the sealing performance and operation flexibility of the valve, and extend the service life of the valve. 

 During the maintenance process, a comprehensive inspection of the valve stem should be carried out, including the thread root, surface treatment, etc. Problems found should be dealt with in a timely manner, such as replacing damaged valve stems, adjusting stroke assembly, etc. In addition, other parts of the valve, such as valve core, valve seat, packing, etc., should also be checked and maintained to ensure the overall performance of the valve.