Since the reform and opening up, the valve industry has developed rapidly, and has made great progress in manufacturing capacity and complete sets of technologies. As an integral part of the pipeline system, the working state of the valve affects the normal operation of the system to a great extent. When the valve is used, there are many failures. Failures of valves are that the valve cannot perform its intended function due to its inability to adapt to the working conditions and its structure being damaged in the service process.
To reduce the probability of failures of valves and reduce the possibility of accidents, it is necessary to learn from historical accidents to find out the nature of failures of valves, inspire designers to make corresponding improvements to valve parts, ensure that valves have sufficient stability and reliability to obtain more stable performance and better quality, which is of certain significance to ensure the safety of the pipeline system.
Failures caused by leakages of valves
Failures caused by leakages are failures generated by the sealing structure losing its ability to stop fluid flow after the sealing is damaged. There are many forms of damage to the sealing surface structure, which can be divided into failures caused by fractures of flanges and sealing surfaces of valve seats, failures caused by surfacing layers and failures caused by sealing elements.
Surfacing metal on the surface of parts to form a surfacing layer is a common measure to improve the wear resistance and corrosion resistance of valves. The failure of the surfacing layer refers to the failure in which the metal layer surfacing on the surface of the valve parts falls off and causes leakages. Hao Wang and others analyzed the failure characteristics of the surfacing layer on the sealing surface of the check valve, and found that the surface cracks penetrated through the entire surfacing layer in the macroscopic state. The crack fracture has a gray-brown oxide layer, and it cracks along the grain boundary direction in the microscopic state, as shown in Figure 1.
Figure 1 Characteristics of failures caused by fractures of surfacing layers
The failure of the sealing element refers to the failure caused by leakages of valves due to the deformation or damage of the packing, gasket and sealing ring. Xiaoying Dai and others observed the macroscopic characteristics of the failure of the О ring, and found that the sealing ring was deformed, and there were visible dents on the surface.
There are many parts of the valve where failures occur, and the characteristics of the failure of the sealing element are often not easy to identify; the classification of different types of failures caused by leakages has not been unified. In the future, developing new identification methods to classify and identify failures caused by leakages will be an important research direction.
The cause of failures caused by leakages
Table 1 shows the cause analysis of valve failures caused by leakages. It can be found that the causes of leakage due to the rupture of the surfacing layer include processes and materials; the reasons for the failure of sealing elements mainly include environments, structures, loads and materials.
Table 1 Analysis of cases of failures caused by leakages
To reduce the probability of failures of valves and reduce the possibility of accidents, it is necessary to learn from historical accidents to find out the nature of failures of valves, inspire designers to make corresponding improvements to valve parts, ensure that valves have sufficient stability and reliability to obtain more stable performance and better quality, which is of certain significance to ensure the safety of the pipeline system.
Failures caused by leakages of valves
Failures caused by leakages are failures generated by the sealing structure losing its ability to stop fluid flow after the sealing is damaged. There are many forms of damage to the sealing surface structure, which can be divided into failures caused by fractures of flanges and sealing surfaces of valve seats, failures caused by surfacing layers and failures caused by sealing elements.
Surfacing metal on the surface of parts to form a surfacing layer is a common measure to improve the wear resistance and corrosion resistance of valves. The failure of the surfacing layer refers to the failure in which the metal layer surfacing on the surface of the valve parts falls off and causes leakages. Hao Wang and others analyzed the failure characteristics of the surfacing layer on the sealing surface of the check valve, and found that the surface cracks penetrated through the entire surfacing layer in the macroscopic state. The crack fracture has a gray-brown oxide layer, and it cracks along the grain boundary direction in the microscopic state, as shown in Figure 1.
Figure 1 Characteristics of failures caused by fractures of surfacing layers
The failure of the sealing element refers to the failure caused by leakages of valves due to the deformation or damage of the packing, gasket and sealing ring. Xiaoying Dai and others observed the macroscopic characteristics of the failure of the О ring, and found that the sealing ring was deformed, and there were visible dents on the surface.
There are many parts of the valve where failures occur, and the characteristics of the failure of the sealing element are often not easy to identify; the classification of different types of failures caused by leakages has not been unified. In the future, developing new identification methods to classify and identify failures caused by leakages will be an important research direction.
The cause of failures caused by leakages
Table 1 shows the cause analysis of valve failures caused by leakages. It can be found that the causes of leakage due to the rupture of the surfacing layer include processes and materials; the reasons for the failure of sealing elements mainly include environments, structures, loads and materials.
Table 1 Analysis of cases of failures caused by leakages
| Researchers | Types of leakages | Failure locations | Reason for failures |
| Hui Duan | Failures of surfacing layers | Valve seat surfacing layers | Welding materials and processes are not selected well and there is residual stress. |
| Min Zhang | Failures of the surfacing layer | Gate valve discs | The welding process is improperly selected, and crystal shrinkage cavity appears. |
| Jun Cao | Failures of sealing elements | Pressure regulating valve seats | There are impurities contained in the flowing medium at the sealing element. |
| Peng Li | Failures of sealing elements | End faces of switching valves | The size of the sealing ring is unreasonable, and the heat is generated during friction. |
| Bin Zhang | Failures of sealing elements | Gate valve stems | The good hardness of gate valve stem packing causes the valve stem to wear. |
The main reason for the failure of the surfacing layer is the improper selection of the welding process. There are many reasons for the failure of the sealing element, which are mainly the improper selection of the material and structure of the sealing element. In the future, the development of new welding and sealing materials for different valve parts will be the focus of research in this field.
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