Safety valve spring failure

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As the most critical component of a safety valve, the spring, once it fails, renders the entire safety valve unusable. In this issue, we will briefly discuss the failure of safety valve springs. It should be specifically noted here that the failure of a spring does not refer only to its breaking; it also includes failures resulting from changes in the spring. The main reasons for spring failure include the following: First, internal defects in the spring lead to fracture. During the production of spring wires, especially those made from high-strength materials with large diameters, physical defects such as voids and cracks may exist within the wires. Under shear forces, these defects can easily cause cracks to propagate, ultimately leading to the fracture of the spring. Furthermore, the uneven internal structure of the wire material, along with high levels of trace elements such as S and P, can easily lead to the formation and propagation of defects within the spring. For such failure events, the cause of the failure can be determined through final chemical composition analysis of the spring material and port analysis. With the advancement of material processing techniques and spring coiling processes, the number of springs that break due to substandard materials or internal defects is decreasing year by year. II. Breakage of springs due to environmental corrosion: Under normal circumstances, the springs in safety valves do not come into contact with the medium before the valve opens. If there is a corrosive environment present at the outlet of the safety valve, the springs can be protected by using bellows. The presence of corrosive agents in the air environment often occurs in chemical processing systems, and it is mainly divided into uniform corrosion and localized corrosion. Uniform corrosion of springs can be avoided through regular inspections. Local corrosion is difficult to detect and causes greater damage; among various factors, the presence of chloride and sulfur ions has the greatest impact on springs, as these two substances can easily induce stress corrosion cracks in them. Spring corrosion and fracture caused by environmental factors can be avoided through the proper selection of materials. Additionally, proper surface treatment processes applied to springs at the time of manufacture, such as painting and metal plating, can also effectively prevent corrosion of the springs. Thirdly, the reduction in spring stiffness leading to failure is actually the issue that receives the least attention in practice, yet it has the widest impact. As we have mentioned in previous articles, a very important parameter for the spring used in safety valves is the spring stiffness; an inappropriate spring stiffness can directly lead to abnormal performance of the safety valve, such as overpressure conditions or failure to return to its normal position. After the safety valve is put into use, its spring stiffness may change to some extent (mainly decrease) due to factors such as usage time and temperature. This process is very slow and difficult to detect. During the periodic verification process, the impact of the reduced spring stiffness from the previous cycle on the tuning parameters is continuously corrected through this verification process; it is only when the safety valve activates prematurely that people become aware of the decrease in spring stiffness. The impact of reduced spring stiffness on performance cannot be measured or corrected during periodical inspections; as a result, the spring stiffness of safety valves that have been in service for a long time (especially those operating under high-temperature conditions) may have decreased to a level that no longer meets the required performance standards, yet this issue cannot be detected in a timely manner. Strong compression and heat-induced compression can effectively improve the stability of springs over long periods of use, but there is a lack of data regarding whether these processing methods can meet the stiffness requirements of springs that are to be in service for extended periods (more than 5 years) in high-temperature environments. It should be noted that the proper selection of spring material also has a significant impact on the stability of spring stiffness, and this factor needs to be given due consideration when selecting springs.