某输气管线安全阀连接螺栓全部断裂的案例分析

天然气管线安全阀是重要的泄压保护装置，而连接阀盖与阀体的螺栓则是安全阀最关键的零部件。虽然连接螺栓设定的安全系数较高，但螺栓由于材料、环境、过载等原因容易发生断裂或其他失效形式，失效螺栓一般为1根或小部分，螺栓发生全部失效的情况比较少见。针对某油田输气管线上一弹簧式安全阀8根双头螺栓全部断裂事故，采用理论与试验相结合的方法，对螺栓进行了光学显微分析、扫描电子显微镜微观分析、尺寸变化分析以及材质理化性能分析，并对螺栓的受力作了详细计算和分析。结果表明：预紧力矩过大和氧腐蚀导致4根螺栓产生了疲劳断裂，另4根螺栓的断裂则属于过载断裂。最后提出了有针对性地解决措施并取得了良好效果。

A case history of stress corrosion cracking in all connecting bolts of a safety valve in a natural gas pipeline

A safety valve is an important pressure relieving device in gas pipelines, while those bolts connecting the valve cap with the valve body are essential parts of a safety valve. Although the safety coefficient is designed rather high of the connecting bolts, crackings or other failure accidents easily occur to the bolts due to many reasons like material, environment, and overloading, etc.. It is common that one or several bolts happen to be in failure state, but rarely in such case that all connecting bolts cease to be available. A study was carried out on an accident that all the 8 double headed bolts became cracked of a spring safety valve in a gas pipeline of an oil field. By use of a combination method of theories and experiments, optical microscopic analysis, SEM microstructure analysis, dimensional change analysis and material's physical and chemical property tests were made on eight bolts and their force analysis and calculation were also performed in great detail. This case history showed that an overlarge pre tightening torque and oxygen corrosion caused the cracking of 4 bolts, while overloading fracture occurred to the other 4 bolts. Finally some corresponding countermeasures presented herein help achieve good results in avoiding such similar problems.