湿气输送管线热电偶套管失效分析

长庆气田某段天然气输送管线在役热电偶运行仅10个月就发生突然失效，无法进行温度测量，停输后卸下，可见热电偶套管外壁出现了严重的点蚀现象。采用宏观分析和剖管检测观察套管内外壁腐蚀情况，采用扫描电镜 (SEM) 分析微观形貌，采用直读火花光谱仪分析套管材质，采用能谱分析 (EDS) 和X射线衍射仪 (XRD) 对腐蚀产物进行成分分析。结果表明：热电偶套管未按设计要求使用304奥氏体不锈钢，而使用了2Cr13马氏体不锈钢，且材质存在设计要求较低的问题。另外，湿气输送时，天然气中含有一定饱和度的水分，在CO₂、H₂S、Cl^-等多种腐蚀因素的协同作用下，2Cr13热电偶套管很快出现点蚀穿孔，进而导致热电偶腐蚀失效。最后，针对湿气输送工况下热电偶套管的腐蚀机理，提出了相应的改进措施。

Failure Analysis of Thermocouple Sleeve in Wet Gas Transmission Pipeline

A sudden failure emerged for a thermocouple in a section of pipeline at Changqing gas field after only 10 months of operation. Then the temperature measurement could not be carried out and the thermocouple was removed. It was found that there existed serious pitting corrosion on the outer wall of the thermocouple sleeve. The cause of the failure was comprehensively analyzed by means of macroscopic inspection of corrosion on both the inner- and outer-side of the sleeve wall, the SEM observation of the morphology of corrosion pits and EDS- and XRD-characterization of corrosion products etc. The results showed that instead of the designed 304 austenitic stainless steel, an improper material of 2Cr13 martensitic stainless steel was used in the manufacture of the thermocouple, accordingly which possesses weak resistance to corrosion. Meanwhile, there existed a certain amount of moist steam in the wet natural gas in the transmission pipeline, therefore, it may essentially be expected that due to the synergistic action of CO₂, H₂S, Cl^- and other corrosion factors, the pitting corrosion was easy to occur on the 2Cr13 sleeve. Finally, aiming at the peculiar corrosion of thermocouple sleeve under this working condition, the relevant prevention measures were proposed.