Analysis and control on the internal corrosion of natural gas pipelines with micro-sulfur

There is slight H2S in natural gas. If water and hydrocarbon are not removed thoroughly, wet H2S environment is formed inside the pipeline and consequently the pipe is suffered from electrochemical corrosion. In this paper, Liangdong Pipeline and Chaibeiyuan Pipeline were taken as the examples to analyze the internal corrosion causes and mechanisms of micro-sulfur natural gas pipelines by means of internal detection data analysis, welding procedure evaluation and corrosion mechanism analysis. And then, the suggestions on corrosion control were proposed. It is shown that the average corrosion rate at local corrosion points in the internal wall is 0.126 mm/a in Liangdong Pipeline and 0.008 6 mm/a in Chaibeiyuan Pipeline if the error of internal detection is not taken into account. According to the laboratory corrosion simulation test results, the uniform corrosion rates of Liangdong Pipeline and Chaibeiyuan Pipeline in the working conditions are 0.006 9 mm/a and 0.007 mm/a, respectively. The analysis on welding technology indicates that metallographic and hardness satisfy the related standards and requirements and the current welding technology is satisfactory. Compared with Chaibeiyuan Pipeline, the local corrosion in Liangdong Pipeline is more serious due to its higher H₂S content and wetter environment. As for this type of pipeline, the water dew point of transported natural gas shall be controlled strictly 5 ℃ lower than the ambient temperature and pigging shall be carried out regularly to eliminate liquid loading. Besides, it is recommended to set corrosion monitoring points to test the wall thickness regularly.