Improvement of Direct Evaluation Method for Internal Corrosion of Natural Gas Pipelines Containing H₂S

In order to determine the influence of H₂S on internal corrosion of natural gas pipelines, the direct evaluation method for internal corrosion of natural gas pipelines proposed by NACE SP0110-2010 standard (including pre-evaluation, indirect evaluation, direct evaluation and post-evaluation) was improved by introducing CO₂-H₂S corrosion rate prediction model and ponding probability in the indirect evaluation of the method, which improved the accuracy of the corrosion degree of pipeline at recommended excavation points. Taking a natural gas pipeline containing H₂S as an example, the improved evaluation method was demonstrated. The results show that according to indirect evaluation (multiphase flow corrosion simulation calculation), extremely severe corrosion occurred 500 m in front of the pipeline, and 23% of the pipeline sections had a ponding probability of 1. According to the direct evaluation (on-site excavation), the pipelines at all 5 excavation points were severely corroded, with a maximum corrosion rate of 0.337 mm/a, which was consistent with the predicted results of the indirect evaluation. According to the post-evaluation, the pipeline reassessment interval was determined to be 1 a. The improved evaluation method can provide certain reference for predicting high-risk corrosion points in multiphase flow natural gas pipelines containing H₂S.