Numerical analysis on the formation of pyrophoric iron sulfides and their oxidation to spontaneous combustion in gas pipelines

Abstract

When the sour gas is transported through the pipeline, the active sulfur in it will corrode the inner wall of the pipeline, forming pyrophoric iron sulfides which will be attached to the pipe wall or move downstream with the transportation medium, and eventually spread throughout the whole pipeline transportation system. When the pipeline is corroded and perforated or in the state of pipeline shutdown and disassembly, if there is negative pressure inside the pipeline, the pyrophoric iron sulfides will contact the incoming air and be oxidized to spontaneous combustion. At this time, if there are substances with combustion or explosion tendency in the pipeline gathered around the pyrophoric iron sulfides that occur spontaneous combustion, major safety accidents such as fire or even explosion are easy to happen, which seriously threaten the safe operation of long-distance gas transmission system. Generally speaking, the greater the accumulation of corrodes inside the pipe, the higher the probability of spontaneous combustion will be due to their contact with oxygen. However, when pyrophoric iron sulfides occur oxidation to spontaneous combustion, it is difficult to be directly perceived from outside the pipe and can only be determined by indirect measurement based on the temperature change of the pipe wall. Therefore, in order to predict the accumulation of pyrophoric iron sulfides inside the pipeline and find out the temperature change of the pipe wall during the spontaneous combustion of pyrophoric iron sulfides inside the pipeline, based on theoretical analysis and computer numerical simulation method, this paper firstly used COMSOL Multiphysics software to build the corrosion model of pyrophoric iron sulfides inside the pipeline and the heat transfer model of pyrophoric iron sulfides inside the pipeline during spontaneous combustion and verify the feasibility of the model; Then, based on the actual operation condition of the long gas transmission pipeline, the formation process of the pyrophoric iron sulfides on the inner wall of the pipeline was simulated, and the heat transfer process of the pipe wall was simulated during oxidative spontaneous combustion of pyrophoric iron sulfides under different factors; after that, according to the simulation results, the corrosion law of the inner wall of the pipe and the heat transfer law of the pipe wall when the pyrophoric iron sulfides occur in oxidation spontaneous combustion were analyzed. The influence of different influencing factors on the heat transfer process of the pipe and their synergistic coupling effect was discussed. Thus, it provides a theoretical basis for early intervention in the oxidation and spontaneous combustion of iron sulfide corrosion products through abnormal temperature monitoring during the shutdown period of the pipeline.