A computational modelling of natural gas flow in looped network: Effect of upstream hydrogen injection on the structural integrity of gas pipelines

In this paper a numerical simulation of transient hydrogen-natural gas mixture flow in a looped network is studied based on a mathematical model that considers the variation of the compressibility factor of the gas mixture with pressure under isothermal gas flow. The transient analysis is preceded by a steady state modelling. Moreover, the distribution of gas flow rates in pipes and the pressure values in nodes are determined in the steady state and served as initial conditions for the time dependent study. The transient regime is created by a periodic evolution of consumers demand versus time. The numerical computation is performed using the finite element method based software, COMSOL Multiphysics, and examined on a single gas transmission pipe as well as on a looped gas network system. Due to periodic changes of gas demands over a day, the pressure undergoes periodic oscillations around its mean value. The numerical computation is also investigated to focus on the effect of hydrogen injection, with a defined mass ratio, into the natural gas flow on the gas mixture behaviour. The impact of hydrogen gas on the integrity of the pipe material is followed up through comparison between the allowable stress and the resulted circumferential stress. It is observed that overpressure values in transient regime are proportional to the hydrogen mass ratio in the gas mixture. Such overpressure raises the circumferential stress and thereafter the internal load applied to the pipeline wall increases. Both steady and transient results numerically obtained were compared with the experimental results available in the literature.