A CFD study of the unsteady leakage and dispersion of natural gas pipelines containing high-H2S in mountainous terrain

Abstract

The leakage and diffusion of high-H₂S natural gas pipelines in complex terrain poses significant toxic risks. Previous studies often assumed uniform gas diffusion coefficients and mass flow rates, leading to substantial errors in energy and mass flux calculations, and ultimately, inaccurate assessments of leakage impact and hazard levels. This study corrects the multicomponent gas diffusion coefficient matrix, reducing errors to 5.14 %, and then derived the unsteady mass flow rate through small pipeline leaks, applying these to the flow governing equations. Results show that the effect of wind speed on toxic gas diffusion in small or 2D calculations does not apply to actual 3D conditions. The H₂S SCC (Safety Critical Concentration) range far exceeds methane’s LEL (Lower Explosion Limited) range, highlighting the importance of H₂S toxicity in the incidents. The leakage direction and range are mainly influenced by Fick coefficient, unsteady state mass flow rate, complex terrain, and wind field. Close to the leakage hole, jet effects dominate gas distribution, while farther away, diffusion from concentration gradients prevails. Quantitative analysis shows that H₂S concentration initially rises then falls due to transient mass flow rate. While increases in initial pressure P₀, leakage diameter D_leak, and H₂S content $x_{H_{2}S}$ all expand the diffusion range, $x_{H_{2}S}$ has the least impact. The exponential effect of D_leak on leakage time means smaller hole may lead to a larger H₂S SCC range. Therefore, the condition of living organisms at a specific point, will be suffered more hazardous condition when the HSGTP has a higher P₀ or $x_{H_{2}S}$, and the larger D_leak may have the opposite effect. Higher wind speed u_wind shift the diffusion range from the vertical to the radial direction, while wind direction changes are significantly influenced by terrain barriers. Trees, acting as obstacles, significantly hinder gas diffusion due to the severe kinetic energy loss of the process. These findings provide significant insights for safety insights in future incidents.