Objective Industrial CO₂ pipeline transportation typically operates in a supercritical/dense-phase high-pressure mode. In the event of a pipeline leakage, highly concentrated CO₂ will be generated in the vicinity of the leakage port, presenting a significant risk to nearby persons and other creatures.

Methods Currently, there is a research gap in quantitatively analyzing and evaluating the range of influence and safety distance related to CO₂ pipeline leakage. To investigate the leakage and diffusion behavior of supercritical/dense-phase CO₂ pipelines and determine safety distances for various CO₂ exposure levels, a self-designed, large-scale CO₂ pipeline relief and diffusion experimental facility was used to conduct CO₂ pipeline relief experiments under varying pressure and temperature conditions, with the aim of examining the change pattern of CO₂ concentration in the diffusion zone after relief and the CO₂ influence range under typical volume fractions.

Results Under the experimental pressures and temperatures, the volume fraction of CO₂ near the relief outlet was predominantly influenced by jet action. Across all experimental conditions, the volume fraction of CO₂ reached up to 70% at a distance of 5 m, while remaining below 8% at 10 m. The relief duration of CO₂, with a volume fraction exceeding 4% at a distance of 5 m from the relief outlet, was calculated to be up to 125 s. Moreover, higher operating pressures in the pipe resulted in prolonged relief duration. The volume fraction of CO₂ at a distant location from the relief outlet was influenced by turbulent diffusion and ambient wind conditions. For CO₂ with a volume fraction of 4%, the maximum diffusion distance was 32 m at pressures ranging from 7.5 MPa to 10.5 MPa and 20 m at operating temperatures ranging from 20 ℃ to 40 ℃. Following the CO₂ pipeline relief, the temperature in the diffusion zone decreased, and the temperature drop in the diffusion zone increased with higher initial pressure or lower initial temperature in the pipe.

Conclusion The diffusion distance of CO₂ is influenced by both the operating pressure and the initial temperature in the pipe. Higher operating pressure or lower initial temperature will result in a greater diffusion distance. Additionally, the diffusion distance can also be influenced by ambient wind conditions. After the relief of an industrial CO₂ pipeline, the hazardous area must be clearly demarcated, taking into account the operating status of the pipeline as well as a comprehensive evaluation of ambient wind speed and direction.