Investigation of the release characteristics of dense-phase CO 2 pipelines with different orifice diameters

Dense-phase CO2 pipelines are essential for efficient CO2 transport, yet operational venting or accidental leaks can induce dangerous localized cooling. In this study, we experimentally investigated the release characteristics from such a pipeline using orifices of varying diameters. Analysis of the transient pressure and temperature data revealed that a larger orifice diameter significantly shortens the total release time and the duration to reach stable depressurization, while also attenuating initial pressure oscillations. Critically, the lowest temperature was recorded at the pipeline’s distal end, and a larger orifice was found to increase the maximum rate of temperature drop. This cooling was severe enough to depress the pipe wall temperature at the venting orifice below the material’s ductile-to-brittle transition temperature (DBTT), posing a direct risk of brittle fracture. Consequently, for ensuring pipeline safety, particular attention must be paid to the wall temperature at both the venting orifice and the distal end. Our findings suggest that appropriately reducing the venting orifice diameter can effectively mitigate the cooling rate, thus alleviating the embrittlement of pipeline steel caused by extreme low temperatures.