Numerical calculation model for decompression waves in CO₂ pipeline leakage and effect of initial temperature and pressure

Objective Supercritical/dense-phase CO₂ pipelines serve as the primary mode for long-distance CO₂ transportation. Given their high-pressure operation, it is crucial to calculate and evaluate the crack arrest toughness of pipe materials. Understanding the characteristics of decompression waves is vital for evaluating the crack arrest toughness of pipe materials. However, CO₂ leakage involves a multi-phase decompression process that complicates the propagation behavior of decompression waves, posing challenges for accurate prediction of decompression wave and evaluation of crack arrest toughness.

Methods To investigate the propagation behavior of decompression waves in CO₂ pipeline leakage, an experimental setup was constructed specifically for this purpose. A numerical calculation model for decompression waves was then developed using the homogeneous flow model as the framework, integrating the state equation of gas, sound velocity model, and outflow velocity model. The accuracy of the model was verified through a comparison of calculated outcomes with experimental data. On this basis, numerical calculations were conducted at different initial temperatures and pressure levels.

Results According to the experimental results, the wave velocities at the initial point of CO₂ decompression wave plateau varied directly with pressure and inversely with temperature across different phase states. In contrast to gaseous CO₂, the plateau height of decompression waves for dense-phase and supercritical CO₂ decreased with rising pressure and increased with rising temperature. The effect of temperature and pressure on the decompression wave plateau is essentially the effect of initial entropy and density. The height of the decompression plateau depends on the initial entropy. Specifically, a lower initial entropy value in the gas phase results in a higher plateau, while higher initial entropy values lead to higher plateaus in dense-phase and supercritical states. Furthermore, higher initial density prolongs the plateau duration across all phase states.

Conclusion In practical engineering, close attention should be paid to pipe crack arrest at the onset of high temperature and high pressure. The experimental method and calculation model for decompression waves can offer theoretical support for evaluating CO₂ pipeline crack arrest toughness and informing pipeline design.