Venting design for block valve station of supercritical CO₂ pipeline

Objective Supercritical CO₂ pipeline transmission is deemed the most cost-effective solution to address the carbon sourcesink mismatch in Carbon Capture, Utilization, and Storage (CCUS) technology. However, during venting operations required for pipeline maintenance, the vented pipeline segment might drop below － 20 ℃ due to the phase change and Joule-Thomson effect, potentially leading to brittle fractures in the pipeline.

Methods OLGA software was used to establish a venting model for supercritical CO₂ pipelines. The subsequent comparison between the simulation results and experimental data demonstrated the accuracy of OLGA software in predicting CO₂ phase changes, temperature reductions, and pressure drops. On this basis, an intermittent venting design was proposed. In addition, simulations were conducted to analyze the influence of vent valve openings on the durations of the venting process and the temperature escalation process post valve closure under varying initial pressures and temperatures.

Results Within the vented pipeline segment of the entire trunk pipeline, the vent point was identified as the most dangerous as it is the first to experience temperature drops under － 20 ℃. Decreasing valve openings led to an exponential increase in the duration required to reach － 20 ℃ at this point. As the vent valve openings decreased, the total venting volume for a single valve opening operation increased, resulting in lower average venting rates and elevated time costs for venting. Conversely, excessively large vent valve openings led to a reduction in the total venting volume for a single valve opening operation. Moreover, high pressure levels throughout the vented segment of the trunk pipeline also prolonged the duration needed to fully vent the medium in the pipeline until it reached atmospheric pressure. The temperature escalation of the medium in the pipeline following valve closure may be divided into two stages respectively dominated by axial heat transfer or radial heat transfer, and the former stage was observed at a higher rate of temperature rise. As the valve openings increased, the time taken for temperature escalation after valve closure tended to stabilize following initial rapid increments.

Conclusion Both excessively large or small vent valve openings lead to prolonged venting durations. In engineering applications, the temperatures or pressures at the vent point may be linked with the vent valve actions. However, to guarantee control reliability and minimize total venting durations, it is crucial to rationally choose vent valve openings for each action.