Study on the effect of valve openings and multi-stage throttling structures on the pressure and temperature during CO2 pipeline venting processes

The venting operation is an important measure for rapidly eliminating the risk of overpressure in CO2 transportation pipelines. However, the low-temperature phenomenon and vibration generated by the rapid discharge can affect structural reliability. Multi-stage throttle structures have been proven to effectively alleviate vibration phenomena, but their impact on low temperatures remains unclear. In this study, venting tests were conducted on eight groups of single-stage and two-stage throttle structures based on a large-scale pipeline. The results indicate that the nonlinear variation of the valve resistance coefficient when the valve opening changes is the fundamental reason affecting the downstream temperature. Two-stage throttle structures can effectively mitigate the low-temperature conditions in certain spaces within the pipeline. For the remaining low-temperature sections, three-stage throttle structures can provide effective compensation. However, this significantly reduces the pressure drop rate in the main pipeline, contradicting the original purpose of the venting operation. Additionally, considering the temperature of the throttle structure and the pressure within the main pipeline, this study recommends increasing the valve openings for the two-stage throttle structure. The results of this study provide important references for the design of throttle structures and the formulation of on-site venting schemes, with strong prospects for engineering applications.