Objective Batch transportation of methanol through existing refined oil pipelines enhances transportation efficiency, reduces costs, and addresses challenges in hydrogen energy storage and transportation. While existing models can roughly predict the mixed oil characteristics of methanol and refined oil during stable flow, their understanding of hydraulic and mixed oil characteristics under complex transient flow conditions remains limited, presenting significant challenges to the safety design and risk assessment of methanol-refined oil batch transportation.

Methods A numerical model for the batch transportation of methanol and refined oil in horizontal and undulating pipelines was developed based on a coupled flow and mixing calculation method. The model’s static and dynamic accuracy was validated through miscibility experiments and comparisons between simulation results and on-site data from refined oil pipeline transportation. The analysis focused on the variations in pressure and mixed oil characteristics under shutdown, water hammer, and leakage conditions.

Results During pipeline shutdown, the density difference between methanol and gasoline intensified natural convection, leading to a more significant increase in shutdown mixed oil compared to other conditions. Under two scenarios—batching methanol followed by gasoline with mixed oil in the up-dip segment, and batching gasoline followed by methanol with mixed oil in the down-dip segment—the mixed oil volume during shutdown reached approximately 20％ of the pre-shutdown volume within 24 hours. During water hammer conditions, the pressure wave from valve-closing water hammer had a greater impact on the pipeline system than pump-stopping water hammer, with the pressure difference being more pronounced on horizontal terrain. Different water-hammer pressure waves caused a minor increase in the volume of mixed oil at the methanol-gasoline interface, with relatively weak overall effects. During leakage conditions, a high leakage flow rate resulted in a decrease in both pipeline pressure and the volume of mixed oil passing through the leakage point. Keeping the leakage flow rate constant, the higher the conveying flow rate, the more significant pressure loss, while the flow rate diminished toward the middle and end of the pipeline, resulting in a more pronounced decrease in mixed oil volume.

Conclusion Compared to horizontal pipelines, pipelines with significant elevation variations and undulations are more significantly influenced by transient conditions, such as shutdown and water hammer, and require special attention. The research findings provide valuable guidance for safety design and mixed oil optimization in methanol-refined oil batch transportation. Further transient testing, aligned with real-world pipeline transportation challenges, is recommended to ensure the safety and reliability of the transportation process.