A crude oil pipeline transporting heated Daqing crude in the Northeast of China had not carried out pigging for four years and the wax deposition in it was very serious. The existence of wax deposition layer decreased the heat loss of the crude in the pipeline during its normal running, but this did not mean it would also make the pipeline cool down slowly after the shutdown of the pipeline, that is, the wax deposition does not always benefit to the heat preservation of a long-term unpigged hot crude pipeline. So the restart safety of a long-term unpigged hot crude pipeline could not be derived directly from that of the same pipeline pigged before shutdown, even qualitatively. In this paper, study on the safety of the restart of a long-term unpigged crude pipeline is conducted based on numerical simulation and significant conclusions are drawn.

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