Abstract: Due to the rapid rise of system pressure drop，the naphthene-base VGO hydrocracking unit with a capacity of 2.1 Mt/a of CNOOC Ningbo Daxie Petrochemical Co. Ltd. had to be shut down. It was confirmed that the system pressure drop occurred mainly in the high pressure heat exchanger and partially in the refining reactor. The white dirt accumulated in the inlet distributor of high pressure heat exchanger and also deposited in the scaling basket and in the upper part of the guard catalyst of refining reactor, resulting in a rapid rise of system pressure drop. The white dirt is Fe2P4O12 formed by the reaction of iron element in the VGO feed and the phosphorus compound in the high temperature corrosion inhibitor in the upstream crude distillation unit. Countermeasures, such as controlling phosphorus content in hydrocracking feed below 0.5 μg/g，meanwhile adopting the phosphorus-free corrosion inhibitor or upgrading equipment materials in the upstream crude distillation unit, were proposed.

Key words: naphthene-base VGO, hydrocracking, pressure drop, scaling, phosphorus