Abstract: There are two forms of coke（flake and powder）found in filters downstream of coker fractionator in 1.2 Mt/a delayed coking unit in PetroChina Dushanzi Petrochemical Company.It is found that the flake coke is from the spalling of the coke layer in large oil and gas pipeline due to a small blow after the coke drum switching； while the powdered coke is from the foam entering the fractionator at the EOR of coking drum and after drum changing.Therefore，preventing the coke powder from entering the fractionator and reducing the wall temperature of large oil and gas pipeline to promote the coke spalling on the tube wall is the key to solve the problem.Adjusting coking drum switching operation is an effective measure to promote further the spalling of coke in large oil and gas lines.The filter SR-109B downstream of the coking drum is switched to a larger filter and the filter SR-109A is changed to a new horizontal-back flush filter，respectively，and the coking drum operation conditions are adjusted as well.The results indicate that the SR-109B can run for 32 days and the SR-109A reaches 4 years long.By adjusting the defoamer injection at different stages of coking drum operation，the coke powder entering the fractionator can be reduced further，resulting in much longer operation period of the filters.

Key words: delayed coking, filter, defoamer, fractionator, flake coke