During the second overhaul of the 800 kt/a ethylene plant of a petrochemical company in Sichuan, some chemical cleaning methods were used to remove the ferrous sulfides, heavy oil components and hydrocarbon polymers in quench oil tower system as well as the waste alkali grease, rust and other scale deposits in alkaline washing tower, waste alkali collection tank and waste alkali tank. After the cleaning, the equipment met the requirements of maintenance/production, and the goal of clean surface was achieved.