Abstract: Based on the service of the existing nitrogen seal in the vertical arch-roof oil tank, the process of the nitrogen seal system is analyzed and a problem of excessive O₂ content is observed in the gas mixture within the tank. After modification, the O₂ content in the gas mixture is less than 0.5%, thus the N₂ consumption is reduced, which solves the shortened lifetime of the catalyst because feed oil oxidizes and deteriorates in the downstream units.