Purge gas in flare systems is used to avoid burnback in the flare stack. We have used computational fluid dynamics (CFD) methods to calculate the purge gas flow rate in a flare stack with a velocity seal. We constructed a simulation model of the purging process for different purge rates. As the results show, the baffle size has a significant effect on the oxygen distribution in the flare. When the first baffle/seal diameter ratio is d (1)/D = 0.92 and hydrocarbons are discharged in the flare, the minimum safe purge gas flow rate is 0.05 m/s. However, when the hydrogen content in the discharge gas is high, the minimum safe purge rate is higher: 0.1 m/s. But if the purge rate is maintained at the level recommended by API 521-2007 (0.012 m/s when discharging hydrocarbon gases), the ratio d (1)/D should not be greater than 0.75. In this case, when discharging gas with high hydrogen content, the minimum safe purge rate is 0.02 m/s.