Determination of the minimum safe purge gas flow rate in flare systems with a velocity seal

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 d₁/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 d₁/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.