If air or oxygen enters into the flare system and forms a flammable mixture of gases within the system, it can be ignited by the pilot burner at the flare tip. If the flash back velocity of the mixture exceeds the efflux velocity, the flame will propagate backward into the flare stack and explosion is likely to result. If the flashback velocity is close to the forward flow there could be a steady combustion inside the flare stack that could lead to overheating and mechanical failure.
Some of the reasons leading to flashback:
- Vacuum system is connected to the flare.
- Lighter gas compare to air is being flared, especially Hydrogen.
- Relief valves removed for servicing.
- Air or oxygen is used in the processes connected to the flare system.
- Keep efflux velocity higher than the flashback velocity.
- Adding purge gas if necessary or use of a velocity accelerator can increase efflux velocity.
- Mixing an inert gas close to the flare tip will reduce the flashback velocity, provided flare gas heating value is above allowed minimum.
- It is economical to use combination of velocity accelerator and inert gas.
- Add liquid seals between the two or more flare open to a common header.