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Sonic tips are designed to discharge the flare gas at sonic velocities. The most conventional technology is the multi-arm design, with fixed or variable exit areas.
Variable exit tips allow greater flexibility in dictating the velocity of gas, and therefore mixing with air for low emission combustion. However, additional moving parts adds complexity to the flare design.
Flares are a safety device and many designs require the flare to remain permanently lit. To ensure that there is no ingress of air into the system (which may lead to an explosion) a minimum amount of gas passing through the flare is required. This purge gas is part of safe and reliable operations. For flares that spend extended periods in this standby state the amount of gas consumed is a significant contributor to overall emissions including methane. This may be reduced in a range of flare designs.