Understanding Flare Exit Velocity and Its Impact on Performance

Hi

Sizing the flare stack diameter is based on velocity, although pressure drop should also be checked. As per API 521, it can be acceptable to allow a velocity up to 0.5 Mach for a peak, short-term, infrequent flow, while maintaining 0.2 Mach for more normal and possibly frequent conditions in low-pressure flares.

The exit velocity of the gas from the flare tip has a significant impact on the flare performance in several ways:

Combustion efficiency: The US EPA with John Zinc performed testing and determined that emissions could be correlated with diameter and gas velocity through the tip. To achieve 98% combustion efficiency, refinery flares are typically limited to an exit velocity of 400 ft/sec at standard conditions. Higher velocities can lead to smoking, especially with certain compounds like olefins, aromatics and long-chain hydrocarbons. This requires steam addition which increases noise.

Flame stability: Low gas exit velocities and buoyancy-dominated flames are preferred for successfully burning low heating value gases. For higher heating value hydrocarbon gases or hydrogen-rich gases, higher exit velocities are desirable. A properly designed flare burner can have exit velocities over Mach 0.5 if other factors allow.

Noise: Higher exit velocities tend to produce greater combustion turbulence and noise. The highest noise levels occur when the flare tip operates at velocities causing combustion instabilities.

Flame shape: At higher exit velocities, the flame uses the discharge energy to entrain combustion air, resulting in a shorter, more vertical flame that resists wind deflection better. Lower velocities produce a softer, longer, more buoyant flame that is affected more by wind.

So the exit velocity needs to be optimized considering the waste gas composition, heating value, combustion efficiency requirements, noise limits and desired flame characteristics. The flare tip design also plays a key role in the range of velocities that can provide stable combustion.

I hope you enjoy these tips, if you'd like a specific subject to talk about, just hit reply to this email.

Happy learning!

Mohamad