In many gas plant, flares are always installed to burn waste gases containing combustible components, such as methane, carbon monoxide, and hydrogen. The waste gases are piped to a remote, usually elevated location, and burned in open flame in ambient air using a specially designed burner tip, auxiliary fuel, and in some cases, assist gases like steam or air to promote mixing for nearly complete.
As mentioned in several posts, combustion requires three ingredients: fuel, an oxidizing agent (typically oxygen in air), and heat (or ignition source).
Flare Purge Gas and Flare Sweep Gas
In flare system, a pipe connection for purging or sweeping purpose is usually installed. Flare purge gas means gas introduced between a flare header’s water seal and the flare tip to prevent oxygen infiltration (backflow) into the flare tip. For a flare with no water seal, the function of flare purge gas is performed by flare sweep gas.
Flare sweep gas means the gas intentionally introduced into the flare header system to maintain constant flow of gas through the flare header to prevent oxygen build up in the flare header.
Sweep or Purge Gas Requirement
The total volumetric flow to the flame must be carefully controlled to prevent low flow flashback problems and to avoid flame instability. Typically, natural gas, fuel gas, nitrogen, or carbon dioxide, is used as sweep gas or purge gas. Nitrogen or another inert gas must be used to prevent the formation of an explosive mixture in the flare system if there is a possibility of air in the flare manifold.
To ensure a positive flow through all flare components, sweep gas injection should be at the farthest upstream point in the flare transport piping. For flare without flare gas recovery, the sweep gas also acts as purge gas to prevent air ingress and the flare tip. For simple flare collection headers where a single emission source is controlled by a flare, the amount of sweep gas required can be estimated by the gas flow rate needed to prevent oxygen ingress at the flare tip.
The minimum continuous purge or sweep gas flow rates required to prevent oxygen ingress the flare tip is determined by the design of the stack seals, which as usually proprietary devices. Modern labyrinth and internal gas seals are stated to require a gas velocity of 0.001 to 0.04 ft/s (at standard conditions). For example, buoyancy seal normally reduces purge-gas velocity requirement through the tip to 0.003 m/s. But much higher gas exit velocities are needed to avoid the burn back of the flare tip.
The use of velocity seal normally reduces the purge gas velocity through the tip between 0.006 m/s to 0.012 m/s and manages to keep oxygen level below the seal under 4-8% level. The same case with buoyancy seal, purge gas requirement to prevent burn back of the flare tip is much higher than this.
Equation below can be used to estimate hourly purge gas requirement. By assuming velocity is 0.04 ft/s, then
D = flare diameter (in)
144 = conversion from square feet to square inches (in2/ft2)
3600 = conversion from seconds to hours (sec/hr)
There is another minimum flare tip velocity for operation without burn lock or instability. This minimum velocity is dependent on both gas composition and diameter and can range from insignificant on small flares to 0.5 ft/second in greater than 60-in diameter units.