Estimate Wind Speed At Flare Tip At Different Height

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Flare is commonly installed in oil and gas process plant to burn hydrocarbon and/or toxic gas to avoid formation of combustible mixture, to minimize green house effect (GHE), to minimize health hazards to personnel on site, etc. There are several earlier posts related to Flare :

• Quick Estimate Flare Stack Diameter
• Estimate Subsonic Flare Tip Pressure Drop With Graph Derived Correlation
• Quick Estimate Flare Tip Pressure Drop
• Flare Tip Type Quick Selection Chart
• ANSI/API Std 537 / ISO 25457 2nd edition, Dec 2008 is Released
• Guideline on Quick Determination of Flare Stack Support Type
• Flare Smokeless Ringlemann Chart
• Must Read Flare Handbook
  
• FLARE combustion efficiency
• Quick Estimate Ground Level Unburnt Flammable Gas For Vent Pipe or Flame-out Flare Stack
• Elevated Flare (SI Unit) Datasheet
• Enclosed Flare (SI Unit) Datasheet

Wind speed affect flare performance
Wind speed affects flare performance at least in few ways :

1. Wind drag flame to downwind, lower the flame center and radiation level of downwind receptor
2. Wind carry unburnt component to downwind
3. Wind potential put-off flame on flare under low gas flow and pilot

Therefore, correct wind speed at flare tip (elevated flare) is important in order to obtain correct estimate of radiation level and unburnt component concentration at downwind location, determination of minimum vapor flow to avoid flame-out and performance of pilots.

Wind speed change with flare height
One of characteristic of wind speed is wind speed increase with height from grade. Thus, higher the flare stack, flare tip will expose to higher wind speed. Generally wind map will be obtained from local authority or organization. The wind map may be measured at specific height i.e. 10m from grade.

In case flare stack height is different than wind speed at specific height, how shall an engineer determine wind speed flare tip ?

Corrected wind speed (without return duration correction)

Where
U_Z = Wind speed at Z m at return duration of t0 hour (ft/s)
U₀ = Wind speed at specified height of 10m at return duration of t0 hour (ft/s)
t₀ = Wind speed at return duration i.e. 60 minutes, 1 minutes, etc
Z = Flare stack height (ft)

Corrected wind speed (with return duration correction)

Where
t = Wind speed at return duration i.e. 60 minutes, 1 minutes, etc (different than t₀ )

Example :
A flare stack with height of 200m, expose to wind speed (at 60 minutes return duration) of 10 m/s measured at 10m from grade. Determine wind speed at flare tip if (a) the return duration stay as 60 minutes (b) return duration change to 1 minute.

Solution (a)
Z = 200 m = 200 x 32.8 ft
U_Z = 10 m/s = 10 x 32.8 ft/s

U_Z = 41.7 ft/s = 12.7 m/s
Wind speed at flare tip with return duration stay as 60 minutes = 12.7 m/s


Solution (b)

u_Z,t = 44.8 ft/s = 13.66 m/s

Wind speed at flare tip with return duration at 1 minutes = 13.66 m/s


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