The United States Environmental Protection Agency provides several guidlelines as well as codes for environmental protection.

A code of federal regulations (CFR) with the title CFR 40 Section 60.18 provides guidelines for determining the flare exit or tip velocities as a function of the calorific value of the flared gas.

The flare exit velocity is calculated based on two different equations for steam-assisted flares and air-assisted flares. The following are the equations:

Steam-Assisted Flares:

log

_{10}(v

_{max}) = (B

_{v}+ 1212) / 850.8

where:

v

_{max}= maximum permissible velocity at the flare tip, ft/s

B

_{v}= Heating value of the gas being flared, Btu / scf

Air-Assisted Flares

v

_{max}= 28.56 + 0.087*B

_{v}

where V

_{max}and B

_{v}are defined as above

Notes:

1. v

_{max}not to exceed 60 ft/s for flare gas having heating value of 300 Btu/scf

2. v

_{max}to be calculated based on above equations for flare gas having heating value between 300-1000 Btu/scf

3. v

_{max}up to 400 ft/s is permitted for flare gas having heating value >1000 Btu/scf

The flare tip is designed considering a velocity of 80% of the max. permitted velocity i.e. 0.8*vmax

Based on the calculated maximum velocity v

_{max}the flare tip daimeter can be easily calculated as follows:

Q = A*V

where:

V = 0.8*V

_{max}, ft/s

Q = Volume flow rate of the total gas passing through the flare tip (sum of the main flare gas plus the purge gas), scfm

A = cross-sectional area of the tip, ft

Simplifying the above equation in terms of flare tip diameter we get the following equation:

D

_{tip}= 1.95*(Q / V

_{max})

^{0.5}

where:

D

_{tip}= diameter of the flare tip, inches

Q = flow rate, scfm

V

_{max}= maximum velocity as calculated above, ft/s

If you know the flare gas composition you can calculate the calorific value using any simulator or using the law of proportions calculate the calorific value of the gas for each component and multiply it with the individual mole fraction and add up the individual values to get the total calorific value.

The following links are given for further reading and guidance on "40 CFR Section 60.18"

http://edocket.acces.../40cfr60.18.pdf

http://gcs.regscan.c...W0EJSKNKSLE5CAD

Hope this proves useful to the esteeemed readers of this forum.

Comments are welcome.

Regards,

Ankur.