FLAME STABILITY
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                          ACCORDING TO THE U.S. EPA, A FLARE HANDLING GASES WITH HEAT CONTENT GREATER THAN 1000 BTU/std ft3 CAN HAVE A STABLE FLAME AS LONG AS THE EXIT VELOCITY OF FLARE IS BELOW 400 ft/Sec.

                         EXIT VELOCITY OF FLARE GAS CAN BE CALCULATED USING THE FORMULA GIVEN BELOW :-
 

Vex = [ 5.766 * 10-3 * Qf * ( Tf + 460 ) ] / Di2

 

WHERE

Vex = EXIT VELOCITY OF FLARE GAS, ft/Sec.

Qf = FLARE GAS FLOW RATE, std.ft3 /Min.

Tf = FLARE GAS TEMPERATURE, 0F

Di = FLARE TIP INSIDE DIAMETER, inches

 

Example : Determine if a flare handling 30,000 std. ft3 /Min. of gas will have a stable flame.  The flare gas temperature is 1500F and the flare tip diameter is 18 inches.

SOLUTION :  Given  Qf = 30,000 std.ft3 /Min.

                                       Tf = 1500F

                                       Di = 18 inches

                             So     Vex = 326 ft/Sec. ........ from the above formula

 

Since this flare gas exit velocity is less than 400 ft/Sec., the flame is expected to be stable for the conditions given in this example.