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FLARE SIZE OR CAPACITY
IF DURING FLARE DESIGN THE FLOW RATE IS OVERESTIMATED, THE FLARE WILL BE OVER SIZED. IN AN OVER-SIZED FLARE, BURNING INSIDE THE FLARE TIP MAY OCCUR AND DUE TO LOW VERTICAL MOMENTUM, THE FLAME MAY EASILY BE EXTINGUISHED BY THE WIND.
ON THE OTHER HAND, IF FLOW RATE IS UNDERESTIMATED, THE FLARE WILL BE UNDERSIZED. HENCE, FLAME WILL NOT BE STABLE. EXCESSIVE FLOW RATE ( OVER THE DESIGN RATE ) MAY CAUSE THE FLAME TO LIFT OFF THE FLARE TIP OR BLOW OUT, ALLOWING UNBURNED COMPOUNDS TO ESCAPE IN THE UNTREATED EXHAUST.
THE MAXIMUM GAS HANDLING CAPACITY OF A FLARE CAN BE ESTIMATED USING THE FOLLOWING APPROACH SUGGESTED BY API. IF THE FLARES ARE EQUIPPED WITH SOME KEY ACCESSORIES, SUCH AS FLAME ARRESTERS, A FLAME WIND SHIELD, AND A RELIABLE PILOT AND IGNITION SYSTEM, THE ALLOWABLE EXIT VELOCITY OF GASES CAN BE AS HIGH AS 0.5 TIMES THE SONIC VELOCITY.
THE SONIC VELCOTY OR MACH NUMBER, Ma IS ESTIMATED AS FOLLOWS :-
| Ma = 1.702*10-5 *[ W/ (P2*D2) ] * [ Z*T/ (K*MW ) ]0.5 |
WHERE W = FLARE GAS FLOW RATE, lb/Hr.
P2 = PRESSURE AT THE FLARE TIP, 14.7 PSIA
D = FLARE TIP INSIDE DIAMETER, ft
Z = FLARE GAS COMPRESSIBILITY FACTOR
T = FLARE GAS TEMPERATURE, OR
MW = FLARE GAS MOLECULAR WEIGHT, lb/lb mole
SO, FROM THE ABOVE EQUATION, IF WE ASSUME Ma=0.5, & OTHER PARAMETERS ARE KNOWN, WE CAN CALCULATE FLARE GAS FLOW RATE OR IF FLARE GAS FLOW RATE IS GIVEN, WE CAN FINDOUT THE DIAMETER OF FLARE STACK.
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