Chemical & Process Technology

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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. Although there are many benefits in using flare for proper disposal of hydrocarbon gases, combustion efficiency is still a common concern about flare. It is commonly accepted a flare combustion efficiency can reach approximately 98% (refer API Std 521). This would still results some remaining unburnt hydrocarbon gases release into atmosphere. Thus, there are operators especially in high environment concern area, zero emission and flaring principle is implemented.

Flare is normally lit and a highly reliable pilot system is maintaining the flame at the flare tip. In an extreme condition i.e. storm, heavy rain, strong wind, long serviced pilot, etc, flare may lose the flame. This situation commonly called as flame-out condition. Any release during this period would lead to flammable gas (heavier than air) release and settle to ground level in process area. Wind blowing may disperse the flammable gas and reduce concentration of flammable gas at ground level. However, there is still possibility of flammable gas settled and form combustible mixture. Thus, it is very important to estimate maximum concentration of flammable gas at ground level.

Nowadays with sophisticated software development, software such as PHAST may be used to estimate concentration of flammable gas at ground level at specific location. However, quick estimation method may be important during conceptual phase. Following will present a simple estimation method to calculate maximum concentration of flammable gas at ground level.

Maximum Ground Level Concentration of flammable gas,


where
C = Flammable gas concentration (g/m³)
Q = Mass flow of flammable gas (g/s)
U = Wind speed (m/s)
H' = Effective height (m)

Effective flare height can be determined with


where
H_s = Flare stack height (m)
d_s = Flare tip diameter (m)
V_ex = Exit velocity (m/s)
U = Wind velocity (m/s)

Concentration conversion g/m³ to ppm,


where
[C in ppm] = Concentration in ppm
[C in g/m³] = Concentration in mg/m³
MW = Gas molecular weight

Example
Estimate the maximum ground-level concentration, C, if a flammable gas is accidentally released unburned from a flare, if the release rate to the atmosphere, Q, is 25,200 g/s, the exit
velocity is 83.8 m/s, and flare tip diameter is 0.46 m. The flare stack height is 61 m. Assume that the wind speed 3.1 m/s. The molecular weight of the gas is 54.

H' = H_s + 3 d_s V_ex / U
H' = 61 + 3 (0.46) (83.8 / 3.1)
H' = 98.3 m

C = 0.23 x Q / (U x H'²)
C = 0.23 x 25200 / (3.1 x 98.3²)
C = 0.193 g/m³

[C in ppm] = [C in mg/m³] x 24.45 / MW
[C in ppm] = 0.193 x 1000 x 24.45 / 54
[C in ppm] = 87.6 ppm

Ref : Section 15.11, Handbook of Chemical Engineering Calculations, 3rd Edition, Nicholas P. Chopey