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334 Lawrence K. Wang et al.

2.4. Steam Requirements
Steam requirements for steam-assisted flare operation depend on the composition of
the flare gas and the flare tip design. Typical design values range from 0.15 to 0.50 lb
steam/lb flare gas. In this handbook, the amount of steam required for 98% destruction
efficiency is assumed as 0.4 lb steam/lb flare gas. The following equation is used to
determine steam requirements (5):
Q = 1.03 × l0 −3 × Q × MW (4)
s flg flg
where Q is the steam requirement (lb/min), MW is the molecular weight of the flare
s flg
gas (lb/lb-mole)
MW = [(Q )(16.7) + (Q )(MW )]/Q (5)
flg f e e flg
MW is the molecular weight of the emission stream (lb/lb-mole).
e
3. ENGINEERING DESIGN
3.1. Design of the Flame Angle
The flare tip diameter, D , should be rounded up to the next largest commercially
tip
available size (14–16). The minimum diameter is 1 in. with larger diameters available
in 2-in. increments between 2 and 24 in., and 6-in. increments between 24 and 60 in.
The flame angle, θ, is calculated using
−1
1/2
θ = tan [1.47V /(550(∆P/55) ] (6a)
w
where θ is the flame angle (deg), V is the wind velocity (assumed to equal 60 mph),
w
2
and ∆P is the pressure drop (in. H O) = 55 (U /550) , where U is obtained from
2 flg flg
Section 2.3. This reduces to
θ = tan −1 (88.2/U ) (6b)
flg
3.2. Design of Flare Height
The flare height is calculated using
−3
H = (0.012185)(Q × h ) 1/2 − (6.05 × 10 )(D )(U )(cos θ) (7)
flg flg tip flg
where H is the flare height (ft), Q is the flare gas flow rate (scfm), h is the flare gas
flg flg
heat content (Btu/scf), D is the flare tip diameter (in.), and U is the exit velocity of
tip flg
flare gas (ft/s).
3.3. Power Requirements of a Fan
The electricity cost results mainly from a fan needed to move the gas through the
flare. Equation (8) can be used to estimate the power requirements for a fan. This
equation assumes a fan-motor efficiency of 65% and a fluid specific gravity of 1.0:

F = 1.81 × 10 −4 (Q )(P)(HRS) (8)
p flg,a
where F is the power requirement for the fan (kWh/h), Q is the actual flare gas flow
p flg,a
rate (scfm), P is the system pressure drop (in. H O [typically 16 in. of H O]), and HRS
2 2
is the annual operating hours (h/yr).