Example 8.10:Calculate Flare Stack Draft


Gas Mass Flow Rate (M, lb/hr):
Gas Density(ρg, lbm/ft3):
Gas Viscosity(me, cp):
Air Density(ρa,lb/ft3)
Stack Inside Dia. (ID, ft):
Stack High(Hs,ft)
Total Equivelent Length for Fraction Loss Calculation(Le,ft)
Roughness (e,in):
Result
Gas Velocity in Stack(V, ft/sec):   Normal 15-20 ft/second
Epsilon/Dia.Ratio (e/D):
Reynolds No. (Re):
Moody Fraction Factor (fm):
Stack Draft(Hav, inH2O):
(Psi):
Pressure Drop (Δp, Psi):
Velocity Head at Exit(vp, Psi):


Example 8-10 Find the available draft in a 2 ft ID by 20 ft long stack

Hav=0.192 * Hs * (ρg - ρa) + 0.0029 * V2 * ρg * ( fm *
Hs / Ds
+ 1 )       Eq 8-18

Fraction Loss based on Darcy-Weisbach
hL=fm*
L*V2 / 2*g*D
        or        ΔP=fm*
L*ρ*V2 / 144*(2*g)*D


Re=
D*V*ρ / μe


1) When the fluid flow is laminar (Re<2000)

fm =
64 / Re


2) When the fluid flow is turbulent (Re>4000)

Chen Equation

A =log{
(ε/D)1.1098 / 2.8257
+
5.8506 / Re0.8981
}

1 / fm
= { -2*log(
ε/D / 3.7065
-
5.0452*A / Re
) } 2






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