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Density
r =
The density computation assumes perfect gas behavior which is fine for these
purposes.
Velocity
V = 
Critical Velocity (Sonic Velocity)
V s
= 68.1
Critical velocity is evaluated at the downstream end of each segment, where
the pressure and density are lower resulting in the highest velocity within
the segment.
Reynolds Number
N RE
= 
Friction factor
f D
= 4
squared
where A =  +
 ^ 0.9
Equivalent length
This is the concept of equating the friction loss in fittings to an
equivalent length of straight pipe.
Definitive pipe sizing
G = 1335.6 d 2
where K = f D[Leq/D]
This formula is applicable to isothermal flow conditions. Vapor flow in
manifolds will more closely resemble adiabatic conditions. Solution of the
adiabatic equation is complex due to the requirement for trial-and-error for
both pressure and temperature. Mak showed that for pipelines of length fL/D>10
the isothermal solution for pressure drop is never more than 4% greater than the
adiabatic solution (Ref 4). Thus, the use of isothermal methods is slightly
conservative and well within the accuracy required for practical engineering
calculations.
Gas mixture viscosity
µ mix
=  +
where f12
= 
and f21
= f12
This is the Wilke estimation method (Ref 5) for the viscosity of gas
mixtures. The molar average method used in this article may be 5 to 10%
erroneous (resulting in a pressure drop calculation that is 1% off). The Wilke
method predicts the mixture viscosity within about 1% of actual.
NOMENCLATURE
d = pipe internal diameter (in.)
D = pipe internal diameter (ft.)
f = Chen friction factor
f D
= Darcy friction factor
G = mass flow rate (lb/h)
k = ratio of specific heat capacities (C p/Cv)
K = resistance coefficient due to friction loss
L EQ
= equivalent length of piping system (ft.)
M w
= molecular weight of fluid (lb/lb-mol)
N Re
= Reynolds Number
P 1
= upstream pressure (psia)
P 2
= downstream pressure (psia)
t = fluid temperature (°F)
V = fluid velocity (ft/s)
V s
= fluid sonic or critical velocity (ft/s)
y i
= mole fraction of component i
r = fluid density (lb/ft3)
µ = fluid viscosity (cP)
e = absolute pipe roughness (ft)
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