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The pressure drop in saturated steam distribution pipes can be calculated in metric units as
dp = 0.6753 106 q2 l (1 + 91.4/d) / ρ d5 (1)
where
dp = pressure drop (Pa)
q = steam flow rate (kg/h)
l = length of pipe (m)
d = pipe inside diameter (mm)
ρ = steam density (kg/m3)
Note that the pressure drop calculated might be higher than the actual values at higher velocities.
The Unwin formula can also be expressed in imperial units as
dp = 0.0001306 q2 l (1 + 3.6/d) / (3600 ρ d5) (2)
where
dp = pressure drop (psi)
q = steam flow rate (lb/hr)
l = length of pipe (ft)
d = pipe inside diameter (inches)
ρ = steam density (lb/ft3)
Steam, 4000 kg/h at 10 bar (1000 kPa) pressure with density 5.15 kg/m3 flows through a schedule 40, ND 100 steel pipe, inside diameter 102 mm and length 100 m. The pressure drop can be calculated as
dp = 0.6753 106 (4000 kg/h)2 (100 m) (1 + 91.4/(102 mm)) / ((5.15 kg/m3) (102 mm)5 )
= 36030 Pa
= 36 kPa
The diagrams below can be used to indicate pressure drop in steam pipes at different pressures. The calculations are made for schedule 40 pipe.




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