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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)

Example - Pressure Drop in Steam Pipe

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

Steam Pressure Drop Calculator - Metric Units

Steam flow rate - q (kg/h)

Length of pipe - l (m)

Pipe inside diameter - d (mm)

Steam density (kg/m³)

Pressure Drop Diagrams - metric units

The diagrams below can be used to indicate pressure drop in steam pipes at different pressures. The calculations are made for schedule 40 pipe.

• 1 bar = 100 kPa = 10,197 kp/m² = 10.20 m H₂O = 0.9869 atm = 14.50 psi (lb_f/in²)
• 1 kg/h = 2.778x10^-4 kg/s = 3.67x10^-2 lb/min

Steam - pressure 1 bar gauge

Steam - pressure 3 bar gauge

Steam - pressure 7 bar gauge

Steam - pressure 10 bar gauge

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