Calculate pressure drops in steam distribution pipe lines.
The pressure drop in a saturated steam distribution pipe line can be calculated in metric units as
dp = 0.6753 106 q2 l (1 + 91.4/d) / ρ d5 (1)
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)
The Unwin formula can also be expressed in imperial units as
dp = 0.0001306 q2 l (1 + 3.6/d) / (3600 ρ d5) (2)
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)
Use values for the specific volume corresponding to the average pressure if the pressure drop exceeds 10 - 15 % of the initial absolute pressure.
Note that for elevated velocities the Unwin's formula is known to give pressure drops higher than the actual.
Example - Pressure Drop in a Steam Pipe
A steam flow 4000 kg/h with pressure 10 bar (1000 kPa) and density 5.15 kg/m3 flows through a schedule 40 ND 100 steel pipe with 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 Pressure Drop Calculator - Imperial Units
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 (lbf/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