Steam Pipes - Sizing
Sizing of steam pipe lines - major and minor loss in steam distribution systems.
The pressure available for distribution of steam is the pressure difference between the initial pressure at the boiler, and the required final pressure at the end of the line - at the steam consumer.
The pressure difference available for the distribution system can be expressed as:
p = pj - pk (1)
where
p = available pressure drop (Pa (N/m2), psi (lb/ft2))
pj = initial or boiler pressure (Pa (N/m2), psi (lb/ft2))
pk = final pressure (Pa (N/m2), psi (lb/ft2))
The total pressure drop in the distribution system is a result of friction (major loss) and pressure loss in fittings (minor loss), and can be expressed as:
pt = pmajor + pminor (2)
where
pt = total pressure drop in the system (Pa (N/m2), psi (lb/ft2))
pmajor = pressure loss in pipes due to friction (Pa (N/m2), psi (lb/ft2))
pminor = pressure loss in fittings (Pa (N/m2), psi (lb/ft2))
Friction - Major Loss
Major loss - the pressure loss due to friction in a low pressure steam distribution system can be expressed as:
pmajor = pa l (3)
where
pa = pipe friction resistance per unit length of pipe (Pa/m (N/m2/m), psi/ft (lb/ft2/ft))
l = length of pipe (ft, m)
The pressure drop in a steam pipe can be expressed as
pa-100 = 0.01306 q2 (1 + 3.6/di) / (3600 ρ di5) (3b)
where
pa-100 = pressure drop per 100 ft pipe (psig / 100 ft)
q = steam flow rate (lb/h)
di = inside diameter of pipe (in)
ρ = density of steam (lb/ft3)
Loss due to Fittings - Minor loss
Loss due to fittings - minor loss can be expressed as:
pminor = ξ 1/2ρ v2 (4)
where
ξ = minor loss coefficient
pminor = pressure loss (Pa (N/m2), psi (lb/ft2))
ρ = density (kg/m3, slugs/ft3)
v = flow velocity (m/s, ft/s)
Equivalent length
Minor loss - loss due to fittings can also be expressed as equivalent length:
pminor = pa le (5)
or
pt = pa(l + le) (6)
where
le = equivalent length of the fittings (ft, m)
As a rule of thumb the total pressure drop is about 5 -10% of initial pressure per 100 m pipe.
Typical Steam Velocities
- Exhaust steam - 20 to 30 m/s (70 - 100 ft/s)
- Saturated steam - 30 to 40 m/s (100 - 130 ft/s)
- Superheated steam - 40 to 60 m/s (130 -200 ft/s)
