Steam Pipes - Sizing
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) / (3600ρ 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)
Related Topics
• Sizing of Steam and Condensate Pipes
Dimensions of steam and condensate pipe lines. Calculate pressure losses, recommended velocities, capacities and more.
• Steam and Condensate
Design of steam & condensate systems with properties, capacities, sizing of pipe lines, system configuration and more.
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