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Steam Pipes - Sizing

Pipe sizing steam systems - major and minor loss in steam distribution systems

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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)
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