Steam Pipes - Sizing

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

The pressure drop available for the 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 = p_j - p_k (1)

where

p = available pressure drop (Pa (N/m²), psi (lb/ft²))

p_j = initial or boiler pressure (Pa (N/m²), psi (lb/ft²))

p_k = 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:

p_t = p_major + p_minor (2)

where

p_t = total pressure drop in the system (Pa (N/m²), psi (lb/ft²))

p_major = pressure loss in pipes due to friction (Pa (N/m²), psi (lb/ft²))

p_minor = 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:

p_major = p_a l (3)

where

p_a = pipe friction resistance per unit lengthof 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

p_a-100 = 0.01306 q² (1 + 3.6/d_i) / (3600 ρ d_i⁵) (3b)

where

p_a-100 = pressure drop per 100 ft pipe (psig / 100 ft)

q = steam flow rate (lb/h)

d_i = 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:

p_minor = ξ 1/2ρ v² (4)

where

ξ = minor loss coefficient

p_minor = 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:

p_minor = p_a l_e (5)

or

p_t = p_a(l + l_e) (6)

where

l_e = 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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