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 = 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 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

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