# 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 = p_{j}- p_{k}(1)

where

p = available pressure drop (Pa (N/m^{2}), psi (lb/ft^{2}))

p_{j}= initial or boiler pressure (Pa (N/m^{2}), psi (lb/ft^{2}))

p_{k}= final pressure (Pa (N/m^{2}), psi (lb/ft^{2}))

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/m^{2}), psi (lb/ft^{2}))

p_{major}= pressure loss in pipes due to friction (Pa (N/m^{2}), psi (lb/ft^{2}))

p_{minor}= pressure loss in fittings (Pa (N/m^{2}), psi (lb/ft^{2}))

### 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/m^{2}/m), psi/ft (lb/ft^{2}/ft))

l = length of pipe (ft, m)

The pressure drop in a steam pipe can be expressed as

p_{a-100}= 0.01306 q^{2}(1 + 3.6/d_{i}) / (3600ρ d_{i}^{5}) (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/ft^{3})

### Loss due to Fittings - Minor loss

Loss due to fittings - minor loss can be expressed as:

p_{minor}= ξ 1/2ρ v^{2}(4)

where

ξ = minor loss coefficient

p_{minor}= pressure loss (Pa (N/m^{2}), psi (lb/ft^{2}))

ρ = density (kg/m^{3}, slugs/ft^{3})

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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