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Minor Pressure Head Loss in Pipe and Duct Components - Equivalent Lengths

Minor pressure and head loss in pipes, tubes and duct systems

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Pressure loss in straight pipes or ducts are called the major, linear or friction loss. Pressure loss in components like valves, bends, tees and similar are called the minor, dynamic or local loss.

Minor loss can be significant compared to major loss. In fact - when a valve is closed or nearly closed - the minor loss is infinite. For an open valve the minor loss can often be neglected (typical for a full bore ball valve).

Minor Loss

The pressure drop or the minor loss in a component correlates to the dynamic pressure in the flow and can be expressed as

Δpminor_loss = ξ pd

      = ξ ρf v2 / 2                                      (1)

where

ξ = minor loss coefficient

pd = dynamic pressure in fluid flow (Pa (N/m2), psf (lb/ft2))

Δpminor_loss = minor pressure loss (Pa (N/m2), psf (lb/ft2))

ρf = density of fluid (kg/m3, slugs/ft3)

v = flow velocity (m/s, ft/s)

The minor loss can be expressed as head water column by dividing the dynamic pressure with the specific weight of water

Δhminor_loss,w = (ξ ρf v2 / 2) / γw

ρf v2 / 2) / (ρw g)   

= ξ ρf v2 / (2 ρw g)                                         (2)

where

Δhminor_loss,w = minor head loss as water column (m H2O, ft H2O)

γw  = ρw g = specific weight of water or reference fluid (9807 N/m3, 62.4 lbf/ft3)

g = acceleration of gravity (9.81 m/s2, 32.174 ft/s2)

  • 1 psf = 0.00694 psi (lb/in2)

Note! - in the equation above the head is related to water as the reference fluid. Another reference fluid can be used - like Mercury Hg - by replacing the density of water with the density of the reference fluid - check Velocity Pressure Head.

If the flowing fluid has the same density as the reference fluid - typical for a water flow - eq. (2) can simplified to

Δhminor_loss = ξ v2 / (2 g)                 (2b)  

where

Δhminor_loss = minor head loss (column of flowing fluid) (m fluid column, ft fluid column) 

Minor Loss Coefficient

The minor loss coefficient - ξ - values ranges from 0 and upwards. For ξ = 0 the minor loss is zero and for ξ = 1 the minor loss is equal to the dynamic pressure or head. The minor loss coefficient can also be greater than 1 for some components.

The minor loss coefficient can be expressed by rearranging (1) to

ξ = 2 Δpminor_loss / (ρf v2)                                             (3)

The minor loss coefficient can alternatively be expressed by rearranging (2) to

ξ = 2 ρw g Δhminor_loss,w / (ρf v2)                                            (4)

The dynamic loss in components depends primarily on the geometrical construction of the component and the impact the construction has on the fluid flow due to change in velocity and cross flow fluid accelerations.

The fluid properties - in general expressed with the Reynolds number - also impacts the minor loss.

Minor loss information about components are given in dimensionless form based on experiments.

Equivalent Length

The dynamic minor loss in a component can be converted to an equivalent length of pipe or tube that would give the same major loss.

Major loss in a fluid flow can be expressed as

Δpmajor_loss = λ (l / dh) (ρf v2 / 2)                        (5)

where

Δpmajor_loss = major (friction) pressure loss in fluid flow (Pa (N/m2), psf (lb/ft2))

λ = Darcy-Weisbach friction coefficient

l = length of duct or pipe (m, ft)

v = velocity of fluid (m/s, ft/s)

dh = hydraulic diameter (m, ft)

ρf = density of fluid (kg/m3, slugs/ft3)

If we want the minor loss to be equal to the major loss for a given equivalent length of pipe or duct - then

 Δpminor_loss  = Δpmajor_loss, eq                               (6)

or by combining (1) and (2)

ξ ρf v2 / 2 = λ (leq / dh) (ρf v2 / 2)                      (6b)

where

Δpmajor_loss, eq = equivalent major loss (Pa (N/m2), psf (lb/ft2))

leq = equivalent pipe length (m, ft)

(6b) can be reduced and rearranged to express equivalent length as

leq = ξ dh / λ                                                  (7)                                  

The total head loss in a pipe, tube or duct system, is the same as that produced in a straight pipe or duct whose length is equal to the pipes of the original systems - plus the sum of the equivalent lengths of all components in the system.

Example - Equivalent Length of Gate Valve

The equivalent length of a 50 mm gatevalve with loss coefficient 0.26 when 1/4 closed located in a steel pipe with friction coefficient 0.03 can be calculated with (7) as

leq = 0.26 (0.05 m) / 0.03 

     = 0.4 m        

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