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Pressure drop in
pipes is caused by:
-
Friction
-
Vertical pipe
difference or elevation
-
Changes of
kinetic energy
-
Calculation of
pressure drop caused by friction in circular pipes
To determine the
fluid (liquid or gas) pressure drop along a pipe or pipe
component, the following calculations, in the following order.
Determine
Reynolds Number:
Where:
Re = Reynolds Number
 = Velocity of Flow
D = Diameter of Pipe
V = Kinematics Viscosity
If the Reynolds
number < 2320, than you have laminar flow.
Laminar flow is characterized by the gliding
of concentric cylindrical layers past one another in orderly fashion. The velocity of the
fluid is at its maximum at the pipe axis and decreases
sharply to zero at the wall.
The pressure drop caused by friction of laminar flow does not
depend of the roughness of pipe.
If the Reynolds
number > 2320, you have turbulent
flow.
There is an irregular motion of fluid
particles in directions transverse to the direction of the
main flow. The velocity
distribution of turbulent flow is more uniform across the
pipe diameter than in laminar flow. The
pressure drop caused by friction of turbulent flow depends on
the roughness of pipe.
Select pipe
friction Coefficient:
The pipe friction coefficient is a dimensionless
number. The friction factor for laminar flow
condition is a function of
Reynolds number only, for turbulent flow it is also a
function of the characteristics of the pipe wall.
Determine Pipe friction
coefficient at laminar flow:
Where:
= Pipe Friction Coefficient
Re = Reynolds number
Note: Perfectly smooth pipes will have a
roughness of zero.
Determine Pipe friction
coefficient at turbulent flow (in the most cases):
Where:
= Pipe Friction Coefficient
g = Acceleration of Gravity
Re = Reynolds Number
k = Absolute Roughness
D = Diameter of Pipe
lg = Log
The solutions to this calculation is plotted vs.
the Reynolds number to create a Moody
Chart.
Determine Pressure drop in
circular pipes:
Where:
= Pressure Drop
= Pipe Friction Coefficient
L = Length of Pipe
D = Pipe Diameter
p = Density
= Flow Velocity
If you have
valves, elbows and other elements along your pipe then you
calculate the pressure drop with resistance coefficients
specifically for the element. The
resistance coefficients
are in most cases found through practical tests and through
vendor specification documents. If the
resistance coefficient is known, than
we can calculate the pressure drop for the element.
Where:
= Pressure Drop
= Resistance Coefficient (determined by test or vendor
specification)
p = Density
= Flow Velocity
Pressure drop by gravity or vertical
elevation
Where:
= Pressure Drop
p = Density
g = Acceleration of Gravity
= Vertical Elevation or Drop
Pressure drop of gasses and vapor
Compressible fluids expands caused by
pressure drops (friction) and the velocity will increase.
Therefore is the pressure
drop along the pipe not constant.
Where:
p1 = Pressure incoming
T1 = Temperature incoming
p2 = Pressure leaving
T2 = Temperature leaving
We set the pipe friction number as a constant
and calculate it with the input-data. The temperature, which is used in the equation, is the average
of entrance and exit of pipe.
Note: You can calculate gases as liquids, if
the relative change of density is low (change of density/density
= 0.02).
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