Pipe Sizing Calculation – Part 8 – Determining the Actual Pressure Drop

We have reached the final part of the water piping system design calculation example tutorial. We will use most of the data calculated so far (refer Part-1 to Part-7) as input for the D’Arcy-Weisbach pressure loss equation. From this equation we will know the actual pressure or head loss in the pipe system. Then we will check whether the actual pressure drop in pipe is less than the already calculated permitted pressure loss in the piping system.


Darcy–Weisbach Equation for pressure head loss can be expressed as:

Δh = λ*(l/d­h)*(v2/2*g)…………………………….Eq.1



Δh –  Pressure head loss for the piping system

λ – Moody friction factor

l – Effective pipe length

dh – Hydraulic diameter

v – Velocity of the fluid

g – Gravitational acceleration (9.81 m/s2)




Getting Back to  our problem:

For using the D’Arcy Weisbach formula (Eq.1) for our water piping system example, we have the following inputs:

λ = Moody friction factor = 0.035 (refer part-7)

l = Effective pipe length = 22 meter (refer part-2)

dh = Hydraulic diameter = 0.02 meter (refer part-4)

v = Velocity of the fluid = 1.591 m/s (refer part-6)

g = Gravitational acceleration (9.81 m/s2)

By inputting the above values to the Eq.1, we will get the total head loss for our case as

Δh = 4.96 meter


Pressure head drop per unit length of pipe = Δh / l = 4.96/22 = 0.225 meter of head per meter of pipe run.


Now refer the permissible head loss calculation of Part-3 and you will find that permissible value for our case is 0.22 meter of head per meter of pipe run.



So, we can conclude that the pipe diameter of 20 mm, we assumed at the beginning of this tutorial, is safe for the water piping system problem considered for this tutorial.


3 thoughts on “Pipe Sizing Calculation – Part 8 – Determining the Actual Pressure Drop

  1. two issues with this tutorial:
    1. For me the roughness calculation page is not loading. Perhaps my browser, but all others worked fine
    2. In part 3 you noted that the figure calculated was the MAX permissible head loss allowable, yet in step 8 your head loss per meter is larger than this figure…

  2. According to the step-3 max permissible head loss per unit length of pipe is 0.22 where as as per the step-8 the calculated head loss value/length of pipe = 0.225.
    I have ignored the third decimal and concluded that the 20 mm dia pipe is OK. However to be precise, you are right, we  should select the next higher diameter of the pipe.

    Thanks for your input

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