Pipe diameter follows velocity, and velocity follows service. Suction lines run 0.8-1.5 m/s, discharge lines 1.5-2.5 m/s, and long transfers sit near 1 m/s. Doubling velocity quadruples friction, so the 36 m3/h example needs 92.3 mm ID at 1.5 m/s and selects DN100 at an actual 1.27 m/s.
Formulas
Mean velocity
v = Q / A
Q in m3/s, A in m2.
Diameter from velocity
D = sqrt(4Q / (pi v))
Round up to the next standard DN.
Worked example
36 m3/h at 1.5 m/s
Inputs: Q 36; target 1.5.
Calculation: Required ID 92.3 mm, so select DN100 at 1.27 m/s.
Lesson:
Frequently asked questions
What velocity should I target?
1-2.5 m/s for water-like liquids; suction lower than discharge.
What if velocity exceeds 3 m/s?
Roughly 4x the loss for 2x the velocity. Resize the line.
When do I need Reynolds?
Always before any loss calc: Re decides laminar vs turbulent.
Sources and further verification
- Reynolds number Wikipedia
- Bernoulli's principle Wikipedia
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