Pipe Flow Capacity from Pressure Drop Calculator
Turn the usual pipe calculation around: for an allowable pressure drop over a length of pipe, find the maximum liquid flow and velocity, using Darcy Weisbach with the Colebrook friction factor.
- ΔP = f × (L ÷ D) × ρv² ÷ 2 → v = √[2ΔP D ÷ (f L ρ)]
- f from Colebrook with Re = ρvD ÷ µ, iterated to convergence
- Q = v × A = 8.2845 m³/h
Darcy Weisbach Solved for Flow
1 ÷ √f = −2 log₁₀[ε ÷ (3.7D) + 2.51 ÷ (Re √f)]
Q = v × π D² ÷ 4
ΔP is the allowable friction pressure drop, L the pipe length plus the equivalent length of fittings, D the inside diameter, ρ and µ the density and viscosity, ε the roughness and f the Darcy friction factor. Because f depends on velocity, the calculation is iterated. Static head changes are not included.
Worked example
Water with 0.5 bar allowable drop over 200 m of 2" Sch 40 steel (52.5 mm) can carry about 8.3 m³/h at 1.06 m/s.
Frequently Asked Questions
How do I include fittings?
Convert elbows, valves and tees to equivalent length (or use the fittings pressure drop calculator) and add it to the pipe length.
Does this work for gas?
Only for small pressure drops (below about 10 % of inlet pressure). For larger drops use the gas pipeline pressure drop calculator.
Why is iteration needed?
The friction factor depends on Reynolds number, which depends on the unknown velocity.
Learn more on the blog
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