Pipe Flow Pressure Drop

Single-Phase Pipe Pressure Drop

Overview

Computes Darcy-Weisbach pressure drop with the Clamond friction factor (via fluids) as primary, and the Haaland explicit form as fallback when fluids is unavailable or fails. Outputs include Re, friction factor f, velocity, head loss, and pressure drop.

Prerequisites

1. uv available.

When to Use

• Sizing a pump suction or discharge line.
• Checking line size given a design flow.
• Screening pressure drop across a long straight run before adding

fittings.

Don't use for

• Two-phase flow (two-phase-flow).
• Compressible / choked gas flow.
• Pressure drop across fittings, valves, expansions, contractions (those

need K factors).

• Non-Newtonian fluids.

Utility Scripts

• uv run scripts/pipe_pressure_drop.py --length-m 100 --diameter-m 0.05 --flow-m3-s 0.002 --density-kg-m3 998 --viscosity-pa-s 0.001 --roughness-m 1.5e-6 --output /tmp/pdrop.json

Procedure

1. Get fluid density and viscosity at the operating temperature.
2. Pick the pipe inside diameter (after accounting for wall thickness and

schedule).

1. Pick the pipe absolute roughness (commercial steel ~ 45 μm, drawn

tubing ~ 1.5 μm, PE ~ 0.0015 mm).

1. Run the script. The output reports Re; check that the flow is in the

regime you expect.

1. Add fitting / valve K-factor losses separately if needed.
2. If the pressure drop is large, iterate to a larger diameter and rerun.

Pitfalls

• Using nominal pipe diameter instead of the inside diameter for a given

schedule.

• Using roughness too low (e.g. ε = 0 for smooth pipe) for commercial

steel.

• Forgetting elevation change when the outlet elevation differs; pass

--elevation-m explicitly.

• Treating the result as the total dP across a system unless you supplied

representative minor-loss K values and elevation change.

• Using water properties for a hydrocarbon line.
• Mixing kinematic and dynamic viscosity.
• Reporting head loss in meters of water when the fluid is something else.
• Using the formula in the transitional regime (2300 < Re < 4000) without

acknowledging the uncertainty band.

Fallback Strategies

• If fluids is missing, the script falls back to Haaland explicitly and

flags the substitution in the JSON output.

• If the calculated Re is borderline transitional, the script attaches a

warning so downstream design margins can be increased.

Verification

• Run the listed script with representative inputs and an --output file when a deterministic calculation is available.
• Confirm the JSON result contains ok: true, expected units, and no unhandled warnings.
• Check result magnitudes against the stated assumptions, references, and a hand calculation or known operating range before reporting them.

References

• references/limitations.md — full assumption list.
• Crane TP-410 (procure separately) for fittings / valve K-factors.

Anti-Patterns

• Designing a pipe with an estimated dP and zero margin.
• Reporting Δp in psi without naming whether it is gauge or absolute (it

is a Δ, but the reader still needs to know the basis).

• Using this skill for the long-run flare line where two-phase or

compressible effects matter.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: jskherman
• Source: jskherman/engg-skills
• License: Apache-2.0

Install and usage instructions live in the source repository linked above.