Two Phase Flow

Two-Phase Pressure Drop

Overview

Three established gas-liquid two-phase pressure-drop correlations from the fluids.two_phase module. Each correlation has its strengths:

• Lockhart-Martinelli: classical horizontal, widely cited, ±30% typical.
• Beggs-Brill: handles inclination (essential for risers, hilly

pipelines).

• Mueller-Steinhagen-Heck: smooth interpolation between all-liquid and

all-vapor frictional gradients; recommended starting point for refrigerant evaporator design.

Prerequisites

1. uv available.
2. On first use, the script writes LICENSE_NOTIFICATION.txt listing the

fluids library terms.

When to Use

• Two-phase frictional pressure drop is needed in a pipe (flare, reboiler

return, riser, knock-out drum inlet, slug catcher manifold).

• Comparing two correlations to bracket the design pressure drop.

Don't use for

• Single-phase pipe flow — use pipe-flow-pressure-drop.
• Choked / sonic flow at a relief valve outlet — use relief-valve-sizing-api520.
• Two-phase across a control valve — consult vendor methods; the control-valve

sizing skill only covers liquid/gas ISA sizing, not two-phase trim rating.

Utility Scripts

• uv run scripts/two_phase.py lm --m 5.0 --quality 0.3 --rho-l 600 --rho-g 30 --mu-l 0.0002 --mu-g 1.0e-5 --D 0.1 --L 50 --output /tmp/lm.json
• uv run scripts/two_phase.py beggs-brill --m 5.0 --quality 0.3 --rho-l 600 --rho-g 30 --mu-l 0.0002 --mu-g 1.0e-5 --sigma 0.02 --P 1000000 --D 0.1 --L 50 --angle 30 --output /tmp/bb.json
• uv run scripts/two_phase.py msh --m 5.0 --quality 0.3 --rho-l 600 --rho-g 30 --mu-l 0.0002 --mu-g 1.0e-5 --D 0.1 --L 50 --output /tmp/msh.json

Procedure

1. Establish phase mass flow rates and quality x = m_g / (m_g + m_l).
2. Get phase densities and viscosities at operating conditions (use

vle-flash-calculations if needed).

1. Pick a correlation:

• Horizontal, dilute liquid: Lockhart-Martinelli.
• Inclined, any angle, oil/gas pipeline: Beggs-Brill.
• Refrigerant / process evaporator, smooth quality range: Mueller-

Steinhagen-Heck.

1. Run two correlations and compare; use the higher dP for the design

margin unless one is clearly outside its validity range.

Pitfalls

• Using a single-phase friction factor on a two-phase stream.
• Treating the slip ratio as 1 (homogeneous) for high-quality steam — fine

for refrigerant boiling, not for low-pressure flashing systems.

• Ignoring vapor density variation along the pipe; for long lines the

acceleration term can dominate at the outlet.

• Picking Lockhart-Martinelli for a vertical riser; it does not include

gravity head.

• Picking Beggs-Brill horizontal mode at 0° without verifying the

correlation switches to the appropriate flow regime.

• Forgetting that quality changes when heat is added or pressure drops

(flashing or condensing). The correlations assume a constant quality over the pipe length L; for large dP / large dT, segment the pipe.

• Reporting only one correlation's number; a 2x spread between correlations

is normal for two-phase flow.

• Using these correlations for entrained-droplet flow at high vapor velocity

with a demister downstream; entrainment loading matters and is not in the correlation.

Fallback Strategies

• If fluids is not installed, escalate to the user; there is no clean

pure-Python fallback for Beggs-Brill that captures all flow regimes.

• For very high quality (x → 1), the Lockhart-Martinelli parameter X

approaches 0 and the correlation degrades; switch to a homogeneous-flow approximation or to Mueller-Steinhagen-Heck.

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/correlation_choice.md — when each correlation is most suitable.
• Beggs & Brill, Two-Phase Flow in Pipes (Univ Tulsa Press, 1991).
• Lockhart & Martinelli, Chem Eng Prog 45, 1949.
• Mueller-Steinhagen & Heck, Chem Eng Proc 20, 1986.
• fluids documentation: https://fluids.readthedocs.io/fluids.two_phase.html

Anti-Patterns

• Picking one correlation and not documenting why.
• Designing a flare line without checking back-pressure on multiple PRV

cases.

• Treating two-phase dP as if it were 1.2x single-phase dP — that

shortcut fails dramatically in slug flow.

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.