Gas / Compressible Flow

Compressible pressure drop for single-phase gas — isothermal, Weymouth & Panhandle

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Engineering Tips

Compressible pressure drop for a gas line. The isothermal method is the fundamental compressible-flow equation; Weymouth and Panhandle are the empirical transmission-line correlations for long pipelines.

Isothermal (Crane TP-410):

• P1² − P2² = (ṁ/A)²·(ZRT/M)·(f·L/D + 2·ln(P1/P2))

• Ma = v₂/√(k·Z·R·T/M) — exit Mach

• Ve = 1.122·C/√ρ — API RP 14E

Recommended values

Method: Isothermal for plant/near-field piping; Panhandle B for large high-pressure transmission lines.
Z & k: Z ≈ 0.85–0.95 for natural gas at 30–80 bar; k ≈ 1.3 natural gas, 1.4 air.
Pressure: Enter P1 as absolute (bar a = bar g + 1.013).

Limitations

Single step: Above ΔP/P1 ≈ 40% march the line in segments; the single-step result loses accuracy.
Choked flow: Results are invalid at/above sonic (Ma ≥ 1). Isothermal (not adiabatic) basis; ignores elevation.
Empirical constants: Weymouth/Panhandle constants follow published SI forms — confirm against GPSA/Menon for your basis.

About the standards

Crane TP-410: Flow of Fluids — isothermal compressible flow.
GPSA / Menon: Weymouth & Panhandle transmission equations.
API RP 14E: erosional velocity limit.

Gas / Compressible Flow

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