Gas pipeline pressure drop
Outlet pressure, pressure drop and velocity for a gas line by the isothermal compressible-flow equation — with the choked-flow check and every Schedule 40 size.
Inputs
Results
Need the full version for your plant?
The Pipe Sizing and Pressure Drop Excel tool does everything here and more: pressure drop or allowable flow for liquid, gas and steam lines, with fittings, elevation, choked-flow and flashing checks and a size table over every NPS of your schedule. Print-ready report, any units, visible formulas, no macros.
Join PlantCalcKit — free
Be first to hear about new calculators, Excel tools and field guides — one short email when something new is ready. The calculators stay free with or without an account.
By joining you agree to our privacy policy. Unsubscribe or delete your details any time.
You are a PlantCalcKit member
Signed in as . We will email you when new tools and guides are ready.
How this calculator works
For a gas, density falls as the pressure falls, so the liquid-line method does not hold once the drop is more than about 10 % of the inlet pressure. This calculator integrates the flow along the line at constant temperature — the isothermal compressible-flow equation with the kinetic-energy term: P1² − P2² = G²·(Z·R·T/M)·(f·L/D + 2·ln(P1/P2)), where G is the mass flux and f the Darcy friction factor (Colebrook). It is solved for the outlet pressure, and flags choked flow if the gas would reach its isothermal sonic velocity before the end of the line.
Example: 50,000 Nm³/h of natural gas (M = 17.4, Z = 0.90, 0.011 cP) at 20 °C entering an 8-inch Schedule 40 line (202.7 mm) at 50 bar a, 10 km long: outlet pressure 38.9 bar a — a drop of 11.1 bar — and an outlet velocity of 10.8 m/s. A 10-inch line drops only 3.1 bar. The results were checked against the isothermal-flow solution of the open-source fluids library for six lines, including a choked one: outlet pressures agree to better than 0.001 %.
The table shows the outlet pressure and velocity for every Schedule 40 size, for quick line sizing. The erosional velocity is from API RP 14E with C = 100 (continuous service). Limits: constant temperature and Z along the line, no elevation term — for long buried pipelines use a pipeline simulator or the Panhandle/Weymouth forms with your company's efficiency factor. References: Crane, Flow of Fluids (TP-410) ch. 1; E. S. Menon, Gas Pipeline Hydraulics (2005); API RP 14E.