Flare Sweep Gas Calculator | Header Velocity & Purge Flow
Flare Header Sweep Gas Flowrate Calculator
Calculates continuous sweep gas flow to maintain a minimum velocity in the flare header to reduce air ingress / stagnant pockets. This is different from flare tip purge. Final sweep gas requirements should be validated with your flare system design basis and vendor.
Sweep Gas Inputs (Header)
Actions
Sweep Gas Results
Selected Vmin (applied):
- m/s
Internal Diameter (D):
- m
Area (A = ΟΒ·DΒ²/4):
- mΒ²
Sweep Flow (Actual) Qact:
- mΒ³/h
Sweep Flow (Standard) Qstd:
- NmΒ³/h
Sweep Flow (SCFM):
- scfm
Mass Flow (αΉ):
- kg/h
Resulting Velocity at ID:
- m/s
Gas Density (Ο, at P,T,Z):
- kg/mΒ³
Reynolds Number (informational):
-
Good practice reminders:
Ensure sweep gas reaches potential
stagnant pockets (dead-legs/low points), consider check valves
and injection location, and evaluate oxygen ingress risks during
low/no-flow conditions.
Technical Notes
Variable Definitions
- D: controlling header internal diameter (m).
- A: cross-sectional area, A = ΟΒ·DΒ²/4 (mΒ²).
- Vmin: target minimum sweep velocity (m/s).
- Qact: sweep volumetric flow at operating conditions (mΒ³/h).
- Qstd: sweep flow at standard conditions (NmΒ³/h @ 15Β°C, 1 atm; and SCFM @ 60Β°F, 1 atm).
- Pabs: absolute header pressure (kPa(a)) = Patm + Pg.
- Tabs: absolute temperature (K) = T(Β°C)+273.15.
- Z: compressibility factor (dimensionless).
- MW: molecular weight of sweep gas (kg/kmol).
- αΉ: sweep gas mass flow (kg/h).
- Ο: sweep gas density at P,T,Z (kg/mΒ³).
- Re: Reynolds number (informational screening).
Formulas / Logic
- Velocity-based sweep flow (screening): Qact = VminΒ·A.
- Actual β Standard (ideal gas with Z): Qstd = QactΒ·(Pabs/Pstd)Β·(Tstd/Tabs)Β·(Zstd/Z).
- Molar flow: nΜ = (PabsΒ·Qact)/(ZΒ·RΒ·Tabs), with R = 8.314 kPaΒ·mΒ³/(kmolΒ·K).
- Mass flow: αΉ = nΜΒ·MW.
- Density: Ο = (PabsΒ·MW)/(ZΒ·RΜΒ·Tabs) (consistent units).
This calculator is a velocity-criterion screening tool. It does not
explicitly model diffusion-based air ingress, wind-driven
entrainment, or detailed header mixing/dispersion.
Assumptions / Notes
- Vmin targets are project/vendor dependent and depend on injection point(s), header geometry, and dead-legs/low points.
- Ideal-gas relationships are used with an optional Z correction; for low pressure sweep gas, Zβ1 is typically adequate.
- Ensure sweep gas reaches potential stagnant pockets; consider check valves, low points, and whether multiple injection locations are required.
- Use this tool for documentation and preliminary checks; confirm final sweep gas with the flare vendor and flare philosophy.
Standards / References
- API 521 β Pressure-relieving and Depressuring Systems (flare system considerations, air ingress discussion).
- Flare vendor design manuals / flare philosophy (governing for minimum sweep/purge rates and devices).
- Basic ideal-gas relationships for flow conversion and density (screening).