Flare Purge Gas Calculator | Flare Tip & Stack Purging
Flare System Purge Gas Calculator
Flare / Purge Inputs
Actions
Purge Gas Results
Selected Vmin (applied):
- m/s
Internal Diameter (D):
- m
Area (A = ΟΒ·DΒ²/4):
- mΒ²
Purge Flow (Actual) Qact:
- mΒ³/h
Purge Flow (Standard) Qstd:
- NmΒ³/h
Purge 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):
-
Technical Notes
Variable Definitions
- D: controlling internal diameter at purge point (m).
- A: internal flow area, A = ΟΒ·DΒ²/4 (mΒ²).
- Vmin: selected minimum purge velocity criterion (m/s).
- Qact: purge volumetric flow at operating conditions (mΒ³/s or mΒ³/h).
- Qstd: purge volumetric flow at reference standard conditions (NmΒ³/h @ 15Β°C, 1 atm).
- Pabs: absolute pressure at purge point (kPa(a)) = Patm + Pg.
- Tabs: absolute temperature (K) = T(Β°C) + 273.15.
- Z: compressibility factor (dimensionless).
- MW: molecular weight (kg/kmol).
- αΉ: mass flow rate of purge gas (kg/h).
- Ο: gas density at P,T,Z (kg/mΒ³).
- Re: Reynolds number (informational).
- RF: reduction factor when molecular seal/purge reduction device is installed.
Formulas / Logic
- Velocity-based purge (screening): Qact = VminΒ·A.
- Seal reduction: if seal present, Qact,final = QactΒ·RF.
- Actual β Standard: Qstd = QactΒ·(Pabs/Pstd)Β·(Tstd/Tabs)Β·(Zstd/Z).
- Molar flow: nΜ = (PabsΒ·Qact)/(ZΒ·RΒ·Tabs).
- Mass flow: αΉ = nΜΒ·MW.
- Density: Ο = (PabsΒ·MW)/(ZΒ·RΜΒ·Tabs) (consistent units).
- Reynolds (informational): Re = ΟVD/ΞΌ (ΞΌ estimated for light gas).
This calculator uses an engineering screening approach. Continuous
purge rate is strongly dependent on flare tip design, wind,
diffusion, molecular seal performance, and vendor philosophy.
Assumptions / Notes
- Velocity criteria values are practice/vendor-dependent. Confirm Vmin and RF with the flare vendor and site flare philosophy.
- Ideal-gas relationships are used with an optional Z correction. For near-atmospheric purge, Zβ1 is typically acceptable.
- Does not model air ingress by molecular diffusion explicitly (e.g., API 521 diffusion-based methods) or wind-driven entrainment; use vendor data for final design.
- Gauge-to-absolute conversion uses the entered Patm (default 101.325 kPa).
Standards / References
- API 521 β Pressure-relieving and Depressuring Systems (purge/air ingress discussion and design considerations).
- Flare vendor design manuals for purge reduction devices/molecular seals (governing for RF).
- Ideal-gas and basic gas property relationships (screening).