Nitrogen Purging Calculator

Technical Notes

Variable Definitions

• V = vessel/equipment internal volume.
• C₁ = initial oxygen concentration in the vessel before purging (e.g., air ≈ 21% O₂).
• C₂ = target oxygen concentration after purging.
• C₀ = oxygen impurity in nitrogen supply (use 0 for ideal/pure N₂).
• Pₗ, Pₕ = lower and higher pressure in a purge cycle (absolute basis in the calculations).
• j = number of purge cycles (rounded up to an integer for practical use).

Formulas / Logic

• Sweep-through (continuous dilution, well-mixed model):
  V_N₂ = V × ln[(C₁ − C₀)/(C₂ − C₀)]
• Pressure purging (cyclic pressurize/vent, imperfect N₂ allowed):
  (C_j − C₀) = (C₁ − C₀) × (Pₗ/Pₕ)^j
  j = ln[(C₂ − C₀)/(C₁ − C₀)] / ln(Pₗ/Pₕ)
• Vacuum purging (cyclic evacuate/backfill with pure N₂):
  C_j = C₁ × (Pₗ/Pₕ)^j
  j = ln(C₂/C₁) / ln(Pₗ/Pₕ)
• N₂ consumption (approximate, isothermal):
  N₂ per cycle (as equivalent volume at 1 atm) ≈ V × (Pₕ − Pₗ) / P_atm
  Total N₂ ≈ j_rounded × V × (Pₕ − Pₗ) / P_atm

Assumptions / Notes

• Calculations are screening-level and assume ideal mixing in the vessel for each step (well-mixed model).
• Pressures are converted to absolute internally (psig/bar(g) are shifted by atmospheric pressure).
• N₂ consumption formula assumes isothermal behavior and reports an equivalent volume at ~1 atm (use for supply estimates).
• Real systems may require additional allowance for leaks, stratification, dead legs, diffuser effectiveness, and vent piping restrictions.
• For hazardous service, ensure target O₂ meets your plant safety requirements (e.g., inerting below limiting oxygen concentration for the mixture) and follow applicable site procedures.
• Reference anchors (industry practice): NFPA 69 (Explosion Prevention Systems) for inerting concepts; EIGA/CGAs inerting and purging guidance; plant/owner standards for nitrogen purging procedures.