What does mixing efficiency K represent?

K (0.25–1.0) accounts for imperfect sweep mixing. Use lower K for dead-legs or poor ventilation; 0.75 is the default field screening value.

How is piping volume calculated?

From ASME B36 Sch 40 inside diameter and run length: V = π (ID/2)² L. Confirm schedule if the line is not Sch 40.

Can pressure-cycle purging exceed MAWP?

No. P_high must remain below equipment MAWP with relief protection. This calculator does not check MAWP — confirm the written procedure.

Is continuous O₂ monitoring required?

Yes. Per NFPA 69 / API RP 2016, theoretical volumes are screening only — verify outfall O₂ before introducing hydrocarbons or confined-space entry.

Nitrogen Purging & Inerting Volume Calculator

1. Core Formula & Variable Definitions

NFPA 69 — Standard on Explosion Prevention Systems (Purging and Inerting, Chapter 7) · API Recommended Practice 2016 — Entering and Cleaning Petroleum Storage Tanks · CGA G-8.1 — Standard for Nitrogen and Inert Gas Purging Procedures · ASME B36.10/19 — Pipe inside diameter for piping geometry

Dilution: V_N2 = V_sys · ln((C₀−C_in)/(C_t−C_in)) / K · t = V_N2 / Q

Pressure-cycle: C_n = C₀ (P_low/P_high)ⁿ · V_N2 = n · V_sys · (P_high,g/P_std)

NFPA 69 Chapter 7 · API RP 2016 · CGA G-8.1 screening

Default K 0.75Target O₂ 5%Margin +20% N₂Cylinder 50 L @ 200 bar ≈ 10 Nm³

Piping V_sys uses ASME B36 Sch 40 inside diameter. Vessel V_sys = π r² (L + 0.5·Di) for cylinder + 2:1 SE head equivalent. Dilution applies mixing efficiency K. Pressure-cycle n = ceil(ln(Ct/C0)/ln(Plow/Phigh)) with absolute pressures; N₂ inventory uses gauge ΔP to atmosphere over P_std (1.0 bar / 14.7 psi). +20% procurement margin on gas supply rows. Screening only — continuous O₂ monitoring required.

V_sys (System Internal Volume) — Piping, vessel, or custom free volume (m³ / ft³).
C0 / Ct (Initial / Target O₂) — Starting and maximum residual oxygen concentration (%).
K (Mixing Efficiency) — Sweep mixing factor 0.25–1.0 (NFPA 69 screening).
Q (Purge Flow) — Nitrogen flow for dilution timing (Nm³/h / SCFM).
n (Cycle Count) — Pressure or vacuum cycles to reach Ct from C0.
V_N2 (Required Nitrogen) — Theoretical purge gas inventory (Nm³ / SCF).

2. Allowances, Tolerances & Standards

Theoretical gas demand only. Continuous O₂ monitoring and MAWP compliance are mandatory field controls.

Personnel / process safetyO₂ analyzer at vent

Do not introduce hydrocarbons or enter confined spaces until measured O₂ ≤ target (NFPA 69 / API RP 2016).

Pressure-cycle MAWPP_high < MAWP

Never exceed vessel/piping MAWP. Provide relief protection before pressurizing.

Procurement margin+20% N₂

Supply rows apply a 20% contingency on theoretical V_N2 for cylinder / trailer / LN₂ screening.

Quick Reference Lookup Table

Nitrogen purging volume screening — engine-computed V_sys, V_N2, time / cycles
Case	Geometry	Method	V_sys	V_N2	Time / n
Default metric	NPS 12 · 100 m	Dilution	7.22 m³	13.82 Nm³	16.6 min
Vessel metric	Di 2000 · L 6000	Pressure cycle	21.99 m³	131.9 Nm³	n = 2
Piping imperial	NPS 24 · 500 ft	Dilution	1396 ft³	5666 SCF	56.7 min
Custom imperial	1000 ft³	Pressure cycle	1000 ft³	6122 SCF	n = 2
NPS 6 · 50 m	Piping	Dilution · Ct 8%	0.93 m³	1.20 Nm³	2.9 min
Custom 50 m³	Custom	Dilution · Ct 2%	50 m³	147.0 Nm³	110 min

Rows from the FEK nitrogen-purging-volume solver (B36 Sch 40 ID; vessel L + 0.5·Di). Spec ≈22.1 m³ / 132.6 Nm³ cases round to engine 21.99 m³ / 131.9 Nm³. Not a certified NFPA 69 procedure — confirm O₂ analyzers and MAWP.

3. Material & Code Limitations

Ideal-gas inventory estimates for field logistics — confirm supplier fill pressure and trailer capacity.

Material Group	Temperature Range	Allowable Stress / Limit	Engineering Notes
50 L cylinder @ 200 bar	—	≈ 10 Nm³	Water volume × fill pressure / P_std screening.
Tube trailer	—	≈ 2000 Nm³	Generic high-pressure trailer inventory for equivalent count.
Liquid nitrogen	—	≈ 0.682 Nm³ / L LN₂	Expansion ≈ 682:1 at ambient screening conditions.

4. Step-by-Step Worked Example

Field VerificationShow

Worked Example — NPS 12 · 100 m Dilution Sweep · 21% → 5% O₂

Default metric piping duty: NPS 12 Sch 40, L = 100 m, K = 0.75, Q = 50 Nm³/h.

Geometry:NPS 12 Sch 40 · L = 100 mO₂:C0 = 21% · Ct = 5% · Cin = 0%K / Q:K = 0.75 · Q = 50 Nm³/h

1

Compute system volume

B36 Sch 40 ID ≈ 303.23 mm → V_sys = π r² L ≈ 7.22 m³.

Result:V_sys ≈ 7.22 m³

2

Dilution nitrogen demand

V_{N 2} = V_{sy s} ln (C_{0} / C_{t}) / K

7.22 · ln(21/5) / 0.75 ≈ 13.82 Nm³.

Result:V_N2 ≈ 13.82 Nm³

3

Purge duration

t = V_{N 2} / Q

13.82 / 50 h ≈ 16.6 min.

Result:t ≈ 16.6 min

4

Procurement margin

Apply +20% → ≈ 16.6 Nm³ ≈ 2 × 50 L / 200 bar cylinders (screening).

Result:+20% supply ≈ 16.6 Nm³

Conclusion: Default dilution duty screens at ≈13.82 Nm³ N₂ and ≈16.6 min at 50 Nm³/h — verify with continuous O₂ monitoring.

5. Frequently Asked Questions & Technical References

Show

Screening follows NFPA 69 Chapter 7 purging/inerting methods, with field practice notes from API RP 2016 and CGA G-8.1. It is not a certified site procedure.