Argon Accumulation in a Tank Compartment
You are the lead GTAW welder on a 12 m³ rectangular stainless steel tank compartment, 3 m long × 2 m wide × 2 m high, at a food plant. The only openings are a 500 mm entry hatch and a 100 mm vent at the top. You are TIG welding 316L internal baffles. Ventilation is natural only; no forced air is running. Argon is set at 20 L/min on the torch and 10 L/min on the purge line, and the welder has been inside for 60 minutes. The gas monitor clipped at chest level reads 18.5% O₂ and alarms; the welder reports dizziness and exits. The standby person says the monitor was bump-tested two weeks ago. The entry permit is ticked “confined space” but has no mechanical ventilation, no low-level sampling, and no rescue plan. Production wants the tank closed and the job finished tonight.
1. State the immediate actions you take in the next 10 minutes and explain the priority order.
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Immediate actions, in order:
- Stop work and keep everyone out. Treat the space as unsafe until re-tested. 18.5% O₂ is below the usual 19.5% minimum, so entry without respiratory protection is not acceptable.
- Account for personnel and call for help if symptoms persist. Dizziness after argon exposure may indicate asphyxia; do not send a standby person in without SCBA and a rescue plan.
- Shut off the argon supplies at the source and open the hatch and vent fully. Do not rely on natural dilution alone.
- Start forced ventilation with an air mover placing clean air at the low point and exhausting at the high point, because argon is denser than air.
- Re-test before any re-entry with a calibrated monitor at low, mid, and high levels, not only at chest height.
Priority is life safety first, then isolation of the hazard, then ventilation and verification. Production pressure does not override the oxygen deficiency alarm. The permit must be suspended and reissued only after controls are in place.
2. Using 30 L/min total argon flow for 60 minutes in a 12 m³ compartment, estimate the oxygen concentration if mixing were perfect and no air entered. Is the 18.5% reading plausible? Explain the limitation of a chest-level monitor.
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Calculation:
- Argon released = 30 L/min × 60 min = 1,800 L = 1.8 m³.
- Fraction of compartment volume = 1.8 ÷ 12 = 0.15, or 15%.
- If the added argon simply displaces air and mixing is perfect, remaining air fraction = 0.85.
- Estimated O₂ = 20.9% × 0.85 ≈ 17.8%.
The measured 18.5% is therefore plausible; the difference is explained by incomplete mixing and some air entering through the hatch and vent. A chest-level monitor can read higher than the true low-point concentration because argon is about 1.38 times denser than air and tends to collect at the bottom. It can also miss a stratified layer if the sample is not drawn from the breathing zone and the low point. Re-entry testing must sample at several heights, especially near the floor, and should continue while work is in progress.
3. List the permit and monitoring changes required before re-entry, with specific acceptance values.
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Permit changes:
- Revise the confined space permit to require continuous mechanical ventilation and continuous atmospheric monitoring; natural ventilation is not adequate for argon.
- Add a written rescue plan with harness, tripod or retrieval line, radio, and trained attendant at the hatch.
- Require a pre-entry and periodic bump test of the gas monitor; two weeks is too long to assume the reading is valid without a functional check.
- Record cylinder locations, hose routing, and a procedure for shutting off argon outside the space.
Acceptance values:
- Oxygen 19.5%–23.5% before entry and continuously during work.
- Flammable gas below 10% LEL if any is present.
- Argon is a simple asphyxiant, so control is by oxygen measurement and ventilation, not by a direct argon alarm.
- Sample at low, mid, and high levels; do not accept a single chest-level reading.
- If ventilation stops or the monitor alarms, evacuate and re-test before re-entry.