Confined-Space Tank Repair: Permit Versus Changing Atmosphere
A two-person crew is scheduled to repair a leaking seam inside a horizontal cylindrical tank at a food-processing plant. The tank is 2.1 m in diameter and 3.6 m long, with a single 450 mm manway; the interior is drained but a 20 mm layer of sludge remains. The planned work is self-shielded FCAW on a 6 mm shell patch. A 4-hour confined-space permit was issued at 07:00 after a single four-gas reading at the manway: O2 20.4%, LEL 2%, CO 18 ppm, H2S 0 ppm. A 600 m3/h portable blower is available, but the duct reaches only halfway into the tank. The welder enters at 07:40 with a half-face respirator, leathers, and a 10 m lifeline; no supplied air is on site. At 08:00 he reports a headache and leaves the tank. The supervisor wants to resume after a 10-minute airing because the permit is still valid.
1. Evaluate the atmospheric and permit information and decide whether the crew should resume work. Identify the first three actions in order and justify them.
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Stop entry. The readings are not immediately IDLH, but the permit is invalid because it was based on one manway sample, and the worker's headache plus LEL 2% indicate the atmosphere is changing and the space is not adequately ventilated. First actions: (1) withdraw and account for personnel, lock out the agitator and steam lines, and post the manway; (2) re-sample at three heights inside the tank with a calibrated four-gas meter and continuous logging, at the breathing zone, not at the door; (3) ventilate to at least 10 air changes with the duct discharging at the low point and exhaust at the high point, then re-sample before entry. Do not use the half-face respirator as the primary control: it does not supply oxygen or protect against an oxygen-deficient or unknown atmosphere. Resume only when O2 is 19.5-23.5%, LEL is below 10% and stable, CO is below 25 ppm, H2S is below 5 ppm, and the permit is reissued with continuous monitoring and a rescue plan.
2. Calculate the tank volume and the blower time needed for 10 air changes, assuming 50% effective mixing, and state where the sample should be taken.
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Volume = pi x (1.05 m)^2 x 3.6 m = 12.47 m3. Ten air changes = 124.7 m3. At 600 m3/h nominal and 50% mixing efficiency, the effective rate is 300 m3/h, so time = 124.7 / 300 = 0.416 h, about 25 minutes. Add a safety margin: ventilate at least 30 minutes and keep ventilating during the work. Sample at the welder's breathing zone in the lower half of the tank, plus top and bottom, because self-shielded FCAW generates fume and CO and heavier sludge vapors may collect low. Use a remote sample pump before entry and a personal monitor during entry; a single manway reading is not representative of a 12.5 m3 space.
3. Specify the respiratory protection and rescue arrangement you would require if entry proceeds.
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While the atmosphere is not proven, use a supplied-air respirator with an escape cylinder; a half-face APR with P100 is acceptable only after engineering controls are verified and continuous monitoring shows fume below the exposure limit with stable oxygen and LEL. Rescue: full-body harness with retrieval line, tripod at the manway, a trained attendant at the entry, radio or air-horn communication, and a written rescue plan. The lifeline alone is not a rescue method if the worker is collapsed inside. Add a fire watch outside, hot-work permit, GFCI protection, insulated electrode holder, and bonding/grounding checks. The attendant must have no other duties and must be able to summon help without leaving the manway.