Confined-Space Entry Into a Fuel-Oil Day Tank
A three-man crew is to clean sludge from a 6 m³ rectangular steel fuel-oil day tank, 2.2 m deep, entered through a 500 × 400 mm manhole. The tank was drained two days ago and flushed with diesel. The shift electrician has issued a confined-space entry permit valid for the whole shift, and a hot-work permit for welding a bracket on the adjacent pipe rack is also open. Gas test at 09:10 with a portable meter: O₂ 19.3 %, LEL 0 %, H₂S 6 ppm, CO 5 ppm. The blower is on site but not connected. The standby man is also the permit issuer and has gone to the store for a torch. Two workers are ready to enter wearing full-face respirators (not supplied-air) and rubber boots; one wants to take a 24 V hand lamp with a frayed cable. The tank inlet valve is closed but not locked and the suction line shows 0 bar. Sludge depth is about 150 mm, and the crew intends to shovel it into buckets and pass them out.
1. Assess the entry against the gas readings and permit conditions, and state your decision.
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Decision: no entry. The entry fails on several independent counts, any one of which is sufficient to stop it.
Atmosphere. Oxygen at 19.3 % is below the 19.5 % lower limit for entry, so the space is oxygen-deficient before anyone enters. H₂S at 6 ppm is under the usual 10 ppm ceiling but the planned work is sludge shovelling, which will liberate the gas absorbed in the sludge; a reading taken while the tank is still cannot be extrapolated to disturbed conditions. The LEL of 0 % is worthless as a clearance signal because the meter's response to the fuel-oil vapour must be confirmed and the tank was flushed with diesel, which itself produces flammable vapour.
Permit conditions. A permit covering a whole shift is invalid for this work: conditions must be re-tested before each entry and monitored continuously. A hot-work permit on adjacent pipe racking conflicts with sludge disturbance in a tank that has held fuel oil.
Control failures. The standby man cannot be the permit issuer and cannot leave the entry point; a full-face respirator gives no protection in an oxygen-deficient or unknown atmosphere - supplied air or escape sets are needed; the tank is not positively isolated and locked; the blower is not connected.
2. Give the correct sequence of steps that must be completed before a valid entry permit can be issued.
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- Isolate: close and lock or tag the inlet, outlet and suction valves, blank off or double-block-and-bleed, and prove zero energy at the pump; the 0 bar reading on the suction line is only valid once isolation is verified.
- Clean and purge: remove the remaining sludge where possible without entry, wash the tank, then ventilate with the blower connected and running for long enough to achieve at least the target number of air changes, with the blower intake placed in clean air and the exhaust discharging outside.
- Test: re-test oxygen, flammable gas and H₂S from outside and at several depths with a calibrated meter, noting that H₂S is heavier than air and will sit at the bottom. Entry is permitted typically at O₂ 19.5–23.5 %, LEL below 5–10 %, H₂S below the site's ceiling (commonly 5–10 ppm).
- Permit: issue a permit for a defined short period and task, signed by a competent issuer, listing the isolation, ventilation, gas test values, entry supervisor, standby man, communication method and rescue plan; re-test and re-endorse before each re-entry.
- Personnel: a dedicated standby man at the manhole who does nothing else, entrants trained and medically fit, harnesses and a retrieval line fitted, at least one attendant outside with a means of raising the alarm.
- Equipment: intrinsically safe or low-voltage lighting and tools in good condition, no frayed leads, escape sets or supplied-air breathing apparatus as determined by the assessment, and a fire watch if any hot work remains open nearby.
3. The worker's 24 V hand lamp has a frayed cable, and no rescue plan has been discussed. Address both points.
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The lamp. Even at 24 V the frayed cable means exposed conductors, a shock risk in a wet tank and a spark source in a space that has held fuel oil. A reduced-voltage lamp is only acceptable when the cable, gland and lamp glass are intact and the supply comes from an isolating transformer with the tank metalwork bonded; a damaged one is removed from service, tagged and replaced. For a fuel-oil tank, intrinsically safe battery lighting is the better choice because it removes both the mains lead and the spark risk.
Rescue. A confined-space permit without a rescue plan is not a permit. Before entry the team must agree and rehearse: who raises the alarm, how the entrant is lifted out (full-body harness with a retrieval line anchored above the manhole, tripod and winch for a vertical 2.2 m lift), who is trained in the use of escape breathing apparatus, and the route and contact for the emergency services. Roughly half of confined-space fatalities are would-be rescuers, so the plan must state clearly that no one enters after a casualty without breathing apparatus and the same controls. The standby man stays at the manhole with the log; if he leaves, everyone comes out.