Confined-Space Welding Inside a Drained Water Tank
An independent workshop is repairing a 5 m³ mild steel water tank that has been in service for eight years. The tank is 2.0 m long, 1.5 m wide and 1.2 m high, and the only access is a 450 mm × 350 mm hatch on the top. A pinhole leak at a bottom seam is to be sealed by SMAW with E6013 electrodes at about 90 A. The tank has been drained and hosed, but a black sludge remains in the corners. Two workers are on site: a welder and a helper. No forced ventilation is set up, and the workshop door is closed because of rain. After 20 minutes of welding, the welder climbs out complaining of headache and dizziness, then says he feels better. The helper wants to go inside to finish the last 150 mm of weld so the tank can be tested the same day.
1. Identify the main hazards in this tank and list the checks and controls that must be completed before anyone enters.
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Main hazards:
- Oxygen deficiency and welding fume in a 5 m³ tank with only a 450 × 350 mm hatch; the headache and dizziness are warning signs.
- Sludge may release hydrogen sulphide or flammable gas; test for H2S, CO and LEL as well as oxygen.
- Electric shock: damp steel tank, sweat and 90 A SMAW; use 24 V lighting, an RCD, insulated electrode holder, dry gloves and an isolated welding return.
- Heat, burns, sparks, restricted rescue and falling objects through the hatch.
Before entry: isolate and lock off the tank; drain and remove sludge; issue a confined-space permit; test the atmosphere from outside and again inside (O2 19.5–23.5%, LEL below 10%, H2S and CO below exposure limits); provide forced ventilation, never oxygen; assign a trained standby person at the hatch with communication; wear a harness and lifeline; keep rescue equipment and a fire extinguisher at the opening. The welder should not re-enter until the atmosphere is re-tested and the cause of the symptoms is understood.
2. The helper says he will crawl in quickly without breathing apparatus while the welder holds the door open. Evaluate this proposal and state what must happen before the last 150 mm is welded.
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The proposal must be rejected. A quick entry without breathing apparatus is how a rescuer becomes a second casualty. The welder's symptoms suggest fume exposure or oxygen deficiency, so the tank is not proven safe. Before the last 150 mm is welded: stop work, keep the hatch open, ventilate mechanically, re-test the atmosphere, and investigate the sludge and the ventilation. Only trained personnel wearing the correct respiratory protection - airline or SCBA in an immediately dangerous atmosphere - and full entry controls may enter. If the tank cannot be made safe, weld from outside if possible or defer the work. The same-day pressure test must not override the permit. The helper must also understand that feeling fine now does not clear the atmosphere.
3. Outline the emergency actions if a welder collapses inside the tank.
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Do not enter. Raise the alarm, call the site emergency number and give the location. Start forced ventilation from outside. If the casualty is wearing a harness and lifeline, retrieve him horizontally through the hatch with at least two people, avoiding a jam at the opening. If retrieval is not possible, wait for trained rescuers with SCBA; untrained entry is prohibited. Once out, place him in fresh air, give oxygen if trained and he is breathing, and start CPR if he is not. Keep the area clear, preserve the scene and report the incident. Afterwards, re-assess the tank and review the permit system, because the emergency has shown that the original controls were inadequate.