Repairing a Diesel Tank from Inside the Manhole
A 1,200 mm diameter × 2,500 mm long mild steel diesel tank is emptied and water-washed. A 300 mm crack in the bottom shell must be repaired by SMAW from inside. The only access is a 450 mm top manhole; a 300 mm drain opening is at one end. The tank last held diesel and contains a thin sludge layer. Ambient temperature is 38 °C and relative humidity is 80%. The supervisor plans to lower a 220 V AC welding cable through the manhole, place the electrode holder on the wet floor, and use a portable fan at the manhole to blow air in. An oxy-acetylene set for cutting is standing 2 m from the manhole. The welder intends to wear a cotton shirt, a half-face dust mask and ordinary safety glasses. A gas test taken 30 minutes earlier showed 2% LEL and 20.9% oxygen. The welder wants to start immediately.
1. Identify the principal hazards and set out the entry and isolation steps that must be completed before the welder enters the tank.
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The principal hazards are flammable diesel vapour and sludge, possible oxygen deficiency or enrichment, toxic welding fume, electric shock in a wet confined space, burns, and the oxy-acetylene set near the entry. Water washing does not make a fuel tank safe: sludge can release vapour when it is disturbed or heated.
Before entry, isolate and lock out every line connected to the tank, blank or disconnect them, and clean out the sludge. Establish forced ventilation and test the atmosphere at the bottom and top for LEL, oxygen, carbon monoxide and hydrogen sulphide. Oxygen must be within the confined-space limit, typically 19.5–23.5%, and the LEL reading should be essentially zero before hot work. Obtain a confined-space entry permit. The sequence is: isolate → clean → ventilate → gas test → permit → harness, lifeline and standby person → enter with 24 V lighting and the welding return clamped directly to the workpiece. The oxy-acetylene set must be moved outside the area, with flashback arrestors fitted.
2. Evaluate the proposed ventilation, PPE and electrical arrangements, and state the corrections needed.
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A fan blowing air in at the manhole is not adequate ventilation. It can short-circuit across the opening and may push vapour toward the welder rather than remove fume. Use a forced-draft supply plus an extractor positioned to take fume from the weld zone, and confirm that the supply air is clean. Do not use oxygen to ventilate.
The electrical plan is dangerous: 220 V equipment, a wet floor and an electrode holder laid on the floor create a serious shock risk. Keep the welding set outside, earth it correctly, use an RCD/ELCB, keep cables and the holder insulated and off the floor, and use 24 V lighting. A dry insulating mat should be provided.
Cotton shirt, dust mask and ordinary glasses are not suitable PPE. The welder needs flame-retardant coveralls, leather gauntlets and sleeves, a welding helmet with the correct filter, safety boots, and respiratory protection suitable for welding fume-P2/P3 at minimum, or supplied air if ventilation is poor. A harness and lifeline are mandatory. The gas cylinders must be outside and secured, not at the manhole.
3. Explain why the 30-minute-old gas test is not sufficient and what the standby person must do.
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A gas test is a snapshot, not a permanent guarantee. The reading was taken 30 minutes before entry and may change when sludge is disturbed, when the welder enters, or when welding heat drives vapour out of the bottom. A reading of 2% LEL is already a warning in a confined space; it is not zero. Hot work also consumes oxygen and generates fume.
Continuous or frequent gas monitoring is therefore required, with the probe kept in the work area and the instrument alarm set. The standby person must remain at the manhole, maintain communication, keep the permit and rescue equipment ready, watch the welder, and stop the job if the gas detector alarms, ventilation fails, or the welder feels unwell. The standby person must not enter to rescue without breathing apparatus and backup; the emergency team should be called. The standby person also controls entry so that no one else starts work at the opening.