Welding a pad onto a tank that held kerosene
A 20 m³ horizontal mild-steel tank that held kerosene for 12 years has been drained, water-washed and left open through a 400 mm top hatch for four days. A fabrication crew must weld a 150 mm reinforcing pad onto the external shell, 300 mm above the bottom tangent line, to repair a support bracket. The gas tester samples through the top hatch with a catalytic LEL meter and reads 12% LEL, with no other flammable reading. Ambient temperature is 34 °C and the sunlit shell is too hot to hold a hand on. The tank sits on saddles 400 mm above a concrete bund; a black tar-like residue is visible on the bottom through the hatch and there is heavy rust scale on the internal floor. The hot work permit states that flammable atmosphere shall not exceed 5% LEL at the work location and requires continuous monitoring. The contractor argues the tank is empty and washed, and asks for the permit to be issued on the 12% reading plus a fire watch.
1. Should the hot work permit be issued on this reading? Explain what 12% LEL means here and how the kerosene residue changes the risk.
Read the worked answer
No. There are four separate reasons, and the first one alone is enough.
- The permit's own limit. It states 5% LEL at the work location. 12% LEL fails on the number. Even the looser 10% LEL convention used for some hot work in some jurisdictions fails.
- The number is not at the work location. It came from the top hatch. The weld is on the shell 300 mm above the bottom, which is where vapour from the residue will concentrate once heat is applied.
- The kerosene is still there. Kerosene's flashpoint is roughly 38–42 °C. Ambient is 34 °C and the shell is hot to the touch, so the liquid is at or near its flashpoint and is actively generating vapour. Heating the shell drives hydrocarbon out of the tar and out of the rust scale, which holds it like a sponge. 12% LEL corresponds to only about 0.08% by volume, so a small rise in temperature moves the number a long way.
- Pyrophoric iron sulphide is possible in a tank that has held sour hydrocarbon. Scale that has been opened to air can self-heat and ignite.
A fire watch does not fix a flammable atmosphere. It only shortens the time between ignition and someone noticing.
2. What would you require before the permit could be reconsidered, and how would you verify conditions at the work location?
Read the worked answer
Clean, ventilate, then test where the heat will be applied.
- Clean to bare metal at the work area. Remove the tar residue and the scale by water jetting, steam, chemical cleaning or mechanical means, and collect the washings. Washing a tank without removing the sludge is the classic failure: the sludge keeps emitting for weeks.
- Ventilate. Forced air through the vessel with the inlet and outlet positioned so the whole shell is swept, continued through the work, with the blower intake in a clean area.
- Test at the work location and at the bottom. Sample the internal atmosphere at the level of the weld, low near the residue, and in the headspace. Also test the outside of the shell and the bund, where vapour can travel and where the welder is standing.
- Verify the meter. A calibrated LEL meter of the right range, bump-checked that shift. If the tank were inerted you would need a percentage-by-volume instrument, not a percentage-of-LEL one; a catalytic sensor cannot measure above its range and reads misleadingly in an inert atmosphere.
- Define the acceptance criteria in the permit: 0% LEL at the work location for hot work on a vessel that has held flammable liquid is the defensible target; 5% LEL only if the permit procedure genuinely allows it, with continuous monitoring and a pre-alarm at half the limit.
- Confirm with the permit issuer and record the decision.
If cleaning cannot reach the residue in the area to be heated, the pad should be fixed cold, by bolting or clamping, or the tank replaced.
3. Halfway through the work the continuous monitor alarms at 8% LEL. State your actions and the reasoning behind each.
Read the worked answer
Treat it as a stop-work trigger, not a number to negotiate.
- Stop the welding immediately - torch out, hot metal left to cool, combustibles cleared from the area.
- Keep the fire watch in place and the area barriered. A rising reading means vapour is arriving, and the fire watch is the person who acts if it ignites.
- Do not re-test and carry on. Investigate the cause: heat-driven desorption from the shell, the tank cooling and drawing vapour inward, a wind change putting the vent downwind of the work, or the residue being heated from the outside.
- Ventilate, allow the tank to cool, then re-test at the work location and at every point previously tested, and watch the trend over a sustained period rather than taking one snapshot.
- Notify the permit issuer and the AGT's supervisor. The permit is suspended. The AGT does not amend it.
- Resume only on a fresh authorisation, with the cause identified and corrected and the alarm set so the next excursion is caught earlier.
The reading that matters in hot work is the trend, not the snapshot. A steady 3% LEL that has not moved for an hour is a completely different proposition from 3% climbing to 8% in ten minutes.