Hot Work on a Tank Shell with Vapour in the Excavation
A contractor is removing a 20,000 L single-wall steel UST at a PEI site that has been out of service for eight months. The tank was emptied and roughly cleaned two days ago, but about 100 mm of sludge remains on the floor. The excavation is 3.5 m deep and sheeted, and the tank shell must be cut into sections for lifting. Vapour readings taken at 10:00 with a combustible gas meter calibrated on pentane: 0% LEL at grade, 8% LEL at the bottom of the excavation about 300 mm above the tank top, and 12% LEL from a sample drawn inside the tank through the 40 mm inspection port. Ambient temperature is 22 °C with an 8 km/h wind from the west. Two dispensers 15 m upwind remain in service. The tank's 50 mm vent line was disconnected at the tank and its open end is lying in the excavation. The contractor wants to cut the shell this afternoon with an abrasive saw, and states that the 10% LEL action level applies "to confined spaces only."
1. Interpret the three readings and decide whether hot work may proceed this afternoon.
Read the worked answer
Hot work may not proceed. The three readings, read together, describe an active vapour source, not a cleared tank.
Reading by reading:
- 0% at grade means nothing - gasoline vapour is roughly three to four times heavier than air and will not be found at grade on a breezy day. Grading a reading taken at the wrong height is a common and dangerous error.
- 8% LEL at the bottom of the excavation confirms heavier-than-air vapour pooling exactly where the cutting will happen. It is close to the 10% action level and, on a pentane-calibrated meter, the displayed figure is not a true gasoline concentration - the response factor has not been applied, so the real figure may be higher. Treat 8% as "at or above action level" and stop.
- 12% LEL inside the tank is above any credible action level and shows the sludge is still generating vapour. A tank is not gas-free because it was emptied and "roughly cleaned."
The contractor's argument about confined spaces is wrong. The 10% LEL threshold is a stop-work control for the work area, full stop. Cutting with an abrasive saw produces sparks and heat on a shell containing a flammable atmosphere.
What must happen first: remove the sludge, purge and ventilate the tank, cap or relocate the open vent, and re-test at multiple heights after purging with continuous monitoring during any cutting. No hot work permit should be issued on these numbers.
2. The open vent line is lying in the excavation and two dispensers upwind are still operating. What controls do these facts demand?
Read the worked answer
The vent line is the primary cause of the readings and must be dealt with first. The disconnection was made at the tank, leaving a 50 mm open pipe directly into the work area. Vapour migrates down it and out into the excavation, and the same pipe is a path for ignition to travel back toward the tank. Until it is capped, blinded or relocated to a safe discharge point outside the excavation, the atmosphere will keep regenerating no matter how many times it is purged. Venting must always be terminated away from the work, above head height and clear of openings.
The in-service dispensers:
- Upwind at 15 m is favourable today at 8 km/h, but wind is not a control. It shifts, and a delivery or a drive-off brings an ignition source and a vehicle into the picture.
- Request that the dispensers be taken out of service, or at minimum that deliveries be suspended and vehicle traffic excluded from the area for the duration of any cutting.
- Confirm no tanker delivery is scheduled during the work window.
Additional controls for the permit:
- Hot work permit with a competent fire watch, charged extinguishers and a 30-minute post-work fire watch.
- Continuous LEL monitoring at the cut point and at the excavation bottom, with a defined stop-work level and a designated person empowered to stop the job.
- Bonding and grounding of the shell before cutting.
- Exclusion zone and signage; no smoking; no mobile phones or non-rated equipment in the zone.
- Site-specific emergency plan and a call-out number.
None of this substitutes for making the tank and excavation gas-free; it controls the residual risk around a job done properly.
3. If the contractor insists, what is your decision and what alternative method would you approve?
Read the worked answer
Decision: refuse the hot work permit. Record the refusal, the readings, the time and the reason on the permit form. The readings were taken at 10:00; they will change, and they will not change in a favourable direction while the vent is open and the sludge is in the tank.
Approve cold methods for this phase:
- Complete sludge removal by vacuum, with the waste manifested and disposed of through a licensed receiver.
- Purge and forced-air ventilate; then verify with a calibrated meter, corrected for the gasoline response factor, at multiple heights in the tank and in the excavation.
- Cut the shell only after the tank is verified gas-free - or, better, lift the tank intact where the excavation and crane capacity allow, and cut it at a permitted off-site location. Intact removal removes hot work from the excavation entirely.
Re-entry criteria before any cutting on site: stable readings well below the action level over a defined period with ventilation running, no sludge, vent controlled, dispensers isolated, and a new permit issued on the fresh data.
Paper trail: document the initial readings, the refusal, the corrective actions and the re-test results. If the contractor proceeds anyway, that is a stop-work and a reportable event, and the regulator is notified - a vapour cloud in an excavation next to live dispensers is not a private disagreement between two contractors.