Diesel Tank Cleaning: Stratified Vapour and a Missing Lockout
A 20,000 L diesel storage tank at a depot was drained three days ago and contains about 200 mm of sludge. The only opening is a 1.0 m top manhole. A contractor crew of three plans to enter with a 12 V transfer pump to remove the sludge. The inlet line from an adjacent tank is closed by a gate valve, but the valve handle is missing and the line feels warm. The crew has no permit yet. They opened the manhole for 20 minutes and took a reading at the rim: LEL 0%, O2 20.9%, H2S 0 ppm, benzene 0 ppm. A second reading 1.5 m below the top gives LEL 16%, O2 19.5%, benzene 2 ppm. The supervisor says the rim reading is 'all clear' and proposes dust masks because 'diesel is not toxic'.
1. Why is the rim reading not acceptable, and what sampling strategy should the competent person require before entry?
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The rim reading is not representative because diesel and sludge release hydrocarbon vapour that is heavier than air and can stratify in a tank. The lower reading already proves a flammable atmosphere below the rim: LEL 16%, O2 19.5% and benzene 2 ppm at 1.5 m.
The competent person must require sampling at several depths, top, middle and bottom, and around the sludge surface, using an extension probe and a pump with the correct tubing, before and after ventilation. The instrument should be a multi-gas meter with a PID or a benzene-specific method if benzene is a concern, and it should be calibrated and bump-tested. Readings should be taken from outside the tank; no one enters to sample. If stratified readings persist, the space is not safe for entry and must be purged or ventilated and re-tested.
2. Identify the isolation deficiencies and describe the positive isolation and lockout steps required.
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A closed gate valve with a missing handle is not positive isolation; it can pass product, and the warm line suggests flow or pressure from the adjacent tank. Required steps:
- Identify all energy and product sources: inlet, outlet, drain, vent, sampling lines and any connected pump.
- Isolate by blanking or spading, or by double block and bleed, not by a single valve.
- Lock out and tag the valves and the transfer pump, then verify zero energy and zero flow.
- Drain and vent the tank, and check that the adjacent tank cannot pressurise the line during filling operations.
- Record each isolation point, who applied it and how it was verified in the permit.
Only after isolation should purging, ventilation and re-testing begin.
3. Can the crew enter with dust masks if ventilation brings the LEL below 10%? State the required controls and PPE.
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No. A dust mask gives no protection against hydrocarbon vapour, benzene or oxygen deficiency. If ventilation and re-testing prove the atmosphere stable below action levels, entry may be considered with the correct respiratory protection selected by risk assessment. For tank cleaning with possible benzene and unknown sludge, supplied air or BA is normally required. An air-purifying respirator with organic vapour or benzene cartridges is only acceptable when the contaminant is known, below the cartridge limit and O2 is adequate; it is not suitable for unknown or oxygen-deficient atmospheres.
Other controls: permit-to-work, continuous monitoring, forced ventilation, harness and tripod or winch, trained standby, non-sparking tools, chemical gloves and coveralls, and a rescue plan. If LEL remains 16%, entry is prohibited.