Sump Entry Under an Invalid Permit

Entry must not proceed. Every control the permit depends on is invalid:

• The detector is 45 days past a 30-day calibration interval and has not been bump-tested for two months. A reading of 20.5 % O2, 0 LEL and 0 ppm H2S from an unbumped instrument is not evidence of a safe atmosphere; it is evidence of nothing. Sensors drift, and a poisoned LEL sensor reads zero in a flammable atmosphere.
• There is no forced ventilation. A 4.0 x 2.5 x 4.5 m sump holds 45 m³ with a 600 mm opening; natural ventilation through that opening is negligible, and the residual 200 mm of sludge will keep generating gas after entry begins. The atmosphere can change while the worker is inside - exactly what a reading taken at the opening cannot detect.
• The attendant leaving to fetch a tripod means there is no attendant. Watching, communicating and starting rescue is the whole job.
• 'Two minutes' is not a control. Most confined-space fatalities are would-be rescuers, and exposure time inside is irrelevant to the risk.
• The unbarricaded opening is a fall hazard, and the two smokers are an ignition source.

The first-line answer is elimination: can the grit be removed by vacuum truck, suction head, remote jetting or flushing? Entry is the last resort.

Testing:

• Recalibrate or replace the detector and bump-test it before use, and record the bump test.
• Test from the top down, before entry and after ventilation: oxygen first, then flammable gas (LEL), then toxics (H2S, CO). Oxygen first because many combustible-gas sensors need oxygen to read correctly, and because an oxygen-enriched reading raises fire risk.

Ventilation, with the arithmetic:

• The space is 4.0 x 2.5 x 4.5 = 45 m³. Four air changes is 180 m³.
• A blower rated 0.7 m³/min would need about 257 minutes - over four hours. That number is the point: either bring a much larger blower or ventilate overnight and re-test. It shows why opening the lid and waiting is not ventilation.
• Keep ventilating throughout entry, with the intake at the bottom of the sump, because H2S and most fuel vapours are heavier than air.
• Re-test at the start of entry, at least hourly during the task, after any break, and whenever the worker reports symptoms.

Acceptance: oxygen within 19.5-23.5 %, LEL below 10 %, and H2S below the site action level (commonly 10 ppm, with 5 ppm used on stricter sites).

Before any entry:

• Barricade and sign the opening, and clear the smokers and any other ignition source; designate the permit area as no-smoking.
• Assign a trained attendant who stays at the opening for the entire entry, with communication that actually works - voice is unreliable through a 600 mm opening with a blower running, so agree radio or rope signals in advance.
• Provide tripod, winch and full-body harness so the entrant can be lifted out without a second person entering. The rescue plan must be non-entry first; if entry rescue is needed, the team must be on site and equipped before the entrant goes in, not called afterwards.
• Brief entrant and attendant on the permit, the isolation, the gas results and the evacuation signal, and have both sign.
• Confirm and lock off isolation of any inlet, sludge line or agitator.

Why a deficient signed permit is worse than none: it creates the appearance of control. The crew believes a competent person has verified the atmosphere and the rescue arrangements, so they stop applying their own judgement - which is exactly how a worker ends up at the bottom of a sump with an uncalibrated detector and nobody watching. Stop the job and re-issue the permit only after every item above is verified.