Confined-Space Entry in a Sludge Sump
At a food-processing plant, a 30 m3 concrete sump collects washdown water and sludge. The only access is a 600 mm top opening; the sump is 4 m deep. A contractor must enter to unbolt a blinded screen. Pre-entry readings at the top are O2 20.4%, LEL 2% LEL, H2S 12 ppm. The site permit sets H2S limits at 1 ppm TWA and 5 ppm STEL, and requires entry only after 30 minutes of mechanical ventilation. A single 300 mm flexible duct is placed at the top and connected to a 900 m3/h blower. The contractor's supervisor says that because the reading is below 100 ppm IDLH, a 10-minute entry with a half-mask respirator is acceptable. The plant has no standby rescue team on site; the local fire brigade has a 25-minute response time. Production is behind schedule and the area manager wants the screen removed today.
1. Evaluate whether the proposed entry meets the permit and regulatory requirements. Identify the hazards and deficiencies.
Read the worked answer
The proposed entry does not meet the permit. The H2S reading of 12 ppm is above both the 1 ppm TWA and 5 ppm STEL, so entry is prohibited regardless of the 10-minute duration. The half-mask respirator is not appropriate as the sole control for an unknown or rising H2S atmosphere, and it provides no protection if the cartridge is not selected or fitted correctly. The 2% LEL reading is below 10% LEL only if the instrument is reading %LEL; this must be confirmed. The ventilation is inadequate: a single duct at the top will short-circuit fresh air across the opening, while H2S is heavier than air and will accumulate at the bottom of a 4 m sump. The pre-entry reading was taken at the top, not at the bottom, so it does not represent the breathing zone. There is no standby rescue team; a 25-minute fire-brigade response is not an acceptable rescue plan for a sump. The entry also lacks continuous bottom monitoring, an attendant, a retrieval system and a clear isolation procedure.
2. What engineering and administrative controls should be implemented before any entry? Include monitoring and rescue.
Read the worked answer
First eliminate the entry: use a long-handled tool, vacuum truck or remotely operated unbolting device from outside. If entry is unavoidable: isolate and drain the sump; clean and purge; ventilate with the discharge positioned at the bottom and the intake at the opposite side; retest at top, middle and bottom after at least 30 minutes; use continuous personal H2S/O2/LEL monitors; provide supplied-air or SCBA, full-body harness, tripod and retrieval line; station a trained attendant at the opening; use intrinsic-safe lighting and non-sparking tools; establish two-way communication; prepare a site-specific rescue plan with a standby rescue team; issue a confined-space permit and JSA; brief all workers on the abort criteria. If any monitor alarms or the reading rises, evacuate immediately.
3. How should you respond to the area manager's schedule pressure and document the decision?
Read the worked answer
Stop the work and tell the area manager that the entry is not permitted because H2S is above the STEL, the ventilation is not effective at the bottom, and rescue is not available. Do not negotiate the limit or accept a short entry. Escalate to the plant EHS manager and operations manager in writing; record the stop-work decision, readings and deficiencies in the permit log and shift report. Offer schedule recovery options: outside specialist contractor with rescue capability, remote tooling, or a planned shutdown entry after cleaning and ventilation. If the manager insists, state that you cannot authorize the entry and ask for a written direction, while making clear that you will escalate. The immediate priority is to prevent exposure, not to protect the schedule.