Sump Entry with a Single Gas Reading
A maintenance worker is asked to clear leaves and sludge from a 2.8 m deep, 1.2 m diameter pump sump in the basement of a shopping complex. The sump receives drainage from a food court and a grease trap; a faint rotten-egg smell is noticeable. The supervisor lowers a four-gas meter on a rope and takes a reading at the top opening: oxygen 20.9%, LEL 0%, hydrogen sulphide 0 ppm, carbon monoxide 0 ppm. The meter's calibration sticker expired 14 months ago. There is no forced ventilation. The worker is issued a disposable dust mask, a body harness and a tripod, but no one is assigned as attendant. A permit to work has not been raised. The worker says he has entered the sump 'many times' without problems. The pump has not been isolated and the discharge line is still live. He is about to descend the fixed ladder with a bucket and scraper.
1. Can the worker rely on the gas reading and proceed? Explain the weaknesses in the test.
Read the worked answer
The reading cannot be relied on. The meter's calibration is 14 months out of date, so its accuracy is unknown. More importantly, the sample was taken only at the top opening; gases can stratify, and the sump is 2.8 m deep. The rotten-egg smell suggests hydrogen sulphide may be present, and the meter may not respond correctly if it is faulty. The dust mask gives no protection against gases. A single reading at the top is not a valid test of the atmosphere at the bottom, particularly with sludge that can release gas when disturbed. The correct decision is: do not enter. Re-test with a calibrated meter, from outside, at several levels, after forced ventilation, and with continuous monitoring.
2. What controls should be in place before entry?
Read the worked answer
Isolate and lock off the pump and discharge line; raise a confined space permit; ventilate with a blower; test from outside at top, middle and bottom; assign a trained attendant at the opening; provide harness and tripod with retrieval line; use continuous gas monitoring; provide breathing apparatus for the entrant where the atmosphere cannot be made safe; establish communication and a rescue plan; train and brief the entrant. The standby person must stay at the opening and never enter to rescue. The worker's experience is not a control. Entry only after the permit is signed and the atmosphere is proven safe.
3. If the worker is later found collapsed at the base of the sump, what should the standby person do?
Read the worked answer
The standby person should not enter. He should call the site emergency team and fire and rescue, start the retrieval system from outside if the casualty is attached and it is safe, and ventilate. Rescuers need SCBA and a harness. A would-be rescuer entering without protection is likely to become a second casualty.