Confined-Space Entry With Stratified H2S
At 09:10, a crew of three arrives at a 2.7 m deep, 1.5 m diameter manhole on a 450 mm combined sewer. A debris blockage is causing a backup. The 4-gas meter at the surface reads O2 20.9%, LEL 0%, H2S 3 ppm, CO 0 ppm. A reading taken 1 m below the cover reads O2 19.8%, LEL 1%, H2S 18 ppm. The crew has a tripod, harness, retrieval line, 15 m ventilation hose, and a blower rated 0.6 m³/min. One worker is confined-space certified and wants to descend now while the blower is set up. The attendant is also the only person who can operate the radio. Weather is 28°C and calm. Upstream flow cannot be fully stopped, but a plug and pump are available.
1. Decide whether the certified worker may descend now. Support your answer with the readings and the manhole volume/ventilation rate.
Read the worked answer
No. The 1 m reading of 18 ppm H2S exceeds the common 10 ppm ceiling/5 ppm entry limit, and the surface reading of 3 ppm proves stratification; the top reading is not representative. Oxygen 19.8% is acceptable but close to the 19.5% limit, and 1% LEL warrants investigation.
Manhole volume = π(0.75)^2 × 2.7 = 4.77 m³. At 0.6 m³/min, one air change takes about 8 min. A 15 min purge is under two air changes, so time alone is not proof of safety. Ventilate continuously, retest at 0.5 m intervals, and do not enter until H2S is below 5 ppm (or your permit's limit), O2 is 19.5–23%, and LEL is 0%. If readings persist, use supplied air.
2. Describe the entry controls and rescue plan you would require before any entry, including the radio problem.
Read the worked answer
Require a written entry permit, a dedicated attendant at the top who does nothing but monitor and communicate, and a second means of communication because the attendant cannot operate the radio and monitor the entrant at the same time. Use tripod, full-body harness, retrieval line, continuous 4-gas monitoring, forced ventilation, and non-sparking tools. Isolate or plug upstream and pump flow around the work to reduce the source. Establish a rescue plan: call 911, retrieval first, no unprotected entry rescue. The attendant must be able to summon help without leaving the manhole.
3. Midway through the task, the attendant's meter alarms at 10 ppm H2S while the entrant is 2 m down. State the immediate actions and the sequence.
Read the worked answer
Immediate actions: shout the evacuation signal, order the entrant out, and start retrieval if the entrant is not self-evacuating. Do not enter to rescue without SCBA and a backup team. Move the meter and attendant upwind, continue ventilation, and account for all personnel. Call 911 if the entrant is unresponsive. After evacuation, reclassify the space; no re-entry until the source is controlled and readings are stable below limits. Preserve the meter data and document the alarm time, readings, and actions.