Wet Well Entry With a Marginal Gas Reading
A clogged pump at Lift Station 4 requires entry into a 6-ft diameter, 14-ft deep wet well. The crew's 4-gas meter was bump-tested 40 days ago and last calibrated six months ago. Held at the rim, it reads 20.8% O2, 0% LEL, and 6 ppm H2S. The portable blower is set up, but its duct ends about 3 ft below the hatch. The entry permit lists one attendant who is also the on-call pump operator, and the rescue section says 'call 911.' A tripod, winch, and full-body harness are on site. Two of the three crew members' respirator fit tests are 14 months old. The foreman says, 'It's only 6 ppm - let's just get down there and pull the rag out.'
1. Would you authorize this entry as described? What has to change first?
Read the worked answer
No. The entry as described has at least six deficiencies.
1. The atmospheric test is not representative. The reading was taken at the rim. Hydrogen sulfide is heavier than air (about 1.19 times the density) and will be concentrated at the bottom where the sludge and the pump are; oxygen-deficient pockets behave the same way. Test at the top, middle, and bottom before entry, then monitor continuously during entry with the sensor clipped in the entrant's breathing zone.
2. The instrument is not trustworthy. A bump test 40 days ago and a calibration six months old do not meet good practice - bump test before each day's use and follow the manufacturer's calibration interval. An H2S sensor can be poisoned or drifted and still read a comfortable number. Re-bump or calibrate, then re-test.
3. Ventilation is not reaching the hazard. The duct stops 3 ft below the hatch. Extend it to the bottom of the well, ventilate, and re-test before entry. Keep ventilating while the space is occupied.
4. The attendant has another job. The attendant must stay outside, maintain communication, and have no other duties. The on-call pump operator cannot be the attendant.
5. The rescue plan is not a plan. 'Call 911' is a phone number. Rig the tripod, winch, and harness for non-entry retrieval before anyone descends, and confirm the fire department actually has confined-space capability and a response time compatible with the hazard. Most confined-space fatalities are would-be rescuers.
6. The permit is incomplete. Isolation (lockout/tagout of the pump and the inlet), the hazards, communications, PPE, the atmospheric results, and the rescue provisions all go on the permit, with a pre-entry briefing.
2. After ventilation, the bottom of the well still reads 40 ppm H2S. Can the crew enter? What does that reading require?
Read the worked answer
Not under the plan on site.
Where 40 ppm sits: above the 15 ppm short-term exposure limit and far above the 10 ppm 8-hour limit, but below the 100 ppm IDLH. That does not make it enterable with an air-purifying cartridge - cartridge respirators for H2S are limited to atmospheres at or below 100 ppm with adequate oxygen, and more practically, any atmosphere above the exposure limits demands a supplied-air decision, not a cartridge decision.
The right answer is usually to not enter:
- Keep ventilating. A properly placed duct at the bottom will often clear the space. Re-test and document.
- If ventilation cannot clear the space, the maintenance should be done without entry - pull the pump by guide rail from grade, or use a portable mixer/eductor to knock down the H2S source.
- If entry is truly unavoidable, it requires supplied air (SCBA or airline with escape bottle), which means the full respiratory protection program applies: medical evaluation, training, and annual fit testing. Two crew members are 14 months out - they are not cleared to wear a tight-fitting respirator until they are re-fit-tested. That alone removes them from the entry team today.
Practical sequence: ventilate and re-test; if the reading will not come down, reschedule the work with the right equipment and the right paperwork. A clogged pump can wait a few hours; a retrieval body bag cannot be undone.
3. What must the permit and the supporting records contain, and how long do you keep them?
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On the entry permit
- The space identified and the purpose of the entry, plus the date and duration authorized.
- The hazards identified, including the sewage atmosphere, H2S, oxygen deficiency, and any engulfment or mechanical hazard from the pump.
- Atmospheric test results with the instrument identifier and the calibration/bump-test date, including the pre-entry readings at each level and the periodic readings taken during the entry.
- The isolation performed - lockout/tagout of the pump and the inlet - and the person who did it.
- Names of entrants, attendant, and entry supervisor.
- Communication method, PPE, and rescue and emergency provisions (the retrieval system plus the rescue service's name and phone number, and confirmation of their capability).
- Any additional permits required, such as hot work.
- The entry supervisor's signature authorizing entry, and cancellation of the permit when the work is done or when conditions change.
Supporting records
- Bump test and calibration logs for the gas monitor.
- Inspection records for the tripod, winch, lifeline, and blower.
- Respirator medical clearances and annual fit-test records for anyone who may wear a respirator.
- The written confined-space program, with the annual review.
Retention: keep each canceled entry permit for at least one year, and review the permit program at least annually or after any incident. These are exactly the documents an inspector asks for after an injury - and exactly the documents that do not exist if nobody writes them down.