Sewer Manhole Entry With Borderline Readings
A utility crew is preparing to enter a 6 ft deep, 4 ft diameter manhole on a live sewer main to remove a debris blockage. The manhole has one 24 in. opening. A calibrated four-gas monitor at the top reads oxygen 19.5%, hydrogen sulfide 10 ppm, carbon monoxide 0 ppm, and LEL 5%. The monitor was calibrated two weeks ago. The crew plans to lower a 20 in. axial blower, start it after the entrant is inside, and keep the attendant at the truck 20 ft away because traffic noise makes communication difficult. The entrant will wear a half-face respirator with organic vapor/acid gas cartridges and a 5-minute escape SCBA. A tripod and retrieval line are on site but not set up. The entry supervisor says the readings are under the OSHA limits and the job will take 10 minutes.
1. Classify the space and determine whether entry may proceed on the current readings.
Read the worked answer
The manhole is a permit-required confined space. It is a sewer with potential hydrogen sulfide, methane, and oxygen deficiency, and it has a restricted opening and limited egress. The readings at the top do not clear the space:
- Oxygen at 19.5% is exactly the minimum acceptable level, with no margin. It can drop quickly in a sewer.
- Hydrogen sulfide at 10 ppm is below the 20 ppm OSHA ceiling, but it is at the NIOSH recommended exposure limit and may be higher at the bottom.
- LEL at 5% is below 10%, but methane can accumulate.
- The reading was taken only at the top. Gases may stratify or be released when the debris is disturbed.
Entry should not proceed. The crew must ventilate before entry, test at multiple levels, continuously monitor, and set up the tripod and retrieval system. The attendant must be at the opening, not 20 ft away.
2. Specify the required controls for ventilation, monitoring, communication, and rescue.
Read the worked answer
Required controls:
- Ventilation: start forced-air ventilation before entry and keep it running during entry. Lower the blower duct to the bottom of the manhole and re-test after ventilation.
- Monitoring: test before entry and continuously during entry for oxygen, hydrogen sulfide, carbon monoxide, and flammable gas. Test at the top, middle, and bottom because gases can stratify. Record results.
- Attendant: station a trained attendant at the opening with continuous communication. The attendant must not leave and must not enter to rescue.
- Retrieval: set up the tripod and retrieval line before entry. The entrant should wear a full-body harness connected to the retrieval system.
- Rescue: have a written rescue plan and trained rescue personnel or call 911. Non-entry retrieval is preferred; if entry rescue is needed, rescuers need supplied air.
- Permit: complete a permit and have the entry supervisor verify all controls before entry.
3. Evaluate the proposed respiratory protection and state what is acceptable for this entry.
Read the worked answer
The proposed protection is not acceptable. A 5-minute escape SCBA is for escape only and cannot be used for entry. A half-face air-purifying respirator with organic vapor/acid gas cartridges is also questionable for an unknown sewer atmosphere: the cartridge has limited capacity, hydrogen sulfide has poor warning properties, and the atmosphere may become oxygen-deficient or IDLH. The space should be ventilated and re-tested before entry.
If ventilation reduces contaminants below exposure limits and oxygen is acceptable, an air-purifying respirator may be used only as part of a full respiratory protection program: medical evaluation, fit testing, training, cartridge change schedule, and written procedures. If the atmosphere cannot be made safe, use a supplied-air respirator with an escape SCBA. Do not enter on the current readings with the equipment described.