Topic : Confined Space
A confined space is an area with limited entry and exit containing known or potential hazards that is not intended for continuous human occupancy. The most common confined spaces in the utility industries are manholes and vaults.
At times, manholes and unventilated vaults must be entered for work. Confined-space operations in manholes pose hidden hazards. The most common of these hazards are exposure to:
- Combustible or flammable vapors and gases from underground storage or piping facilities.
- Oxygen deficiency by displacement with other gases and the introduction of nitrogen from cable pressurization.
- Toxic gases from decomposing vegetation and soil, chemical spills, and enginecombustion exhaust (from vehicles and equipment).
Manhole Entry Equipment
Several pieces of equipment are required for safe entry into a confined space:
- Work-area protection devices, such as traffic (reflective) vests, traffic cones, manhole guards, and work-area protection signs.
- Manhole cover lifter and manhole hook.
- Atmospheric tester for oxygen deficiency, combustible gas, and other toxics.
- Power ventilator (blower).
- Manhole pump to remove water.
- First-aid kit.
- Portable fire extinguisher (dry chemical).
Atmospheric Conditions
Atmospheric conditions are considered unacceptable if oxygen levels are less than 19.5 percent or greater than 23.5 percent. The following levels of other hazards are unacceptable:
- A flammable gas, vapor, or mist greater than 10 percent of its lower flammable limit (LFL). LFL means the minimum concentration of the flammable material that will ignite if an ignition source is present.
- An airborne combustible dust at a concentration that obscures vision at a distance of 5 feet or less.
- An atmospheric concentration of a substance greater than the allowed limit in the material safety data sheet for that substance.
Testing Procedures
After all work-area protection devices are in place and the atmospheric test equipment
has been tested as operational, proceed as follows:
- Lower the sampling hose approximately 6 inches through the hold in the manhole cover to make the first pre-entry test. If no combustible gas is found, remove the cover and proceed. If the flammable gas concentration is above 60 percent LEL (lower explosive limit), suspend operations and follow company procedures.
- After the manhole cover is removed, continue to test for combustible gas by lowering the sampling tube within 12 inches of the manhole or vault floor. If no flammable gas is found, purge the manhole with the power blower for the period of time based on the blower capacity and manhole configuration, following company procedures.
Ventilation
If flammable gas is detected and is above 10 percent but below 60 percent LEL, purge and ventilate the manhole for a minimum of 10 minutes, depending on the manhole or vault size. If combustible gas was detected but subsequently cleared, continue to monitor and ventilate to maintain the flammable gas level below 10 percent LEL.
When ventilating a manhole, insert the blower hose in the manhole opening, positioning the blower hose to direct the flow of air horizontally, midway between the manhole’s floor and roof toward an end-wall, and away from the work area, if possible. To minimize the intake of exhaust fumes from passing vehicles, the blower intakeshould be positioned away from the flow of traffic. Wind direction also must be considered. Ventilation of the manhole must continue as long as the manhole is open to avoid the possible development of a hazardous atmosphere.
If the confined space is vacated for any period of time, the atmosphere of the confined space should be retested before re-entry is permitted. Further testing shouldbe conducted with ventilation systems turned on to ensure the contaminants are removed and that the ventilation system is not causing a hazardous condition.
Remember, when entering any manhole, follow the written confined-space entry
procedures.
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