The federal Occupational Safety and Health Administration (OSHA) requires a written permit entry program to be established for confined space operations. What effect does this have on fire departments? Photo by Bob Stella Operating 250 feet under Boston Harbor during a tunnel...
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- Atmospheric conditions are to be monitored before entry and throughout the time the confined space is occupied and shall include a test for stratifying levels of gas.
- Conditions to be monitored are that of oxygen, hydrogen sulfide, carbon monoxide, flammable gas or vapors, toxic or poisonous vapors, and physical hazards.
- Oxygen-deficient atmospheres will be considered to be levels of 19.5 percent or less. The use of respiratory protection is mandatory.
- Oxygen-enriched atmospheres will be considered to be a level of 23.5 percent or greater. Again, the use of respiratory protection is mandatory.
- OSHA defines a hazardous atmosphere as an atmosphere where greater than 10 percent of the lower explosive limit (LEL) of a flammable gas or vapor is present. Levels greater than 10 percent of the LEL require the use of respiratory protection and the proper tools and equipment for that environment. The LEL is defined as the lowest concentration of flammable gas or vapor of a material that can be ignited in air.
- OSHA also requires a test for toxicity. Toxicity levels (measured in parts per million, or ppm) which exceed defined permissible exposure limits (PEL) create an atmosphere that is immediately dangerous to life or health (IDLH).
- Airborne combustible dust that is at or exceeds its lower flammable limit is considered to be an atmospheric hazard. This is approximated when the dust obscures vision at a distance of five feet or less. Respiratory protection is mandatory in this environment.
Because some of the most common chemicals that create hazardous atmospheres within confined spaces are colorless, odorless and tasteless gases, failure to properly monitor the atmosphere before entry can be deadly to the rescuer. Atmospheric monitoring equipment should be properly maintained, carefully handled, and calibrated according to the manufacturer's specifications.
Flammable atmospheres are detected by measuring the percentage of the LEL. Most atmospheric monitors are calibrated with methane or pentane; they have conversion charts that will convert the monitor reading from methane to a specific gas. Different gases have varying lower explosive limits. If you are monitoring a gas with a different LEL than methane, and your meter is calibrated with methane, the meter will give you an inaccurate reading. You must use the conversion chart to obtain the proper atmospheric reading.
It is important to test the atmosphere in the following order:
Photo by Glenn Drake/Philadelphia Fire Department Visual Communications Unit
Atmospheric monitoring is a significant part of a confined space rescue operational plan. This multi-function meter simultaneously monitors oxygen, carbon monoxide and combustible gases.
The first test will determine whether there is a normal or acceptable level of oxygen. One reason the oxygen level is tested first is because most combustible gas/flammability monitors are accurate only within a specified range of oxygen content. An atmosphere that is oxygen deficient or oxygen enriched may give a false flammability reading.
The initial tests of the atmosphere should be performed by an extension or remote probe before entry into the space is attempted. Every level of the space should be tested; some gases may stratify and give different readings at different levels of the space.
When monitoring for entries involving a descent into atmospheres that may be stratified, the atmospheric envelope should be tested a distance of approximately four feet in the direction of travel and to each side.
The final test is for toxicity. If you have knowledge of what chemical is in the confined space, you can use a "chemical-specific" monitor for that particular chemical. If you are dealing with an unknown chemical within the confined space, a special meter that narrows the spectrum of chemicals until it identifies the existing chemical(s) in the space must be used.
Atmospheric monitoring must occur throughout the rescue operation. Keep in mind that these situations are dynamic, and changes in the atmosphere can frequently occur due to ventilation operations, wind or humidity changes, stratification or other factors.
Once a hazardous atmosphere is identified, consideration should be given to ventilating the space. The primary goal of ventilation is to change the environment within the space to as "near normal" an atmosphere as possible. This can be accomplished by:
- Decreasing the chance of an explosion or fire by eliminating the LEL of a flammable gas within the space.
- Replacing a non-life-supporting atmosphere with an oxygen-sufficient, survivable atmosphere.
- Eliminating the toxicity of the confined space by decreasing the ppm concentration of any toxic substance present.
It is advisable to use positive pressure ventilation to transfer safe air into and displace flammable and/or toxic vapors out of the confined space. Ventilation should be done to force air into the confined space by positive pressure instead of exhausting from the confined space. Exhausting from confined spaces may be appropriate at times in very specific situations to remove heavier vapors and gases that accumulate in lower areas but the introduction of fresh air should be established first and in greater cubic foot volumes. Purging of a confined space should be at a level to dilute the interior atmosphere sufficiently.