What combustible gases are associated with confined spaces?
Combustible gases and vapors can be produced by materials used or stored in confined spaces, as well as natural processes. The most common combustible gas found in confined spaces is methane (CH4), which can be easily produced by microbial decomposition of materials like sewage and rotting vegetation. Methane is lighter than air, and tends to rise. It often accumulates immediately beneath lids or hatches at the top of the confined space.
Combustible gas explosions often occur at the time the confined space is initially opened or disturbed. OSHA 1910.146 specifies that untested permit confined spaces may not be disturbed in an unsafe manner. Use of sampling pumps and other special techniques are often required to sample the atmosphere in the space before proceeding with pre-entry procedures.
Gases in confined spaces tend to form density-dependent layers. Lighter-than-air gases such as hydrogen and methane accumulate near the top of the space. Heavier-than-air combustible gases and vapors such as propane, butane, and solvent vapors accumulate near the bottom of the space.
It is important to sample all levels in the space during evaluation procedures. Failure to sample near the bottom of the space may result in missing or underestimating the true concentration of combustible gas in the space.
Even after they are drained, the atmosphere in chemical or fuel storage tanks often continues to contain significant concentrations of potentially flammable vapor. It is critical to make sure that instruments used to monitor the atmosphere are able to detect and measure the gases and vapors that are potentially present.
Besides the explosive hazard, many combustible gases are also toxic at very low concentrations. A good example is n-hexane (the “n” indicates the straight chain or “normal” form of the hexane molecule). The LEL concentration for n-hexane is 1.1% volume ( = 11,000 ppm).However, the OSHA permissible toxic exposure limit is only 500 ppm (calculated as an 8-hour TWA exposure limit). In the case of n-hexane, the toxic exposure limit is reached long before the concentration reaches 10% LEL ( = 1,100 ppm).
Introducing Hazards into Confined Spaces
The work performed in confined spaces can produce dangerous atmospheric conditions. Many accidents result from hazardous conditions that did not exist at the time the entry was initiated. Procedures such as welding, painting, de-greasing, scraping, sandblasting, mucking, and inerting can easily create hazardous atmospheric conditions.
It is important to continue to monitor the atmosphere according to the requirements of the entry permit as long as the entry is underway. Welding, heating, burning, and other hot work procedures which involve the use of torches, rods, and other sources of ignition, or fuel gases such as acetylene or propane, require a special permit or a section of the general confined space permit that addresses the additional hazards.
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