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Vapors that are heavier than air, like propane, do not dissipate when they are released. Instead, they sink, accumulating along the ground or in enclosed spaces, and building up to explosive or toxic concentrations. Yesterday, we discussed how to identify heavier-than-air gases and their hazards. Today, let’s talk about how to protect workers.
Worker Protection Strategies
To control the hazards of heavier-than-air vapors, use a combination of awareness, source control, ventilation, and updated code compliance.
Awareness. Pay attention not only to chemicals and activities that are part of normal production processes but also to chemicals and activities that are unusual. The decommissioning of a process vessel, for example, may put hot work and flammable chemicals together, while painting an office could fill a normally habitable room with noxious vapors.
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Remember also that not all hazardous, heavier-than-air vapors may be generated on your site. If your site is located in an industrial area, vapors may flow from off-site onto your site, collecting in low-lying areas and creating a hazard. What sorts of chemicals may be flowing downhill toward you? Are you situated near a landfill or a petrochemical facility? Know what your neighbors are doing.
Source control. Preventing chemicals from entering the air is an important way to control the levels that accumulate outside of process tanks and vessels. Proper maintenance of process vessels and piping can prevent leaks. Safe work practices, such as permitting an oven to completely vent before it is opened, will help minimize releases of chemicals. Proper storage (for example, inside a flammables cabinet) helps control the release of vapors, as does minimizing the use of hazardous, heavier-than-air chemicals.
Ventilation. Properly designed ventilation is key to the control of heavier-than-air vapors. Ensure that vents are positioned correctly. In laboratories, for example, chemical storage cabinets may need to be ventilated from the bottom rather than from the top to collect accumulated vapors. In dry-cleaning establishments, however, floor-level ventilation was found to be ineffective; ventilation worked better when it was positioned to capture vapors at the source.
For flammable vapors, ventilation systems must be explosion-proof. For toxics, care must be taken not to recirculate the vapors within the facility. Do your homework carefully when designing ventilation systems for heavier-than-air vapors.
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Code compliance. Another issue—pointed out by the Little General explosion and by a long history of residential explosions in which gas cans were stored too close to natural gas water heaters equipped with open pilot lights—has to do with up-to-date code compliance. In the Little General explosion, the existing propane tank was located against the back wall of the convenience store, under the eaves. When propane was released, it immediately infiltrated the store. Since that tank was installed, codes had changed to require propane tanks to be located at least 10 feet from any structure.
Codes for natural gas water heaters have also changed to require that they be installed off the floor and to require that pilot lights be protected. If you have heavier-than-air flammable vapors in your workplace, make sure that your equipment and storage are up to current codes.
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