Not all “simple asphyxiants” are created equal; thus, not all make for good purging or blanketing. Would you believe me if I told you that one very common simple asphyxiant gas should NEVER be used for purging or blanketing flammable atmospheres?
Many folks view Nitrogen (N2) and Carbon Dioxide (CO2) as very similar. This is based on how we, as humans, respond to both of these gases. CO2 has been getting a lot of attention lately due to its increasing use in the food and beverage retail industry and industrial refrigeration. It has some great qualities for its intended uses, but it comes with a significantly increased risk to workers who normally do not work around hazardous chemicals.
Inerting with compressed carbon dioxide gas may be dangerous. The contents are under pressure, and static electricity can build up during gas discharge. More importantly, the compressed CO2 has a much larger temperature difference with the outside atmosphere than bottled nitrogen. This difference leads to condensation, which increases the generation of static electricity.
If CO2 cylinders are used, the nozzle should be properly bonded or grounded, and the gas should be introduced slowly to reduce static.
CO2 fire extinguishers should not be used to inert flammable atmospheres. Since solid and gaseous CO2 is discharged from the nozzle, this “two-phase now” generates static electrical charges.
From NFPA 77, 14.5.1:
| For example, CO2 discharged under pressure will form charge solid “snow”. In an ignitable atmosphere, this phenomenon can create an ignition hazard. For that reason, carbon dioxide from high-pressure cylinders or fire extinguishers should never be used to inert a container or vessel. |