All too often, terms like “Class-I/Division-2” or “Class-II/Group-E” stir up confusion in the minds of makers and users of industrial heating equipment. Furnaces use fuel, and fuel creates a risk of fire and explosion. Furnaces also have flames, and flames can ignite combustibles. Why aren’t all high-temperature processing areas designated as “hazardous-classified”?
The best way to think about hazardous-area classification is that it is a tool for explosion protection that is applicable to one face of the fire tetrahedron – ignition. If an environment is at least somewhat likely to contain the dangerous combination of (a) dispersed fuel, (b) oxygen and (d) the possibility of uninhibited chain reactions, then steps should be taken to eliminate or mitigate (c) ignition sources in the area. A frequent default conclusion is that area classification (and use of explosion-proof electrical devices) provides the “definitive solution” for preventing explosions, but this is a faulty paradigm.
Known Likelihood of Fuel
The purpose of designating an area “hazardous-classified” is because there is a known likelihood of fuel gas or combustible dust in the area (Division 1 = high likelihood; Division 2 = low, but finite likelihood). Accordingly, steps should be taken to reduce the likelihood of an ignition source being present concurrently with the fuel release.
On one side of the coin, when it’s impossible to fully prevent the release of fuel gas or flammable vapor into an area (let’s call it a “fuel inevitable” area), ignition sources must be eliminated by area-classification. This is typically accomplished by (a) modifying electrical devices to be non-incendive (incapable of causing ignition), (b) shielding devices from contact with the released fuel gas or (c) moving devices out of the area entirely.
On the other side of the coin, when it’s impossible to fully eliminate or shield ignition sources in an area (let’s call that an “ignition inevitable” area), area-classification can’t be used as a solution because the ignition sources can’t be moved or altered. Instead, these areas must be designated “non-classified,” and steps must be taken to prevent or minimize the likelihood of unintended fuel-gas releases.
Designating and Equipping
The two steps required for effectively implementing a hazardous-area classification strategy are designating and equipping.
NFPA 497 (which addresses flammable liquids, gases and vapors) and NFPA 499 (which addresses combustible dusts) provide guidance to facilities that are contemplating whether or not an area should be designated as hazardous-classified and, if so, how to define its boundaries.
NFPA 70, Article 500 (which addresses electrical equipment installed in hazardous-classified areas) enumerates accepted ignition mitigation techniques and gives design and performance requirements for listed electrical equipment that is installed in hazardous-classified areas.
One question that is misguided, but often asked, is whether the interior of a combustion device should be designated as a hazardous-classified area. Actually, the correct question is not “should” but “can” the interior of a combustion device be designated hazardous-classified, and the answer is emphatically NO.
Since the interior of a combustion device has the potential for several major ignition sources to be routinely present (flames, reacting gases, ignition sparks, hot surfaces, etc.), there is no way it could ever be designated hazardous-classified because area designation is a technique used to eliminate ignition sources. In order to prevent furnace explosions, precautions that prevent unintended releases of fuel (e.g., redundant safety shutoff valves, purging, ventilation, trial-for-ignition, etc.) must be taken, as required by NFPA 86.
Next month, the applicability of classifying areas exterior to furnaces will be analyzed in the context of Class A, B, C and D furnaces/ovens. IH
Demystifying Hazardous (Classified) Areas
March 2, 2011