A fire erupted on May 4, 2009, at the Veolia ES Technical Solutions Hazardous Waste Facility in West Carrollton, Ohio, injuring two workers. The U. S. Chemical Safety Board suggested that poor zone classification might have contributed to the incident.
The mission of Electrical Area Classification (EAC) is to prevent a leak from leading to something far worse. However, a hazard classification has consequences beyond safety -- it can complicate efforts to fully utilize expensive plant real estate.
EAC is an arcane science. As an "expert" in EAC, let me attempt to make it less mysterious.
The first step in classifying a zone is to identify the chemistry there. Are you dealing with dust, flammable or combustible liquid, flammable gas or something harmless? If material balance information isn't available, use Material Safety Data Sheets (MSDSs) to define chemicals in the area.
The National Fire Protection Association (NFPA) provides several references to explain EAC: NFPA-30, "Flammable and Combustible Liquid Code;" NFPA-499, "Recommended Practice for the Classification of Combustible Dusts and of Hazardous Locations for Electrical Installations;" and NFPA-497, "Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous Locations for Electrical Installations in Chemical Process Areas." In addition, the American Petroleum Institute (API) offers "Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Division 1 and Division 2."
Another resource, the National Electric Code (NEC) Standard 500, divides hazards into three classes: Class I -- gases and liquids, Class II -- dusts, and Class III -- fibers. It further categorizes liquids from A to D in order of decreasing risk. In contrast, NFPA-30 Section 1.7 defines liquids in the following categories: Class I -- flammable liquids with flash point <100°F, Class II -- combustible liquids with flash point ≥100°F and <140°F, and Class III -- combustible liquids with flash point ≥140°F. The NEC-500, upon which NFPA-497 and API-500 are based, subdivides ignitable vapors and gases into groups: A -- acetylene; B -- 1,3 butadiene, ethylene oxide, propylene oxide and H2; C -- acetaldehyde, CO, ethylene and H2S; and D, the most common -- acetone, ethanol and other hydrocarbons.
Classifying dusts and fibers is more difficult. If a dust layer forms, is less than 1/32-in. thick after 24 hours and its surface color is discernable, an area can be unclassified. Accumulated dust is more dangerous than clouds; risk is greatest when conditions are dry -- as in winter.
Another means of identification for gases and vapors used by both NFPA and API is the relative density compared to air. Heavier-than-air vapors pose a greater risk because they hug the ground. Light or hot gases rise and disperse.
Vapors and gases aren't the same. In EAC work, a vapor is a gas that condenses at 100°F or less at ambient pressure. At these conditions, a gas has a compressibility factor, Z, of 1; a wet gas has a Z less than 1. Vapors are especially dangerous because they can evaporate and recondense somewhere they're likely to start a fire -- that's why butane, with a boiling point of 31°F, is so dangerous!