Don't Zone Out On Area Classifications

Electrical ratings must start with analysis of chemicals present.

By Dirk Willard, Contributing Editor

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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.

Vapors are especially dangerous because they can evaporate and recondense.

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!

Determining whether a chemical is a hazard is the most difficult challenge in EAC work. Here're some general guidelines for classification: 1) only consider NFPA-30 Class I liquids -- Class II and Class III liquids don't produce enough vapor, unless heated; 2) treat a fire danger of at least 3 in an NFPA diamond (refer to NFPA-704) as Class I; 3) categorize mixtures with >30% H2 by volume as Group B (API-500, 5.5.5) and mixtures with >25% H2S as Group C (API-500, 5.5.4) -- NFPA-497 Annex B provides a rule for estimating the NEC group for mixtures of vapors or gases but it doesn't work for H2 and H2S; 4) when in doubt about the chemistry use the worst case in evaluating a hazard; and 5) if you can't measure the molecular weight (MW) of a complex organic, you can get a good estimate for n-alkanes with MWs from 80 to 1,400 via the equation in "Select the Right Hydrocarbon Molecular Weight Correlation" by Donald Schneider of Stratus Engineering: MW = 3.3955×10-15Tbf 6 - 1.2416×10-11Tbf5 + 1.8256×10-8Tbf4 - 1.3234×10-5Tbf3 + 5.2285×10-3Tbf2 – 0.741692Tbf + 116.19, where Tbf is the boiling point.

EAC is based on the auto-ignition temperature (AIT) -- the minimum temperature at which a combustible material will burst into flame without an external ignition source. AITs reported in MSDSs often aren't tested for the particular mixture but reflect testing of a pure compound. There are no mixing rules for the AIT, which severely handicaps studies. Use the following simplification if no other data are available: for compositions containing compounds with MW exceeding 50 use AIT = 280°C; for others, use 200°C.

In later columns, I'll discuss enclosures, preparation of EAC drawings, defining envelopes and remediation.


DIRK WILLARD is a Chemical Processing Contributing Editor. You can e-mail him at dwillard@putman.net

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  • Class II is for combustible dusts and Class III is for combustible fibres and flyings, NOT liquids as stated in this article.

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  • Never mind that's for NFPA-30 classes of liquids not NEC area classes.

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