Passing Gas: Air Circulation for Hazardous Locations

By Mark C. Ode | Feb 15, 2017




For many years, the mystery of area classification has resided with a select group of experts within the electrical, petrochemical and industrial sectors. Many electricians, electrical engineers, fire inspectors and electrical inspectors have remained outside of this group. Instead, designers and installers rely on National Electrical Code (NEC) articles 500 through 517 for installation requirements regarding hazardous (classified) locations. Rightfully so, but often, these industry personnel do not understand the original basis for these requirements.

Understanding the reasons for these requirements will provide the user with much greater capabilities to deal with hazardous (classified) locations and any abnormalities or issues that are not specifically covered in these locations.

Section 500.5 of the NEC provides the classification of hazardous locations based on the properties of the flammable gases, flammable liquid-produced vapors, combustible-liquid-produced vapors, combustible dusts and fibers/flyings that may be present and the likelihood that a flammable or combustible concentration or quantity is present. Let’s concentrate on flammable and combustible liquids and gases. I’ll cover other combustible material in a future article.

The chemical properties, chemical concentrations and air circulation surrounding the chemical are extremely important in determining the classification or area. If the chemical is a gas, vapor or liquid that has reached its flash point, determining the amount of air circulation necessary to drop the concentration of the gas or vapor becomes critical to rendering the area as unclassified or reclassifying the area.

When the amount of gas or vapor, where mixed with air (normal concentration of oxygen in air is about 21 percent with any concentration in excess of 22 percent considered to be highly oxygenated) reaches the lower flammable limit (LFL), the mixture of gas or vapor and air becomes an ignitable concentration. The ignitable concentration may be a percentage value of low LFL to an upper value of concentration called the upper flammable limit (UFL). An example of the LFL and UFL for hydrogen would be a level of 4 to 75 percent. A level of 4 percent hydrogen with 96 percent air is an ignitable concentration, and anything over 75 percent is considered to be too rich for ignition.

Another determining factor in hazardous area classification is whether the gas or vapor molecular weight is heavier than air, the same weight as air, or lighter than air, where air is considered to be a vapor density of 1. If the gas or vapor is heavier than air (e.g., gasoline at a vapor density of 3), the gas or vapor will fall to grade level because it is three times heavier than air. If the vapor density is the same as air (vapor density of 1), the gas or vapor will suspend close to its release point and neither fall nor rise. If the vapor density is less than 1 (e.g., hydrogen at a vapor density of 0.1), the gas or vapor will rise above grade level.

The location of the concentration of gas or vapor is critical because dispersal or removal of the gas or vapor at the point of concentration would be best closer to ground or grade level for heavier-than-air materials and at the ceiling for lighter-than-air materials. A level of 25 percent of the LFL of most gases or vapors will ensure that an ignitable concentration does not exist.

An example of this 25-percent concentration level would be hydrogen with an LFL of 4 percent concentration. Reduced to a 1 percent concentration, the hydrogen is not ignitable. Adding heat to a gas or vapor or cooling the gas or vapor can affect the molecular weight of the material. A lighter-than-air gas, such as hydrogen, can be super-cooled to the extent that it changes from a gas to a liquid and only reverts back to a gas when it returns to normal temperature or the flash point of the liquid hydrogen. This extremely cold level for combustible material is called “cryogenic material.”

An area’s ventilation is important in determining its classification. A flat floor can be ventilated with four air changes per hour or 1 cubic foot per minute per square foot of area to ensure the combustible gas or vapor is adequately dispersed or removed. Where the floor area has pits, below-grade work areas, or subfloors, the ventilation should be within 12 inches of the pit floor, below grade and subfloor work area.

Next month’s column will cover the method of determining the airflow necessary based on the molecular weight of a chemical or a mixture of chemicals.

About The Author

ODE is a retired lead engineering instructor at Underwriters Laboratories and is owner of Southwest Electrical Training and Consulting. Contact him at 919.949.2576 and [email protected]

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