Calculating a Hazardous (Classified) Area: Determining air-circulation requirements for gases and vapors

Shutterstock / Mrs_YA
Shutterstock / Mrs_YA
Published On
Nov 15, 2021

When an area containing flammable or combustible material is properly classified and the electrical installation is appropriately designed and installed, these areas are no more dangerous than any other location.

The key to ensuring the hazardous (classified) location’s safety is based on the chemical characteristics of the material being handled, processed or stored in these areas and often the amount of air circulation in the area. Electrical hazardous (classified) locations are covered in Chapter 5, which deals with special occupancies, in the National Electrical Code. Articles 500 through 504 provide the scope for electrical and electronic equipment requirements and wiring for all voltages where fire or explosion hazards may exist due to flammable gases and liquid-produced vapors, combustible liquid-produced vapors, and dusts or ignitable fibers/flyings.

Hazardous location requirements

Code-Making Panel 14 uses information from other NFPA codes and standards, as well as the vast knowledge of the individuals on the panel, to determine the necessary requirements for electrical installations in these areas. Determining the flammability or combustibility of a material is necessary to correctly identify the proper classification of the area under consideration, and applying the appropriate air circulation will help determine the extent of the classified area.

Flash point is defined as the minimum temperature of a liquid at which sufficient vapor is given off to form an ignitable mixture with air. A flammable liquid has a flash point below 100°F, and a combustible liquid has a flash point at or above 100°F.

A flammable or combustible material must normally reach its flash point before an explosion can occur. A common example is gasoline, a flammable liquid, which has a flash point at around –50.8°F (46°C). However, diesel fuel, a combustible liquid, has a flash point of 125–180°F, depending on the type. Diesel fuel would require heating above the normal ambient temperature for ignition to occur, while gasoline will reach its flash point in extremely low temperatures.

Once a material has reached its flash point with enough vapor mixed with air, an ignition source is often required, with some flammable or combustible materials igniting more easily than others. Autoignition temperature is the minimum temperature required to initiate or cause self-sustained combustion of a solid, liquid or gas independently of the heating or heating element. Autoignition can occur where the material increases in temperature enough to ignite without an external flame source.

The lower flammable limit (LFL) and upper flammable limit (UFL) apply to the material’s vapor concentration in the air. If the concentration is below LFL, then it is normally too lean to ignite, but if it is above the UFL, then it is too rich to ignite.

NFPA 497, Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas, as well as other NFPA documents, state that keeping the concentration of the gas or vapor to 25% or less of the LFL will reduce the probability of an ignitable concentration. For example, hydrogen has an LFL of 4%, so keeping the concentration at 1% or lower would ensure the concentration is not ignitable. The priority for the safest applications for hazardous (classified) locations is designing a ventilation system that would keep the gas or vapor concentration to the appropriate level of 25% of the LFL.

The appropriate air-circulation system design requires determining the molecular weight of the flammable or combustible gas, vapor or mixture where there are complex mixtures. The molecular formula of the gas or vapor can be used, the material safety data sheet can be obtained from the material manufacturer or the chemical abstract service number can be used to identify the material and locate the molecular weight.

The material’s vapor density is compared to air, which is 1. Gasoline has a vapor density of 3, so it is three times heavier than air. Hydrogen is 0.1, so it is 10 times lighter than air. To figure the molecular weight of a hydrocarbon, one must determine it using the molecular weight of carbon (12) and hydrogen (1). To determine the air circulation necessary for most hydrocarbons, the fugitive emissions calculation located in Annex F in NFPA 30, the Flammable and Combustible Liquids Code, can be used as a guide.

The calculation for determining the hazardous (classified) area and the necessary air circulation involved for the chemicals are complex and require some studying and guidance, but I have tried to provide a basis for understanding the requirements found in the NEC , NFPA 497 and NFPA 30.

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and

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