The Fire Tetrahedron: High oxygen levels change how a fire burns

By Mark C. Ode | Oct 15, 2021

A chief electrical inspector friend wanted to know about the construction of an oxygen-generating facility in the Raleigh/Durham, N.C. area. His question was whether the construction should be based on the hazardous (classified) locations based on the National Electrical Code.

He wanted to know whether to require the entire facility to be installed as a Class I, Division 1 area or as a Class I, Division 2 area. He explained that there were multiple, existing, single-story, wooden buildings at the facility. The buildings were originally a manufacturer’s industrial facility and were being renovated by the new occupant as a medical-grade oxygen supplier. The renovation plans were under review by the city, and he raised the question of requirements for explosion-proof equipment. He had looked everywhere in articles 500 and 501 and could not find any mention of installation requirements for this type of facility.

I explained that oxygen is not flammable by itself, but it is one of the elements or ingredients necessary for most fires. Many people are familiar with the concept of the “fire triangle” or “combustion triangle,” with the triangle being the three materials needed for a fire: an oxidizer (usually oxygen), fuel and an ignition source. But most are not aware that this concept is more correctly a fire tetrahedron.

The fourth piece in the tetrahedron is a chemical chain reaction. Once a fire is started, an exothermic chain reaction sustains the fire and allows it to continue until or unless at least one of the elements in the fire triangle is blocked or goes away. If the fuel is burned away, the fire stops. If the temperature falls below the ignition temperature by applying water to fire, then the fire is extinguished. If the fire is deprived of oxygen by closing off the space, using a foam extinguisher or applying an inert gas (one that is not ignitable under any circumstances), then the fire is put out.

Nitrogen gas is a common inert gas that is also present in air, but it can be used in higher percentages to reduce the amount of available oxygen in the surrounding air and reduce the flammability of the combustible material.

If a closed area limits the amount of oxygen below the levels normally present in air (approximately 21% by volume), then the fire may not go out. However, it will smolder and then re-ignite when a new supply of oxygen is re-introduced. This is an example is what firefighters call “rich flashover” or “backdraft,” where a fire source in an enclosed area superheats the material and the enclosed area is extremely hot, and then a new source of oxygen is introduced by opening a door or window, resulting in a rapid, explosive fire.

Higher-than-normal oxygen levels in the air can change the combustible materials’ burning characteristics by causing the flame temperature to be hotter than normal. Any nitrogen in the area can absorb the added heat caused by the increased oxygen levels and act as a heat transfer to combustible materials, with a resulting decrease in burning time.

Any location where there is an increase in normal oxygen levels, such as in a hospital, can therefore be a concern. In intensive care units, some patients receive supplemental oxygen through a tent over their head and shoulders or even their entire body. Other hospital patients are given oxygen directly in the nose through a nasal cannula or mask, with oxygen levels at 24%–40%. The oxygen levels may be fairly high in close proximity to the nasal cannula or mask, but fall back to around 21% within a short distance from the patient’s face or nose.

I told my chief electrical inspector friend that introducing a higher level of oxygen (above normal air levels) into a building made of wood could change the building’s burning characteristics, but it would not involve an electrical hazardous (classified) location. Of course, high levels of oxygen can certainly migrate into the wood and would need to be treated differently by firefighters who might be responding to a fire.

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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