“The name is the thing, and the true name is the true thing. To speak the name is to control the thing.”  — Ursala K. Le Guin, The Rule of Names

Process safety has three concerns: fires, explosions, and toxic releases. The most ancient of these concerns is fire.

Because fire has been our servant and our master for so long, it has the largest vocabulary associated with it of the three. And because there are so many terms for fire—blaze, conflagration, combustion, flame, flash, ignition, inferno, holocaust, and spark, to list a few—most talk about fire is imprecise.

The purpose of technical jargon is to give precision to otherwise imprecise terms. Fire is no exception. There are words in common usage that safety professionals have taken and narrowly defined. Writers, reporters, and pundits may not use these terms with technical precision, but we safety professionals should.

What’s Burning?

One of the ways we classify fires is by what is burning. This is especially important when choosing how to extinguish the fire. Water, the firefighter’s extinguishant of choice, is great for some classes of fire and terrible for other. These are the five classes of fires and how they behave with water.

  • Class A. Ordinary combustible solids, like paper, wood, fabrics and some plastics. What makes them “ordinary” is not that they are harmless, but that water is perfectly appropriate for extinguishing them.
  • Class B. Flammable liquids. There are several things that distinguish Class B fires from Class A fires. First, the fire spreads across an accumulation of the fuel much faster. In either Class A or Class B fires, it is not the solid or liquid that is burning, but the vapors they emit. Liquids vaporize much more readily, so a fire will spread across the surface of liquid much faster than it will across the surface of an ordinary combustible solid. Second, most flammable liquids are less dense than water and are immiscible with water, meaning that they float on top of water. Applying water as an extinguishant spreads the fuel, spreading the fire. The term, “Class B,” applies to all flammable liquids, even those miscible with or denser than water.
  • Class C. Electrical fires. Class C fires are either Class B fires, or more likely, Class A fires, but where live electrical equipment is present. Obviously, there is a concern about trying to extinguish a fire involving live electrical equipment with an electrically conductive extinguishant. Like water.
  • Class D. Combustible metal. When we think of combustible metals, most of us think of lithium, sodium, magnesium, and titanium. All metals oxidize, however. When divided finely enough, metals will oxidize very quickly, becoming a fire. That means that metal powders are combustible. This includes aluminum and zinc, but also includes nickel and iron. The problem with combustible metal is that it is water reactive, frequently producing hydrogen—a flammable gas—as the reaction product.
  • Class K. Cooking oil and grease. In Europe and Australia, these are called Class F, fires, but in the U.S., we like “K” for kitchen. Strictly speaking, Class K fires are a subset of Class B fires, but special extinguishing agents react with the fatty acids, forming a saponified (soapy) crust on the burning liquid that extinguishes the fire.

Flammable vs. Inflammable

William Strunk wrote The Elements of Style in 1918, and we’ve been bullied ever since about the use of the word “flammable”:

  • An oddity, chiefly useful in saving lives.  The common word meaning “combustible” is inflammable.  But some people are thrown off by the in- and think inflammable means “not combustible.”  For this reason, trucks carrying gasoline or explosives are now marked FLAMMABLE.  Unless you are operating such a truck and concerned with the safety of children and illiterates, use inflammable.

Since we use the word to save lives and are concerned with the safety of children and illiterates (and English professors for that matter) let’s all feel free to use the word “flammable” without apology.  Even Strunk and White favored clarity over style.

Deflagration vs. Conflagration

In addition to terms describing what is burning, we have terms to describe how the fire is burning. Three of them end with “-ation.” Deflagration and detonation have specific meanings in the context of process safety. Conflagration? Not so much.

  • Deflagration. Combustion where a flame front propagates through a flammable mixture in gas at subsonic speeds. The mixture can be a mixture of flammable gases or vapors and oxidizing gases (like air), a mixture of flammable liquid aerosols and oxidizing gases, or a mixture of combustible dusts and oxidizing gases. The flame front propagates by thermal conductivity. Vapor cloud fires and fireballs are deflagrations.
  • Combustion where a flame front propagates through a flammable mixture in gas at sonic or supersonic speeds. The flame front propagates by shock compression. It is this shock wave that is characteristic of explosions. In addition to dispersion, it takes confinement to accelerate a flame front past the speed of sound to a detonation.
  • A raging, destructive fire. In urban settings, a conflagration involves several structures and at least one full city block, although some don’t consider a fire a conflagration unless it jumps across streets to engulf more than a city block. In rural settings, like a forest, a conflagration is characterized as being big. Very big. Whatever that means.

In process safety, we encounter a couple of other terms to describe how fires are burning.

  • Pool fires. A fire burning at the surface of a pool of flammable liquid. What is burning is the flammable vapor released from the liquid, mixed with air. For a more thorough discussion of the technical aspects of pool fires, you can see the piece we put together on pool fires.
  • Jet fires. An ignited flammable fluid released under pressure through an orifice. Anyone who has ever seen a welding or cutting torch has seen a jet fire. Most gas heaters rely on jet fires as well, although at very low pressures. As a hazard, however, we don’t see jet fires often, which probably explains why the EPA does not address jet fires in its Risk Management Program Guidance for Offsite Consequence Analysis.

Flash Point vs. Autoignition Temperature

There are two terms that are often confused: “flash point” and “autoignition temperature.”

  • Autoignition temperature. The temperature at which a material ignites in dry air at atmospheric pressure in the absence of any ignition source. Only combustible or flammable materials have an AIT. The measured AIT depends on the test method, the amount of time allowed for ignition to occur, and the form of the material. ASTM E659 stipulates that AIT is in dry air at atmospheric pressure. Humidity makes ignition harder, raising the ignition temperature, and increased oxygen concentration makes ignition easier, lowering the ignition temperature.
  • Flash point. The temperature at which the vapor pressure of a flammable liquid is exactly high enough to form a mixture with air at the lower flammable limit (LFL) of the flammable liquid. Flammable gases do not have flash points.

The flash point for any flammable liquid is much lower than the AIT. For instance, while the AIT for pure ethanol is 365 C (689°F), the flash point is 13 C (55°F). An ignition source can set ethanol alight well below room temperature, but ethanol won’t spontaneously burst into flames until it is hotter than the capability of most residential ovens.

For more detail on the technical aspects of flash point, autoignition temperature, and related concepts, you can see the piece we put together on autoignition temper.

Words Matter

Fire is an important tool for all of industry, including the chemical process industries. As Asimov said, “The foundation of all technology is fire.” It can also be a scourge. Process safety professionals need to be concerned about fire. If we’re going to address those concerns, however, we need to make sure that we’re all talking about the same thing.


  • Mike Schmidt

    With a career in the CPI that began in 1977 with Union Carbide, Mike was profoundly impacted by the 1984 tragedy in Bhopal and has been working on process safety ever since.