Home | Business News | Browse by Publication | C | Chemical Engineering

Don''t get burned--know the limits of flammable and combustible fluids: it is important to know what mixtures are ignitable and when they have been diluted enough to be considered safe from ignition. (Feature Report).

Article Excerpt
What are flammable and combustible liquids? Obviously, they are liquids that burn. They may be miscible with water, like ethanol and acetone, or immiscible, like hexane and xylene. If immiscible, they may be denser than water, like chlorobenzene. More likely, though, they float on water. They may also be toxic, corrosive, or reactive. They are almost always organic. But specifically, they burn.

Not everything that burns is a flammable or combustible liquid. For instance, solids and gases may burn, but when burning, behave differently from liquids. Solids do not flow, but can send off glowing embers. Gases, on the other hand, flow quite easily and do not rely on evaporation across a liquid interface to enter a burning state.

Flammable vs. combustible. The words "flammable" and "combustible" have very specific meaning within the context of fire prevention and suppression. In general, a flammable liquid burns more easier than a combustible liquid. More specifically, the Flammable and Combustible Liquids Code of the National Fire Protection Assn. (NFPA; Washington, D.C.; nfpa.org) makes the distinction between a flammable and a combustible liquids based on the liquid's boiling point and flashpoint (Figure 1).

[FIGURE 1 OMITTED]

The division between flammable and combustible is at a flashpoint of 100[degrees]F. That temperature represents the upper limit of ambient temperatures one might expect to encounter in all but the hottest climates.

Liquids with a flashpoint below 100[degrees]F are flammable. More precisely, they are called Class I flammable liquids. Liquids with a flashpoint above 100[degrees]F, that is, liquids that do not burn readily until they are above ambient temperatures, are combustible. Liquids with a flashpoint between 100[degrees]F and 140[degrees]F are Class II combustible liquids. Liquids with flashpoints above 140[degrees]F--coincidentally, the same temperature above which safety engineers begin requiring insulation for personnel protection against thermal burns--are defined as a Class III combustible liquids.

There is more to this classification system, though. Class I flammable liquids are further divided into three sub-classifications. Class III combustible liquids are divided into two sub-classifications. Only Class II combustible liquids are left without further sub-classification.

NFPA divides Class III combustible liquids into Class IIIA (flashpoints between 140[degrees]F and 200[degrees]F) and Class IIIB (flashpoints above 200[degrees]F). The dividing temperature, 200[degrees]F, is once again a nice round number, this time approaching the boiling point of water or, more importantly, the temperature of steam condensate at atmospheric pressure. The distinction is interesting from the perspective of code compliance, because in many regards, Class IIIb combustible liquids are treated much the same as noncombustible liquids.

The sub-classification of Class I flammable liquids is first by flashpoint, and then by boiling point. Class I flammable liquids with flashpoints between 73[degrees]F (room temperature) and 100[degrees]F (hottest normal ambient temperature), are Class IC flammable liquids. If the flashpoint of a liquid is less than room temperature, then the NFPA looks at boiling point. If its flashpoint is less than room temperature and its boiling point is higher than 100[degrees]F, that is to say it does not boil at normal ambient temperatures, then a liquid is defined as a Class IB flammable liquid.

If its flashpoint is less than room temperature and its boiling point is less than 100[degrees]F (it boils at an ambient temperature) then a liquid is defined as a Class IA flammable liquid--the most dangerous of all the flammable and combustible liquids. Storage, handling, and use of these chemicals require the greatest restrictions.

To fully appreciate the NFPA Classification System, it is important to understand how the terms "boiling point" and "flashpoint" are defined by the codes. Boiling point, at least for a pure component, is exactly what a chemical engineer would expect it to be: the temperature at which the vapor pressure of a liquid equals 1 atm (14.696 psia or 760 torr). For mixtures where the vapor pressure changes as the mixture evaporates, the code considers the boiling point to be the temperature of the mixture that has a vapor pressure of 1 atm after the first 20% has evaporated.

Flashpoint is the minimum temperature of a liquid at which sufficient vapor is given off to form an ignitable mixture with air. So, by definition, neither solids nor gases can have flashpoints. For liquids, it is important to understand that flashpoint refers to mixtures in air, not mixtures that have been enriched with oxygen, or purged with an inert gas, such as nitrogen.

The method for determining flashpoint is quite specific. The code refers to the Tag Closed Tester, the Pensky-Martens Closed Tester, the Setaflash Closed Tester, and Small Scale Closed Tester, each of which is called for under specific conditions. For our purposes, however, the specific method is less important than what the value tells us.

Ignitable mixtures

It is this term, ignitable mixture, upon which an understanding of flashpoint, and of fire prevention and suppression, hangs. Remember in elementary school, when the fire marshal visited your class and talked about the "fire triangle." The marshal talked about fire depending on all three sides of the fire triangle: fuel, oxidizer,...