Flammable and combustible liquids rank close to the top among workplace fire hazards. They burn readily and intensely, and can be highly explosive under the right — or shall we say, wrong — conditions.

These liquids include gasoline, diesel fuel, kerosene, benzene and alcohol, however the full list is much more extensive. Flammable liquids are used so frequently that we sometimes lose sight of their awesome destructive force. We regard them as helpful friends while neglecting their uglier side — the side that results in fire, explosion, injury and death.

What is it about flammable liquids that creates the hazard? What characteristics must be controlled to reduce catastrophic results?

The chemistry of fire
A review of the elementary chemistry of fire will be helpful. Although combustion is more complex than the phenomenon represented by the customary “fire triangle,” we can nevertheless illustrate the hazard in this way. Three essential ingredients must be present to permit a fire to occur: fuel, oxygen and heat. Remove any one of these elements and there can be no fire. Safety equipment for flammable liquids is designed to control one or more legs of this “triangle.”

Actually, it is the vapors from flammable liquids that present the hazard. Flammable liquid vapors, when mixed with air, can ignite so fast that witnesses often describe the ignitions as explosions. Once ignited, the speed of heat generation causes additional vapors to form even more rapidly. Thus, uncontained flammable liquid fires are extraordinarily fast-spreading and violently destructive.

Let’s look at some key definitions to help gain a better understanding of this potential workplace hazard.

Flashpoint — Flashpoint is the lowest temperature at which a liquid gives off enough vapors to form an ignitable mixture with the air near the surface of the liquid. It is the primary measurement used in classifying liquids. This concept divides liquids that will burn into three categories. It is assumed that indoor temperatures can reach 37.8°C (100°F). Therefore, liquids with flashpoints below 37.8°C are called “Class I liquids” and are classified as “flammable.”

In some areas, ambient temperatures might exceed 37.8°C. Only slight heating would be required to raise the temperature of the liquid to its flashpoint. An arbitrary division of 37.8°C (100°F) to 60°C (140°F) was established for liquids in this flashpoint range, known as Class II liquids. Since liquids with flashpoints higher than 60°C would require considerable heating from a source other than ambient temperatures before ignition could take place, they have been termed Class III liquids. Both Class II and III liquids are further designated as “combustible.”

Explosive or flammable range — Explosive or flammable range is used to define the limits of flammability. It may be explained as the range of concentration of a combustible in an oxidant through which a flame, once initiated, will continue to propagate at the specified temperature and pressure.

For example, gasoline vapors mixed with air will propagate flame between 1.4 and 7.6 percent by volume of gasoline at 21°C (70°F) and atmospheric pressure. As the temperature increases the flammable range widens. A decrease in temperature can cause a previously flammable mixture to become non-flammable by placing it either above or below the limits of flammability for the specific environmental conditions. The extremities of the “range” are referred to as the lower and upper flammable limits.

Ignition temperature — Ignition temperature is the minimum temperature to which flammable liquid vapor in the air must be heated in order to initiate or cause self-sustained combustion independently of the original heat source. It is too easy to discount the high hazards of flammable liquids when noting relatively high ignition temperatures. Gasoline’s ignition temperature is commonly shown above 500°F. You must constantly remind yourself that an extremely small area and duration of temperature contact is all that’s needed to set flammable vapor aflame.

Specific gravity — Specific gravity is important in fire prevention planning in order to anticipate the behavior of the liquid where water is present under fire conditions. Many flammable liquids with specific gravity less than 1 (lighter that water) are also insoluble in water. Water, therefore, may be totally ineffective as an extinguishing agent.

Vapor density — Vapor density is another characteristic that must be recognized when establishing standards and procedures for the storage of flammable liquids. The term refers to the weight of a volume of pure gas (vapor) compared to the weight of an equal volume of dry air at the same temperature and pressure. A figure greater than 1 indicates the gas is heavier than air. The vapor density of gasoline is 3.4, indicating that these vapors will settle to the floor, seek low pockets and accumulate. Almost all flammable liquid vapors have a vapor density greater than 1 and behave in a similar way.

Proper containment
Controlling flammable liquids begins with containment. The safe handling and storage of flammables demands specific practices as well as specially designed equipment.

OSHA’s Standard 29, Parts 1910 and 1926, and NFPA 30, Flammable and Combustible Liquid Code, cite the nationally mandated do’s and don’ts for storage and handling procedures.

Underwriters Laboratories and Factory Mutual Research have developed equipment standards and testing procedures.

Safety cans — FM’s approval standard covers performance requirements relating to container capacity, materials, flame arresters, openings, vacuum venting, pressure venting, nozzle, handle, seams and bottom.

Cans are examined and tested for stability, leakage, pouring spout and handle strength, fire exposure, abuse and flame arrester effectiveness. Containers molded of plastic materials undergo additional examination and testing.

Flammable liquid storage cabinets — UL’s cabinet standard indicates it covers cabinets intended to be used in compliance with the flammable and combustible liquids code, NFPA 30.

Cabinet construction including wall thickness, joint fastening, leakage sill, door locking and latching, venting, means for grounding and air space requirements are all designated.

Cabinets are tested for fire endurance as prescribed by Underwriters Laboratories’ own protocol. Load tests are conducted to assure the cabinet will meet the manufacturer’s rated liquid capacity.

Note: UL also has a standard for safety cans, and FM has its own standard and test procedure for cabinets.

The UL or FM label assures that the equipment meets independently established requirements. Look for the label.

For safety’s sake, treat flammable liquids with respect, storing and using them properly.