Fire is the visible heat energy being released from rapid oxidation of a fuel. Something is "on fire" when the exothermal release of heat from the oxidation reaction reaches visible light level.

Basically, combustion takes place within two modes flaming and flameless. The flaming mode can be viewed as a tetrahedron, with each side representing one of the four basic components of flaming combustion: fuel, heat, oxygen and uninhibited chain reaction. Here the practical emphasis is on extinguishing an existing fire.

The classic fire triangle illustrates the flameless mode, the three legs representing fuel, oxygen and heat. Air provides oxygen for combustion. The heat required for ignition varies with the characteristics of the fuel. Fuel is any substance that will sustain combustion after the initial application of heat to start it. Here the practical emphasis is on preventing a fire from starting by prohibiting the formation of the triangle.

How Flammable Liquids Safety Equipment Defeats Fire

Safety equipment for use in storing, transporting or dispensing flammable liquids is designed to control one or more of the legs of the Fire Triangle.

Containment of the liquid fuel to prevent it from spreading in event of fire is a primary function of all safety containers. These containers include safety cans, rinse and wash tanks, bench and plunger cans, drip cans, storage cabinets, waste containers and others.

Dissipation of heat to prevent flammable liquid vapor from reaching ignition temperature is another function built into certain types of safety equipment. This is accomplished by the flame arrester which is common to safety cans and faucets, bench cans and other equipment. The flame arrester, in the form of a wire mesh screen or perforated baffle plate, permits escaping vapor to burn but dissipates heat so that vapor inside the container will not ignite or explode.

Closing out oxygen is still another function of certain safety containers. For example, when the lids of self-closing rinse and cleaning tanks shut, they snuff out fire by closing off the oxygen supply.

Characteristics of Flammable Liquids

In order to best understand the hazards of flammable liquids and the control the following definitions will prove useful.

Classifications: Flammable and Combustible Liquids.
A flammable liquid is one having a flash point below 100 F and having a vapor pressure not exceeding 40 lb/sq in. absolute at 100 F.

A combustible liquid is one having a flash point at or above 100 F.

Vapor. In any liquid there is a constant movement of molecules. As temperature increases, the molecules speed up, some acquiring enough energy to escape from the liquid surface as a vapor. When vapor escapes from a flammable liquid into the air, a flammable or explosive situation can occur, dependent upon the proportions of the air/vapor mixture.

Flash point is the lowest temperature of a flammable liquid at which it gives off vapor sufficient to form an ignitable mixture with the air near the surface of the liquid or within the vessel used. Combustion is not continuous at the flash point Flash point is the basic characteristic used by NFPA to classify the relative hazards of liquids.

Ignition temperature is the minimum temperature to which flammable liquid vapor in air must be heated in order to initiate or cause self-sustained combustion independently of the original heat source.

One might tend to discount the high hazards of flammable liquids when noting the relatively high ignition temperatures [gasolines, for example, are in the 500 to 800 F ranger. But what must be recognized is that an extremely small area and duration of temperature contact is all that's needed to set flammable vapor aflame. A static spark with the duration of a few thousandths of a second, contacting a few molecules of the vapor/air mixture is enough to raise the temperatures above the ignition point.

Flammable (explosive) range of flammable liquids is the percentage range of liquid vapor in air, by volume, within which ignition can occur. Gasoline, for example, has an explosive range between 1.4% and 7.6%. This indicates that any concentration of gasoline vapor in air between these percentage limits will ignite at any temperature above -45F (flash point) when an ignition source provides a contact temperature in the range of 500 to 800 F (ignition temperature, depending on type of gasoline). Auto engine carburetion is designed to keep the gas/air mixture fed to the cylinders within this explosive range. Too 'lean' a mixture (near or below 1.4%) or too 'rich' (at or above 7.6%) causes engine firing failure or faulty operation because the gasoline vapor/air mixture is at the limits of the explosive range.

Explosive range figures are based on normal atmospheric temperatures and pressures. There may be a considerable variation in explosive range where other temperatures and pressures are present. Increases in temperature will widen the explosive range.

Reprinted from "How to Handle Flammable Liquids Safely." Form 162021285. Justrite Manufacturers Company, 2454 Dempster Street, Des Plaines, IL, 60016