FIRE LOSS MANAGEMENT
A Series by
Part 8: EXTENSION -KINDLING, continued
WE WILL NOW Continue our discussion of kindling—a combustible material that can be decomposed and ignited by the amount of heat provided by the initial cause. Fire extension starts when initial ignition is applied to kindling.
THE DEATH CLOUD
Whenever a flammable liquid or gas container is opened to the atmosphere, there is a cloud of ignitable vapors—a “death cloud.” This death cloud exists in the area in which the flammable vapor is mixed with the air in the proper proportions to ignite when a source of ignition of sufficient heat is brought into the cloud. Immediately surrounding the surface of the flammable liquid or the source of the vapor the mixture is usually too rich; that is, there is so little oxygen in proportion to the amount of flammable vapor that ignition cannot occur. Some distance away the flammable vapor is so diluted that ignition can no longer occur; it is too lean. Between those two, ever changing in shape, is the death cloud, and a single spark of the proper temperature—generally about 400° to 500° F—is sufficient to ignite the entire cloud.
The size of this cloud depends on various factors. The first of these is the explosive range of the material. The explosive range is the percentage of material by volume in the air that will make an explosive mixture. There is always a lower and upper limit. For instance, for gasoline vapor the lower explosive limit is about 1.6 percent and the upper explosive limit is about 5.9 percent. In other words, gasoline vapor and air mixtures between these two limits will absolutely ignite if a portion of this mixture is brought up to 495° F. Other factors include whether the gas is heavier or lighter than air and the amout and direction of air flow.
Often, people who work with flammable liquids, such as in gas stations and laboratories, are unaware of the certainty of ignition under the right circumstances. If it were not for this absolute certainty we could not, for instance, operate a motor vehicle. Yet very often people ignore this certainty of explosion of a vapor and air mixture. Sometimes a disaster is prevented only because explosive limits are quite critical and the death cloud area is often very small in size so it doesn’t happen to come in contact with the necessary heat source.
The combustion of a liquid actually takes place in the fuel/air mixture above the liquid surface rather than at the surface. The temperature of the liquid at which ignitable vapors are produced varies from substance to substance and with other conditions, such as whether the surface of a pool or a spray is involved.
The flash point of a liquid is the minimum temperature at which vaporization of a flammable liquid is sufficient to produce a flammable fuel/air mixture. You can measure the flash point by gradually heating a sample of the liquid in contact with the air and periodically bringing a small ignition source near the surface. The minimum temperature at which a flash of flame spreads over the surface is the flash point. It is one of the most important properties in determining the fire hazard of a liquid.
The initial flash seen when the ignition source is brought near the liquid surface may consume the fuel in the fuel/air mixture and the flash will die out. At a slightly higher temperature, vaporization of the liquid is sufficient to sustain the flame.
The fire point is the lowest temperature that a surface of flammable liquid can evolve vapors fast enought to support sustained combustion. It is usually a few degrees above flash point. The flash point, however, is more widely used than the fire point to characterize a liquid’s flammability.
A liquid fuel is not readily ignited by a small ignition source if the liquid’s temperature is below the flash point, but it can be ignited by a larger ignition source that is capable of heating the surface of the liquid to the flash point. Once ignited, the fire will increase the temperature of the liquid, which in turn increases vaporization rate and the fire’s intensity.
(Photo by author.)
Liquid sprays and mists (perhaps from a leak under pressure) ignite more easily and burn more vigorously than the bulk liquid because a larger surface area is in contact with air. An example of this is the pressurized oil burner of an oil-fired heating plant.
The ignition temperature of a liquid fuel is that temperature to which the fuel, in air, must be heated to ignite spontaneously. The ignition temperature of a specific substance can vary widely depending on many conditions.
BEWARE OF VARIABLES
People working in the physical sciences draw needed figures from reference books. They consider flash point and ignition temperature unchangeable physical constants. However, they are not constants. They are observed phenomena and as such can vary greatly from published observations.
Ignition temperatures observed under one set of conditions may change substantially when conditions change. For this reason, written references to ignition temperatures should be considered approximations. Some of the variables known to affect ignition temperatures of flammable liquids and gases are percentage composition of the vapor or gas-air mixture, shape and size of the space where the ignition occurs, rate and duration of heating, kind and temperature of the ignition source, catalytic or other effect of materials that may be present, and oxygen concentration. As there are many variables in ignition temperature test methods, such as size and shape of containers and method of heating and ignition source, it is not surprising that laboratories report different ignition temperatures for the same substance. Investigators should keep such variations in mind in fires where ignition of a liquid fuel by an external heat source is thought to be involved.
Process operators often rely on our old friend “Falsenza Security.” “The fluid in this system has an ignition temperature of 400° F. We operate at 350° F. It’s perfectly safe.” Not so. A pinhole leak can ignite at a much lower temperature than the stated ignition temperature. Explosions due to leaks in hydraulic catapults with “safe” hydraulic fluid forced the Navy to change over to steam catapults on aircraft carriers.
Flash points are used, among other criteria, to provide for legal regulation of liquid fuels. The regulatory situation is confusing. NFPA classifies liquids with flash points below 100° F (38° C) as “flammable.” These Class I liquids are further subdivided according to flash point and boiling point. Those liquids with flash points above 100° F are classified as “combustible”—Class II liquids—and are subdivided according to flash point and boiling point. (See NFPA Handbook, 16 Ed., p. 5-29 for details.)
Consumer products are labeled as follows: “Danger Extremely Flammable”—materials with flash points below 20° F ( — 7° C); “Warning Flammable” — flash points 20° F to 80° F ( – 7° to 27° C); and “Caution Combustible”—flash points above 80° F. These are as measured in the Taglibue (Tag) Open Cup Tester. There are several ways to determine flash point. Each will give a different result for the same sample. (See NFPA Handbook, 16 Ed., Section 5, Chapter 4 for details.)
Years ago the word “inflammable” was often used as a synonym for flammable. The prefix “in” generally signifies a negative, and many people believed inflammable meant that a material would not burn. The NFFA campaigned to get regulatory authorities and labelers of products to drop the use of the word “inflammable.” This effort has been largely successful.
Flammable liquids shipped in interstate commerce carry the familiar red label. This label is to inform transportation workers, so it is applied to the outer shipping container. If two fivegallon cans of flammable liquid are shipped in a cardboard box, the label is on the box, not on the cans. The absence of a red label does not necessarily indicate that no hazard is present.
(The placarding regulations for shipments of flammable liquids and the NFPA 704 system are too detailed to be discussed here. Consult your local haz-mat expert.)
Regulations for the handling and storage of combustible liquids are more lenient than for flammable liquids. But like flammable liquids, the tested values assigned to properties of combustible liquids are often misunderstood. They are not physical constants. They are merely observed phenomena and are truly valid only for the sample tested in the apparatus in which they are tested.
If a substance has a flash point of 150°F, the material must be heated to this temperature in order to evolve flammable vapors. However, it is dangerous to play this flash point game too closely. Kerosene has a nominal flash point, for instance—just above 100° F. However, many samples of materials with kerosene-type solvents in them have been found by actual test to have flash points quite a bit lower. Although the flash point is a broad guide to the degree of hazard, do not place implicit faith in it.
It is just not safe to handle a 100° F flash point material at 90° temperatures without considering it to be a low flash point material.
A change in the physical form of a material (such as to a mist) may greatly change the flash point. In the early days of die casting, the hydraulic oils used had relatively high flash points. However, when these oils were relieved under pressure, disastrous fires occurred. In recent years, less hazardous synthetic oils have been developed to replace these hazardous oils.
Floor wax, specified as “combustible,” ignited from the spark of a floor polisher, causing the loss of several lives in a military hospital ward. Tests of 2 samples showed actual flash points of 74° F (24° C).
A mixture of a flammable liquid (such as alcohol and water) may or may not ignite under ordinary temperatures depending on the percentage of alcohol. Wine will not ignite at room temperature. However, if the wine is heated, the alcohol will ignite. In one of the worst disasters in fire suppression history, 40 firefighters died in a fire in a Glasgow, Scotland whiskey warehouse.
In short, “flammable” and “combustible” are legal terms, not precise technical terms. Consider published charac, teristics of flammable liquids with caution when attempting to determine what part a particular flammable liquid played in the development of a fire.