A New Look for the Old Fire Triangle

A New Look for the Old Fire Triangle

Figure 1. The fire triangle (circa 1920)

DR. R. L. TUVE

OVER THE PAST FEW YEARS there have been advancements in the science of fire extinguishment involving chemical reactions which take place in fires. New information has been unearthed about fires being chain reactions involving free radicals, activation energy, negative catalysts, etc. There has even been some discussion concerning remodeling the old reliable fire triangle into a four-sided shape so that the new developments can be united with our older concepts of fire and its extinguishment.

We need a new version of the old fire triangle, a version which is dynamic in concept, not static like the old picture of fire. Combustion is a dynamic, continuous, chemical reaction and should be explained in some manner showing motion and mixture. Fire protection people must understand the new information we know about combustion which is largely concerned with chemical kinetics, or the speed and stepwise reactions which occur within flames and with extinguishing agents on fires and fuels.

The old fire triangle involving the three components of fuel, oxygen or air, and heat or temperature, is a reasonable explanation of the simple happenings which occur when cooling water is applied to a wood fire or when fcam is directed onto the surface of burning gasoline. The modem concept of combustion as a chain reaction can be indicated and illustrated in some rudimentary fashion with omnidirectional arrows such as Figure 1.

This is the old fire triangle with its corners labeled instead of labeling each leg. With these arrows we get a much more dynamic picture of combustion but we come up against a real problem when we’re asked to show the action of highly efficient dry chemical powders or the older vaporizing liquids on fire. With these agents we don’t necessarily remove any leg of the triangle, but the fire is extinguished!

In recent years there has been considerable scientific research toward isolation of intermediate chemical and atomic products which occur when materials break down and undergo interrelated reactions with their environment under the influence of temperature, time of exposure, or other forces. We have learned this breakdown process involves the generation of very active substances with unsatisfied urges to combine with other matter. These substances or particles of activated matter are free radicals. They are a sort of fragmented molecule and in the case of molecules of diatomic elements such as hydrogen or oxygen, they are atomic varieties of the original element. This is very complicated from a physicochemicaltime viewpoint, but it can be simplified.

Figure 2. The chain reaction of combustion

In order to portray these separate, active fragments of the dynamic reaction of combustion, Figure 2 can be used. Here we personify the fire triangle with skilled jugglers who pass Indian clubs to each other in every direction. The back-and-forth action of these pins can be used to represent the trading around of the vital characteristics of each component—fuel, oxygen and heat—which added together, results in combustion.

This pictures in a very simple manner the most important factors in the chain reaction of combustion, represented as the three-man team of jugglers. You will note that each man or component possesses a supply of materials which he uses to accelerate the game.

Mr. Heat or Temperature has a supply of activation energy pins. These are the stuff which raise the level of the reaction of fire. As he passes them to Mr. Oxygen or to Mr. Fuel, things begin to happen. The more activation energy he puts into the game the faster it becomes.

Mr. Oxygen, in turn, has a supply of available oxygen molecules which, under the influence of Mr. Heats activation energy becomes tremendously active atoms, and eager to unite with something. He passes them to Mr. Fuel, hoping that he’ll take them off his hands and to Mr. Heat who just makes the atoms more anxious to find something to react with.

Mr. Fuel gets some pins from Mr. Oxygen and Mr. Heat and, not to be outdone, adds a few of his own in the form of activated free radicals, ready for marrying up with other atoms. Mr. Fuel gets pretty excited by this time and the more pins he puts into the game the faster and more furious it becomes.

The result of this rapidly accelerating game is an extremely furious reaction proceeding at almost explosive rates under certain conditions and taking the form of a chain reaction of combustion. At this point it is well to spend a moment on just what is meant by chain reaction, for it seems to mean different things to different people.

Figure 3. A simple chain reactionFigure 4. The chain reaction of hydrogen-oxygen combustionFigure 5. Free radical quenching of fireFigure 6. The effect of cooling

Everyone is familiar with the use of the words chain reaction as applied to the atomic bomb. The Walt Disney representation of the multitude of cocked and ready mousetraps inside a plastic box, each with its little mortar ball ready to be ejected inside the enlosure is a dramatic one. When a single activating ball is thrown in, it bounces around and sets off a couple of mouse traps from which these balls in turn set off many more traps and very quickly the entire layout of mouse traps is set off in one big explosion. That’s a chain reaction!

Another instance of a chain reaction concerns the combustion of gasoline in the automobile engine. The mixture of compressed gasoline vapor and air contained in the combustion chamber of the engine bums with explosive velocity, yielding an audible knock, unless its rate of burning (time vs. components) is controlled or suitably slowed up with lead tetraethyl compound. When the rate of explosion is held down, maximum power and thnist is obtained without a knock. This is the chain reaction of combustion controlled by an inhibitor.

Figure 3 is a graphic explanation of a very simple chain reaction. Here we have a container of a mixture of gases, molecules of hydrogen and chlorine. These gases do not react with each other at ordinary temperatures and pressures until a third component is brought in; this is energy in the form of light.

When light falls on the container it constitutes activation energy and causes the molecules to become active atoms, ready to continue with different molecules and atoms. (Keep in mind at this point that this is very similar to the new fire train gle in Figure 2 where Mr. Heat is passing out his Indian club activation energy to Mr. Oxygen, who then picks up his active atom clubs for passing on to Mr. Fuel.)

The active atoms of chlorine (which we can also call chain carriers) in the gas mixture, then react with molecules of hydrogen, producing the finished product, hydrogen chloride gas; but observe that in doing this, an active atom of hydrogen is cut loose, which then reacts with a molecule of chlorine to set free more finished product (HC1) and more active chlorine, which then reacts, etc., etc. The chain depicts the simple reaction, which continues as long as there are any hydrogen or chlorine molecules available and as long as photons of activation energy continue to be supplied. If we halt the source of light, the reaction stops, just as the fire triangle operates.

The chemical symbols and equations at the bottom of the picture show the simple chemical reactions involved in this chain reaction. This reaction can go rapidly in an explosive manner if enough light is added (bright sunlight), or it can go slowly if only very little light is put on the mixture (lots of Indian clubs from Mr. Heat or just a few in the juggling act at one time).

The chain reactions which occur in the combustion of fuels are a great deal more complicated than the one pictured in Figure 3. Basically, they contain the same form of contributing factors in a three-cornered partnership, and they are capable of the same type of analysis. Many subfactors begin to enter when such oxidations take place and the reaction kinetics (the amounts of materials and heat present at certain time intervals) of both the subfactors and the principal factors must be considered and studied by the chemist.

Figure 4 pictures a much simplified version of the chain reaction of combustion (or oxidation) of hydrogen— one of our simplest, yet quite complicated, oxidation phenomena. This picture leaves out a great deal of the physical chemistry of the combustion of hydrogen, but the simple form explains a special phenomenon of combustion which we will take up later.

Here we have gaseous hydrogen fuel and gaseous oxygen uniting to form water. The process requires (again) activation energy in the form of an electrical spark of some energy level or an ionized flame containing thermal energy, to initiate it. It is obvious that because this chain reaction begins to branch out, and each branch likewise branches out, the reaction is a rapid one and releases much energy in a short period of time. The reaction begins with the oxidation of hydrogen atoms (active atoms) to produce a fragmented molecule called a hydroxl radical, OH. This is a free radical and at this point in the chain, a side chain is produced, branching off from the original chain of combustion. This is a vulnerable point in the combustion and becomes very useful in our picture of extinguishment, as we will see later.

The continuously branching chain reaction of combustion of hydrogen in Figure 4 can sustain itself only so long as the activation energy level is sustained either from its own heat of reaction or from some external source of energy.

In this picture of combustion you will note the recurrence of the new fire triangle with its active or atomic oxygen Indian clubs, its free radical Indian clubs produced by the fuel, and the whole reaction urged on to proceed by means of activation energy, or, in our new triangle, activation energy Indian clubs!

Now, you might reasonably ask, “Where are we with respect to explaining the extraordinary extinguishing ability of potassium bicarbonate dry chemical, for instance, when compared with inerting agents like carbon dioxide or nitrogen?”

Figure 7. The removal of fuel

Recently, the results of a very interesting series of experiments were published by Friedman and Levy1 concerning the reaction kinetics of potassium and its compounds in flames. Very simply put, these investigators found that the most likely reactions which account for potassium bicarbonate’s extinguishing effectiveness are due to its ability to break down under heat to give potassium hydroxide vapor, which reacts quickly with the OH radical in the branched combustion chain and the atomic hydrogen such as found in the chain reaction in Figure 4. The chemical reactions are as follows:

(1) 2KHCO3 + Heat = H2O + CO2 + KO2

(2) Then: KO2 + H20 (from combustion) = 2 KOH

(3) The flame-quenching reactions are:

KOH + OH (free radical from the branched chain) KO + H2O

(4) and:

KOH + Active H (from the branched chain) K + H2O

As can be seen in the above reactions, the extremely fast breakdown and vaporization of potassium bicarbonate in the heat of the fire reacts to halt (or quench) the free radical OH groups and the active hydrogen atoms, l>oth of which are produced in the process of burning hydrogen or hydro gen-containing compounds. The reactions of KOH vapor are exothermic ones (they give off heat) and take place quickly and easily at the 2,200 F (and above) temperatures of flames.

1Friedman, R. and Levy, J. B, Inhibition of Opposed-Jet Methane-Air Diffusion Flames. The Effects of Alkali Metal Vapours and Organic Halides, Combustion and Flame, Vol. 7, No. 2 (June 1963) Page 195.

Obviously, there are other things happening while this free radical quenching is taking place. Among these important but subordinate factors is the absorption of heat by the bulk of the powder, and the release of carbon dioxide which helps to diminish the oxygen supply.

In Figure 5, the invisible shield free radical quenching action is illustrated using the new fire triangle. This shows the interruption of dynamic exchange of activated oxygen atom Indian clubs and the free radical Indian clubs by the dry chemical agent as long as it is in position to quench the reaction. If it should be removed the reaction could immediately start up again.

The same analogy can be used with the vaporizing/liquid agents, where bromine and hydrogen bromide vapor from breakdown of the agent reacts efficiently to quench free radical production.

If we use the new fire triangle to show fire extinguishment when older methods are employed, we might have something like Figures 6 and 7. In Figure 6 the removal of heat from a fire by the use of water as a cooling agent, shows a little more than the old fire triangle. Obviously, the activation energy Indian clubs have been diminished in number and dissolved by the cooling action of water. At the same time production of free radicals by the fuel diminishes. The atomic or active oxygen particles have less and less to unite with at the slower rate at which they are produced by the diminished (or cooled) source of heat.

Figure 7 is a dramatization of the process halting gasoline fires using foam blankets. The isolation of the fuel is quickly seen.

There are many uses for a dynamic picturization of the old fire triangle in fire protection training and education. The factor of speed or time at which events take place and the chemical character of these events in the fire situation, is extremely important. The three jugglers led the viewer to a more accurate knowledge of the factors involved in the complicated chemical kinetic existence of combustion and its termination.

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