The Community Oil Fire Hazard

The Community Oil Fire Hazard

A Demonstration of the Principles Governing the Conduct of Flammable Liquids and Vapors, and Their Fire Hazards

(Concluded from last issue)

PROBABLY the greatest fear having the least foundation is the almost universal belief that a spark may get inside an underground tank and cause it to explode.

Such fear is shared not only by the public and a large portion of their

Fig. 5 (from Previous Article)

fire authorities, but also by some oil operators. The first group may be excused because of its ignorance and because of what it has read in the newspapers; the second, because of lack of opportunity to secure actual records of such fires; while the third has no excuse at all, except stupidity.

No man in this room or elsewhere can point to a single instance where an underground tank in service, has exploded. Let us look into the possibilities.

In Fig. 10 let the flask “A” represent an underground tank. Notice that it has a small quantity of gasoline in the bottom. I think it is fair for us to assume that any underground tank in service will never be completely empty. The funnel “B” can represent a compartment in a tank truck. Let line “C” represent a hose leading from the tank truck to the fill pipe of the tank. The tube marked “V” will represent the conventional vent pipe running to some point above the ground.

The space above the gasoline in the flask will naturally be filled with vapor, the same sort of vapor that was above the gasoline in the jar (Fig. 5), a vapor too “rich” to burn. Being heavier than air, as all petro-

(Lecture delivered to the graduating class of the Portland Fire College, Portland, Ore.)

leum vapors are, it simply lies quietly until something occurs to force it out of the tank. This is the condition which prevails during normal operation. Linder such conditions a match flame or any other source of ignition may be produced at or near the vent terminal without fire occurring at that point.

Sooner or later a dump of gasoline will be made into that tank. As the gasoline fills the tank, vapor is displaced to make way for it. This vapor is forced up the vent (“V”). Just so long as it is in the vent it is still too rich to burn. There is no air present. But as it issues from the vent, air is supplied from the surrounding atmosphere and a burnable mixture is formed. This time if we strike a match, a fire will occur—there will be burning at the vent.

All this can be demonstrated by simply opening the valve on funnel “B.” I do so and gasoline runs into the flask, vapor is forced from the vent, it combines with air and ignites from the match flame. Here we have an unfriendly fire burning, say at a service station. We do not, however, call out the Fire Department to make a $25 run for a 25c fire. Instead we simply close the truck faucet, shut off the flow of gasoline into the tank. Vapor is no longer forced through the vent and so the fire goes out for lack of fuel.

A Word About Vents

Obsolete practice was to provide these vent terminals with a gauzelike screen to guard against flames or sparks entering the tank through the vent pipe. This is an unwarranted precaution. I dare say every man in this room has seen a service station or garage fire in which the vents and even the vent pipe itself became so hot they practically melted and yet the tank did not explode—did not, because it would violate all physical laws if it did.

Why not have vent terminals free and open, in order to insure adequate venting of these vapors at a point where they are less subject to ignition.

rather than have the openings clogged with screens, blocked with scale, paint, wasp nests, etc.

Storage Tank Explosions

Now we come to the third type of explosion about which there, is great concern, but like the others there is little if any evidence to support the popular theory. The contrary is true. As a matter of record, there is no instance in the history of oil storage plants on the Pacific Coast where an explosion has occurred in a tank, because of a spark or flame entering the tank. There have been two instances where tanks have ruptured from internal pressure (“boiler explosions” again) generated by heat from flames outside the tank. In both of these instances, which occurred several years ago, the failure of the tank was caused by inadequate venting—a fault that has long since been corrected.

The storage hazard in a hulk oil plant should not be confused with the greater hazard of tanks in a refinery or other processing plant. In the latter facilities a great deal of experimental work is carried on be-

Fig. 1O

cause of necessity. The development of the automobile has come along, hand in hand, with petroleum refining technique. The car manufacturer cannot build a better car unless the oil industry can make a suitable product with which to operate it. Because of this experimental work and its accompanying handling of what are sometimes unfamiliar products, whose characteristics are not definitely known, as they are known in the finished products handled in hulk plants, explosions may occur. Whenever they do, here is an illustration of what usually happens. Let a tube represent the tank (See Fig. 11). I shall drop a few drops of gasoline into it and insert the cork in the top. Anchored at the bottom of the tube I have a piece of small chain. This chain serves to break up the drops of gasoline, causing them to vaporize, and the resultant vapor combines with the air already present in the tube. Now we fire the spark plug and we see the cork is blown from the tube. We have had an explosion, the same sort of explosion that takes place in a tank. In this explosion the cork was blown out. In the case of a tank the roof would have blown off. There would not be and never has been a rupture of the shell in a conventional vertical storage tank. After the roof is gone the tank stands until all its contents are burned out.

Fig. 11

Even a tomato can filled with gasoline will burn itself out without melting its soldered seams. Try it sometime—you will find that to be true.

Let us try again. This time instead of putting in seven or eight drops, let us try 20 drops. We repeat the shaking process. Now we fire the plug and to our surprise, nothing happens. Why? Because this time the tube is filled with a mixture too rich to burn, the same sort of mixture that was in the glass jar and in the underground tank; a mixture containing too much gasoline vapor and not enough air. We can prove that by inverting the tube and pouring out some of the vapor. The vapor will pour because it is heavier than air. Notice that no liquid comes out.

Now we have changed the mixture and we replace the cork. This time we get an explosion. We learn from this that we not only need a combination of air and vapor to get a burnable mixture, we need an almost exact proportion of both. In every 100 cubic feet of mixture, if we have more than six cubic feet of vapor it will be too rich to burn. If we have less than 1 1/2 cubic feet, the mixture will be too lean to burn. This fact will almost entirely explain why explosions never occur in bulk plant tanks. Such a tank in service will probably never have an explosive mixture in its vapor space. So much for explosions.


We have learned much in the analysis we have made. We know a great deal more of the actual hazards. Perhaps we can sum it all up in one simple test. We have learned :

  1. That the fuel for every fire must be in vapor form.
  2. Gasoline will give off that vapor without heating.
  3. These vapors are heavier than air and accordingly will seek a level as water does, and
  4. Exposed to the surrounding atmosphere will combine with air to form a mixture which will burn when
  5. It comes in contact with heat.

Now let us demonstrate all these. We take a ball of cotton (See Fig. a trough. At the lower end of the trough we place a lighted candle. We moisten the ball of cotton with gasoline. Vapor flows down the trough, combines with air and finally reaches the candle flame. There is a slight puff, flame travels back up the trough, and the cotton bursts into flame. With that we conclude our search

Fig. 12

into the problem. We have said nothing of fire control, because time does not permit it. You gentlemen are expert firemen. You probably know a great deal more about that than I do. I should like, however, to make one suggestion. Take this triangle and look at it. One glance will indicate the necessary control measures. Either get rid of fuel (vapor) or air or heat. We may have any two we like, any time we like, but never all three at the same time. Finally, I call your attention to the

greater hazard of handling and use rather than the storage and keeping, by the responsible oil operator. Here today we have had numerous fires, but all of them have been outside of containers. Let me illustrate it this way: If you went to a service station and

had a “safety” can filled with gasoline and then took it home, put it in your closet or on the back porch, there would be no hazard in that. No more so than if it were water. But in the can the gasoline would have no value, before it can become valuable it must be poured out of the can. (Just as it must be drawn from tank or tank truck). Once out of the can, where its vapors can combine with air, it is just as hazardous as if it had been poured from a paper bag. So the hazard is not in the keeping or storage, it is in the handling; in the use, or misuse rather, on the part of the indifferent, uninformed person and irresponsible service establishments. Let us concentrate on the develop-

ment of a more intelligent use of these very necessary commodities. Let us level our heaviest guns in a campaign of education. To that end the oil industry is dedicated, and to that end we pledge you our unstinted support. Chief Hayes Recovering

Chief J. David Hayes of the Milburn,

New Jersey, Fire Department was recently taken to the hospital for an operation. Latest reports are very encouraging, and Chief Hayes is expected to be on duty again within a few weeks. 12

Previous articleTwo Injured in Blast

The Community Oil Fire Hazard


The Community Oil Fire Hazard

A Demonstration of the Principles Governing the Conduct of Flammable Liquids and Vapors, and Their Fire Hazards

THE Standard dictionary gives eleven definitions for the word “fire.” There are only two which are common; the “friendly” fire and the “unfriendly” fire. We are all familiar with the “friendly” fire and there is no debate on that score. There may be some differences of opinion regarding an “unfriendly” fire. Regardless of that possibility we can agree on the following statement:

Fig. 1

“Before we can have fire at all we first must have something that will burn. We call it fuel.”

Let us now draw a horizontal line on the blackboard and identify with the word “FUEL”—–:


Here we have the base of every fire; something that will burn.

On the table here we have a number of pieces of common fuels, things which burn every day somewhere. Yet I can take this block of wood and hold it over the flame of a burning candle (Fig. 1) and we see it does not take fire. Of course the block is heavy and solid but here is a piece of cloth. We substitute it for the black of wood and the results are the same.

Now let us try a liquid. In this bottle there is some kerosene. I shall pour some of it out in a pan. The

(Lecture delivered to the graduating class of the Portland Fire College, Portland, Ore.) pan is filled to a depth of about 1/4-inch.

If we hold a match over the surface (Fig. 2) there is no burning. Even when we dip the match directly in the kerosene the result is the same. Complete immersion of the match flame extinguishes it. If we substitute a lighter product such as Stoddard Solvent the results are the same. A match flame held directly above the liquid causes no burning; dipped quickly into the liquid, it is extinguished. The same with a burning card.

To carry this experiment further I shall moisten this calling card with a little of the liquid—just enough to form a thin film. If we hold a match to the card we find that it burns slowly. But again by dipping the card in the very liquid with which it is moistened, we can extinguish it.

Fig. 2

Now we are beginning to learn something. When we held that match flame against the calling card, a tiny portion of the thin film of solvent was heated to the point where it gave off a vapor, and it was the vapor which burned. Because the solvent in the pan has not been heated, it did not give off vapor, so there was nothing to burn. We know now that solids or liquids do not burn, it is only their vapors which do so. If we heat all of this solvent on a stove then all of it will be at a temperature that will produce vapor. Let us try it. I place it on this stove. It will not take very long for it needs to be heated to about the temperature of the water you use for shaving,—105° to 110° F. A match flame held at edge, after solvent has been heated will ignite vapors.

Note: Take pan off stove before attempting to ignite vapors with match flame.

Not All Oils Require Heating

Some oils will give off vapor at the temperature of the surrounding air. This may be as low as 40° F. Included in these are gasoline, naphtha, benzine, alcohol, paints and numerous other household articles. If we pour gasoline into a pan and hold a match flame above it (as in Fig. 2) the vapors will immediately burst into flame. So let us go back to that line we drew on the blackboard and revise it so that it looks like this:


Further Proof

Here I have a jar of gasoline. On top of the jar is a sparking device connected to a battery. With it we can produce the same sort of spark we get in the motors of our automobiles. I shall hold it up so that all may see. Notice that when a spark is produced beneath the liquid level (Fig. 4), there is no burning. Here we have a positive proof that the liquid does not burn.

But, we say the vapors do burn, so let’s move the spark up into the vapor space. Again we see (Fig. 5) there is no burning. That may be surprising. Let us try something else. This time we move the spark to a point just above the mouth of the jar (Fig. 6) and there burning occurs because that is the only point where the vapors inside the jar have an opportunity to combine with air. So we establish another fact: Unless there is air present, there will be no burning.

Fig. 3

Now we go back to that line on the blackboard and make it look like this:

What else do we need to build a fire? Here in this pan of gasoline (Fig. 7) we have fuel (vapor) above the gasoline and air all around that vapor—and yet we have no fire and won’t have until we add one more thing—heat. When this is done our “fire” looks like the triangle, as illustrated. Whenever we put that triangle together we have a fire whether we want it or not. That combination will cause it regardless of what other condition may exist; whether it be hot or cold, wet or dry, makes no difference. Also, the fire will be the same whether the fuel was gasoline vapor, wood vapor, coal vapor, or vapor from any other combustible substance. This is good to remember—to save for the time when that fire is to be controlled.

Fig. 4

The fires which we have seen here were relatively small, yet they came into being and behaved in exactly the same way they would have had they been larger. The gasoline vapors, in the small pan, combined with air and ignited from the match flame in the identical way that vapors from a large body of gasoline would take fire.

The behavior of the spark in the jar of gasoline, below the liquid, above the liquid, and in the mouth of the jar is no different from what would happen in a tank truck under similar circumstances. Compare Fig. 8 with Figs. 4, 5 and 6 and this is easily understood. Notice the physical conditions are identical. This would be true of any other container, such as a delivery bucket, a drum or fuel tank. Fire exists solely because of an immutable natural law. There is no way in which we can create it in another manner. For example, just because we are afraid a fire will occur does not cause it to occur.

Fig. 5


Now that we have gone through the problem of ordinary fires let us take up another and more popular phase of our subject,—Explosions. In using the term “popular” I am thinking of the newspapers and the public more than you and me. Certainly the term is not popular with the oil industry and I am sure it is no more so with you gentlemen. With the average newspaper reporter no gasoline fire can occur unless it originates as or terminates in an explosion. The story of an open tub of gasoline burning quietly in the middle of the street lacks spectacularity and therefore has a relatively low news value, but if it can be said that the gasoline “exploded” and thus endangered the lives and property of everyone within blocks of the site, it becomes “front page” news. This is especially true if the item is supplemented by a picture showing great clouds of billowing black smoke just as if the color or quantity of smoke given off by a burning substance is an indication of the severity of the fire. In naval operations, great smoke screens are laid down by airplanes and ships without fire at all. You and I know that a gallon of carbon tetrachloride, an incombustible liquid, when heated will give off more black smoke than a drum of burning gasoline.

Fig. 6Fig. 7

Tank Truck Explosions

Every so often we hear of a tank truck “explosion.” Did you ever hear of one exploding while it was traveling down the highway? While parked at the curb? Neither have I! We do know of trucks which have had fires on them because of burning brakes, flat tires, faulty wiring, collisions or overturning. You nor I have ever known of even one case where the fire originated in or because of its gasoline cargo. Yet in almost every instance we are told “the gasoline exploded.” Let’s see what is behind that statement.

Fig. 8

Here I have an ordinary 100 cc. capacity test tube. The liquid in the tube is plain water. We all know that when plain water or any liquid is heated it expands, increases in volume. So if we hold the tube over this candle flame the water will heat up and expand. If we get it hot enough the expansion will be sufficient to cause the water to overflow the tube. I shall draw a crude picture of the tube. Alongside, I shall sketch a section of a tank truck carrying gasoline. Due to a collision, a spill or other reason, gasoline is burning underneath the truck (Fig. 9).

Notice that the general conditions in each case are the same. Therefore, we have an identical situation to deal with; in the case of the tube, expanded water would overflow the top while in the truck the gasoline would overflow the dome. If we were to put a cork in the tube, which I will now do, we know that just as soon as expansion takes up all the vacant space in the tube a pressure will be built up. Eventually this pressure would increase until the cork would be blown out of the tube. The same thing would happen in the case of the truck if we were to place a tight cover over the dome,—-but if we drill a hole up through the center of the cork, any pressure which tends to build up will be relieved through that hole. Likewise if we cut in opening in the tight dome cover of the truck (a pressure relief valve), any pressure generated will be relieved through that opening. Provided, however, that the opening in the cork or the dome cover is large enough; certainly a pin-hole opening, would not he sufficient for adequate relief. But so much experience and competent knowledge is at hand that calculation is easy. The most generally accepted data for emergency relief venting is found in the N.F.P.A. Recommended Ordinance.

Fig. 9

Whenever adequate relief has not been provided and the truck is involved in a fire, accompanied by a rupture of the tank, we are told “the gasoline exploded.” Such a statement is definitely untrue. What really occurred was identical to what happens to a boiler that has no “pop valve” or an air compressor tank whose relief valve is stuck. The presence of the gasoline had nothing whatever to do with it except to follow the natural law of expansion just as water, milk, or any other liquid might do. These “explosions” are no more than “boiler explosions.”

(To be concluded)