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