US2730178A - Method of controlling oil tank fires - Google Patents

Description

United States Patent O METHOD OF CONTROLLING OIL TANK FIRES Joe L. Risinger, Maplewood, N. J., assignor to Socony Mobil Oil Company, Inc., a corporation of New York Application December 11, 1951, Serial No. 261,158

2 Claims. (Cl. 169-1) This invention relates to a method of controlling and combatting oil tank lires.

One of the most serious features of oil tank fires is the boil-over, experienced particularly with crude oils and heavier oils. Practically any tank used for the storage of oil has a certain amount, varying from a fraction of an inch up to several feet, of water at the bottom of the tank, below the oil. When the contents of the tank are permitted to burn away, the oil within the tank becomes heated, and when the tank contents have burned down enough, and the heat of the oil becomes great enough, this water vaporizes, suddenly, giving rise to great quantities of oil-steam froth and foam which rise up and overilow the tank, and spread the fire. It is this feature which gives rise to the dikes usually seen surrounding large oil tanks, particularly those used for the storage of crude oil and the heavier oil fractions. The initiation of the fire is usually accompanied by an explosion which damages the tank roof, frequently removing it entirely over at least a portion of the tank. While there exist many devices for permanent attachment to a tank, at the roof edge, for introducing foamite or similar lire lighting chemicals, these are so frequently damaged by the initial ame burst that many tank installations do not use them, but depend rather upon portable foam towers and the like to apply fire fighting chemicals through openings made by the initial blast. Additionally, many crude oil tank farms are placed in relatively isolated localities, and equipment and man-power, particularly man-power, are not available for lire fighting until the lire is quite well established. In such circumstances, even if a fire is then put out, the upper portion f the sides of the tank are usually so damaged that the tank must be entirely rebuilt, excepting possibly the bottom.

This invention is particularly concerned with a novel concept in combatting and controlling such lires, capable of utilizing equipment which is normally installed in the tank for other purposes, which equipment is seldom damaged or rendered inoperative by the tire until its very late stages are reached.

This invention is based upon the fact that to support combustion, the oil immediately adjacent the burning surface must be at or above its flash point, and asecond fact, that the remainder of the oil in the tank is not that heated, taking usually a period of several hours to heat up to any temperature near the combustion supporting level.

This becomes most clear when we consider a tank full of kerosene. The Hash-point of kerosene, the temperature at which a small cup-full of kerosene will give olf enough vapors to support a combustion upon its surface, is usually at least 125 F. The normal temperature of the body of kerosene in storage in a large tank is usually about the mean atmospheric temperature existing in the locality of storage at the time in question. This temperature is usually of the order of not above 70-90" F., even in summer in very hot climates. In a tank ot' kerosene, burning at its surface, only an inch or so of the kerosene, at the top, is at the ash point until after the tank has ICC been burning for a time of the order of an hour or more. (Of course it is obvious that the fire is usually started by a mechanism involving the presence of vapors in a combustible mixture of vapor and air, which does not necessitate the surface of the oil being above its flash point.) Opposed to this slight layer of hot oil, there is much depth, usually many feet, of oil well below the ash point. Now if we agitate the oil suiiciently to bring about mixing and cooling of the surface oil, it should be possible to stop combustion.

In actual trial, an open-top tank, thirty feet in diameter, containing about twenty feet of kerosene, was set on fire. After it was burning well, a stream of compressed air, rather small in volume, was introduced into the water draw-off line-a small pipe entering the tank near its bottom, and not extending into the tank beyond the side wall-and within thirty seconds the lire was out.

In this test the air applied was at a rate of from 40 to C. F. M., at a pressure of about 5-7 p. s. i. for a duration of less than a minute. From this may be seen that the air requirements are quite low.

It is apparent that the air requirements set forth above may alternatively be expressed as an amount varying from about 6 cu. ft. to about 13 cu. ft. of free air per minute per sq. ft. of oil surface.

In a storage tank containing kerosene, gas-oil, lubricating oils, fuel oils, or any oil having a ash point more than a few degrees above atmospheric temperatures the same thing will happen Thus it may be seen that this invention provides a positive and economical method for the rapid, safe, and complete extinguishment of lires in tanks containing oils having a flash point above atmospheric temperature.

With gasolines, naphthas, and most crude oils, where the flash point of the oil is below the storage temperature, that is, below the average ambient temperature during the period of storage, the method provides only a form of control. In most crudes there is a suieient gasoline fraction to support combustion. The greater portion of the crude oil (usually at least 75% of it by volume), is high flash material. Consequently the same technique, applied to a crude oil lire, will so considerably diminish its activity that it becomes easily susceptible to extinguishment by other methods.

In a test made with crude oil, the same thirty foot tank was filled to a depth of about twenty feet with crude oil, leaving about seven feet of side wall above the crude. This was ignited, and after it was burning well, air was injected through the water draw-off line. Within a space of 45 seconds the lire had so subsided to only a flickering flame above the side wall. Using a spider device to secure air distribution over the bottom of the tank, the fire immediately subsided so that it was possible to stand at the upper edge of the tank wall and observe the lire icking across the oil surface. This is a thing that to my knowledge has never before been done with a crude oil re. This lire was allowed to burn for several hours, while under such control, to determine at what rate the oil in the tank below the re became heated. Using a paint on the outside of the tank wall which changes color with temperature change, it was not possible, through the duration of the test, to find any heating of the oil below the burning surface. In similar tests, without agitation, heat can be seen to be moving downward at an appreciable rate. During this period the introduction of air through other means, up to and including a rather elaborate layout of perforated pipe on the bottom of the tank was tested. The conclusion drawn from these tests was that the better distribution of air except in crude oil tanks did not usually secure effects enough better to warrant expense of installation and maintenance.

In this crude oil test, the air supply was again at a rate from 40 to 90 C. F. M. and at a pressure of from 5-7 p. s. i. Had the desire been solely to so control the fire as to render it readily extinguishable with foam, etc., the duration of air supply would have been less than a minute to minimize the ame, plus whatever time would be necessary to supply and apply the extinguishing agent.

As another point of interest, when a tank of heavy oil has been burning for some time, and the upper foot or so of the oil is quite well heated, the application of any of the water-borne foam type extinguishing compounds is very apt to result in the formation of oi1-foam and slopover of burning oil and foam, for the same reasons that cause slop-over when the heat level has reached the bottom water, and with consequences equally dangerous. Tests have shown that with a tank so heated, the introduction of air will so cool the surface oil, even after extensive burning, as to permit the introduction of waterborne foam extinguishing compounds without slop-over.

The air agitation of crude oil tanks to prevent boil over has been previously proposed, the proposal being based upon tests in small tanks. When submitted to the industry, it encountered adverse independent tests and this proposal failed of adoption. It has been proposed to mechanically agitate the tank contents to arrive at the same result. To mechanically agitate the contents of tanks more than a few feet in diameter, (most oil industry tanks are over 100 feet in diameter), requires equipment and investment of a size that makes this proposal absurd. It has been proposed to pump tank contents from a low level and spray them upon the burning surface. This requires a spray system, widely and evenly distributed over the tank oil surface and floatably supported to remain at that tank oil surface. quirements for this equipment likewise render this proposal practically absurd.

In contrast, the method here proposed is of the utmost simplicity, using only equipment already installed and present in the tank for other purposes.

This may be seen by reference to the drawing which is made a part of this specification.

In the single diagrammatic ligure of this drawing, 1 denotes the shell of a tank, shown in section, and containing oil, 2. This tank is equipped with the usual product drawoi line 3, which extends to the middle of the tank at a point adjacent the bottom thereof. External to the tank shell, drawoff line 3 is usually equipped with a valve 4, and at some point in the length of pipe 3, beyond valve 4, there will be another valve 5. For convenience, this valve 5 is here shown as adjacent valve 4. It is assumed that the oil 2 in tank 1 has been ignited, and has been burning at its surface. Compressed air has been introduced through valved pipe 6, with valve 5 closed and valve 4 open. Passing through pipe 3, this air rises through the oil 2 at the point indicated by character 7, creating an active upward motion of the oil, which surges upward at 8 and tlows outward at 9, 9, cooling the surface layer of the oil and causing the flame to retreat ahead of it to the position 10, 10, along the tank sides, where the lire is soon chilled out of existence.

The tank 1 is also tted, customarily, with one or more water draw-otf connections, such as that shown at 11. In a large tank there will be several of these, located around The investment and maintenance rethe periphery. Each, or all, of such water draw-off connections may also be used for the introduction of compressed air.

This method will work with pools of oil, such as those in open or roofed earthen storage pits, as well as with oil in tanks.

Since any tank or pool in which oil is stored will customarily be equipped with piping leading to a point near the bottom of the oil, for oil handling purposes, and since such piping is customarily open at the oil end and so arranged at points outside the tank or pool that it can be broken into and' arranged for air introduction, any such piping can be used. On tank storage, the water draw-olf line is most convenient, since it is a pipe of relatively small diameter, valved outside the tank, and outside the valve, the water drawoff pipe ends in an open end, quite convenient for attaching to a compressed air supply. Since in almost any location, portable gasoline-motor driven compressors are at hand or procurable, the method is adaptable to widespread use.

I claim:

l. That method of extinguishing a combustion upon the surface of a stored body of oil, which oil has a ash point above atmospheric temperature, which comprises introducing into the oil, at a point well below the surface thereof, at least one stream of compressed air, the amount of such air being from about 6 cubic feet to about 13 cubic feet of free air per minute per one hundred square feet of oil surface, the pressure of such air being sufficiently above atmospheric pressure to insure ready entry of air into the oil, whereby submerged portions of the oil, existing at a temperature below the ash point of the oil are brought into contact with and mixed with the surface portion of the oil to cool the surface below the ash point and extinguish the combustion by depriving it of oil vapors.

2. The method of extinguishing a combustion upon the surface of a stored body of oil which comprises introducing into the oil, at a point well below the surface thereof, at least one stream of compressed air, the amount of such air being from about 6 cubic feet to about 13 cubic feet of free air per minute per one hundred square feet of oil surface, the pressure of such air being suiciently above atmospheric pressure to insure ready entry of air into the oil, whereby submerged portions of the oil body are brought up to and admixed with the surface thereof to cool said surface oil, diminish the evolution of vapors therefrom and lessen the combustion, and thereafter applying a fire extinguishing agent to the surface of said body of oil, by which two-step method the combustion may be extinguished in a shorter time and with less lire extinguishing agent than had the agent alone been used.

References Cited in the le of this patent UNITED STATES PATENTS Aitken Mar. 20, 1951 OTHER REFERENCES

Claims (1)

1. THE METHOD OF EXTINGUISHING A COMBUSTION UPON THE SURFACE OF A STORED BODY OF OIL, WHICH OIL HAS A FLASH POINT ABOVE ATMOSPHERIC TEMPERATURE, WHICH COMPRISES INTRODUCING INTO THE OIL, AT A POINT WELL BELOW THE SURFACE THEREOF, AT LEAST ONE STREAM OF COMPRESSED AIR, THE AMOUNT OF SUCH AIR BEING FROM ABOUT 6 CUBIC FEET TO ABOUT 13 CUBIC FEET OF FREE AIR PER MINUTE PER ONE HUNDRED SQUARE FEET OF OIL SURFACE, THE PRESSURE OF SUCH AIR BEING SUFFICIENTLY ABOVE ATMOSPHERIC PRESSURE TO INSURE READY ENTRY OF AIR INTO THE OIL, WHEREBY SUBMERGED PORTIONS OF THE OIL, EXISTING AT A TEMPERATURE BELOW THE FLASH POINT OF THE OIL ARE BROUGHT INTO CONTACT WITH AND MIXED WITH THE SURFACE PORTION OF THE OIL TO COOL THE SURFACE BELOW THE FLASH POINT AND EXTINGUISH THE COMBUSTION BY DEPRIVING IT OF OIL VAPORS.

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