Oil Fires, Their Cause and Prevention
Lightning as a Cause of Tank Fires—Advantages from the Safety Standpoint of All Steel Tanks—Fires from Outside Exposure
THE following paper deals in an unusually thorough and analytical way with a subject that is of great interest to fire chiefs. It is full of important information well presented and will be found of great help to those “who have oil tank fires to contend with:
Statistical evidence shows that lightning is the cause of greater fire loss to the oil industry than all other fire causes put together. As it is my purpose to tell how to prevent fires, it is obvious that 1 must first contend with lightning. On the face of it this appears to be a task of great magnitude. As we are not in league with His Satanic Majesty, we certainly cannot prevent lightning. However, we can prevent fires caused by lightning and to that end I am going to direct my remarks.
Lightning Fire Prevention in Oil Tanks
To understand the whys and wherefores of lightning fire prevention, .it is first necessary to consider the fundamental conditions which almost invariably arise in lightning fires. A wooden, or non-gas tight tank roof is almost invariably a preliminary condition to oil tank ignition by lightning.
There are several theories regarding the ignition of wooden roofed tanks. Undoubtedly there are a number of “direct hits”. Experiments made by the General Electric Company, indicate that a tall object will only draw the direct hits that would otherwise fall within a zone whose radius would be four times the height of the object.
Thus, a tank 30 feet high will probably receive discharges that would otherwise strike the earth within a circle concentric with the tank but 120 feet greater in radius. With an understanding of this point you will see that the tank of itself, even though highly conductive to electricity does not draw direct hits from lightning to any appreciable extent. The evidence at hand indicates that a large proportion of tank fires attributed to lightning are caused in an indirect manner. It is commonly accepted that a column of gas vapor ascends from such tanks at least several hundred feet into the atmosphere. Lightning, when flashing through this vapor zone, is presumed to cause a flash-back to the tank.
It is highly improbable that a tank would be ignited in this manner. In the first place, the vapors from crude oil and gasoline are largely pentane and hexane, with perhaps smaller portions of propane and butane. All of those gases are from one and one-half to three times heavier than air. and in consequence thereof it is not probable that they would rise to any extent. Secondly, a thunder storm not accompanied by more or less wind is unusual. In fact the production of wind is part of the storm itself. Any wind or breeze would very rapidly dissipate the column of gases which might be hovering above the tank. Thirdly, even assuming the gases would rise, it is not probable that a mixture rich enough to propagate flame would extend several hundred feet into the atmosphere. But even so, the thunder cloud is usually from 2,000 to 5,000 feet above the earth, and this being true, of what account would be a column of vapor a few hundred feet high.
It is much more likely that the lightning strikes the ground in the vicinity of the tank, and the heavy vapors trailing along the ground in a rich enough mixture, flash-back to the tank itself. But here again the wind and breeze serve to dissipate and diffuse the vapors quickly.
Is it usual to detect odors of hydro carbon cases in the vicinity of such tanks? It is not. While this is not conclusive evidence of the non-existence of an inflammable mixture, still it is a very fair indication that any vapor air mixture which is present, is too lean to propagate flame. To my mind this refutes the theory of flash-back from the ground—except in exceptional cases where distinct gas pockets are known to exist.
Explanation of What Really Happens
In all probability ignition takes place through what is known as an “induced charge.” One often hears of a man being knocked unconscious, or being shocked by lightning as he stood under a tree which was struck. One can hardly fail to appreciate that not the faintest part of the bolt of lightning which struck that tree touched the man. He was shocked by an “induced charge.” We know that positive electricity attracts negative and vice versa. A cloud overhead, charged with negative will destroy the electrical equilibrium on the earth beneath by attracting charges of positive electricity to the tops of promontories on the earth. The man standing under the tree had a heavy positive charge drawn to the top of his head. When the cloud discharged to the tree the attraction upward for the electricity in the man’s head no longer existed, and it immediately rushed back to earth to restore equilibrium. The man was thereby shocked by the current passing through his body. On a wooden roofed tank covered with sheet iron, the respective sheets of same not being well bonded together, the same thing happens, only the electricity in rushing back to the ground jumps over a joint between two sheets not bound together, and produces a spark. This spark is produced right where a rich mixture of gas exists and it need only be a few hundredths of an inch long to fire the contents of the tank. This is, I believe, the true explanation for the ignition of so many wooden roofed tanks.
Safety of All Steel, Gas Tight Tanks
At first thought, it is remarkable how few steel roofed tanks are ignited by lightning. Occasionally, we hear of one. Certainly the steel roofed tank is not less subject to being struck by a direct hit than a wooden roofed tank. Undoubtedly lightning actually strikes a great many steel roofed tanks and we hear nothing about it. The discharge is immediately conducted to ground by the great conductivity of the steel roof and the shell of the tank. If the tank has no holes in it, that is, if the different hatches are closed and the vent is properly protected with a fire screen, an all steel tank is really a very safe proposition. Experimentally demonstrated it is impossible for a spark to be produced within such a container by an external electrical manifestation. I am confident that every steel roofed tank fire caused by lightning can be traced directly back to an open gauge hatch, open manhole, defective vent screen or other non-gas-tight condition in which vapor was ignited outside and flashed back to the interior of the tank. I do not believe it is possible to set fire to an all steel gas-tight tank by lightning and I actually know of one case where lightning was seen to strike such a tank and no ignition resulted therefrom. To prevent tank fires from lightning I am satisfied that it is only necessary to provide gas-tight all-steel construction and supply reasonable maintenance.
Tank Fires from Outside Exposure
Another cause of tank fires is exposure to other fires, but again let me point out that here also the wooden roof is the primary evil. The gas-tight, steel-roofed tank will withstand very severe conditions of prolonged exposure. Instances can be cited where such tanks have been surrounded for hours by blazing seas of oil. and except for the loss of paint and distillation of some oil, there has been no damage. Under such conditions a gas-tight tank will act like a large still, and the vapors produced will burn from the vents like huge torches.
The subject of tank exposure, both to adjacent tanks and adjoining property has been the source of much difference of opinion between certain of the fire insurance interests and the oil companies. The whole story is too long to relate as it had its beginning as far back as 1913. In substance, however, most of the discussion has arisen since 1920 when the regulations of the National Board of Fire Underwriters applying to oil containers came up for revision. It is still unsettled. The American Petroleum Institute has performed yeoman service in collecting information, opinions and experiences from the oil industry from coast to coast. Branch advisory committees of oil men were formed in every section of the United States to study the situation. As a result of this very extensive investigation the American Petroleum Institute submitted on behalf of the oil industry their findings— to the effect that gas-tight steel tanks need not be spaced further apart than one tank diameter shell to shell to provide every reasonable and safe precaution against the exposure hazard. This conclusion was substantiated by numerous cases of actual fires.
Tank Spacing Submitted to Arbitration Committee
The fire insurance interests, however, could not agree to that proposition; at least as far as adjoining property was concerned. A compromise was reached which divided oils into three general grades according to volatilty and provided distances to adjoining property equal to 2, 1 1/2 and 1 1/4 tank diameter depending upon grade of oil. with maximum requirement of 175 ft., 150 ft. and 125 ft. distance respectively for each of the three grades. Tank to tank spacing was permitted to remain substantially as recommended. When this compromise was presented to the convention of the National Fire Protection Association a serious debate was raised by other fire insurance interests with still more conservative views, with the result that the matter has now been referred to a special arbitration committee for decision.
Unfortunately the numerous cases of non-gas-tight tanks being ignited, and the crude oil boil-overs following certain of such fires have produced vivid impressions in the minds of many people of the ease with which an oil tank is ignited and the consequences following such ignition. An oil tank is regarded as a veritable bomb and a menace to the entire community. As a matter of fact, the gastight steel roofed tank is an entirely different breed of cats, and the few scattered instances of fires in such tanks indicate that oil so stored is a far less fire hazard to adjoining property, than almost any other kind of structural exposure that can be erected. In view of this the extreme penalties of isolation imposed upon oil tanks, certainly should not be made to apply to this safe type of tank construction, which is to the non-gas-tight tank, as the most modern reinforced concrete fire resistive building is to the frame shack.
In a recent paper on static electricity, some very interesting information was given on the production of static charges caused by streams of oil falling into a tank. It was demonstrated that a spray of falling oil under certain conditions, could produce extraordinary electrical discharges—sparks several feet long. I have found this to be a rare source of fire. It is a hazard, however, which may be removed toy merely submerging all connections with discharge oil into the tank. The over-shot connection is a serious offender as regards evaporation loss besides being a fire hazard. There is no particular reason for its existence and all connections of this type should be corrected.
Steam as Cause of Static Electricity
A very unusual case of static was called to my attention last winter. A certain large oil company was engaging in process of steaming out a tank. It was a steel roofed 55,000 barrel tank with steel supporting members. A 1 1/2 steam line had been discharging into the tank for about three-quarters of an hour when an explosion occurred, blowing off the roof of the tank and igniting the non-gas-tight tank next to it, about 10 feet away. It is of interest to know that sufficient gas remained in the 55,000 barrel tank after it had been steamed for three-quarters of an hour to permit the fire to burn for a period of three minutes. The adjoining tank contained gasoline and was extinguished with the foam system. What is of great interest, however, is the cause of the fire. Subsequent experiments in an empty tank proved that the steam jet unquestionably generated a great deal of electricity. A corona discharge was observed all around a similar jet in a darkened tank. Luminescence appeared at tips of fingers of individuals approaching near the jet and from objects pushed within the zone of the steam discharge. It is of interest to know that Lord Armstrong many years ago, developed an electro-static machine which consisted merely of a row of spikes which acted as a collector, and a steam jet. With this device Lord Armstrong was able to draw electric sparks from 5 to 6 feet long. Just what condition obtained which caused the ignition of the 55,000 barrel tank referred to. is not known. It is possible that the steam jet discharged against an insulated upright roof support, or it is possible that a miniature thunderstorm was caused within the tank by the steam jet, the rapid condensation of the steam and formation of drops of water being analogous to conditions which develop in a thunder shower. The extreme rarity of such fires is proof positive that the hazard is very small. Of course, it is evident that the steam hose should be properly grounded and that the steam should not be blown directly against any supporting members of the tank roof. Beyond this, there appear to be no further precautions that should be taken, although I would be interested in having this point discussed by your readers.
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Oil Fires, Their Cause and Prevention
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Few Tank Fires Caused by Carelessness
It is remarkable how few tank fires are caused directly by carelessness. Only once in a great while do we hear of a tank fire being caused by the match or smoking hazard. Occasionally gross carelessness with welding torches is the fire cause. In one instance, that I am familiar with, an employee cut into an overhead run-down line with a welding torch about 200 feet away from the run-down tank. Fire flashed back through the pipe and that ended the career of that tank. One classification of tank fires in particular has been of considerable interest to me during the past year. Because of lack of evidence it seems proper to classify these fires as of unknown cause. It is possible that some manifestation of static may he the cause in certain cases, but in most of these cases nothing that we know about static and the conditions under which it is produced, would apply. 1 believe that spontaneous ignition is the answer, at least occasionally. In tanks containing cracked distillates, or crudes of high sulphur content, there is a considerable formation of sulphur deposit on the under side of the tank roofs. 1 have had some samples of this material analyzed and have found it to contain free sulphur, organic sulphides, iron sulphide and iron oxide. It is definitely known that iron sulphide when freshly formed is pyrophoric, that is, it will greedily absorb oxygen until it becomes heated to the point of incandescence. We know that the gases given off from cracked distillates and high sulphur crudes contain hydrogen sulphide and organic sulphides. It is probable that these gases react with the bare iron, or iron rust incrustion on the inside of the tank to the extent of producing under certain conditions a considerable amount of iron sulphide. It is then supposed that this materia! absorbs oxygen and reaches a glowing temperature, at which temperature it may burst into flame and ignite the explosive vapor in the tank. The fact remains that tanks are set afire now and then for no reason whatever except that which might be explained by the information just presented.
What can be done to prevent the occurrence of this spontaneous ignition? In reply to that question I will honestly say that I do not know that it can be effectively prevented. It appears to be one of the hazards of the oil business. Fortunately it does not happen often. The only remedy I know of is regular cleaning, scraping, and painting of the interior of tank roofs and structural members inside of tanks where this formation occurs.
Inspection and Care of Vent Screens
There is one detail of tank construction that has a close bearing on fire prevention, and to this I desire to make allusion. I wonder how many realize that vent screens as usually installed and maintained are quite ineffective as flame arresters. Unless a systematic method of inspection of vent screens is pursued, it is probable that a large percentage of the vent screens at any location will be found to be defective, that is clogged or corroded through. In some locations, particularly on tanks containing sour distillates, I have found it necessary to renew the vent screens as often as once a month. A monthly inspection should be maintained on the vent screens in every tank and where the gases coming from certain grades of oil are particularly corrosive, this inspection should be made at least once every two weeks.
Metal vent screens will arrest flame only by reason of their ability to conduct the heat away very rapidly to adjacent solid metal parts thereby preventing the formation of ignition temperature in the other side of the wire screening. If the distance from the central part of the wire screen to solid metal is too great, a single thickness of gauze will not serve, but a double or triple thickness is required. Beyond certain dimensions even a triple thickness of wire screening is unsatisfactory. All of this was demonstrated by the Underwriters Laboratories of Chicago, I11., in a far reaching investigation made by them in 1919 in conjunction with the E. I. du Pont de Nemours Co. According to my interpretation of their results, in order to provide enough venting area and still have the screen dimensions suitable, it is necessary that a number of small 2 inch diameter screens be provided or that the wire gauze screen be clamped between a grill work of heavy metal and that the ports in the grill work should not exceed the area of a 2 inch pipe. Single sheets of wire gauze 6 inch to 18 inch in diameter as will be found on many old tanks today are inadequate as flame arrestors.
Foam in Fighting Fires
Now, I wish to say a few words about extinguishing fires. After a tank fire starts there are two methods of fighting it, the first is with steam and the second with foam. Steam as it is normally applied is not effective unless the roof remains on the tank, which it usually does not. Tank fires usually start with an explosion, and then off comes the roof. In such cases if steam is applied through one or several pipes at the shell of the tank it is absolutely ineffective. Steam is of value, however, in preventing the ignition of exposed tanks nearby.
To extinguish the average oil tank fire there in only one agent which is tried and proven. That is Foam. I do not intend to dwell on the essentials of the successful application of Foam. That is a whole story in itself. I do want to call your attention to the fact that there have been quite a few failures of so-called foam systems. These failures have prejudiced many against the use of foam. May I take this occasion to state that I do not know of a single instance where a foam system, designed in accordance with accepted standards, maintained in proper condition, and operated with fair judgment has failed to extinguish a tank fire.
Other methods of handling oil tank fires have been and are being developed. I refer to the use of water, carbon dioxide, and carbon tetrachloride. These methods as yet in my opinion are in the development stage. They do not have any record of actual fires to prove their merit. They work alright under test conditions but may I say that a “test” tank fire is a very unsatisfactory way of proving what will happen in a real fire under the conditions imposed by an initial explosion, or a driving storm.
Fires in Stills
Next to tank fires, the most frequent and expensive fires are those which occur in stills. It is not realized usually that of all the fires which occur in oil properties, perhaps 50 per cent, of them occur in the distillation process. Most frequently these are small fires and are caused by small oil or vapor leaks. They are extinguished with hand extinguishers. steam jets, water hose, or by “pumping out” the still, with negligible loss. Occasionally, however, something lets go, a still cracks open, a high pressure drum explodes, or an oil line breaks and a conflagration is started. The wonder of it is that such fires are so few. The least leak, defect or wearing out of any part is almost certain to cause a fire. Only continual inspection and expert operation will prevent still fires. However, a great deal can be accomplished in providing facilities to prevent the spread of such fires, and means for controlling them rapidly under the severest of conditions. Feeling that this phase of my subject is of considerable interest I will outline some of the more important features of this problem.
In the first place fire walls 24 inch to 30 inch high should be provided around every battery of stills as far as practical. It is usually possible to use the near wall supporting the condenser boxes as part of this surrounding fire wall. This has the double advantage of economy, and subdivision of values as it places the condensers outside of the fire wall. To protect the condensers further, and to protect any towers which maybe mounted over them, a fire barrier of brick, tile, asbestos board, or sheet steel, (the latter equipped with water sprays), may be erected from the ground up, adjacent to the condensers, all openings in it being protected with standard fire doors. This forms a splendid fire barrier, protects the condensers and towers and gives advantageous position to those who may subsequently have to fight a fire there.
So far as practical by means of fire walls, it is further desirable to break up the fire areas in and among the units of any battery of stills so that a flood of hot oil pouring from a break will endanger as few adjoining stills as possible.
Particular attention should be paid to the method of supporting vapor lines, flow lines, charging lines, in fact all oil and vapor lines. Most frequently these pipe lines are carried overhead on unprotected structural steel work. Any severe fire will bring them down in short order, often even before effective fire fighting can be commenced. These oil lines may drain other stills, and the additional fuel added to the blaze may cause it to become uncontrollable. All supports for such lines should be protected with material known to have at least a three hour fire resistance period.
Pumping Out the Stills in Case of Fire
One of the simplest and most effective ways of handling a serious still fire is quickly to pump out the burning still and those which are menaced. Therefore, the importance as a fire protection measure of having large pumps lor this purpose and large pumping-out lines is apparent. But it is essential that the pumping-out valves shall be equipped with remote controls that so they can be operated safely and surely—usually from in back of the condensers. Likewise, to pump out a still the tar plug, if there is one, must be up. As tar plugs habitually stick it is desirable that the still be run with these plugs in open position, reliance for closure of the pumping out line being placed on the pumping-out valves, which may be in duplicate. Remote control and frequent inspection of the tar plugs should likewise be provided so that they may be operated surely and quickly from a safe position in case of emergency.
Glass gauge columns are serious offenders. It is most often impossible to use the customary safety ball-check valve for these gauge glasses. The ball checks corrode or coke up and will not operate. Where a gauge column is necessary, a pipe column fitted with closelyspaced pet cocks is safe, if in addition the gauge connections to the still are fitted with well designed level operated globe valves, the levers being held in open position by a chain and fusible link.
Strength of Steel under Certain Temperatures
A great many still fires are the result of carelessness and are caused by pumping over when charging, or permitting water to get into the charging stock thereby causing the still to froth over. Improper inspection or testing of safety and vacuum valves, clogging of traps and run back lines are all hazards, which may be relieved by proper inspection. In high pressure stills, particularly, the importance of inspection is doubly significant. I do not believe it is generally realized how high temperatures affect the tensile strength and physical properties of the metals used. I have investigated this problem and believe a few of my findings may be of interest. Ordinary steel plate becomes very brittle in the two temperature zones—• 200 to 400 and 800 to 1000° F. respectively. The ultimate strength of steel increases up to about 400’ F. at which temperature certain grades of steel actually become 40 per cent, stronger than at ordinary temperatures. At 700° F. the steel has about the same ultimate strength as at 70° F. At 1000° F. it has lost 1/2 of its initial ultimate strength. The elastic limit, however, which should determine the design and strength of parts rather than ultimate tensile strength does not increase materially above its initial value. At 600° F. the elastic limit is only one-half of its value at 70° F. At. 900° F. it has fallen to one-third.
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Oil Fires, Their Cause and Prevention
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It is usual to measure the thickness of the steel in different parts of a high pressure still at regular intervals because the hot oil apparently erodes the steel until it becomes quite thin. Naturally, since steel is much weaker at working temperatures of high pressure stills than at ordinary temperatures this matter must be watched very carefully and worn parts must be replaced before they become dangerously thin, due consideration being taken of the elastic limit of the steel at the working temperatures of the still.
Foam Most Effective Medium in Still Fires
As with tank fires, the most effective medium for handling a serious still fire is foam—applied this time through hose streams. At one very large plant that has had a very sucessful record in handling still fires, the superintendent told me he would not permit a drop of water to be used on such a fire. Unless water is used very carefully around a still it may very easily crack a hot fitting. We all know that water is not effective in extinguishing burning oil—that it acts as a vehicle for spreading an oil fire. However, it is of advantage in keeping structural steel work nearby cool. For this reason I feel the cautious use of water in keeping things cool until the foam system can be brought into action is advisable. Then, it is my opinion that the use of water should be discontinued and that the foam hose streams should be used. As the simultaneous application of water and foam will result in the water breaking down the foam it is apparent why this course should not be pursued.
Importance of Plant Fire Brigade
An obviously important factor in dealing with any fire, is the plant fire brigade. No matter how small the plant or station, the men who may be called upon to fight fires should be trained in what to do and how to do it in the emergency of fire. Where the plant is of sufficient size a definite military type of organization should be formed headed by a fire chief who has good judgment and is a quick thinker, who knows the plant intimately from one end to the other, whose strong personality enables him to dominate such a situation and secure action and co-operation from his men. Each man on his force should be drilled regularly and instructed in everything he may be called upon to do. His assistant and battalion chief should be of the same general calibre and be fully able to assume charge if necessary.
The brigade should be organized, drilled and maintained with the idea that fires shall be controlled immediately at their start, so far as possible. The nearest man belonging to the fire brigade should give the alarm immediately. The local captain in the area should take immediate charge. He should proceed to get the other men to work on the fire using local first aid apparatus, see that an alarm has been turned in, and delegate men previously selected to attend to the salvage such as saving stock.
Certain men should be delegated to remain in their several departments, closing fire doors, shutting down power, closing fire shutters, skylight, and taking other necessary precautions to guard their particular departments in case the fire should spread.
Method of Drilling the Brigade
Drills should he held once a month during paid time. The location of drills should be changed each time so that the men will become familiar with all conditions and parts of the plant. The handling of apparatus should be thorough in every respect and approximate actual fire conditions. It should embrace the operation of portable apparatus, the making of hose connections with hydrants, unreeling and stretching hose without kinks, coupling and uncoupling, carrying hose up ladders, over roofs and through the interior of buildings, reaching inaccessible and out-of-the-way places, including basements, attics, etc. It is important that the men should become proficient in holding play pipes, also carrying hose lines while under pressure. As a general rule, water should be turned on for all practice work, except during freezing weather. Certain men should be assigned to the definite duties of shutting off fuel oil and gas mains in case of emergency. Others should be instructed on methods of resuscitation. The crews selected to operate fire pumps, foam systems and other special apparatus likewise should be drilled regularly in their duties by the fire chief. And so, a great deal of invaluable time can be saved having these men fully trained for this emergency work.
Construction of building, subdivision of fire areas within buildings, external exposures and their reductions, installation of electrical equipment, heating equipment and power equipment, fire protection equipment from extinguishers to sprinkler systems and fire prevention education, are all matters of vital importance to the fire prevention man.
Fire prevention is principally a process of checking up. Fires can be prevented. To do it we only have to form and maintain three lines of fire defense, which are:
First—Not allowing fires to start.
Second—Minimizing combustible material and making construction such, that if fire does start, it cannot spread.
Third—Extinguishing fires that do start and spread in spite of our best efforts.
It is dangerous to rely solely on the third and last line. Fire cannot be driven back of our from trenches and kept there, unless we strengthen and maintain all three.
Work of a Fire Protection Department
A few years ago the company with which I am connected, organized a fire protection department. To obtain an accurate understanding of the conditions existing at each property, an original survey was made of every building, tank, still, agitator, loading rack, etc. All the details of construction, occupancy, internal and external exposures, fire hazards and fire protection were considered. As a result of. these surveys, detailed recommendations for the improvement of unsatisfactory conditions were made and submitted to the Operating Managements, and followed through to a ‘yes or no’ decision. Since that time reinspections have been made of each structure covered in the original survey at periods varying from three to twelve months for the purpose of detecting new hazards, and checking up on former ones. This department further has collaborated with the engineering and purchasing departments in the development of proper designs and specifications for fire protection materials and installations, thereby making use of the best materials and practices available. Reports of fires, no matter how small, occurring on our own properties are collected and analyzed. Information and lessons obtained thereform, which are of value, are distributed to those interested. Educational work is carried on by means of bulletin boards, direct contact, and through the mails.
It is too soon to forecast what will be the ultimate saving to the company through this work. A severe fire may occur tomorrow in spite of our precautions. It may be said, however, our chances of severe loss have been considerably reduced and should continue to grow less as the work further advances, until the limit of improvement is reached—at which time, as with accident prevention, the problem becomes one of maintaining the headway accomplished.
In its final analysis, fire protection is a problem of administration and organization. This important work must be properly organized, thoroughly studied, conscientiously executed, and last but not least, inspired by attention from the management if it is to achieve its deserving success.
(Excerpt from paper read before the Petroleum Section, National Safety Council.)