INDUSTRIAL AND ENGINEERING CHEMISTRY
November, 1925
the ash from Alabama coke. I n fact, the temperature was so high a t times that globules of melted ash were seen falling down through the coke. Since the ash from Alabama coal melts around 1650’ C., which is nearly as high as any coal ash in this country, it seems very probable that all coal ash would melt in the oxygen process. Calculations
The results of Run A in Table I1 using the gas analysis of Tank I1 have been figured on the basis of a ton of coke and are given in Table 111. With raw coal as the initial material the reactions discussed would not be essentially different; the results therefore
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referred to coal would be a matter of calculations, which are not here included. Table 111-Results Obtained on Gasification of Coke Using Oxygen a n d S t e a m w i t h a Vacuum on the Generator 66,000 Cubic feet of gas per ton of coke 920 Pounds steam required per ton of coke 13.95 Pound steam required per M of gas 20,562.0 Cubic feet of oxygen required per ton of coke 311.6 Cubic feet of oxygen required per M of gas Composition of gas, per cent: Carbon dioxide 10.4 Illurninants 0.0 0.6 Oxygen -~ 57.6 Carbon monoxide 20.7 Hydrogen 0.0 Methane 10.7 Nitrogen 100.0 253.5
Total B. t. u.
Fire Hazards of Static Electricity‘ By Rollin M. Clark2 CHAMBER OF COMMERCE OF THE UNITEDSTATESOF AMERICA, WASHINGTON. D. C.
STUDY of fire prevention methods discloses that the causes of all fires may be divided into two broad classifications; first, common hazards, so called because they are to be found generally; and second, special hazards which are inherent to specific types of occupancy. As a result of the comprehensive educational program which is being undertaken throughout the country by agencies interested in fire prevention, an appreciation of most of the common hazards has been engendered, but thus far the public has acquired little knowledge regarding the more unusual causes of fire. It is well known that defective electrical installations and improper use of electric current are prolific sources of fire. On the other hand, it is not widely recognized that under certain conditions static electricity discharges can cause explosions and start fires which may develop into serious proportions. The former is a common hazard, while the latter may well be termed a special hazard. Static electricity-or frictional electricity, as i t is sometimes designated-is a form of electrical energy generated by the contact and separation of two substances, one or both of which are insulators. One body will take positive charges and the other negative. Insulated materials retain these charges on their surfaces until contact is made with the ground through a conductor or until the intensity is sufficiently strong to overcome the resistance of the air, with the result that a spark jumps to the ground or to an object having a lower potential or a n opposite charge. If the substances which are brought into contact and then separated are both conductors, there is still a possibility that static electricity is generated, but the positive and negative charges are immediately neutralized or dispersed. Naturally there are no sparks in such instances. Formerly it was believed that friction was necessary to produce static electricity, but subsequent experiments have indicated that the rubbing of one body upon another is merely a convenient method of generating a charge. According to one authority, “It seems highly probable that the friction between the two bodies is merely incidental and the charge results from the fact that in the process of rubbing the materials are first brought into intimate contact and are then separated.” The rubbing evidently tends to distribute the charge over the entire surfaces of the two substances.
A
1Receive.d April 25, 1825. 9 Chief, Conservation Bureau.
Static electricity is often developed upon belts, pulleys, shafting and moving parts of machinery, as well as upon fabrics or other material, either liquids, solids, or gases, passing through machines and conveyors. I n order to be dangerous, static electricity must be discharged in the presence of some readily ignitible material such as an inflammable vapor or organic dust. If such vapors and dust are confined in closed or nearly air-tight systems so as to prevent their escape into rooms, the static hazard does not exist. This precaution should be adopted wherever possible. Static electricity fires are found to occur in the presence of cotton fiber or cotton dust; in machinery which handles or treats cereals, starch, and the like; in machines Finding sulfur and organic materials; and in locations where benzine, gasoline, or other inflammable liquids are being evaporated or discharged through pipes and orifices. The occupancies affected by the hazard include rubber mills, cloth-coating plants, japanneries, dry-cleaning establishments, and other places where inflammable volatiles are used, cotton gins, card and picker rooms of cotton mills, threshing machines, grain elevators, grain cleaning machines, and in sulfur and dye grinding operations. Dust Hazards
Experiments and investigations carried on under the direction of the Bureau of Chemistry of the United States Department of Agriculture have demonatrated that static electricity is the cause of many fires and explosions in plants where inflammable dust is found in quantities. Numerous examples of dust explosions attributed to static electricity may be cited. Within the last few years disastrous losses have been reported in the following occupancies: a grain elevator in Texas; starch plants in New Jersey and Louisiana; sulfur-grinding plants in Ohio and Ontario; and rubber mills in Massachusetts and Connecticut. A dust explosion is analogous in many respects to a gas explosion. I n both there are at least two stages, ignition and propagation. If a static spark of sufficient intensity should come in contact with a proper mixture of inflammable dust and air, ignition of the dust would undoubtedly ensue. The heat given off by the burning of the first dust particles ignites surrounding particles, which in turn ignite others near them. Thus the propagation proceeds in concentric circles with a rapidity dependent upon the mixture of dust
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and air and the inflammability of the dust. If the amount of dust suspended in the air is within certain limits, an explosion will occur. In clean plants the explosion may be localized, but where dust has been allowed to accumulate, the force of the initial explosion may possibly jar loose large quantities of dust, which are immediately ignited, thereby permitting the explosion to spread through the entire plant. Such explosions are accompanied by excessive pressures which can cause great life and property damage. The presence of static electricity in plants having a dust hazard is often revealed by the passage of sparks between pulleys and belts on the side where the latter leave the pulleys. It is also found on rapidly moving machinery such as grinding machines, elevators, conveyors, and screens. The movement of a dry product through conveying and grinding equipment may likewise cause the generation of static charges. Dust particles, when blown in a cloud, often have a static potential. It has been found that a potential is developed in filter type dust collectors by virtue of the passage of dust-laden air through porous cloth. To prevent dust explosions caused by static, it is essential that all moving parts of machinery be connected metallically and the machines themselves grounded. The frictional electricity will be neutralized through the connections between the moving parts, and if there is any excess charge it will be dissipated through the ground wire. Rotating parts of machinery may become insulated because of the film of oil surrounding the bearings, for lubricating oil is a nonconductor. Thus i t is possible for rotating parts to be charged even though the stationary parts are well grounded. A solution is to affix a small metal brush, such as a piece of piano wire, so that it will rub against a portion of the moving machinery. Such a brush should obviously be well grounded. Belts can be connected to the earth by installing metal combs in contact with their inner surfaces or by providing a network of grounded wires close to the belts and parallel to them. Various belt dressings have been used with the idea of making the inner belt surface conducting. Equal parts of glycerol and water are probably effective in some instances, the glycerol tending to retard evaporation of the water which acts as a conductor between the belt and the grounded pulley. If such a dressing is used, it should be applied every few days. Another method which may be more adaptable to dusty locations is the use of an idle metal roller which presses against the inside surface of the belt. I n addition to being grounded, the roller should be connected to the pulley by means of an electrical conductor. Cotton Gin Fires
The major cause of fires in cotton gins has been found to be static electricity. When it is considered that the annual loss from gin fires approximates a million and a quarter dollars, the importance of eliminating this bothersome hazard is obvious. The passage of cotton through the equipment and the operation of the ginning machinery are known to produce electrostatic charges. Such parts as the unloading and conveyor pipes, the cleaner and boll breaker, the gin stands, and belts are usually affected. Static charges on them are most dangerous when the relative humidity is low and the cotton being ginned is both dry and dirty. The most effective preventive measure is a complete wiring system for grounding the ginning machinery. Tests conducted by the Bureau of Chemistry have demonstrated the feasibility of grounding. The bureau has issued a circular in which complete details of such protective systems are described.3 8
Depl. Agr., Circ. 271.
Vol. 17, No. 11
Handling Volatile Liquids
Many mysterious fires and explosions which have occurred in the presence of vapors of gasoline, benzine, and similar volatiles are now thought to have been caused by static sparks. If these liquids are pumped through pipe lines or hose, poured through filters of chamois or cloth, or are kept in motion in tanks and other containers, static charges are created. As clean (anhydrous) gasoline and bemine are poor conductors, it is impossible for charges built up on ungrounded materials with which they are in contact to be dissipated. It is therefore important in handling these petroleum products that suitable grounds be attached to hose, pipe lines, tanks, and other receptacles in order that static charges may not accumulate. Buckets or pails used as gasoline containers should have metal handles. The use of metal-lined hose with fittings soldered at each end is advocated. This provides a direct electrical connection throughout the length of the hose. The nozzle should always be in contact with the grounded tank or container into which the gasoline is being pumped in order to afford a couple between the hose and the earth for the outlet of static charges. Permanent storage tanks located above ground and unloading racks may be safeguarded by having the piping grounded to another pipe driven into the earth below the frost line. When trucks are filled or unloaded, a stranded copper wire should be connected from the faucet or the piping near it to the chassis. Through the use of metallined hose or a direct attachment between the fill line and the storage tank, a complete metallic connection will be provided. The opinion of experts regarding the efficacy of dragging chains on gasoline trucks as a method of providing a ground is divided. Some claim the chain is needed to insure protection, while others believe that contact between the chain and the street pavements in common usage today will not effect a substantial ground, except possibly on wet days. Although there may be some justification in this contention, it is not likely that such a practice is harmful, particularly if it is not the sole dependence for dissipation of static electricity. A case is on record where an explosion was caused by a steam jet when used to clean an empty oil-storage tank. The vapor which remained in the tank was ignited by the charges which were set up. As a precautionary measure, the ends of the steam hose should have been connected metallically and grounded. Dry Cleaning Establishments
A serious accident occurred last fall in a middle-western city when naphtha vapor exploded in a dry cleaning establishment. The superintendent of the plant had taken a party of visitors into the cleaning room to show them the inside of a washer which had been running about half an hour. As he attempted to raise the cover, a sheet of flame shot out, striking him and the visitors in the face, arms, and body. Twelve persons were burned, some quite seriously. ii careful investigation indicated that during the operation of the machine static electricity had been generated. The lifting of the cover had broken the electrical contact between the cover and shell of the washer, causing a spark discharge. As the cover had been dropped immediately and the fumes had been consumed a t once, the fire consisted of nothing more than the one flash. The room itself showed no evidence of fire. It was discovered that the machine was grounded to a water pipe, but this had been rendered ineffective because the paint on the pipe had not been scraped a t the point of connection and the ground wire had not been soldered to the pipe. At its other end the ground wire was in poor contact with a painted and greasy part of the machine.
Iiovember, 1925
INDUSTRIAL A N D ENGINEERING CHEMISTRY
I n the same city another dry cleaning plant, fire was caused when an employee dragged a silk dress over the edge of a washer. A static spark resulted and naphtha fumes in the room were immediately ignited. These are typical examples of fires caused by static electricity in dry cleaning establishments. It is apparent that tjhe contact of silk and woolen cloth with naphtha introduces a real hazard in such businesses through the production of electrostatic charges. Wire connections between all metal parts of each dry cleaning machine, together with the grounding of the metal frame of the machine, should be effective. The precautions mentioned previously to eliminate the possibility of parts rotating in bearings becoming insulated should be followed in this type of machine. Belts should likewise be grounded, as discussed in a preceding paragraph. Static spark discharges are quite common when the relative humidity is low. On the other hand, fires from this source are rarely experienced when the humidity is fairly high. The physical theory underlying this phenomenon of high relative humidity is that many nonconducting solids and liquids such as benzene and gasoline in equilibrium will absorb on their surfaces a minute film of moisture which acts as an electrical conductor and thus carries away the static charges previously held by the material. Following a series of fires and explosions in its dry cleaning plant, the management of a n Illinois concern lately made a study of probable causes and remedies. It was decided that static electricity was the source of trouble and that the introduction of steam into cleaning rooms, with an attendant increase in relative humidity, offered the best solution of this perplexing problem. The conclusions to be drawn from the fires which have already occurred in the dry cleaning industry are: 1-Metallic connections between parts of machinery and ground wires from machine frames should be installed. 2-High relative humidity should be maintained. 3-Care should be exercised in removing cloth and garments from washers. C h u r n and Spreader Rooms
One of the foremost fire hazards in the rubber industry is that introduced through the softening of a rubber compound by means of an inflammable solvent and the subsequent spreading of this softened substance upon fabric and cloth. As is true whenever volatiles of this nature are present, static electricity becomes one of the more serious fire dangers. One authority states that the most frequent cause of fire in spreader and churn rooms and other processes involving readily ignited solvents is static electricity. He continues: An examination of fire records shows t h a t about three-quarters of the rubber factory fires in processes in which benzine is used, occurred in the winter time. This indicates that more than onehalf of the fires in rubber factories can be prevented by maintaining a n absolute humidity in the winter, in rooms in which benzine is used, of about the same amount that is provided by nature in the summer months, t h a t is, about 4 or 5 grains of moisture t o the cubic foot of air.
Electrostatic charges probably originate in spreader rooms coincident with the evaporation of the solvent benzine. The charges remain on the cloth unless drawn off by a proper ground connection to the knife of the spreader. This ground wire should connect the steam pipes of the table and the machine frame to the knife. Because evaporation of the solvent continues after the cloth has passed the knife, additional charges may be formed and retained by the cloth when it is rolled. High relative humidity will tend to keep the potential of the charges a t a minimum. The electrostatic capacity of men makes it advisable to
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provide additional precautions in order that no sparks may jump from the bodies of workmen to metal parts. The combination of metal floors and copper- or brass-pegged shoes for workmen affords one remedy. Grounding
I n the preceding paragraphs numerous references have been made to grounding as a means of dispersing static electricity. Properly grounded equipment is of unquestioned assistance’in preventing spark discharges, but in order that maximum protection may be obtained it is necessary to make frequent inspections of the connections. Corrosion, the presence of oil and grease, or indiscriminate painting may often make useless what was once a good bond. A recent incident shows how difficult it is successfully to ground some types of equipment. In this particular case a collodion solution was being made up, using anhydrous solvent in a cast-iron kettle of about 100 gallons capacity. The vertical shaft of the agitator passed through the center of the cover and was driven in the usual way by belt and pulley through a pair of bevel gears. Both the kettle and the driving mechanism were grounded, but in spite of this a spark could be drawn from the agitator shaft. The solution evidently was a very poor conductor and the static therefore accumulated on the agitator, the film of oil in the bearings preventing it from being conducted away. I n this case the shaft itself had to be grounded. A similar kettle next to it, containing the same collodion with the addition of aluminium powder, never showed any signs of the accumulation of static. Apparently, the metal powder made the solution conducting and any static formed grounded itself through the kettle. It is advisable that ground connections be made by a workman of more than average intelligence and then carefully checked by the plant superintendent or a competent engineer. The latter should see to it that thorough inspections are made periodically. Too much care cannot be taken. Fire Protective Facilities
Despite all efforts made to combat static dangers, fires may still occur. Sole reliance should therefore not be placed in grounding, high relative humidity, and other preventive measures; suitable fire protection should also receive due consideration. The installation of automatic sprinkler systems is recommended generally in buildings possessing an inherent static hazard. If fire does occur, a deluge of water is immediately released over the seat of the blaze. Where large quantities of solvents are used, floor drains or curbs should be provided to prevent danger from overflowing. The lives of two workmen in an Atlanta dry cleaning plant were saved through the opening of overhead sprinklers following a fire caused by a static discharge. Extinguishers of the foam type are suggested as first-aid protection against oil fires. It is possible to arrange such devices directly over tanks containing volatile liquids so that their contents will be discharged into the tanks automatically if fire occurs. Their effectiveness was demonstrated by a recent fire in Wisconsin. Static electricity generated on aluminium basins which were being cleaned in tanks of naphtha caused an explosion of vapor and ignited one of the dip tanks. Automatic foam extinguishers suspended above the tank operated and a t the same time a quick-opening drain valve in the bottom of the tank was actuated. These devices were so effective that no automatic sprinklers were opened and the tumbler was not damaged. Hose streams and steam jets are other forms of protection which may be utilized, but they are not to be recommended over sprinklers and foam extinguishers because of practical difficulties they present in fighting inflammable liquid fires.
