7 in the Chemical Iaborafory I Edited by N O R M A N V. STEERE, 140 Melbourne Ave., S.E. Minneopolir, Minn. 554 14
XLIX. Dangers due to Increased Flammability of Materials in Oxygen-Enriched Atmospheres 1. J. WILK, Ph.D., P.O. Box 5006, Stonford, Calif. 94305 Introduction T h e use of pure oxygen, both in the liquid and gaseous state, has been extensive for many yearn, increasing constantly. Whereas in olden days proper respect was paid to the dangers inherent in handling pure oxygen, particularly if pressurized, and oxygen-enriched atmospheres, continued use over many years brought about a contempt of the dangers, bred from familiarity. Unfortunately, i t took s. major tragedy, resulting in the death of three astronauts on Jannary 27, 1967, to draw attention to the ever-present dangers when pure oxygen, or oxygen-enriched atmospheres are handled. The simplest. way to demonstrate these dangers is to list a. number of case histories, all from the open literature. The first part of this report will cite a. number o f accident8 where injury, or death, to personnel was due to flame-propagation in an oxygen-enriched atmosphere. The second part will be concerned with effects of such atmospheres on the combustion of various matorisls. Part,icular emphasis will be placed on the flammability of clothing in oxygen-enriched atmospheres. Caases of ignition, and the possibility of spontaneous combust,ion in w e h xtmospheres will be covered. I n conclusion, b summary of pertinent safety regulations will be presented.
Case Histories A. Vent stacks in an rmdergronnd launching complex had been sprayed with a polyurethane foam ta protect the steel against occasional splashes of liquid oxygen or nitrogen. I t was realized, after application, that the foam, although fire-rebardant, was not compatible with oxygen. I t was, therefore, decided to remove t,he foam. Unfortunately, p l y urethane foam adsorbs and retains oxygen vapors. The workman who applied a spark-proof sand in a blasting gun to the foam was immediately engulfod in flames, and last his life ( 1 ) . B. An oxygen compressor fire in an air separabion plant in Ecorse, hfich., led to the death of three persons, as well as extensive damage to the plant. A eompressor which had given some trouble had been shut dawn. Whon it was started no again, ignition took place, presumably n{ the center cylinder (8). C. An air separation plant in Germany had been shut down for repairs. I t was started up again. Smoke began ~~
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to come out of the cold box jacket, and a small fire appeared. This fire was extinguished. A short time lat,er an explosion took place which destroyed the plant, killed faorteen workers, and injured an additional thirteen persons (4). I n order to properly undentand t,he following cases one m w t keep in mind that clothing can absorb oxygen, and retain fairly high concentration of this gas for half an hour or longer. D. A person who was wearing the proper protective, safety elothing was working in an oxygen-enriched atmosphere. H e went to a safe smoking area, and lit a cigarette, whereupon his clothing also ignited. No indications were given as to the extent of injuries (4). E. Due to spillage or leakage liquid oxygen was discharged into the sail, or a cinder fill. A laborer was digging a hale about 150 f t from the point of discharge. He lit a cigarette and was burned. Apparently it was not est,ablished whether the oxygen migrated through the fill, or along the surface (4). F. Rupture of s liqnid oxygen line caused the flawing of axygen into the soil. The oxygen migritt,ed t,hraugh the soil for 200-300 ft, and accumulated in an excavation. A workman went into the excavation a. day after the rrqture of the oxygen line. He lit a cigarette, and was burned to deabh (6). G. Oxygen from a leaking 6 in. valve had saturated the clothing of two men working in a furnace in x hydrogen plant. The clothing of the two men was ignited, due to spontaneaw combustion, a spark, or lighting of a cigarette. Both men were hurned to death. Analysis of the atmosphere immediately after the accident showed that t,he oxygen concentration in the furnece varied from 41 to 5.37,. The gas escaping from the valve had an oxygen content of 70% ( 6 ) . I A welder was working in a tank car. After a while he interrupted his work in order to renew t,he air in the tank by introducing oxygen. When be resumed his welding a 8park ignit,ed his clothing. The worker succumbed t,o fatal burns ( 7 ) . I. A boiler contained some tarry, not easily ignitible, residue. I t was sapposed to be welded from the outside only. Hawever, the welder wanted t,o reinforce the welds from the inside. Prior to doing so he allowed oxygen from a pressure battle to enter the boiler for 10 mi", in order to remove some "bad air." The welder climbed into the boiler, and st,arted the electrodes. His clothing ignited im-
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mediately. He tried to beat the fire out, and two of his fellow workers poured water over him. I t was to no avail. After a few hours he died of bums (7). J. A 10-m deep well was to be exeavnted some more. I n order to remove toxic gases the well had been pnrged with oxygen. Wheu the dynamiter ignited the cord, his elothing also ignited. 'Ie was able to leave the well prior to the explosion; however, t,he burns were fatal (7). K. I n an accident similar to the one cited above a 23-m deep well was purged with oxygen for 45 min. Two workers, a t the bottom of the well, were working wit.h hammer and chisel to remove some sandstone. Nothing happened for several hours. Suddenly a big spark ignited their clothing. Both were found dead a t the hot,tom of the well. Analysis of the air in the well, carried out the following day, gave a concentration of 5.8y0 for carbon dioxide, and 23.7% for oxygen (7). L. An irritant gas, an organic compound, was dispersed in a room designed for testing gas masks. Oxygen was introduced into this room in order to displace the irritant gas. Four persons with gas masks entered the room, whereupon a gas pistol was discharged, filling the room with the irritant gas. A large flame formed immediately, ignited other materials in the room, eventually lending to fatal bums for three of the persons in the room. The irritant gas readily burns in oxygen (7). M. A leak in an oxygen line was detected. A door leadina from the leakage area to an adjoining ronm was locked, &d all cracks sealed, except for the keyhole. A worker in this room lit a cigarette, whereupon the flame from the match also ignited his trousers and underwear. He was burned severely. His recovery toak seven monbhs. The clothing which burned was made out of cotton. T h e worker's woolen socks, and woolen sweater, were not damaged by fire (7). N. An oxygen line had been depressurized, but not purged. A worker who was carrying out repairs under the line soddenly found a. fire hurning around his neck, ears, and face. He was able to extinguish the fire without difficulty. He suffered minor burns, as well as the loss of his shirt collar. His shirt had been saturated with oxygen. It is likely that a. welding spark ignited it,. I n the regular atmosphere a welding spark would cause a small hole in the shirt only (8). A number of accidents have been reported when oxygen was used as s pressarising agent in place of an inert gas (7). I n most cases no injury to personnel toak place. Organic substances, such as oils, and other easily oxidizable compounds may
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easily ignite in oxygen-enriched surroundings. (7, 9, 10) Organic dusts must be brested with special care under such conditions (11). Tho extreme danger of oxygen atmospheres is demonstrated in n report which cites the complete destruction of a stainless steel high-pressure oxygen pump. Particular note should be taken of this accident, because, under the conditions of operation of this pump, stainless steel should not he subject to combuslion (8).
Causes of Ignition The probability of autoignitiun taking place goes up with an increase in the oxygen concentration of the atmosphere, , and bhe temperature of the medium. A number of reports discuss the spontaneous ignition of hydrocarbons.(lb-14) As was stated above, certain organic compounds, among them oils, and other easily oxidizable materials, may satoignite under proper conditions. When rust particles are exposed t o a n oxygen stream, they may begin to glow. The result would be ignition.(?) Carbonaceous material, or carbon, readily auto-oxidizes. (15, 16) Another source of danger would be organic peroxides, farmed in oxygen-rich atmospheres.(l?) The most likely sources of ignition would he flames and sparks. Whereas sparks a t atmospheric oxygen concentrations present little or no danger, they are a serious hasard a t oxygen concentrations higher than those found in air. Although everyone is familiar with the generation of static electricity, e.g., when removing s. sweater or combing one's hair, most ere not aware that charges of up to 10,000 volts may be built up. Grounding of this charge will lead t o spark farmstian. I n an oxygen-enriched stmosphere, where even fire-retardant materials burn vigorously, serious accidents may hzppen. Escaping gases generate and carry an electrostatic charge. Formation of this charge is not prevented by grounding of the equipment,, e.g., oxygen cylinders. The potential of these charges will depend on: 1. Degree of perturbation of the escaping gas, 2. The number of liquid particles present, 3. The escape velocity of tho gas.(l7)
When the cold oxygen gas stream contains ice particles, a relatively high electrostatic charge is built up.(18) By reducing the humidity in the s o n r m ~ n d i n g atmosphere, formation of ice particles can be reduced. Ohvioidy, mixing of the released oxygen with copious qllsntities of air, or, better vet. . , an inert -eas.. will reduce ienition dangers. Q~.o~rndiogof all clothing, possibly tlrrnugh cnnduclivc shocs, and conduct,ive floors, would make working in oxygenenriched atmospheres milch safer.
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Flammability of Clothing in Oxygen-Enriched Atmospheres As was pointed out in case history M, wool is less likely to burn in oxygen-rich atmospheres than cotton. I t is reported that cotton fibers contain cavities which retain oxygen or air.(lY) hIent,ion has been made of the fact that ignition of cotton can he ascribed to oxidation of fats cont,aining maaturnlion. (19, 20) A number of studies have been carried out to dotermine the effeot of oxygenenriched atmospheres on t.he ignit,ion and bnrning rate of various fabrics and materials.(Sl-27) A comprehensive stody, concerned with burning rates of a. number of materials at various pressures, and wing a nnrnhcr of different gas mixtures, is presently under wny.(S8)
Safety Considerations I n atmospheres enriched with oxygen flash, fire, and autoignition points are lowered, and explosive limits of cerbain compounds may be widened. The vigor of combustion processes increases eonsiderably. I n a ~ c ntmospheres h materials will be consumed by flames in a very short time. Fire-rerardant clothing will add a. safety factor hut, if conditions are too adverse, will not be able to guarantee the safety of personnel. An increase in the oxygen content of air of only 1-2% can lead to serious accidents. Many materials, such as clothing, have a t,endency to soak up oxygen, and retain it for extended periods of time. Such clothing must he aerated immediately. A change of clothing should be availahl.ble at all times; oily and greasy contsminants must he avoided. Constant monitoring of oxygen levels in potentially dangerous areas is mandatary. Once the oxygen level passes 21%, appropriate safety measwes must, be taken. Since oxygen is slightly heavier than air, oxygen clouds will accumulate and move along t,he floor level. The colder oxygen vapors are, the heavier they are. Such cold clouds may cause candensat,ion of the moisture in the air; the condensed moistore will be visible. The coneent,~atimof oxygen in the clouds will he highest in the center of the cloud, decreasing toward t,he outer edges. However, the concentration of oxygen a t the ooter edges will exceed atmospheric concentration by about 1 9 The longeviiy of such clouds, even in the open, may be considerable, particularly when there is no or lirt,le wind, the temperature is low, and a low inversion layer exists. General safety regulstians are pnhlished by the National Fire Protection Association, and a number of companies concerned with the handling of liquid and gaseous oxygen.
References (1) Air Products and Chemicals Safety Bulletin No. IF, Ang. 3, 1962. (2) Ibid., No. 3, July 7, 1961. (3) Ibid., No. 2, July 7, 1961.
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(4) Cryogenic Safety, A Summary Iteport of the Cryogenic Safety Conference, Allentown, Pa., 1959. (.5) Z.\IIETI
