REGION OF NONCOMBUSTION IN NITROGEN-OXYGEN AND HELIUM-OXYGEN DIVING ATMOSPHERES G E R H A R D A. C O O K , ’ V I C T O R A. D O R R , A N D BRUCE M. S H I E L D S
Linde Division Research Laboratory, Union Carbide Gorp., Tonawanda, N . Y .
Minimum oxygen concentrations required for the combustion of solid flammables in the presence of nitrogen or helium over a range of pressures were determined. The limit of noncombustion was defined as the highest percentage of oxygen in a mixture of oxygen and diluent gas a t which combustion of an ordinarily flammable solid will not take place upon ignition. This limit varies from 8 to 5.6y0 oxygen in mixtures of nitrogen and oxygen as the total pressure increases from 0.5 to 1 6 atm. absolute and from 10.8 to 3,6y0 oxygen in mixtures of helium and oxygen as the total pressure increases from 0.5 to 42 atm. absolute. The limit also varies with the temperature of the ignition source and the nature of the combustible solid.
sea diving is of current interest in connection with the exploration of the oceans, commercial underwater activities, and naval operations. Divers who explore or carry out work at appreciable depths must be supplied with breathing gas at the same pressure as the hydrostatic pressure to which they are subjected. Since the breathing gas contains oxygen, and the presence of sparks, hot spots, or other ignition sources cannot be completely ruled out, there is possible fire hazard, which has been well described by Harter (1966). I t is therefore important that either the oxygen in the breathing gas be sufficiently diluted by one or more inert gases (usually nitrogen for shallower operations, helium for greater depths) or that divers’ clothing and other materials in the chamber be nonflammable. After a diver has completed his underwater operations and desires to return to the surface, he must be gradually decompressed to relieve his body fluids and tissues of dissolved gases. If the decompression is not carried out slowly enough, the gases may form painful bubbles inside the body. Formation of the bubbles is called “the bends” or “decompression sickness”; it is exceedingly painful and may be fatal if not promptly treated. Because of the necessity for slow decompression, quick egress from decompression chambers is usually impossible without grave risk to the divers. I n case of fire, there is not only the danger of burns to the body, but of inhalation of the toxic products of combustion and pyrolysis. I n this article, one aspect of our study of the fire hazard problem in hyperbaric breathing gases is described. EEP
The Problem
When a candle burns in air a t atmospheric pressure in a confined space, the flame goes out before all the oxygen is used up. Burning paper and other solid combustibles behave in a similar manner, not only at atmospheric pressure but at greater pressure. When a candle burns at decreasing oxygen concentrations, there is an intermediate combustion period in which the rate of combustion is low and the candle flame flickers before it finally goes out. This intermediate combustion zone can be fairly quantitatively and reproducibly explored by use of strips of filter paper. 1
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Present address, c / o American Embassy, Asuncibn, Paraguay l&EC PROCESS DESIGN A N D DEVELOPMENT
In mixtures ofcombustible gases or vapors with air or oxygen, fairly definite composition limits for flammability can be given, and there is a considerable body of literature (Coward and Jones, 1952; Zabetakis, 1965) on the determination of these limits a t atmospheric pressure. A few data are also available for higher pressures (Yantovskii and Chernyak, 1966). We have, however, found very few data on the minimum oxygen concentration at superatmospheric pressures required for the combustion of solid flammables. We have experimentally determined the maximum oxygen concentration at given pressures at which filter paper and other ordinarily flammable solids will not burn, even in the presence of a strong ignition source. We have defined this maximum oxygen concentration as the limit of noncombustion. When the limit is plotted against pressure, it gives a line which delineates the conditions of gas composition and pressure which will not support combustion-Le., the region of noncombustion. The noncombustion limit varies somewhat with the nature of the flammable material being tested, but it is remarkably reproducible, being of the same order of reliability as the values found in the literature for flammability limits in mixtures of combustible gases with air or oxygen. Apparatus and Technique
Experience in various oxygen-containing atmospheres has shown that ordinary filter paper will burn at about the same rate as other kinds of paper and combustible cloth fabrics. Use of filter paper in combustion studies has the advantages that it is universally available, fairly uniform, and readily cut into strips of closely controlled dimensions. Most of our experiments were conducted with Whatman No. 1 filter paper strips, 10 mm. wide by 160 mm. long, in equilibrium with the moisture in the air of the room. The strips were suspended vertically inside a stainless steel pressure vessel 6 inches in diameter by 20 inches long. The volume of gas inside a vessel of this size is sufficiently large so that if a strip is completely burned, the mole per cent of oxygen in air at atmospheric pressure is only reduced from the initial value of 20.95 mole yo to about 20.17$, a consumption of about 470 of the available oxygen. At 2 atm. pressure, only about 2% of the oxygen would be used, and at higher pressures, correspondingly less. The paper strips Lvere ignited at the lower end by a grid of Chromel-A wire in good contact with the paper. Initial experiments showed that the extent of combustion of the
strips varied somewhat with the temperature of the igniter wire. When the igniter temperature was approximately 1200" to 1300' F., a vertical strip of filter paper in a 15.6% oxygen and 84.4% helium mixture a t 4.03 atm. absolute did not burst into flame, but only smoldered slightly and extinguished itself within 1 cm. past the igniter. However, when the igniter temperature was increased to l6OO0 to 1700' F., a paper strip under the same conditions burst into flame and burned completely. For all the results given here, the initial temperature of the wire was 1400' =t 50' F., measured by means of a Chromel-Alumel thermocouple welded to the wire. This was the highest temperature which may be repeatedly used without burning out the igniter. Initial experiments also showed that the combustion rate of filter paper strips varied with the width of the strips. I n air a t atmospheric pressure, the burning rate of a strip 12 mm. wide was almost double the rate of a strip 6 mm. wide (Cook et al., 1367a). T h e width of the filter paper strips was rigidly controlled in order to obtain consistent results. A more complete description of the apparatus and technique is available in a report on combustion safety in diving atmospheres (Cook et al., 1967a). Gas mixtures of known composition were prepared from U.S.P. oxygen and pure nitrogen or helium. After the gases had been admitted to the stainless steel vessel at the desired partial pressures, they were mixed by means of a small electric blower inside the vessel. T h e oxygen percentage was verified by direct analysis on a n Orsat analyzer until the preparation technique was perfected. No moisture was added to the gas mixtures; neither was any special effort made to dry them. After the data with filter paper were obtained, a few experiments were run with strips of waxed paper and cotton terry cloth of the same width and length. as nearly as possible, as the paper strips.
completely, shown by points in diamonds in the figures; ( 2 ) ignited and burned for a length greater than 1 cm. above the igniter grid, but the flame extinguished itself before the strip was entirely consumed, shown by triangles; (3) ignited to produce either a flame or smoldering or both; but there was no combustion above a point 1 cm. above the igniter wire, shown by circles, or (4) not ignited a t all by the glowing igniter, shown by squares. Points in category 4 are in the pressure-composition region of noncombustion. T h e results obtained with waxed paper
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Preliminary tests indicated that combustion is most rapid when the sample strips are in the vertical position and that small variations in the moisture content of the filter paper or of the gas mixture did not make an important difference in the rate of combustion. O n the basis of this experience, it was decided that the time-consuming steps that would be required for more exact moisture control in the paper were not necessary.
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Figure 1 . Three combustion zones for vertical paper strips in nitrogen-oxygen atmospheres
T h e results obtained with the strips of filter paper were classified into one of four categories and the values were plotted in Figures 1 and 2 : the strip was (1) ignited and burned
In absence of material more flammable than paper, AD€ m a y b e considered a flre-safe and physiologically satisfactory breathing zone for divers. DBCE is fire-safe but not physiologically acceptable because of danger of nitrogen narcosis
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Three combustion zones for vertical paper strips in helium-oxygen atmos-
ABC is both fire-safe and physiologically acceptable up to a pressure of at least 20 atm.
The upper limit for physiologically desirable helium breathing pressures has not yet been determined
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and cotton terry cloth were so close to those obtained with filter paper that they are not shown on the graphs. Figure 1 gives the experimental points for mixtures of nitrogen and oxygen in the pressure range 0.5 to 16 atm. absolute. T h e area below the heavy lower curve is the region cf noncombustion, the area above the heavy upper curve is the region of complete combustion, and that between the two curves represents the intermediate region (that of smoldering and selfextinguishment). An intermediate region of incomplete combustion has also recently been found for gaseous mixtures of hydrogen and oxygen, in connection with a determination of the explosive limits (Yantovskii and Chernyak, 1966). Figure 1 also includes two isobars, shown as broken lines; the lower one represents a constant partial pressure of 0.21 and the upper one of 1.5 atm. of oxygen. Figure 2 gives the corresponding results for helium-oxygen mixtures. The experiments with helium as diluent were carried out to a pressure of nearly 42 atm. absolute, the pressure a t an ocean depth of about 1350 feet. The lower heavy curve (noncombustion limit) was in each case drawn conservatively-Le., toward the lower percentage of oxygen when the exact location was in doubt. The true location of the noncombustion limit curves may, in places, be higher by 1 or 2 mole % oxygen. I n general, the curves are believed to be accurate within about 1 mole oxygen. Discussion of Results
There are fairly well-known physiological breathing limits for both oxygen and nitrogen. Prolonged human exposure to atmospheres containing a partial pressure of oxygen greater than about 1.5 atm. may result in convulsions or produce lung damage, and the breathing of gas containing more than about 5.5 atm. partial pressure of nitrogen tends to produce narcosis, described below. A partial pressure of 0.21 atm. of oxygen is generally sufficient for human respiration, although a t higher than atmospheric pressure, it is sometimes desirable to supply a little more oxygen than this as a safeguard against hypoxia in the event of a malfunction in the oxygen-control system. T h e upper boundary of the safe oxygen-breathing zone cannot be defined exactly (Lambertsen, 1966), but for practical purposes the human breathing range may be considered to be between the 0.21 and the 1.5 atm. partial pressure lines for oxygen shown in Figures 1 and 2. Area ABC is the area in which the oxygen concentration is sufficient for breathing, yet not enough to support the combustion of paper and cotton terry cloth. I n Figure 1 there is, however, in the ABC area, a region in which the danger of nitrogen narcosis must be considered. When a diver works for more than a few minutes at a nitrogen partial pressure of over 5.5 atm. absolute, he usually suffers from symptoms similar to those of anesthesia or alcohol intoxication, and becomes ineffective as a worker. There is actually, therefore, only a small region (ADE in Figure 1) in which a diver may be considered to be quite safe from both fire and physiological hazards. If a diver is to go to depths greater than about 200 feet (7 atm. absolute total pressure), the nitrogen is partially or entirely replaced with helium. Helium does not produce narcosis until some (as yet undetermined) pressure corresponding to a depth greater than 720 feet of sea water (23 atm. absolute pressure) is reached (Hamilton et al., 1966). When a diver works a t depths of less than 200 feet of sea water while breathing nitrogen-oxygen mixtures, the oxygen concentration is usually well above that of the noncombustion 310
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(lower heavy) line. Precautions against fires should therefore be taken. T h e worst fire hazard in ocean diving operations arises during the final stages of the decompression process. As sea level pressure is approached, it is customary to enrich the atmosphere with oxygen (Harter, 1966) in order to hasten the safe elimination of inert gases from body fluids and tissues. During this period, therefore, clothing, oxygen masks, bedding, and other items in the chambers should be made, if possible, from noncombustible fabrics, noncombustible elastomers, etc. (Cook et al., 1967, a and b). As a further precaution, a suitable water spray or fog system should be provided. Combustion of Solids
There is as yet no well-established theory from which the limits of noncombustibility for solid flammables could be calculated. The heat capacity correlation suggested by Huggett and his associates (1966) was derived from data obtained over only a small range of pressures and for combustion in either the horizontal or downward direction. Huggett’s correlation shows no pressure dependence and does not apply to our results. If combustion of a solid, such as paper or cotton cloth, is accompanied by flame (rather than just by smoldering), it is assumed that pyrolysis of the solid to give combustible gases precedes combustion. When the combustion is carried on only as smoldering, it may be assumed that the solid surface is oxidized directly by oxygen gas. Noncombustion l i m i t
We may visualize the phenomena associated with this investigation by use of the following simplified model. The hot igniter grid pyrolyzes some of the solid sample to form combustible gases, or preheats some of the solid to a temperature a t which smoldering can take place. T h e hot gases or the hot surfaces react exothermically with oxygen in the atmosphere of the container. If the rate of heat production brought about by the gaseous or surface combustion process is equal to or greater than the sum of (A) the heat required to preheat or pyrolyze an additional quantity of the solid, and (B) the various kinds of heat loss, combustion will continue up the vertical strip of solid material. If the rate of heat production in the initial combustion is less than the sum of A and B, there will be either no combustion past the igniter grid or incomplete combustion. We see that once the solid surface or the pyrolysis gases have reached combustion temperature, the rate of heat production by oxidation \Till be determined largely by the rate at which oxygen molecules are brought into contact with the combustible material by diffusion and convection. The lower the oxygen concentration a t any given pressure, the lower will be the rate a t which oxygen molecules can be supplied to the combustion zone. The heat losses include conduction along the combustible material and its ash, conduction by gas molecules, convection, and radiation. U p to a pressure of about 8 atm. absolute, the noncombustion limit occurs a t a higher percentage of oxygen when the diluent is helium (Figure 2) rather than nitrogen (Figure l ) , but at pressures above 12 atm. absolute, the reverse is true. Perhaps ignition and burning take place less easily at the lower pressures when helium is the oxygen diluent, because the greater thermal conductivity of the helium brings about such high heat losses that preheating the sample to the ignition temperature becomes more difficult.
At pressures above about 12 atm. absolute, the safe oxygen limit is somewhat lower in helium-oxygen than in nitrogenoxygen mixtures. Here, apparently the fact that oxygen molecules can diffuse more rapidly through helium than through nitrogen (at the same temperature and total pressure) becomes more important than the greater rate of heat loss in helium as compared to nitrogen. Conclusions
We have conveniently defined the limit of noncombustion as the highest percentage of oxygen (in a mixture of oxygen and diluent gas) at which combustion of a n ordinarily flammable solid will not take place upon ignition. This limit varies with the nature of the diluent gas and with total pressure, as shown in the lower heavy curves in Figures 1 and 2. The limiting oxygen percentages shown in the figures vary somewhat ivith the temperature of the ignition source and with the nature of the combustible solid such as sample size and bulk. O u r tests were conducted on a small-scale basis and this may underestimate the degree of flammability in large-scale fires. The authors will perform scaled-up experiments in a 250-cu. foot capacity decompression chamber in the near future. Since our data are based on a rather high-temperature ignition source (1400’ F.) and on readily flammable materials (filter paper, iraxed paper, and cotton terry cloth), the results are believed t o be useful in diving and other operations in \rhich human beings have to function in confined spaces filled with gases a t a pressure of 0.5 to 42 atm. absolute.
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Materials that are suspected of being more readily combustible than paper should be tested in the laboratory before they are used in diving chambers or other confined spaces. literature Cited
Cook, G. A , , Meierer, R. E., Shields, B. M., “Screening of FlameResistant Materials and Comparison of Helium with Nitrogen for Use in Diving Atmospheres,” Defense Documentation Center No. AD 651583 (March 31,1967 a). Cook, G. A , , Meierer, R . E., Shields, B. M., Textile Res. J. 37, 591 (1967 b). Coward, H . F., Jones, G. W., “Limits of Flammability of Gases and Vapors,” Bur. Mines Bull. No. 503 (1952). Hamilton, R . W., Jr., MacInnis, J. B., Trovato, L. A., Schreiner, H. R., Aerosfiace Med. 37,281 (1966). Harter, J. V., “Fire at High Pressure,” Third Symposium on Underwater Physiology, March 1966, National Academy of Sciences, Washington, D. C. Huggett, C., von Elbe, G., Haggerty, W., “Combustibility of Materials in Oxygen-Helium and Oxygen-Nitrogen Atmospheres,” USAF School of Aerospace Medicine Publication SAM-TR-66-85, Defense Documentation Center No. AD489728 (December 1966). Lambertsen, C. J., “Oxygen Toxicity,” in “Fundamentals of Hvuerbaric Oxvaenation.” National Academv of Sciences. NAiional Reseaich C o u n h , \I7ashington, D. ’C., N.4S-NRC Publ. 1298 (1966). Yantovskii, S.A . , Chernyak, M. V., Russ. J . Phys. Chem. 40, 1557 (1966). Zabetakis, M. G., “Flammability Characteristics of Combustible Gases and Vapors,” Bur. Mines Bull. 627 (1965). RECEIVED for review .4ugust 28, 1967 ACCEPTED January 8, 1968 Studies aided by Contract N00014-66-C0149 between the Office of Naval Research, Department of the Navy, and Union Carbide Corp.
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COM MUN ICATIONS EMPIRICAL EXPRESSION FOR T H E TURBULENT FLOW VELOCITY DISTRIBUTION An equation permits determination of the velocity distribution as well as the eddy momentum diffusivity profile for turbulent flow in both smooth and rough pipes. The empirical nature of the equation bypasses the simplifying assumptions often used in previously reported turbulent velocity profile expressions. Comparison of experimental data from Nikuradse with predicted data over the Reynolds number range of 9000 to 3,000,000 indicated average deviations of only z!= 1 % over the entire flow region. A simple linear relationship exists between the maximum and average velocities, with surface roughness as the primary parameter.
problems associated with the.analytica1 determination of the velocity distribution for turbulent flow in circular ducts have confronted research and industrial people for a number of years. Two difficulties immediately arise: The turbulent mechanism is not sufficiently well understood to fit a reasonable model, and for the complex models that are presented the mathematics cannot be solved without making some sort of simplifying assumption. These difficulties often cause the velocity distribution expressions based on the mixing length of Prandtl (1933), the similarity hypothesis of von Karman (1931b), and the universal equations of von Karman (1331a) to be incompatible with the physical situation existing in the turbulent flow process. An empirical expression is presented here which permits THE
calculation of the turbulent velocity distribution in such a way as to fit the physical boundary conditions found in pipe flow. I n addition. the velocity distribution expression can be used to obtain the eddy diffusivity as a function of radial position. An examination of the velocity distribution data of Nikuradse (1932, 1933) indicates that if the ratio of the time-averaged point velocity to the average center line velocity is plotted against the square of the radial position ratio, the resultant curve has two regions corresponding to the wall and turbulent core portions of the velocity distribution. The functional relation
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