Chemical Data Notebook Series:
Molecular oxygen, a colorless, odorless gas that is If essential for life as we know it, presents a potential danger to emergency responders when involved in an accidental release.
Oxygen is a very reactive material, hence its ability to support combustion, which is an oxidation reaction. However, not all oxidation requires mitigation by emergency personnel. Oxidation reactions may take place at much slower rates, such as in the process of spontaneous ignition, or even slower as in the rusting of iron and other metals containing iron.
Oxygen is used principally for its strong oxidizing ability. It is used in the manufacture of nitric acid, ethylene oxide, propylene oxide, and many other chemicals. Oxygen is crucial to the manufacture of steel, and is used in the cutting and welding of many metals in conjunction with acetylene. It is also used to purify waste water.
Oxygen will be discussed as a gas, which is its natural state of matter, and as a cryogenic liquid, which is how it is most often shipped and/or stored.
The Department of Transportation (DOT) requires the yellow oxidizer placard or label to be used on shipments of oxygen. The oxidizer placard has black lettering saving “OXIDIZER,” with a large bail or “O” with flames around the upper half. The UN/NA designation for oxygen is 1072 for the gas and 1073 for the liquid. The Standard Transportation Commodity Code (STCC) is 4904350 for the gas and 4904360 for the liquid.
The UN/NA class is 5 for oxidizers. There is also a special oxygen placard, again black letters on a yellow background. The word “Oxygen” appears in the center, and the ball or “O” with flames appears above it. This placard is used whenever compressed oxygen is being shipped. The UN/NA class number for compressed gases is 2, and this appears at the bottom of the placard.
When liquified oxygen, a cryogenic material, is shipped, the cryogenic or oxidizer placard may be used.
Oxygen makes up approximately 20.95% of the earth’s atmosphere (dry air) at sea level. Its chemical formula is O2 and its molecular weight is 32 AMU (atomic mass units). It boils at -297°F and has a vapor density of 1.10. It is moderately soluble in water (1 volume of gas in 32 volumes of water).
Pressurized oxygen is shipped and/or stored in steel cylinders or tubes. The cylinder is a familiar container, tapered on one end to hold a relief valve. The tubes used for shipping or storing oxygen are tapered on both ends, which may be fitted with connections and/or safety valves. The cylinders are often stored in groups to form portable banks. The tubes are usually mounted on trailers, railcars, or placed at stationary locations.
Safety relief devices will usually be found in the cylinder valves or attached to tubes. Spring loaded valves may or may not be used on these containers. Usually, frangible discs or fusible plugs are used. A frangible disc is a small device that will break under high internal pressure (considerably below the rated strength of the tank), while the fusible plug is designed to melt at a certain temperature. Both types of devices, when activated, allow the tank to empty.
Spring loaded valves, when used on other types of containers, activate at a predetermined pressure, allowing the contents of the tank to vent to the atmosphere. When the internal pressure drops to below the design strength of the spring, the valve reseats itself and the container is re-sealed.
The major hazard of pressurized oxygen is its oxidizing power. Although oxygen itself does not burn, it does support combustion, and its presence in the atmosphere is what supports the overwhelming majority of fires. Oxygen is, as an oxidizer, one of the legs of the fire triangle.
In any concentration above its normal percentage in air, oxygen will make normal combustible materials burn very rapidly; and in essentially 100% oxygen atmospheres, most materials not ordinarily thought of as combustible, like metals, burn with relative ease, while ordinary combustibles seem to “flash” into flame and are totally consumed almost immediately (explosively).
Since oxygen is essential to fires (there are some fires that burn due to the presence of some oxidizer other than oxygen), its removal is a key to fire extinguishment. Most fires inside a building are oxygen regulated, that is, they have an abundance of fuel to consume, and the fire is regulated only by the amount of oxygen available to support the combustion.
One ordinarily thinks of oxygen as being non-toxic, and it certainly would have to be for us to depend on it for our very lives. Our physiology is such that we breath the 20% to 21% oxygen in the atmosphere with no ill effects. However, at higher concentrations, from 50% to 100%, severe coughing and chest pains may appear in 8 to 72 hours. If one breathes normal concentrations of oxygen at higher pressures for 2 to 3 hours, several problems arise, such as confusion, impaired judgment and coordination, visual and hearing problems, muscle spasms and even seizures.
SOURCES OF OXYGEN
Manufacturing pure oxygen
The principal source of oxygen (just as in a fire) is the atmosphere. Prior to 1971, the sole method of producing pure oxygen in large quantities was from the distillation of liquified air.
The liquified air was produced by compressing cleaned, dried air drawn in from the atmosphere, cooling it, and allowing it to expand into a larger container. Thus, by taking advantage of the natural gas laws, it is very simple to liquify air. The liquid is then allowed to warm up slightly and the various gases making up the local atmosphere will boil off. This is collected and re-liquified, providing a source of pure gas.
All the principal commercial gases present in air were produced in this way. In addition to oxygen, this includes nitrogen and the inert gases (argon, neon, helium, krypton, and xenon).
The source for oxygen, the atmosphere, is the same today as it always was, but the method of separation of the gases from each other is different. Instead of distilling the various gases from liquified air, a “molecular sieve” is used. Air is drawn through a synthetic mineral, a precipitated aluminosilicate, which has the ability to adsorb nitrogen, thus leaving almost pure oxygen (the inert gases and some pollutant gases will remain). This mixture is then cooled to just below -297°F, whereupon oxygen condenses out of the mixture. Any pollutant gases are distilled out, and pure oxygen is left.
The atmosphere is not the only source of oxygen. As mentioned in the sidebar on elemental oxygen, oxygen is present in many chemical compounds. Some compounds release their oxygen readily, and are known as oxidizers or oxidizing agents.
These oxidizing agents appear to be harmless materials, but when mixed with or stored near anything that burns, they can and will release their oxygen to the fuel, usually when some sort of energy (heat or shock) is applied. If these compounds are present at an uncontrolled incident, emergency responders must be advised, since they will be a source of oxygen that will support a fire, even if atmospheric oxygen is removed or prevented from being near the burning substance. And when these oxidizers release their oxygen, a serious fire or explosion is possible.
The element oxygen, one of the 92 natural elements that makes up the universe, has the chemical symbol O and an atomic weight of 16. It is a non-metallic gaseous element with atomic number 8 and sits atop Group VIA of the periodic table.
As an element, oxygen does not exist in nature, due to its activity. The chemistry of elemental oxygen is determined by its six valence electrons, and it will combine with any other element from which it can take or share two electrons to fill its outer ring. It will do this with another atom of oxygen to form molecular oxygen, two atoms of hydrogen to form water, and literally hundreds of thousands of other reactions to form one of the largest groups of compounds in nature and/or made by man.
Elemental oxygen exists in compounds such as oxides, peroxides, nitrates, nitrites, perchlorates, chlorates, chlorites, hypochlorites, sulfates, sulfites, carbonates, phosphates, and many other ionic compounds, usually making it an oxidizing agent. It also exists in such covalent compounds as alcohols, ethers, aldehydes, ketones, acids, esters, and carbohydrates.
Oxygen is by far the most abundant of the elements, making up 49.2% of the earth’s crust (including the atmosphere and oceans). Oxygen was discovered by Joseph Priestly, an English chemist, in 1774, whose further research led to the discovery of carbon monoxide.
An oxidizing agent is usually an inorganic compound whose name starts with a metal (sodium, potassium, aluminum, magnesium, barium) or ammonium, and whose second name ends with -ate or -ite (chlorate, chlorite, nitrate, nitrite, and hypochlorite). Sometimes the second name starts with perand ends with -ate or -ite (such as perchlorate, persulfate, permanganate, and perborate).
The presence of the -ate and -ite endings usually signify the presence of oxygen in the compound, but does not always mean it will give up the oxygen easily. The presence of peraccompanied by -ate or -ite always signifies an oxidizer. Some common compounds that end in sulfate, carbonate, bicarbonate, and phosphate are not usually oxidizers. It may be confusing to you now, but you should make some effort to learn what chemical compounds will provide oxygen when stressed. This will eliminate a surprise source of oxygen that occurs when you least want it.
ACCIDENTAL RELEASE OF GASEOUS OXYGEN
Where a leak has developed in a cylinder or tube, the threat of fire is great if there are combustible materials in the area and ignition sources present. The flow of gas must be stopped as soon as possible. If the flow of gas cannot be stopped and no fire is present, the gas may be dispersed by using high-pressure fog or spray patterns.
While this is occurring, all ignition sources should be eliminated, and if the fuel can be removed without danger of a flash fire or explosion, it should be considered. Wetting down the fuel in place may be the best strategy. Evacuation should always be considered.
Threat of pressure relief explosion
Remember, gaseous oxygen is contained under pressure in cylinders and tubes and, as such, will be sensitive to heat. Keep all containers cool to reduce the threat of a catastrophic failure (explosion) of the pressurized containers.
Liquid oxygen is a very cold (boiling point -297°F), pale blue liquid. The specific gravity of the liquid is 1.14. It has a latent heat of vaporization of about 92 Btu/pound, and an expansion ratio of 857:1. That is, 1 cubic foot of liquid oxygen will produce 857 cubic feet of gaseous oxygen.
Synonyms for liquid oxygen include LOX and LO2.
The only reason for liquifying a gas is economic. It is very expensive to try to ship and/or store a gas in its gaseous state, even under pressure. Since so much more of the gas can be shipped and/or stored as a liquid, it is logical to do this to save transportation and storage costs.
The containers for shipping and storing liquid oxygen are significantly different than those for gaseous oxygen (see schematic). Liquid oxygen is stored and/or shipped in a container called a Dewar, a non-pressurized, insulated container. It is really one container sitting inside another, with an airspace or vacuum as insulation. The Dewar usually holds 5 to 200 liters of liquid.
A special insulated cylinder may also be used to hold liquid oxygen. The cylinder is used to store the liquid, but dispense the oxygen as a gas, whereas the Dewar (smaller than 50 liters) may be poured. Liquid may be withdrawn from the cylinder under its own vapor pressure. The cylinder is equipped with safety relief valves or frangible discs, and may hold 100 to 200 liters.
Liquid oxygen tanks are powder and vacuum insulated, and may be stationary or mounted on railcars or truck trailers. The tanks range from 300 gallons to 420,000 gallons and are equipped with safety relief valves.
Transfer lines are used to remove liquid from all three container types, with the longer lines being insulated to prevent ice build-up on the line from moisture in the air.
The major hazard of liquid oxygen is the amount of oxidizer produced by a relatively small amount of liquid (its vapor-to-liquid ratio of 857:1). Even a small release of the liquid will produce enough 100% pure oxygen to present a major fire hazard anywhere. The vapor density of 1.1 is just slightly higher than air, and you would not expect any rapid sinking of the gas to the ground. However, remember that the gas just generated by the liquid is very cold, somewhere near -295°F, and this gas will be very dense and will sink to low spots rapidly. A visible vapor cloud will form caused by condensed water vapor, but the visible droplets will quickly re-evaporate on a warm day. It is a mistake to assume all the oxygen is within the vapor cloud.
Another hazard is that liquid oxygen is really super-concentrated, that is, it is much more than 100% oxygen. It may seem strange to think of something being more than 100% concentrated, but it is true. An atmosphere of 100% pure oxygen contains only the gas, nonpressurized. But the same space filled with liquid oxygen actually contains 857 times as much oxygen. This will produce explosive mixtures with organic materials.
The third hazard of liquid oxygen is its extreme cold. At -297°F, this is 369°F below room temperature. Needless to say, any living tissue will be instantly destroyed when in contact with liquid oxygen. Also, hypothermia may occur if enough of the liquid is contacted.
ACCIDENTAL RELEASE OF LIQUID OXYGEN
An accidental release or spill of liquid oxygen will be extremely hazardous because of all three problems above. Even though oxygen does not burn, you must approach a release or spill in the same manner as a very volatile flammable liquid. Approach from upwind, and remove all ignition sources. If the apparatus were to enter an atmosphere of 100% pure oxygen, the internal combustion engine might begin racing to the point that the engine burns itself up.
Logically, one would expect the liquid oxygen to begin boiling vigorously once it contacts the air or ground. Although it does begin to boil immediately, there is no vigorous action, nor any “flashing” of liquid instantaneously to vapor. This is due to the latent heat of vaporization of the liquid. For a liquid to evaporate, it must absorb heat energy from its environment. After initial withdrawal of heat from its surroundings, everything is so cold that there is very little energy available to the liquid, thereby allowing relatively slow evaporation. Just remember the term “relatively.” There will still be tremendous amounts of gaseous oxygen being generated and, of course, the larger the area of the spill, the more gas will be generated.
Covalent—A compound containing atoms that are bonded together by covalent bonds, which are formed by the sharing of electrons between non-metallic atoms.
Cryogenic—Any material manufactured or used at temperatures of -150°F and below.
Element—A pure substance that cannot be broken down into simpler substances by chemical means.
Emulsion—A stable mixture of two or more immersible liquids held in suspension.
Ionic—A compound containing ions, which are atoms or groups of atoms bound together chemically, that have gained or lost electrons and are electrically charged. The force holding the ions together in a compound is the electrostatic attraction between opposite charges.
Latent heat of vaporization—The
amount of heat that must be absorbed by a liquid as it evaporates.
Molecule—The smallest particle of a covalent compound that can still be identified as the compound; two or more atoms bound together chemically by covalent bonds and electrically neutral.
Non-metallic—Elements that are not metals and that will accept electrons to form negatively charged ions. Elements that will share electrons to form covalent compounds. The nonmetallic elements are: boron, carbon, silicon, nitrogen, phosphorus, arsenic, oxygen, sulfur, selenium, tellurium, fluorine, chlorine, bromine, iodine, astatine, helium, neon, argon, krypton, xenon, and radon. All the other elements are metals.
Periodic table—A systematic arrangement of all the known elements by their atomic numbers, which demonstrates the periodicity, or regular repeating of chemical properties of the elements.
Any water applied to the pool of liquid oxygen will be at least 330°F warmer than the liquid, so water application will speed up the evaporation. If no hazard exists to exposures (that is, the spill is in an isolated, uninhabited area), one strategy is to promote the evaporation of the gas by applying water.
A strange behavior of liquid oxygen is that it will seep into porous materials, such as asphalt and soil. Asphalt is a hydrocarbon— and an excellent fuel. The liquid and solid form an emulsion, a very intimate mixture. We now have two legs of the fire triangle present, and energy in any form is all that is missing. There are documented instances of sections of a street exploding where the apparatus has driven over a section of asphalt soaked with liquid oxygen, or the butt end of a hose line dropped onto the asphalt. The soil also contains a great deal of organic material, and it is now ready to burn or explode.
Wherever possible, the spill or leak must be contained. Spread of the liquid will increase generation of vapors and contamination of soil or other organic material that could produce explosions. Entry into sewer systems is very dangerous because of the organic material present. Entry into waterways is less hazardous from a contamination view, but it will still generate tremendous amounts of gaseous oxygen as the cryogenic liquid hits the relatively warm water. Fires are always a threat because the gas is so cold and will stay within the banks of the waterway.
Whenever dangerous quantities of the gas are being generated from the liquid, dispersion into the air is the quickest and easiest method of lessening the hazard. This can be accomplished by sweeping the air with high-pressure spray or fog patterns. The runoff presents no particular problem. Evacuation should be considered if there is no control of the gaseous oxygen.
Whether or not an attempt should be made to recover the spilled product is a decision to be made by the shipper, who, of course, should be one of the first notified in the event of an incident. The shipper will also advise on emergency procedures that should be taken by the first responders.
PERSONAL PROTECTIVE GEAR
The greatest danger to emergency personnel (aside from the threat of fire and explosion) is the extremely low temperature of the liquid. Prevention of contact with the skin and other organs is of primary concern. Turnout gear will offer adequate protection against the low temperature of the gas as it escapes, but if it is not impervious to the liquid, the wearer must be able to get out of the clothing rapidly in case of contact. Chemical splash goggles, faceshields, and loose fitting gloves of impervious material must be used. No skin should be exposed.
Any material will become extremely brittle when in contact with the liquid and will consequently shatter if bumped (this includes hands and fingers).
Flush affected areas with tepid water in large volumes. Do not apply heat. Get the victim medical treatment immediately.
The major hazard of oxygen is its oxidizing power. Any releases may cause a flash fire or explosion of fuel if ignition sources are present. The added hazard of extremely low temperatures is present when dealing with liquid oxygen and, of course, a spill of the liquid will produce tremendous quantities of gaseous oxygen. There is also a danger of a pressure relief explosion if a cylinder or tube of gaseous oxygen is exposed to heat. Treat gaseous oxygen containers as you would any other pressurized gas container.
Since liquid oxygen containers are insulated, the application of water to a tank will be ineffective as a coolant. If water is to be used on a fire approaching a liquid oxygen storage tank, be careful to keep moisture from any safety relief devices. Any water near a valve that might be venting will immediately freeze and prevent the valve from operating.