Unique Hazard Posed by Oxygen-Enriched Atmosphere
Situations in which oxygen-enriched atmospheres (OKA) are present are potentially dangerous and becoming more common.
Although oxygen is colorless, odorless and tasteless, its oxidizing properties and associated fire hazard cause fire departments to become involved. And not all OEA incidents are unusual or spectacular ones like the 1967 flash fire that killed the three American astronauts in the Apollo training exercise or the $7 million Titan missile that exploded in its silo after a liquid-oxygen leak developed during a fuel discharge operation.
Any fire fighter could become exposed to an oxygen-enriched atmosphere. Fire fighters in Los Angeles County were routinely servicing their resuscitator oxygen cylinder in the fire station last December. A full replacement cylinder was ident ified by a piece of tape placed over the valve. When the tape was removed this time, adhesive clung to the valve and prevented a tight seal around the gasket. Flammable adhesive came into contact with an oxidizing agent under pressure and ignited. The oxygen-enriched flame quickly burned through the steel cylinder.
Fortunately, the cylinder was small and the fire was quickly extinguished without injury. But the fire fighters had witnessed the awesome quickness with which an OEA incident occurs.
An oxygen-enriched atmosphere can occur in many other locations where oxygen is used. Normally oxygen makes up slightly less than 21 percent of the earth’s atmosphere at sea level. Thus when the oxygen concentration exceeds 21 percent, or the partial pressure of oxygen exceeds 3.09 psi (14.7 psi normal atmospheric pressure X 21 percent), an oxygen-enriched atmosphere exists. Atmospheric pressure may also be expressed as 760 mm of mercury.
An oxygen concentration of ,21 percent is quite adequate for most forms of life and also for combustion. However, there are situations that have been brought into common usage by modern technology for which the usual atmospheric concentration and partial pressure of oxygen are not adequate. Therefore, the OEA comes into being.
The following are other examples of OEA incidents.
An inexperienced employee was told to inert an aircraft’s fuel lines with nitrogen, a common practice. Instead of nitrogen, however, he carelessly connected an oxygen cylinder to the lines. The resulting explosion killed two men and destroyed the airplane.
A hospital patient died in a fire that started within the oxygen tent covering her bed. She had tried to light a cigarette in the oxygen-enriched atmosphere of the tent.
A mechanic installed a pressure gage on an oxygen cylinder. The gage, contaminated with hydraulic fluid, exploded instantly.
The severity of the OEA problem was more formally recognized in 1969 with publication of the first edition of NFPA 53M, which address OEA hazards. Other pertinent information is to be found in academic research journals and in United States military and British Royal Air Force publications. Occasional articles on specific aspects of this subject have appeared also in NFPA periodicals. The information for this article was gleaned from these sources.
The combustion reaction occurs between a fuel and an oxidizer. The reaction rate, measured in terms of flame propagation, is governed partly by the concentration of fuel and oxidizer relative to each other, by the temperature of the reactants and by the ambient atmospheric pressure. This relationship is usually altered in an OEA, so a fire in a class A or class B fuel may not behave as we have learned to expect it to under usual atmospheric conditions.
So great is this effect of elevated oxygen levels and pressures that materials generally considered to be nonflammable may suddenly become highly flammable. Slow-burning materials may be consumed with seemingly explosive quickness. In short, virtually every material will burn in 100 percent oxygen, even some fire extinguishants!
The fire and explosion hazards of flammable and combustible liquids and gases depend upon factors in addition to those listed above, such as the temperature required for formation of flammable vapor mixtures and the critical fuel concentrations for flame propagation (the lower and upper explosive or flammable limits). The lower explosive limit usually varies little with oxygen concentration but the upper explosive limit is often much higher at elevated oxygen levels. That is, the flammable range widens as the oxygen level increases. Also, autoignition temperatures typically decrease at higher oxygen concentrations.
Ignition temperatures for combustible solids are usually lower in an OEA as compared to those in 21 percent oxygen. Flame propagation rates are typically much greater in an OEA also.
Fire extinguishment, like combustion, also differs in an OEA. Additional requirements are placed upon extinguishing agents and extinguishing systems because of the increased dangers.
An OEA incident can be handled properly only if you know in advance through prefire inspections where OEAs exist. Do not allow yourself to lapse into the belief that these situations are so rare as to make them of little concern.
OEAs are used daily in hospitals and other medical facilities (operating rooms, hyperbaric chambers for victims of carbon monoxide poisoning and the “bends,” incubators and resuscitators). Medical uses for OEAs are not restricted to medical facilities, though, because many outpatients use oxygen therapy equipment in their homes and offices.
Industrial applications of OEAs are common also. You’ll undoubtedly be able to think of many such uses, such as welding and metal-cutting operations, chemical manufacturing, etc. Incidentally, the most serious hazards in welding and metal-cutting operations are not likely to occur in major industrial plants. Rather, small repair shops and home hobby shops are apt to hold greater dangers because safety precautions may be poorly understood or even ignored.
Medical and military
An obvious OEA environment surrounds the preparation of oxygen used in medicine and industry, and its subsequent transfer and transportation.
Almost every military and commercial aircraft has an oxygen breathing system on board, a system that provides the potential for either inflight or on-ground OEA emergencies. OEAs are also used in spacecraft and on-the-ground flight simulators, in the fueling and de-fueling of rockets and in deep-sea diving vessels.
Lastly, don’t forget the OEA environments that cannot be planned for, the accidental ones. These can occur when oxygen is inadvertently substituted for some other gas, for example. A large number of people confuse the terms “air” and “oxygen,” and believe that these terms and the gases are synonymous. The misunderstanding has had fatal results on several occasions.
Other potential causes of unwanted OEAs include use of oxygen in poorly ventilated spaces and leakage from oxygen apparatus.
What can you, as fire fighters, do to minimize the effects of an OEA mishap to which you are summoned! How do you handle an OEA incident?
Planning a must
Basic to the successful handling of an OEA emergency,’like just about every other type of incident, is planning. You must know where these OEAs exist. Only then can you recognize that you are indeed confronted with an out-of-theordinary situation. Get out and conduct on-site inspections.
The second step in controlling an OEA incident is in-depth training of fire fighters and also of employees and other people who are routinely in or near an OEA area prior to the emergency. And not only must they be trained, but the training must be frequently updated and maintained at a high level of proficiency. Fires in OEAs happen with explosive rapidity. Rarely will there be sufficient time for the fire department to reach the scene before serious injury occurs and extensive property damage is done. Serious burns can be inflicted -upon a person wearing cotton clothing within two seconds in 100 percent oxygen!
A fire in an OEA will burn with much greater intensity than in the ambient atmosphere. Extremely high temperatures and pressures will build up much faster in an OEA, especially in a fixedvolume OEA environment such as a sealed hyperbaric chamber. The pressure increase can cause the chamber to rupture explosively, and the abnormally high temperatures will contribute to rapid fire spread. For these reasons, a fire in an OEA cannot always be extinguished in the conventional manner.
More water needed
Water is effective on fires in class A fuels under OEA conditions—if it can be applied in sufficient quantities in a very short time. A “sufficient quantity” of water under OEA conditions is much greater than that needed in 21 percent oxygen at normal atmospheric pressure.
The technique of water application in an OEA, especially in a confined area OEA, is just as important as the volume applied, because all surfaces within the closed chamber must be covered by the minimum water spray density virtually simultaneously and immediately. This minimum density, incidentally, has been reported by some British researchers to be 1.25 gpm per square foot under their experimental conditions. It remains to be seen whether this application rate will suffice under other circumstances.
Don’t count on using halogenated hydrocarbon extinguishants in OEAS, either, unless you have carefully evaluated their effects beforehand. Several that are useful in normal atmospheres fail in OEAs for various reasons. Some may actually be flammable themselves in an OEA.
Bromotrifluoromethane (Halon 1301) appears to be the most promising of these extinguishants for use in OEAs. However, even Halon 1301 must be applied at much higher than usual concentrations in the OEA. And herein lies another potential problem, because Halon 1301 may be toxic at high concentrations.
No studies have been published concernting the effectiveness of carbon dioxide, dry chemicals and low-expansion foams in an OEA environment. High expansion foams show promise, but it takes too long to apply them with today’s equipment. □ O
photos by the author