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The Hazards of Oxygen and Oxygen-Enriched Mixtures

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The Hazards of Oxygen and Oxygen-Enriched Mixtures Safetygram 33 The hazards of oxygen and oxygen-enriched mixtures Most people who use pure oxygen are aware of the hazards of and special precautions for handling this material. Unfortunately, fewer are aware that these hazards extend to oxygen mixtures. This Safetygram reviews how to distin- guish between the various mixtures of oxygen, the hazards of pure oxygen and oxygen mixtures, and how to safely handle these products. The Safetygram concentrates on mixtures above 23.5% oxygen and pure oxygen because opera- tionally they should be treated with the same precautions and requirements. Oxygen-enriched classification In the United States regulations define oxygen-enriched mixtures or atmo- spheres as those containing more than 23.5% oxygen by volume. In oxygen- enriched atmospheres, the reactivity of oxygen significantly increases the risk of ignition and fire. Materials that may not burn in normal air may burn vigorously in an oxygen-rich environment. Sparks normally regarded as harmless may cause fires. And materials that burn in normal air may burn with a much hotter flame and propagate at a much greater speed. Oxygen mixtures are classified, labeled and valved differently. As with all gases, the primary method of product identification is the label. Hazards of oxygen and mixtures One of the challenges in educating individuals about the dangers associated with oxygen is their perception of the product. When one thinks about oxygen, medical applications usually come to mind. Doctors, dentists, nurses, paramed- ics and first aid teams administer oxygen to people when they are sick or in- jured. These professionals would not allow us to intentionally inhale a hazard- ous material. This perception can downplay or at least minimize respect for the hazards of oxygen. But let’s look at oxygen from a different perspective. Oxygen and its mixtures are packaged In an oxygen-enriched environ- Let’s look at the basic fire triangle. as compressed gases. They are sup- ment, fire chemistry starts to change. All three legs of the triangle must be plied at pressures up to 4500 psig (300 Materials become easier to ignite present to produce a fire—a fuel, an bar). In addition to pressure, oxidizers because their flammable ranges start oxidizer, and an ignition source. If carry other hazards. To fully under- to expand and their autoignition tem- asked to name some fuels, materials stand these hazards, we must first peratures begin to drop. This includes like wood, coal, oil, and gas would be understand some of the terms used: the materials of construction used mentioned. But would anyone list in oxidizer systems, such as metals. materials like aluminum, steel, stain- • Adiabatic Heat: A process in which This reactivity continues to increase less steel? What is the primary reason there is no gain of heat to or from not only with the concentration of we can light a piece of wood with a the system. With respect to this oxygen, but also with pressure and/or match but not a steel rod? The igni- document, it is the heat picked up temperature. In other words, oxygen tion temperature of the wood is much by the gaseous oxygen from the contacting a material at 2000 psig is lower than that of the steel rod, and rapid pressurization of a system. more likely to react with the material the heat from the match is sufficient • Autoignition Temperature: The low- than at atmospheric pressure. In the for ignition. Remember what we said est temperature required to ignite case of a contaminant in a system, the about fire chemistry and oxygen—as or cause self-sustained combustion contaminant may react and generate the oxygen concentration increases, in the presence of air and in the enough heat to start another material the autoignition temperature decreas- absence of a flame or spark. reacting. This is called the kindling es. So materials that cannot be ignited chain. When temperature increases, in normal air may burn readily in • Flammable Range: The range of it can lower the amount of energy oxygen-enriched atmospheres. With concentration in volume percent of required to initiate a reaction. this in mind, it is easy to see that in flammable gas or vapor between an oxidizer system we have two legs the upper and lower flammability of the fire triangle present. All that is limits. required for an ignition is an energy source. • Kindling Chain: The promotion of ignition from materials of low igni- tion temperatures to materials of higher ignition temperatures. • Limiting Oxygen Concentration: The minimum oxygen concentration in volume percent (at a given tempera- ture) in a gaseous mixture contain- ing a fuel below, which a flame will not propagate. • Lower Flammability Limit: The minimum fuel mixture in volume percent with air through which a flame will just propagate. • Upper Flammability Limit: The maximum fuel mixture in volume percent with air at which a flame will just propagate. 2 Now let’s consider ignition sources. Adiabatic heat is sometimes con- All of these energy sources can be Typical sources of ignition would be fused with the heat of compression. enhanced by the presence of a con- fire, open flames, sparks, or cigarettes. The heat of compression causes the taminant. Contaminants are typically But that is in the world of normal air, temperature of a system to rise. An easier to ignite than the components not oxygen-enriched atmospheres. example would be a tire pump. The of the system. If they react with the Remember the definition of autoigni- barrel or compression chamber builds oxygen, they may generate sufficient tion temperature—the lowest temper- heat as the pump compresses air. heat to propagate a reaction to the ature required to ignite a material in This process occurs relatively slowly, system. Or as in the case of the pipe in the absence of a flame or spark. Could and the system takes on the heat. Figure 1, they may react so strongly as gas velocity, friction, adiabatic heat, Adiabatic heat is caused by the rapid to compromise the system. or contamination provide ignition pressurization of a system where sources? Yes. the gas absorbs the energy and the gas temperature rises. This heating In the case of gas velocity, it is not the occurs at the point of compression flow of gas that can cause ignition, or the point where the flow of gas is but a particle that has been propelled stopped, such as at a valve or regula- by the gas and impacts the system tor seat. Depending on the material in with sufficient force to ignite. The use where the hot gas impinges, the heat generated may be sufficient to heat may be sufficient to ignite the start a fire, depending on the material material. impacted. Friction from a component malfunctioning or operating poorly can generate heat. Friction between two materials generates fine par- ticles, which may ignite from the heat generated. 3 Figure 1 depicts a section of an oxygen Figure 1 pipeline that ruptured. Here’s how this happened. The oxygen supply line at an installation needed to be extended an additional 150 feet (45.7 meters). The line was solvent washed, but shop air was used to dry the line rather than clean, dry nitrogen. Most shop air is compressed with a hydro- carbon compressor, and in this case, the compressor did not have a clean- up system to remove any trapped oil. When the line was purged, a thin film of hydrocarbon oil coated the interior of the pipe. The pipe was put into service, and the operation went smoothly until at the end of the shift when a valve at the downstream end was closed. This stopped the flow, and the oxygen heated as it compressed against the valve seat. The compres- sion provided enough energy to Let’s take a look at a carbon steel The above examples show how con- react with the hydrocarbon oil, and pipeline used to provide oxygen to a tamination in a system can enhance a deflagration occurred. The speed of customer. Because most of these pipe- the potential for a reaction. It must the pressure wave was such that it du- lines have large diameters, economy be stressed that systems must not plicated the line rupture, as depicted and availability make carbon steel the only be cleaned to oxidizer service in Figure 1, every 15 feet (4.57 meters). material of choice. Carbon steel is an requirements on initial construction This is a very good example of how excellent fuel when used in oxygen but must be maintained in that condi- materials that burn in normal air can service. In fact, due to the operat- tion of cleanliness. Contamination of react in oxygen. ing pressure, the pipelines contain a materials like hydrocarbons or con- flammable mixture. To prevent fires taminants that may be in the form of It is critical to keep nontypical igni- in these pipelines, ignition sources particles can initiate a system fire. tion sources in mind when designing must be considered. Since these lines systems for oxygen use. Some applica- are underground, external sources tions are very vulnerable to ignition. are usually not a problem. However, For example, the elastomers used in carbon steel is prone to rust, which valves and regulator seats have lower can generate particles. If a particle is ignition energies than metals. Since picked up in the flow of oxygen, the particle impingement or adiabatic particle may impinge on part of the heat can be directed at these valves system. If the impingement gener- and regulators, they are particularly ates sufficient heat, it may provide a susceptible to ignition.
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