Oxygen alone is not flammable, but if there is enough heat, oxygen aids in the burning of a fuel. Oxygen, heat, and fuel comprise the familiar “combustion triangle.” The three factors of the combustion triangle must be present for a fire to burn.
In certain circumstances, a few milligrams of debris can be enough to start a fire. If the temperatures are hot enough, even stainless steel can act as a fuel.
Fires in oxygen-rich surroundings can rapidly become disastrous. The Apollo 1 capsule fire in 1967 is possibly the most ill-famed example. In that disaster, a moderately small spark acted as the trigger source, and the pure oxygen inside the capsule increased the materials' flammability.
Even in less unusual settings, eliminating contamination from oxygen delivery units or oxygen-supplemented surroundings is a priority, irrespective of application or industry. As a result of the danger posed by oxygen fires, both ASTM International and the Compressed Gas Association (CGA) have published rules stipulating the best practices for correctly cleaning equipment and areas where high oxygen concentrations are used.
ASTM’s publication, ASTM G93/G93M-19, is called “Standard Practice for Cleaning Methods and Cleanliness Levels for Material and Equipment Used in Oxygen-Enriched Environments” and is aimed at a wide range of industries. The CGA’s document has a more concise title, “CGA G-4.1, Cleaning of Equipment for Oxygen Service,” and a smaller audience, comprised mainly of gas suppliers and the medical field.
Regardless of the varied focus, both documents stress the need for meticulously clean systems in areas where pure or pressurized oxygen is used. At adequate pressure, even a combination of 21% oxygen and 78% nitrogen used for breathing can support combustion once a fuel source has caught fire.
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Small Source, Big Fire
Non-volatile residues are often the fuel source for oxygen fires. These materials can be solid or liquid and frequently contain a high carbon content.
Under specific conditions, even metal particulates or other non-carbon sources can be the initial fuel for a fire. Even a little non-volatile residue is enough for ignition, resulting in a larger inferno. A contamination level of 6.0 mg/square foot (0.1 m2) can be sufficient in the right settings.
No specific cause of oxygen fires stands out from the others. Fuel in the form of debris will not burn without something triggering it. While a spark from faulty wiring is the probable suspect in the Apollo 1 tragedy, the standard function of a system can also start a fire.
Compressed gas produces heat. If that heat is not spread fast enough, it can spike, triggering any existing non-volatile residue to catch fire in a process known as adiabatic compression.
If a pipe system has a sharp bend and an unfastened piece of contamination strikes the surface with sufficient kinetic energy, it too can create adequate heat to trigger ignition in a pure oxygen setting.
Irrespective of the source for ignition, a higher level of oxygen can mean that an otherwise small fire turns into an inferno within seconds. At extreme temperatures, anything can become a source of fuel. The non-volatile residue acts as the kindling, producing sufficient heat to burn nearby rubber gaskets, for instance.
If a fire becomes sufficiently hot and the velocity of the oxygen flow is substantial, even stainless steel can ignite.
Astro Pak Oxygen Fire
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Cleanliness is Prevention
Both ASTM G93/G93M-19 and CGA G-4.1 describe the hazard's nature and severity in detail. They both state that the solution is precision cleaning. No single technique can be used as a standard solution due to the opposing nature of the systems that face the threat of oxygen fires. Numerous aqueous and solvent-based cleaning approaches are referred to in the two guidelines.
Both documents also discuss the pros and cons of different approaches. Environmental issues have further complicated matters, necessitating the need to create eco-friendly substitute solutions that still accomplish the proposed results.
Recognizing the dissimilarities in systems that may have the same purposes, the guidelines permit a certain level of interpretation. Stating that a system “requires cleaning per CGA G-4.1 or ASTM G93” is not sufficient and is too un-specific.
A respected precision cleaner will collaborate with the system owner to stipulate the cleanliness standard, establish the contamination, and identify the material, finish, substrate, and other features of the component to be cleaned.
This “Cleaning Triangle” will provide the required information concerning the combination of operating environments and applications, as well as the temperature, pressure, concentration, and volume of the oxygen-supplemented gas flowing through the system. Only then can the ideal cleaning technique be established.
Failing to obtain these answers can result in inadequate cleaning or the incorrect type of cleaning. In the worst scenario, this can lead to an oxygen fire.
Systems with comparatively low pressure and oxygen concentrations may be cleaned for oxygen service harmlessly and do not require a special environment. They can be certified by employing qualitative visual means. However, parts of systems with higher pressures, flow rates, or oxygen concentrations must be meticulously cleaned and packaged inside cleanrooms.
Those cleanrooms must follow stringent standards to prevent particulate or NVR contamination from being re-deposited on components after precision cleaning. In most cases, an ISO 7 cleanroom (Class 10,000) will meet the requirements, with no contaminants measuring over 0.5 μm. There are, however, cases where even more rigorous measures are necessary.
After cleaning, the parts must be protected against recontamination with particular packaging processes and materials. Setting up oxygen-cleaned system parts in non-clean environments can pose major challenges. Such reassembly should be carried out under carefully regulated environmental settings.
For several years, technicians and engineers at Astro Pak have worked through challenging field installations and gained an in-depth understanding of the necessities and processes that enable even large outdoor installations in less-than-ideal circumstances to preserve the stipulated cleanliness levels, guaranteeing safe operation.
Experience Makes the Difference
Due to the variety of factors for establishing the precise cleaning technique of a specific system, collaborating with the right provider is very important. Owners must depend on the skill and analysis of the service vendor to detect the determining factors, carry out the process correctly, and attain the desired results.
Astro Pak is well acquainted with ASTM G93/G93M-19, CGA G-4.1 and is even featured in the latter. Over the last six decades, technicians at Astro Pak have processed equipment ranging from small filters for ventilators to large-width valves and piping for liquid oxygen delivery systems catering to the launch pads at Kennedy Space Center, offering them unmatched expertise in the industry.
This information has been sourced, reviewed, and adapted from materials provided by Astro Pak Corporation.
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