Oxygen compatibility is the issue of compatibility of materials for service in high concentrations of oxygen. It is a critical issue in space, aircraft, medical, underwater diving and industrial applications. Aspects include effects of increased oxygen concentration on the ignition and burning of materials and components exposed to these concentrations in service.
Understanding of fire hazards is necessary when designing, operating, and maintaining oxygen systems so that fires can be prevented. Ignition risks can be minimized by controlling heat sources and using materials that will not ignite or will not support burning in the applicable environment. Some materials are more susceptible to ignition in oxygen-rich environments, and compatibility should be assessed before a component is introduced into an oxygen system.
Prevention of fire
Fires occur when oxygen, fuel, and heat energy combine in a self-sustaining chemical reaction. In an oxygen system the presence of oxygen is implied, and in a sufficiently high partial pressure of oxygen, most materials can be considered fuel. Potential ignition sources are present in almost all oxygen systems, but fire hazards can be mitigated by controlling the risk factors associated with the oxygen, fuel, or heat, which can limit the tendency for a chemical reaction to occur.
Materials are easier to ignite and burn more readily as oxygen pressure or concentration increase. so operating oxygen systems at the lowest practicable pressure and concentration may be enough to avoid ignition and burning.
Use of materials which are inherently more difficult to ignite or are resistant to sustained burning, or which release less energy when they burn, can, in some cases, eliminate the possibility of fire or minimize the damage caused by a fire.
Although heat sources may be inherent in the operation of an oxygen system, initiation of the chemical reaction between the system materials and oxygen can be limited by controlling the ability of those heat sources to cause ignition. Design features which can limit or dissipate the heat generated to keep temperatures below the ignition temperatures of the system materials will prevent ignition.
An oxygen system should also be protected from external heat sources.
Assessment of oxygen compatibility
The process of assessment of oxygen compatibility would generally include the following stages:
- Identification of worst-case operating conditions.
- Evaluation of flammability of system materials. Geometry should be considered as most materials are more flammable when they have small cross-sections or are finely divided.
- Assessment of the presence and probability of ignition mechanisms. These may include:
- Chemical reaction: An exothermic reaction between chemicals that could release sufficient heat to ignite the surrounding materials.
- Electrical arc: Electric current arcing with enough energy to ignite the material receiving the arc.
- Engine exhaust
- Explosive charges
- Flow friction: Heat generated by high velocity oxygen flow over a non-metal
- Note:Flow friction is a hypothesis. Flow friction has not been experimentally verified and should be considered only in conjunction with validated ignition mechanisms.
- Fragments from bursting vessels
- Fresh metal exposure: Heat of oxidation released when unoxidized metal is exposed to an oxidizing atmosphere. Usually associated with fracture, impact or friction.
- Galling and friction: Heat generated by rubbing components together.
- Mechanical impact: Heat generated by impact on a material with sufficient energy to ignite it.
- Open flames
- Particle impact: Heat generated when small particles strike a material with sufficient velocity to ignite the particle or the material.
- Personnel smoking
- Rapid pressurization: Heat generated by adiabatic compression.
- Resonance: Acoustic vibrations in resonant cavities that cause rapid temperature rise.
- Static discharge: Discharge of accumulated static electrical charge with enough energy to ignite the material receiving the charge.
- Thermal runaway: A process which produces heat faster than it can be dissipated.
- Estimation of the ignition risk and the consequences of ignition. The further development or dissipation of the fire.
- Analysis of the consequences of a fire
Compatibility analysis would also consider the history of use of the component or material in similar conditions, or of a similar component.
Oxygen compatible materials
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Hazards analyses are performed on materials, components, and systems; and failure analyses determine the cause of fires. Results are used in design and operation of safe oxygen systems.
- Rosales, KR. Shoffstall, MS. Stoltzfus, JM. (2007) Guide for Oxygen Compatibility Assessments on Oxygen Components and Systems. NASA, Johnson Space Center; White Sands Test Facility, NASA/TM-2007-213740 http://archive.rubicon-foundation.org/4861 accessed 4 June 2013