It's important to understand Oxygen enriched atmospheres to ensure safety.
Above is an example of compression heating in an oxygen system that occurs when a valve is quickly opened and the gas stream compresses the oxygen downstream against an obstruction. For more information on oxygen system safety, download the catalogue.
There are three elements that are required to create a fire: fuel, ignition energy, and an oxidizer. When the oxygen is increased beyond the normal 21% level in the atmosphere, the risk of fire is greatly increased. It’s important to note that many materials that may not be combustible in a normal atmosphere will burn in an oxygen-enriched atmosphere. Combustible materials are easier to ignite and burn faster and hotter. Ignition sources that have no effect in air can be of critical importance in oxygen enriched systems.
Obviously, with all these issues, we must be extremely careful when dealing with oxygen enriched systems. A ‘Kindling Chain’ occurs when a small amount of energy ignites a material which creates a small fire. Once that fire is ignited, a chain reaction occurs to larger materials that generate more heat until the fire becomes self-sustaining. Because of this, every effort must be made to eliminate any potential cause of ignition. Some of the main ignition sources can include:
- Mechanical impact: When one object strikes another, heat is produced at the point of impact. The heat produced by a mechanical impact can act as an ignition source. For example, in an oxygen system, a mechanical component may break off and strike a pressurized container, producing heat upon impact. If the surface of the container is contaminated with oil, it can ignite and initiate the kindling chain sequence.
- Particle impact: Small particles can be carried along with a flowing oxygen stream, often at high velocity. When the particles strike a surface in the system, the impact energy is released as heat, and because of their small mass, the particles become hot enough to ignite larger materials.
- Friction: When two solid materials rub together, they generate heat which can ignite other materials
- Compression heating: When a gas flows through an orifice from high to low pressure, it expands and its velocity can reach the speed of sound. If the gas flow is blocked, it recompresses to its original pressure and becomes hot. The greater the pressure difference, the higher the gas temperature. This effect can be seen when pumping up a bike tire. As the pressure rises in the tire, the pump gets hot. In an oxygen system the oxygen temperature can be high enough to initiate the kindling chain. For this reason, fast opening valves should not be used in oxygen systems. Ball valves, for example, can give 80% flow when only 20% open. Slow opening valves should be used instead.
Risk management in oxygen systems
ASTM G128 discusses the hazards of oxygen service in much more depth and also gives design considerations and ignition sources in greater detail while G88 and Manual MNL36 provide specific design guidance. ASTM G4 Standards Technology Training course Controlling Fire Hazards in Oxygen Handling Systems provides detailed instruction in hazard analysis and risk management in oxygen systems.
In summary, the first and foremost rule for the safe use of oxygen is to consult an expert. Although oxygen systems present serious and unusual hazards, they are used safely throughout the industry because the risk of injury and economic loss can be managed and controlled.
More information on oxygen safety can be found in Swagelok’s Technical Bulletin Oxygen System Safety (MS-06-13)
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