Combustion: Oxygen fire
The use and handling of high pressure Oxygen can be extremely dangerous if done without knowledge; you have to be aware that it can cause fires or explosion if handled improperly.
Oxygen is not a fuel or explosive by itself, but it can decrease the ignition temperature of the materials and increase the combustion rate. To cause a fire or explosion, it is necessary to have three things: fuel, an oxidizer and heat; this is known as the "Fire triangle" or "Combustion triangle".
In Oxygen filling all the system components must be Oxygen clean and Oxygen compatible.
Air compressors are designed and built to run as AIR compressors, not with enriched Oxygen mix !
Follow equipment manufacturer guidelines, in terms of maximum operating conditions and maintenance.
Air compressors are full of small pipes where adiabatic compression can concentrate in unpredictable "hot spots" and are full of materials that are not compatible with Oxygen: remember the fire triangle: if you want to reduce the chance of accidents when compressing Nitrox with the Air compressor, you have only one option of the triangle: keeping the Oxygen fraction low.
Pay attention to homemade gas boosters; I know, Haskels are expensive ... but accidents can also be extremely expensive.
Overconfidence can be very dangerous: search “incident+Oxygen+fire+explosion"on the web, think about these horror stories and consider this topic again.
• Oxygen cleanliness - it's simply the absence (in the overall system) of contaminants which could act as fuel or ignition.
The cleaning procedures are managed with chemical solutions in appropriate concentration and temperature and sometimes with specific processes (ex. ultrasonic cleaning).
It is widely accepted in the diving industry that systems with Oxygen concentration greater than 40 % are considered and managed as pure Oxygen to minimize the risk of fire; this is due to the well known "40 Percent Rule" based on an OSHA standard written for commercial diving (OSHA = Occupational Safety and Health Administration, an agency of the United States Department of Labor).
• Oxygen compatibility - the materials are full Oxygen compatible if they are not combustible at the working pressure in presence of pure Oxygen.
A concept which is greatly misunderstood regarding Oxygen fire hazard is that many materials used in diving rigs and filling systems are not flammable in air, but can burn in high Oxygen environments. See Ref. ASTM standard test D2863 (minimum concentration of oxygen in a flowing mixture of oxygen and nitrogen that will just support flaming combustion)
Pay attention: Nitrox fires with less than 40% Oxygen have been reported, in particular with aluminum compressor parts, fill station panels and in equipment for continuous flow systems.
Also beware of lower thresholds for Oxygen enriched environments that have been established by some authoritative organizations: U.S. Navy, NASA, CGA, NFPA, ASTM,OSHA.
This means that fire hazards are real, even in systems with less than 40% Oxygen.
This part is not a disclaimer, it's a real call to proceed with caution: fire hazards are real.
The thresholds defined in the industry are not "magic values" , they are accepted risk limits.
It is interesting to note that the commercial diving industry has most of the experience in brass components and 2500 psi environments (172 bar), whereas some technical divers use aluminum 4500 psi (310 bar); like in decompression issues, a successful protocol requires big databases.
Risks are not simply related to the Oxygen fraction and pressure, cleanliness and material compatibility. You must also consider all the environment designs because fire ignition can be triggered in "hot spots" with local temperatures raised for a number of reasons: system design , adiabatic compression, fluid velocity (ex.. opening valve quickly), obstructions, high flow coefficient, burrs etc.
How to fill minimizing the hazards in high fraction Oxygen systems
A more advanced step
• Cleanliness and materials compatibility: consider and treat as 100% Oxygen all the systems with more than 21% Oxygen with appropriate materials, lubricants and cleanliness, it's a good investment !
• Compressors and boosters: use only oil free, high reputation, specifically dedicated Oxygen apparatus
• Use high quality Oxygen valves: using the wrong valve can seriously increase the dangers of adiabatic heating
• Check and try the valves before filling to get a feeling how smoothly they work
• Always open the valves very slowly
• Keep the flow rates low
• Keep the temperature low
• Assemble a Swagelok excess flow valve XS series on the Oxygen storage tank valve: it will stop uncontrolled gas flow in case of line rupture; if properly selected and tuned with your system, the Oxygen is immediately shut !