To remove contaminants that can become the fuel for an oxygen-fed fire.
Oxygen, an oxidizing gas, does not burn, but it supports combustion and causes other materials to burn. In the presence of pressurized oxygen concentrations exceeding that in standard air, most materials become progressively more flammable and ignitable as the oxygen concentration and pressure increase. Even materials that would not easily catch fire in standard air under normal atmospheric pressure will ignite and burn furiously in a pressurized oxygen-enriched environment. It cannot be overemphasized that rising pressure increases the risk of ignition.
For an oxygen-fed fire to occur, three things must be present: oxygen, fuel (something to burn), and heat (an ignition source)—this is the classic “fire triangle.” When dealing with pressurized oxygen or EAN in a scuba cylinder, remember this important variation on the classic three factors needed for a fire:
- Oxygen or gas containing oxygen, under pressure (higher concentrations of oxygen and significant pressure greatly increase the need for oxygen cleanliness and diligent safety practices).
- An ignition (flame, spark, heat source, etc., including impact from dropping or striking a pressurized cylinder).
- A contaminant (i.e., fuel—but the reason the term “contaminant” is preferred in this case is that in an properly cleaned oxygen system, no fuel is present. If fuel is present, it is contaminating the system and making it no longer safe for use with oxygen). Common contaminants include:
- Machining oils (including residual oil film)
- Hydrocarbon-based grease and lubricants (including compressor oil)
- Some soaps, detergents, solvents and cleaning solutions, especially those that contain organic compounds
- Skin lotions and emollients and cosmetics
- Sun-tanning oils and lotions
- Human skin oil and bodily fluids
- Insects and insect body parts
- Paint, wax, and marking crayons
- Carbon dust from filtration systems
- Metal fines, filings, scale and burrs
- Chrome chips (usually from valves and other chrome-plated parts)
- Rust particles and dust
- Metallic oxides in general
- Airborne soot and dust
- Pipe thread sealants
- Residue from soapy water and leak-detection fluids used to check for leaks
- Lint from cloths used in cleaning
- Any other material containing organic compounds and hydrocarbons
Any of these contaminants—many of which are very difficult to detect—can be the initial fuel for an ignition event, the technical term for starting a fire. Once an oxygen-fed fire gets going, even metal components—including a scuba cylinder itself—can burn vigorously. Aluminum melts at 1,220 degrees F (660 degrees C); after an oxygen-related cylinder fire, it is not unusual to find large areas of melted aluminum—which shows just how hot and energetic such a fire can be.
That’s why it’s so essential to remove contaminants and to keep the oxygen system clean. Proper oxygen cleaning need not be excessively costly or time-consuming, but it does need to be effective. Removing contaminants and keeping equipment oxygen-clean is the best way to make sure that an oxygen-fed fire never starts.
A number of agencies and organizations in the United States provide certification courses in oxygen cleaning. For example, American Nitrox Divers International (ANDI) offers a particularly comprehensive training course that defines “oxygen cleaning” as “the process of contaminant removal from component parts or assemblies intended for contact with gases containing Oxygen concentrations of 23.5% or more by volume.”1
1EAN and Oxygen Equipment Servicing Procedures, ANDI International: 1999, p. 19.