Aircraft Oxygen Systems
Some safety precautions
By Joe Escobar
Oxygen — the life-sustaining gas we need in order to survive. It makes up around 21 percent of the air in the atmosphere. Although this amount is sufficient at or near sea level, at higher altitude we require supplemental oxygen due to the decreased density of the air. At higher altitudes in non-pressurized aircraft, pilots require it to avoid hypoxia, a physiological condition where the brain gets sluggish due to the decreased oxygen supply. Pressurized aircraft rely on oxygen systems as a supplement when pressurization problems occur. Although it is a lifesaving gas, oxygen can be a hazard to life when improper handling or servicing practices are used. This article will cover some of the issues to be aware of when working with oxygen systems.
During maintenance, all open ports and fittings should be capped to prevent moisture and contamination from being introduced.
The two most common types of cylinders used in aircraft gaseous oxygen systems are steel cylinders and composite cylinders. Steel cylinders can be divided into two categories: 3AA and 3HT cylinders.
DOT 3AA 1800 (3AA) cylinders are standard industrial size steel cylinders found mostly in older airplanes. They are being replaced with 3HT cylinders.
3HT cylinders are high-tensile steel cylinders. They have a thinner wall than 3AA cylinders, thereby affording a weight savings.
Even lighter than either of the steel cylinder types are composite cylinders. They are typically aluminum-lined cylinders with a Kevlar™ overwrap. Many airframe manufacturers are now either installing composite cylinders in their aircraft or offering them as an option due to the weight savings.
For the purpose of identification, letters and numbers are found around the neck of cylinders. The beginning letters are ’DOT’, which indicates Department of Transportation approval. This MUST be stamped on the cylinder or it may not be able to be commercially filled.
After DOT, there will be four numbers that identify the rated pressure of the cylinder — 2015 and 2216 are common. Following the rated pressure identification will be two numbers, followed by a letter that looks like an inverted capital ’A,’ then two more numbers. This is the cylinder’s date of manufacture. The first two numbers are the month and the last two are the year of manufacture. Although many oxygen servicing vales have filters incorporated in them, they should not be solely relied upon to control contaminants from entering the system.
Oxygen cylinders are scheduled for hydrostatic testing on a regular basis. This testing is required by the DOT to ensure the continued safety of the cylinder. During a hydrostatic test, the cylinder is visually inspected for stress cracks and corrosion and is pressurized to 1.67 (5/3) times its rated pressure to ensure its continued ability for safe operation. The time frame for required hydrostatic inspection varies from cylinder to cylinder as follows:
3AA steel cylinders are required to be tested every five years and have an indefinite service life provided they meet service life requirements and inspection criteria.
3HT steel cylinders must be tested every three years and have a service life of 24 years, after which they must be destroyed.
Composite cylinders must be tested every three years and have a 15-year service life, after which they must be destroyed.
It is illegal to charge a cylinder if it is past its hydrostatic due date. Hydrostatic due dates are based on the most recent test date stamped on the cylinder. An important item to note is that the hydrostatic due date for new cylinders is based on the date of manufacture, not on the date of purchase or installation. Even though buying a new cylinder, it may have already lost a significant amount of time from the manufacturing date just sitting on a shelf.
Types of oxygen
There are four basic types of oxygen that are marketed for various users: aviators, medical, welding, and research.
Norm Laschinger is an engineer and Product Line Manager for Lancaster, NY-based Scott Aviation, a division of Tyco International, which develops and manufacturers oxygen systems for everything from general aviation aircraft to large commercial fleets. Laschinger explains the differences in oxygen.
"Oxygen all comes from the same place. Physiologically, aviators breathing oxygen and medical oxygen are both pure oxygen. But the aviators breathing oxygen has a control on moisture. Medical oxygen may have that control, but it’s not tested for that. Aviators breathing oxygen has a dewpoint of about minus 85 degrees Fahrenheit, so there’s hardly any moisture in there to begin with. So it starts off extremely dry."
Tom Harmon, Director of Sales for Scott, adds, "The important thing is to know your oxygen supplier. Know where it is coming from and make sure it is within the specifications. There are some shops out there that are hanging out the DOT sign, but they may not be authorized by us or our competitors. And, there are some that may cut corners by not following recommendations as put forth by either the DOT or the manufacturer."
Moisture present in aircraft oxygen systems is a safety concern, not necessarily from a health issue, but from the danger it poses to system operation — specifically valves and regulators. The problem has to do with the expansion of the oxygen at altitude. That is why starting off with a pure, dry product is essential.
"Depending on where the oxygen cylinder is stored," explains Laschinger, "it may be subjected to subzero temperatures, especially at altitude. When the system pressure reducer operates and expands this cold oxygen gas from 1800 psi down to 70 psi, the gas further cools, allowing the gas temperature to drop to -80 to -100 degrees Fahrenheit. If you’ve got moisture in the cylinder, that moisture will condense and freeze. Now you’ve got ice in the system, ice granules that can jam up regulators or valves."
Cover those openings
Moisture, as well as particulate contamination, can get into the oxygen system during maintenance actions. Any time the oxygen system is open for maintenance, it is a good practice to cap all open lines and ports. This keeps moisture and contamination out of the system. In relation to caps, Laschinger states, "Our preference is to cap with either a clean aluminum or PVC cap — NOT polyethylene. If you take a polyethylene cap and move it back and forth over threads, it will shred. You don’t want these shreds getting in the system." Harmon adds, "We also recommend that when working on a system, all components remain in their sealed containers or packages until ready for use."
Moisture may also be introduced into oxygen systems when they are empty. In fact, most maintenance manuals address this by requiring actions such as purging of systems that have been left in a depleted state.
"That is because empty cylinders can breathe," says Laschinger. "For example, on a high pressure day, pressure will go into the cylinder and it can pick up moisture. It is best to leave some residual pressure in a cylinder so this doesn’t happen."
He goes on to say, "Here, if a cylinder comes in empty, we assume it’s contaminated and take it down and clean it. There are many steel cylinders out there, and they’re going to corrode if they get wet on the inside."
The triangle of fire
One of the dangers inherent in working with oxygen is fire. In order for a fire to exist, there must be three components: fuel, oxygen, and heat. This is commonly referred to as the triangle of fire. When pure oxygen is involved versus normal air, virtually anything, including metal fragments, dust, and dirt, can act as fuel. The situation becomes significantly worse if highly combustible materials like hydrocarbons are involved.
Anytime oxygen systems are involved, there is a critical need to keep all components of the system free from contamination of hydrocarbons like oil and grease. If escaping oxygen contacts a hydrocarbon, then the result can be a violent fire.
All tools used to service oxygen systems need to be kept clean. The best situation would be to have a set of tools dedicated for use with oxygen that can be kept extra clean. Non-sparking tools would be even more beneficial. However, this is not always practical. If tools that are used for other maintenance on the aircraft must be used on the oxygen system, always ensure that they are thoroughly cleaned with a recommended cleaner prior to beginning work. This also goes for hands, and any other material that may be in close proximity to the area.
A former co-worker of mine shared his first-hand experience with the danger posed when hydrocarbons and oxygen mix. During an oxygen cylinder change, he loosened a fitting that had some residual pressure in it. As the oxygen ran across a flight control cable nearby that had some residual grease on it, a shower of sparks erupted. It was a scary adrenaline rush for him that day, and he is quick to share that story with others in the hopes they won’t make the same mistake.
Obviously, the oxygen system should be kept free from any lubricants. In fact, if a lubricant is required — such as in hinge points of shutoff valves or O-rings — the only lubricant approved by Scott is DuPont’s Krytox®, a product manufactured specifically to be compatible with oxygen. Keep in mind that even though it is an approved lubricant, it should always be used sparingly.
A few safety items need to be remembered when servicing oxygen systems. The servicing cart should be clean and free from any contamination, especially hydrocarbons. If exposed to hydrocarbons, it should be condemned until it can be properly cleaned. Always use caps, bags, and other protective coverings to guard against dust and dirt. The equipment should be handled with care and stored in clean, dry locations. Equipment that is visibly dirty, in poor repair, or damaged should not be used. Also, never smoke while servicing oxygen or while around oxygen servicing equipment.
Prior to hooking up the line to the service fitting, the line should be purged. Although most service fittings have filters of some sort incorporated into them, they should not be relied upon to catch all particles. Purging the line helps ensure any contamination present is expelled from the line.
Another important safety item to remember when servicing is to service slowly. Again, the heat issue comes into play. With very rapid servicing, the oxygen is quickly compressed into the cylinder. The resulting rise in temperature can instantaneously elevate the temperature of the regulator to hundreds of degrees. Even if there is not a catastrophic result from this extreme heat, once the oxygen in the bottle cools down, the pressure will drop, causing a lower serviced pressure than originally indicated.
The bottom line
When it comes to safety, the bottom line is having properly trained personnel performing oxygen servicing and maintenance. They should be aware of all of the dangers associated with working on oxygen systems, and be knowledgeable of proper servicing and maintenance methods required by the manufacturer. Attention to detail at all times when working with oxygen will help ensure properly maintained systems and that no catastrophic oxygen-related incident occurs at your facility.
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