Oxygen tanks are high-pressure vessels designed to store pure oxygen for various applications, including medical treatment, industrial processes, and deep-sea diving. Filling these cylinders requires a precise, multi-step operation to ensure the gas is pure and safely contained under extreme pressure. This procedure is meticulously controlled due to the inherent flammability risks associated with high-concentration oxygen and the mechanical stress placed on the storage tanks.
Obtaining Medical-Grade Oxygen
Oxygen production begins in large industrial facilities known as Air Separation Units (ASUs). Atmospheric air is the raw material, which is first filtered to remove contaminants like dust, moisture, and carbon dioxide. The air is then cooled and compressed before processing.
The most common method is cryogenic distillation, which relies on the different boiling points of air components. The filtered air is cooled to extremely low temperatures, often near -200°C, causing it to liquefy. This liquid air is then slowly warmed in a specialized distillation column.
Nitrogen, having a lower boiling point, vaporizes first and is drawn off the top of the column, while the more stable liquid oxygen remains toward the bottom. This fractional distillation process is repeated to achieve the necessary high concentration. Medical-grade oxygen must meet stringent standards, typically requiring a minimum purity of 99.5% to ensure it is safe for patient inhalation.
The final product is usually stored as liquid oxygen (LOX) in large, insulated cryogenic tanks at the production facility. This liquid form is then transported to filling plants where it is converted back into a gas and compressed for cylinder filling. Storing oxygen as a liquid allows for a much greater volume of the gas to be transported efficiently.
Preparing the Tank for Refill
Before any gas transfer begins, the oxygen tank must undergo a thorough inspection to ensure its structural integrity and purity. The process starts with a detailed visual examination of the cylinder exterior for signs of damage. Technicians look for dents, excessive corrosion, deep scratches, or any evidence of fire or heat exposure that could compromise the metal.
A mandated check involves verifying the hydrostatic test date, which is stamped onto the cylinder shoulder. This test, required periodically (often every five years for most cylinders), confirms the tank can safely withstand its maximum rated pressure. If the test date has expired, the cylinder must be removed from service until a new pressure test is performed.
The internal environment is carefully controlled to prevent contamination. If an empty tank has drawn in ambient air, the moisture must be removed through an evacuation and purging procedure. The cylinder is put under a vacuum and then flushed with pure oxygen or dry nitrogen. This step prevents corrosion or dangerous reactions with high-pressure oxygen.
The Process of High-Pressure Filling
Gas transfer occurs at a filling manifold or rack. Tanks are securely connected using specific high-pressure hoses and connectors designed to be oxygen-clean, meaning they are free from oils and grease that could spontaneously ignite under high pressure.
Many facilities use a technique called a cascade filling system to maximize efficiency, especially when refilling smaller portable tanks. This system utilizes a bank of supply cylinders, each holding oxygen at a different pressure. The empty cylinder is first connected to the supply tank with the lowest useable pressure, allowing the gas to flow naturally until the pressures equalize.
The partially filled cylinder is then moved sequentially to supply tanks with progressively higher pressures, maximizing the amount of gas transferred through simple pressure differential. Once the tank has received all the gas possible from the cascade bank, a specialized high-pressure compressor is engaged. This machine boosts the remaining oxygen supply pressure dramatically to reach the cylinder’s maximum safe working pressure, often 2000 pounds per square inch (psi) or higher.
High-pressure filling generates heat as gas molecules are compressed into the small volume. This temperature increase is managed by filling slowly or using cooling systems, such as water baths, to keep the cylinder stable. Controlling the temperature prevents cylinder damage and ensures an accurate final pressure reading.
Once oxygen transfer is complete, the cylinder is sealed and allowed to cool to ambient temperature for a final pressure check. The cylinder is weighed or the gauge is inspected to confirm it has reached the correct capacity without exceeding the maximum fill pressure. The valve is then checked for leaks using a specialized liquid solution before the tank is labeled and returned to service.