Medical Gas Supply

Describe the supply of medical gases (bulk supply and cylinder) and features to ensure supply safety including pressure valves and regulators and connection systems

Production

Fractional Distillation

Oxygen is produced on the industrial scale by fractional distillation of atmospheric air. This process:

• Relies on the fact that different gases have different boiling points
  By liquefying air and then heating it gradually, each gas can be removed separately as it boils.
• Occurs in stages:
  • Atmospheric air is filtered
    Removes dust and other contaminants.
  • Air is compressed to 6 atm and then cooled to below ambient temperature
    Water vapour condenses and is removed.
  • Compressed air passed through a zeolite sieve which removes CO₂
  • Compressed air is allowed to re-expand
    As it does so it loses heat energy as per Gay-Lussac's Law, and liquefies.
    • Air must be cooled below the boiling point of the desired gases
      This requires getting gases very cold, and so the process may be mechanically assisted using a turbine, and/or a heat exchanger. Key boiling points (at 1 atm):
      • Nitrogen: 77°K
      • Oxygen: 90°K
      • Helium: 4°K
        Helium can be produced by fractional distillation, but liquefying it is understandably difficult given the very, very low boiling point. Helium can also be mined, as helium produced by alpha decay of radioactive materials may be trapped in gas pockets under the earth.
  • Liquid air is then fractionally distilled
    Temperature of liquid air is raised slowly.
    • As the boiling point of each gas is reached (e.g. 77°K for nitrogen), that gas will begin to vapourise from the liquid, and can be collected
    • The remaining liquid can then be further heated, until the boiling point for the next gas is reached
    • This process can be repeated until all the desired gases have been separated

Oxygen Concentrator

Oxygen concentrators:

• Produce up to 95% oxygen from air by removing nitrogen
• Built using two zeolite lattices
  • Pressurised air is filtered through one lattice
    • Nitrogen and water vapour are retained in the lattice
    • Oxygen and argon are concentrated
      Produces a 95% oxygen/5% argon mixture.
  • The unused column is heated to release the bound nitrogen and water

Pros

• Cheap
• Reliable
• Avoid need for oxygen delivery

Cons

• Result in an accumulation of argon when used at low flows on a circle system
• Require continuous power
• Fire and explosion risk

Storage

Medical Gas Cylinders

Gas cylinders are:

• Made from chromium molybdenum or aluminium
• Used as:
  • Backup for a piped supply
  • When a piped supply is not available (transports)
  • When the gas is uncommonly used (e.g. nitric oxide)

• The common cylinder used in hospital is CD
  This contains 460 L of oxygen at 15°C and 137 bar.

• Cylinders are not completely filled, to reduce risk of overpressure and explosions if the temperature rises
  • The filling ratio is the weight of liquid in a full cylinder compared to the weight of water that would completely fill the cylinder
    • In cool climates, the filling ratio is ~0.75
    • In warmer climates, the filling ratio is reduced to ~0.67
• Cylinders are tested for safety every 5-10 years
  Tests include:
  • Endoscopic examination
  • Tensile tests
    1% of cylinders are destroyed to perform testing on the metal.

Pros

• Portable
• Reusable

Cons

• Heavy
• Limited supply

Cylinder Manifolds

Cylinder manifolds are formed of sets of large gas cylinders used in parallel.

• All cylinders in a group are used together
• When the pressure falls below a set level, a pressure valve will switch and gas will be drawn from another cylinder group
• The first (now empty) cylinder group is exchanged for full cylinders

Pros

• Cheap
• Useful as a backup supply

Cons

• Less capacity than a VIE
• Fire and explosion risk

Vacuum Insulated Evaporator

The VIE:

• Stores liquid oxygen
  It is vacuum insulated as it must keep oxygen below its critical temperature (-119°C). The VIE typically stores oxygen between -160°C and -180°C, and at 700kPa.
  • The gas is stored below its critical temperature and above its boiling point
  • The amount of oxygen remaining is calculated from its mass
• Does not require active cooling
  Instead it is cooled by:
  • Insulation
  • Evaporation
    Heat entering the VIE causes liquid oxygen to evaporate. Oxygen vapour is drawn off the VIE to the pipeline supply, so the VIE remains cool and at a steady pressure provided oxygen is being drawn from it.
• Has a pressure relief valve to prevent explosions if oxygen is not being used
• Has an evaporator to evaporate large volumes of oxygen rapidly if demand is high
  • This is simply an uninsulated pipe exposed to the outside temperature

Pros

• Cheapest option for oxygen delivery and storage
  • Storing oxygen as a liquid is much more efficient than as a gas
  • Does not require power

Cons

• Set-up costs are expensive
• Requires a back-up setup
• Will waste large volumes of oxygen if not being used continuously
• Fire and explosion risk

Safety in Medical Gas Supply

Many systems exist to ensure safety:

• Colour coding of cylinders and hoses
  • Oxygen is white
  • Nitrogen is black
    • Air is black with white shoulders
  • Nitrous oxide is blue
  • Helium is brown
    • Heliox is brown with white shoulders
  • Carbon dioxide is grey-green
• Labeling of connections
• The pin index system
  Used to prevent the wrong gas yoke being connected to a cylinder.
  • Pins protrude from the back of the yoke
  • Holes exist on the valve block
  • Pins and holes must line up for the cylinder to be connected
  • There are six positions, divided into two groups of three
    Common combinations include:
    • Oxygen: 2-5
    • Air: 1-5
    • Nitrous oxide: 3-5
• Sleeve Index System
  Used in Australia when connecting pipeline gases.
  • Wall block contains a sleeve when prevents fitting the incorrect gas hose to the wall
  • Screw thread is identical in all cases
• Non-Interchangeable Screw Thread (NIST)
  Used (but not in Australia) when connecting pipeline gases.
  • NIST connectors have a probe and a nut
  • Probe diameter is gas-specific, preventing the wrong gas from being connected

• Testing
  • Must demonstrate
    • Correct oxygen concentrations
    • Absence of contamination
    • Delivery of adequate pressure when several other systems on the same pipeline are in use
  • Testing must be performed twice on a new installation:
    • First by engineers
    • Second by a medical officer
      In theatres, this should be the director of the anaesthetic department or their delegate, who should hold fellowship of ANZCA.

References

1. Aston D, Rivers A, Dharmadasa A. Equipment in Anaesthesia and Intensive Care: A complete guide for the FRCA. Scion Publishing Ltd. 2014.
2. ANZCA August/September 2016
3. The Essential Chemical Industry Online - Oxygen, Nitrogen, and the rare gases.
4. Thompson, C. The Anaesthetic Machine - Gas Supplies. University of Sydney.