Tubing and Fittings for Compressed Gas Systems

Systems dispensing gases must be compatible with the gas in use. Consult the manufacturers’ guidelines and requirements for the equipment before use.

Content

Tubing

Material Compatibility

  1. Determine the type of gas that will be used (e.g., inert, corrosive, flammable).
  2. Select tubing materials that are compatible with the gas. Bear in mind the following when selecting tubing materials:
    • Use metal piping (copper, stainless steel, etc.), when possible. NOTE: Metal piping is mandatory for service with hydrogen outside of a fume hood.
    • DO NOT use copper or copper alloys with acetylene due to possible formation of explosive acetylides.
    • Ammonia corrodes copper and copper alloys in the presence of air.
    • Tubing for oxygen service shall be certified “oxygen cleaned” or cleaned before use according to a recognized standard (e.g., NASA or CGA oxygen-clean standard). NOTE: Trace contaminants such as hydrocarbons can cause ignition or explosion when exposed to high pressure oxygen.

Pressure Rating

  1. Ensure that tubing and fittings have pressure ratings well above the maximum pressure of the gas system. Consult the manufacturers’ guidelines for applicable pressure ratings.
  2. Working pressure is the maximum pressure the tubing or fittings can endure in sustained use and is temperature-dependent. Working pressure shall NOT be exceeded at any time! Bursting pressure is the pressure at which failure is expected.
  3. Consult manufacturer’s technical data if tubing or fittings are to be used above room temperature.
  4. Note that the working pressure of a tubing-fitting combination may be lower than the working pressure of either the tubing or the fitting taken alone.
    • ALWAYS consult manufacturer’s data to ensure tubing and fittings are compatible, and to determine if there are restrictions on the working pressure of tubing-fitting combinations.
    • Common example – Swagelok™ fittings used with polymer tubing can have an extremely low working pressure.

Size and Diameter

Select the appropriate tubing size and diameter based on the required flow rate. Smaller diameter tubing creates higher resistance and might not be suitable for high-flow applications.

Flexibility

When tubing flexibility is required, three options are available:

  • Small Diameter Metal Tubing
    • Limited flexibility, though more movement can be obtained if the tubing is formed into loops or a spiral.
    • May be a good option where pressures are high, flow is low, gases are hazardous, high gas purity must be maintained, or flexibility requirements are limited.
    • Risk — Fatigue cracking and eventual failure may occur if the tubing is frequently bent past its elastic limit.
  • Corrugated (“convoluted”) Stainless Steel Tubing
    • Use instead of polymer tubing for hydrogen and other flammable, corrosive, or toxic gases.
    • Good option where heat resistance is required, e.g., near furnaces.
  • Polymer and Elastomer Tubing
    • Low-cost and most flexible
    • Numerous types available, with and without internal reinforcing fibers, steel wire jackets, chemical resistant liners, etc. Always consult manufacturer’s technical data to understand tubing characteristics and specifications.
    • Avoid use for hydrogen and other flammable, corrosive, or toxic gases, unless:
      • Inside a fume hood, OR
      • Tubing is rated for flammable gas use and is feeding a portable burner, e.g., Bunsen burner or glassblowing torch.

Additional notes on flexible tubing

  • Consider the flexibility and bend radius of the tubing, especially if your setup requires tight bends. Flexible tubing might be necessary to navigate complex lab setups without kinking or breaking. Minimum bend radii may often be found in manufacturer’s technical data sheets.
  • Do not run flexible tubing through walls, doorways, or across walking paths.
  • Ensure flexible tubing terminations are appropriately made and clamped, in accordance with manufacturer’s instructions.
    • If pressures are high, it is recommended to use professionally manufactured and pressure-tested tubing assemblies, purchased with the terminations already installed – see commercial examples.
  • Most flexible tubing degrades over time and with exposure to chemicals, UV light, and other common lab hazards.
    • Avoid using natural rubber tubing.

Regular Inspection and Maintenance

  • Regularly inspect the tubing for signs of wear, damage, or degradation. Promptly replace any damaged tubing to prevent leaks or accidents.
  • Always leak test when using hazardous gases.
    • Leak test new assemblies and after making any modifications.
    • Where hazards are high (e.g., hydrogen or toxic gases), leak test on a regular schedule, or on every use, as most appropriate for the specific setup.
    • Log maintenance and leak testing results.

Special Considerations

  • Engineer permanent installations with safety in mind.
  • Test newly installed systems for leaks before initial use.
  • Always consult manufacturer information for the parts being used.
  • Non-metallic tubing:
    • Shall NOT be used outside of a fume hood for conveyance of hydrogen.
    • Is NOT recommended to convey other, non-hydrogen flammable gases outside of a fume hood. Exception: Tubing rated for LPG (propane/butane) or natural gas used to supply lab burners.

Fittings

Gas Cylinder-to-Regulator Connection (CGA-Standard Fittings)

  • Never use PTFE tape.
    • CGA fittings already incorporate a gas seal (metal-to-metal or gasket, depending on the fitting).
    • The threads of a CGA cylinder connection are NOT a sealing surface and do not require tape.
    • PTFE tape can break down to smaller pieces which can contaminate the fitting and enter the regulator.
    • If tape pieces enter the regulator, they may prevent internal valves from completely closing, causing dangerous over-pressurization of downstream tubing and equipment.
  • Do not over-tighten the fitting when connecting to a gas cylinder.
    • Sealing surfaces may be permanently damaged or deformed.
    • Proper torque recommendations by CGA fittings can be found in online literature.
    • If leaking occurs despite recommended tightening:
      • Fittings with a gasket — Replace the gasket.
      • Fittings with a metal-metal seal — The sealing surface on the cylinder or regulator fitting or both may be damaged. If on the cylinder, return to the vendor. If on the regulator, replace the regulator or send to the vendor for professional repair.
  • DO NOT change or modify CGA fitting on a regulator. Industry standards use specific CGA fittings for specific gases. Changing the fitting on a regulator to be used for a specific gas/gas cylinder is extremely dangerous. Adapting a regulator not designed for the specialty gas could lead to over-pressurizing, gas venting, and possible regulator failure. Severe injuries may result.
  • Do not interchange or intermix fittings. Each manufacturer follows its own design and process and these are not intended to be mixed with parts from other manufacturers.
  • Do not install or tighten fittings when the system is pressurized.

Threaded Connections

  • It is a user’s responsibility to understand pipe thread standards, the differences between and appropriate applications of tapered and parallel threads, and the correct use of sealing materials (e.g., PTFE tape or anaerobic sealing compound for sealing a tapered-threaded joint). If in doubt, consult online pipe-fitting resources from a reliable source, consult a subject matter expert, or contact the manufacturer.
  • Note that joints between parallel threads are NOT satisfactorily sealed by PTFE tape; an appropriate elastomeric or soft-metal gasket is required (as per manufacturer’s recommendations for the specific fitting) for assured long-term sealing against any significant pressure. For more information, consult the Parallel (Straight Threads) Vs Tapered Threads article and Types of Tube Fittings: Learn the Key Differences webpage.

Swagelok™ Fittings

Swagelok™ products are commonly used in laboratories and are discussed here. They are not specifically endorsed by USC EH&S, however: