2.3 Joining Methods: Permitted and Prohibited Joints

The Permitted Joining Methods

For positive-pressure patient gas systems, medical support gas systems, and vacuum systems built from hard-drawn seamless copper or stainless steel tube, NFPA 99 requires every turn, offset, and change in direction to be made with fittings or techniques that suit one of these methods:

Why 1000°F? NFPA 99 requires brazing alloys to melt above 1000°F (538°C) so the piping keeps its integrity if a fire exposes it. Mechanical fittings must meet the same temperature to be treated as equivalent. The same thinking explains a catch-all clause. Listed or approved metallic gas tube fittings are permitted if, once made up, they form a permanent joint with the mechanical, thermal, and sealing integrity of a brazed joint.

Welding details worth memorizing

• The GTAW autogenous orbital procedure and the welder qualification are both qualified to ASME BPVC Section IX. Welder qualification includes a bend test and a tensile test on each tube diameter.
• Each welder qualifies to a welding procedure specification (WPS) for each tube diameter.
• Orbital welds are purged during welding with a commercial mixture of 75% helium (plus or minus 5%) and 25% argon (plus or minus 5%). The shield gas is the same mixture.
• Test coupons are welded and inspected on the ID and OD by a qualified quality control inspector:
  • at the start of work, every 4 hours after that, and at the end of the work period;
  • after the machine sits idle for more than 30 minutes;
  • whenever the operator, weld head, welding power supply, or gas source changes.
• When the autogenous orbital process is used, joints are purged with inert gas inside and outside per the qualified welding procedure.

Threaded Connections: Only Where the Code Lists Them

Threaded fittings are limited to:

1. Connections for pressure and vacuum indicators
2. Alarm devices
3. Gas-specific demand check fittings
4. Source equipment on the source side of the source valve

Threads must be tapered pipe threads per ASME B1.20.1. Joints must be made up with PTFE tape or another oxygen-service thread sealant, applied to the male threads only, and the sealant must not enter the pipe.

Special Fittings

• Dielectric fittings are permitted only where the manufacturer of special medical equipment requires electrical isolation of the equipment from the distribution piping. They must be brass or copper with an approved dielectric, may be a union, and must be clean for oxygen when used on medical gases or medical support gases.
• Extruded tee branches: vacuum and WAGD systems made of copper tubing may use mechanically formed, drilled, and extruded tee-branch connections formed per the tool manufacturer's instructions. The branch connection is then brazed. This exception does not exist for positive-pressure gas piping.
• Metallic flexible joints for expansion, seismic protection, thermal expansion, or vibration control are covered in Section 4.2 of this guide.

Prohibited Joints

NFPA 99 prohibits all of the following throughout medical gas and vacuum distribution systems:

Choosing a Method in Practice

• New construction: brazing with BCuP filler under a nitrogen purge remains the default for copper.
• Tie-ins near occupied patient areas: listed memory metal or axially swaged fittings eliminate open flame. They must meet the 300 psi and 1000°F ratings and be installed by technicians trained in that product.
• Stainless systems: welding by qualified welders to ASME Section IX.
• CMT: only where the adopted code edition and the design allow it, installed by ASSE 6010-qualified installers per the manufacturer's instructions.

Whatever the method, the finished joint still goes through the same installer pressure tests, standing tests, and verification. A non-brazed joint does not get a lighter test regime.