12.2 Medical Gas Piping Materials, Brazing & Nitrogen Purging

Medical Gas Piping Materials, Brazing & Nitrogen Purging

Medical gas piping fails in one specific way: copper oxide. When copper is heated to brazing temperature in the presence of air, the inside wall of the tube forms a black, friable cupric oxide scale. That scale does not stay put. It flakes loose in the pipeline, travels downstream on 50-psi oxygen, and lodges in the check valves, regulators and flowmeters of ventilators and anesthesia machines — or in a patient's airway. Every unusual procedure in this section exists to prevent that scale from ever forming.

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1. Approved Piping Materials

Positive-Pressure Medical Gas Piping

Distribution piping for oxygen, nitrous oxide, medical air, carbon dioxide, nitrogen and instrument air shall be hard-drawn seamless copper tube conforming to ASTM B819, Standard Specification for Seamless Copper Tube for Medical Gas Systems, Type L or Type K.

The tube is not ordinary Type L water tube with a different label. ASTM B819 tube is cleaned for oxygen service at the mill, capped or plugged at both ends, and shipped sealed. NFPA 99 requires the manufacturer to identify it with the markings "OXY," "MED," "OXY/MED," "OXY/ACR" or "ACR/MED", printed in:

• Green for Type K (the heavier wall)
• Blue for Type L (the standard wall for medical gas distribution)

[!IMPORTANT] Memorize the color pairing backwards and forwards, because it inverts the plumbing convention many candidates carry in from water piping. In ASTM B88 water tube, green is Type K and blue is Type L — and ASTM B819 uses the same green/blue pairing for the same two types. What changes is the marking text: water tube is not stamped OXY or MED, and installing B88 tube on a medical gas pipeline is a rejectable defect even if the wall thickness is identical, because it has not been cleaned and sealed for oxygen service.

The installer must furnish documentation certifying that all installed piping materials comply. Bring the mill certifications to the verification; a verifier who cannot document material compliance will not sign off the system.

Vacuum and WAGD Piping

Vacuum is the one place NFPA 99 relaxes. Field-installed medical-surgical vacuum and WAGD piping may be any of:

• ASTM B88 seamless copper water tube, Type K, L or M
• ASTM B280 copper ACR tube (air conditioning and refrigeration field service)
• ASTM B819 copper medical gas tube

Vacuum piping does not carry an oxidizer, so oxygen-service cleaning is not required. It is still brazed, still purged, and still tested.

Fittings and Joining Methods

NFPA 99 permits three joining methods, and only three:

Soldered joints are never permitted on medical gas piping. Threaded joints are permitted only where specifically allowed at equipment connections.

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2. Preparing the Joint

The preparation steps are short, mechanical and heavily inspected:

1. Cutting (5.1.10.4.2): Tube ends shall be cut square with a sharp tubing cutter so the tube is not deformed. The cutting wheels shall be free of grease, oil or other lubricant not suitable for oxygen service — a wheel oiled for steel work contaminates every cut it makes. Cut ends shall be rolled smooth or deburred with a sharp, clean deburring tool, taking care to keep chips out of the tube.
2. Cleaning (5.1.10.4.3): Interior surfaces of tubes, fittings and components cleaned for oxygen service shall be stored and handled to avoid contamination before assembly and brazing — no bare hands inside a fitting, no laying tube in the mud. Exterior surfaces of tube ends shall be cleaned before brazing to remove surface oxides, using an abrasive pad or cloth and then wiping away the grit.
3. Filler metal and flux: Copper-to-copper joints are brazed with a BCuP (copper-phosphorus) filler metal without flux. Flux is permitted only where dissimilar metals are joined — for example a bronze valve body to copper tube — using BAg-series filler. Flux left inside a medical gas pipeline is itself a rejectable defect.

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3. The Nitrogen Purge — The Heart of the Trade

This is the procedure that most distinguishes a certified medical gas installer, and the exam knows it. NFPA 99 Section 5.1.10.4.5 reads as a sequence:

1. Continuous purge. When brazing, joints shall be continuously purged with oil-free, dry nitrogen NF to prevent the formation of copper oxide on the inside surfaces of the joint. ("NF" is National Formulary grade — welding-grade nitrogen is not acceptable.)
2. Monitored source. The purge gas source shall be monitored, and the installer audibly alerted when the source content is low. Running out of nitrogen mid-joint ruins the joint.
3. Regulator and flowmeter. Purge gas flow rate shall be controlled by a pressure regulator and flowmeter, or a combination thereof. NFPA 99 then adds the sentence candidates miss: "Pressure regulators alone shall not be used to control purge gas flow rates." A regulator sets pressure; only a flowmeter sets flow.
4. Verify with an oxygen analyzer. To ensure all ambient air has been removed before heat is applied, an oxygen analyzer shall be used to verify the effectiveness of the purge, and it shall read below 1 percent oxygen concentration before brazing begins. This is the single most quotable number in the procedure.
5. Discharge opening opposite the joint. While a joint is being brazed, a discharge opening shall be provided on the opposite side of the joint from where the purge gas is introduced, so the nitrogen actually sweeps across the joint rather than stagnating.
6. Hold the purge until cool. The flow of purge gas shall be maintained until the joint is cool to the touch. Copper oxidizes as it cools just as readily as it does while heating.
7. Seal afterward. After the joint has cooled, the purge discharge opening shall be sealed to keep the nitrogen atmosphere inside the pipe and prevent debris from entering. During and after installation, all openings in the system shall be kept sealed.
8. The final tie-in exception. The final brazed connection of new piping to an existing pipeline containing the system gas is permitted to be made without a nitrogen purge — you cannot purge a live oxygen main. In exchange, after such a connection an outlet in the immediate downstream zone of the affected portions of both the new and existing piping shall be tested in accordance with the final tie-in test.

Heating and Inspecting the Joint

• Insert the tube end into the socket fully, or to a mechanically limited depth not less than the minimum cup depth (overlap) specified by ANSI/ASME B16.50.
• Where flux is permitted, heat slowly until the flux liquefies; then — or where flux is not used at all — heat quickly to brazing temperature without overheating. Heating, filler application, and horizontal, vertical and large-diameter joint technique follow the "Brazed Joints" chapter of the CDA Copper Tube Handbook.
• After brazing, wash the outside of every joint with water and a wire brush to remove residue so the joint can be inspected (hot wash water where flux was used), then visually inspect each joint. Joints showing flux or flux residue, or base metal melting or erosion, shall not be permitted.

Orbital Welding as an Alternative

Where gas tungsten arc welding (GTAW) autogenous orbital welding is used instead of brazing, the procedure and the welder qualification shall be qualified under Section IX of the ASME Boiler and Pressure Vessel Code, joints shall be purged inside and outside with a commercially available mixture of 75 percent helium and 25 percent argon (each within 5 percent), and test coupons shall be welded and inspected at the start of work, every 4 hours, after the machine is idle more than 30 minutes, and at the end of the work period.

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4. Who Is Allowed to Do This Work

Florida inspectors verify credentials before they verify pipe:

• Installers of medical gas and vacuum piped distribution systems — including all appurtenant piping supporting pump, compressor and manifold source systems — shall be certified in accordance with ASSE/IAPMO/ANSI 6010, Professional Qualifications Standard for Medical Gas Systems Installers. A 6010 installer shall not use that certification to oversee installation by non-certified personnel.
• Brazers shall be individually qualified, and prior to any installation work the installer shall provide and maintain documentation on the job site for the qualification of both the brazing procedures and the individual brazers.
• Bulk and cryogenic central supply installers are qualified to CGA M-1 or ASSE/IAPMO/ANSI 6015; those systems are verified by ASSE 6035 verifiers.
• Inspectors meet ASSE 6020; verifiers meet ASSE 6030. Both must be a party other than the installing contractor.