3.2 Nitrogen Purge: Setup, Control & Final Tie-In Exception
Key Takeaways
Brazed joints must be continuously purged with oil-free, dry nitrogen NF to keep copper oxide from forming on the inside surfaces of the joint.
The purge gas source must be monitored, and the installer must be audibly alerted when the source contents are low.
Purge flow must be controlled with a pressure regulator and a flowmeter, or a combination of them; a pressure regulator alone is not permitted.
An oxygen analyzer must read below 1% oxygen before brazing begins, and the purge must keep flowing until the joint is cool to the touch.
The final brazed connection of new piping to an existing pipeline that contains system gas may be made without a purge, but it then requires a final tie-in test.
What the Purge Prevents
At brazing temperature, the inside of a copper tube full of air quickly forms copper oxide scale. That scale later flakes off and travels as particulate to regulators, flowmeters, ventilators, and patients. It is the reason the verifier runs a particulate test and the reason oxidation inside the joint is tracked during brazer qualification. NFPA 99's answer is simple: while being brazed, joints shall be continuously purged with oil-free, dry nitrogen NF.
The Purge Rules, Step by Step
| Step | NFPA 99 requirement |
|---|---|
| Purge gas | Oil-free, dry nitrogen NF, continuous during brazing |
| Supply monitoring | The purge source must be monitored, and the installer must be audibly alerted when the source contents are low |
| Flow control | Use a pressure regulator and a flowmeter, or a combination of them; a pressure regulator alone is not permitted |
| Verify the purge | Use an oxygen analyzer to confirm the ambient air has been removed; it must read below 1% oxygen before brazing begins |
| Discharge | Provide a discharge opening on the opposite side of the joint from where the purge enters |
| Cooling | Keep the purge flowing until the joint is cool to the touch |
| Seal up | After the joint cools, seal the discharge opening to keep contaminants out and keep the nitrogen atmosphere in |
| Between joints | Keep openings in the piping sealed during and after installation so the piping holds a nitrogen atmosphere and debris cannot enter |
Why a regulator alone is not enough
A regulator controls pressure, not flow. A purge needs a steady, low flow that displaces air without pressurizing the joint. Too much pressure can blow molten filler out of the capillary gap or leave pinholes. A flowmeter shows and controls the actual flow, which is why the code requires one. The purge flow rate is not a single code number. It is one of the variables that must be stated in the qualified brazing procedure specification, and it is recorded on the procedure and performance qualification records.
Why an analyzer instead of a timer
Purge time depends on pipe volume, flow rate, and how many openings the run has. Rather than guess, NFPA 99 requires an oxygen analyzer to show that oxygen at the discharge has dropped below 1% before heat goes on the joint.
A Practical Purge Setup
- Connect a cylinder of nitrogen NF to a regulator. Make sure the cylinder has a low-contents audible alert and is restrained.
- Run from the regulator through a flowmeter to a clean, dedicated purge hose and a clean fitting that feeds the section being brazed.
- Open a discharge opening on the far side of the joint, such as the open end of the run, a vent cap with a small orifice, or an open outlet rough-in.
- Flow nitrogen until the analyzer at the discharge reads below 1% O2.
- Braze the joint while the purge continues.
- Keep flowing until the joint is cool to the touch, then cap or seal the discharge opening.
The diagram after this text shows the same arrangement.
The One Exception: Tying Into a Live System
NFPA 99 allows the final brazed connection of new piping to an existing pipeline containing the system gas to be made without a nitrogen purge. The live system gas cannot be displaced without shutting down and venting the existing system, and that may not be clinically possible.
The exception comes with a price. After a final brazed connection in a positive-pressure medical gas pipeline is made without a purge, an outlet in the immediate downstream zone of the affected portions of both the new and the existing piping must receive the final tie-in test (see Section 7.4). That test includes:
- leak testing every joint in the final connection with the system gas at normal operating pressure, using a leak detectant that is safe for oxygen and contains no ammonia;
- purging an outlet in the new piping and an outlet in the existing piping immediately downstream of the intrusion point;
- operational pressure and gas concentration tests before the new work is used for patient care;
- permanent records of these tests.
Warning
The exception covers only the final brazed connection to a pipeline that contains system gas. It does not excuse unpurged joints elsewhere in the new work. It also does not permit skipping the purge just because the existing system has been shut down and vented.
Welding and Mechanical Joints
For autogenous orbital welding, joints are purged inside and outside with inert gas per the qualified welding procedure. Section 2.3 gives the helium-argon mixture. Memory metal and axially swaged fittings need no torch and no internal purge for the joint itself, which is one reason they are chosen for work next to occupied areas. They must still meet their own 300 psi and 1000°F requirements, and every installer test still applies.
Common Purge Failures and Their Fixes
| Problem | Likely cause | Correction |
|---|---|---|
| Analyzer reads 4% oxygen after several minutes | Discharge opening too large, or air leaking in at an upstream joint | Reduce the discharge opening and seal upstream openings, then wait for the reading to fall below 1% |
| Pinholes or filler blown out of the joint | Purge pressure building up because a regulator was used without a flowmeter | Control flow with a flowmeter and keep a discharge opening |
| Black scale found when a joint is sectioned | Purge stopped before the joint cooled, or no purge at all | Keep flowing until the joint is cool to the touch |
| Purge cylinder ran empty mid-joint | No monitoring of the source | Use a monitored source with an audible low-contents alert |
What must an oxygen analyzer read at the purge discharge before brazing a medical gas joint begins?
Below 0.5% oxygen
Below 1% oxygen
Below 5% oxygen
Below 19.5% oxygen
An installer controls the brazing purge with a nitrogen cylinder regulator set to a low outlet pressure and no flowmeter. How does NFPA 99 treat this setup?
It is acceptable if the regulator is set below 5 psi
It is acceptable for joints of DN20 and smaller
It is not acceptable; a regulator alone may not control purge flow
It is acceptable if the oxygen analyzer at the discharge reads below 1%
New piping is joined to an existing oxygen pipeline that still contains oxygen, and the final brazed joint is made without a nitrogen purge. What does NFPA 99 then require?
Nothing more, because the tie-in exception removes all of the extra testing requirements
A 24-hour standing pressure test of the entire existing system at 20% above normal
Replacing the joint later during a planned shutdown
A final tie-in test at outlets just downstream in the new and existing piping
Sections you finish are checked off in the contents.