3.1 Joint Preparation, Fittings, Filler Metals & Flux

Fittings: Wrought, Socket-Type, Never Cast

NFPA 99 requires brazing fittings to be either of the following:

• Wrought copper capillary fittings complying with ASME B16.22, or
• Brazed fittings complying with ASME B16.50, the standard for wrought copper braze-joint pressure fittings.

Cast copper alloy fittings are not permitted. Brazed tube joints must be the socket type. Butt joints and other non-socket designs are not brazed tube joints under the code.

The tube end must go into the socket either fully or to a mechanically limited depth that is at least the minimum cup depth (overlap) specified by ASME B16.50. The brazing procedure specification (BPS) must address both joint clearance and overlap.

Filler Metals

Why BCuP needs no flux on copper

The phosphorus in BCuP alloys deoxidizes copper at brazing temperature, so the alloy effectively fluxes itself on clean copper. Some BCuP grades contain silver; for example, BCuP-5 contains about 15% silver. Silver improves the alloy's flow and ductility but does not change the no-flux rule for copper-to-copper joints.

Why flux is so restricted

Flux is corrosive, and flux residue inside an oxygen pipeline is a contaminant. NFPA 99 therefore says flux shall only be used when brazing dissimilar metals, such as copper and bronze or brass, using a silver (BAg series) brazing filler metal. When flux is used:

1. Clean the surfaces for brazing first, as described below.
2. Apply flux sparingly to keep it out of the tube. Work it over the cleaned surfaces with a stiff bristle brush so the surfaces are completely covered and wetted.
3. On joints DN20 (NPS 3/4, 7/8 in. OD) and smaller, flux-coated brazing rods may be used instead of brushing flux onto the joint.
4. Where possible, braze short sections of copper tube onto the non-copper component and clean the flux out of that subassembly before it is installed in the piping system.

Cutting and Deburring

1. Cut square with a sharp tubing cutter so the tube is not deformed.
2. Keep the cutter's wheels free of grease, oil, or any lubricant not suitable for oxygen service.
3. Roll the cut end smooth or deburr it with a sharp, clean deburring tool, and take care that chips do not enter the tube. Many installers hold the cut end facing down while deburring so chips fall out instead of in.

A hacksaw or abrasive cutoff wheel produces chips and dust and cannot give the square, undeformed end that capillary brazing needs. Use a tube cutter.

Cleaning for Brazing

NFPA 99 sets a specific cleaning sequence:

• Remove surface oxides by cleaning the exterior surfaces of tube ends before brazing, and keep all matter from entering the tube.
• Clean the tube ends with clean, nonshedding abrasive pads.
• Steel wool and sand cloth are prohibited. Cleaning must not leave grooves in the surfaces being joined, because grooves upset capillary flow.
• After abrading, wipe the surfaces with a clean, lint-free white cloth.
• Fitting sockets that become contaminated must be recleaned for oxygen service and then cleaned for brazing with a clean, oil-free stainless steel or brass wire brush.
• Visually examine tubes, fittings, valves, and components internally before joining. Confirm they are still oxygen-clean and free of obstructions or debris.

The 8-hour rule

Joints must be brazed within 8 hours after the surfaces are cleaned for brazing. Oxides reform on clean copper, and surfaces left exposed overnight collect dust. Cleaned joints that sat through a shift change or overnight must be cleaned again.

Accessibility and Planning

Joints to be brazed in place must be accessible for preparation, assembly, heating, filler application, cooling, cleaning, and inspection. Plan fabrication so that joints in tight chases are made on the bench where you can reach and inspect them. Those joints must still be purged and brazed to the same procedure.

The Brazing Procedure Ties It Together

NFPA 99 requires the brazing procedure specification to address cleaning, joint clearance, overlap, internal purge gas, purge gas flow rate, and filler metal. On the job, the BPS your employer qualified sets the clearance range and the filler alloy. Follow it exactly. A joint made outside its procedure is not a qualified joint, even if it looks good.