Pipe Bending, Jointing and Fabrication
This domain tests practical fabrication: bending pipe to shape without flattening it, calculating cut lengths, making sound joints in copper, steel and plastic, and testing the finished work. Expect questions on bend geometry, soldering sequence and joint faults.
Bending rules
- Cold-bend annealed copper 15–22 mm with a spring or hand bender; use a hydraulic bender for larger sizes; heat only where the material and procedure allow.
- Minimum radius about 2.5 × outside diameter for hand bending, 1.5D for machine bends. Tighter bends flatten the bore and restrict flow.
- Terms: take-up (set-back from the corner to the mark), gain (material saved by bending instead of using an elbow), developed length = radius × angle in radians.
- For a 45° offset, the travel between bend centres = set × 1.414, so a 60 mm set needs 85 mm of pipe between bends.
Jointing methods
- Capillary soldering: clean to bright, flux sparingly, heat the fitting not the pipe, let solder draw in; use lead-free solder on potable water.
- Compression: insert fully, hand-tighten plus 1–1.5 turns; do not use PTFE on the olive.
- Solvent weld: primer, then cement, quarter-turn and hold for 30 seconds; respect the cure time, which lengthens in cold weather.
- Threaded steel: PTFE tape or hemp and paste, three to five turns, first thread left bare.
- Pressure test before boxing in at 1.5 × working pressure for at least one hour.
Worked example
Bend 22 mm copper through 90°, legs 400 mm and 300 mm measured to the intersection of centrelines, bend radius 75 mm. Arc length = πR/2 = 1.571 × 75 = 117.8 mm. Cut length = 400 + 300 − 2(75) + 117.8 = 667.8 mm, so 668 mm. Reason: each leg overlaps the bend by one radius, so subtract 2R, then add the true arc. Forgetting this makes the pipe about 32 mm short.
Traps
- Omitting the gain and cutting the pipe too short.
- Overheating copper so solder will not draw or the tube anneals.
- Soldering with water still in the pipe - the joint will not take.
- Skipping reaming and deburring, leaving a turbulent, leak-prone edge.
- Pressure-testing only after the pipe is buried or boxed in.