Compressed-air pipe, hose and fittings must each be rated above the relief valve setting, laid to slope toward drain points and tested with water, because plastic pipe can shatter under air and a joint that fails under air throws fragments. Leaks are not harmless: one 1 mm hole at 6 bar gauge wastes about 65 L/min, the output of a quarter of a small compressor, and about 310 W of its power.
Part of Pneumatic Systems
The pipework carries compressed air from the receiver to the tools. Its strength, slope and joint quality decide whether the air arrives, and whether a failure is a leak or a hazard.
Every Part Must Outlast the Relief Setting
The relief valve protects the weakest part, so check each part’s rating against the relief setting, and set the relief from the weakest part’s rating, not from what the compressor can reach. The corpus gives:
- Wrought-iron pipe made from strip, 25 mm bore, 2.5 mm wall, welded seam: 3-5 bar. Seamless 25 mm with a 3 mm wall: 6-8 bar (Metal Pipes).
- Single-braid fabric rubber hose: 3-10 bar (Hoses and Tubing).
So a single-stage compressor reaching 8 bar gauge would exceed the rating of welded-seam pipe; either cap the pressure below the weakest rating or use a stronger pipe. Salvaged steel or copper pipe and fittings should carry their own marking, and any part with no known rating is treated as having none.
Do Not Use Plastic Pipe for Air
The corpus’s pipe articles warn against testing plastic pipe with compressed air because rigid plastic can shatter and throw sharp fragments (PVC Pipe, Salvaged Modern Pipe). The same applies to running air through it: a plastic pipe that fails under compressed air fails by shattering, because the stored air energy goes into the fragments. Use steel, iron or copper for the main, and a short length of reinforced hose only where flexibility is needed.
Slope, Drains and Branches
- Slope the main 1-2° toward drain points, per the carrier, so water runs to a drain and not into the tools.
- Take branches off the top of the main, so water travelling along the bottom does not enter them.
- Put a drain valve at the lowest point of every section. See Moisture Removal.
- Size the pipe for the flow, not just the pressure; see Pipe Sizing.
Leaks Cost Real Power
The flow through a small hole is choked when the pressure ratio exceeds about 1.9. With the choked-flow formula used in Safety Devices:
Worked example: a 1 mm leak at 6 bar gauge (7 bar absolute).
- Hole area = π / 4 x 0.001² = 7.85 x 10⁻⁷ m²
- Mass flux at 7 bar absolute = 1,674 kg/(m² s), so mass flow = 1.31 x 10⁻³ kg/s
- Free-air flow = 1.31 x 10⁻³ / 1.204 = 1.09 x 10⁻³ m³/s = 65 L/min (an upper limit; real holes pass less)
- Compressor power wasted = 65 / 235.6 x 1,130 W = 310 W, using the adiabatic power for the reference compressor in Gas Laws in Practice
A pinhole leak on a weekend runs the compressor all day. A 3 mm hole at the same pressure passes up to 583 L/min, more than the whole reference compressor delivers.
Method: Test a New Line
Step 1: Make the joints with threads sealed by a suitable sealant, flanges with gaskets (Gaskets and Seals), or brazed or welded joints.
Step 2: Test with water. Fill the line completely, bleed all the air, and pump to 1.5 times the working pressure with a hand pump. Water stores almost no energy.
Step 3: Drain, dry and then test for leaks with air at working pressure, brushing soapy water on each joint and watching for bubbles, with everyone clear of the line’s ends.
Common Mistakes
- Finding leaks with a hand. A jet can break skin; use soapy water.
- Mixing pipe ratings. The weakest section sets the limit for the whole line.
- No end cap or open hose. Every end of a pressurised line should be a closed valve, a coupling or a tool, never an open hose.
- A pipe run that traps water in a low sag.
Safety
A failed joint throws fragments and a whipping hose
Pipe, threaded joints and hose clamps fail under air by bursting and whipping, and the stored energy of the air makes them dangerous well beyond a water leak. Secure every hose with a restraint, use only parts with a known rating, and never exceed the weakest rating. Stand clear of line ends when first pressurising. Never use a flame to look for a leak.
What’s Next
- Learn next: Pipe Sizing — how much pressure drops along the pipe
- Combine with: Quick-Connect Couplers — detachable joints