Compressed Air Piping Materials, Layout, and Common Problems

Piping isn't the high-tech part of a compressed air system. Just material selection, pipe sizing, route planning, drainage. Standards and textbooks spell it all out. Follow them and you're fine. Yet a lot of projects have their problems right in the piping.

Pipe diameter selection is the most commonly overlooked step. Main line DN100, branch DN50, sub-branch DN25. Step down gradually. Looks reasonable. Question is whether anyone actually calculated that step-down. Same DN50 branch feeding one cylinder versus feeding a row of spray guns. Flow requirement differs by several times. Pipe too small, velocity goes up, pressure drop follows, end user doesn't have enough pressure. Complaints about insufficient air. Investigate and find some section of pipe was undersized.

Flow velocity is an indicator that gets overlooked. Compressed air isn't water. Can't see it, can't touch it. Pipe is connected and people assume it's fine. 6 to 8 m/s is the comfortable range. Above 10 m/s the pipe whistles. Vibration too. Pressure drop increases noticeably. More problematic is velocity too low. Below 3 m/s, condensate in the pipe can't be pushed along by the airflow. Just sits there. So bigger pipe isn't always better either. Oversized has its own problems.

Pipe routing, a lot of people only look at drawings and never visit the site. Drawing shows the shortest path. Through the warehouse, through the corridor. Saves pipe, saves labor. Nobody cares that the warehouse has no heating in winter. Minus ten or fifteen degrees. Water in the pipe freezes. Pneumatic valves freeze up. Downstream equipment all stops. Afterwards everyone says they should have routed along the heated areas, just a detour. A few thousand extra dollars in piping material. Nobody thought of it during design. Or thought of it but figured the probability was low.

Similar details are many. Where pipe passes through a wall, from an air-conditioned room to an unconditioned area. That short section through the wall isn't insulated. Condensate concentrates right there. Large-area insulation done well. This corner spot missed. Some projects outsource insulation. The wall penetration gets treated as civil finishing. Insulation crew doesn't cover it. Civil crew doesn't cover it. Becomes a no-man's-land.

Drainage sounds simple. Put a drain at every low point. Execution goes sideways. Low points on the drawing and actual low points on site often don't match. Piping installation has tolerances. Building settles. Supports deform over time. Low point locations shift. Manual drain valves installed but nobody opens them. Same as not installed. Automatic drains are convenient. Mechanical float types fail less. Electronic timed types can be connected to monitoring. Which one to use depends on site conditions and management capability.

Ring loop vs. tree layout. This gets discussed a lot. Tree layout is simple, intuitive, easy to build, uses less pipe. Downside is low end pressure. Any point in the middle gets serviced or fails, everything downstream loses air. Ring loop has even end pressure. One section under maintenance, air can route around from the other side. High reliability. Cost is more pipe, more joints, bigger investment. For scattered use points where lost air means big losses, ring loop is worth it. Concentrated use points that can tolerate brief interruption, tree works fine. Some projects do ring for the main trunk and tree for branches. A compromise.

Support spacing has standards to look up. Different diameters and materials correspond to different spacing. During actual installation, walls, columns, and beams don't always cooperate. Adjustments are constant. Plastic pipe has low stiffness. Slightly too much span and it sags. Sag becomes a low point that collects water. Metal pipe is better, but excessive span still causes problems.

A few details that tend to get overlooked.

One more issue. Pipe slope. Standards require pipes to have slope so condensate gravity-flows to drain points. In practice this is very hard to achieve. Factory ceiling height is limited. Piping has to dodge all kinds of obstacles. Up and down. Maintaining consistent slope is essentially impossible. Many projects just give up on slope. Compensate by adding more drain points and running automatic drains frequently.

Piping design life is typically twenty to thirty years. A lot of facilities change layout within ten years. Equipment moves. New equipment added. Piping needs modification and extension. The materials chosen originally, the margin left, the pre-installed connections, that's when the differences show. Some projects design piping only for current needs. Exactly right, not one bit of margin. The slightest change requires re-running pipe.