When compressed air piping sags or has low points without drain legs, water collects in these traps. Each time the compressor cycles, a burst of air pushes collected water through the pipe and into the tools. Fixing the piping layout eliminates the water traps.

What you'll see

Periodic bursts of water at the tools, rather than a constant mist. The water comes in slugs -- especially when the compressor starts up or after a burst of heavy air use. If you disconnect the hose at a low point, water pours out. The piping may visibly sag between supports, especially with rubber or plastic hose. Copper or iron pipe may have been installed level instead of sloped, creating hidden water traps at every union and fitting.
Before you assume this is the problem

If water is equally present everywhere in the system (not just at low spots), the problem is upstream -- not enough draining, no filtration, or excessive moisture production. See: Infrequent Tank Draining, No Point-of-Use Filtration.

How to diagnose

  1. Inspect the piping layout

    Follow the air piping from the compressor to each outlet. Look for sags, dips, and low points. Any place where the pipe goes down and back up creates a water trap. Rubber and plastic hoses sag between supports. Copper and iron pipe should slope consistently toward drain points -- if it goes up and down, water collects at every low point.
    Result: Consistent downward slope toward drains = good layout. Sags and dips = water traps.

  2. Check for drain legs at low points

    Proper compressed air piping has drain legs (tees with a valve at the bottom) at every low point, change of direction, and end of line. These let you drain collected water. If there are no drain legs, water accumulates with no way to escape except through the air outlets.
    Result: Drain legs present at low points = drain them. No drain legs = install them.

How to fix it

  1. Re-support piping to eliminate sags

    Add hangers or supports to prevent hose and pipe from sagging. The piping should slope slightly downward (about 1-2% grade) in the direction of airflow, toward drain points. This way, water flows by gravity to the drains rather than collecting in traps.

  2. Install drain legs at low points

    At every unavoidable low point, install a tee fitting with a short pipe extending downward (a drain leg) and a drain valve at the bottom. The main airflow continues through the straight run of the tee, and water drops down into the drain leg by gravity. Open the drain valves periodically to empty collected water. For best results, use automatic drain valves.

  3. Take air off the top of the main line

    When branching from the main air pipe to an outlet, take the branch off the top of the main pipe, not the side or bottom. This way, water flowing along the bottom of the main pipe doesn't get diverted into the branch. It's a simple detail that makes a big difference in keeping water out of the tool connections.

Common mistakes

Taking branch lines off the bottom or side of the main pipe sends all the water straight to the tool. Always take branches off the top. Also: don't slope the piping toward a dead end with no drain. The water will accumulate at the end with no way out. Slope toward the compressor (in a loop system) or toward drain legs. And don't use corrugated flexible hose for permanent piping -- the corrugations create hundreds of tiny water traps.

Parts & tools

Pipe hangers and supports. Tee fittings for drain legs. Drain valves (ball valves work best). Level or inclinometer for checking slope. Pipe wrenches and thread sealant.

Review safety precautions before starting →

Safety

Depressurize the system before modifying piping. Pipe under pressure can whip when disconnected.