One of the most frequent questions we receive about compressor room layout concerns the proper location of the centrifugal (AKA cyclone) moisture separator.
A common query goes something like this: “In our planning drawings, the separator is located quite high up relative to the compressor. Should we be worried about water building up before the separator and flowing back to damage the machine?”
It is a valid concern. Water is the enemy of pneumatic tools and compressors alike. However, the answer lies less in a strict "maximum height" specification and more in understanding the physics of airflow. It is a constant tug-of-war inside your pipes between two opposing forces: Gravity and Towing Force.
The Physics: The Tug-of-War Inside Your Pipes
To understand the risk of water backup, we have to look at what happens to liquid condensate in a vertical pipe.
- Gravity: Naturally, water wants to flow down.
- Air velocity: The compressed air carries water droplets in the air stream as well as water films on the pipe wall in the direction of compressed air flow.
Who wins this battle depends entirely on the velocity of the air.
- Below 2 m/s gravity wins and water runs down the pipe walls.
- From 2 to 5 m/s, the air begins to slowly push the water upwards against gravity.
- Above 6 m/s the air generates enough force to push the condensate upwards. This velocity is typical for a compressor operating under full load. Note that 6 to 10 m/s is usually considered the ideal range for untreated compressed air flow for efficiency purposes but higher velocities are common.
During normal operation, even if your separator is installed high up, the air velocity is usually sufficient to carry the condensate up the vertical pipe and into the separator, preventing water from collecting over time.
The Real Danger: "Water Shock"
As suggested above, the condensed water in the compressed air leaving the compressor isn’t a problem under full flow. The concern is that when the compressed air system is turned off or consumption drops significantly, the moving force disappears. Gravity takes over immediately. Any water suspended in that vertical pipe flows back to the lowest point.
If that water accumulates in a low spot without drainage, the next time the compressor starts up, that slug of water is launched downstream and can wreak havoc on valves, cylinders, and downstream tools.
3 Design Rules to Prevent Water Damage
While we always recommend installing separators as low as possible to minimize this risk, you can prevent water shock regardless of separator height by adhering to three "Golden Rules" of piping design:
1. The "Gooseneck" Rule
Piping each compressor to the main header with a gooseneck is a best practice (shown below). Downstream, at take-offs and user drops, connections to horizontal headers should be at the top (looping over in an arc) or from the side so that only dry air is pulled out, leaving the liquid water along the bottom of the pipe to flow to a low point in the system.
2. The Drip Leg (Water Trap)
At the lowest point of any vertical rise, we recommend a drip leg. This is a short piece of pipe directed downwards that collects the water falling back by gravity. If the pipe from compressor to header is a significant vertical run, a drip leg at the compressor discharge is recommended if the moisture separate cannot be installed there for some reason. Either solution should be fitted with an automatic condensate drain (preferably an electronic, level-controlled type) to eject the water before it accumulates. Note that many industrial compressors already have integrated moisture separators with auto drains at the discharge.
In some systems there are many small drip legs downstream at points of use. For these, ball valves may be sufficient as long as they are checked periodically.
3. Slope Horizontal Pipe Runs
Horizontal pipes should never be perfectly flat. They should have a slight (1% to 2%) downward gradient in the direction of the flow. This uses gravity to your advantage, guiding water away from the supply and toward toward your designated drains. These can be drip legs.
The Verdict
There is no specific height restriction for vertical pipe runs prior to a centrifugal separator. However, relying on air velocity alone to carry liquid past the vertical run is risky. Design your system so that when gravity does occasionally win (and it does), the water has somewhere else to go besides your compressors, tools and products.
Additional Resources