The Valve That Stuck (Dirt in the Air)
We used a solenoid valve (article 90) on a machine. After a month, the valve stuck (it didn’t switch). We opened it. The valve’s spool was dirty (the air had carried rust and dirt into the valve). The problem: the FRL filter (article 119, 128) was 5 micron (standard). But the plant air had rust (from the old pipes). The 5 micron filter didn’t catch the fine rust (it passed through). The valve’s spool (which needs clean air) got dirty. We either upsized the filter (1 micron, a fine filter) or replaced the plant pipes (no more rust). We added a 1 micron filter (after the standard 5 micron). The valve stayed clean. The mistake was using a standard filter (5 micron) on a dirty air supply.
Pneumatic FRL filter micron selects the filter fineness. This article covers the choice.
The Filter Micron Ratings
- 40–5 micron (standard): Catches water, dirt, rust. For general cylinders (article 85). The standard FRL filter (article 119). Fine enough for a cylinder (it tolerates some dirt).
- 5–1 micron (fine): Catches fine particles (rust dust). For a solenoid valve (article 90) or a sensor. The valve’s spool needs clean air (1 micron).
- 0.01 micron (coalescing): Catches oil mist (for a lubrication-free application). For a paint gun, or a food application. Expensive.
Step 1: What’s Downstream?
The filter fineness depends on what’s downstream:
- Cylinders (article 85): Tolerate 5 micron (they’re not sensitive). Standard filter.
- Solenoid valves (article 90): Need 5 micron (the spool). A standard filter is fine (if the air is reasonably clean).
- Proportional valves (article 90): Need 1 micron (the precision orifice). A fine filter.
- Vacuum ejectors (article 73): Need 5 micron (the nozzle). A clogged nozzle loses vacuum.
The filter micron rule: For a solenoid valve on dirty air, use a 5 micron (standard) + a 1 micron (fine) after. The valve that stuck had 5 micron on rusty air. Add a 1 micron fine filter. For a cylinder, 5 micron is enough. Match the micron to the downstream component.
Step 2: Water (Article 119)
The filter also catches water (from the compressed air). The FRL filter bowl (article 119) collects the water (auto-drain, article 119). A filter that’s full of water doesn’t filter (the air pushes the water through). Drain the water (article 119). For a wet air supply, add a dryer (after the compressor).
Step 3: Two-Stage Filtration
For a dirty air supply (rust, water), use two filters: a 5 micron (coarse, catches the big stuff) then a 1 micron (fine, catches the rest). The coarse filter protects the fine filter (it lasts longer). The two-stage filter is bigger (and more expensive) but needed for dirty air.
| Downstream | Filter |
|---|---|
| Cylinder (article 85) | 5 micron (standard) |
| Solenoid valve (article 90) | 5 micron (standard) |
| Proportional valve | 1 micron (fine) |
| Rusty/wet air | 5 + 1 micron (two-stage) |
| Oil-free (food) | 0.01 micron (coalescing) |
A Filter Checklist
- What is downstream? (Cylinder? Valve?)
- Is the air clean? (Or rusty?)
- Is the filter 5 micron? (Standard?)
- For a valve on dirty air: add 1 micron?
- Is there water? (Auto-drain? Article 119?)
- Is the filter element replaced? (Article 128?)
- Is the pressure gauge OK? (Article 149?)
- Does the valve stick? (Dirty?)
- Is a dryer needed? (Wet air?)
- Is the filter accessible? (Service?)
The Bottom Line
Pneumatic FRL filter micron matches the fineness to the downstream. The valve that stuck had 5 micron on rusty air. Add a 1 micron fine filter. For a cylinder, 5 micron is enough. Drain the water (article 119). The valve that didn’t stick wasn’t the best valve — it had a fine filter.
