The Cylinder That Slowed at the End of the Line
We plumbed a new machine to the plant air. The FRL (filter-regulator-lubricator, article 119) was sized for one machine. As we added more machines on the same line, the pressure dropped at the farthest machine. The cylinders slowed (the pressure was 4 bar, not 6). The problem: the FRL (and the piping) couldn’t handle the flow. The filter element clogged (article 128), and the FRL’s internal restriction dropped the pressure. The farthest machine starved. We upsized the FRL (larger flow rating), replaced the filter, and enlarged the main header (article 135). The pressure held. The mistake was using a small FRL on a high-demand line.
FRL pressure drop is the pressure lost across the filter and regulator. This article covers the sizing.
The FRL’s Flow Rating
An FRL has a flow rating (NL/min, article 135) at a given pressure drop (e.g., 700 NL/min at 0.5 bar drop). If your demand exceeds the rating, the pressure drops (the FRL can’t pass the flow). The downstream machines starve.
The pressure drop across an FRL is:
- Clean filter: About 0.1–0.2 bar (small).
- Clogged filter (article 128): Up to 1 bar (or more). The filter traps dirt, it clogs, the flow restricts.
- Regulator: A small drop (0.1 bar) at rated flow.
Sum the drop across the FRL. If the supply is 6 bar, and the FRL drops 1 bar (clogged filter), the downstream is 5 bar. The cylinders see 5 bar (slower force, article 85).
Step 1: Size the FRL for the Flow
Calculate the machine’s air consumption (article 135). Pick an FRL whose flow rating is at least 1.5× the demand (margin). For a demand of 500 NL/min, pick an FRL rated for 750 NL/min (or more). The margin covers the clogged filter (as it clogs, the drop rises — the margin keeps the pressure up).
The FRL sizing rule: Pick an FRL rated for 1.5× the machine’s air demand (article 135). The cylinder that slowed had an undersized FRL (and a clogged filter). The margin covers the filter clog. If the FRL is at the edge of its rating, the pressure drops as the filter clogs.
Step 2: Monitor the Filter (Article 128)
The filter clogs over time (article 128). The pressure drop rises. Add a pressure gauge (or a differential gauge) across the filter. If the drop is over 0.5 bar, replace the filter element. Don’t wait for the machine to slow — the gauge tells you.
Step 3: The Regulator (Article 119)
The regulator sets the working pressure (downstream). If the supply pressure (upstream) drops (because of the FRL drop, or the plant), the regulator compensates (it holds the downstream pressure). But if the supply drops below the setpoint (e.g., supply is 5 bar, setpoint is 6 bar), the regulator can’t hold (it’s already open). The downstream drops. Keep the supply (upstream) at least 1 bar above the setpoint.
Step 4: Piping (Article 135)
The FRL is only as good as the piping. A small pipe (6 mm) on a high-demand run drops the pressure (article 135). The FRL might be sized right, but the pipe after it is too small. Size the pipe for the flow (article 135). The FRL and the pipe work together.
| Component | Typical Drop |
|---|---|
| Clean filter | 0.1–0.2 bar |
| Clogged filter | 0.5–1+ bar |
| Regulator (at rated flow) | 0.1 bar |
| Small pipe (6 mm, long run) | 0.2–0.5 bar |
An FRL Sizing Checklist
- What is the machine’s air demand? (Article 135?)
- Is the FRL rated for 1.5× the demand?
- Is there a gauge across the filter? (Drop?)
- Is the filter element replaced? (When clogged?)
- Is the supply 1 bar above the setpoint?
- Is the downstream pipe large enough? (Article 135?)
- Does the pressure hold at peak demand? (Test?)
- Are multiple machines on one FRL? (Split them?)
- Is the FRL accessible? (For service?)
- Is the working pressure right? (Article 85?)
The Bottom Line
FRL pressure drop sizes the FRL for the flow. The cylinder that slowed had an undersized FRL (and a clogged filter). Pick an FRL rated for 1.5× the demand. Monitor the filter (gauge). Keep the supply above the setpoint. Size the piping. The machine that held pressure wasn’t the biggest FRL — it was sized for the flow.
