For closed‑loop nitrogen inerted sulfur grinding & classification systems referenced on sulfur-mill.com, the target operating condition is a low, stable slight positive pressure across the main grinding chamber, while the full circuit naturally develops a pressure gradient from inlet to dust collector outlet.
Optimal setpoint (grinding chamber / mill body, gauge pressure)
+2 kPa ~ +5 kPa (+20 ~ +50 mbar) is the preferred steady operating window for ACM / impact type sulfur mills in closed nitrogen circulation.
- Below +1 kPa: Risk of negative pressure excursions; atmospheric air can leak inward through gaskets, shaft seals or inspection hatches, pushing oxygen above the 2% safety threshold.
- Above +6 kPa: Accelerated nitrogen escape through every small joint. Nitrogen consumption rises sharply, seal wear increases, and workshop hypoxia hazard grows.
Full loop pressure gradient (typical real operating profile)
A closed-loop sulfur nitrogen circuit is not uniform pressure:
- Grinding chamber: +2 ~ +5 kPa (main control point)
- Heat exchanger / cooler outlet: up to +10 ~ +13 kPa
- Baghouse / dust collector outlet: slightly negative, around −1 kPa (driven by circulating fan)
This gradient is normal and intentional. The mill body stays mildly positive to block air ingress, while the fan pulls gas through the filter, creating a slight vacuum at the collector tail.
Control logic rules
- N₂ makeup valve: Activates when mill chamber pressure drops below +2 kPa, automatically adding nitrogen.
- Vent / relief valve: Opens when pressure exceeds +5 kPa to bleed surplus nitrogen back to the recovery circuit, avoiding continuous overpressure.
- The core safety target is O₂ ≤ 2%, pressure is the control variable to maintain this oxygen setpoint, not an independent target.
Startup & standby pressure
- Purging phase: Pressurize and hold at +5 ~ +8 kPa for pressure tightness test before production.
- Standby/idle: Maintain a low positive blanket of +1 ~ +2 kPa to keep air out without wasting nitrogen.
Key warnings
- Never run the grinding chamber under sustained negative pressure. Even tiny seal gaps will pull oxygen inside, triggering explosion hazard.
- Pressure values are gauge pressure, not absolute pressure.
- The relief and burst disc must follow equipment design rating; the operating setpoint is far below the explosion protection relief threshold.
Quick summary table
| Zone | Optimal Gauge Pressure | Purpose |
|---|---|---|
| Mill / grinding chamber | +2 ~ +5 kPa | Primary control point; slight positive pressure to prevent air ingress |
| Cooler / classifier outlet | +8 ~ +13 kPa | Process side pressure rise from fan delivery |
| Dust collector outlet | ~ −1 kPa | Fan suction side, normal mild vacuum |
| Standby idle state | +1 ~ +2 kPa | Maintain inert blanket with minimal nitrogen loss |
