Graphite grinding systems are mostly closed-loop pneumatic conveying & classification systems, consisting of grinding chamber, air classifier, ductwork, baghouse filter, induced draft (ID) fan and pulse-jet compressed air supply. Air pressure directly determines grinding efficiency, classification accuracy, dust collection effect and operational safety.
Graphite dust is conductive, fine, easy to adhere and has explosion risks, so pressure management must combine process performance, dust control and explosion protection. This guide covers optimal pressure setpoints, routine maintenance, periodic inspection, fault troubleshooting and graphite-specific control rules.
1. Standard Optimal Pressure Setpoints (Key Reference Values)
All values apply to conventional ultra-fine graphite grinding lines (airflow mill, vertical grinding mill + classifier + bag filter system). Two independent air pressure systems need separate control: process airflow pressure (main system) and pulse-jet compressed air pressure (bag filter cleaning).
| Monitoring Point | Optimal Pressure Range | Alarm Threshold | Core Function |
|---|---|---|---|
| Grinding Chamber (Main Cavity) | -50 ~ -150 Pa (micro negative pressure) | > -200 Pa (over negative); > 0 Pa (positive pressure) | Prevent graphite dust leakage; stabilize material suspension and grinding |
| Classifier Inlet & Outlet Differential Pressure | 80 ~ 200 Pa | > 250 Pa | Guarantee particle classification precision; avoid coarse powder escaping |
| Main Ductwork Pressure Drop | ≤ 30 Pa per 10m pipeline | Local drop > 50 Pa | Ensure smooth material conveying; reduce energy loss |
| Baghouse (Bag Filter) Differential Pressure | 0.8 ~ 1.5 kPa | > 2.0 kPa (blockage); < 0.5 kPa (bag damage/air leakage) | Judge filter bag working status; core index of dust removal |
| ID Fan Inlet Pressure | -800 ~ -1200 Pa | Out of range → adjust fan speed | Provide total power for the whole pneumatic system |
| Pulse-Jet Compressed Air (Filter Cleaning) | 0.5 ~ 0.7 MPa | < 0.4 MPa (insufficient cleaning); > 0.8 MPa (bag damage) | Remove graphite dust cake from filter bags |
| Hopper & Discharge Section | -30 ~ -80 Pa (micro negative pressure) | Positive pressure → dust re-entrainment | Prevent secondary dust overflow |
Core principle for graphite system: Run under stable micro negative pressure overall; strictly prohibit long-term positive pressure operation.
2. Pre-Operation Calibration & Preparation
Complete these steps before startup to lay a foundation for stable air pressure:
- Calibrate pressure instruments Verify differential pressure transmitters, pressure gauges and negative pressure sensors regularly. Since fine graphite dust easily blocks pressure sampling ports, install anti-dust covers and clean sampling tubes before each shift.
- Full system air tightness test Check all flanges, access doors, observation windows, flexible connectors and shaft seals. Air leakage will cause pressure fluctuation, reduce airflow volume and bring in humid air (leading to graphite dust caking). Seal gaps with gaskets and tighten loose bolts.
- Preset parameters according to production requirements Adjust initial fan frequency and pulse air pressure based on graphite fineness, output and raw material moisture. Ultra-fine graphite has higher flow resistance, so slightly increase negative pressure within the standard range.
- Commission compressed air system Start air compressors, dryers and filters first; drain all condensed water in pipelines and air storage tanks to avoid water mixing into pulse airflow.
3. Routine Operation & Daily Pressure Maintenance
Split maintenance into two independent systems: process airflow pressure (grinding & conveying) and pulse-jet compressed air (bag cleaning).
3.1 Maintain Stable Process Air Pressure (Main Grinding System)
- Stabilize induced draft fan operation
- Use variable frequency drive (VFD) for ID fans (preferred solution) to adjust speed steplessly and keep pressure constant. Avoid frequent start-stop of fixed-speed fans.
- Clean fan impellers every week: Graphite dust adheres to impellers, causing unbalanced rotation and violent pressure fluctuation.
- Inspect fan bearings and belts regularly; worn parts reduce fan efficiency and lead to insufficient air pressure.
- Match feeding rate with airflow Use variable-speed feeders to maintain uniform feeding. Sudden overfeeding will increase system resistance and make pressure rise sharply; insufficient feeding causes excessive negative pressure and material loss. Keep feeding quantity stable within rated range.
- Clear pipeline blockages timely Elbows, reducers, branch pipes and low-position sections are prone to graphite dust accumulation. Material buildup increases local pressure drop and disrupts overall pressure balance. Conduct visual inspection every shift and dredge blockages immediately.
- Control inlet air temperature and humidity Humid air makes fine graphite powder stick to pipe walls and equipment, raising flow resistance. Equip air intake with dehumidification devices; avoid introducing outdoor humid air in rainy or foggy weather.
- Inspect dynamic seals Focus on main mill shaft seals and classifier shaft seals. Seal failure is a major cause of air leakage. Replace aging packing or air-tight seals in time to prevent pressure loss.
3.2 Maintain Pulse-Jet Compressed Air Pressure (For Bag Filter)
Unstable pulse air pressure directly leads to filter bag blindage and rising baghouse differential pressure.
- Keep constant air pressure Install pressure regulating valves and buffer air storage tanks on compressed air main pipes to eliminate pipe network pressure fluctuation. Lock the working pressure at 0.5 ~ 0.7 MPa and do not adjust arbitrarily.
- Strictly control compressed air quality Graphite dust will harden rapidly when mixed with oil or water. Equip the system with:
- Pre-filter, fine filter and refrigerated air dryer; keep air dew point ≤ -20 ℃
- Automatic drainers for all tanks and pipelines; drain condensed water every class
- Prohibit oil-contaminated compressed air from entering pulse pipelines
- Inspect pulse valves and blowpipes Stuck solenoid valves or blocked blowholes result in incomplete dust cleaning and local high differential pressure. Test each pulse valve manually every day to ensure normal action; clear blocked nozzles.
- Optimize pipeline layout Reduce elbows and long thin pipes for compressed air lines to avoid unnecessary pressure loss.
4. Scheduled Periodic Maintenance Plan
Follow this cycle to prevent potential pressure faults:
Daily
- Record pressure data of all monitoring points every 1 hour, compare with standard values
- Drain condensed water from compressed air system
- Check for visible air leakage and pipeline dust accumulation
Weekly
- Recalibrate pressure and differential pressure instruments
- Clean ID fan impellers and pressure sampling tubes
- Test all pulse jet valves; tighten loose flange bolts
- Inspect flexible connectors for aging and air leakage
Monthly
- Fully dredge all pipe elbows and dead corners
- Deeply inspect shaft seals, filter bag gaskets and tube sheets
- Maintain air dryers and filters of compressed air system
- Lubricate fan bearings and transmission parts
Quarterly
- Conduct overall air tightness test for the whole grinding system
- Inspect internal condition of air storage tanks and pulse blowpipes
- Recheck VFD parameters of ID fan and optimize pressure setting
- Evaluate filter bag aging status (damaged bags cause pressure drop anomaly)
5. Troubleshooting Common Pressure Abnormalities
| Abnormal Phenomenon | Root Causes | Solutions |
|---|---|---|
| Grinding chamber over negative pressure (> -200 Pa) | Low feeding volume; excessive fan speed; partial pipeline blockage | Increase feeding rate appropriately; reduce fan frequency; dredge blocked pipes |
| Chamber positive pressure (dust leakage) | Serious air leakage; fan power insufficient; severe bag blockage | Repair seals; check fan performance; clean or replace filter bags |
| Baghouse differential pressure keeps rising | Pulse air pressure too low; compressed air with oil/water; filter bag blindage | Adjust pulse pressure; maintain air dryer/filter; perform offline cleaning for bags |
| Baghouse differential pressure too low | Filter bag broken; loose bag installation; tube sheet leakage | Find and replace damaged bags; reinstall bags tightly; repair tube sheet gaskets |
| Violent pressure fluctuation in whole system | Fan impeller fouling/unbalance; uneven feeding; large-area air leakage | Clean impeller; stabilize feeding; repair all leakage points |
| Pulse compressed air pressure insufficient | Air compressor failure; pipeline leakage; tank pressure loss | Inspect air compressor; repair compressed air pipes; check drain valves |
| Excessive local pipeline pressure drop | Severe dust accumulation in elbows/branches | Stop the system and thoroughly clear material buildup |
6. Graphite-Specific Special Control Rules
Due to the physical and chemical characteristics of graphite dust, extra safety and process rules must be followed:
- Anti-static & explosion protection linked with pressure
- All equipment, pipelines, filter cages and bags must be reliably grounded to eliminate static electricity. Drastic pressure change will stir up dense graphite dust cloud and raise explosion risk.
- Set high/low pressure interlock alarm: When pressure exceeds the threshold, the system automatically alarms, reduces feed rate or shuts down. Never run the system with pressure alarm ignored.
- Avoid pressure frequent adjustment Frequent pressure change causes airflow turbulence, makes graphite powder adhere more easily to equipment inner walls, and accelerates filter bag blindage. Keep pressure parameters stable after normal production.
- Different pressure settings for different graphite fineness
- Coarse graphite: Slightly lower negative pressure (within standard range)
- Ultra-fine nano graphite: Appropriately increase negative pressure and keep pulse air pressure at the upper limit of 0.7 MPa for better cleaning
- Forbid over-pressure operation Do not blindly increase fan pressure to pursue higher output. Excessively high airflow will destroy classification effect, cause fine graphite dust to escape and wear pipelines severely.
7. Standard Management Suggestions
- Post the optimal pressure range, alarm values and handling methods on site for operators’ reference.
- Establish operation logs to record pressure, airflow, feeding volume and maintenance items every shift, and track long-term pressure changes.
- Train operators to judge abnormal conditions by pressure data and master basic troubleshooting skills.
By implementing the above measures, the graphite grinding system can maintain long-term stable air pressure, ensure high grinding efficiency, qualified dust emission and safe continuous operation.