This guide is site-practical for graphite grinding/milling/purification systems, focusing on anti-dust, anti-condensation, explosion-proof requirements, and stable readings for grinding pressure, air pressure, temperature, and differential pressure.
General Safety & Pre-Requisites (Must Follow First)
- Lockout / Tagout (LOTO)Isolate power, air supply, and nitrogen purge before disassembling any instrument; no live/depressurized operation.
- Graphite Dust SafetyWear P100 respirator, anti-static coveralls, conductive gloves; no iron knocking, friction sparks.
- Environment ControlKeep instrument room/box humidity 40%–55% to avoid condensation; clean graphite dust regularly.
- Explosion-Proof ComplianceAll instruments in grinding zones must be Ex d IIC T4 ATEX-certified; keep grounding resistance ≤4Ω.
1. Pressure Gauges & Pressure Transmitters Inspection & Maintenance
Covers: ordinary pressure gauges, digital pressure transmitters, differential pressure (DP) meters for mill air pressure, bag filter DP, system negative pressure.
1.1 Daily Visual Inspection (Shiftly, No Shutdown)
| Inspection Item | Standard Requirement | Abnormal Judgment |
|---|---|---|
| Appearance & Glass | No crack, no fogging, no graphite dust covering | Blurred reading, glass broken |
| Pointer Status | Pointer stable, no jitter; zero-position accurate | Jitter violently, stuck, zero drift |
| Pressure Tube/Connector | No air leakage, no graphite clogging | Air bubble (soapy water test), pressure fluctuation |
| Reading Consistency | Consistent with DCS/PLC display | Deviation >5% of full scale |
| Bag Filter DP Gauge | Normal: 800–1500 Pa | >1800 Pa (clogged) or <500 Pa (bag broken/leakage) |
1.2 Routine Maintenance (Weekly)
- Dust CleaningUse a soft brush + explosion-proof vacuum to wipe gauge surface, junction box, and pressure pipe; no high-pressure air blowing (avoids dust spreading).
- Seal CheckApply soapy water to pipe joints; tighten loose threads, replace damaged gaskets.
- Zero Calibration (On-Site)Vent the gauge to atmospheric pressure; adjust zero screw if pointer deviates.
- DP Meter PurgeBlow the pressure tapping port with dry nitrogen (0.2–0.3 MPa) to clear graphite fine powder clogging.
1.3 Periodic Calibration & Overhaul (Mandatory Metrology)
| Instrument Type | Calibration Cycle | Calibration Standard | Disposal Standard |
|---|---|---|---|
| Mechanical Pressure Gauge | 6 months | Dead-weight tester | Precision ≤±1.6% FS |
| Digital Pressure Transmitter | 12 months | Precision pressure calibrator | Precision ≤±0.5% FS |
| Differential Pressure Meter (Baghouse/Mill) | 3 months | Micro-pressure calibrator | Deviation ≤±50 Pa |
Overhaul Steps:
- Dismantle → clean internal graphite dust with dry cloth
- Replace damaged spring tube/sealing ring
- Calibrate zero & full range
- Seal and affix metrology qualification label
1.4 Common Faults & Fixes
| Fault | Cause | Solution |
|---|---|---|
| Pointer stuck / no response | Graphite dust clogging in pressure tube | Purge with dry nitrogen; replace clogged capillary |
| Severe pointer jitter | Pipeline vibration, air pressure instability | Install damping valve; fix gauge firmly |
| Zero drift / reading inaccurate | Spring fatigue, temperature influence | Recalibrate or replace gauge |
| Air leakage at connector | Thread wear, gasket aging | Replace connector/gasket; apply thread sealant |
2. Thermocouples & Temperature Sensors Inspection & Maintenance
Mainly K-type thermocouples (mill inlet/outlet, dryer, purification kiln, bearing temperature) and armored temperature sensors; graphite mills focus on anti-dust, anti-wear, and anti-condensation.
2.1 Daily Visual Inspection (Shiftly)
| Inspection Item | Standard | Abnormal |
|---|---|---|
| Thermocouple Probe | No bending, no graphite buildup, protective sleeve intact | Sleeve worn/ broken, probe covered by thick graphite |
| Junction Box | Sealed tight, no dust/moisture ingress | Dust accumulation, water marks, loose terminals |
| Temperature Reading | Stable, matches DCS; no sudden jump/drop | Temp jumps >20°C, fixed value, no change |
| Cable & Insulation | No damage, insulation good | Cracked cable, short circuit |
2.2 Weekly Maintenance
- Surface CleaningWipe the thermocouple protective sleeve with dry cloth; remove caked graphite powder (soft scraper only).
- Junction Box ServiceOpen the explosion-proof box → clean dust with dry brush → tighten wiring terminals → re-seal with gasket.
- Insertion Depth CheckEnsure probe insertion depth 100–150mm into air duct/mill wall (not too shallow, no contact with liner).
- Anti-Condensation TreatmentApply sealant to cable entry; install small heating pad in junction box for winter.
2.3 Periodic Calibration & Replacement
| Sensor Type | Cycle | Calibration Method | Replacement Standard |
|---|---|---|---|
| K-type Thermocouple | 12 months | Dry block calibrator (compare with standard temp) | Deviation >±3°C |
| Armored Temperature Sensor | 18 months | Multi-point calibration (100°C/300°C/500°C) | Protective sleeve worn through |
| Bearing PT100 | 6 months | Precision resistance calibrator | Resistance value abnormal |
Calibration Key Points:
- Calibrate at actual working temperature range (graphite grinding: 20–300°C; purification: up to 1200°C)
- After calibration, fix the probe position to avoid displacement
2.4 Common Faults & Fixes
| Fault | Cause | Solution |
|---|---|---|
| Temperature reading too low | Probe loose, poor contact, sleeve covered by graphite | Reinsert probe; clean sleeve; tighten terminals |
| Reading jumps randomly | Loose wiring, short circuit, damaged insulation | Rewire; replace cable; seal junction box |
| No temperature change | Thermocouple broken, open circuit | Replace thermocouple core |
| Reading drifts high | Protective sleeve worn, probe in direct heat | Replace sleeve; reposition probe |
3. General Instrument Installation & Long-Term Protection (Graphite-Specific)
- Pressure Tapping Port DesignUse inclined upward tapping (45°) to prevent graphite powder deposition; install filter plug at the port.
- Thermocouple Protective SleeveUse stainless steel / ceramic sleeve for grinding zones; avoid direct wear by graphite particles.
- Sealing & DustproofAll instrument junction boxes use IP65/IP67 waterproof & dustproof grade; add silicone sealant for gaps.
- GroundingAll instruments, pipes, and junction boxes must be reliably grounded to eliminate static electricity.
- Spare Parts PreparationStock spare pressure gauges, thermocouples, DP sensors, sealing gaskets, and terminal blocks.
4. Complete Maintenance Cycle Summary (Easy for Workshop Posting)
| Frequency | Tasks for Pressure Instruments | Tasks for Temperature Instruments |
|---|---|---|
| Per Shift | Check reading, pointer, leakage | Check temp stability, probe appearance |
| Weekly | Clean dust, purge tapping port, zero check | Clean probe, tighten wiring, seal box |
| Quarterly | DP meter calibration, pipe inspection | Thermocouple function test |
| Half-Year | Pressure gauge full calibration | Sleeve wear inspection |
| Yearly | Transmitter calibration, replacement | Thermocouple calibration & replacement |
5. Key Notes for Graphite Processing
- Never use water to clean instruments → causes graphite caking and short circuit.
- Always use dry nitrogen for purging pressure pipes (no oil/water in air).
- Replace instruments immediately if they fail calibration to avoid wrong grinding pressure/temperature control.
- Record all calibration data in the instrument maintenance log for traceability.
