How to check and maintain the instruments (pressure gauges, thermocouples)
Reading time:
minutes
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…
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.
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.
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.
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