Magnetic Pump Dry Start Temperature Spikes to 250°C: Thermal Failure Mechanism of Carbon Graphite Seal Rings and Prevention Measures

1. Why Magnetic Pump Dry Start Is Especially Severe

Magnetic drive pumps transmit torque contactlessly through inner and outer magnetic couplings, eliminating the dynamic seal point of traditional mechanical seals — theoretically achieving "zero leakage" — making them the first choice for transporting flammable, explosive and highly toxic media. But precisely because the dynamic seal leakage warning channel is eliminated, magnetic pump dry liquid startup shows no visible signs in the early stages, and by the time of detection the seal faces are typically already burned out.

Measured data (Huahao Sealing laboratory magnetic pump test bench, model CQB65-50-160 with M120K carbon graphite thrust washer):

| Time After Start | State | Face Temp | Friction Coefficient | PV Value |

|-----------------|-------|-----------|--------------------|---------|

| 0 s | Start instant | 24.3°C (RT) | — | — |

| 0.3 s | Full dry friction | 28.5°C | 0.22 | 2.1 MPa·m/s |

| 2 s | Rapid heating | 95.6°C | 0.18 | 1.9 |

| 4 s | Resin softening begins | 152.1°C | 0.15 | 1.8 |

| 8 s | Past Tg point | 248.7°C | 0.28 | 2.3 |

| 15 s | Resin decomposition | 325.4°C | 0.41 | 2.8 |

| 30 s | Severe burnout | 402.0°C | 0.55 | 3.4 |

This table reveals the core paradox: PV value reaches 2.1 MPa·m/s at 0.3s after startup — far exceeding the 0.8-1.2 MPa·m/s recommended upper limit for phenolic-impregnated carbon graphite — then temperature climbs exponentially, crossing resin Tg in just 8 seconds.

2. Three-Stage Thermal Failure Mechanism

2.1 Stage One: Glass Transition (Tg Crossover, 120-200°C)

The impregnation layer in resin-impregnated carbon graphite is a polymer three-dimensional cross-linked structure. Below the glass transition temperature Tg, large molecular chain segments are "frozen" and the material exhibits high modulus and low creep; when temperature exceeds Tg, chain segments begin to thaw and move, and the impregnation layer transitions from glassy state to high-elastic state, manifesting as:

• Modulus drops 2-3 orders of magnitude, impregnation layer adhesion to carbon graphite matrix decreases from ~8MPa to below 1MPa
• Friction coefficient surges from 0.04-0.08 under normal liquid film lubrication to 0.20-0.30
• Impregnation layer is sheared off the carbon graphite matrix by face shear action, exposing porous carbon graphite matrix and creating three-body abrasive wear with resin debris

Tg values by impregnation type:

| Impregnation Type | Tg (DSC method, N2 atmosphere) | Recommended Max Temp |

|------------------|-------------------------------|---------------------|

| Phenolic Resin | 160-190°C | ≤150°C |

| Furan Resin | 260-300°C | ≤220°C |

| Epoxy Resin | 120-150°C | ≤100°C |

| PTFE | N/A (crystalline) | ≤260°C (dry start not recommended) |

| Antimony Metal | N/A (melting point 630°C) | ≤500°C |

2.2 Stage Two: Resin Thermal Decomposition (230-400°C)

When temperature exceeds the thermal decomposition temperature Td of the impregnated resin, C-C and C-O bonds in the polymer backbone undergo homolysis, generating large numbers of free radicals that further accelerate oxidation reactions. TGA thermogravimetric analysis data (N2 atmosphere, 10°C/min ramp):

• Phenolic resin Td5 (5% weight loss temp): ~260°C, char yield at 700°C ~50%
• Furan resin Td5: ~340°C, char yield at 700°C ~65%
• Epoxy resin Td5: ~320°C, char yield at 700°C ~15%

Higher char yield means a thicker carbonaceous protective layer remaining on the carbon graphite face after decomposition, which can somewhat mitigate subsequent wear. Furan resin's 65% char yield far exceeds epoxy's 15%, making it the preferred impregnation type for high-temperature service.

2.3 Stage Three: Carbon Graphite Matrix Oxidation (Above 420°C)

Above 420°C, even in air atmosphere, the carbon graphite matrix undergoes significant oxidation:

C(graphite) + O2 → CO2 (ΔH=-393.5 kJ/mol)

The oxidation reaction is exothermic; once initiated it creates positive feedback: rising temperature → accelerated oxidation → more heat release → further temperature increase, forming "thermal runaway". Above 600°C, carbon graphite oxidation rate in air can exceed 0.1mm per hour, and the seal face can wear away completely within minutes.

Carbon graphite oxidation onset temperature in air strongly depends on impregnation type:

• Un-impregnated: ~420°C (high porosity allows easy O2 diffusion)
• Resin-impregnated: ~500-550°C (impregnation seals pores)
• Antimony-impregnated: ~600-650°C (antimony forms dense Sb2O4 oxide film covering surface)

3. Four Pre-Startup Mandatory Checks (Standard SOP)

Huahao Sealing jointly developed this magnetic pump startup checklist with multiple chemical design institutes. Recommend entering it into DCS or PLC startup interlock conditions:

3.1 Check One: Inlet Liquid Level ≥200mm Above Pump Centerline

Purpose: Prevent air ingestion into inlet line at startup, creating air locks that prevent liquid film formation.

• Installation: Inlet tank level gauge high/low switch
• Interlock logic: Startup prohibited when level <200mm
• Special condition: For self-polymerizing media (e.g., styrene), additionally verify inlet filter differential pressure ≤0.03MPa

3.2 Check Two: Vent Valve Discharges Liquid 3s Without Bubbles

Purpose: Evacuate trapped air from pump cavity and seal cavity.

• Procedure: Fully open inlet valve → slowly open pump body top vent valve → close after continuous liquid flows without string bubbles
• Pass/fail: Bubble diameter ≤1mm with sporadic appearance = pass; continuous bubbles = fail
• Alternative: Automatic float-type air release valve, set leakage rate ≤1mL/h

3.3 Check Three: Inlet Pressure ≥0.05 MPa (Gauge)

Purpose: Guarantee required NPSHa (Net Positive Suction Head available) to prevent impeller inlet vaporization at startup instant.

• Installation: Inlet line pressure transmitter with accuracy ≥0.5% FS
• Interlock logic: Startup prohibited when inlet pressure <0.05 MPa gauge (self-priming pumps excepted)
• Apply atmospheric pressure correction at altitudes >1000m

3.4 Check Four: Magnetic Coupling Gap Temperature Difference <10°C

Purpose: This is a Huahao Sealing patented monitoring method. Under normal magnetic pump operation, the temperature difference across the containment can (inner rotor side / outer rotor side) stabilizes at 5-8°C. When abnormal heat is generated near the containment can from dry friction, the temperature difference rises before seal face temperature, providing 2-3 seconds of advance warning.

• Installation: PT100 RTDs embedded on both sides
• Yellow alert: Difference ≥12°C, DCS alarm
• Red alert: Difference ≥15°C or either side >220°C, interlock shutdown

4. Temperature Monitoring and Interlock Protection Plan

4.1 Sensor Selection and Installation

Recommend direct face-embedded PT100 (Class A accuracy, ±0.15°C + 0.002×|t|):

• Stationary ring side drilling: 0.8-1.2mm from seal face (do not penetrate through face)
• Rotating ring side: via wireless telemetry or slip ring output (accuracy slightly lower, ±1°C)
• Sampling frequency: ≥20Hz (to capture 8s temperature rise spike)
• Protection sheath: 316L stainless steel, φ1.5mm armored

4.2 Two-Stage Alarm Logic

| Level | Temperature Threshold | Action | Response Time |

|-------|----------------------|--------|---------------|

| Yellow (warning) | 150°C | DCS audible/visual alarm, operator confirms within 60s | ≤100ms |

| Red (interlock) | 180°C | Main motor contactor trip + outlet valve close + alarm | ≤500ms |

Note: 180°C as red alarm threshold uses phenolic Tg=180°C; for furan-impregnated M180K this may be raised to 250°C, but we recommend keeping 180°C as the yellow warning level.

4.3 Material Upgrade as Last Line of Defense

Even with perfect protections, human error can still occur. Upgrading thrust bearing material from phenolic M120K to furan M180K:

• Tg raised from 180°C to 280°C, dry-start tolerance extended from ~4s to 13.5s — 2.4× improvement
• Td5 raised from 260°C to 340°C, 80°C safety margin increase
• Mating SiC ring surface Ra degradation rate reduced 68% (furan's high char yield forms carbonaceous lubricating film)
• A pesticide plant statistics: After upgrading 22 magnetic pumps to M180K, annual seal failures dropped from 17 to 3

Huahao Sealing provides magnetic pump dry-start bench test services that can simulate dry-start scenarios per client conditions, issuing complete evaluation reports with temperature-time curves, friction coefficient changes, and face wear morphology.