Mechanical Seal Failure: Causes & Troubleshooting
Mechanical seal failures are the leading cause of pump downtime. This guide covers the common failure modes — from dry running to chemical attack — and how to diagnose each one from the seal face condition.
How Mechanical Seals Work
A mechanical seal consists of two flat, polished faces — one rotating with the shaft and one stationary in the pump housing — pressed together by spring force. A thin film of the pumped liquid between the faces provides lubrication and cooling. The seal works because the liquid film is only a few microns thick — too thin to leak visibly, but thick enough to prevent metal-to-metal contact.
When the liquid film is disrupted — by dry running, contamination, vibration, or chemical attack — the seal faces run dry, overheat, and crack. The seal starts leaking, and the pump must be taken out of service for repair.
Common Failure Modes
- 1. Dry running: The #1 cause. If the pump runs without liquid (cavitation, loss of prime, or starting an empty pump), the seal faces have no lubrication. They overheat within seconds, the elastomer hardens, and the faces crack. Fix: install dry-running protection and ensure the pump is primed before starting.
- 2. Abrasive wear: Solids in the liquid (sand, rust, catalyst fines) get between the seal faces and score the surfaces. The leak path widens and the seal fails. Fix: use hard-face materials (silicon carbide vs silicon carbide) and consider a flush plan that filters the seal fluid.
- 3. Chemical attack: The liquid is incompatible with the seal materials. Elastomers (O-rings) swell, harden, or dissolve; seal faces etch. Fix: verify material compatibility — consult the chemical resistance chart for each seal component.
- 4. Heat degradation: High operating temperature breaks down the elastomer O-rings — they harden and lose their seal. The maximum temperature for standard Viton is 200°C; for Kalrez, 320°C. Fix: select materials rated for the operating temperature.
- 5. Vibration: Excessive pump vibration (from cavitation, misalignment, or bearing wear) chatters the seal faces, causing rapid wear. The vibration also fatigues the spring and damages the elastomers. Fix: address the root cause of the vibration.
- 6. Pressure spikes: Sudden pressure transients (water hammer, rapid valve closure) can exceed the seal's pressure rating, blowing the faces apart. Fix: install surge protection and verify the seal is rated for maximum system pressure.
- 7. Improper installation: The seal is damaged during installation — faces touched by bare hands (oil contamination), springs compressed incorrectly, or the shaft sleeve scratched. Fix: train technicians on proper seal installation procedures.
Diagnosing from Seal Face Condition
| Face Condition | Likely Cause |
|---|---|
| Heat checking (cracks radiating from centre) | Dry running or inadequate cooling — faces overheated |
| Grooving (parallel scratches) | Abrasive particles between faces |
| Chipping at edges | Vibration or impact damage |
| Etching / pitting | Chemical attack of face material |
| Discoloration (brown/blue) | High temperature — lubricant film burned off |
| Wavy / uneven wear | Distortion from pressure or temperature |
| Blistering on carbon face | Thermal cycling — trapped fluid expands in carbon pores |
| Bond failure (elastomer separated) | Chemical incompatibility or high temperature |
Seal Flush Plans
For difficult applications (high temperature, abrasive liquids, crystallising fluids), a seal flush plan circulates clean fluid to the seal faces to provide cooling and lubrication:
- Plan 11: Discharge recirculation — pumped liquid is tapped from the discharge and circulated to the seal. Simple and common.
- Plan 21: Discharge recirculation through a cooler — for hot liquids, the recirculated fluid is cooled before reaching the seal.
- Plan 31: Cyclone separator — removes solids from the recirculated liquid before it reaches the seal. For abrasive liquids.
- Plan 53A: Dual seal with barrier fluid — a pressurised barrier fluid between two seals. For hazardous or toxic liquids.
Prevention
- Never start a pump that isn't primed — dry running destroys seals in seconds
- Install dry-running protection (flow switch or current monitor) on all pumps
- Select seal materials compatible with the pumped liquid — consult chemical resistance charts
- Use hard face materials (SiC/SiC) for abrasive liquids, carbon/SiC for clean liquids
- Install a seal flush plan for difficult applications (hot, abrasive, crystallising)
- Address pump vibration promptly — vibration is the enemy of mechanical seals
- Train technicians on proper seal installation — use clean hands, correct tools, and follow procedures
- Keep spare seals in their original packaging — contamination before installation is a common cause of early failure