How a Mechanical Seal Works
A mechanical seal prevents leakage along the pump shaft by maintaining a precise gap (typically 0.25–0.5 microns) between two extremely flat, lapped faces: a stationary seat fixed in the pump casing, and a rotating face on the shaft. The seal faces are separated by a thin film of process fluid which acts as both lubricant and coolant. When this fluid film breaks down, the faces contact — and heat, wear, and failure follow rapidly.
7 Mechanical Seal Failure Modes
| Failure mode | What you see on the face | Root cause | Prevention |
|---|---|---|---|
| Dry running | Carbon face: heat crazing, radial cracks, blackening. Ceramic face: thermal shock crazing. | No fluid at seal faces — cavitation, dry start, vapour pocket | Ensure pump is primed before starting; check NPSHa; install dry-run protection |
| Cavitation damage | Pitting erosion on seal faces and seal hardware; fine craters | Cavitation bubbles collapsing at seal face | Correct system NPSHa; maintain minimum flow; avoid operating far below BEP |
| Misalignment | Uneven, asymmetric wear pattern on faces; higher wear on one side | Shaft/coupling misalignment; bent shaft; soft foot | Laser-align pump to motor; check for soft foot; inspect shaft run-out |
| Installation error | Chipped or cracked seal face (from dropping); incorrect compression; wrong orientation | Physical damage or incorrect setting dimension | Handle seal faces with care; set compression per manufacturer drawing; use installation jig |
| Product crystallisation | Hard deposits on secondary seals (O-rings), springs, shaft sleeve; spring clogged | Product drying around seal due to poor flush, intermittent operation | Increase flush rate; use double seal with clean flush fluid for crystallising products |
| Abrasive wear | Uniform wear across both faces; matt finish instead of mirror-polish | Solid particles in product passing through seal face | Install cyclone separator or strainer on flush; upgrade to harder face materials (SiC/SiC) |
| Chemical attack | O-rings swollen, brittle, cracked, or dissolved; elastomer blistering | Seal elastomers incompatible with process fluid | Verify O-ring material compatibility with fluid at operating temperature |
Seal Examination Procedure
Before discarding a failed seal, conduct a root cause examination:
- Photograph seal faces before cleaning — record face patterns, discolouration, and contamination
- Examine faces under magnification (10× loupe minimum) — look for cracking, pitting, uneven wear, chips
- Check O-rings — are they swollen, brittle, or chemically attacked?
- Check secondary seals for deposits (crystallisation)
- Check springs — clogged? Corroded? Broken?
- Measure shaft run-out at seal location (<0.05 mm TIR acceptable)
- Record findings and compare against the failure mode table above
Seal Face Material Selection
| Material pair | Best for | Not for |
|---|---|---|
| Carbon vs ceramic (Al₂O₃) | General water, light chemicals | Abrasive slurries; dry running; HF acid |
| Carbon vs silicon carbide (SiC) | General chemicals, hot water, moderate abrasives | Strong caustics; hydrofluoric acid |
| SiC vs SiC | Abrasive slurries; heavy chemicals; high pressure | Applications where carbon self-lubricating properties are needed |
| Tungsten carbide vs SiC | Severe abrasive and corrosive service | Light-duty general service (expensive) |