- ▸Carbon-graphite-related failures fall into three classes — wear, thermal cracking and fracture; the face tells you which one you have
- ▸Wear is driven mainly by dry friction and solids; a healthy face should wear at well under the 0.05 mm per 1,000-hour class
- ▸Thermal cracking stems from excessive local temperature gradients; dense radial cracks signal thermal stress exceeding material strength
- ▸Fracture is mostly installation stress concentration, hammer assembly and poor O-ring groove design; carbon graphite resists 150-300 MPa in compression but only about one-tenth of that in tension
- ▸Troubleshooting order: duty conditions first (cavitation, deadheading, solids), then installation, material last
- ▸API 682 requirements for seal chamber temperature and flush plans target precisely the two root causes of thermal cracking and dry wear
The first step in failure analysis is opening the seal and reading the face. Carbon graphite failure modes leave distinctive marks, and the marks point straight to the root cause.
I. Wear: Material Removed, Not Broken
Wear shows up as excessive face wear and gradually increasing leakage. Two cases:
Dry-Friction Wear
1.Root cause: flush interruption, pump deadheading, or media flashing — the liquid film is lost
2.Face signature: uniformly bright, mirror-polished wear across the whole face
3.Prevention: keep the flush plan active; avoid prolonged low-flow operation; specify dry-running grades (PTFE or antimony impregnation)
Particle Wear
1.Root cause: solids in the media embedding or rolling between faces
2.Face signature: circumferential scoring and grooves
3.Prevention: add flush filtration or a hydrocyclone; a hard/soft pair (graphite vs. silicon carbide) tolerates solids better than a soft/soft pair
II. Thermal Cracking: Radial Cracks and Heat Distortion
Thermal cracking is a thermal-stress failure. Local face temperature changes sharply; the hot surface expands while the interior stays cooler, and tensile stress exceeds the material's tensile strength (roughly one-tenth of the 150-300 MPa compressive strength).
1.Cause 1: dry friction or PV exceeding the limit — friction heat cannot escape
2.Cause 2: thermal shock — cold media hitting a hot face, a delta over 100℃
3.Cause 3: poor heat-path contact, heat accumulating at the faces
4.Face signature: fine radial cracks perpendicular to rotation, star- or comb-shaped
5.Prevention: control seal chamber temperature (a core aim of API 682 flush plans); choose high-conductivity grades — carbon graphite at 70-150 W/(m·K), higher conductivity means lower thermal stress; avoid quench cycles on hot faces
III. Fracture: Chunks Breaking Off
Fracture is a mechanical-stress failure — the ring splits into pieces or chips at edges.
1.Installation stress: uneven gland-bolt tightening, or oversized interference on secondary seals concentrating load
2.Hammer assembly: striking the ring directly; graphite has no plasticity, so stress releases as cracks
3.Poor O-ring groove design: insufficient groove depth pushing compression beyond 25%-30%, wedging the ring apart
4.Cavitation and vibration: axial thrashing and repeated face impacts
5.Prevention: tighten diagonally and evenly; assemble with soft tools, heat or pressing; design grooves to standard compression ratios; eliminate cavitation sources in the pump
IV. Troubleshooting Order
1.Duty records first: temperature, pressure, deadheading history, solids content
2.Face evidence next: wear, cracks or chipping map to the three classes
3.Installation records: tightening sequence, seal compression, axial float
4.Material last: chemical compatibility, strength and PV margins
In our failure analysis experience at Huahao Sealing, duty and installation issues account for most cases while the material itself is usually sound. Correct material selection only rules out material problems; engineering management of duty and installation determines actual seal life.