You identify mechanical seal face damage by reading the pulled faces in two layers: first the wear track, measured against a normal one, then the marks on the face surface. Read the two faces as a pair, with the springs, O-rings and drive parts beside them.
Handling the Pulled Seal
Photograph and record each face before you clean it, and keep every part labeled with its position, because cleaning and lapping destroy evidence the faces carry.
The residue is part of that evidence. Product still tacky on the faces means something different from residue that is clean and dry.
Keep the rotating and stationary parts apart, and on a double seal keep the inboard and outboard sets apart too, so each pair of faces can be read together.
Record the service history with the parts: how long the seal ran and whether it leaked from start-up or only later. Note any part damaged while the seal came out, so a chip from removal is not later read as service damage.
What a Normal Wear Track Looks Like
A normal wear track is a smooth, even band that runs unbroken all the way around the wider face, concentric with it and as wide as the narrow face that ran on it.
Even wear is normal only if it took about as long as the seal’s expected life. The same track worn deep in a fraction of that time is premature wear.
A worn-in face can read out of flat under light bands at rest without being damaged.
If both faces show a normal track and the seal still leaked, the leak came from somewhere else: an O-ring, or rarely a porous ring.
Abnormal Wear Track Patterns
A track that differs from the normal one in width, continuity or position shows how the faces actually ran: tilted out of parallel, distorted out of flat, off-center, or not touching at all. Measure the track’s width against the narrow face before reading anything else.
Narrow Wear Track
A track narrower than the narrow face means the faces touched on one edge only, all the way round: they ran out of parallel.
- Track at the outside edge: pressure distortion, from a process upset or a seal run above its pressure limit.
- Track at the inside edge: thermal distortion.
The outside-edge and inside-edge readings assume the process fluid is at the outside diameter. On a seal pressurized from the inside, the edges swap.
A one-edge track can also come from poor support of the stationary face in its gland, swollen O-rings, or inadequate hydraulic balance.
Wide Wear Track
A track wider than the narrow face means the narrow face moved radially on its mate as it turned and swept a band wider than itself. Either the stationary face sat cocked in its gland, or the shaft moved: runout, a bent shaft, worn bearings, misalignment, pipe strain on the casing, vibration or cavitation.
Shaft runout and vibration readings on the pump show whether the shaft moved. If they are within limits, look at how the stationary face sits in the gland.
No Wear Track
A face with no wear track never ran against its mate. Either the two faces had no motion between them, or they were held apart.
- No relative motion: the rotating unit slipped on the shaft at its set screws or rubber bellows and stopped turning, or a broken anti-rotation pin let the stationary face spin with the rotating one.
- Faces held apart: deposits, coke or packed solids around the springs or bellows held the flexible face back so it could not follow its mate, or the rotating unit rubbed on the stuffing box or gland.
Intermittent Wear Track
A track that comes and goes, with heavy contact at two or more spots and little or none between them, means the face was distorted out of flat and the high spots carried the load. When the pattern is too faint to see, a light pass on a lapping plate brings the high spots out.
The distortion comes from one of five places:
- Gland bolts: tightened unevenly on a rigidly mounted or clamped stationary face. Contact spots that line up with the bolt pattern mean uneven bolt torque.
- Cooling: uneven cooling that stressed the face thermally.
- The face itself: lapped or machined with a saddle shape or high spots.
- Gland seat: debris, deposits or damage in the gland that load the stationary face unevenly.
- Stuffing box face: corrosion or damage on the face the gland bolts against.
Uneven Wear Track
A track that runs all the way round but changes width means the face was distorted. The causes are the ones listed under Intermittent Wear Track, plus excessive pressure on the face.
A track of even width that sits off-center on the face, nearer the outside edge on one side and the inside edge on the other, is a different fault. The stationary face sat off-center to the shaft, from too much clearance between gland and stuffing box or a gland not concentric with the bore.
Uneven tracks are often blamed on shaft misalignment. The Fluid Sealing Association’s guidance on misalignment puts the evidence of misalignment elsewhere on the seal: broken springs, worn or extruded dynamic O-rings, fretting on the metal next to the dynamic O-ring, and worn drive parts. Without those, read an uneven track as distortion or an off-center face, not misalignment.
Damage to the Face Surface
Each mark on the face surface traces back to solids, impact, heat or the fluid’s chemistry.
Grooves and Scoring
Grooves and scoring around the face mean something cut it as it turned: hard particles between the faces, or a hard pair that lost its liquid film.
Particles score a carbon face most readily. They can also embed in a resin-impregnated carbon face and grind grooves into its silicon carbide mate, so grooves on the hard face can come from solids too.
A grooved hard-on-hard pair from a clean fluid lost its film through flashing, dry running or running on one edge after a pressure upset.
Fine, even scratches on a newer face are not always damage. Some faces are matte lapped, which leaves microns-wide scratches on purpose, and the texture wears off in service.
Chipped Edges
Chipped edges mean the face was knocked, or the faces were forced apart and slammed back together.
- Isolated chips at random points: handling. The face was knocked before or during assembly, or while the seal came out.
- Chipping around both the inside and outside edges, with pitting across the face: the liquid flashed to vapor between the faces, which pushed them apart and let them slam back together.
Silicon carbide is more brittle than tungsten carbide and chips more easily, sintered grades most of all.
Cracks
A crack through a seal face means the face was overstressed by start-up torque, impact or heat. The crack’s starting point and the residue on the face separate torque from impact:
- From the drive pin slot of the stationary face, with sticky product on the faces: the product glued the faces together while the pump stood, and the torque at the next start cracked the face at its pin.
- Elsewhere, with residue that is clean and not tacky: impact, which means the seal was not installed properly.
Heat cracks faces as well. A hard-on-hard pair that runs dry can heat fast enough to crack, and a ceramic face that cracked after a sudden change in temperature has failed by thermal shock.
Heat Checking and Discoloration
Heat checking, fine radial cracks in the wear track, means the face ran too hot. Either the faces lost their liquid film, through dry running or vaporization, or they were short of cooling. Pressure and speed beyond what the faces can carry overheat them too.
Tungsten carbide heat checks more readily than silicon carbide.
Discolored spots in the track mark where the film broke down and the faces ran hot. On a carbon face the same thing shows as shiny marks.
Blistering
Blistering on a carbon face means the carbon cracked just below its surface, most often under the shear of starting up in a viscous fluid rather than from running hot. It starts as shiny raised spots in the track, which grow into bubbles ringed by cracks and then break away, leaving pits.
Viscous oils are the usual service, and light hydrocarbons that flash in the carbon’s pores are the other. Resin-impregnated carbon blisters more readily than antimony-impregnated carbon.
Pitting and Corrosion
Pits and corrosion mean the face material did not suit the chemistry it actually saw, and what that chemistry dissolves is the phase holding the face together. The chemistry includes cleaning: in a food or beverage plant, the clean-in-place cycle runs acids and alkalis that the product never would.
On a carbon face, rule out blistering first. If the track carries raised blisters elsewhere, the pits are blisters that broke away.
What corrodes depends on the face material:
- Carbon: carbon itself resists most chemicals, but strong acids leach its impurities and additives, and strong acids or bases attack an antimony impregnant.
- Reaction-bonded silicon carbide: caustics, other high-pH fluids and strong acids eat out its free silicon. Sintered silicon carbide has no free silicon.
- Tungsten carbide: the cobalt or nickel binder dissolves from between the carbide grains, and the unsupported grains can come loose.
A leached ceramic or tungsten carbide face looks dull and matted, and its hardness reads 5 points or more lower on the Rockwell A scale.
Flaking Coatings
Flaking means a coating has come away from the base metal under it, so it happens only on coated faces: hard overlays, tungsten carbide coated onto stainless steel, and diamond coatings.
Dry running, flashing and edge running after a pressure upset load the bond between coating and base metal hardest. Chemical attack at the bond also delaminates a coating, and so does a poor bond from the start.
Heat checks or a discolored track beside the flaking point to heat. Flaking on a seal that ran cool in an aggressive fluid is more likely chemical attack.
