What a mechanical seal does
A rotating pump shaft has to pass through the casing without letting the pumped fluid escape. Soft packing can do this, but most modern pumps use mechanical seals. A mechanical seal consists of two very flat rings, one stationary and one rotating with the shaft, pressed together by springs. The contact between their faces is the main barrier. A very thin film of fluid, often only a fraction of the width of a human hair or less, separates the faces slightly, allowing them to run with limited friction while still keeping leakage low.
Pump seals work well when that film is clean and stable. Abrasive service threatens the film in several ways, and that explains much of the material selection.
What goes wrong in abrasive flow
When the pumped fluid is a slurry (mine tailings, dredged material, ceramic slip, mineral processing streams, ash handling), the liquid carries hard particles. Several problems follow.
- Particles enter the gap. Grit that gets between the faces scratches them, causing leakage paths and rough surfaces that wear faster.
- Erosion around the seal. Fast-moving slurry hitting seal parts and nearby casings gradually removes material, a form of erosive wear.
- Particle embedding and packing. Fine solids can pack into springs, dead spaces and gaps, preventing the seal from moving as intended.
- Heat build-up. If the film breaks down, friction heats the faces and can cause cracking, blistering or fluid vaporization.
- Corrosion. Many slurries are also chemically aggressive, so wear and corrosion work together.
Bearings in pump equipment, particularly sleeve bearings lubricated by the process fluid, suffer from the same issues, because the process fluid itself is the lubricant and it is full of grit.
Why hard ceramics and carbon are common
A traditional choice for seal faces is a hard-against-soft pair, often a carbon-graphite face against a harder face. In abrasive service, designers frequently move to two hard faces, since a soft face would be quickly embedded with or cut by particles.
Silicon carbide. Silicon carbide is a very hard ceramic with good chemical resistance and high thermal conductivity, which helps carry away friction heat. It is a common choice for faces in abrasive and corrosive duties. Some grades contain free silicon or are sintered in different ways, and their chemical and wear behavior differs. The sintering process used affects the final structure, so grades are not interchangeable.
Tungsten carbide. Cemented carbide is tough for a hard material, so it tolerates shock and pressure better than most ceramics. It is a frequent choice in high-pressure or heavy-duty pairs, although the metal binder can limit chemical resistance in some fluids.
Alumina. Alumina ceramics are lower in cost and give acceptable service in moderately abrasive duties, with good chemical resistance. They conduct heat less well than silicon carbide.
Carbon-graphite. Industrial graphite and carbon grades are self-lubricating and run well against hard faces, tolerating brief dry running better than most ceramics. In clean fluids, a carbon face against silicon carbide is a common pairing. In highly abrasive slurries, carbon may wear too fast, which is when hard-against-hard pairs come in.
Diamond-coated faces. Diamond coatings, including those deposited by chemical vapor deposition, are used on some faces where extreme wear resistance is needed. The coating must bond well and the substrate must be suitable, so these remain a specialist choice.
The common thread is hardness above that of the typical particles, plus stability when hot and in contact with chemicals. The same logic explains why ceramic bearings are used in other harsh moving-part duties.
Trade-offs of hard-against-hard pairs
Two hard faces last longer in grit, but they are not free of downsides.
- Brittleness. Hard ceramics can chip or crack under impact, thermal shock or poor installation. Running dry, even briefly, can cause rapid heat damage.
- Lower tolerance for dry running. Carbon provides some self-lubrication. Two hard ceramics generate more friction if the fluid film fails.
- Squeal and noise. Hard pairs can be noisy in some conditions.
- Cost. Silicon carbide and related parts typically cost more than conventional face materials, so the economics depend on how often seals fail and how costly downtime is.
Design measures beyond material choice
Material is only part of the answer. Typical measures include:
- Clean flush. Injecting clean fluid near the seal keeps grit away from the faces.
- Dual seals with barrier fluid. A second seal with a clean fluid between keeps the process slurry from reaching the primary faces.
- Seal geometry. Designs that sweep solids away from the gap and avoid dead pockets reduce packing.
- Hard bushings and sleeves. Shaft sleeves and throat bushings made of hard ceramic or cemented carbide protect the shaft and limit particles reaching the seal.
- Flow management. Reducing velocity and avoiding sharp turns lowers erosion.
Bearings in process fluid
Pumps used for slurry may use bearings that run in the process fluid itself, with no oil bath. Silicon carbide sleeve bearings and silicon nitride components are used for this type of duty because they tolerate grit, resist corrosion and keep close tolerances. They need fluid for cooling and lubrication; running dry usually damages them quickly. Tough materials such as cemented carbide may be preferred when shock is likely. In the oil and gas and energy fields, the use of diamond components in pumps and valves follows similar logic: put the hardest material where the particles hit.
Realistic expectations
Seal and bearing life in abrasive service depends heavily on slurry concentration, particle size and hardness, pressure, temperature, installation quality and operating practice. Published life figures should be treated as rough guides at most. The practical approach is to start with materials that experience suggests for similar duties, monitor performance, and examine worn parts to see whether the damage is abrasion, thermal cracking or chemical attack before changing anything.