On a slurry pump seal, make both faces a hard material, and for most abrasive slurries that means silicon carbide against silicon carbide, preferably the sintered grade. Nickel-bound tungsten carbide belongs on a face that large solids or impacts can chip, as long as the slurry is not strongly acidic and the faces never run short of liquid. Carbon against a hard face works only where the faces run on clean barrier fluid.
Seal faces are commonly made from carbon, tungsten carbide, silicon carbide or alumina ceramic, and no one of them is best in every service; carbon is the most common, and in abrasive slurry it is the first to wear out.
Why Abrasive Slurry Needs Two Hard Faces
Abrasive slurry needs a hard face on both sides because solids get between the faces, and a particle harder than a face can score it.
A carbon face is much softer than the sand and rock in a mineral slurry and scores first. Grit also embeds in the carbon and grinds grooves into the mating ring, so a carbon against silicon carbide pair loses both faces.
A face must be harder than the hardest solid in the slurry, and in most mineral slurries that is quartz, at about 1,200 HV. Both carbides sit above it, silicon carbide by the wider margin.
Alumina is harder than quartz but rates only fair in abrasive service and cracks under thermal shock, and stainless steel faces are softer than quartz, so neither belongs on a slurry face.
Two hard faces need a liquid film between them: without one, they damage each other quickly and overheat. A slurry that dewaters and packs around the faces, as wood pulp does, leaves them close to running dry.
When Carbon Against a Hard Face Still Works
Carbon against a hard face, the default pair in clean service, still works in slurry only when the faces run on clean liquid. The dependable way to get that is a pressurized dual seal.
In that arrangement the outboard faces see only barrier fluid, so carbon against silicon carbide is the normal pair there. Keep the inboard pair hard against hard, because those faces meet slurry whenever barrier pressure is lost.
A flushed single seal is a weaker case for carbon. Even with a Plan 32 flush, some solids collect behind the throat bushing and build up with every stop and start.
Selection charts that list carbon against silicon carbide for “mild slurry” do not say what makes a slurry mild: no solids content, particle size or hardness. Every carbon grade, resin- or antimony-impregnated alike, wears poorly in abrasive service. Treat any hard grit that can reach the faces as abrasive.
Silicon Carbide or Tungsten Carbide
Silicon carbide against silicon carbide is the default for abrasive slurry. Nickel-bound tungsten carbide is the better choice for a face that large solids or impacts can chip, provided the slurry is above pH 5 and the faces stay lubricated.
Tungsten carbide is the tougher of the two, and light impacts generally do not fracture it. A silicon carbide face can chip when large solids wedge or strike at its edge. Judge the need from the solids the pump actually passes, such as coarse ore or gravel.
Silicon carbide has the wider chemical range. Strong acids, strong bases and ore-processing chemicals attack tungsten carbide’s metal binder, nickel or cobalt. One published specification proposal puts the line at pH 5: tungsten carbide above it, solid silicon carbide at pH 5 or below.
That pH line holds at best for nickel binders. Specify nickel-bound tungsten carbide, and keep cobalt-bound grades out of chloride-bearing slurry even above pH 5.
Tungsten carbide also needs the faces kept wet. A tungsten carbide face that runs short of liquid against silicon carbide or another tungsten carbide face develops fine surface cracks, known as heat checking. Where the flush can fail or the slurry can dewater at the faces, run silicon carbide against silicon carbide.
Which Silicon Carbide Grade to Use
Use sintered silicon carbide for abrasive slurry. The sintered grade also survives the caustics and strong acids that attack reaction-bonded silicon carbide. Drawings and quotes call it sintered, self-sintered, direct-sintered or alpha-sintered silicon carbide (SSiC, DSSiC), and these are the same material.
Where a hard pair may run short of liquid, consider graphite-loaded silicon carbide, either grade with a graphite phase added. The sintered version holds its graphite better.
