The primary sealing point is the interface between the rotating and stationary seal faces, and it leaks in five ways: faces out of flat, faces pushed apart in service, damaged faces, solids between the faces, and faces running out of square or off centre. Most seal leakage comes through this interface. The rest comes through the other sealing points, the O-rings and gaskets behind the faces, and in rare cases through a porous face ring. A carbon face pressed into a metal holder adds one more path, along the joint between the two.

Pressurizing the seal with the shaft stopped separates face leaks from the rest. Healthy faces touch at standstill and leak almost nothing, so a leak that continues with the shaft stopped normally points to an O-ring or gasket.

The exception is a seal that has already run. Running can wear a non-flat pattern into the faces, and a Fluid Sealing Association column on seal leakage notes that such a seal may leak more stopped than running. On a seal that has been in service, a standstill leak does not rule out the faces.

Faces That Are Not Flat

A face that is out of flat leaves a gap that the closing force cannot shut, and fluid runs through it.

Seal face flatness is commonly specified at two to three helium light bands (one helium light band is 11.6 microinches, or 0.29 µm).

Distortion in service takes three shapes:

  • Thermal distortion: heat generated at the faces bends them into a gap that narrows from the pressure side toward the leak side.
  • Pressure distortion: pressure on the face rings bends them into a gap that widens toward the leak side.
  • Mechanical distortion: uneven clamping, uneven O-ring loading or a holder out of tolerance pushes high and low spots into the face.

A face that checks flat on the bench can still be the leak path once installed.

A new seal, especially one with two hard faces, leaks more at start-up than after it has worn in.

Faces That Open in Service

The faces open in service when the forces pushing them apart exceed the spring and hydraulic force holding them together, and the seal then leaks heavily.

The usual causes:

  • Liquid flashing between the faces: a liquid close to its boiling point vaporizes between the faces, and the seal pops, chatters and leaks, getting worse as the seal chamber temperature rises.
  • Lost spring load: broken or clogged springs leave the faces free to separate when the pump is stopped or seal chamber pressure is low.
  • Hang-up: on a pusher seal, deposits or slurry particles packed around the dynamic O-ring or the springs stop the flexibly mounted face from sliding, and the faces stay apart. A bellows seal has no sliding secondary seal to hang up.
  • Worn faces: faces that are badly worn change the seal’s loading, and fluid pressure can then force them open.
  • Reversed pressure on a double seal: if barrier pressure falls below seal chamber pressure, the inner seal is loaded from the wrong side, and an inner seal not designed for reverse pressure can open.

Damaged Faces

Any damage that breaks the lapped surface opens a path the fluid film cannot seal, and the pattern on the face usually shows which kind of damage it was.

  • Blistered carbon: shiny raised splotches on the carbon face grow into bubbles ringed by cracks, which break away and leave pits.
  • Heat cracks: hot spots at the interface spread cracks through the face. A ceramic face can also crack outright from thermal shock.
  • Scored hard faces: two carbide faces run without lubrication score each other with fine concentric grooves, known as phonographing, and each groove is a leak path.
  • Chemical attack: strong caustic or acid attacks the free silicon in reaction-bonded silicon carbide, and process chemicals can leach the nickel or cobalt binder out of tungsten carbide.
  • Lost coating or broken edges: a surface treatment that separates from its substrate, or an edge breaking down around a pocket cut into the face, releases debris that damages the interface quickly.
  • Cracks from the drive slot or from impact: a crack that starts at the drive pin slot of a stationary face can mean the product stuck the faces together and the extra torque cracked the face at the pin. A crack from a chip or an edge mark points to impact during installation.

Solids Between the Faces

Solids that get between the faces are trapped in the interface and abrade the lapped surfaces. The particles reach the faces in three ways:

  • Carried in the fluid: slurries and other solids-laden fluids deliver particles to the faces directly.
  • Formed at the faces: dissolved material precipitates out of the sealed fluid nearest a heat source, which in a seal chamber means the faces. Antioxidants in glycol cooling systems are a known case.
  • Circulated in the seal chamber: flow that sets up recirculation vortices in the seal chamber keeps sweeping the same particles across the faces.

The softer face of the pair, usually carbon, wears first.

Faces Running Out of Square or Off Centre

When the shaft is misaligned, bent or running out, the rotating face no longer runs square to the stationary face, and leakage across the faces rises.

Static misalignment, the kind that can be measured with the pump stopped, comes from parts out of tolerance, static shaft deflection or a seal installed out of square. Misalignment also leaves marks away from the faces: broken springs, worn or extruded dynamic O-rings, fretting corrosion on the metal next to the dynamic O-ring, and worn drive parts.

A stationary face mounted off centre to the shaft shows a wear track as wide as the rotating face’s nose but pushed toward one edge.