The gap between the faces of a mechanical seal is the space between the rotating and stationary faces, about a micron to a few microns wide, and it is filled with a film of the liquid being sealed.
How Wide the Gap Is
In a pump seal running on liquid, the gap runs from as thin as 1 µm (39 microinches) up to a few microns.
Flatness errors are on the same scale as the gap. A face lapped to a two-light-band flatness spec may be 22 microinches out of flat, more than half of a 1 µm gap.
Too thin, and the faces lose their film, run dry and wear. Too wide, and the pressure-driven leak rises with the cube of the gap.
Do the Faces Touch While the Seal Runs
In most pump seals they touch partly: the liquid film carries most of the load, while the high points of the two lapped surfaces still meet through it. This is mixed lubrication.
Higher face load and lower speed push the faces into more contact, toward boundary lubrication, where the film is broken in places. Higher speed can build the film until it lifts the faces fully apart. Even a seal that normally runs on a full film goes through periods of contact when operating conditions change.
What Holds the Gap at Its Size
Two opposing forces hold the gap at its size. The closing force, from the spring and the sealed pressure acting on the back of the floating face, pushes the faces together. The opening force, from the pressure in the film plus the load on any high points in contact, pushes them apart, and the floating face moves axially until the two are equal.
Hydraulic balance is how the design sets the closing force. A seal balanced at 60 to 90 percent takes part of the sealed pressure off the faces and runs a thicker film than an unbalanced seal.
The Shape of the Gap
Across the face, the gap can be parallel, converging or diverging. A seal runs stably only when it converges, widest at the high-pressure edge and narrowest at the low-pressure edge.
Faces are lapped flat, so the convergence comes from the running seal. Pressure and heat deform the faces and their holders into a slight cone, and the seal is designed so that this deformation gives the right convergence at its design point.
Around the circumference the gap is uneven too. A face that is slightly tilted gives a gap that converges over half the turn and diverges over the other half.