Mechanical seal operation is the result of a complex integration between solid friction due to asperity contact and hydrodynamic forces generated by the fluid in the interface due to the relative movement of the faces. The combination of these two effects generates the forces that keep both faces slightly apart when in operation.
Liquid film formation is similar to a hydrodynamic thrust bearing, except that the axial load usually is much smaller, the fluid often is a poor lubricant, temperatures can be very high, and the fluid film cannot always separate the faces completely and often are partially vaporized, as in the case of liquified petroleum gas (LPG) or hot water seals.
There always is some leakage past the seal when it is operating. This leakage is very small, invisible to the naked eye.
This does not mean that it is impossible to avoid all leakage of the pumped fluid. Some of the fluid that contacts the seal will leak, but it is possible to avoid the leakage of even minute quantities of the pumped fluid if double seals are used.
When a mechanical seal begins to rotate under pressure, the sealing faces are subject to the forces of the springs, the static pressure forces of the sealed fluid, and the hydrodynamic forces generated by the fluid at the interface. The factor most influential on seal life is the fraction of total face load supported by hydrodynamic fluid pressure. This is the well known lubricated friction problem.
As usual, if the film thickness is large enough to completely avoid solid contact, wear is very low. The hydrodynamic forces are generated by the interaction of the fluid with the moving irregularities of the rotating face. Unlike a hydrodynamic bearing, the mechanical seal normally is not designed to provide a thick fluid film, as this would lead to high leakage. This is the mechanical seal dilemma.
The lubrication conditions that result in longer life also result in more leakage. These, clearly, are conflicting requirements. The necessity of little leakage means that the film thickness should be very thin. To obtain good lubrication conditions with a thin film, surface asperity height should be very small, too.
Seal surfaces usually are lapped to a mirror like finish to reduce asperity contact. Even when correctly lapped, the seal faces have some asperities.
The normal operating condition of mechanical seals causes some solid contact. This solid contact causes the removal of the highest spots of the softer surface.
Special face configurations sometimes are used to improve lubrication, when poor lubricity fluids have to be sealed. This can be useful in poor lubricity fluids, like hot water, LPG, and even gases. Note that a portion of the surface, close to the inside diameter, is flat. This is done to limit the seal leakage.
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