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Stuffing Box Sealing

An Unbalanced Seal exposes the full cross-sectional area of the rotating
face to the stuffing box pressure and creates a high closing force
between the seal faces.
If a pressure of 50 pounds per square inch is acting on the area of 2
square inches on the reverse side of the rotating face, then the force
trying to close the faces is 50 p.s.i., multiplied by 2 square inches,
which equals 100 pounds.

Although the nature of the pressure drop across the seal faces is
unknown, it is generally considered to be almost linear.
Consequently, the average pressure between the faces will be 25 pounds
per square inch, resulting in a force trying to open the seal faces of 25
p.s.i, multiplied by 2 square inches, which equals 50 pounds. Therefore
the magnitude of the force trying to close the seal faces is twice that
trying to open them. This constitutes an unbalanced seal, the effects of
which can include an increased operating temperature and an
accelerated wear rate. These conditions would dramatically reduce the
seal life in services that were already at a high temperature and/or in an
aggressively abrasive liquid.
Balancing a mechanical seal reduces the closing force and extends the
life of the seal. This is usually achieved by reducing the effective cross-
sectional area of the rotating face by using a stepped shaft or sleeve.
However, this is never taken to the point that there is a Net Closing
Force of Zero, as it is possible that the condition between the seal faces
can become unstable, and they may be blown open by any sudden
change.

Figure 6.7: Pressures on an unbalanced seal