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The Practical Pumping Handbook ................... 7 ........................................................................ i---iiii iiiiiiiiii:ii-~a~ ::::::,e,::~

your local expert will normally identify the best material combination
available for your particular application.
Regardless of the face material used, a thin film of liquid must exist
between the faces to provide some lubrication. But a combination of
the spring load and the liquid pressure in the stuffing box, creates a
closing force on the seal faces. If that closing force is too high, it can
substantially reduce the amount of liquid between the faces. This will
result in increased heat generation and premature wear on the faces. If
the closing force is too low, the faces can open easily and permit
leakage.
Seal manufacturers are constantly trying to improve the seal design with
particular reference to the flatness of the faces. Currently, the industry
standard for face flatness tolerance is 2 helium light bands which
translates to 23.2 millionths of an inch (0.0000232 inch)

In order to achieve this degree of accuracy, seal faces are machined on
Lapping Machines with special polishing plates. The finish is then
checked on an optical fiat, using a monochromatic light source.
In view of this, it is obvious that careful handling of these faces is
essential. It is also apparent that the manufacturers' installation
instructions should be carefully followed to ensure that the seal faces
are suitably protected and precisely located.

6.3.3 Seal flexibility options
Any axial or radial movement of the shaft will require some flexibility
from the spring(s) in order to keep the faces closed.
This flexibility however can only bc carried to a certain degree, and
the mechanical condition of the pump, including a low value of the
slenderness ratio, also plays an important role in the reliability of the
SCal.
This seal flexibility is usually supplied by
9 a single large spring,
9 a series of small springs, or

9 a bellows arrangement.
The process pumping industry has traditionally used seal designs where
the springs were applied to the rotating face. This is known as a 'Rotary
Seal' as the springs or bellows rotate with the shaft.
More recent designs apply the springs or bellows to the stationary face
of the seal. In fact, it is now quite common to find both stationary and
rotating faces of a mechanical seal having some kind of flexible
mounting arrangement.

9z