Page 111 -

The Practical Pumping Handbook

TERTIARY
_~,~' SEAL/

Stuffing Box ,...........,....<

~/////////] Rot at ln g [~!~ii:i-i-i~:!i:!-!-i-i~
Stationary
Face

PRIMARY
Pump Shaft SECONDARY SEAL
SEAL

Figure 6.3: Labeled mechanical seal

Between the Stationary Face and the Gland.
.
Between the Gland and the Stuffing Box.
.
The last two seal paths are usually 'Static' seals as there is no relative
motion between the two parts. They are frequently referred to jointly as
the 'Tertiary Seal', and may consist of a flat gasket or an 'O'-ring in
materials compatible with the pumpage.
In the older seal designs, the Secondary Seal under the Rotating Face
will move marginally back and forth on the shaft, thus causing fretting
corrosion and premature failure. However, in the newer seal designs,
the Secondary Seal will be 'Static', thus avoiding fretting corrosion
problems on the shaft. This will be discussed in greater detail when we
review the 'Fretting Seals' in Chapter 6.3.4.

In normal pump operation, the rotating and stationary faces are held
closed by the pressure of the liquid in the stuffing box acting as the
closing force. During startup and shutdown, the stuffing box pressure
is augmented (and even possibly replaced) by the spring force.

6.3.1 Match the seal to the service
While some liquids are fairly simple to work with, others can be very
difficult. It is essential that all the individuals involved (including the
seal supplier) are made aware of all the factors that will influence the
seal selection. These factors should include the following.
9 pressure 9 temperature
9 corrosiveness 9 abrasiveness

9 viscosity 9 tendency to crystallize

m 90