Page 225 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

Figure 14-3 Chemical attack Figure 14-4 Extrusion

Extrusion
Extrusion is deformation under pressure. An O-ring extrudes when the
pressure is too high. Maybe the O-ring needs back-up rings to tighten
tolerances. Maybe the design of the O-ring groove is inadequate. Some
O-ring compounds get softer as the temperature rises so temperature-
linked-to-pressure is also a factor to consider. An O-ring also may soften
if under chemical attack. This should also be checked as a possible
source of the extrusion.

Split, cracked or hard O-ring
Although an O-ring softens with temperature, too much heat will
harden the O-ring. Too much heat is the usual cause, but chemical
attack may produce the same result. The O-ring must deal with the
temperature of the application and also from the heat generated
between the mechanical seal faces. Many mechanical seal designs place
the O-ring where it receives a lot of heat. A good idea is to find a
mechanical seal with the O-rings and other secondary elastomer seals
placed away from the heat of the faces.

Compression set
This is a good indicator that the O-ring was exposed to too much heat.
Compression setting means that the round cross-sectioned O-ring came
out of its groove with a squared cross section. The heat caused the o-
ring to re-cure in the groove, taking the shape of the groove.

Figure 14-5 Split, cracked or hard O-ring Figure 14-6 Compression set

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