TP80-40/4 Shaft Seals Manual

18
Seal face materials
Shaft seals
Silicon carbide
The ceramic silicon carbide (SiC) has been manufac-
tured for many years.
There are three main types of SiC:
• Reaction-bonded and liquid-phase sintered grades
have limited corrosion resistance in alkaline water
due to the content of free silicon.
• Converted carbon is produced from carbon graphite
and can also be made as a thin SiC layer on the
surface of the carbon.
• The most common SiC for seal rings are direct-sin-
tered silicon carbide.
Direct-sintered SiC
The direct-sintered SiC has a typical porosity below
2%, but also grades with pores have been developed.
These pores are discrete, non-interconnected and dis-
persed in a controlled manner throughout the body of
the material. The spherical pores act as fluid or lubri-
cant reservoirs helping to promote the retention of a
fluid film at the interface of sliding component surfaces.
This pore-based lubrication mechanism allows porous
SiC to outperform conventional reaction-bonded and
sintered silicon carbides in hot water.
Extended sintering or adding different fillers can imply
variations in these standard SiC grades. Fillers can be
added to obtain better electric conductivity, more
toughness or lower friction.
Carbon or graphite inclusions can be used as dry lubri-
cant to reduce friction. Low friction of graphite can only
be achieved with appropriate impurities because the
intrinsic friction of graphite is high. For graphite to suc-
cessfully act as a lubricant, the bonding between the
SiC and the graphite as well as the size and amount of
the graphite inclusions must be optimized. In a working
seal the graphite must be smeared out in the seal face
to reduce friction, and it must be possible to remove
some of the graphite from the inclusions.
Performance of seals with different SiC variants
Evaluation of materials for seal faces requires thorough
testing at many different testing conditions.
SiC seal materials can be subjected to the following
tests:
• Performance in hot water applications
• Dry running
• Water containing abrasive particles
• Water containing glycol
• Demineralized water
• Seizure test.
The tests are described in detail as from page 19.
During the last 15 years almost 50 different SiC grades
have been tested at Grundfos and categorized in
groups according to performance.
Q
s
is a conventional dense-sintered SiC with a porosity
of less than 2%. This grade has poor performance in
hot water and high dry friction.
Fig. 20
Direct-sintered, dense SiC, Q
1
s
Q
p
is a sintered SiC with discrete, non-interconnecting
pores. The pores give better performance in hot water,
but the dry friction is high. Graphite-containing SiC with
poor dry running capability or poor performance in dem-
ineralized water are also categorized in this group.
Fig. 21
Direct-sintered, porous SiC, Q
1
P
Q
G
is a dense-sintered SiC with bimodular grain size
and pores of carbon/graphite or other low friction mod-
ifiers.
It has good performance in hot water and demineralized
water, and has low dry friction.
Fig. 22
Direct-sintered, porous SiC, Q
1
G
TM02 7726 3603TM02 7727 3603TM02 7728 3603