TP50-80/2 Shaft Seals Manual

20
Seal face materials
Shaft seals
Fig. 26 Accumulative wear of seal rings running in
2,000 ppm sand
It is not recommended to use a porous SiC seal face
against a carbon face in water containing a high level of
dissolved solids.
Abrasive wear may be observed on seals with hard seal
faces in corrosive liquids. Wear on SiC may occur in
demineralised water due to corrosion in grain bounda-
ries.
Wear on SiC faces may occur in hot water. It may look
like abrasive wear because SiC grains are pulled out
due to thermal fatique of SiC. This type of wear is only
seen above the pressure and temperature limit for sta-
ble friction.
Water containing glycol
Water containing glycol may cause problems with leak-
ing seals. The problems often arise due to additives
such as inhibitors, antioxidants and alkalines.
Some additives, for instance silicates, may crystallize in
the seal face as hard particles. Hard crystallites such as
silicates or phosphates will cause wear on seals with
one face in carbon.
Organic film binders, so-called inhibitors, adhere to all
surfaces in contact with the liquid, including a major
part of the seal face. Many inhibitors may build up sticky
layers on the seal face resulting in leakage.
Seals with both seal faces of tungsten carbide or SiC
have better self-cleaning properties regarding deposits
than seals with a carbon seal face.
A high closing force and a narrow width of the seal face
reduce the risk of deposits building up. Comparisons of
seal face combinations in water containing glycol with a
high content of additives have been performed and
results can be seen in the following graph.
Fig. 27
Leakage of seals in water containing glycol
Surface roughness is an important factor for the leak-
age. A smooth surface finish is preferred. On the other
hand a too smooth surface finish may cause problems
in terms of seizure between the seal faces. Thus some
roughness of the seal faces is required to prevent the
seal rings from seizing up. Therefore seals have at
least one lapped seal face.
After the shaft seal has been fitted, the leak rate will be
high until the seal faces have become smooth. As a
result seals with one seal face in carbon often have low
leakage in the running-in period because this period is
shorter as compared to a seal with both seal faces in
hard materials.
Seals with a high closing force have a shorter running-
in period as a result of a thinner lubricating film.
Demineralised water
Pure water can be aggressive to many ceramics.
Regarding sintered SiC, the grain boundaries contain-
ing sinter additives may be attacked in pure water.
Corrosion attacks are only observed on the seal face
where high temperatures may be achieved where
asperity contacts.
By controlling the sintering process it is possible to
achieve SiC grades that are resistant to pure water.
Tests of seals in demineralised water at +40°C have
been carried out for different SiC grades. The result of
standard sintered SiC grades can be seen in the follow-
ing figure.
For grades not sensitive to demineralised water no fail-
ure has been observed during 11,000 hours of testing.
Only grades resistant in demineralised water can be
categorized in group Q
G
.
TM02 7286 3203
0
500
1000
2000 [h]
1500
Q
Q
S
P
TM02 7287 3203
0
1
2
3
4
5
6
Q
P
/Q
P
Q
S
/Q
S
Q
G
/Q
G
Low closing force
High closing force
Leak rate (comparative)
Tungsten c./ tungsten c.
Tungsten c./ carbon