TP50-160/2 Shaft Seals Manual

21
Seal face materials
Shaft seals
Fig. 28 Failure of SiC seals in demineralised water
Seizure of seal faces caused by storing
Very smooth and flat seal faces will easily adhere to
each other. In extreme situations the adhesion
becomes so strong that the shaft on the motor of the
pump cannot rotate.
There are different mechanisms acting on the adhesion
between the seal faces.
Physical adhesion
Vacuum may occur when two flat and smooth surfaces
are pressed tightly together.
The force created by the vacuum is axial, meaning that
the force needed to separate the two surfaces has to be
large whereas the shear force needed to rotate the sur-
faces is lower. The size of the shear force at start-up is
the same as the force needed for a very low rotational
speed. The coefficient of friction at low rotational speed
for different surface combinations can be seen on the
graph below.
Start friction in water
Fig. 29
Start friction in water
Chemical adhesion of surfaces
All surfaces subjected to the atmosphere have an oxide
layer. The equilibrium of the oxide layer may change
when the surface is in close contact with another sur-
face or when it is exposed to the pumped liquid. This
change in equilibrium may involve chemical bindings to
oxides from other surfaces. The more inert the oxide
layer is to the surroundings, the weaker will the bind-
ings to the counter surface be. If the liquid is aggressive
to the seal face material, the seal faces may corrode
together and create immensely high adhesion forces.
To prevent such adhesion mechanisms, highly inert
dissimilar materials of the seal faces are preferred.
Chemical adhesion involving adhesive agents
If the pumped liquid contains ions that may precipitate
on the seal face, these precipitations may act as glue
between the seal faces.
This adhesion mechanism may occur in hard water and
can be reduced by having one seal face in carbon. Also
hard materials containing solid lubricants will reduce
adhesion because the dry lubricant will be smeared out
as a thin layer on the seal face, providing low shear
forces.
Conclusion
The sensitivity to abrasive particles is low when using
hard materials in both seal rings such as SiC against
SiC.
Friction of SiC against SiC in hot water are reduced by
the porosities in the seal face material. Generally, the
resistivity to corrosion of sintered SiC is good except in
pure water, but with SiC containing long grains also
resistance against pure water can be obtained.
Incorporating small pockets filled with solid lubricants
can reduce dry friction of SiC.
Mechanical seals with improved SiC grades Q
G
are
thus able to handle many different applications, thereby
increasing the reliability of the pump.
TM02 7288 3203TM02 7289 3203
0
10
20
30
40
50
60
70
80
90
100
0 2000 4000 6000 8000 10000 12000
Failures [%]
[hour]
0
50
100
150 [°C]
0
0.1
0.2
0.3
0.4
0.5
0.6
TC/SiC
TC/Carbon
TC/TC
SiC-G/SiC-G
[°C]
Coefficient of friction