TP50-80/2 Shaft Seals Manual

19
Seal face materials
Shaft seals
Performance in hot water
The lubrication of the seal faces in hot water is scarce
due to the low viscosity of water at high temperatures
and evaporation in the seal gap.
Limits of temperature and pressure ranges are based
on tests where factors such as friction, torque and leak-
age are measured.
Above these limits noise from the seals may be
expected and fatique wear may occur.
The figure below shows the limits from the different SiC
groups and tungsten carbide for a ø16 mm Grundfos
type A seal at 3,000 min
-1
. At lower speeds the limits
are shifted towards lower temperatures.
Fig. 23
Limits for stable friction of a seal
For information about the SiC variants (Q
S
, Q
P
and Q
G
)
see page 16.
The hot water tests are performed in tap water. Under
these conditions the seal faces are exposed to very lit-
tle wear in the stable region, whereas some wear might
be expected above the limit for stable operation.
The figure below shows the wear rate as a function of
temperature.
Fig. 24
Wear rate of seal faces
Dry running
Mechanical seals running completely dry can be
destroyed within one minute if the friction between the
seal faces is high. The heat dissipated in the seal face
will raise the temperature of the seal to above +200°C
and the rubber parts will burn off.
The friction of SiC against SiC depends of the fillers
added to the SiC and the way these are bonded to the
SiC matrix. Relative humidity affacts the coefficient of
friction for SiC materials even though this has little
practical effect on a mechanical seal because the tem-
perature quickly rises above +100°C at which point
humidity has no effect.
Temperatures measured on the stationary seat on
selected dry running seals can be seen on figure 26.
Fig. 25
Dry-running performance of seals
As will be seen on figure 26, SiC
S
/SiC
S
and SiC
P
/SiC
P
show poor dry-running performance, similar to tungsten
carbide/tungsten carbide.
The two SiC
G
/SiC
G
grades show better dry-running
performance. The dry-running performance may vary
slightly, even within the same grade of SiC
G
.
Although it contains graphite, the SiC
P
grade shown
cannot be categorized as SiC
G
, due to its poor dry-run-
ning properties.
Water containing abrasive particles
SiC is a hard material and is therefore less affected by
abrasives. Wear on the seal faces caused by abrasives
are rarely observed for seals with both faces in SiC.
The seal gap in a mechanical seal is typically below 0.3
micron. Theoretically this means that only particles
smaller than 0.3 micron can enter the seal gap. In prac-
tice the edge of a seal face is not completely sharp.
This means that particles measuring a few microns are
able to enter the seal gap. Normally such small parti-
cles will only cause a polishing wear on a hard seal
face. When one of the faces is a carbon ring, the edge
of the seal face will wear and permit larger particles to
enter the seal gap.
Such larger particles can be trapped in the carbon seal
face and cause wear on the counter face.
TM02 7284 3203TM02 7283 3203
0
50
100
150 [°C]
0
5
10
15
20
25
30
[bar]
Q
Q
P
Q
WC/WC
S
G
0
20 40
60
80
100 120
140 [°C]
HQQE (Q )
HQQE (Q )
G
HUBE
S
TM02 7285 3203
0
500 1000 [sec]
0
50
100
150
200
250
[°C]
Q (with graphite)
P
Q
G
Q
G
Q
S