TP80-240/2 Shaft Seals Manual

Shaft seals
16
Seal face materials
Seal face material combinations
The choice of seal face materials is decisive of the func-
tion and life of the mechanical shaft seal. Below is a
description of the possible material combinations.
Noise is generated as a result of the poor lubricating
conditions in seals in connection with low-viscosity liq-
uids. The viscosity of water decreases with increasing
temperature. This means that the lubricating conditions
are deteriorated with increasing temperature. A speed
reduction has the same effect.
If the temperature of pumped liquid reaches or exceeds
the boiling temperature, the liquid on part of the seal
face evaporates, resulting in a further deterioration of
the lubricating conditions.
Fig. 19
Relation between duty range and speed
The seal face lubricating conditions depend on the
adaptation/running-in wear properties and material
structure of the various seal face materials.
Tungsten carbide/tungsten carbide
Cemented tungsten carbide is the designation of the
type of hard metals based on a hard tungsten carbide
phase and usually a softer metallic binder phase. The
correct technical term is "cemented tungsten carbide",
however the abbreviated term "tungsten carbide" is
used for convenience, "cemented" being implied.
Cobalt-bonded (Co) tungsten carbide is only corrosion-
resistant in water if the pump incorporates base metal
such as cast iron.
Chromium-nickel-molybdenum-bonded tungsten car-
bide has a corrosion resistance equal to EN/DIN
1.4401.
Sintered binderless tungsten carbide has the highest
resistance to corrosion, although it is not resistant in
substances such as hypochlorite.
The material combination has the following features:
Extremely wear resistant.
Very robust, resists rough handling.
Poor dry-running properties. In case of dry running,
the temperature increases to several hundred de-
grees Celsius in very few minutes with a conse-
quent damage to O-rings.
If certain pressure and temperature limits are
exceeded, the seal may generate noise. The noise is an
indication of poor seal operating conditions, causing
wear of the seal in the long term. The limits of use
depend on seal diameter and design.
The pressure/temperature diagrams of the various
seals show areas where noise may occur.
To a tungsten carbide/tungsten carbide seal face com-
bination, the running-in wear period with noise may last
3 - 4 weeks, although, typically, no noise occurs during
the first 3 - 4 days.
Silicon carbide/silicon carbide
Being an alternative to tungsten carbide/tungsten car-
bide, silicon carbide/silicon carbide (SiC/SiC) is used
where higher corrosion resistance is required.
For differentiation purposes, the various variants are
designated as indicated below.
Q
1
S
, dense, sintered, fine-grained SiC
A direct-sintered, fine-grained SiC with a small amount
of tiny pores (below 3%) and a density above 3.11 g/
cm
3
.
For a number of years, this SiC variant has been used
as a standard shaft seal material. This material stands
slightly lower pressure and temperature than tungsten
carbide/tungsten carbide.
Q
1
P
, porous, sintered, fine-grained SiC
A variant of the dense, sintered SiC. This SiC variant
has large, circular, closed pores. The degree of porosity
is 5 - 15% and the size of the pores 10 - 50 µm.
The density is around 2.7 - 3 g/cm
3
. This material
stands higher pressure and temperature than tungsten
carbide/tungsten carbide.
Consequently, in warm water, the Q
1
P
/Q
1
P
face mate-
rial combination generates less noise than the tungsten
carbide/tungsten carbide combination. However, noise
from porous SiC seals must be expected during the run-
ning-in wear period of 3 - 4 days.
TM02 4278 0402