TP40-240/2 Shaft Seals Manual

17
Seal face materials
Shaft seals
Q
1
G
, self-lubricating, sintered SiC
Several variants of SiC materials containing dry lubri-
cants are available on the market. In this document, the
designation Q
1
G
applies to a SiC material which is suit-
able for use in distilled or demineralized water, as
opposed to the above materials.
Q
1
G
/Q
1
G
stands the same pressure and temperature as
Q
1
P
/Q
1
P
.
The dry lubricants, i.e. graphite, reduce the friction in
case of dry running, which is of decisive importance to
the durability of a seal during dry running.
SiC/SiC features
The SiC/SiC material combination has the following
features:
• Very brittle material requiring careful handling.
• Extremely wear-resistant.
• Extremely good corrosion resistance. SiC (Q
1
S
, Q
1
P
and Q
1
G
) hardly corrodes, irrespective of the
pumped liquid type. However, an exception is water
with very poor conductivity, such as demineralized
water, which attacks the SiC variants Q
1
S
and Q
1
P
whereas Q
1
G
is corrosion-resistant also in this liq-
uid.
• In general, the material combinations have poor dry-
running properties (like tungsten carbide/tungsten
carbide), however, the Q
1
G
/Q
1
G
material withstands
a limited period of dry running because of the
graphite content of the material.
Carbon/tungsten carbide or carbon/silicon
carbide
Seals with one carbon seal face have the following fea-
tures:
• Brittle material requiring careful handling.
• Worn by liquids containing solid particles.
• Good corrosion resistance.
• Good dry-running properties (temporary dry run-
ning).
• The self-lubricating properties of carbon make the
seal suitable for use even with poor lubricating con-
ditions (high temperatures) without generating
noise. However, such conditions will cause wear of
the carbon seal face leading to reduced seal life.
The wear depends on the pressure, temperature,
liquid, diameter and seal design.
Low speeds reduce the lubrication between the seal
faces; as a result, increased wear might have been
expected. However, this is normally not the case be-
cause the distance that the seal faces have to move
is reduced.
• Metal-impregnated carbon (A) offers limited corro-
sion resistance, but improved mechanical strength,
heat conductivity and thus reduced wear.
• With reduced mechanical strength, but higher corro-
sion resistance, synthetic resin-impregnated car-
bon (B) covers a wide application field. Synthetic
resin-impregnated carbon is approved for potable
applications.
• The use of carbon/SiC for hot-water applications
may cause heavy wear on the SiC, depending on
the quality of the carbon and water.
This type of wear primarily applies to Q
1
S
/carbon.
The use of Q
1
P
, Q
1
G
or a carbon/tungsten carbide
combination causes far less wear. Thus, carbon/
tungsten carbide, carbon/Q
1
P
or carbon/Q
1
G
are
recommended for hot water systems.
Carbon/ceramic (aluminium oxide)
Good all-round seal for not too demanding applications.
The seal has the following features:
• Brittle material requiring careful handling.
• Worn by liquids containing solid particles.
• Limited corrosion resistance, 5 < pH < 9, depending
on ceramic type.
• Relatively good dry-running properties. However,
thermal cracks may occur in case of a sudden influx
of water to a hot seal after a period of dry running or
similar condition.
• The carbon of the seal offers properties very similar
to the carbon/tungsten carbide seal. However, com-
pared to the carbon/tungsten carbide seal, the pres-
sure and temperature ranges are limited.
Tungsten carbide/hybrid
Combining the positive properties of the tungsten car-
bide/tungsten carbide and carbon/tungsten carbide
seal face combinations, the tungsten carbide/hybrid
combination has the following features:
• Extremely wear-resistant.
• Resistant to rough handling.
• Certain dry-running properties (temporary dry run-
ning).
• Corrosion resistance equal to EN 1.4401, corre-
sponding to the resistance of a CRN pump. In cer-
tain corrosive liquids and solvents, the resistance is
restricted.
• Application limits as to pressure and temperature
are similar to tungsten carbide/tungsten carbide due
to the risk of seizure. However, exceeding the limits
may damage the hybrid.
Some of the additives used in anti-freeze liquids on gly-
col basis may cause precipitation, especially at high
temperatures. In such cases, tungsten carbide/tung-
sten carbide should be used, if possible.