Protecting
tubes, as their name implies, are used to protect sensors,
usually thermocouples, from contaminating atmospheres and/or
mechanical damage. Closed on one end and open on the termination
end they usually incorporate some means by which the tube,
sensor, and terminal are assembled and mounted into the process.
Ceramic
Protecting Tubes
Dense,
fine grained, nonporous compositions that remain gas tight
even at temperatures near their melting point. Ceramic tubes
are generally used at high temperatures with platinum type
thermocouples although use with base metal thermocouples is
prevalent in atmospheres harmful to metal tubes but not subjected
to mechanical damage. Ceramic tubes will sag at temperatures
below their maximum working temperatures, so if they are installed
horizontally and used above their sagging temperatures they
should be fully supported. Sag temperature is temperature
at which the tube will sag 1/4" in one hour. Ceramics will
retain moisture at room temperature. This moisture may become
trapped in the ceramic and cause the destruction of the tube
when the tube is thermally shocked. It is recommended that
preheating or slow heating of ceramic tubes to 400 to 800
¡ÆF be done in order to drive off this moisture before high
heat is introduced.
- Alumina
(Al
2 O3) tubes in their
purest forms have very good thermal shock and strength
characteristics and are virtually chemical resistant.
For long term use Alumina 998 is very compatible for use
with platinum type thermocouples. Its use is evaluated
by examination of these features versus its relatively
high cost.
- Mullite
(3Al2O3 · 2SiO2)
has good thermal shock and strength characteristics and
is chemically resistant. This low cost ceramic is also
used with platinum type thermocouples usually for shorter
term applications in which mechanical damage is more likely
to be encountered rather than the long term detrimental
effects of the silica in the mullite on the platinum type
thermocouple.
TYPICAL
CHEMICAL ANALYSIS |
TYPE |
Al2O3 |
SiO2 |
MgO |
Na2O |
CaO |
Fe2O3 |
Cr2O3 |
TiO2 |
B2O3 |
K2O |
998 |
99.8 |
0.060 |
0.035 |
0.008 |
0.040 |
0.025 |
<0.003 |
0.004 |
<0.001 |
<0.001 |
997 |
99.7 |
0.1 |
0.05 |
0.06 |
0.04 |
0.05 |
- |
- |
- |
- |
Mullite
60 |
60.0 |
38.0 |
0.2 |
0.2 |
0.1 |
0.5 |
- |
0.5 |
- |
0.7 |
TYPICAL
PHYSICAL PROPERTIES |
Material |
998 |
997 |
Mullite
60 |
Constitution |
99.8%
Al2O3 |
99.7%
Al2O3 |
85%
Mullite / 15% SiO2
|
Bulk
Specific Gravity |
3.85 |
3.65 |
2.8 |
Impenetrability |
gas
tight |
gas
tight |
gas
tight |
Maximum
Working Temperature |
1950¡ÆC
(3542¡ÆF) |
1800¡ÆC
(3270¡ÆF) |
1600¡ÆC
(2912¡ÆF) |
Sagging
Temperature (Unsupported) |
1600¡ÆC
(2912¡ÆF) |
1500¡ÆC
(2730¡ÆF) |
1400¡ÆC
(2552¡ÆF) |
Thermal
Conductivity
@24¡ÆC (75¡ÆF)
@800¡ÆC (1472¡ÆF) |
(BTU/ft2/hr/¡ÆF/in.)
230
60
|
(BTU/ft2/hr/¡ÆF/in.)
125
30
|
(BTU/ft2/hr/¡ÆF/in.)
40
25
|
Dielectric
Strength
@24¡ÆC (75¡ÆF) |
(V/Mil)
230 |
(V/Mil)
250 |
(V/Mil)
250 |
Thermal
Expansion
(24 to 1000¡ÆC) |
(per
¡ÆC x 10-6)
8.5 |
(per
¡ÆC x 10-6)
7.7 |
(per
¡ÆC x 10-6)
5.0 |
Silicon
Carbide Tubes (SiC)
Porous
and highly refractory. They are used to temperatures of 1650¡ÆC
(3000¡ÆF) as secondary protection against extreme temperature,
abrasive atmospheres and direct flame impingement. Silicon
Carbide tubes are moderate in cost. Primary mullite or alumina
tubes are recommended with these tubes.
Single-Phase
Silicon Carbide Tubes (SA SiC)
A
pressureless, sintered form of alpha silicon carbide with
a density greater than 98% theoretical. Having a very fine
grain structure and being 50% harder than tungsten carbide
makes it resistant to erosion. It contains no free silicone,
which makes it highly chemical resistant in both oxidizing
and reducing environments. For use in air to 1650¡ÆC (3000¡ÆF).
SA SiC tubes are high in cost.
Metal
Ceramic Tubes
A
high cost combination of chromium and alumina for use to temperatures
of 1205¡ÆC (2200¡ÆF) that provides excellent oxidation resistance,
thermal conductivity comparable to that of stainless steel,
good resistance to wetting by most molten metals. A primary
alumina tube is recommended when this tube is used in conjunction
with platinum thermocouples.
Refractory
Laminated, Metal Tubes
Offer
the mechanical protection of metal tubes and the corrosion
resistance of ceramics. For molten aluminum and zinc applications,
they resist erosion, will not contaminate metal melts, and
may outlast iron tubes by many times depending on the application.
Metal
Tubes
Offer
good mechanical protection for base metal thermocouples up
to 1150¡ÆC (2100¡ÆF) in oxidizing atmospheres. All metals are
porous after about 870¡ÆC (1600¡ÆF) so it may be necessary to
provide a ceramic primary tube to protect the thermocouple
from detrimental vapors.
- Mild
Steel
Provides good protection at lower temperatures against
oxidizing and reducing atmospheres and non-corrosive liquids
and vapors. Maximum working temperature 700¡ÆC (1300¡ÆF).
- 304
SS (18% Chrome/8% Nickel)
A general purpose material that has good resistance to
corrosion and oxidation.
Maximum working temperature 875¡ÆC (1600¡ÆF).
- 316
SS (16% Chrome/10% Nickel)
A material that has superior corrosion resistance as compared
to 304 SS with improved oxidation resistance and a higher
hot strength. Maximum working temperature 925¡ÆC (1700¡ÆF).
- 446
SS (28% Chrome)
Has excellent resistance to corrosion and oxidation. It
is highly resistant to sulphur atmospheres, salt baths
and molten non-ferrous metals. maximum working temperature
1100¡ÆC (2000¡ÆF).
- Inconel
600TM (75% Nickel/15% Chrome)
Combines good mechanical strength at elevated temperatures
with high resistance to oxidation, corrosion and scaling.
Not suitable for use in sulfurous atmospheres above 875¡ÆC
(1600¡ÆF).
Maximum working temperature 1150¡ÆC (2100¡ÆF).
- Inconel
601TM (60% Nickel/23% Chrome)
Has similar properties of Inconel 600 and offers improved
resistance to sulfur attack at elevated temperatures.
- Cast
Iron
A low cost material used in molten aluminum and aluminum
alloy applications and also has good resistance to acid
and caustic solutions. Maximum working temperatures 875
¡ÆC (1600¡ÆF) reducing; 700¡ÆC (1300¡ÆF) oxidizing.
TM
- International Nickel Co., Inc.
For
more information regarding Protecting Tubes and Thermowells,
Consult your Marlin Catalog, pages D-0 to D-13.
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you do not have a Marlin Catalog - Click
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