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Thermowells provide maximum protection for thermal sensors
from corrosion, pressure and flow induced stresses. When
selecting thermowells these parameters determine the type
and material that should be used. In general, thermowells
are machined from solid bar stock for "A" dimensions to
24" but for londer lengths a built-up design is used.
General Application Considerations
- Select sensor location for representative temperature
measurement.
- Provide sufficient depth of immersion so that heat transfer
along the instrument does not influence temperature measurement.
- Select materials that are compatible with corrosive
media elements.
- Select thermowell with sufficient stiffness to resist
destruction from flow induced stresses.
For Mechanical Application
Considerations -Click
Here!
Thermowell Materials
Strength at operating temperature and resistance to corrosion
are the primary considerations in material selection. A
corrosion guide
is available for viewing.
Carbon Steels
Can be used to 1300¡ÆF (700¡ÆC) usually in oxidizing atmospheres.
Austenitic Stainless Steels
(300 Series)
Can be used to 1600¡ÆF (870¡ÆC) mostly in oxidizing atmospheres
although type 316 can be used in some reducing environments.
Ferritic Stainless Steels
(400 Series)
Can be used to the 1800¡ÆF (982¡ÆC) - 2100¡ÆF (1149¡ÆC) range
in both oxidizing and reducing atmospheres.
High Nickel Alloys
Can be used to 2100¡ÆF (1149¡ÆC) in oxidizing atmospheres.
Material
Code |
MATERIAL |
Melting
Point (°F)
|
Recommended
Operating
Atmosphere |
Max.
Operating Temperature in
Atmosphere (¡ÆF)
|
304 |
304
SS |
2560 |
ORNV |
1650 |
310 |
310
SS |
2560 |
ORNV |
2100 |
316 |
316
SS |
2500 |
ORNV |
1700 |
321 |
321
SS |
2550 |
ORNV |
1600 |
347 |
347
SS |
2600 |
ORNV |
1600 |
446 |
446
SS |
2700 |
ORNV |
2000 |
CS |
Carbon
Steel |
2500 |
ON |
1300 |
INC |
InconelTM |
2550 |
ONV |
2100 |
INX |
Inconel
XTM |
2620 |
ONV |
1500 |
INY |
IncoloyTM |
2500 |
ON |
1600 |
HTX |
Hastelloy
XTM |
2300 |
O |
2200 |
HTC |
Hastelloy
CTM |
2310 |
O |
1800 |
HTB |
Hastelloy
BTM |
2375 |
OR |
1400 |
MON |
MonelTM |
2460 |
OR |
1000 |
BR |
Brass |
1850 |
O |
650 |
AL |
Aluminum |
1220 |
O |
700 |
NCK |
Nickel |
2647 |
O |
1400 |
TRN |
Tantalum |
5425 |
V |
5000 |
TIT |
Titanium |
3035 |
VN |
2000 |
O=Oxidizing, R=Reducing, N=Neutral, V=Vacuum
Velocity Rating
Once the selection of materials is made attention should
be given to the parameter of flow induced stresses. Fluids
flowing by a well form a turbulent wake called the Von
Karman Trail. This wake has a definite frequency based
on the diameter of the well and the fluid velocity. It
is important to provide a well with sufficient stiffness
so that the wake frequency will never exceed the natural
frequency of the well itself. Should the natural frequency
of the well coincide with the wake frequency, the well
would vibrate to destruction. Tapered shank wells provide
greater stiffness for the same sensitivity than a straight
shank well. The higher strength to weight ratio gives
these wells a higher natural frequency and therefore are
able to operate at higher fluid velocities. Recommended
maximum velocity rating can be found for every standard
well length and material catalogued. Ratings are based
on operating temperatures as shown in the table below.
MATERIAL |
Velocity
Rating
Operating Temperature |
Carbon
Steel (C1018)
304 SS
316 SS |
1000¡ÆF
(538¡ÆC) |
Monel |
900¡ÆF
(482¡ÆC) |
Brass |
350¡ÆF
(177¡ÆC) |
Single values that appear in the velocity tables may
be considered safe for water, steam, air or gas. Double
values distinguish between water (paranthesized) and steam,
air and gases. These values are intended as general guides
to selection. If you have operating conditions requiring
special well designs our engineering
staff is available to assist you.
Pressure Rating
The limit pressure versus temperature ratings are tabulated
for various materials for each thermowell series. Here
is a typical table.
LIMIT
PRESSURE (lbs/in2) vs TEMPERATURE (¡ÆF) |
MATERIAL |
CODE |
TEMPERATURE
(¡ÆF) |
70 |
200 |
400 |
600 |
800 |
1000 |
1200 |
Brass |
BR |
5000 |
4200 |
1000 |
- |
- |
- |
- |
Carbon
Steel |
CS |
5200 |
5000 |
4800 |
4600 |
3500 |
1500 |
- |
A.I.S.I.
304 |
304 |
7000 |
6200 |
5600 |
5400 |
5200 |
4500 |
1650 |
A.I.S.I.
316 |
316 |
7000 |
7000 |
6400 |
6200 |
6100 |
5100 |
2500 |
Monel |
MON |
6500 |
6000 |
5400 |
5300 |
5200 |
1500 |
- |
NOTE: Selection of material and/or equipment
is at the sole risk of the user of this site. The data presented
does not and should not preclude professional engineering
design and consulting for your particular application. Marlin
Manufacturing Corporation, its distributors, representatives,
and the contributors to this site specifically deny any
warranty expressed or implied.
For more information regarding
Protecting Tubes and Thermowells,
Consult your Marlin Catalog, pages D-0
to D-13.
If you do not have a Marlin Catalog - Click
Here!
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