Temperature Sensing Devices
SECTION - 7
Protection Tube Selection
To insure longer life with unimpaired accuracy, thermocouples must be adequately protected. Corrosion, contamination, and abrasion all shorten the life of bare thermocouples. Protecting tubes are capable of withstanding these actions.
For continuous duty the temperatures shown are the maximum recommended. For intermittent duty tubes may be worked at slightly higher temperatures than shown, but total length of life may be less.
Thermocouple protecting tubes are available in a wide range of types and sizes. The tubes listed here are
those which have been in greater demand and many of these are stocked for last delivery. Other tubes, for special purposes, are available.
Note: The following comments, temperature ratings, and application information are offered as a general guide only. This information is not to imply a guarantee of adequate or successful use of any of the listed materials in any specific application. Numerous variables such as impurities, concentrations, vibration, shock, temperature cycling, abrasion, etc., greatly influence the satisfactory life experienced from a protection tube material.
The length of service life considered satisfactory in one application may be unsatisfactory in another. Testing of selected materials should be performed In the application for final determination of a suitable material.
Tube Specifications and Applications
|Typical Applications||Tube Description|| Composition
|304 Stainless Steel||
0.08 Carbon Max
2% Manganese max.
1% Silicon Max., traces of Phosphorous & Sulfur, balance Iron
1260oC Non Oxidizing
|Good resistance to corrosion. For wet process applications such as steam, oil and many chemical solutions.|
|316 Stainless Steel||
2% Manganese max.,
0.08% Carbon max.,
1% Silicon, max., traces of Phosphorous & Sulfur, balance Iron
|871oC Oxidizing||Good resistance to corrosion. Resists pitting corrosion. More resistant to acids than 304 SS.|
High Temperature Hardening
Non-ferrous Metals such as Tin, Lead, or Babbit Metal Smelting
Low Temperature Blast Furnaces
|446 Stainless Steel||
0.25% Nitrogen max.,
0.20% Carbon max.,
1.50% Manganese max.,
1.00% Silicon max., traces of Phosphorous & Sulfur, balance Iron
1260o Non Oxidizing
|Good resistance to corrosion at high temperatures. Impervious to sulphurous atmospheres, salt bath or low temperature molten metals.|
High Temperature Heat Treating
Blast Furnace Operations
|1260oC Oxidizing||Excellent resistance to corrosion and oxidation at high temperature. Good mechanical strength. More resistant to sulphur than Inconel 600. Hydrogen causes embrittlement.|
Potassium Cyanide Salt Baths 1094oC
Caustics and Brines
Drawn or Drilled
|1204oC Oxidizing 538cC Reducing 1316oC Neutral||For High temperature applications. will withstand many chemical actions, but must not be placed in the presense of sulphur. Frequently placed in caustic and molten salt baths. Drilled tube recommended for hydrogen atmospheres.|
|Neutral Salt Baths||Chrome Plated Steel||Chrome Plating over Steel||848oC||Can be used in Cyanide Salt Baths and in presence of Sulphur.|
Molten Non-ferrous Metals
(Babbit, magnesium, tin, etc.)
SAE 1020 or
0.035 Phosphorous Max.
0.045 Sulfur, Max.,
1093oC Non Oxidizing
|Economical for low temperature applications. Scales easily at higher temperature. For non corrosive atmospheres.|
Power Plant Pre heaters
Food Baking Ovens
|Low Alloy Steel||
traces of Phosphorous & Sulfur, balance Iron
For non-corrosive atmospheres and in low temperature molten metals
This is a replacement for wrought iron.
|Similar applications as Inconel, but less well suited to nitriding and molten caustics||Incoloy 800||
982oC Oxidizing 649oC Reducing.
|Oxidation resistance similar to Inconel, but has superior resistance to sulphur attach. Incoloy is a trade name of International Nickel Company.|
Die Cast Metals
1093oC Non Oxidizing
|To 871oC in reducing atmospheres. Will withstand sulphuric acid and caustic solutions. For extra long life, process coated tubes are available. Cast Iron tubes should be painted daily with shiting when measuring aluminum or die cast metal temperatures .01 tubes have a special coating to extend tube life in some operating conditions.|
High temperature heat treating:
Molten copper base alloys to 1150oC, Blast furnace and stack gases to 1316oC,
Sulphur burners to 1093oC
Cement kilns to 1204oC
Chemical process reactors to 1371oC
(Slip cast composite of Chromium and Aluminum Oxide)
23% Aluminum Oxide
Superior oxidation resistance to 1371oC. Thermal conductivity
equal to that of stainless steel. Good resistance to most molten
metals to 1149oC. Not suitable in molten aluminum.
With nobel metal element, a ceramic primary tube is required.
|1510oC||Impervious to gases at high temperature. Possesses good thermal shock but poor mechanical shock. Often necessary to provide secondary tube protection. Should be mounted vertically. Usable in Barium Chloride salt baths to 1288oC|
meting up to 1760oC
Applications for metal and ceramic
industry requiring extreme temperatures.
|1760oC||Fair resistance to thermal and mechanical shock. For very high temperature processes. impervious to gases up to 1760oC|
and Ceramic Kilns
Steel Soaking Pits
Applications requiring resistance
to cutting action of flames and gases
|Silicon Carbide|| 90%
9% Silicon Dioxide,
Oxide & Ferric Oxide
|1649oC||For molten non-ferrous metals. Also is a secondary protecting tube for resistance to thermal shock.|
Reinforced with Steel Pipe
Outer Tube Low
|843oC||High Heat Transfer. Superior resistance to wetting by molten aluminum.|
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