How do you protect your thermocouples in high temperature processes?

The below describes the way in which contact temperature thermocouples can be protected from the arduous process environments present in modern high temperature industrial processes

Contact thermometry is still the most successful means of measuring temperatures to enable control of industrial processes. Its success is, however, largely dependent on how well the sensing element is protected from the process environment.

SELECTION OF THERMOCOUPLE TYPE

For processes operating at temperatures above 1000oC it has been common practice for platinum-based thermocouples (Types R, S & B) to be used. With the improvement in performance of mineral insulated thermocouples, more specifically the NicrobellÒ sheathed type N mineral insulated thermocouples, industry has been given in many instances an alternate to the platinum-based thermocouples up to temperatures of 1200oC to 1250oC.

OPERATING ABOVE 1250oC

Once above this threshold temperature of 1250oC the only choice in contact thermometers for industry is to use platinum-based thermocouples. Typically, these are installed in ceramic sheaths made from recrystallised alumina (Alsint, Rubalit, AL23 or KER 710). A common practice in many industries, where temperatures are not excessively high, is to use aluminous porcelain (Pythagoras, Dimulit or KER 610) because the cost of the sheaths and insulators is substantially less than that of recrystallised alumina. This practice is certainly not recommended as aluminous porcelain contains free silica which will poison the platinum thermocouple. Aluminous porcelain is also high in free Na2O3 (soda), which in combination with the slightest trace of reducing agent will cause the platinum and aluminous porcelain to interact. It should also be noted that recrystallised alumina has a much greater thermal conductivity than aluminous porcelain allowing the thermocouple to respond quicker to changing temperatures.

At Pyrosales, we advocate that, for standard ceramic sheathed thermocouples, only recrystallised alumina should be used to ensure that maximum service life and performance is maintained.

In many applications ceramic thermocouple protection sheaths are not able to withstand the process environment. Recrystallised alumina, by nature of its construction is very prone to thermal shock and must be preheated before installing in an operating process.

HEXOLOY Ò SA

It is well known that Silicon Carbide affords excellent resistance to erosive and chemical attack in reducing atmospheres. Its use as a heating element also confirms its suitability for use in high temperature applications. In its standard form, though, because it contains free silica, it is readily attacked in strong oxidising environments.

With the development of Hexoloy SA sintered silicon carbide, a silicon carbide material with no free silica now offers superior chemical resistance in both reducing and oxidising environments.

The properties of Hexoloy SA make it an ideal candidate for thermocouple protection sheaths: –

1. a) High temperature performance to 1650oC.
2. b) High thermal conductivity many times greater than recrystallised alumina.
3. c) Thermal shock resistance 10 times greater than recrystallised alumina.
4. d) Impermeable to gases at 31MPa.
5. e) Abrasion resistance: – 50% harder than tungsten carbide.
6. f) High strength and modulus of elasticity.
7. g) Excellent creep resistance.

Areas where Hexoloy SA has been successfully used include: – mineral processing, non-ferrous melting, and refining, high temperature process furnaces, chemical incinerators, recuperators, and corrosive chemical processing streams. Its high temperature capabilities have made it extremely effective in the coal processing industry, including gasification at temperatures above 1600oC.

METAL CERAMIC (LT-1)

Another protection sheath, which is available, is metal ceramic. It is a composite material made up of chromium and aluminium oxide. This combination of metal and ceramic exhibits excellent resistance to oxidation above 1200oC and makes it resistant to wetting by many metals and alloys.

It has high thermal conductivity and is superior to ceramics in thermal shock capabilities but should still be preheated before immersion into molten metals.

It is recommended for use in the following process environments: –

1. a) Molten copper, brass, zinc, and lead.
2. b) Excellent in atmospheres containing SO2, SO3.
3. c) Calcining kilns.
4. d) Blast furnace stove domes and bustle pipes.
5. e) Gas and Ethylene cracking atmospheres.
6. f) Oil fired furnace chambers.

As the chromium phase becomes reactive at elevated temperatures metal ceramics are not recommended for use in carburising or nitriding atmospheres, or in molten aluminium. It has a maximum continuous temperature rating of 1400oC.

CONCLUSION

While there may not be an ideal protection sheath for every process environment, Pyrosales will select, after analysis of the process conditions, the most suitable thermocouple/sheath combination to best suit the needs of the customer. By this the customer can be assured that the most cost-effective solution to their temperature sensing problems has been applied.

Pyrosales engineers are available to consult on various stages of any project, from the development and planning of a new project to modifying or upgrading an existing facility. Our staff is here to provide you with assistance and recommendations to ensure the best outcome and value. We are happy to visit our clients on-site or conduct meetings from one of our offices.