By J M SHANAHAN B.Sc., B.E. (SYD), CEO, Pyrosales Pty Ltd,Padstow,Australia
This article describes the ways in which contact temperature sensors 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 1000°C, it has been common practice for platinum-based thermocouples (Types R, S, and B) to be used. With the improvement in performance of mineral-insulated thermocouples, more specifically the Nicrobell® sheathed type N mineral-insulated thermocouples, industry has been given an alternative to the platinum-based thermocouples for temperatures of 1200°C to 1250°C.
Operating above 1250°C
Once above this threshold temperature of 1250°C, 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). Where temperatures are not excessively high, a common practice in many industries 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 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.
Pyrosales advocate only recrystallised alumina should be used to ensure that maximum service life and performance is maintained for standard ceramic sheathed thermocouples.
Ceramic thermocouple protection sheaths are not able to withstand the process environment in many applications. Recrystallised alumina, by nature of its construction, is highly prone to thermal shock, and must be preheated before installing in an operating process.
Silicon carbide affords excellent resistance to erosive and chemical attack in reducing atmospheres. The use of silicon carbide as a heating element also confirms its suitability for use in high-temperature applications in its standard form. 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:
- High temperature performance to 1650°C.
- High thermal conductivity; many times greater than recrystallised alumina.
- Thermal shock resistance ten times greater than recrystallised alumina.
- Impermeable to gases at 31MPa.
- Abrasion resistance: 50% harder than tungsten carbide.
- High strength and modulus of elasticity.
- 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 1600°C.
METAL CERAMIC (LT-1 or LT1)
Another protection sheath is metal ceramic, a composite material made up of chromium and aluminium oxide. This combination of metal and ceramic exhibits excellent resistance to oxidation above 1200°C, and also makes it resistant to wetting by many metals and alloys.
The high thermal conductivity makes it 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:
- Molten copper, brass, zinc and lead.
- Excellent in atmospheres containing SO2, SO3.
- Calcining kilns.
- Blast furnace stove domes and bustle pipes.
- Gas and Ethylene cracking atmospheres.
- Oil fired furnace chambers.