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Topics Covered
Background
The Design Criteria
MIMS Sheath Materials
Type K Thermo-Elements
The Integral Designed Thermocouple System
Thermoelectric Stability
Nicrosil as a Thermocouple Protection Sheath Material
Nicrobell as a Thermocouple Protection Sheath Material
The Effect of Magnesium Addition to Nicrobell B
The Effect of Niobium Addition to Nicrobell B
The Effect of Chemical Contamination on the Performance of Nicrobell B
Nicrobell B Clad Type N Thermocouples
Nicrobell C - An addition to the Nicrobell Family
Development of Nicrobell C Sheathing Alloys
Advantages of Nicrobell C Materials
Implications of Using Nicrobell Sheaths with K-Type Thermo-Elements
Nicrobell Clad Type K Thermocouples and Conventional Sheathed Type K Thermocouples
Summary of Design Factors for Nicrobell Thermocouple Sheathing Materials
Conclusions
Background
Mineral-insulated metal sheath (MIMS) thermocouples were introduced more than 30 years ago, and represented a major advancement in thermocouple technology which has been the most common method of measuring temperature for over 75 years. The MIMS thermocouple offered a compact design where the thermo-element was housed in a protected environment. However, with the every increasing demand for higher levels of precision and accuracy as well as high temperature performance, major deficiencies have been discovered in the conventional designs of MIMS thermocouples using AISI 300 series and Inconel 600.
The design of a thermocouple system, where both the sheath material and the element are designed to be mutually compatible, as developed by Nicrobell® Pty Ltd in conjunction with Incotherm Limited, has provided industry with a much needed breath of fresh air which virtually eliminates the problems with traditional MIMS thermocouples.
The Design Criteria
Traditional MIMS thermocouples have been designed where the sheath and element are treated as separate entities. These criteria firstly took into consideration temperature range and then a sheath material was chosen to satisfy the process environment. This design had no consideration for the effects that the sheath has on the thermo-element and in the attempts to use MIMS thermocouples at temperatures above 1000°C (typically type K) evidence of thermal drift and premature element failure are prevalent. Research has shown that four factors are evident in realising the full potential of the MIMS design.
MIMS Sheath Materials
The Common sheath materials, Inconel and AISI 310 will not withstand exposure in air or protective environments much above 1050°C. There is a need to be able to operate up to 1300°C.
Type K Thermo-Elements
The type K thermo-element exhibits substantial thermoelectric instability on long term exposure to temperatures above 1050°C.
The thermo-element conductor wires can be contaminated by chemical elements, which thermally diffuse through the MgO, causing substantial drift in emf.
By using dissimilar sheath materials with differential thermal coefficients of expansion as compared to the element wires, mechanical stresses imposed by thermal cycling can cause failure of the element (particularly the negative leg)
The Integral Designed Thermocouple System
Using the design criteria established and the four factors highlighted, a novel MIMS thermocouple with optimum thermoelectric and environmental stability must be based on the fact that it is a truly integral system.
It must also take into consideration the problems evident in the conventional 300 series and Inconel 600 thermocouples (typically type K thermocouples) presently being used.
Thermoelectric Stability
If we firstly look at thermoelectric stability, it has been shown that the type N bare wire thermocouple is substantially superior to that of E, J, K, T up to about 1250°C. Thus the use of type N thermo-element wires in a MIMS format has the potential of providing a solution to the problems of stability, however any design must adopt a sheath alloy which is compatible with the type N wires.
Nicrosil as a Thermocouple Protection Sheath Material
In initial tests Nicrosil was used because of its superior oxidation resistance and being the same material as the positive leg of the thermocouple solved the problem of dissimilar metals and differential thermal co-efficients of the element wires and sheath. However the thermomechanical, aqueous corrosion resistance and carburisation resistance of Nicrosil are substantially inferior to conventional sheath materials.
Nicrobell as a Thermocouple Protection Sheath Material
Through a number of stages of development the Nicrobell® B sheath was developed. Nicrobell® B is essentially Nicrosil with a small percentage of Magnesium and Niobium added.
The Effect of Magnesium Addition to Nicrobell B
The addition of magnesium has the effect of increasing the oxidation resistance at elevated temperatures by combining with the Silica contained in Nicrobell® B to form a tenacious passive oxide layer of Magnesium oxysilicate which locks into the grain boundaries of the metal.
The Effect of Niobium Addition to Nicrobell B
The addition of niobium has the effect of improving the thermomechanical properties of Nicrobell® B and tests show that the ultimate tensile strength, rupture stress and ductility as a function of temperature to 1250°C are superior to Inconel and 310SS.
The Effect of Chemical Contamination on the Performance of Nicrobell B
It can be seen that three of the factors have now been provided with solutions in the use of Nicrobell® B. The fourth factor being chemical contamination causing drift in the emf over time has been shown by Bentley and Morgan to be mainly attributable to the migration of Aluminium and Manganese by vapour diffusion through the magnesium oxide insulation. Nicrobell® B has no Aluminium or Manganese thus this source of contamination is eliminated.
Nicrobell B Clad Type N Thermocouples
Also by using the type N thermo-elements the short circuit migration of Aluminium and Manganese typical of type K thermocouple is eliminated by virtue of the fact that these elements are not present in the type N conductors. Through the development of Nicrobell® B sheathed type N MIMS thermocouples, industry now has available a base metal thermocouple with excellent oxidation resistance, high temperature strength and high thermoelectric stability to temperatures in the range of 1250°C. Its performance stands up well against that of the Platinum based thermocouples without the inherent cost associated with rare metal thermocouples.
Nicrobell C - An addition to the Nicrobell Family
Although the Nicrobell® B sheathed thermocouple exhibits many favourable attributes for high temperature measurement in oxidising environments it is not ideal in carburising, nitrating or sulphidising atmospheres present in processes using fossil fuels for their heat source.
Development of Nicrobell C Sheathing Alloys
The Nicrobell® C sheathing alloy was thus developed by modifying the Nicrobell® B sheath by the addition of chromium. Experience has shown that nickel chrome alloys with an excess of 20% chrome possess an enhanced resistance to Chromium Carbide formation.
This has bee achieved while retaining most of the advantages of the Nicrobell® B alloy apart from a slightly low high temperature strength.
Advantages of Nicrobell C Materials
Thus we have the emergence of a high temperature sheath alloy which is resistant to carburising atmospheres and also exhibits better resistance to sulphidising atmospheres.
Implications of Using Nicrobell Sheaths with K-Type Thermo-Elements
It suffices to say that all of the tests to date indicate that the N-clad-N approach provides the highest stability and performance for a MIMS thermocouple construction.
Nicrobell Clad Type K Thermocouples and Conventional Sheathed Type K Thermocouples
There are however, many instances where it is not practicable to change existing installations to type N thermocouples due to the cost of re-instrumenting or cabling the plant. In these cases it is certainly worth considering the improvements available by changing to Nicrobell® clad K thermocouples. Although not ideal it supplies a partial solution to the problems associated with conventional sheathed type K thermocouples.
Summary of Design Factors for Nicrobell Thermocouple Sheathing Materials
Analysing the factors represented in the design criteria:
- · Use of the Nicrobell® sheath provides extended high temperature resistance to oxidation.
- · The Nicrobell® sheaths do not possess Aluminum or Manganese thus this source of element contamination and effect on emf is eliminated.
- · The Nicrobell® sheath has a thermal expansion co-efficient very close to that of the type K elements, thus improving the thermocouple's resistance to mechanical failure during thermal cycling.
- · It should be noted that the type K thermo-elements still contain quantities of aluminium and manganese, which at high temperatures still have the effect of causing thermal emf instabilities.
Conclusions
A full range of Nicrobell® alloy sheathed type N and K thermocouples have been introduced to the temperature measuring industry for over two years. They have been tried and tested in many environments where they have exhibited either far superior performance, stability or resistance to thermal emf drift than conventional sheathed thermocouples, or nearly equivalent performance to the rare metal thermocouples traditionally used for temperatures over 1000-1100°C.
In the case of platinum base thermocouples systems the cost savings available in using N-Clad-N up to 1250°C are substantial due to the high cost of platinum based assemblies.
The economic advantages evident in having improved long term stability and accuracy whether it be related to plant life of product quality necessitate looking closely at the advantages available in the Nicrobell® approach to MIMS thermocouples.