In most processes, temperature is a substantial parameter for both process control and product quality. In addition, monitoring of the temperature is critical for plant safety as well as the plant components lifespan and ultimately plant availability.
The application field for high-temperature sensors is mainly suited to energy, primaries, metal and glass processing industries. The burning, heating or melting processes usually run at temperatures of up to 1700°C and the sensors generally operate at pressures and in atmospheres that can be highly aggressive and corrosive. The temperature sensors used generally have a short lifespan of a few days or weeks. The lifespan of the sensor and the quality of the measurement are affected both by the thermocouple as well as the thermowell.
Here, not only the temperature but also the medium being measured is important. Only the correct selection of the thermowell material and dimensions will lead to an optimal temperature measurement and thus ideal productivity and effectiveness of the plant.
Thermocouples
Generally thermocouples are used when measuring temperatures higher than 600°C. This is based on the effect discovered by Seebeck in 1821: if two metallic wires of different materials are bonded together when the junction is heated an emf is generated. The emf then can be interpreted into temperature by the back end instrumentation (DCS/Scada).
Different thermocouples and the accompanying thermo-electric forces are standardised according to IEC 60584.Thermocouples can be either MI (mineral insulated) or wire and bead in construction.
The thermocouple types available in the TAF sensors are J, K, N, R, S and B, each one has a different material composition in order to suit the specific application range. The type N is new and has an advantage over type K, as it offers a higher thermo-electrical stability and is less inclined to oxidise.
Thermowells
In addition to the form and dimensions the choice of the thermowell material is an important factor in the configuration of the measurement point. The material must withstand the process media and show sufficient strength at high temperatures to protect the thermocouple from mechanical stress and chemical erosion.
Metallic thermowells are generally impervious and therefore can protect the thermocouple from pollution and resulting drift. In addition, they have a high mechanical strength which can withstand mechanical shock.
In the TAF family, in addition to the proven austenitic and ferritic steels, nickel based alloys and new high-resistance materials are also available. Incolloy 800 HT, Kanthal AF or nickel cobalt based alloy thermocouple reached considerably higher operating times than conventional materials in special application field tests.
Ceramics are generally used where the temperature exceeds the specification for metal thermowells. However, process conditions must be taken into account due to ceramics being susceptible to mechanical shock. Their main application area is when the temperature range is between 1100 and 1800°C. Ceramic materials are brittle and sensitive to both impacts and temperature shocks. The use of an outer tube which is less sensitive to temperature shocks and mechanical stress can be recommended in combination with a gastight inner tube.
The outer thermowell protects the internal tube from temperature shock by a layer of air between the two tubes. This extends the lifecycle of the thermocouple, however response time is decreased.
In addition to the well-known ceramics C530, C610 and C799 thermowells made from sintered silicon carbide, Kanthal super and silicon nitride will also be available in the near future. These materials offer a higher resistance against thermal shocks and are very suitable for abrasive conditions.
With this complete offering the TAF line is able to operate in a cross section of different high temperature applications. The wide choice of resistive thermowell materials for different applications generates an increased lifespan of the sensors. The benefits are numerous and include savings costs due to higher plant availability, increased plant safety and improved product quality.
For more information contact Trevor Fletcher, Endress+Hauser, +27 (0)11 262 8000, [email protected], www.za.endress.com
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