The thermocouple or resistance thermometer is protected against mechanical and chemical effects by the protective fitting. With regard to the temperature resistance, the requirements fulfilled by resistance type thermometers are not as high as those for thermocouples due to the naturally lower maximum temperature. Moreover in both cases it is necessary to fulfil mechanical requirements which determine the wall thickness as well as the shape and therefore the associated resistance to pressure and bending.
Design of thermowells
Thermowells made of tubular or solid material must satisfy the following functions:
- Positioning of the temperature-sensitive probe tip during the process
- Protection of the thermometer against mechanical and chemical influences
- Termination of the process chamber to the surroundings
Failure of these components can result in downtimes, release of combustible, explosive or toxic substances, property damage and personal injury. For this reason, careful risk and load analysis is required. Protective fittings are available in proven and standardized designs with various process fittings. The two basic versions can be characterized as follows:Schutzarmatur zum Anflanschen
Protective fittings of tubular material consist of welded piping and connection parts, which can be produced economically and in long lengths. However the mechanical stability is limited. Structural changes and heat influence zones on the welds can have a negative effect on corrosion resistance.
Protective fittings of solid material are produced using solid rods with drilled centre. The conical shape ensures highly stable protective fittings with optimal material utilization. No welds are present in areas subject to high thermal loads.
In addition to version and dimensions, the material used is of decisive importance. The material must be resistant to temperature and corrosion in the operating medium. It is necessary to reliably eliminate the possibility of premature failure resulting from certain types of corrosion - intercrystalline corrosion, stress corrosion or material embrittlement - reduction of elongation at break and impact resistance. Appropriate wall thickness allowances are necessary to compensate for wall thickness erosion resulting from surface corrosion or abrasion. When selecting the right material, we recommend using at least the same material quality as used for the container / piping. For cost or production reasons, the protective tube can be used with a correspondingly resistant jacket, for example ECTFE or TANTAL, as well as an abrasion-resistant coating such as STELLITE.