Thermometer protection tube
Thermometer protection tubes or immersion sleeves are important components of temperature measuring points. They seal off a process from the environment and keep aggressive media as well as high pressures and flow velocities away from the actual temperature sensor. Only then do they enable temperature measurement in highly stressed processes or in aggressive or toxic media.
Since the number of possible uses for protective tubes is almost unlimited, there are different ways in which protective tubes can be constructed or which materials they are made of. The way in which thermowells are connected to the process is a constructive differentiating criterion. They can either be screwed in, welded in or mounted using a flange . Furthermore, a distinction is made between multi-part and one-part protective tubes.
Multi-part protective tubes are built up from a tube that is closed at the top by means of a welded-on bottom piece. A screw-in spigot, weld-in piece or flange is used as a connection to the process. One-piece protective tubes are made from a solid round or hexagonal bar material. After the inner bore has been drilled using the deep hole drilling technique as the first step, the outer contour of the protective tube and the connection thread for the thermometer are produced. In the case of flange protection tubes, this is now connected to the protection tube. Each of these variants has its own advantages and disadvantages in terms of structural design or attachment with regard to load limits or special materials that can be used.
Thermowell calculations are mostly carried out in accordance with ASME PTC 19.3 TW-2016. For this purpose, the flow velocity in m / s, the density in kg / m³, the process temperature in ° C and the process pressure in bar are required. The necessary protective tube dimensions are installation length, root / tip diameter, bore diameter and the material.
The static results of the WFC (wake frequency calculation) according to ASME PTC 19.3 TW-2016 are maximum pressure and installation length. The natural frequency fn and the excitation frequency fw are calculated as dynamic results. The frequency ratio fw / fn should not exceed 0.8 (20% confidence in the case of resonance fw / fn = 1)
- DIN 43772: Metal protective tubes and neck tubes for machine glass thermometers, dial thermometers, thermocouples and resistance thermometers - dimensions, materials, testing
- DIN 43763: Measurement, control, regulation; Electrical temperature sensors; Metal protective tubes for thermometers with measuring insert (replaced by DIN 43772)
- DIN 16179: Screw-in sockets and protective tubes for machine glass thermometers (replaced by DIN 43772)
- ASME PTC 19.3 TW-2016: Thermowells - Performance Test Codes