ASME PTC 19.3 TW-2016 - Thermowells

This Standard applies to thermowells machined from bar stock and includes those welded to or threaded into a flange as well as those welded into a process vessel or pipe with or without a weld adaptor. Thermowells manufactured from pipe are outside the scope of this Standard. 

Thermowells with specially designed surface structures (e.g., a knurled surface or a surface with spiral ridges) are beyond the scope of this Standard, due to the difficulty of providing design rules with broad applicability for these types of thermowells. 

Thermowell attachment methods, standard dimensions, parasitic vibration of a sensor mounted inside the thermowell, and thermal equilibrium of the sensor relative to the process stream are beyond the scope of this Standard. In addition, thermowells fabricated by welding, including flame spray or weld overlays, at any place along the length of the shank or at the tip are outside the scope of this Standard. 

The application of the overlay to a bar-stock thermowell may affect any number of critical attributes such as natural frequency, damping, material properties, or surface finish. These changes are difficult to account for in the calculations, therefore, there is risk that an inappropriately designed thermowell could be installed. 
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