For the safe operation of temperature measurements with thermowells, the following must be observed:
Thermowells are placed in a state of spatial stress by static pressure. Bending and shear stresses are added when flow is applied.
Behind flow-loaded thermowells, vortices are periodically detached. However, this effect, which is required for vortex flow meters, also leads to undesired oscillations of the thermowell and thus to its destruction if the frequency of the vortices does not deviate significantly from the resonant frequency of the thermowell.
For strength calculations, all stresses are combined into one equivalent stress, which should have a safety factor > 1.7 with respect to the hot yield strength or the creep strength of the material.
The risk of a thermowell failure can be significantly reduced by calculating the stresses prior to design.
