Intro4u2u

Fluid flowing past a thermowell creates a turbulent wake, which causes alternating lateral forces on the well perpendicular to the direction of flow. This wake (or Strouhal) frequency is proportional to the fluid velocity and well dimensions. If the wake frequency coincides with (or comes within 20% of) the natural frequency of the well, the resultant vibration could cause mechanical failure of the well. Thermowells are normally safe if the natural frequency is well below the wake frequency or if the fluid velocity continually fluctuates through the critical velocity point.

If vibratory stress is a potential problem, tapered wells should be used. The tapered shank provides greater stiffness and higher strength-to-weight ratio than the equivalent straight shank well, which gives them a higher natural frequency and permits operation at higher fluid velocities.

The maximum length of a thermowell for given service conditions is dependent upon both vibratory and steady state stress. The ASME calculations (PTC 19.3) are used to determine if the selected well dimensions provide a well strong enough to withstand the stresses imposed by static pressure, steady state flow and vibration. Separate evaluations of each are made to determine which is the limiting factor. The calculations are applicable to tapered wells with a fluid velocity less than 300 fps, and meet the dimensions given in PTC 19.3. For a nominal ¼” sensor, these are:

Bore Diameter: 0.258 ± .004”
Minimum Tip Diameter: 0.625”
Minimum Root Diameter: Tip + .188”

The results of these calculations are very conservative and are intended to be used as a guide in the selection of a thermowell to meet the specific requirements of a specified application.

Thermo-Kinetics assumes no responsibility for failure of a well based on the results of these calculations, and accepts
no liability, direct or consequential, arising from error or misinformation supplied herein, or from program misuse.