This lesson and the calculator are a screening method in the spirit of ASME PTC 19.3 TW. The full standard (a paid document) adds more corrections, cyclic-stress and fatigue checks. Final thermowell designs should be checked by the supplier's PTC 19.3 TW calculation.
Why thermowells fail
Flow passing a cylinder does not slide smoothly round it. Swirls (vortices) break away from alternate sides, one after the other: a von Kármán vortex street, the same effect that makes wires "sing" in the wind. Each vortex gives the thermowell a small push. Pushes across the flow happen at the shedding frequency fs; smaller pushes along the flow happen at 2·fs.
Shedding frequency
Doubling the velocity doubles the shedding frequency. A 22 mm tip in steam at 25 m/s sheds at about 250 Hz.
Natural frequency
The thermowell is a cantilever: fixed at the support, free at the tip, like a diving board. Its natural frequency rises with stiffness and falls with length squared and with mass:
- Length matters most: making the well 30 % shorter raises fn by about 2 times.
- Thicker and tapered wells are stiffer.
- Dense liquids add mass (the fluid moves with the well), lowering fn slightly; a flanged mounting is a little less stiff than an ideal fixed support.
- Hot metal is softer (lower E), so fn drops at high temperature.
The frequency ratio
| fs / fn | Meaning | Action |
|---|---|---|
| < 0.4 | Shedding far below resonance | Low risk (still check steady stress) |
| 0.4 – 0.8 | In-line resonance zone: the 2·fs along-flow pushes can match fn | Full PTC 19.3 TW cyclic-stress check required |
| ≥ 0.8 | Transverse (cross-flow) resonance: the large pushes match fn | Redesign |
Also check the startup and maximum velocities, not only the normal one: a well can pass resonance on the way up.
How to fix a failing well
- Shorten it. The sensor only needs to reach the main flow; often ⅓–½ of the pipe diameter is enough.
- Use a thicker root or a tapered well (stiffer, but slower thermal response).
- Add a support collar (velocity collar) in the nozzle; it must fit closely.
- Move it to a lower-velocity location or a larger pipe section.
- Use a surface or helical-strake ("vortex-suppressing") design where available.
Live calculator
Enter the well dimensions and the process conditions, or pick an example. The gauge shows the frequency ratio in green / amber / red, the drawing vibrates when you enter the resonance zone, and the table gives the maximum safe length and velocity. It also works offline in the Instrument Apps Hub.
Q&A quiz
Related: Temperature · RTD vs Thermocouple · Thermowell app