Why thermowells break
A thermowell is a cantilever beam sitting in a flowing stream. As fluid passes, vortices peel off alternately from each side, pushing the well first one way then the other. That alternating force has a frequency — and if it lands near the well's own natural frequency, the amplitude grows until the metal fatigues at the root and the tip departs downstream, taking the sensor with it and leaving a hole in the process.
This is not theoretical. Thermowell failures have caused major incidents, which is why ASME PTC 19.3 TW exists as a dedicated standard rather than a paragraph in a general code.
The two frequencies
St is the Strouhal number, about 0.22 for a cylinder across a wide Reynolds range; B is the tip diameter; E the elastic modulus; I the second moment of area; ρ the material density; L the unsupported length. The acceptance criterion is fs / fn < 0.8.
Worked example
316 stainless straight-shank well, 150 mm long, 22 mm root, 6.6 mm bore, in 5 m/s flow:
- I = π/64 × (0.022⁴ − 0.0066⁴) = 1.140×10⁻⁸ m⁴
- A = π/4 × (0.022² − 0.0066²) = 3.459×10⁻⁴ m²
- fn = 0.5595 × √(193×10⁹ × 1.140×10⁻⁸ ÷ (2.767 × 0.15⁴)) ≈ 701 Hz
- fs = 0.22 × 5 ÷ 0.022 = 50 Hz
- Ratio = 0.071 — comfortably safe
The length trap
Natural frequency scales with 1/L², and that inverse square relationship is what catches people out. Take the same well and the same fluid, and only change the insertion length:
| Length | fn | At 30 m/s: ratio | Verdict |
|---|---|---|---|
| 150 mm | 701 Hz | 0.43 | pass |
| 250 mm | 253 Hz | 1.19 | fail |
| 400 mm | 99 Hz | 3.04 | fail badly |
Somebody specifying "insert it well into the flow for a good measurement" can turn a safe design into a failure without changing anything else. Immersion depth and mechanical survival pull in opposite directions, and the wake frequency check is where that conflict gets resolved.
Field notes
- Calculate at maximum velocity, including upset and start-up cases — not the normal operating flow. Get velocity from the true pipe bore using the pipe schedule lookup and velocity calculator.
- Steam and gas are the dangerous services. High velocity plus low density is exactly where shedding frequencies climb into resonance territory. Water lines rarely fail this check.
- Tapered and stepped shanks raise natural frequency for the same immersion, which is why they exist. This calculator uses the root diameter for stiffness and the tip diameter for shedding, which is a reasonable approximation for tapered wells.
- Support collars change the sum entirely by shortening the unsupported length — often the cheapest fix on an existing installation.
- Immersion depth still matters for accuracy. Too short and stem conduction pulls the reading toward pipe wall temperature; the usual guidance is at least ten times the tip diameter of immersion. Balance that against this result rather than maximising either alone.
Frequently asked questions
What is thermowell wake frequency?
The frequency at which vortices shed alternately from the sides of a thermowell in flowing fluid. If that shedding frequency approaches the thermowell’s natural frequency, resonance builds and the well can fatigue and break off inside the pipe.
What is the frequency ratio limit in ASME PTC 19.3 TW?
The standard limits the ratio of shedding frequency to natural frequency to 0.8 for the in-line excitation case where a scalar approach is used. Staying below that keeps the well clear of the resonance region across its operating flow range.
Why do long thermowells fail?
Natural frequency falls with the square of unsupported length. Doubling the insertion length quarters the natural frequency, so a well that is comfortably safe at 150 mm can be in resonance at 400 mm in the same service.
How do I fix a thermowell that fails the wake frequency check?
Shorten the insertion length, increase the root diameter, use a tapered or stepped shank, add a support collar, or move to a velocity collar design. Shortening is usually the cheapest and most effective because of the inverse square relationship.
Simplified screening calculation for reference and education. It omits the fluid added-mass factor, pressure and stress checks, and the full tapered-shank treatment of ASME PTC 19.3 TW. Final thermowell design must follow the complete standard. See our disclaimer.