The vortex shedding frequency Strouhal Frequency increases linearly with fluid velocity, but the forces increase with the square of the velocity. When the Strouhal Frequency approaches the natural frequency of the thermowell, it can lock-in to the natural frequency causing resonance, with greatly magnified forces.
To prevent lock-in, the natural frequency of the thermowell must be higher than either the in-line or the transverse resonance condition. Operation through the in-line resonance is acceptable only if the cyclic stresses at the resonance condition are acceptably small. The fluid velocity at which resonance occurs is referred to as a velocity critical. There are two velocity criticals for each natural frequency of the thermowell: one describing the transverse and the other describing the in-line response.
Since in-line force fluctuates at twice the frequency of the transverse force, the corresponding velocity critical is approximately one half that requires for transverse resonance. If the natural frequency of the thermowell overlaps either fs or 2fs, a large resonant buildup in vibration amplitude can occur.
The major cause of thermowell failure is fatigue due to resonance. A high enough level of damping may allow the thermowell to operate at the in-line or even the transverse resonance frequencies. In addition to frequency limits, the stresses within the thermowell and forces applied are also critical to evaluating the suitability of a thermowell for a specific process application.
The 4 quantitative criteria to be evaluated are:. Resonant frequency of the thermowell must be sufficiently high so that destructive oscillations are not excited by the fluid flow. The steady-state s-s fluid velocity should meet one of the following conditions:. Steady-state stresses are the result of hydrostatic fluid pressure and non-oscillating drag forces on the thermowell, and are calculated at the location of maximum stress.
If the thermowell is partially shielded or has a reduced tip, the calculation must be performed with those considerations. The maximum steady- state stress on the thermowell at design velocity must not exceed the allowable stress as determined by the Von Mises Criteria. Dynamic stresses are a result of the periodic drag forces that cause in-line oscillations and the periodic lift forces that cause transverse oscillations.
If the thermowell is intended to operate above the in-line velocity critical, there are cyclic stresses at the in-line resonance to consider as it passes through that point on the way to the design velocity. The maximum dynamic stress must not exceed the allowable fatigue stress limit.
The magnification factors are calculated and applied to the cyclical stress equations, then the cyclic drag and lift forces are calculated at the design velocity. The maximum combined lift and drag stress must not exceed the fatigue stress limit. Please contact us for assistance. Register Size. Tag analysis Fast, true design with no guesswork. An error in your tag results? No problem. Intuitive Interface Safely breeze through thermowell designs with blazing speed. Pop-up description feature tells you what data to input Thermowell drawings dynamically update on-the-fly Visual indicators show errors with details for making informed decisions Built-in help feature assist with "How-to" questions Temperature Specialists at your service so you can Consider It Solved TM.
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