Thermowell
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Thermowells are used to protect temperature sensors used to monitor industrial processes while permitting accurate measurement. They are typically installed in piping systems and subject to both hydrostatic and aerodynamic forces. Vortex shedding is the dominant concern for thermowells in cross-flow applications and is capable of forcing the thermowell into resonance with the possibility of fatigue failure not only of the thermowell but also of the temperature sensor. The conditions for flow-induced resonance generally govern the design of the thermowell apart from its pressure rating and materials of construction. Flow-induced motion of the thermowell occurs both in-line with and transverse to the direction of flow with the fluid forces acting to bend the thermowell. In most applications the transverse component of the fluid forces resulting from vortex shedding tends to govern the onset of flow-induced resonance, with a forcing frequency equal to the vortex shedding rate. In liquids and in high pressure compressible fluids, a smaller but none the less significant component of motion in the flow-direction is also present and occurs at nearly twice the vortex shedding rate. The in-line resonance condition may govern thermowell design at high fluid velocities although its amplitude is a function of the mass-damping parameter or Scruton number describing the thermowell-fluid interaction.
For drilled bar-stock thermowells, the most common form of failure is bending fatigue at its base where the bending stresses are greatest. In extreme flow conditions (high velocity liquids or high velocity, high pressure gases and vapors) catastrophic failure may occur with bending stresses exceeding the ultimate strength of the material. For extremely long thermowells, the static component of the bending stresses may govern design. In less demanding services, fatigue failure is more gradual and often preceded by a series sensor failures. The latter are due to the acceleration of the thermowell tip as it vibrates, this motion causes the element to lift off the bottom of the thermowell and literally, batter itself to pieces. In cases where the acceleration stresses have been measured, sensor accelerations at resonant conditions often exceed 250 Gs and have destroyed the accelerometer.
The natural frequencies of thermowell bending modes are dependent upon the dimensions of the thermowell, the compliance (or flexibility) of its support, and to a lesser extent dependent upon the mass of the sensor and the added mass of the fluid surrounding the thermowell.
The ASME Performance Test Code PTC 19.3TW (2010), defines the criteria for the design and application of common thermowells and includes improved natural frequency estimates, consideration of the bending forces transverse to and in-line with the flow direction, definition of the maximum allowable stresses, fatigue strength, critical dimensions, and corrosion factors.
While the design of so-called pipe type thermowells or protection tubes is not specifically covered by the ASME Code, due to the range design varieties and questions regarding the degree of spanwise coherence of the vortex shedding process for a given installation, the design procedure offered by PTC 19.3 TW remains a useful starting point.
