Understanding thermowell vibrationMarch 26, 2015 REDWIRE is news you can use from leading suppliers. Powered by FRASERS.
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Thermowells are principally used with thermocouples, resistance temperature detectors and bimetal thermometers in applications where it is necessary to measure temperature at high pressure (above 75 psig) or in hostile environments. They are also used for isolation so a sensor can be replaced without having to shut down the process.
Thermo-Kinetics stocks a complete range of standard tapered, straight and reduced-tip thermowells to meet most applications. Flanged, socket weld, van stone, ground joint and weld-in thermowells are also available. With so many options, how do you determine which thermowell is right for your application? It’s all about vibration.
When fluid flows past a thermowell, the change in fluid momentum creates a turbulent wake behind the well. Vortices form in this wake, and shed from alternate sides of the well. The vortex shedding frequency (or wake frequency) is linear with flow velocity and inversely proportional to thermowell tip diameter.
These shedding vortices impose a periodic force comprising two components: a lift force and a smaller drag force. These vortex-induced forces, which cause thermowell vibration, are normally small with the magnitude of the vibrations generally negligible. However, as the wake frequency (fw) approaches the natural frequency (fn) of the thermowell, it can shift and lock-in to the natural frequency. When fw = fn, the thermowell goes into resonance, and vibrating forces increase rapidly. The resultant vibrations can cause mechanical failure of the well.
The Murdock calculations (and companion ASME PTC 19.3) consider only the oscillating lift force as the cause of thermowell vibration. The ratio of wake to natural frequency is restricted to a maximum of 0.8 to eliminate the possibility of resonance.
Although the oscillating drag force is small, it can force the thermowell into resonance at lower velocities because it occurs at twice the wake frequency. For high-density fluids, the Murdock analysis is not adequate. When the oscillating drag component is included, the velocity rating can be reduced by up to 50 per cent.
Thermo-Kinetics provides guidelines on thermowell velocity calculations, which the company says are only to be used as a guide in the selection of the correct thermowell. Other variables, like corrosion, should be evaluated and influence the decision.
To learn more about thermowell velocity calculations and its selection of thermowells, contact Thermo-Kinetics.