During a recent plant revamp, it was discovered that several thermowells should be replaced. These devices had been in service for years without issues, so it was an uphill battle to justify their replacement. The whole thing boiled down to vibration problems due to vortex shedding, and the potential for fatigue failure.
It turns out that over the years the plant had accumulated so many process changes that the original thermowell sizing was no longer valid. Temperature changes, flow changes, even viscosity changes, combined to push conditions into the red zone. It was just a matter of time until something failed.
As a small part of a series of presentations explaining this problem, the following video illustrates the vortex shedding phenomena. It shows a pipe and thermowell cross section with flow from left to right. The gradient colors represent fluid velocity profiles, and the green lines represent contours of constant pressure. The oscillating pressures on the thermowell can induce fatigue related vibrations if the device is not designed and installed correctly.
The web is full of information on thermowell design considerations that apply to vortex shedding. Below are some useful links:
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