Helical strakes prevent damaging vortices
Helical strakes help to ease turbulence around thermowells, Tom Shelley reports
Japanese company, Okazaki Manufacturing Company (OMC) has put a number of helical strakes around the outside of a thermowell. The idea is to prevent the formation of alternating vortices that can damage traditional smooth walled designs.
Thermowells allow the replacement of temperature sensors in fluid flows, particularly oil and gas streams. Cylindrical designs result in the formation of vortices on both sides, which detach first from one side, then from the other, causing a fatigue loading. If the vortex shedding frequency is close to the natural frequency of the tube, early failure is likely. To date, the solution has been to make the thermowalls either shorter or thicker, or add a collar to move the point of vibration resonance.
What the OMC has done is to add helical strakes similar to those seen on car aerials and steel chimneys. Chris Chant, business development manager at OMC (UK) says: "By using the latest cfd software to visualise the flow behaviour, we were able to accurately compare a standard tapered thermowell and our new VortexWell design which incorporates a helical strake. For the standard tapered well design, an oscillating pressure field was observed around the structure. The VortexWell displayed a constant and stable pressure field, presenting no dynamic variations. As this pressure is the source of vortex-induced vibrations, it can be assumed that the VortexWell would experience a significant improvement in practise compared to the standard thermowell design. "
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