Okazaki Manufacturing Company

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• Aeropak Cable, Resiopak, Tubeskin Thermocouple, Multipoint Assemblies, Thermowell (Vortex Well) Vortex Well: OMC’s VortexWell® incorporates an innovative helical strake design, very similar to the helical strakes seen on car aerials and cooling towers. Chris Chant, Business Development Manager at OMC (UK) comments: “By using the latest CFD [computational fluid dynamics] 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. What we discovered was pretty dramatic.” In the analyses, the standard tapered thermowell showed classic shedding behaviour as expected, whereas the VortexWell® demonstrated no signs of regular flow behaviour. The VortexWell® helical strake design disturbed the flow sufficiently to interrupt the regular formation of vortices. Whilst a small vortex was observed in the wake of the VortexWell® this was a localised stagnation point and didn’t shed. However, the most significant comparison made was with regard to the pressure fields. As Chris Chant continues: “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 experiencea significant improvement in practise compared to the standard thermowell design.” In further tests, this time using finite element analysis (FEA) software, OMC discovered that the ASME calculations (PTC 19.3, 2004) used by thermowell manufacturers could place significant limitations upon the safety of petrochemical applications. Using the ASME calculations (PTC 19.3, 2004) gave the lowest natural frequency of vibration for the standard tapered thermowell to be 68.5Hz. However, OMC’s own FEA results showed a corresponding value of 90.3Hz, a difference of more than 30%. This highlights that the ASME calculations (PTC 19.3, 2004) design rules include assumptions that can do lead to considerable inaccuracies when designing thermowells for petrochemical (and other) applications. The risk of a thermowell failing due to under-engineering, or the extra costs incurred by the end user because of an over-engineered thermowell, can both be avoided if the buyer works with a reputable, experienced thermowell supplier such as OMC.

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