Helical Thermowells

Helical thermowells, as the demand for optimalization of processes gets higher every year, the demand for faster and better control of these processes follows this trend at a short pace. One of the measures to keep the process and process controls safe, is to use the ASME PTC 19.3, the most recent version being the 2016 edition. This standard is a way to safely calculate thermowells with all process variables taken into account. One of the aspects that needs to be calculated is the resistance to Vortex Induced Vibration (VIV). A solution to the vortex induced vibration is the addition of helical strakes to the process exposed part of the thermowell. Helical strakes are aerodynamic stabilizers that reduce forces and deflections experienced on the thermowell due to vortex shedding. Helical Strakes consist of 3 square vanes which are wrapped in a helical pattern to the surface of the thermowell. The vortices generated by the wake of the helical strake are much smaller and non-uniform as compared to the normal thermowell, and are also considered negligible when evaluating vortex-induced vibrations. With the addition of the helical strakes to the thermowell, the VIV can be reduced by 90%. We as Thermo-Electra have conducted our own research in collaboration with representatives from the Technical University of Delft in the Netherlands, to validate the existing studies and come up with a specific calculation method for the helical solution. In this calculation the stress accompanying VIV is reduced by 80%, where others in the industry calculate with a 90% reduction. It is the understanding of Thermo-Electra that there will always be process variables that can differ from theoretic parameters in these calculations. With this extra 10% conservative calculation we want to build in a safety for our customers. When calculating according to ASME PTC 19.3 2016 with a reduced VIV it is far more likely that the thermowell dimensions can come closer to the original dimensions and measurement instead of needing to shorten or thicken the thermowell.