Chris Green presents a thermowell solution for when a design fails the ASME PTC 19.3 TW calculation
Having supplied standard thermowells to the oil, gas and petrochemical industries for many years, Okazaki Manufacturing Company was of course aware of the problems caused by the effect of flow-induced vibrations and was continuously looking to develop a solution that would remove this effect. Doing so would massively improve the accuracy and reliability of the results the thermowell provides as well producing a product that would have a longer lifespan.
The importance of this product development was brought to even greater urgency in 1995 following the failure at the Monju nuclear power plant in Japan. This was caused by the failure of a thermowell that had passed a calculation based on the ASME PTC 19.3 1974 guidelines. And with Okazaki being one of Japan’s largest manufacturers of temperature assemblies, research was stepped up to see how the standard thermowell design could be improved upon.
The breakthrough came in 1998 with the publication of JSME S 012-1998, which along with the company’s in-house preliminary research, led the firm to look at alternative methods to decrease flow-induced vibrations and create a real alternative solution.
JSME S 012-1998 presented a few methods for decreasing the vibrations caused by a Karman vortex street, which include making the thermowell shorter or larger in diameter, but when this isn’t possible then other countermeasures should be used. Several methods are mentioned but after Okazaki’s initial R&D, the helical strake design was found to be an excellent solution on a thermowell.
Helical strakes are not an original idea and have been used in other applications such as industrial cooling towers and car aerials, for example, but it was discovered that this technology would also work on a thermowell to reduce vibrations.
After further in-house research was completed (which included other alternative solutions such as increasing damping and streamlined cross sections), it was confirmed that the best solution for thermowells was to add helical strakes. The research team experimented with various combinations of pitch, quantity and thickness to create the optimum configuration.
Tests were carried out at Tamagawa University in a direct comparison between a standard thermowell profile and one fitted with the optimum helical strake configuration. It showed the latter had no adverse effects from the various flow rates subjected to the test samples. However, at higher flow rates the standard thermowell showed signs of vibration and failed due to metal fatigue.
In 2008 a CFD analysis was commissioned to study a standard thermowell and the solution with helical strakes – now branded VortexWell. It was found that, “the thermowell with helical strakes demonstrated no regular vortex shredding. While the flow behaviour varied significantly along the length of the thermowell it was observed that little or no time dependent changes occurred and the results were considered dynamically stable in all cuts along its length. It’s suggested that the strakes sufficiently disturb the flow in the wake region such that no regular vortexes can form.” The report’s conclusion was that, “the VortexWell offers a credible solution to overcoming increasing failures.”
Chris Green is with Okazaki Manufacturing Company