A few weeks ago, I highlighted the issue of resonance and thermowells in a post, New Thermowell Calculations Help Avoid Damaging Resonance. Thermowells are the circular cylinder installed like a cantilever into process piping. The temperature sensor goes into the thermowell, to be able to measure the temperature of the fluid in the pipe without having to come in direct contact with the process fluid.
A velocity collar is a metal ring machined into the shank of a thermowell and installed tightly into the standoff of a pipe. Its purpose is to eliminate vortex-induced failures by raising the natural frequency of the thermowell. The process fluids also create a drag force on the thermowell causing it to bend. The velocity collar reduces the moment length to reduce this drag force.
While this all sounds good in theory, Emerson’s Alex Cecchini, a member of the Rosemount temperature team, noted that even with the tightest tolerance for a machined velocity collar and standoff, there will still be a small gap between the collar and standoff, which allows failure-inducing vibration to still occur. This gap widens over time due to deformation of the collar and standoff caused by vibration.
The vibration collar is also sensitive to the installation process. I spent a month one summer in Scotland as a college intern and learned what a “wee persuader” (also known as a hammer) was all about from some plant maintenance technicians. This type of “persuasion” can impact the size of these gaps.
The new ASME PTC 19.3 TW – 2010 Thermowells standard addresses the use of these velocity collars:
Support collars or other means of support are outside the scope of the standard. The use of support collars is not generally recommended, as rigid support can only be obtained with an interference fit between the support collar and the installed piping.
Alex noted Emerson’s position that given the nature of thermowell vibration behavior, variability in installation practices, and the position established by ASME with respect to these velocity collars is not to recommend their use. Better means of reducing the risk of failure caused by thermowell resonance include:
- Reduce immersion length in to pipe or standoff height to reduce unsupported length
- Change root diameter to affect the affected frequency due to flow conditions
- Change thermowell style
- Install thermowell at an angle or in an elbow of a pipe
- If all other options are not sufficient for the application, consider surface mounted sensors