Velocity Collars are Not the Answer to Thermowell Resonance

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.

Thermowell Velocity Collar
One of the engineering practices in thermowell design has been to use velocity collars to try to dampen the resonance caused by the fluid flowing through the pipe.

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

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4 comments

  1. Would it be possible to machine an external groove onto the collar and add a vibration dampening elastic seal?

    • Daniel, Thanks for your question! I did some checking and wanted to relay this response back:
      “Yes, that would certainly help dampen the effects of vibration however it would not be easily tuned for the installation. The longer term concern of course is degradation of the damping element due to direct exposure to the process fluid. Care would need to be taken to select a material that is suitable for both the temperature and process medium. A frequent visual inspection regimen would also be required to ensure that the solution was still functioning per design since the disintegration of the damper could cause thermowell failure. This would require removal of the thermowell, so the damper would be limited to processes that are shut down frequently. Since we provide only the instrumentation, we cannot provide any guidance on the design of this and it is outside the scope of ASME PTC 19.3 TW-2010.”

  2. Blake Turner says:

    http://www.orbitalglobalsolutions.com/uploads/ve%20thermowell.pdf This is the answer. My company makes them, so I guess I am biased, but they have solved this problem.

    • The helical strake thermowell referred to by Blake is one alternative for use in applications where a more traditional thermowell cannot be made to work. Velocity collars have been historically used as one means of changing the attributes of a thermowell to produce a suitable design for a given application. As mentioned in the original post, velocity collars have been recognized to be unreliable over the long term so are not recommended in the ASME PTC 19.3 TW – 2010 standard. There are other ways to change a thermowell design to ensure reliable service, and a helical strake thermowell design could be one solution to consider.

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