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A Tool to Simplify Thermowell Design

 Figuring out if a thermowell can provide a responsive temperature measurement while maintain process containment used to be a tedious and error-prone task. A new design tool, built on ASME standard calculations, makes the job much easier and the results more reliable.

Using the new software, it is possible to provide a definite answer to the question, “Is this thing going to work?” Maybe you get asked this question a lot when designing new equipment or part of a process unit. When dealing with piping, valves, fittings, vessels, and the like, this can be particularly important because a failure can result in loss of containment. Something hot, toxic, or flammable may be released if any element—including a thermowell installed for a temperature measurement—isn’t up to the job.

 Thermowells have the potential to be far more dangerous than a casual observer might think. When inserted into a flowing stream, even a thick-profile thermowell can vibrate to the point that it breaks off under certain conditions. Is there a way to know when an installation is likely to produce such problematic conditions? The answer is yes, but only when using analytical tools designed for just this purpose, which is the topic of an article Naveen Kulkarni and I wrote in the October 2019 issue of Process Heating: A Tool to Simplify Thermowell Design.

The success or failure of a given thermowell in a given operational context relates to how closely the frequency of the vortex-induced vibration (VIV) matches the natural resonant frequency of the thermowell. When those two values match, the displacement will be its greatest. Both of these frequencies can be calculated, so the American Society of Mechanical Engineers (ASME) created a formula to help engineers determine if a given thermowell with particular dimensions installed in a certain way would survive a specific set of process parameters (fluid velocity, density, temperature, etc.).

The formula has proven reliable, but the calculations are tedious with nearly 20 variables to deal with, so users typically build it into a spreadsheet or use some kind of online version. Even so, it can still be burdensome because any potential operating scenario must be considered, so a single instance may require three, five, or maybe more calculation runs. And worse, all the formula can indicate is if the installation/scenario combination has passed or flunked. Users are on their own to determine what to change to adopt to receive a pass.

Emerson is moving the thermowell design procedure beyond a simple testing method and turning it into a powerful design tool.

Increasing functionality of the formula requires a broader approach. The basic formula remains, but the mechanisms to apply it must be more sophisticated. There could be tens or even hundreds of temperature tags in a process unit, all needing evaluation. The design platform should add predictive capabilities to help the engineer optimize thermowell design beyond simply delivering a passing grade.

So, a design tool needs to include capabilities beyond meeting the minimal requirements of the formula. It should address dimensional issues for the installation, what other installations in the unit are similar, what company practices may influence the choice, what stock thermowell sizes would suit the application, and many other considerations, rather than simply provide a yes-or-no verification. Also helpful is if it recommends alternatives for difficult applications, such as Emerson’s Rosemount Twisted Square Thermowell with its ability to reduce VIV, or Rosemount X-well Technology for temperature measurement without a thermowell.

It is the ability to analyze failures and suggest solutions that separates a design tool from a simple checking method. To users designing a range of temperature monitoring points in a process unit, this approach represents an advantage. Such tools allow engineers to save design time, ensure correct installations, minimize ongoing maintenance and reduce equipment inventories.

You can find more information like this and meet with other people looking at the same kinds of situations in the Emerson Exchange 365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the Temperature Group and other specialty areas for suggestions and answers.