Imagine a machinist running a screw machine, turning out hundreds of parts with a male thread. To verify the thread size is in tolerance, he or she checks samples using two gauges with corresponding female threads. If a part’s thread is sized correctly, it will screw into the first gauge, but it should not screw into the second. If it screws into both, the diameter is too small; if it will not screw in to either, the diameter is too big. Of course, there could be other problems such as a bad thread profile, but the gauges ca not diagnose those given their limited function.
An instrumentation engineer wanting to insert a thermowell into a flowing fluid stream also has evaluations to make. It is critical to ensure that the thermowell will not be subject to excessive vortex-induced vibration (VIV) caused by wake shedding as the liquid flows past (figure 1). If the thermowell vibrates excessively at the anticipated flow rate, it will likely break, causing a loss of process containment (figure 2).
Can the engineer predict the degree of vibration? Is there a gauge to determine if the thermowell’s length and diameter — when used in the specific pipe at a given flow rate, pressure and temperature — will avoid vibrating to the extent of causing metal fatigue and failure?
The answer is yes; unfortunately, the gauge used by most engineers is not much better than the hypothetical machinist’s tools. The thermowell checking tool indicates pass or fail but little else. This article will look at how this tool operates and how it can be transformed from a simple gauge into a design platform.
The Thermowell Evaluation Formula
The success or failure of a given thermowell in a given operational context relates to how closely the frequency of the vortex-induced vibration matches the natural resonant frequency of the thermowell. When those two values match (figure 3), the displacement will be its greatest.
FIGURE 2. If vortex-induced vibration is severe enough, metal fatigue can cause a thermowell to break.
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.). In 1974, ASME published ASME PTC-19.3-1974, providing a formula based on thermowell behavior and materials characteristics to predict if a thermowell’s natural resonant frequency would be different enough from the vortex-induced vibration frequency.
In 2010, ASME revisited the topic and released a much-expanded version of the formula. The revised calculation methodology took more process information into consideration and applied it to a wider variety of thermowell profiles and mounting methods. It also dug deeper into steady-state, dynamic and pressure stress.
FIGURE 3.The relationship between displacement and flow is not linear. The most damaging displacement is where the vortex-induced vibration frequency matches the resonant frequency of the thermowell. The blue diamond data point indicates the response of a thermowell resistant to vortex-induced vibration at the same conditions.
Even more recently, ASME opened the evaluation process again and released the current ASME PTC 19.3 TW-2016 standard.
The calculations evaluate the component configuration and operating parameter combinations on four points:
- Frequency Limit. The thermowell’s resonance frequency must be high enough that it will not be reached in operation. This avoids the potential for a match with the vortex-shedding frequency.
- Dynamic Stress Limit. Dynamic stress shall not exceed the fatigue stress limit.
- Static Stress Limit. Steady-state stress shall not exceed the stress limit.
- Hydrostatic Pressure Limit. External pressure shall not exceed the ratings of the tip, shank or flange.
All four of those limits must be satisfied for the thermowell design to be deemed safe.
Turning a Formula into a Tool
ASME PTC 19.3 TW-2016 provides the formula and supporting materials. Yet, with nearly 20 variables related to process conditions and thermowell dimensions, doing the calculations by hand can be impractical. Engineers often build the formula into a spreadsheet to facilitate data retention and number crunching.
Various suppliers offer this as an online software tool or as a downloadable version built into a spreadsheet framework. This can add useful data retention and management functions, but it only provides a pass or fail evaluation without comment. The engineer must design the dimensional parameters of the thermowell, place it in the operational contexts (which should include a range of conditions such as startup, normal, etc.), create the implementation and then test it iteration by iteration, gradually working toward a solution able to pass all four evaluation points.
FIGURE 4. If an application is particularly difficult, the design platform can suggest a thermowell profile resistant to vortex-induced vibration. Such designs offer the ability to disrupt vortex formation so they cannot cause vibrations.
For some, this time-consuming, trial-and-error effort may result in thermowells far thicker and more massive than necessary. After all, the formula cannot indicate that a workable result is actually overkill. An oversized thermowell slows temperature-measurement response time while creating a larger penetration and obstruction in the pipe.
Still, it is important to remember: The formula does what it promises. It weighs a given set of parameters and provides a pass or fail grade, but that is all. It does not point the user toward the most right-sized solution. Like the machinist’s thread gauges, it has important, but limited, functionality.
Turning a Tool into a Design Platform
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. What might this type of design platform look like?
Obviously, it must retain basic data-management and calculation capabilities. The engineer should be able to review the dimensional and operational parameters for “Tag XX” and the results of all related test calculations.
What else should a design platform tell us about Tag XX and all the other temperature tags involved in the unit? Here are some basics:
- In/out of scope.
- Dimensional issues.
- Comparison with similar installations.
- Proprietary concerns.
- Available devices that meet the formula criteria.
- Existing, similar devices with the organization.
- Multiple operating ranges.
- Data management.
- Failure analysis and recommendations.
In/Out of Scope. The ASME formula includes dimensional standards such as wall and tip thickness. These are criteria that any thermowell must meet. If any input variables fall outside acceptable ranges, they are flagged, and a more appropriate solution is suggested.
Dimensional Issues. If a thermowell is not compatible with the installation, this will be flagged. (An example might be a design longer than the pipe diameter or too short to extend out of a mounting spud.) A modeling routine can turn the dimensional data into a scale drawing of the thermowell and its mounting to indicate how the assembly fits together, and how close the thermowell tip is to the center of the pipe.
Similar Installations. If a tag calls for a flange mount on a 4” pipe, the system should look for similar installations within the facility. The similarities the system looks for can be defined dimensions, operational parameters or whatever is considered relevant.
Company Practices. If a company or facility has preferences for a certain thermowell profile, mounting method or other consideration, the design platform should suggest these first.
FIGURE 5. In some extreme situations, a thermowell simply is not practical. In such cases, the design tool should recommend an alternative measurement approach.
Previously Used Designs. Thermowells used previously have all their dimensions in the system, so the engineer can pull one easily from a listing. The system should also suggest an existing design if it is close to one the engineer is testing. This lookup function can extend to specific part numbers for the facility — or to a supplier’s catalog — to minimize the number of potential inventory items.
Similar Tags. When there are similar applications with minor variations, it should be easy to copy all the values for a given tag and then change only those variables required for the new application.
Multiple Operating Ranges. Because thermowell lifespan is dependent on process conditions, testing across a range of conditions (normal, startup, maximum, grade change, etc.) is critical. An effective thermowell design tool helps accelerate testing. Most installations could easily see three or four different conditions, and some could experience as many as 10.
Cloud-Based Collaboration and Data Management. If the design tool is hosted in the cloud, it eliminates the need for it to reside on individual computers or servers. Cloud-based computing provides mechanisms for centralized support and access by any number of authorized users. Report generation can be simplified.
Failure Analysis and Recommendations. Simply reporting that a given tag has failed the test at one or multiple conditions is not enough. The system should provide analysis of why it failed and suggest a remedy. In simple situations, this might be an easy change; for instance, shortening the insertion length or adding a millimeter or two to the wall thickness. In more problematic situations, it may suggest a thermowell profile resistant to vortex-induced vibration (figure 4), or an external measurement method (figure 5) to avoid the need for a thermowell entirely.
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.
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