When Should You Use a Vortex Thermowell?

A vortex thermowell should be used when a standard thermowell cannot safely withstand the vibration forces created by high-velocity process flow or fails a wake frequency calculation.

In many process industries, thermowells protect temperature sensors from system pressure, high-velocity flow and corrosive media while allowing the temperature instrument to be removed without shutting down the process. However, selecting the wrong thermowell can have serious consequences. Under certain operating conditions, flow-induced vibration can cause a thermowell to fail, putting your process, equipment and personnel at risk.

Ashcroft has decades of experience helping engineers select thermowells for demanding process applications. While flanged thermowells are the preferred choice for many installations because of their design and cost, there are situations where a vortex thermowell is the safest and most reliable solution.

In this article, you'll learn what causes a thermowell to fail, how wake frequency calculations help determine suitability, and when a vortex thermowell is worth the additional investment.

What causes a thermowell to fail?

In high-velocity applications, a thermowell can fail when process media flows across the shank of the thermowell, creating a phenomenon known as vortex shedding. This occurs when changes in the flow's momentum create alternating vortices behind the thermowell, causing it to oscillate. If these vibrations become excessive, the thermowell can fatigue and eventually fail.

Several factors determine whether these oscillations become a problem:

• Media flowing at higher velocities increases the likelihood of vortex shedding.
• The wake frequency depends on the thermowell diameter and the speed of the process media.
• If the wake frequency falls within approximately 20% of the thermowell's natural frequency, resonance can occur.
• Resonance significantly increases vibration, which can lead to thermowell failure, damage to the temperature instrument and disruption of the process.

When process velocity is relatively low, the likelihood of damaging vibration is significantly reduced.

Figure 1. Example of wake turbulence

To help ensure a thermowell can safely operate under these conditions, ASME developed the Wake Frequency Calculation (WFC) as part of ASME PTC 19.3 TW-2016. The standard evaluates the external, static and dynamic forces acting on the thermowell and compares them to the design limits of the device.

To learn more, read What Is a Thermowell Wake Frequency Calculation?

What information is needed for a wake frequency calculation?

A wake frequency calculation requires six key pieces of application information to determine whether a thermowell can safely operate under your process conditions. When requesting a wake frequency calculation from the manufacturer, you should be prepared to provide the following information.

Figure 2. Information required for a wake frequency calculation

What happens if a thermowell fails a wake frequency calculation?

If a thermowell fails a wake frequency calculation, the design must be modified or replaced with one that can safely withstand the operating conditions.

Once the required application information has been evaluated, the wake frequency calculation determines the probability of failure based on four primary criteria:

• Oscillating (dynamic) stress limit: If the drag and lift forces that cause oscillation exceed the allowable stress of the thermowell, the device can fail.
• Steady-state stress limit: If the continuous loading exceeds the allowable stress of the thermowell material, the thermowell can fail.
• Hydrostatic pressure limit: If the applied process pressure exceeds the pressure rating of the thermowell, failure can occur.
• Frequency limit: If the thermowell's natural frequency is too close to the wake frequency generated by the flowing media, damaging resonance can develop.

The thermowell must successfully pass all four criteria before it can be considered suitable for the application.

Failing a wake frequency calculation doesn't automatically mean you need a vortex thermowell. In many applications, modifying the dimensions of a standard thermowell is enough to satisfy ASME requirements. A vortex thermowell typically becomes the preferred solution when those modifications aren't practical or the application remains outside acceptable limits.

Why should you use a vortex thermowell?

A vortex thermowell is designed to reduce vortex shedding, making it an effective solution for high-velocity applications where a standard thermowell may not safely operate.

When a standard flanged thermowell cannot pass the requirements of ASME PTC 19.3 TW-2016, engineers generally have two options:

• Modify the thermowell to satisfy the wake frequency calculation.
• Replace it with a vortex thermowell.

While the second option comes at a higher initial cost, it can help prevent catastrophic failures that may damage equipment, interrupt production or create unsafe operating conditions.

As a comparison, a standard flanged thermowell typically ranges from approximately $125 to $2,000, while a vortex thermowell generally ranges from $900 to $5,000, depending on the materials, size and application requirements. However, the cost of replacing a failed thermowell or recovering from an unplanned shutdown can far exceed the initial investment.

How does a vortex thermowell reduce vibration?

A vortex thermowell uses helical strakes along the shank to disrupt vortex shedding before damaging vibrations can develop.

Engineered using computational fluid dynamics (CFD), the helical strakes discourage vortex shedding and reduce the amplitude of oscillations by more than 90% compared to a standard round-bar thermowell. Along the length of the thermowell, the strakes also promote cross-plane flow that is not present with a conventional design while still allowing the maximum allowable pressure load using the original stem dimensions.

Figure 3. Aschroft® Vortex Thermowell

Standard thermowell vs. vortex thermowell

Choosing between a standard thermowell and a vortex thermowell depends on the application, process conditions and the results of the wake frequency calculation.

Standard thermowell

Best suited for:

• Most industrial process applications
• Moderate process velocities
• Applications that successfully pass the wake frequency calculation
• Projects where lower initial cost is a priority

May require:

• A wake frequency calculation
• Dimensional changes to satisfy ASME requirements
• Support collars or other design modifications in some high-flow applications

Vortex thermowell

Best suited for:

• High-velocity process applications
• Installations that fail the wake frequency calculation
• Applications where modifying a standard thermowell is impractical

Advantages:

• Significantly reduces vortex shedding
• Eliminates the need for wake frequency calculations
• Accommodates longer immersion lengths
• Simplifies installation and replacement
• Improves long-term reliability in demanding applications

Key takeaways

Selecting the right thermowell is about more than meeting pressure and temperature requirements. Flow velocity, vibration and wake frequency all play an important role in determining whether a thermowell can operate safely and reliably.

While a standard flanged thermowell is the preferred solution for many applications, a vortex thermowell can provide the additional protection needed when high-velocity flow creates conditions that could lead to thermowell failure. Performing a wake frequency calculation early in the design process helps ensure the safest and most appropriate solution for your application.

Learn more about thermowell selection

Every application has unique operating conditions. Understanding how wake frequency, flow velocity and thermowell design work together can help you improve measurement reliability while reducing the risk of premature failure.

For more information about selecting the right thermowell, contact us to speak with a product expert.