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What is a Thermowell Wake Frequency Calculation?

Thermowells | Wake Frequency

When using a thermowell, it’s important to ensure that it will work in the application. Wake turbulence caused by your process could break the thermowell and damage your temperature equipment.

How can you ensure your thermowell is adequate?

Calculating thermowell wake frequency can help. In this article, I will explain what thermowell wake frequency is and how to calculate it.

Defining Thermowell Wake Frequency

A wake is the region of recirculating flow immediately behind a moving or stationary solid body, caused by the flow of the surrounding fluid. The alternating vortices that occur as the media flows across the shank can create turbulence causing the thermowell to resonate or oscillate. This is referred to as vortex shedding or Von Karmen trail.

The shedding rate is calculated and compared to the natural frequency of the thermowell. If the frequency of the wake is too close to the natural frequency of the thermowell then the vortex shedding can destroy the thermowell, resulting in possible damage to the temperature instrument and your process as the broken shank flows through your system.

Ashcroft’s Wake Frequency Calculator

Ashcroft has developed a Wake Frequency Calculator tool to assist you with the proper thermowell selection. This tool was designed following the ASME PTC 19.3 TW and can be found on our website.

What’s Needed to Run a Wake Frequency Calculation?

Five pieces of your process media information and the thermowell dimensions are required to run a calculation. Without all the details you cannot run a calculation:

  1. Velocity: This is the speed of your media, typically measured in feet per second or meters per second. This condition is critical to the calculation as the velocity affects both the lift and drag forces and other stresses applied to the thermowell.
  2. Density: Mass per volume or specific gravity. The density combined with the velocity comes into play in all calculations.
  3. Viscosity: The thickness of your media, or a measure of your media’s resistance to shear stress.
  4. Maximum Pressure: The maximum amount of force applied to an object.
  5. Maximum Temperature: The maximum temperature of your process media.

Within the Ashcroft Wake Frequency Calculator, if you happen to have a standard Ashcroft part number you can pull it from the drop box and your thermowell dimensions will automatically populate:  

wake-frequency-calculator-main

If you do not have an Ashcroft part number but you know your thermowell dimensions, then you can manually enter them. These dimensional fields can be modified when necessary:

wake-frequency-calculator2

Once the process information and dimensional details are complete, you will need to understand the results.

Understanding the Wake Frequency Results

There are four types of stresses the calculator will run through to determine the probability of failure:

  1. Oscillating Stress: This includes the drag and lift forces that cause oscillation. It’s also known as dynamic stress. The thermowell fails if this stress exceeds the fatigue stress limit for the thermowell.
  2. Steady State Stress: The steady state stress should not exceed the allowable stress of the thermowell. The allowable stress is determined using Von Mises criteria within the formula of the calculator to calculate whether the stress combinations at a given point will cause failure.
  3. Pressure: The pressure applied should not exceed the pressure ratings of the thermowell or it will result in a failure.
  4. Frequency: The resonance frequency of the thermowell must be high enough to prevent destructive oscillations caused by the flow. Most failures are seen in this area when the process conditions allow the thermowell to vibrate to the point of failure.

A thermowell must pass all four of these parameters to be considered adequate to use.

Passing the Test Parameters

Let’s say you've entered all your data and clicked the calculate button but your selected thermowell failed for its intended application. Specifically, it failed both the oscillating stress and the frequency. What now?

The typical steps to correct this failure would be to shorten the U dimension. This is always the best solution if it’s possible in your application.

In cases where this is not acceptable, you would need to fatten up the shank. Another option would be to run the test with a stronger material.

If you make the decision to thicken up your root and tip dimensions, please keep in mind where you’re inserting your thermowell. You do not want to thicken it up so much that your well no longer fits into your nozzle or tee.

We don’t like to pressure you, but we have more information.

Now that you understand the process of calculating wake frequency, you should be better prepared to select the best thermowell for your process.

You can always reach out to our temperature specialists here at Ashcroft if you have questions about our Wake Frequency Calculator. You can also research the available thermowells on our site.

To learn more about wake frequency, our Learning Center has a helpful webinar explaining the calculation process.

And for more information on temperature instruments, check out our past articles:

Contact Our Team!

About Dave Dlugos, Product Marketing Leader, Temperature Products

Dave Dlugos has a BSEE degree and 40 years of experience in the measurement industry performing design engineering and product management. He has earned 4 U.S. patents and joined Ashcroft in 2007, currently as the Product Marketing Leader for Temperature products. He is a senior member of the International Society of Automation (ISA ), currently ISA District 1, Vice President and a Past President of the CT Valley ISA Section.