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Twisted Square Thermowell Improves Safety

Oct. 31, 2017
Design prevents failure due to vortex-induced vibrations

Thermowells subjected to flow experience dynamic stresses imposed by oscillating vortex pressures. These dynamic stresses can result in vortex-induced vibration (VIV), a leading cause of thermowell stress failures. To combat this, a new “twisted square” thermowell suppresses VIV, reducing potential for device failure. In addition to preventing thermowell failures that can cause expensive shutdowns, or even worse, result in leaks and explosions, it provides accurate and reliable temperature measurement.

The difficulties of specifying traditional thermowells to avoid issues with VIV inspired the design. Most often, stress failures occur on conventional thermowells that haven’t undergone recommended calculations per ASME PTC 19.3 TW to ensure the thermowell will withstand fluid forces and process pressures (for more on thermowell calculations).

The wake frequency limit is generally the most challenging calculation to pass, especially for long thermowells or high-velocity flows, says the company. Verification of the frequency limit is essential to ensure that the natural frequency of the thermowell is safely away from the Strouhal (vortex-shedding) frequency. As these frequencies converge, the thermowell can “lock-in” to resonant conditions, greatly magnifying dynamic stresses caused by VIV.

A conventional thermowell experiences lock-in frequencies that multiply stress 1,000 times or more, notes the company. The traditional solution to avoid these lock-in regions is to shorten the thermowell or increase the outer diameter. However, these changes can decrease accuracy and worsen the response time of the temperature measurement.

Because process velocities are increasing due to more frequent use of smaller pipe sizes, more end users have been choosing to standardize on short, fat thermowells to save cost on projects, but these devices may not reach far enough into the flow for a representative reading, the company notes.

Instead of stiffening the thermowell, the twisted square thermowell desynchronizes the vortices in its wake so they aren’t uniformly defined or alternating at a consistent phase along the length of the thermowell. This dampens the dynamic stresses from the vortices and suppresses VIV excitation to a safe level.

The design reportedly reduces resonance stresses by more than 90%, so static stresses dominate. With a twisted square thermowell, the static stresses will always be the limiting factor. Without resonance lock-in, the dynamic stresses don’t generate enough energy to be damaging, the company claims.

The twisted square simplifies thermowell calculations, and works in applications where conventional thermowells don’t pass ASME PTC 19.3 TW. It allows optimum penetration using existing nozzles, and is available in all normal configurations and materials. It also allows for growth because plants can change flow rates and other operating conditions without resonant frequencies concerns. In addition, because one design can be used in a wide variety of applications, it reduces stocking requirements.

“Above all, it provides an accurate and reliable temperature measurement while preventing thermowell failures that can cause expensive shutdowns, or at worst result in leaks and explosions.”

Emerson Automation Solutions

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