Thermowells subjected to flow experience dynamic stresses imposed by oscillating vortex pressures. These dynamic stresses can result in vortex-induced vibration (VIV), the leading cause of thermowell stress failures. Emerson is now offering a patented “twisted square” thermowell that suppresses VIV, eliminating the concern of dynamic stress failure.
The new design was inspired by the difficulties of specifying conventional thermowells to avoid issues with VIV. Most often, stress failures occur on conventional thermowells that have not undergone recommended calculations per ASME PTC 19.3 TW to ensure the thermowell will withstand fluid forces and process pressures.
[sidebar id =1]The wake frequency limit is generally the most challenging calculation to pass, especially for long thermowells or high-velocity flows. The limit must be verified 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 the VIV.
“A conventional thermowell experiences lock-in frequencies that multiply stress 1,000 times or more,” says Timchan Bonkat, product manager, temperature, Emerson Automation Solutions. “Every material has its own fatigue limit. After enough reps, it will fail.”
The traditional solution to avoid these lock-in regions is to shorten the thermowell and/or increase the outer diameter. These changes can result in decreased accuracy or increased response time of the temperature measurement.
“Customers try to standardize on a thermowell, so to accommodate the worst application, they choose one that’s short and fat,” Bonkat says. “They may have to use a larger nozzle size and retrofit existing applications. In some of their applications, the short, fat thermowell may not reach far enough into the flow for a representative reading.”
These situations are becoming more frequent because process velocities are increasing due to more frequent use of smaller pipe sizes to save cost on projects.
Instead of stiffening the thermowell, the Twisted Square Thermowell desynchronizes the vortices in its wake so they are not 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 reduces resonance stresses by more than 90%, so static stresses dominate. “With a twisted square thermo-well, the static stresses will always be the limiting factor. Without resonance lock-in, the dynamic stresses don’t generate enough energy to be damaging,” Bonkat says.
“The twisted square simplifies thermowell calculations, and it works in applications where conventional thermowells don’t pass ASME PTC 19.3 TW,” Bonkat says. “It allows optimum penetration using existing nozzles, and is available in all normal configurations and materials. It allows for growth because plants can change flow rates and other operating conditions without resonant frequencies concerns, and it also reduces stocking requirements because one design can be used in a wide variety of applications.
“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.”
For more information, including a link to an eye-opening video comparing deflections of a conventional thermowell and a Twisted Square Thermowell under a wide range of flow velocities, see www.emerson.com/en-us/catalog/rosemount-twisted-square-thermowell.
