One of the perpetual problems in process industries is getting fast-responding temperature measurements of flowing fluids. Extending a temperature sensor into a product stream usually involves a thermowell, and this is where the problem begins. When inserted in a moving liquid stream, a thermowell creates alternating vortices downstream which induce vortex-induced vibration (VIV) perpendicular to the flow. When the liquid velocities and vibration frequencies work together, it’s like somebody is hitting the thermowell from side to side with a hammer. Eventually it fails, creating a potential safety incident.
The conventional answer is to design thermowells as short and thick as possible to make them strong enough to survive the vibrations. This approach creates its own problems, not the least of which is very slow response to a temperature change. The extent of this headache makes people notice when a solution emerges, which is why Control called special attention to Emerson's new Rosemount Twisted Square thermowell in its November 2017 issue. As it points out, calculating the extent of VIV is a challenge:
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. 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.
Calculations, like those mentioned, reflect a single set of conditions, so if something changes, such as an increase in flow, the extent of VIV changes, which leaves users with a difficult choice.
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,” says Timchan Bonkat, product manager, temperature, Emerson Automation Solutions. “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.”
This is where Emerson's Twisted Square technology comes in. The shape makes all the difference.
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.
Since stresses go down by more than 90%, it’s easier to have long thermowells to reach a critical measuring point, and thinner-wall thermowells to deliver quicker response to changing temperatures. This allows users to have more of a one-size-fits-all strategy, with less inventory on the shelf. But most importantly, fewer failure-related outages, resulting in greatly increased safety.
You can find more information like this, and meet with other people looking at the same kinds of situations in the Emerson Exchange 365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the Temperature Group and other specialty areas for suggestions and answers.