If you were at Emerson Exchange last October, you may have seen Ryan Leino deliver his presentation on this topic in person. He’s an engaging speaker, but if you missed it, this article does a good job of covering much of the same material. One of the key take-aways from both the article and presentation is that instrumentation engineers and technicians have more things in their toolbox today than ever before when it comes to making temperature measurements.
The article appears in the January 2018 issue of Processing with the headline, When and When Not to Use Thermowells in Process Temperature Measurement. He begins by defining what a thermowell is and how it gets integrated into a process.
A thermowell mounts through a vessel or pipe wall to extend into the process fluid to the desired temperature measurement point. The sensor is inserted into the thermowell from outside the process so it can be accessed without requiring a shutdown. The thermowell is exposed to the process fluid, so heat transfers through its wall and eventually conducts to the sheath and sensor itself. If the process fluid temperature changes, the new temperature must make its way through all those layers. Depending on the application, this can take a few seconds to several minutes. Few processes experience sudden temperature step changes, so this arrangement is adequate for most real-world applications.
This approach is straightforward enough, and as the article points out, there are lots of different mounting options an engineer can choose from to solve a given application. The thermowell becomes part of the larger process containment, so it needs to use an appropriate material, and be strong enough to withstand the temperatures and pressures involved. This is where things begin to get complicated.
Problems arise when a thermowell is inserted in moving fluid, which is the most common type of application. As the fluid flows past a round thermowell, high- and low-pressure vortices form at both sides. These vortices detach, first from one side and then from the other, in an alternating pattern. This phenomenon is commonly known as vortex shedding. The differential pressure caused by the alternating vortices produces vortex-induced vibration (VIV), resulting in stresses and causing transverse and axial deflection, which can ultimately lead to fracture. It is as if the thermowell is being pulled up and down perpendicular to the flow.
It’s hard to imagine the vortices having much effect on a stainless-steel thermowell, but in a video Ryan showed in his presentation, when the frequency is just wrong, it looks like invisible hammers are banging on the thermowell from both sides. It’s no wonder that so many break in actual applications. This forces users to make massively thick thermowells so they can stand the punishment, or explore a completely different way to solve the problem: Rosemount’s new patented Twisted Square thermowell.
A new design disrupts formation of the long vortices and allows them to form on both sides, so they tend to balance and cancel each other. The result is far less VIV, up to a 90 percent reduction in some cases. Helical geometry like this has been used successfully with wind stacks and deep-sea risers to solve similar problems. It does not depend on a specific orientation when inserted, and reduces the need for excessively thick thermowells and large diameter process penetrations. Moreover, it is effective and suitable across a wide range of operating parameters.
The Twisted Square is a major advance over conventional thermowells, but what if thermowells could be eliminated entirely? Rosemount has that option covered too with its X-well technology, which is able to measure temperature in many applications without a thermowell or even any process penetration at all.
New technology is designed to measure surface temperature, and corrects and compensates for heat transfer and ambient condition effects resulting in an accurate process temperature measurement without requiring any intrusions or penetrations into the process. The unit is installed using a simple pipe clamp assembly and then insulated. The pipe material and wall thickness are entered into the transmitter so it can calculate heat flow and extrapolate the process temperature inside the pipe. The unit also uses an ambient temperature reading taken inside the transmitter enclosure to account for changing ambient conditions.
The bottom line to the discussion is that Rosemount has you covered for temperature measurements, whether using conventional thermowells, new Twisted Square thermowells, or X-well with no thermowell at all. You can choose the exact technology you need to solve any measurement problem.
You can find more information like this, and meet with other people looking at the same kinds of situations in the Emerson Exchange365 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.