Thermowells protect temperature sensors from the process conditions being measured. They also serve to form a process seal.
In normal applications, process fluid passes around the thermowell, creating low-pressure vortices on the downstream side of the flow. This phenomenon generates static and dynamic stresses on the thermowell. If these stresses are high enough, there is a potential for fatigue-induced mechanical failures of the thermowell. Piping designers use a variety of tools to predict and avoid thermowell failures in their systems; ASME PTC 19.3TW-2010 is the internationally recognized standard for thermowell design.
The scope of ASME PTC 19.3TW-2010 is limited to single thermowell installations. However, process designers often employ multiple thermowells in close proximity for redundancy purposes and/or to meet safety requirements
Applications with thermowells in close proximity can suffer from uncertainty due to the interaction between them. As fluid flows from one thermowell to another thermowell, the fluid can experience increased turbulence. Rather than discuss multiple thermowell installations, ASME PTC 19.3TW-2010 refers the designer to a portion of the ASME Boiler Pressure Vessel Code dealing with banks and arrays of thermowells.
|Figure 1. Vortex shedding in side-by-side installation|
While this reference provides some guidance for specific applications, it doesn’t cover the wide range of design conditions and applications that are prevalent in the process industry. To understand these applications, peer-reviewed journal articles provide some insight using experimental results. These experiments cover a variety of cylinder arrays, but the two configurations most applicable to process control applications are "side-by-side" and "tandem."
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In side-by-side installations, the velocity of the fluid increases as it passes between the two thermowells due to constriction of the flow. With an increase in velocity comes an increase in the vortex-shedding rate, along with an increase in the forces acting on the thermowells. As the space between thermowells increases, the effect is reduced.
In tandem thermowell installations, the downstream thermowell is within the vortex street generated by the upstream thermowell. This changes the forces on the upstream thermowells and induces additional forces on the downstream thermowell. Since the fluid velocity is unchanged, the shedding frequency is unaffected; only the forces on the thermowells are different. This means that if the thermowells individually pass the frequency limit evaluation per ASME PTC 19.3TW-2010, they may still be acceptable for use in tandem installations.
|Figure 2. Vortex shedding in tandem installation|
Much of the available experimental data is taken in the "close proximity" region (within eight cylinder diameters) to generate the most dramatic results. The data suggest that when using two or more thermowells in conjunction, the best thermowell installation orientation is side-by-side with at least five thermowell diameters spacing between them.1 However, the geometry of side-by-side installations can be difficult to fabricate and verify. This fact alone might cause the designer to consider tandem orientation.
|Figure 3. Spacing requirement for side-by-side installation|
If the designer chooses the tandem thermowell orientation, then the spacing between the thermowells must be greater than the distance required for the vortex street to dissipate. This distance varies with fluid density and velocity, but the experimental results show vortices generally decay within 100 thermowell diameters.2 For typical thermowells with a tip diameter of 0.50" (12.7 mm) or 0.75" (19.1 mm), the center-to-center distance between thermowells would need to be anywhere from 4’ to 6’ (1,200 mm to 1,800 mm).
Elbows or other in-situ flow disruptions help to dissipate vortices and can reduce the spacing requirements. However, this is highly variable and should be evaluated on a case-by-case basis. The designer can only use ASME PTC 19.3TW-2010 guidelines to evaluate the thermowell installation if the vortices can be dissipated through spacing or flow disruptions. The designer can reference the same open source documents for other orientations and applications that may need evaluation.
Danjin Zulic is a temperature marketing engineer at Emerson Process Management, Rosemount Measurement in Chanhassen, Minn. He has a Mechanical Engineering degree from Iowa State University and is currently pursuing his MBA at the University of Minnesota. Recently, he helped create "The Engineer’s Guide to Industrial Temperature Measurement," a comprehensive handbook on temperature measurement in industrial processes. He also helped develop a free online thermowell calculation tool that is available at www.rosemount.com/thermowellcalc. Mr. Zulic can be reached at Danjin.Zulic@Emerson.com.
Dirk Bauschke is an engineering manager in Temperature Product Development at Emerson Process Management, Rosemount Measurement in Chanhassen, Minn. He has a bachelor’s degree in Mechanical Engineering from Minnesota State University and an MBA from University of St. Thomas, St. Paul, Minn. He has authored several white papers associated with Thermowell Calculations, discusses the topic in a video, and contributed to "The Engineer’s Guide to Industrial Temperature Measurement." Dirk led the development of the Emerson software tools for Thermowell Calculations available at www.rosemount.com/thermowellcalc. He is the ASME PTC 19.3 representative for Emerson, helped develop the PTC 19.3 TW standard released in 2010, and serves as the current PTC 19.3 committee chair. Mr. Bauschke can be reached at Dirk.Bauschke@Emerson.com.
1. Zdravkovich, M.M., "Flow Induced Oscillations of Two Interfering Circular Cylinders" Journal of Sound and Vibration (1985).
2. Cimbala, J.M., Nagib, H.M., and Roschko, A., "Large Structure in the Far Wakes of Two-Dimensional Bluff Bodies" Journal of Fluid Mechanics (1988).
3. ASME Standard, Performance Test Codes 19.3TW-2010.