Last week, I visited our Emerson division that makes the Rosemount brand family of instrumentation. One of the things I learned coming out of those meetings was a new release of thermowell design standards. For those not already familiar with a thermowell, it’s essentially a circular cylinder installed like a cantilever into process piping. The temperature sensor goes into the thermowell, to be able to measure the temperature of the fluid in the pipe without having to come in direct contact with the process fluid. The governing standard for design of these thermowells for the past 30+ years has been the ASME PTC 19.3-1974 standard.
While this standard was straightforward to understand and apply to thermowell designs, it had limitations. The biggest issue with the 1974 standard was that it only considered the transverse resonance and completely ignored the possibility of thermowell mechanical failure from in-line resonance. The forces generated by the alternating vortices formed downstream of the thermowell in the fluid flowing through the pipe drive the thermowell vibration.
At certain flow rates, the frequency of this alternating “vortex shedding” (Strouhal Frequency) can coincide with either the natural in-line or transverse resonant frequencies of the thermowell. Over time, operating at these resonance frequencies can cause fatigue-induced mechanical failures at the thermowell/piping junctions. Depending on the type of fluid flowing through the pipe, this breached-pipe condition can have disastrous consequences.
Dirk Bauschke, engineering manager for temperatures and thermowells, represented the Emerson team on the ASME PTC19.3 committee, which was reformed in 1999. Its formation not only was in response to some catastrophic failures that had occurred from mechanical failure, but also because of advances in the knowledge of thermowell behavior, and the increased application of Finite Element Analysis, used for stress modeling.
The committee determined that due to the significance of the changes, a new standard was required instead of an update to the existing standard. The new standard, ASME PTC 19.3TW-2010 was approved in February of 2010 and fully released in July 2010.
Dirk and the Rosemount team have developed a whitepaper, Thermowell Calculations, which describes the practical implications of this new standard. The standard:
…models the thermowell as a simple beam and applies a series of correction factors to account for the differences from the ideal beam, including added fluid mass, added sensor mass, non-uniform profile beam, and mounting compliance.
From the model, the installed or “in-situ” is calculated and used for the rest of the frequency analysis. Vibrations may not only be induced in parallel with the flow, but also through a transverse (lift) force perpendicular to the flow. In the whitepaper, Dirk and the authors note:
The in-line vibration is approximately twice the frequency of the transverse and the in-line “velocity critical” (where the Strouhal frequency equals the natural frequency) is approximately half the velocity as the transverse.
The thermowell designer must understand the Strouhal Frequency compared to the thermowell natural frequency and set a margin of safe operation between these frequencies, to avoid resonance. The nature of a thermowell is that it is easy to get into a resonant condition, but it requires a large change in flow rate to stop the resonance–therefore it is vital in the design to avoid this resonant condition.
The standard calls for a 20% guard band between the frequencies to account for variability in the thermowell elastic response, thermowell manufacturing tolerances, material property variation, and process variations in flow rate, temperature, density, and/or viscosity of the fluid flowing in the pipe.
There is much more to this topic than I can address in a single post, so I’ll be checking back with Dirk. It’s important for instrumentation and automation engineers to understand that this new standard and calculations are available so that they can be incorporated into the thermowell design. The whitepaper is a great place to start this familiarization process.
