The Best Approach to Reliable, Safe Thermowell Design

Thermowell TemperatureThe design, integration and specification of thermowells in achieving reliable temperature measurement is a complex process requiring more than 65 percent of the engineering effort in a given installation. But this process doesn’t have to be a difficult one, and users can follow a levelled decision-making approach to reduce the complexity and design time involved in thermowell usage.

Level one is prevention, utilizing known process parameters, product designs, best practices and calculations. These are based on ASME PTC 19.3 TW, the “wake frequency calculation” standard. At some point, a second level, known as “detection”, will be added to the continuum. Emerson is currently underway in development of active in-situ notification techniques should thermowells approach resonance conditions.

Level three is suppression, and it’s a major step engineers can use today to suppress the effects induced by process conditions on the thermowell. Emerson’s Rosemount Twisted SquareTM technology nearly eliminates fluid induced vibration. In a typical cylindrical thermowell, the fixed separation point allows ribbons of vortices to synchronize and shed uniformly. Alternating low-pressure cells apply forces to the thermowell that can lead to failure. The Twisted Square design continuously changes the separation point causing chaotic vortex shedding and the upstream vortex disrupts those downstream and vice versa. As Fig. 1 shows, the Twisted Square design exhibits mean and max stresses that are much lower than the fatigue limit.

Level four is the penultimate decision in securing reliable, leak-free thermowell integration – eliminate the thermowell. Integrating even the most effective and reliable thermowell is a complex problem:

  • Design complexity (material, length, stem style…)
  • Extremely expensive if exotic materials are required in corrosive processes
  • Installation (welding) and maintenance are challenging
  • Possible leak points are inherent to the penetration of the pipe
  • Wake Frequency Calculations are needed

And all this must be done to achieve accurate process temperature. Emerson has made it easy to eliminate a lot of this challenge and complexity with its Rosemount X-well technology. Rosemount X-well is a complete point temperature measurement solution that provides a process temperature measurement without the need for a process penetration or thermowell.

Rosemount X-well, however, is not traditional surface temperature measurement technology, and such measurement does not give an accurate or repeatable representation of how the process is behaving and its performance is negatively impacted by environmental conditions.

Rosemount X-well technology:

  • Compensates and corrects for environmental effects
  • Measures ambient temperature (T1) and pipe surface temperature (T2)
  • Uses the measurement assembly and process piping thermal conductivity properties to calculate heat flow through assembly
  • Extrapolates accurate process temperature inside the pipe (T3)

More information on Rosemount X-well technology can be found here and Rosemount Twisted Square technology can be found here.