Design Criteria of Thermowell Dimensions (ASME PTC 19.3-2016)

For the credible design of thermowell against the process conditions, a Performance Test Code (PTC) was developed by American Society of Mechanical Engineers (ASME). According to this code, a vibration calculation should be performed before proposing any thermowell made from solid barstock without any weld joint for any process. This vibration calculation is called as Wake Frequency Calculation (or WFC) which should be passed and accordingly the dimensions of thermowell (i.e. Parameters such as root diameter, tip diameter, immersion length, process connection, tip thickness, and bore diameter, among others, are determined. Tempsens is a leading company that ensures this process is seamlessly carried out, providing reliable thermowell solutions.

ASME introduces 3 versions of this code.

The first code was developed in 1974. A serious incident of sodium leakage was occurred in Monju nuclear power plant in year 1995 due to which the generation affected for the next 15 years. Sodium leakage was detected from the main pipeline of the SHTS (secondary heat transport system) in the Monju fast breeder reactor (FBR) caused by a failure of the thermowell during the operation.

This thermowell was harmed as a result of significant cyclic fatigue failure.

Flow induced vibrations associated with vortex shedding was the main source of alternating stress on the thermowell.  In 1999, it was found that thermowells designed according to PTC-1974 were no longer dependable for steam applications and resulted in catastrophic failures.

The second version was launched in year 2010 in which evaluation of forces caused by external pressure of the critical process and the combination of static and dynamic forces were included. The third version was launched in year 2016 which is the latest one and accepted worldwide with calculation of design criteria named wake frequency calculation, steady stress calculation, dynamic stress calculation and hydrostatic pressure calculations.  The input process parameters to perform WFC includes design temperature, design pressure, medium velocity, medium density, viscosity, nature of medium, immersion length of thermowell, nozzle heigh and diameter etc and results are noted as wake frequency computations due to different forces influencing the thermowell.

Diag-I: Flow of thermowell design and Wake Frequency Calculation

Flow of thermowell wake frequency calculation can be described by Diag-X. Each thermowell needs to be passed in WFC as per ASME-PTC 19.3-2016. Sometimes wake frequency calculations are not required in low process applications with velocity lesser than 0.5m/sec. There are 5 basic process parameters based on which thermowell WFC can be performed. If thermowell consist of pipe, any weld joint, any coating, rough surface or with a collar then WFC is not applicable as per ASME standards. We can perform WFC only for those thermowells which are made from solid barstock rod without any kind of weld joint.

There are following corrective actions we can opt while WFC is failing as per inputs:

1. Increase root diameter and tip diameter of thermowell but it should be in line with the nozzle internal diameter in which it is to be installed.
2. Reduction in immersion length of thermowell but it should be in line with the sufficient inside line length (In between 1/3rd to 2/3rd of the line ID) for accurate sensation.
3. Alteration in the tip thickness and bore diameter of thermowell (Thicker thermowells with a smaller internal diameter are preferable compared to thermowell with larger inner diameter).

If applying all three methods, WFC is not passing then we have to change the design of thermowell. We can either opt the use of velocity collar or we can use helical design thermowell which reduces the vibration oscillations by 90%. Currently, helical design thermowells are widely used, and the application of velocity collars is limited due to ASME regulations.

Diag-II, Diag-III and Diag-VI shows the layout of flanged thermowell with velocity collar and helical design thermowell types for critical process applications. Tempsens offers advanced helical design thermowells, ensuring exceptional performance and reliability in challenging conditions.

                      Diag II : Flanged thermowell with velocity collar

  Diag-III: Helical design thermowells (Solid machined)

      Diag-IV: Thermowells with helical design (Solidly machined)

Conclusion:

Before the manufacturing of any barstock thermowell, it is a necessary requirement to perform its vibration analysis by doing wake frequency calculation as per ASME PTC19.3-2016 edition. To ensure a dependable thermowell design, it must succeed in the WFC.

It is never recommended to install any thermowell without performing WFC. To pass WFC, corrective measures involve shortening the thermowell’s length, increasing the diameter based on nozzle size, and implementing velocity collars.

A special helical design well found suitable credibility for critical process applications in comparison to a normal barstock thermowell.

Watch our video on thermowell design solutions and WFC(Wake Frequency Calculation):

At Tempsens, we specialize in customizing thermal solutions according to your specific needs, ensuring that every project receives the attention and precision it deserves.

For more information or to discuss your project needs, feel free to reach out to us at enquiry@tempsens.com. Our team is ready to assist you with the Best Thermal Solutions catered to your requirements.