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Thermowell Design ASME PTC 19.3 TW

This is for the Thermowell Specialists.. Need a bit of Advice.. I am in the process of updating the Specs for an end user regarding ASME PTC 19.3 TW.  Some of us are aware that PTC 19.3 methodology required the user to calculate the natural frequency through a simple formula that did not account for pipe wall flexibility. The user calculated the ratio of vortex shedding frequency about the tip of the thermowell (wake frequency) to the natural frequency of the thermowell. This design was acceptable if the frequency ratio was less than 0.8 to guard against vortex induced vibrations.A number of thermowells designed in accordance to ASME PTC 19.3 have failed in non-steam service leading to catastrophic consequences. Subsequently ASME issued PTC19.3 TW and updated in 2016, I would like to add the following criteria in the Specs, Pl confirm this approach is good enough for a robust design.

Calculated wake frequency shall not exceed the number in Table below based on information on pipe schedule in data sheets.

Table - Limits of thermowell frequency ratio

Parent pipe schedule (Note 1)

Thermowells

Schedule 160 or greater

< 0,384

Schedule 80 to less than Schedule 160

< 0,372

Schedule 40 to less than Schedule 80

< 0,340

Less than Schedule 40

< 0,168

Note:

1. Treat stainless and non-stainless pipe the same (i.e. Schedule 40S and 40 have the same limit)
LV RAMAN
  • timchan bonkat
    LV Raman,

    Your interpretation of the ASME PTC 19.3 standard is correct, there have been changes that now consider different parameters to ensure the thermowell is not compromised. We adhere strictly to the ASME PTC 19.3 TW 2016 method which shows that the pipe schedule by itself does not directly impact the wake frequency calculations (WFC) calculations. Instead, the standard accounts for the flexibility of the thermowell attachment to the pipe (mounting compliance factor). The 2016 version of ASME PTC 19.3 TW uses an improved frequency calculation with more adjustment factors and accounts for a 0.4 inline frequency limit when necessary (only low density/velocity gas are allowed up to 0.8).).

    The 4-frequency ratio values you provided (.384, .372, .340, and .168) are all under 0.4 so they would still be acceptable per the standard. What you’re doing is making the calculations more conservative which in turn may lead to an over designed thermowell. We have a free online software called the Rosemount Thermowell Design Accelerator which ensures your thermowell designs meet the latest ASME PTC 19.3-TW standard plus does a lot more to make sure you have the best product for your application.

    I do have one question for you, where did the frequency ratio values you provided come from? Are they part of a standard or vessel code table?

    Let us know if you have more questions.

    Thanks
    Timchan Bonkat
  • LV RAMAN

    In reply to timchan bonkat:

    Hi Timchan
    Many thanks, The frequency ratio I have provided are in consultation with our Mechanical team, From the vessel code..
    Regards
    LV Raman

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