ASME PTC 19.3 2016 code does not provide clear guideline on how to deal with the vibration problems of existing thermowells that are currently in operation and may be operating near inline or transverse vibration zone.
While the code allows passing through the in-line vibration zone, it prohibits operation completely in transverse vibration due to lock-in phenomenon. Once lock-in occurs, the thermowell gets into resonance and in the absence of adequate damping in the system, the thermowell vibration amplitude would keep on building with every cycle till eventual failure. It was identified that several operating assets had thermowells operating in the prohibited zone as per ASME PTC 19.3 and were facing a greater process safety risk.
Damping though difficult to predict, plays very crucial role in amplitude when thermowell is operated in critical zone i.e. within 20% of natural frequency. Hence it is very important to estimate the damping factor. ASME PTC 19.3 2016 have suggested conservative damping factor (ζ) of 0.0005 based on the lab studies. The test set-up assumes the piping system as rigid component, whereas, in the field piping systems are flexible. Using a conservative damping in the stress calculations leads to a high fictitious stress indicating failure of the thermowell.
In the present paper, a method is suggested to quantify the damping in the system by utilizing actual site vibration measurement of thermowell in the finite element analysis and thus a more realistic assessment of the stresses in the thermowell can be made. This assessment presented a much larger damping present in the system than ASME suggested and led asset to continue operate the plant with no risk of unplanned downtime as well as technical integrity of equipment. The results are presented for one sample thermowells.