For measurement of the gas temperature inside process piping, temperature sensors are installed in a thermowell. In most cases these cylindrical thermowells are installed perpendicular to the flow direction. Declining pressures of many aging oil & gas assets results in higher gas velocities. This will increase the so-called Strouhal frequency and amplitude of the dynamic forcing at the thermowell. If the frequency of this force coincides with the installed resonance frequency of the thermowell, this can lead to fatigue failure.
The design of thermowells is comprehensively described in ASME PTC19.3 TW Code. In a recent version the excitation of the so-called in-line resonance condition was introduced, which already occurs at half the flow velocity of the original transverse resonance condition. As a result, many already installed thermowells do not pass the so-called ‘frequency criterion’ anymore. However, when all other criteria in the ASME Code are passed, the thermowell may operate at higher flow velocities, only if the fatigue stress remains acceptable.
When calculating the fatigue stress with the tools as provided in the Code, in many cases this will lead to prediction of unacceptable fatigue stresses. Therefore, here a more detailed fatigue assessment method has been developed. It consists of in-situ vibration measurements at the tip of the thermowell and a numerical evaluation with a detailed FE model of the installed thermowell. With this approach the fatigue stress is then calculated and judged against a fatigue limit.
In the meanwhile, this detailed assessment method has been successfully applied to numerous high pressure, gas filled installations. In most cases this led to acceptable fatigue stresses and preservation of the desired gas production profiles.