Failure Analysis of TG/TT (Temperature Gauge/Transmitter) in a Gas Refinery(Part 2) Vibration-measurement-based

As was mentioned in the previous article (https://www.dhirubhai.net/pulse/failure-analysis-tgtg-temperature-gaugetransmitter-gas-moshayedi/?published=t) FIV or Flow Induced Vibration is a usual phenomenon in the Temperature Gauge or Temperature Transmitter due to thermowell vibration in the oil and gas refineries.

One of the main codes for thermowell design is ASME PTC 19.3TW. However measuring Vibration could also be useful. Two types of forces are applied to the thermowell due to pressure, velocity and density of service, geometry and roughness of thermowell, in-line and transverse forces.

As a result of these oscillating forces two types of vibrations affects the thermowell. These vibrations has it's own resonance and if the thermowell oscillation occur on each of these resonance due to fluid parameters, failure would happen.

Considering the picture above from ASME PTC 19.3TW and the picture below, in-line and transverse vibrations and safe frequency limitation of thermowell are shown.

As shown in the picture above, vibrations of in-line and transverse has to be less than that of resonance. Now, let's see the actual frequency curve of thermowell operating in a gas refinery.

It was mentioned in the previous article that Finite Element Method could also be used to investigate that whether a thermowell is design properly or not! In our model the in-line frequency is about 200 Hz which was close to that of measured and far less than natural frequency of thermowell which is 250 Hz. As mentioned in ASME PTC19.3TW, frequency of thermowell has to be less than %80 of natural frequncy. As you can see in-line vibration is about 230 Hz and is NOT less than %80 of natural frequency! So failure will happen soon.

This was a brief history of what actually happened. For more information do not hesitate to message. Share this article to your friend and comment if you have similar experience.

#inspection #oiland gas #thermowell #failure #RCA