Failure Analysis of TG/TT (Temperature Gauge/Transmitter) in a Gas Refinery (Part 1)
TG/TG (Temperature Gauge/Transmitter) are one the most important equipment in refineries for controlling temperature of the different processes. These instrument are mounted on refinery equipment usually by small bore diameter piping (nipple), fitting and a flange. There is similar instrument devices for measuring pressure of the process which are called PG/PT (Pressure gauge/Transmitter). But Failure of TG/TT are more common, But why?
As shown in the Picture above, most of the cracks types are “Longitudinal on Center-line of the Weldmetal” joining pipe (nipple) to the flange or pipe to the fitting (olet). Longitudinal cracks happen as a results of dissimilar welding or fatigue .The results of hardness and chemical composition of the welds are in the acceptable tolerances. So dissimilarity could not have happened. But Fatigue? How?
By cross sectioning and performing metallographic analysis on the crack zone “ratchet marks” and “beach marks” was observed which are clearly the results of fatigue. As shown in the pictures below, the left is “ratchet marks” and the right is “beach marks”.
The main question is that where the fatigue came from? Well, in measuring temperature, as we need to know the temperature inside the pipe due to gradient of temperature from center to wall of pipe, Thermowell is used to protect the sensor and help us measure the temperature accurately as inside as possible.
As thermowell makes turbulence in the fluid, vibration would be induced in the thermowell itself. The more the length of the thermowell, the higher the vibration. If the induced vibration overlaps the natural frequency of the thermowell, The resonate is the result and the thermowell will fail. If the vibration is close to the natural frequency, fatigue would happen. The latter is called Flow Induced Vibration (FIV) damage mechanism. find more information on API RP 571.
As you see above, by the time flow passes the thermowell, vortices are made. The picture below also shows the the temperature gauge assemblies.
Thermowell is the main source of fatigue in the TG/TTs. It activate the Flow Induced Vibration (FIV) damage mechanism.
The detailed method for designing the length of thermowell is described in ASME PTC 19.3TW. We also developed Finite Element Method (FEM) to verify the results. Both of them show that the length of thermowell was not proper for the designated system. The picture below, shows the FEM analysis of the FIV in the system.
The through information of this article will soon be published in a journal. Share if you have similar experience of failure.