Failure Analysis of TG /TT (Temperature Gauge/Transmitter) in a Gas Refinery (Part 1)
Saeed Moshayedi
Deputy Head Of Inspection Department at 6th Refinery of South Pars Gas Complex
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.
Vibration & Integrity Engineering
4 年Neil good article I performed some tests on some thermowells in situ at UK gas plant about 18 years ago. I bonded strain gauges inside the thermowell bore at the location of the external fillet radius and we ran a flow rate trial and the results were very good with the FEA predictions. We managed to give the client an acceptable flow range to operate in as they required a shut down to remove the thermowell.
Consultancy & Training for Pipework Failure Prevention
4 年Good article Saeed. The problem is all about resonance caused frequency matching between in-line and transverse vortex forces and the thermowell natural frequency. The vortex frequency is very dependant on flow velocity and thermowell probe diameter. The NF is very dependant on TW length and diameter. Ideally these will be selected to ensure resonance will never occur, at least at design flow rates. Without physically changing the TW, the only real mitigation is to avoid critical flow velocities, at least until the next maintenance opportunity.
Section Head-Inspection, Asset Integrity at RIL,Ex-HMEL, Certified API571,API 510, API 570, API 653.NDT -LV-II RTFI and UT C&I engineer.
4 年Hope restraints are provided at this location.
Advanced Project Completions Engineer / Quality Manager / Quality & Risk Engineer / Quality Advisor / Quality Lead / ISO Auditor with over 20 years experience. Flexible to both working location and rotation.
4 年Excellent post, I have reviewed similar case studies. Very interesting article and something that needs to be reviewed for those in service but more importantly reviewed by engineering in the design phase as a lesson learnt for a preventative action.
COSCO (Lianyungang) Liquid Loading & Unloading Equipment Co., Ltd - QA
4 年Your writing is very professional and very good