Metallurgical Lab:

Case Study – Thermowell Failure Analysis
Authors:

Structural Integrity (SI) was recently asked to examine a fractured thermowell and determine the damage mechanism.  The thermowell had been removed from bypass line piping in a heat-recovery steam generator (HRSG) that ran from the High Pressure (HP) bypass valve to the cold reheat section, and sent to the SI Materials Science Center. As reported by plant personnel, the fracture was located within the pipe wall. The pipe material was specified as ASME SA-335, Grade P22, and the thermowell was specified to be ASME SA-182, Grade F22.

Examination Procedure and Results

The fractured thermowell sections were visually examined and photographed in the as-received condition, as shown in Figure 1. The thermowell was comprised of two pieces: the thermowell housing itself which protruded into the steam stream, and a fitting connection to the pipe into which the thermowell housing was inserted.  The fitting was fillet-welded to the pipe.  Areas on the fitting and the housing were analyzed using X-ray fluorescence spectroscopy (also known as PMI), and the nominal compositions of both components were consistent with Type 316 stainless steel.

FIGURE 1. The fractured thermowell sections shown in the as-received condition. The mating fracture surfaces are facing down on each section.

The thermowell housing had fractured below the fillet weld at a diameter transition, reportedly within the opening in the pipe wall. In addition to the fracture across the thermowell housing, the fillet weld was partially fractured and contained a crack that was visible on the surface that had been cut for sample removal. These features are shown in Figure 2.

FIGURE 2. Overview of the fractured end of the thermowell housing looking toward the fitting and the fractured fillet weld joining the fitting to the pipe.

Figure 3 shows the thermowell housing fracture surface, which exhibited crack progression markings (“beach marks”), indicative of high-cycle fatigue crack growth, across the fracture surface and around the center hole. The fatigue crack origin area was located at the external surface of the thermowell housing adjacent to the weld buildup. The area of final overload was associated with the center hole in the thermowell housing; this hole was rounded and enlarged at the fracture plane, indicating movement of the housing relative to the thermocouple element that ran through the center hole. The center hole remote from the fracture did not exhibit similar damage. The housing fracture origin area exhibited relatively severe secondary mechanical damage.

FIGURE 3. The thermowell fracture surface on the thermowell fitting section. Beach marks (white arrows) across the fracture surface are evident. The beach marks show the direction of fatigue crack propagation, which goes around the center hole.

The section of the thermowell containing the housing and fillet weld fractures was cross-sectioned through the housing fracture origin area. The cross-section was mounted, prepared for metallographic examination using standard laboratory techniques, and examined using a metallurgical microscope for evaluation of the housing and fillet weld fracture morphologies. The prepared sample is shown in Figure 4. The length of the housing contained in the metallographic mount had weld buildup around its outer surface, and the fillet weld was connected to this weld buildup.

FIGURE 4. The prepared cross-section through the fracture on the fitting side. The heavy black arrow indicates the direction of fatigue crack propagation. Note that the pipe wall would be at the top of this image.

Cross-sectional views of the thermowell housing fracture are shown in Figures 5 and 6.  The fracture was relatively smooth and flat, which is consistent with fatigue. No secondary cracks or corrosion were observed at the origin area; however, small, secondary cracks were present along the remainder of the fracture surface. These secondary cracks were more numerous and larger on the side of the fracture opposite the origin area.The secondary cracks had an appearance consistent with stress corrosion cracking (SCC).

FIGURE 5. An overview of the fatigue fracture origin area. The fracture across the thermowell housing surface was relatively flat and smooth. The arrow indicates the direction of fatigue crack propagation.

Secondary branched cracks were also present in the fillet weld at the locations indicated in Figure 5 (page 17); an example of the fillet weld cracking is shown in Figure 7. The branched cracks propagated across the dendrites of the weld structure.

Figure 6. The small, secondary branched cracks that were present along the fracture surface outside of the origin.

Following the metallographic examination, the deposits on a portion of the thermowell housing and fillet weld fracture surfaces were analyzed in a scanning electron microscope (SEM) using energy dispersive X-ray spectroscopy (EDS) to identify the elements present. Potentially corrosive contaminants identified on the fractures surfaces included chlorine (chlorides) and sodium (sodium hydroxide). While both constituents were present in minor amounts, chlorine was present in more of the areas examined.

Figure 7. An example of the branched cracking emanating from the fillet weld fracture

Discussion

The thermowell failure was due to high-cycle fatigue. The thermowell housing clearly experienced fatigue crack propagation as indicated by the presence of beach marks across the fracture surface. However, secondary branched cracks were present along the housing fracture surface and within the fillet weld. The fatigue crack appears to have been the primary damage mechanism of the thermowell housing: no evidence of SCC was observed at the fatigue origin area, and the secondary branched cracks were larger and more numerous on the side of the fracture opposite the origin. The presence of beach marks across the thermowell housing with a relatively small area of final overload indicate the actual fracture was a fatigue rather than SCC failure. The fact that the center hole in the fractured area had enlarged also indicates there was movement of the thermowell relative to the thermocouple contained in it, and this movement is another indication of fatigue. The presence of SCC could have aggravated and accelerated the fatigue cracking. The SCC in the fillet weld would have been the cause of any steam leaks that occurred.

The scope of this analysis was limited to determining the damage mechanism and did not include modeling the thermowell to determine the root cause.  It is worth noting, however, that avoiding fatigue failures due to flow-induced vibration was the driving force to updating the thermowell design code in ASME PTC 19.3 TW-2010.  Fatigue caused by flow-induced vibration is typically affected by the following thermowell parameters: shank radius (the fatigue crack in this sample initiated from this radius), wall thickness of the shank, unsupported length of the shank (distance into the pipe and fluid flow), root and tip diameter of the shank, maximum allowable stress, and fatigue endurance limit. The fluid velocity also has a significant effect on flow-induced vibration. In this case the hexagonal thermowell housing was modified to fit into the round port in the piping, as indicated by the weld buildup on the outer surface of the housing.  Many of these factors are addressed in the new code, so if it had been designed and installed under the new code, some of the parameters that likely contributed to the fatigue damage may have been different.

According to ASME, Key enhancements over the 1974 edition include:

  • Expanded coverage for thermowell geometry;
  • Natural frequency correction factors for mounting compliance, added fluid mass, and sensor mass;
  • Consideration for partial shielding from flow,
  • Intrinsic thermowell damping;
  • Steady state and dynamic stress evaluations;
  • Improved allowable fatigue limit definition.

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