Diagnosis of HRSG tube failure causes is not always straightforward. Sometimes, a relatively clear-cut cause such as a gross weld defect or classic tube wall thinning by internal corrosion or a corrosion fatigue crack at a tube-to-header weld joint makes the job easy. But often, the actual cause of damage is not easily identified by visual examination without removing the damaged section of tube. In some instances, such as failures at the upper tube bends, it is very difficult to view the failure location because it is behind other rows of tubes, etc.
The location of the failure can be classed as:
- Weld: Either tube to tube, tube to header, or seam weld in tube
- Bend: A bend in the tube body, usually a 30-45° or 180° bend
- Body: The straight portion of tube, finned or bare.
Once the tube failure is located, visual inspection will show the type of failure. Typically these are classed as:
From design data the material of the tube is obtained. This may have to be confirmed by testing if there is any doubt that the correct material was used. The next step is to classify the operating environment of the tube at the tube location to aid in diagnosis. These factors include:
- Thermal Conditions
- Chemical Factors
- Mechanical Factors
Consideration of the operating environment and history aids in developing a root cause. All these can be evaluated without removing tube samples for metallurgical and chemical analysis. However, confirmation of the root cause is often not possible without a detailed analysis of the failed material itself. This would include chemical analysis of the tube and any corrosion products or deposits, microscopic inspection (optical and electron microscope) of the failure area, and other tests.