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The boiler tube failure (BTF) problem still ranks as the number one equipment problem in fossil power plants. According to the North American Electric Reliability Council (Ref. 1), BTF has been in this position in North America for the 28 years that reliability statistics have been kept. Currently BTF represents about 3-4% of lost availability.
Previous analysis (Ref. 2) has indicated that a disturbing feature of these failures is that most of them are repeats. They occur on the same tube, same material, or in the same boiler section over and over again.
This implies that the root cause of the initial failure is infrequently determined. Without a proper understanding of both the mechanism of failure and the root cause, it is not possible to apply permanent engineered solutions.
In fact, in many cases palliative solutions ("the quick fix") are applied, which either require repetitive maintenance actions or the tubes fail again after a certain time interval. Good examples here are the use of coatings and pad welding to repair erosion damage and not removing a high pressure boiler from service to chemically clean it after a serious (low pH) excursion.
Another reason why repeat failures keep occurring has been the lack of statistical documentation to the root cause level.
Over the last 10 years major efforts have been expended to understand the mechanisms of BTF and to determine possible root causes. The initial steps were taken in the early 1980s with the publication of two BTF manuals (Ref. 3 and 4). These manuals delineated the 22 BTFs known at that time (Table 1). (Table 1 omitted)
It was also determined at that time that three of the failure mechanisms were insufficiently understood to enable utilities to apply permanent solutions. These mechanisms were: corrosion fatigue tube failures, which initiate from the waterside of waterwall and economizer tubing; circumferential cracking, which initiates from the fireside of waterwall tubing in the highest heat flux regions of supercritical units; and flyash erosion failures.
Since then much research has been undertaken to almost fully understand these three mechanisms and they will be discussed.
The corollary was that the other 19 failure mechanisms were understood and permanent solutions existed for each. Again analysis indicated that the barrier to achieving...