In today’s electricity generating environment, new demands are being placed on fossil-fueled boilers, including operating in cycling modes for which the boilers were not originally designed. In a growing number of cases, these changes are resulting in boiler tube failures. To avoid such failures, regular inspections with the latest examination tools are becoming increasingly critical for identifying areas of damage that can be repaired before they lead to an unscheduled outage.
The most common failure mechanism is corrosion fatigue, a cracking mechanism in fossil-fueled generating plants that stems from a combination of thermal cycling and water chemistry at high-stress areas. One of the largest contributors to forced plant outages, corrosion fatigue predominately occurs in the waterwalls and economizers of subcritical plants. Cracking attributed to corrosion fatigue has occurred in waterwall tubing of both membrane and tangential designs.
Cracking (particularly in membrane waterwall tubing) presents a challenge for conventional nondestructive evaluation (NDE) because the condition manifests itself as multiple cracks that initiate and grow on the inside surface of the tubes and on the insulated side of the tube—locations not usually accessible due to the presence of insulation, building structural components, and exterior building covering.
A typical location that may experience corrosion fatigue is shown in Figure 1. Notice the presence of the scallop bar and casing structure, which limits the use of conventional NDE, either ultrasonic or radiographic. The outer building structure (not shown in Figure 1) is farther from the waterwall tubes.
1. Tough spot. Here is one location where corrosion fatigue failures can occur on boiler waterwall tubes. Courtesy: EPRI
Nondestructive evaluation techniques
Today the most common types of volumetric examination techniques in power plant environments are ultrasonic and radiographic.
Ultrasonic examination involves the introduction of high-frequency sound waves into a component for the purpose of determining some characteristic of the material, such as flaws or thickness. However, to date, ultrasonic examination has not been successfully applied to detect corrosion fatigue cracking from the fireside of boiler waterwall tubes, though progress is being made by EPRI with the use of ultrasonic phased array probes.
Conventional film radiography has long been a primary weld examination technique and has been accepted as a valid means of crack detection for many years. This technique passes radiation through the component and records the exiting backside radiation by either photographic or electronic means. The radiation may be produced either by a radioactive source (gamma) or by an x-ray machine.
The advantages of film radiography are that it provides a permanent film record of each examination for archiving and later review, and it is the ASME Code–approved method for examining numerous types of welds. Its disadvantages are these:
- It requires significant safety requirements (the test procedure must protect personnel from radiation exposure, and radioactive sources must be handled properly by qualified personnel).
- It is nonquantitative (flaws cannot be located or sized precisely).
- Results for components with complicated, nonuniform section thickness can be difficult to interpret.
- This technique cannot offer quick results because the film needs to be placed, removed, and processed.
In addition, conventional film radiography has limitations specifically with corrosion fatigue cracking because of the loss of sensitivity when passing the radiation beam through heavy components on the outside of the boiler.
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