The unavailability of coal fired electrical generating capacity when needed is often attributable to corrosion in steam generators. Such problems are becoming more widespread as electricity demand grows faster than new plants come online, forcing greater use of old plants. Identifying such corrosion problems before they force boiler shutdowns requires proper inspection tools and techniques. These enable inspectors to take the guesswork out of whether boiler tubes should be repaired or replaced.
The goal of today?s boiler inspection programs is to increase the availability, efficiency, and reliability of the existing power plants and ensure safe and cost effective operation. Power generators are extending the time period between unit shutdowns which makes scheduled outages even more important to completely examine the boiler in order to operate until the next scheduled outage. As the United States utility boiler fleet ages, and the time periods between shutdowns increase, the proper inspection tools and techniques needed to find, observe, and document current and possible failures are paramount. Before large and cumbersome inspection rigs are assembled for inspecting large areas in the firebox, it is the job of the inspectors to inspect all surfaces and focus the inspection rigs on the degraded areas. The following paper outlines the tools and techniques needed for properly conducting and documenting a boiler inspection. The degradation mechanisms will first be introduced, and then the techniques and tools used to find each mechanism will be discussed.
Degradation Mechanisms Observed Using the Proper Inspection Techniques in Utility Boilers
Long Term Overheating: The steam temperature always varies some from individual tube to tube, and the boiler design allows for this variability. However, when the range of temperatures is larger than anticipated, the hottest tubes fail sooner than expected. A more likely cause of premature failure is the slow increase in tube-metal temperatures due to the formation of the steam-side scale. Steam reacts with steel to form iron oxide along the ID surface of the tube. In any event, the thermal conductivity of the steam side scale is about 5% of the thermal conductivity of the steel tube. Thus, an effective insulating layer forms and prevents proper cooling of the tube metal by the steam. The net effect of the scale is to raise the tube metal temperature. Depending on the scale thickness, which is dependent on the time and temperature of operation, tube-metal temperature increases of 25 to 75°F (14 to 42°C) are likely. The presence of scale can increase tube metal temperatures beyond the safe design range. These elevated temperatures result in increased creep deformation rates, more rapid oxidation and corrosion (thinner walls and higher stress) and hasten the onset of creep failures.
Creep failures are characterized by:
1. Bulging or blisters in the tube.
2. Thick-edged fractures often with very little obvious ductility.
3. Longitudinal "stress cracks" in either or both ID and OD oxide scales.
4. External or internal oxide-scale thicknesses that suggest higher-than expected temperatures.
5. Microstructure has intergranular voids and cracks.
In steam touched tubes, often the very first sign of creep damage is longitudinal cracks in the steam-side scale. As creep deformation expands the tube diameter, the brittle ID scale cannot follow the expansion. Cracks develop in the direction, which is perpendicular to the principle hoop stress. With time, the tube continues to expand, and these cracks widen. This wide crack shortens the path from steam to steel; iron oxide forms preferen
