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Structural Integrity used the "Time of Flight Diffraction" (TOFD) technique to inspect the butt welds, of which 37 showed hydrogen damage. The tubes with hydrogen damage were replaced, as well as with some tubes that showed severe wall thinning. After this inspection, the unit ran until the next planned outage in October 2004 with only one tube failure due to hydrogen damage.
In October 2004, the Valmont Station had an outage and hard scaffolding was erected in the furnace.
TesTex inspected approximately 13,740 linear feet with LFET with the inspection focusing on the rear wall, including the bullnose and all 4 corners of the boiler. The inspection detected 87 defects and 11 of these defects showed hydrogen damage that was verified by Ultrasonic Tangential L-Wave Velocity Shift.
The inspection scope was repeated in April 2007, when the inspection found 104 defects. The Ultrasonic Tangential L-Wave Velocity Inspection identified 18 of the defects to have hydrogen damage.
The plant took the following steps to reduce boiler tube failures:
- Performed eddy current on the condenser
- Chemically cleaned the boiler
- Chemistry control
- Paid close attention to burner/fireball alignment.
- Conducted periodic inspections
The steps listed above significantly reduced the boiler tube failures. The coating of the condenser tubesheet cut down on the amount of deposits that formed in the tubes and the eddy current inspection of the condenser tubes and subsequent plugging of damaged tubes reduced contaminants in the boiler water. The chemical clean removed ID deposits in the tubes and the plant carefully observed the water chemistry. The plant also paid special attention to burners and fireball alignment to reduce hotspots on the furnace walls.
Horizontal reheater failures due to oxidation pitting
A western Pennsylvania power plant experienced boiler tube failures in the horizontal reheater section in one of their boilers. The plant has 3 identically designed 835 MW coal-fired Foster Wheeler Units.
Tube samples near the failures showed I.D. pitting approximately 3/16? in diameter on the bottom side of the tubes. Due to the design of the reheater, access to the bottom of the tubes was very limited. The tubes were 2.5? O.D., 0.180? wall thickness, with the material being SA-213T22.
Plant personnel notified TesTex and described the situation. The unit had recently experienced a few tube failures in the horizontal reheater, which were repaired and returned to operation. The plant needed a method to assess the condition on the horizontal reheater. TesTex went out to the site and looked at some drawings and tube samples. There was less than 1? of vertical space between the tubes to place a scanner. Side to side access between the pendants also was limited. The tube samples showed I.D. pitting with an approximate pit diameter of 3/16?. Some of the samples contained clusters of pits, while other samples contained an isolated pit.
The inspection required a scanner that could fit between the tubes and be able to see pits as small as 3/16? in diameter. TesTex already had a low profile scanner but the current design for the detection of a very small single pit was limited. A new LFET scanner with a double driver coil design was manufactured. The new scanner was able to see these small pits, and through the use of calibrations was able to size the defects. Several scanners were manufactured for the upcoming outage.
Once the unit was offline, the reheater section was blown down with air to remove the ash. The inspection focused on 5 different areas that were accessible where the past failures occurred. The scanners were able to inspect 3 tubes deep into the bank without having to spread the tubes.
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