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Thorough visual checks also were performed for (i) contact areas (ii) clearance (iii) run out, and dye penetrant tests were used to determine whether any types of cracks were present.
Valve inspection observations
Main stop and control valves
The main stop valve strainer had foreign particles stuck in it. All valve parts exposed to steam flow were heavily coated with blue blush material. Products of exfoliation from the boiler were clearly visible on valve parts. Valve bushings were honed out until custom made try bars went in perfectly.
The stem back seat and the valve disk back seats were found to be in poor condition because of blue blush and were ground out and cleaned to acceptable status. The pilot valve disc was found to be pitted from impact damage. Non-destructive examination (NDE) work on all valve parts was found to be satisfactory.
When the control valve was being inspected it was found that the connector pin was stuck and it had to be drilled out and replaced. The disc to stem pin also was drilled out and
Figures 3-6 show photos of the valves at various stages of the inspection. Figure 5 shows the reheat stop valve disc and stem, and also the strainer basket. The blue blush deposit is very clear on the reheat stem and disc. The strainer shows oxide material and blue blush.
Like the MSV/CV, the reheat stop and intercept valves had excessive blue blush on them. The disc nut pin was found to be stuck, as well as the disc to stem, which also were stuck and required more than 50 tons of force to separate them. The stem back seat had large amounts of impact damage to it. The seal head back seat was found to be cracked. The dimensional readings taken were found to be within acceptable limits. All blue blush material had to be cleaned out as recommended – primarily by carefully scraping it off in a machine shop. The reheat intercept valve had similar problems observed and also on the reheat stop valve; stuck connector pin, the disc to stem was stuck and excess blue blush on valve parts were present. The seal weld on the main chest valve seat area was found to be cracked all the way around. It was ground off and a new weld was applied. The success of this valve inspection lay in the fact that critical dimensions of the valves were maintained as far as internal and external diameters of valve parts, runouts, clearances, backseats, try bar checks, and all expected clearances were in acceptable ranges.
Afterward, the mechanical inspection valves were reassembled as recommended by the OEM. The valve actuators were reconnected to the valve stems. The valves were calibrated to make sure that they were still within their recommended stroke movement at various levels of openings. The valve calibration was very successful and came in as expected. After plant start-up, the valves were checked to see if they were performing as expected and they passed their expected duty.
Despite the fact that the steam valves at the TS Power plant had only been in service for a relatively short time, the extent of blue blush coatings on valve parts was alarming. It also was obvious that boiler exfoliation had significantly affected these valves because of the presence of magnetite material on strainer baskets and other valve parts. Most plants that utilize austenitic steels and operate above 1,005°F also risk this kind of exposure to steam turbine valves and the turbine itself. The problems of blue blush or anything deposited on valve parts will cause them to stick, and therefore will not be able to close completely following a turbine trip, which could result in an overspeed event.