Pressure pulses aid steam generator cleaning
Virginia Power`s Surry units successfully used a combination of pressure pulses and chemical solvent to clean steam generators more effectively
By Jonathan Barkich and Phillip Battaglia, Westinghouse Electric Corp.
The Electric Power Research Institute Steam Generator Owner`s Group (EPRI SGOG) has been researching secondary-side chemical cleaning in an effort to reduce maintenance and outage costs at nuclear power plants. It identified a group of chemicals that proved to be effective, but prolonged exposure to high concentrations of these chemicals could result in excessive corrosion of steam generator components. A new, unique approach to steam generator cleaning combines the advantages of EPRI`s chemical cleaning formulation with the benefits of Westinghouse`s patented pressure pulse cleaning (PPC) techniques. This combination decreases the corrosive effect of the process while it increases the cleaning efficiency.
Westinghouse Nuclear Services Division developed the patented PPC as part of an intensified, integrated outage program designed to reduce maintenance and outage costs while increasing the overall availability of nuclear power plants. The process, called pressure pulse chemical cleaning (PP/CC), was used successfully at Virginia Power`s Surry Units 1 and 2. The process removed more than 38,000 lbs. of deposits from both sites, with lower chemical and waste processing costs and less structural corrosion. An extensive video inspection conducted before, during and after completion of the process confirmed the success of the operation.
Westinghouse first used the PP/CC process on the Surry Unit 2 steam generators (Westinghouse model 51F, 3 loop). After 14 years of operation, the plant could only operate at approximately 85 percent of rated power because water level fluctuations in steam generator C required power reduction. Thermal-hydraulic analysis attributed the water level oscillations to the accumulation of corrosion products in the quatrefoil support plates, resulting in an excessive pressure drop. Engineers confirmed this blockage of the upper-support plate quatrefoil flow holes through a specially designed visual inspection technique (steam generator in-bundle visual inspection) developed by Westinghouse and R. Brooks Associates.
For the 14 years the Surry Unit 2 steam generators were in service, operators performed tubesheet cleaning on a regular basis. However, they made no attempt to remove the deposits located in the upper bundle region. Initially, operators used the traditional PPC technique, mechanical cleaning without chemicals, in steam generator C in an effort to dislodge the corrosion products from the support plates. The PPC process is Westinghouse`s preventive mechanical cleaning process for the upper bundle region. It is the upper bundle region`s equivalent to sludge lancing the tubesheet.
The PPC process succeeded in returning the plant to 100 percent rated power for only six weeks, after which the problem reappeared. In June 1994, a Westinghouse service team cleaned the three Surry Unit 2 steam generators–two in parallel and one in sequence–using the PP/CC process.
Six months later, the Surry Unit 1 began showing similar steam generator water level fluctuations. Preparations for PP/CC were made immediately, and the unit was chemically cleaned in December 1994. The success criteria were to return both units to full-power operation following the completion of PP/CC with continued long-term, full-power operation.
PPC is a patented Westinghouse service wherein nitrogen is injected into the steam generator through specially designed pulsers and nozzles mounted on each tubelane inspection port. PP/CC combines this process with the EPRI SGOG chemical cleaning solvents. Pulsers automatically fire (release the nitrogen) simultaneously every 10 seconds during the entire chemical cleaning fill, hold and drain cycles. Firing pressures range between 400 psig at mid-fill to about 800 psig at full range of fill (Figure 1). The pulsed solvent solution weakens and mechanically dislodges the sludge that either dissolves into the solution or is removed by on-line filtration.
Solvents readily dissolve the magnetite. Bubbles from the injected pressurized nitrogen rise through the tube bundle, loosening and washing away the more adherent deposits from free tube surfaces and crevice areas to the tubesheet. Sludge lancing then removes this loosened sludge.
The Surry process
Engineers used identical process specifications for both Surry units. The process removed copper (between 10 and 30 percent) from the sludge in two stages–one before and the other after the iron removal stage. Normal EPRI solvent formulations and temperatures were used at a reduced-EDTA (ethylene-diamine-tetra-acetic acid) concentration of 15 percent in the iron removal stage. The copper removal stages lasted six hours, and the iron stage lasted 40 hours. Rinses (pH >9) were performed between each major sludge removal step.
Table 1 shows that station personnel removed 20,630 lbs. of sludge from Unit 1 and 17,422 lbs. of sludge from Unit 2. Thus, an average of 6,342 lbs. of magnetite was removed from each of the Surry steam generators. A total of 15 percent of the solids removed from Unit 1 steam generators and 16 percent removed from Unit 2 steam generators was extracted during sludge lancing following PP/CC. Chemical analyses showed the remaining undissolved sludge to be primarily magnetite, devoid of copper.
The removal of large quantities of potentially soluble materials by sludge lancing shows the beneficial effect of the mechanical (pressure pulsing) part of the process. The pulsing enhances the removal of loosened sludge material from the steam generator tubes and crevices, and this material is both solubilized and mechanically dislodged. Thus, PP/CC requires lower EDTA concentration for a given sludge burden. The lower EDTA concentration decreases process corrosion rates.
The chemically enhanced pressure pulse process used solvent more efficiently–Westinghouse used 60,000 lbs. less V665, the main ingredient in the cleaning solvents, than the EPRI process alone would have used. Lower volumes of solvent equaled lower chemical and waste disposal costs for Virginia Power. For those processes where complete dissolution of magnetite was necessary for complete cleaning, 17 to 19 percent EDTA would have been necessary, resulting in increased chemical costs, corrosion rates and waste processing costs.
Westinghouse used on-line corrosion rate measurement in conjunction with the PP/CC process at both Surry units. The technicians placed probes in the steam generators during PP/CC where hand-hole space was available. They measured linear polarization resistance, zero resistance ammetry and electronic noise. Typical model F steam generator ferrous materials and weldments (A533-A533) were especially ob served during the process. For the 40-hour duration of the iron-removal step, with a 15 percent EDTA concentration, engineers identified no corrosion-related phenomena and no heat-affected zone (HAZ) corrosion.
Return to full power
After completion of PP/CC at each plant, the units were returned to full-power operation and continue to operate successfully in that mode. R. Brooks Associates, specialists in steam generator inspection technology, conducted extensive video inspections before, during and after the chemical cleaning at all elevations in the steam generator. The inspection results showed that both the quatrefoil and land area of the crevices were cleaned by the process. Overall, the videotapes revealed that the Surry steam generators were cleaned of sludge and deposits that existed in the quatrefoil areas of the support plates.
At the Surry Units 1 and 2, Westinghouse successfully applied a new steam generator chemical cleaning process that combines pressure pulsing with the EPRI-developed chemical cleaning solvents. The PP/CC process uses approximately 20 percent less chemical than other cleaning techniques. It contributed to a five-day reduction in Virginia Power`s scheduled outage and an additional 44-hour reduction in the actual outage. According to Michael R. Kansler, Surry 1 station manager, “Considering the short lead time, the complexity of the project and the fact that this was a first for Westinghouse steam generators, this was truly an outstanding accomplishment.”
A total of 20,630 lbs. of sludge was removed from Unit 1 steam generators and 17,422 lbs. of sludge was removed from Unit 2 steam generators through the use of PP/CC. Of this, post-cleaning sludge lancing removed 15 to 16 percent of the total magnetite inventory. This was a result of the combination of cleaning solvents and pressure pulsing that fluidized sludge on the tube surface and crevice areas. The fluidized sludge then dissolved or accumulated as solid material on top of the tubesheet for easy removal during sludge lancing. The direct benefit of using the PP/CC process was to return the plant to full-power operation. Other benefits included lower chemical and waste-processing costs and less structural corrosion.
In-situ corrosion monitoring of typical model F steam generator materials showed that after 40 hours of 15 percent EDTA pressure pulsing there was no HAZ pitting to the A533/A533 weldments.
Dave Christian, Surry 2 station manager, described the project as “… an outstanding accomplishment … Most importantly, the goal was achieved, and the steam generators are performing beautifully.” The Surry units returned to full-power operation following the PP/CC. Video inspection following the PP/CC process confirmed that both the quatrefoil and land area of the crevices were cleaned (Figure 2). z
Jonathan Barkich is manager, chemistry & materials, NSD, Westinghouse Electric Corp.
Phillip Battaglia is supervisory engineer, NSD, Westinghouse Electric Corp.
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