Oxygen levels in just parts per billion dissolved in the feedwater stream for boilers and heat-recovery steam generators (HRSGs) can cause pitting and reduce the operating life of steam cycle components. That’s why reliable steam plant operation relies on low dissolved oxygen levels in boiler and HRSG feedwater systems to limit pitting and related corrosion damage to carbon steel components, including deaerator vessels and storage tanks, piping, and boiler tubes.
Traditionally, many power plants have relied on a deaerator vessel and storage tank to liberate dissolved oxygen in feedwater by raising its temperature by direct injection of saturated steam. Usually, the steam is provided by an extraction line or by dedicated supplies from a low-pressure source such as the low-pressure (LP) drum in a combined-cycle plant.
The deaerator removes oxygen just prior to feedwater entering the boiler economizer section of the HRSG, making conditions near the optimum (approximately 300F) for flow accelerated corrosion (FAC) damage to carbon steel components. Components with a change in flow direction--such as upper tube bends, piping elbows, and high-fluid-velocity regions in deaerator vessels--are especially vulnerable. Also, because operating pressures in deaerators are typically slightly above atmospheric, the vessel shell and head thicknesses required by ASME Code are relatively thin compared with those of higher-pressure vessels such as steam drums. If protective magnetite layers are damaged, a rapid wall thinning can occur in areas that are exposed to high local velocities under adverse water chemistry.
These corrosion mechanisms are well known to the industry, so it is incumbent on plant management to assess the rates of degradation and take actions to correct water chemistry deficiencies that will accelerate FAC, among other failure mechanisms, and to project conservative estimates of remaining equipment life.
The following case study details a recent incident in which FAC damage rapidly degraded the high-pressure (HP) deaerator vessel shell and how management responded to effectively maintain reliability and protect personnel safety.
Isolated plant
The Beluga Power Plant (Figure 1) operates in perhaps the most difficult conditions encountered by a combined-cycle power plant in North America: It is located about 40 miles due west of Anchorage and is accessible only by barge or airplane. POWER profiled the plant’s technical details (March 2006) in a report describing Beluga’s first 25 years of operation and specific measures taken to ensure reliable operation. The plant is owned and operated by Chugach Electric Association (CEA), which is headquartered in Anchorage and is the largest electric utility in Alaska.
1. Wilderness outpost. The 210-MW, 2 x 1 combined-cycle Beluga Power Plant, located 40 miles west of Anchorage, supplies power to the surrounding area. Courtesy: Chugach Electric Association Inc.
More than half of CEA’s thermal generation is produced by its gas turbine fleet operating in simple-cycle operation; the remainder is generated by Beluga Power Plant’s two ABB-11DM gas turbines, which operate in a 2 x 1 combined-cycle configuration with vintage Babcock & Wilcox HRSGs supplying the single-pressure BBC steam turbine. HRSG bypass stacks allow simple-cycle operation with either HRSG out of service. Total power produced by the plant is approximately 210 MW.
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