by Lindsay Morris, Power Engineering
The push for greater efficiency and capacity often drives steam turbine upgrades and the development of new turbine features. Steam turbines used at coal and nuclear power plants require regular upkeep and scheduled maintenance. With a detailed, long-term steam turbine maintenance plan, utilities can ensure their facilities will deliver as much reliable power to the grid as possible.
|A steam turbine is transferred at North Anna Power Station in Louisa County, Va., which recently underwent a steam turbine upgrade performed by Alstom. Courtesy: Alstom.|
Many companies are zooming out to view steam turbine upgrades in light of their entire facilities. Efficiency increases always are welcomed, said Charlie Athanasia, vice president of thermal services, North America for Alstom. An upgrade, however, should not focus simply on ramping up the turbine, but rather optimizing the entire design system, including balance of plant, he said.
In April, Alstom completed a steam turbine upgrade at Dominion Power’s two-unit, 1,863-MW North Anna Power Station in Louisa County, Va. The project entailed a new high-pressure (HP) and two new low-pressure (LP) rotors for each nuclear generating unit to enable the 140-ton units to handle increased steam output. The rotors installed were among the first produced at Alstom’s new Chattanooga, Tenn., turbomachine manufacturing facility.
|A 500-MW-class low-pressure rotor with 40-inch steel Hitachi last-stage continuous cover blades. Courtesy: MD&A Hitachi.|
Alstom also increased the blade length on North Anna’s LP rotors from 48 to 57 inches to maximize energy capture from the steam flow.
“The advent of computational fluid dynamics has allowed us to accelerate our technology and blades,” Athanasia said. “That not only allows for more efficiency and more power outfit, it prolongs the life.”
Alstom’s retrofit work at North Anna Units 1 and 2 resulted in a power output capacity increase of 60 MW per unit. Previous to the North Anna upgrade, Alstom completed a similar project at the Surry Power Station in southeastern Virginia. The uprate was completed in June 2011 for Surry Unit 2, and the Surry 1 uprate was completed in December 2010. Prior to the uprates, each Surry unit was rated at 799 net MW. After the uprates, each is rated at 838 net MW.
The efficiency increase is welcomed, but Alstom’s focus during its steam turbine upgrades is not simply on ramping up the turbine; but rather optimizing the entire shaft line and accessory system configuration, often potentially including balance of plant, Athanasia said.
When conducting a steam turbine upgrade, General Electric (GE) Energy sees improvements in the range of 5 to 8 percent, said Trevor Bailey, general manager of steam products.
In addition to upgrading the turbine with modern technology, complementary upgrades can be executed to improve the entire plant performance, such as boiler modifications and emissions control technology retrofits, Bailey said.
|Laser work is performed on a casing during the steam turbine upgrade at North Anna. Photo courtesy Alstom.|
When performing a steam turbine retrofit or upgrade, the primary return on investment results from the replacement of the steam path to improve efficiency and output, said Leo Molina, vice president of retrofits for MD&A Hitachi. It is critical to ensure the steam path sections are optimized to accommodate specific application and conditions for each power plant, he said, and another consideration for efficiency improvements is the implementation of advanced sealing to minimize losses associated with steam leakage.
It can be difficult to gauge how much of an efficiency and reliability increase any given plant will gain from a steam turbine upgrade, Bailey said. End results will “depend on the start-up position, the condition of the plant and how well it’s been maintained for the first 20 to 25 years of its life.”
Plants that have been very well-maintained will see “marginal improvements” as a result of a steam turbine upgrade; plants that have not been well-maintained will experience more noticeable benefits, Bailey said (see Figure 1).
“You’re typically upgrading power plants that are 20 to 30 years old, and the reliability improvements from a retrofit are really all about correcting issues that have emerged over the first half of the life of the equipment,” Bailey said.
For example, many older units experience performance losses as a result of changes in nozzle and bucket throat areas and blade profiles, as well as deterioration of finishes and increased clearances. By rebuilding the turbine with more modern, high-efficiency components, upgrades can recover degradation. For example, on a unit that has operated more 30 years, GE’s steam turbine efficiency upgrade can recover 10 percent degradation and increase performance 5 percent beyond its original level.
In addition, a steam turbine upgrade can help aid overall plant reliability. Steam turbine reliability decreases after 20 years of service (see Figure 2). For a typical unit, the forced outage rate can increase 3 to 4 percent every 10 years, according to GE Energy; however, steam turbine upgrades can be targeted specifically to increase reliability by replacing the steam path and installing a new generation nozzle, bucket and rotor components, thus reducing forced outage time at the facility.
In addition, a steam turbine might need to be upgraded to complement changes in plant or process parameters, such as new throttle, extraction and admission, exhaust conditions or both. Plant changes might require the turbine to be redesigned or re-rated to accommodate these changes. The turbine might need to be uprated, which entails a modification of the steam path for increased power or flow, or derated, which encompasses optimizing steam path performance for reduced power or flow needs.
|A 900-MW-class lower-half high-pressure section inner shell. Photo courtesy MD&A Hitachi.|
Often, utilities realize a much higher overall benefit to power plant retrofits when steam turbine upgrades are integrated into projects. Emissions control retrofits and steam turbine upgrades usually go hand-in-hand for U.S. plants, Molina said.
“A number of older plants did not address the need for air-quality control systems (AQCS) at initial design and construction,” he said. “In doing so, now they’re recognizing it makes sense to retrofit steam turbines simultaneously with AQCS to offset auxiliary load losses from the addition of new emissions equipment.”
As coal-fired generators have been pushed by the Environmental Protection Agency to retrofit their plants with state-of-the-art emissions controls, many coal plants have witnessed reductions in their plants’ net outputs, Bailey said.
“It requires more megawatts to feed all of this additional equipment to reduce emissions,” he said.
Therefore, many utilities have chosen steam turbine upgrades on the back ends of their emissions control installations to bring the power plants back to net output capacity or even higher.
Bailey said the coal plants will be needed for a while.
“We need coal plants for some time yet, so what can we do to make those power plants as clean and efficient as possible?” Bailey said.
With each upgrade comes an added cost. Alstom, however, has developed a cost solution for its upgrades. Instead of conducting extensive turbine maintenance at one time, Alstom’s spreads out the implementation and cost of maintenance over a long period.
Athanasia said better technology equates to longer lifetimes.
“As we continue to advance technology, we look at component design options to prolong lifetime and thus outage periods,” Athanasia said. “In doing so, customers get much higher value and return on their maintenance costs.”
In addition to implementing a unique cost mechanism, Alstom is focusing much attention on lowering the costs of steam turbine upgrades in an effort to keep coal competitive with natural gas generation. Although the market is perceived “suppressed” for new steam turbines in conventional coal-fired generation, Athanasia said, the need for new gas turbines and steam turbines in combined-cycle configuration plants is increasing. Options for both new and retrofitted steam turbines must be considered.
“Alstom is looking at how to better position steam turbine technologies, application and service capabilities and capacity for what we see as a coming surge in the gas turbine-driven combined-cycle application,” Athanasia said.
Although coal-fired power might not be competitive with $2 per million British thermal unit natural gas prices, assuming natural gas remains at these levels, utilities have options when it comes to improving their existing coal generation.
|A partially assembled 500-MW-class low-pressure section retrofit. Photo courtesy MD&A Hitachi.|
Molina said that improving efficiency is the overriding benefit from a steam turbine retrofit.
In addition, steam turbine upgrades at nuclear plants, such as those performed by Alstom at North Anna and Surry, allow nuclear facilities to produce even more megawatts. By undergoing a steam turbine upgrade, nuclear and coal-fired facilities can gain significant results in efficiency and reliability.
Lindsay Morris is an associate editor for PennWell’s Power Engineering magazine. She covers issues related to EPA regulations for fossil-fired power plants, emissions control, and renewable energy. In addition, she is the co-chairperson for Penn-Well’s Coal-Gen event and the chairperson of an industrywide organization called Women in Power Generation. Prior to joining PennWell in 2010, Morris was the online editor for Oil and Gas Investor in Houston. She also worked as an online editor for Oklahoma’s state newspaper, The Oklahoman. She has a bachelor’s degree in journalism.