By Lindsay Morris, ASSOCIATE EDITOR
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 in place, utilities can ensure that their facilities will deliver as much reliable power to the grid as possible.
Many companies are zooming out to view steam turbine upgrades in light of the entire facility. Efficiency increases are always welcomed, said Charlie Athanasia, vice president of thermal services, North America for Alstom. However, an upgrade should not be simply focused 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 in order 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.
Alstom also increased the blade length on North Anna's LP rotors from 48 to 57 inches in order to maximize energy capture from the steam flow."The advent of computational fluid dynamics has allowed us to accelerate our technology and blades," said Charlie Athanasia, vice president of thermal services, North America."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, they are each rated at 838 net MW.
While the efficiency increase is welcomed, 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 in order to improve the entire plant performance, such as boiler modifications and emissions control technology retrofits, Bailey said.
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 that the steam path sections are optimized to accommodate specific application and conditions for each power plant, he said, and an additional consideration for efficiency improvements is the implementation of advanced sealing to minimize losses associated with steam leakage.
|A 500 MW class low pressure rotor with 40"- steel Hitachi last stage continuous cover blades. Photo courtesy MD&A Hitachi.|
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; while plants that have not been well maintained will experience more noticeable benefits, Bailey said (see Fig. 1 on pg. 100).
"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 examples, many older units experience performance losses as a result of changes in nozzle
|A steam turbine is transferred at North Anna Power Station in Louisa County, Va., which recently underwent a steam turbine upgrade performed by Alstom. Photo courtesy Alstom.|
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 been operating over 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 Fig. 2). For a typical unit, the forced outage rate can increase by 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.
|Laser work is performed on a casing during the steam turbine upgrade at North Anna. Photo courtesy Alstom.|
Additionally, a steam turbine may need to be upgraded to compliment changes in plant or process parameters, such as new throttle, extraction, admission and/or exhaust conditions. Plant changes may require the turbine to be redesigned or re-rated to accommodate these changes. The turbine may 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.
Often times, utilities can realize a much higher overall benefit to power plant retrofits when steam turbine upgrades are integrated into a project. Emissions control retrofits and steam turbine upgrades usually go hand-in-hand for plants in the U.S., said Molina."A number of older plants did not address the need for Air Quality Control Systems (AQCS) at initial design and construction. 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."
|Partially assembled 500 MW class low pressure section retrofit. Photo courtesy MD&A Hitachi.|
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 a reduction in their plants' net output."It requires more megawatts to feed all of this additional equipment to reduce emissions," Bailey said. Therefore, many utilities have chosen a steam turbine upgrade on the back-end of their emissions control installations in order to bring the power plant back to net output capacity or even higher.
"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.
|900 MW class lower-half high pressure section inner shell. Photo courtesy MD&A Hitachi.|
With each upgrade comes an added cost. However, Alstom 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 of time."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. Therefore, options for both new and/or 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.
While coal-fired power may not be able to compete with $2/MMBtu natural gas prices, assuming natural gas remains at these levels, utilities have a number of options when it comes to improving their existing coal generation."Improving efficiency is the overriding benefit yielded from completion of a steam turbine retrofit," Molina said.
Additionally, 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, both nuclear and coal-fired facilities can gain significant results in efficiency and reliability.