By Teresa Hansen, Senior Editor
Steam turbines have undergone many changes in the past 30 years. Operating experience, as well as better design tools and materials, have led to some significant improvements, especially in nuclear power plants. Many early nuclear plant turbine replacements and upgrades were performed to address maintenance and reliability issues. Most turbine upgrades today, however, are driven by operators’ quests for increased capacity, longer turbine life following license extensions and to address primary side changes.
As part of an uprate project, Alstom retrofitted three 500 MW steam turbine units at Aberthaw coal-fired power station in Wales. Photo courtesy of Alstom.
Nuclear power’s value in today’s electricity market has led many plant owners/operators to uprate their plants to generate every megawatt possible (see related story, “Nuclear Plant Uprates,” on page 33 in Power Engineering magazine’s March 2007 issue). Steam turbine upgrades play a big role in these uprates. At the same time, many owners are extending their plants’ operating licenses for another 20 years. This re-licensing trend is occurring when many of these plants’ original components and equipment are reaching their original design life. License extensions also make this a perfect time to upgrade steam turbines.
Steam Turbine History
In the early years, many power plants, especially nuclear power plants, experienced low pressure (LP) steam turbine disc cracking caused by stress corrosion. Engineers and designers determined that this disc cracking, which is a well known problem in the industry, was caused by some “shrunk-on” disc designs. James (Jay) McCracken, Siemens Power Generation’s steam turbine service engineering director, said the designers of the first 1,800 rpm turbines installed in nuclear power plants in the 1970s and 1980s based their designs on fossil turbines built and put into service in the 1950s.
“These (fossil turbines) were 3,600 rpm split train turbines with a 3,600 rpm shaft train for the HP (high pressure) and IP (intermediate pressure), and an 1,800 rpm shaft for the LPs.” Due to limitations in forging size at that time, the large 1,800 rpm LPs were typically constructed with a central shaft with individual discs shrunk on,” he said. In the nuclear steam cycle, the shrunk on designs experienced issues with stress corrosion cracking (SCC) at the interface between the disc and shaft. This had not been seen on the fossil units. “Most of the domestic OEM LP turbines had to be replaced in the ‘80s and ‘90s,” McCracken said.
Many first-generation LP turbines were replaced by second-generation models with a better design. “Most of our customers have retrofit their original rotor configuration to a monoblock to mitigate this (SCC) concern,” said Steve Pock, GE Energy’s steam turbine services product manager.
Although the replacements were made for reliability reasons, modest performance improvements also were obtained from the replaced LPs, said Bill Newsom, Mitsubishi Power Systems’ service sales and marketing director.
Casing erosion posed another serious long-term reliability issue in many original LP turbines installed in nuclear power plants. “Persistent erosion issues, including cylinder distortion-caused erosion, were widely dispersed in the steam turbine population,” Newsom said.
Because these problems were seen throughout the fleet, many of the turbines currently operating in nuclear power plants are not the originals. In fact, some plants have even replaced the replacement turbines and others are looking at doing the same, said McCracken. In the 1990s and early 2000s, as the LP rotors that were installed as replacements in the 1980s and early 1990s were inspected, stress corrosion cracking on the blade attachment areas was often found, he said. This was cause for concern over the life of the turbine because it would likely mean additional scheduled inspections and more maintenance costs than were originally planned. Therefore, many plants looked at a third LP replacement – some for turbines that were only 10 to 15 years old, he said.
While maintenance and reliability of LP turbines in nuclear power plants have been industry issues, the HP turbines have generally been reliable. Many nuclear power plants are still operating with the original HP turbines with no major problems.
Most experts agree that today’s design calculation computational tools have played a big role in turbine advancements. The technology originally used to design turbines, steam flow and related systems was primitive compared to today’s. “Features that could have been developed before were not offered because there was no way to implement the theories behind them,” said McCracken. “Computational fluid dynamics (CFD) provides the ability to take advantage of theories that were known in the past, but could not be proven to the degree necessary to offer in a customer’s unit.”
Exelon contracted with Mitsubishi in 2002 to upgrade the steam turbine in Cromby Unit 2, originally constructed in 1955. The unit burns fuel oil or natural gas. Photo courtesy of Mitsubishi Power Systems.
CFD tools have resulted in improved blading and sealing areas. In turn, these have led to greater efficiency and reduced steam losses. “Our design and modeling tools have improved over the past 30 years,” said Newsom. “In the past, we used two-dimensional flow analysis to design turbine blade paths. Today’s advanced design tools allow us to use three-dimensional flow analysis to reduce losses and improve the HP and front LP blading efficiencies. Today’s fully three-dimensional HP and front LP blading stage efficiencies have improved approximately 4 percent to 5 percent over the last 30 years,” he said.
Richard Klover, Burns & McDonnell Engineering’s associate vice president, coal projects, said that minimizing steam flow losses and leakage through better flow analysis and improved seals and diaphragms has been a major factor in improving steam turbine efficiencies.
Don Stephen, Alstom Power’s general manager, turbine retrofit, agreed. Reducing leakage loss is more than just sealing, he said. “Modern computer modeling tools have allowed design engineers to better direct steam flow so that less is subject to leakage. Alstom has made big advancements by redirecting steam flow and designing the turbines to minimize leakage and reduce dependence on sealing.”
Mitsubishi has also reduced turbine steam leakage, said Newsom. The manufacturer currently uses active clearance control seals (ACC) to reduce degradation over time. The ACCs include spring-loaded seals that hold open clearances during startup and close during base load operation. Mitsubishi also uses leaf seals to reduce leakage flows, improve performance and decrease degradation.
Another major turbine improvement seen in the past 30 years is the development of longer last-stage LP blades. Longer last-stage blades allow the turbine to operate more efficiently using the same amount of steam. This improvement has also been applied to coal-fired plants (see “Carbon Rules Could Drive Coal-fired Turbine Demand” on page 72).
Nuclear Uprates and Efficiency Increases
The efficiency increase realized from a steam turbine uprate in a nuclear power plant can vary widely, depending on the age and condition of the existing turbine, plant operating costs, plant operating conditions, the extent of changes and upgrades to the primary (steam generator) side of the plant and the extent to which the steam turbines and electrical generator are modified.
Exelon’s Quad Cities Nuclear Power Plant began an extended power uprate in 2001. The uprate included installation of this new GE Energy HP turbine. Photo courtesy of GE Energy.
“Steam turbines are optimized around a given steam flow and operating conditions,” said Pock. “Variations in steam flow result in a deviation from the optimal point. Depending on the variation there could be a significant degradation in both mechanical and thermodynamic performance. Nevertheless, this doesn’t necessarily mean that the steam turbine, nor significant components of the steam turbine, must be replaced.” Plant owners’ decisions are ultimately based on the economic value the customer places in this lost performance.
The potential for optimization from different designs can vary greatly, said Trevor Bailey, Alstom Power’s vice president, steam turbines retrofits. “Some can be huge with just a replacement, due to technology improvements alone.”
McCracken said that when comparing new components to the “as new” specifications of existing components, turbine component replacement can result in as much as a 4 percent to 5 percent performance improvement, simply due to replacement component technology improvements made over the years. The percentage can be even higher when the effects of degradation to the existing hardware are considered.
“The percentage of improvement seen with this comparison can vary a great deal based on the age and condition of the old/existing turbine,” he said.
When changes on the primay side of a nuclear power plant are made, resulting in a significant increase in turbine inlet steam conditions, an upgrade to the plant’s turbines to accommodate the change typically is necessary. This type of uprate is usually associated with nuclear plant stretch uprates and extended power uprates (see related story, “Nuclear Plant Uprates,” on page 33 in Power Engineering magazine’s March 2007 issue).
“A plant uprate typically results in more steam that drives the need to re-optimize the steam turbine around the increased flow,” said Pock. “However, numerous customers invest in uprates to extend the life of their equipment independent of incremental steam flow.”
When the steam flow to the turbine is changed, the results from a turbine uprate can vary greatly depending on the magnitude of the thermal power increase. While it is common to see a 5 percent to 12 percent increase in thermal power (megawatt-thermal, MWth), some plants have increased their thermal power by as much as 20 percent.
Most owners partner with a manufacturer and/or engineering firm to ensure the installation goes smoothly. Photo courtesy of Alstom.
“During an extended power uprate, steam conditions change so much that it is normally necessary to modify or upgrade the HP turbine. The LP turbines may need modifications, although in some cases there is sufficient margin in the original hardware,” McCracken said. Extended power uprates require thorough evaluations and may require large investments in other parts of the plant. “A turbine modification budget in an extended power uprate is typically only one-half or less of the total budget.”
Increasing megawatt-thermal input to the turbine adds between 25 and 50 megawatt electric (MWe) of capacity output (on a 1,000 MW plant) when the HP turbine is correctly designed to accommodate added steam flow, said Dave Wilburn, Mitsubishi Power Systems’ steam turbine upgrades sales manager. “The key is designing the HP to match the change in steam flow from the primary side.” If that isn’t done correctly, some of the primary-side increase can be lost through throttling across the inlet valves to the HP. Or, if the steam turbine blade path is not sized adequately, the electrical output after the primary-side upgrade can be lower than before the upgrade, Wilburn said. The combined total of increased MWth and steam turbine efficiency improvements can result in more than 100 MW of electrical power increase for just one nuclear generating unit.
Investing in turbine upgrades to gain additional capacity at existing nuclear power plants is almost always worthwhile (sources for this article report that most plants see a return-on-investment – ROI – of three to five years, with a few plants reporting an 18 month ROI), but it isn’t always easy. The upgrade typically must be completed during a refueling outage. Staging and laydown areas are often small and difficult to access. Even crane hook availability can be critical. “Usually the main crane on the turbine deck hasn’t been used so intensely since the plant was built,” McCracken said.
Workers inspect and prepare a Siemens 50Hz low pressure SST5 9000 turbine for installation in a nuclear power plant. Photo courtesy of Siemens Power Generation.
Most plant owners partner with a competent manufacturer and/or engineering firm to ensure the plant uprate goes smoothly. It isn’t uncommon for the utility engineer responsible for the turbines never to have actually seen inside the machine. “Siemens turbine inspection intervals are normally 100,000 hours of operation, which equates to about 12 years,” McCracken said. “Because the inspection intervals are so long, some engineers have never seen the inside of the turbine. This makes it difficult for them to schedule and plan outages. This is another reason that a team approach with the turbine vendor is needed.”
Other challenges include supplying and staging the steam turbine and related equipment before and during the outage without interrupting plant operations and other jobs. This can lead to aggressive installation time. “Some of the equipment can have long lead times, so the manufacturer must have a robust supply chain,” said Alstom’s Bailey. Most sources suggest that the plant owner select a turbine supplier and/or installation partner at least two years before the turbine upgrade is scheduled to occur and then work closely with that partner until the project is complete.
Although early nuclear power plant turbine replacements and upgrades were performed to improve maintenance and reliability, today’s turbine upgrades are driven by plant capacity uprate goals. Since 2000, more than 3 GW of new nuclear capacity has been added by nuclear plant uprates, said McCracken. Currently, another gigawatt is under review by the Nuclear Regulatory Commission (NRC) and another 1.5 GW is expected to be sought through regulatory filings. Most of these upgrades, if not all, have or will include turbine upgrades.
Operating experience, better design tools and better materials have made turbine upgrades an integral part of the uprate trend.
Carbon Rules Could Drive Coal-fired Turbine Demand
The coal-fired industry hasn’t experienced the kind of industry-wide turbine issues that the nuclear power industry has experienced. “Various issues have surfaced from supplier to supplier, but nothing like the issues seen in the early nuclear steam turbines,” said Richard Klover, Burns & McDonnell Engineering’s associate vice president, coal projects.
Issues that have affected coal-fired plant steam turbines in the past deal with water and steam quality, which is largely a function of water chemistry and solid particle erosion. Older plants contained copper-based condenser and feedwater heater tubes in the condensate and feedwater systems. In these systems, when proper water chemistry was not maintained, the copper would leach from the tubes into the feedwater to the boiler and then carry over in the steam to the steam turbine, resulting in copper plating on the turbine blades. Plants with this problem usually addressed it by periodically cleaning the turbine blades where the elemental copper had been deposited.
Solid particle erosion of the steam turbine blades also was an issue due to the plants’ cyclic operation. With the exception of plants that faced these issues, many coal-fired power plants continue to run with their original turbines.
Some coal-fired plants have opted to upgrade their steam turbines to take advantage of technology advances. Like the nuclear steam turbines, fossil-fueled steam turbines have improved efficiencies in the past 20 to 30 years by minimizing steam leakage through better flow analysis and improved seals and diaphragms. Another advancement has been in last stage blade (LSB) length and design. “In the early 60 hertz coal-fired steam turbines, LSB lengths were up to 33.5 inches,” Klover said. “Today, steel LSB lengths in the 40-inch range are being installed, with even longer LSBs in development.” This means more heat can be extracted from the steam, resulting in efficiency increases. These improvements increase output without increasing boiler steam production.
This is important for at least two reasons. First, output can be raised without increasing fuel consumption, which is important in improving plant heat rate. Second, an output increase without additional fuel consumption is less likely to require a permitting change.
Because most coal-fired plant changes and particularly boiler modifications are tied to permitting and new source review (NSR), steam turbine upgrades are often included in environmental upgrades where best available control technologies are being installed. Back-end flue gas emissions reductions can reduce a plant’s efficiency due to increased auxiliary loads. Turbine upgrades are one way to offset these plant efficiency impacts and raise the plant’s overall electrical output, boosting revenue to pay for the emissions reductions.
In addition, if a plant needs to cut fuel consumption for economic or environmental reasons, it can upgrade its LP turbine blades and improve its efficiency enough to offset the effects of decreased fuel consumption, said Trevor Bailey, Alstom Power’s vice president, steam turbines retrofits. “This is a good way to drive down emissions per megawatt generated.”
Besides size increases in LP blades, turbines used in coal-fired plants have improved in other ways. “Today, manufacturers have a better understanding of materials and failure modes,” said Bailey. “Better materials can eliminate corrosion and erosion problems.”
Some coal-fired turbines have had issues with casing distortion leading to excessive leakage. Solid particle erosion degraded the blades and turbine internals, said Don Stephen, Alstom Power’s general manager, turbine retrofit. This was largely a problem in the United States, but higher-temperature metals have been developed that make rotors and inner casings more durable and less susceptible to erosion.
Several turbine manufacturers have developed and incorporated stronger materials into their designs. These materials have helped mitigate mechanical problems and also have been important to the development of high-temperature, high-pressure coal-fired boiler technologies, which can lead to substantial cycle efficiency increases.
Bill Newsom, Mitsubishi Power Systems’ service, sales and marketing director, said the United States is behind Japan when it comes to fossil-fired technology, including steam turbines. Because Japan imports all of its fuel, efficiency becomes extremely important, Newsom said. Inlet and reheat temperatures have been 1,100 F in Japan since the late ‘80s to early ‘90s. Similar temperatures are just now being seen on new U.S. units.
Although coal-fired steam turbines have improved greatly in the past three decades, many plant owners/operators have elected not to upgrade their turbines and continue to operate with original equipment. Most sources agree that permitting and avoiding triggering NSR are often behind these decisions.
“Permitting and NSR are the biggest issues for coal-fired turbine upgrades,” Klover said. “Any heat input to the boiler equals some level of emissions. The big concern is, will any efficiency improvement trigger NSR?”
A capacity increase without a fuel consumption and emissions increase is unlikely to trigger NSR; however, it is not a certainty. Klover said any capacity increase, even without an increase in boiler heat input, could trigger NSR. “Even if a plant doesn’t change its heat input or its emissions and only changes its capacity/efficiency through a turbine upgrade, the change could affect the plant’s dispatching which could still trigger NSR,” he said.
Klover said he believes something should be done about steam turbine upgrades and permitting issues. “Steam turbine upgrades make perfect sense. They are a sensible way to increase megawatts without increasing emissions.” In today’s electricity-constrained environment, it is important to get every megawatt possible out of existing plants. “It makes sense to get incremental capacity from steam turbines. Steam turbine upgrades need to be easier.”
Klover said the industry needs legislative help. “CO2 legislation will likely help because it could affect NSR requirements,” he said. “Without it, there would most likely be no change to NSR. A lot of steam turbine upgrades should occur if CO2 legislation passes.” – TH
Replacement, Retrofit, Upgrade: Some Definitions
If you ask 10 people knowledgeable about the steam turbine industry to define replacement, retrofit and upgrade, you’re likely to get 10 different answers. While most answers were similar, all of the sources interviewed for this article said the terms are used loosely within the industry.
Here are some definitions.
Replacement. A replacement is usually conducted because the existing turbine has significant problems and a new turbine is needed. In a replacement, the turbine or specific turbine components are replaced with a like-turbine or turbine components. The replacement items are designed to serve the same purpose as the original. Some components might be modified or improved from the original components, but the original parts are replaced with the same currently available parts. For example, a turbine replacement would involve rotor replacement only and would not include the turbine casing.
Retrofit. Retrofit usually involves more significant changes than replacement. These are aimed at changing the turbine’s performance or duty to improve its reliability or performance. Retrofit implies that the existing equipment is being modified.
Upgrade. Upgrade is viewed as a more complex retrofit. It implies that the performance is being increased and that old technology is being replaced with new technology. Performance can be improved, for example, through better blade path efficiency, resulting in increased output. Or seals can be changed for better efficiency and increased capacity. Often, an upgrade involves the rotor and inner casing, but does not change the outer casing. In coal-fired plants the term upgrade is usually used to include a retrofit.
Extensive Modernization/Upgrade. This is a step beyond an upgrade. It involves replacing the entire turbine, including outer casing. It can be complex and requires much preplanning because the new components must fit on the existing foundation, fit the bearing casings and operate correctly with the other system components and equipment. – TH