Modern technology breathes new life into old turbines
By Timothy B. DeMoss,
I keep reading about Russia`s aging power plants and how most of them are well past retirement age. Picture scrappy Russian engineers in soot-covered buildings, holding together these beasts of burden with paper clips and a good, sturdy rope, knowing they`re fighting a losing battle, but refusing to give in. The situation is probably not quite as I imagine it, but the battles these Russian engineers fight, in a country with little money for plant repairs, is not altogether unlike what we face in the U.S. as plant engineers prepare to do battle in the open electricity market. Deciding what to do with aging plants is a problem that is here to stay. Retiring and rebuilding will not always be economical when customers can purchase electricity from whom ever they please.
One way many companies today are breathing new life into their plants, resuscitating their money-making potential, is by fully refurbishing power plant steam turbines, the electricity-producing workhorses for the last 100 years. Today`s retrofits and refurbishments have little to do with paper clips and rope.
As John Zink outlines in “Steam turbines power an industry,” (p. 24) technology can advance very far in one century. At our current speed for technology evolution, applying modern designs to machines, installed only last decade, can bring marked improvement to system performance. Original equipment manufacturers are catering to the needs of turbine owners by offering a smorgasbord of options for retrofit, whether or not they manufactured the original turbine. Not every retrofit requires total replacement of major components, and the diagnostic tools available to manufacturers today allow them to customize separate components to fit existing equipment with the highest efficiency and capacity boost possible.
Higher efficiency and capacity are typical results expected from these retrofits, but the benefits extend beyond these obvious basics. Besides lower fuel costs and lower emissions resulting from increased efficiency, the improvements to the steam turbine can ease the stress on other aging components such as the boiler and emissions controls and reduce costs associated with ash disposal. Furthermore, the improved reliability from replacing life-limited components leads to reduced inspection and maintenance costs. Engineers are lowering overall costs by improving steam path design, controlling leakage, refining blade profiles and controlling erosion and corrosion. These areas consume most of the efficiency-loss pie (Figure 1).
The largest single cause for reduced performance in the high-pressure (HP) stage is leakage. However, even intermediate- and low-pressure (IP and LP) stages suffer losses due to poor sealing. Regular inspections and maintenance do much to alleviate this problem, but there are new seal designs and other modifications which improve the system`s ability to prevent leaks. A recent LP turbine retrofit by Siemens at San Diego Gas & Electric`s (SDG&E) Encina plant in Carlsbad, Calif., used special double-strip seals for interstage sealing. One sealing technique that has been very successful, and one that Siemens used on Encina, is to machine rotate seals directly into the shroud of integrally shrouded blades. Siemens used this design on the LP turbine, caulking renewable seal strips to the opposing stationary surfaces. They used the same sealing arrangement for the stationary blades. Figure 2, from an SDG&E and Siemens paper presented at POWER-GEN `95, shows a profile comparison of the seal used in the Encina retrofit compared to other seal types. The tortuous path in each seal design induces turbulence and restricts flow.
General Electric (GE) has also tested a variety of different sealing configurations, confirming that a stepped or high-low spill strip substantially reduces leakage. The configuration used on most HP and IP bucket tip seals includes steps with double teeth on both the upstream and downstream sides of a recessed tenon. The multi-strip design also reduces efficiency loss due to rub damage compared to a single-strip design.
Many plant operators experience solid particle erosion (SPE) of internal turbine components, with damage estimates up to $150 million per year, according to GE. Efficiency loss, forced outages, extended maintenance outages and increased maintenance cost are only some of the potential problems created by SPE. Although many of the techniques used to eliminate SPE are built into new designs, using coatings in retrofit components or coating existing components can significantly reduce SPE damage. Today`s advanced coatings can last years before erosion again begins to accelerate. The performance improvements afforded by using coatings can be significant (Figure 3) and applying coatings in a retrofit can reduce the chances that SPE will cause major problems, requiring component replacement.
Lab work and testing are requirements for almost every turbine design improvement. New coating technologies are one example. However, many companies are turning to advanced computer methods to both design new turbine components, such as blades, and to analyze existing turbines for optimized retrofit. Using three-dimensional computational fluid dynamics (CFD) for blade design, GE has seen overall stage efficiency gains of 2 to 2.5 percent relative to its original designs, which relied on assumptions about radial velocity components (the third dimension) in design calculations. GE has also applied three-dimensional CFD to design exhaust hoods and valves and to analyze tip seal leakage. Most engineers previously relied on scale model experiments to optimize design of these complex geometric components.
Although advanced components play a significant role in increasing the performance of older turbines, advances in computing power also allow retrofit manufacturers to optimize a retrofit design, even if another manufacturer installed the original equipment. For example, in the Siemens Encina retrofit mentioned earlier, none of the turbines were Siemens designs. Modeling the original equipment on the computer and investigating the effects of different modifications allow retrofit companies to propose the best possible design to the customer in as little time as possible. By avoiding complete replacement of all turbine components, utilities can save thousands of dollars, shorten payback times for the retrofits and make project cost justification much easier.
Kansas City Power & Light (KCPL) is currently upgrading steam turbines at its Jeffrey Energy Center near St. Marys, Kan. KCPL operates Jeffrey, sharing electricity generated with area utilities who all own a share of the plant. Two of the turbine upgrades are complete and Siemens, which is performing the upgrades, expects to finish work on Unit 3 in the spring of 1997.
Siemens offered an upgrade on the 680 MW units, which vary in age from 13 to 18 years, promising higher output and better efficiency. However, it was the fuel cost improvement and avoided maintenance on the older machines which sold KCPL on the project. “The project pays for itself in five years,” said Vic Tolentino, Siemens` Jeffrey retrofit project engineer.
Siemens replaced the HP turbine on Units 1 and 2 and also plans a replacement for Unit 3. The new HP turbines have 12 stages (the previous design had 11) and a bigger design which allows more steam flow. The company only replaced the blades on the IP and LP turbines because the expected improvement with a complete turbine replacement was not cost justified. “This was the optimal value for the customer based on a cost benefit analysis,” said Tolentino. The new blades–Siemens` T4 design–are approximately 20 mm longer than the original T2 style blades. Siemens remachined the IP rotor and replaced the inner casing to accommodate the change. The first three stages of the LP turbine also have new T4 blades and a new matching inner casing.
Besides the new blades and inner casings, the new machine incorporated larger steps in the sealing strips and more seals to improve stage efficiency.
Recent performance testing on Unit 1 showed that all output requirements were met and the capacity factor of the unit has increased by about 8 percent. The efficiency improvement on the first two upgrades (Units 1 and 2) is 7 percent for the HP, 5 percent for the IP and 2.5 percent for the LP, according to Tolentino. Siemens expects similar improvements for Unit 3.
The Jeffrey Center also has a fourth unit on hand which KCPL never installed but is part of the upgrade package. Siemens will reblade the IP and LP rotors from this unit, and KCPL will use them during a major maintenance outage. Tolentino said KCPL plans to install this unit in place of an operating rotor, eliminating the time normally required to inspect and repair the replaced rotor. KCPL will inspect and repair the replaced rotor (if needed) after the outage is complete. Having this spare rotor will also significantly shorten future outage times.
Both upgrade outages completed to date went smoothly due to excellent cooperation between Siemens and KCPL. They completed the first outage in seven weeks and the second outage in six weeks, encountering practically no problems. Commenting on the project`s success thus far, Tolentino said, “The experience with this retrofit shows that with proper planning and execution, an upgrade can be completed without going through a very long outage.”
Siemens Power Corp. recently restructured its fossil division due in large part to the domestic power industry`s unwillingness to build new generating capacity. The restructuring allows the company to focus more on upgrading and servicing aging U.S. power plants, driving home the point that for many older plants, as deregulation nears, refurbishing existing capital is a wise economic choice. As more equipment manufacturers concentrate their efforts on the retrofit market, the options and technologies available for rebuilding should increase. Because companies are taking advantage of the latest design technologies to retrofit competitors` original equipment with their own components, power plants considering the rebuild option will enjoy the savings created from this newly competitive market. z
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