Modernizing steam turbine components
Upgrading an aging turbine`s performance with new steam path components can improve plant efficiency and output
By D. Bergmann, M. Jansen and
Siemens AG Power Generation Group
Stricter environmental protection requirements in recent years (and a subsequent increase in operating expenses) have induced aging power plant operators to think about whether a given facility`s cost-effectiveness could be improved by modernizing its steam turbine generators. A number of options are available for modernizing older steam turbines, both in conventional and nuclear power plants. In such cases, it is useful to contact the original turbines` manufacturer and discuss whether or not the existing unit would benefit from an upgrade.
Turbine upgrades should serve to improve plant efficiency, plant output, extend the plant`s service life, and optimize plant availability. In many cases, a single upgrading option can achieve several improvements at the same time. For example, all the improvements detailed in this article can be achieved by installing a new rotor, inner casings and new blades.
The options described here are based on new, recently-developed designs, some of which are already serving successfully in new plants. Almost all of these contemporary features can be retrofitted to older turbines without difficulty, ultimately bringing poor-performing turbine generators up to the operating standards of modern units.
Over the last few years, new blade profiles and blade clearance seals have been developed using up-to-date computational methods. Extensive laboratory analysis and other experiments prove that older blade designs had considerable room for improvement.
If a 600-MW turbine underwent upgrading today, output could be boosted more than 12 MW, which corresponds to a 2 percent improvement in efficiency, and, depending on the specific upgrade, would compensate for blade surface deterioration losses. In most cases, a turbine modernization project is likely to consist of replacing the rotors and inner casings and installing new stationary and moving blades.
Profile T4 (Figure 1) has been developed for the cylindrical part of the blade. Offering a high section modulus, this blade profile exhibits optimum contours with a smooth, suction-side curvature. The blade`s optimized profile ensures low suction-side deceleration and reduces the danger of flow separation. This design`s profile also produces lower flow losses than earlier designs. Figure 1 (left side) shows a T4 profile cascade and the associated isoMach lines for a specific load condition. Figure 1 (right side) shows the resulting Mach number distribution over the blade contour versus the relative chord length.
The high section modulus of the T4 profile often makes it possible to select a smaller profile than with earlier blade designs. Ultimately, a smaller blade profile provides space for more stages and reduces the stage`s loading coefficient. Surface finish refinements also have contributed to efficiency improvements. A smoother blade surface reduces friction losses and makes the profile less susceptible to deposits.
Twisted blades with integral shrouds are generally used in the last few stages of a turbine`s low-pressure section. By twisting the blade and using different profiles over its length, it is possible to reduce profile losses, optimize the exit flow angle, and produce a longer blade. All of this reduces stage losses and, in the last stage of each turbine, exhaust losses.
Compared to today`s standards, the design and fabrication of low-pressure blades in the 1960s was rather primitive. Some turbines were fitted with cast stationary blades that had wide trailing edges and, at the outer cross-sections, last-stage blades with a very unfavorable pitch-to-chord-length ratio and an unsuitable transonic flow profile. In contrast, the thermodynamic design of modern low-pressure blades pays particularly close attention to mass flow distribution near the end of the blade. Losses are known to be high in the blade`s boundary zones due to wall friction and secondary flow paths. New blade designs focus on keeping the mass flow density low in the boundary zones to promote efficiency.
Low-pressure blades also are designed for optimum reaction distribution. To minimize the unshrouded blades` radial clearance losses, the mean reaction has been reduced. If no countermeasures were taken, there would be a very low reaction at the hub and an increasing chance of flow separation in that area as well.
Because a curved stationary blade manages mass flow distribution better, it has a distinct advantage over a conventional blade. The curved blade`s inclination exerts an additional radial force on the steam flow to counteract flow separation at the hub. In order to improve flow in this region, the last-stage stationary blades are not only twisted but also curved circumferentially (Figure 2).
The effect of the new profile`s design, in terms of improved flow, is depicted in Figure 3 which shows the computed Mach number distributions for the flow cascades off the tips of the blades. The new designs feature a smaller ratio of pitch-to-chord length and straighter suction-side contours. The maximum Mach number at any point along the profile is only slightly higher than the value at the exhaust.
To further reduce exhaust losses, a diffuser was developed that enhances pressure recovery. Laboratory experiments on a variety of diffuser geometries helped to determine which one produced the best results with regard to actual in-turbine conditions (casing geometry and internals).
The effectiveness of the diffuser is governed primarily by the geometry of the inner shell. This means that current diffuser designs can be fitted to older turbines without having to replace or modify the turbine`s outer casing.
Seals at shrouded blades, shaft seals
It is well understood that clearance losses at the blade`s tip account for a major part of a turbine`s overall losses. Figure 4 shows the results of laboratory experiments on a variety of sealing configurations for stationary, low-pressure, drum-stage blades which are subjected to a large axial-relative expansion. The noninterlocking seal type (a), with its straight shroud and caulked-in seal strips, is an older design.
The development of new seal geometries on the basis of these “noninterlocking” seals led, via configurations (b) and (c) to design (d), the double-strip seal design. The double-strip seal offers the lowest clearance losses of all seal types.
With double-strip seals, the seal strips are an integral, machined-in part of the shroud, while the shaft and casings have seal strips that are caulked in place. Because the risk of rubbing is lower than with tip-to-solid-shaft or tip-to-solid-shroud seal designs, radial clearances can be reduced to inhibit losses.
Double-strip shaft seals are also effective at locations subject to large, axial-relative expansion between the shaft and casing. The double-strip configuration makes it possible to accommodate a much larger number of seal strips, which significantly improves the sealing effect. Radial to axial shaft seal conversions have proven to be an effective way to improve an older turbine`s availability.
The addition of an admission section (Figure 5) is an effective way to improve the efficiency of double-flow turbines that have mid-turbine admission. In this case, a shield connected steam-tight to the inlet stationary blade rows prevents clearance losses at this location. The inlet stages are designed for low-reaction states (i.e., there is only a slight pressure drop across the moving blade rows), and that reduces clearance losses in the two moving blade rows. Likewise, the flow-dynamic design of the rotor shield reduces stationary blade vibrations because it diminishes admission turbulence.
The shield also extends the service life of the shaft by protecting it against the high-temperature admission steam. Tangential bores in the shield allow steam to flow into the space between shield and shaft in the direction of shaft rotation, thus reducing the shaft`s surface temperature in an effect known as steam-swirl cooling.
New condenser designs are now providing good heat transfer from steam to tubes, largely due to uniform steam flow to all tubes and reduced low pressure steam losses in the lanes between the tube bundles and in the tube bundle itself.
Other advantages of contemporary condenser design include better-defined steam flow towards the air cooler bundle, better top-to-bottom condensate run-off with minimum interference from heat transfer in the lower tubes and improved gas evacuation.
Older condensers can be upgraded simply by exchanging the tube bundles and the tube plates. Figure 6 shows upgrading work being performed on a condenser at the Unterweser Nuclear Power Plant.
Life extension and availability
With most older turbines, it is a good idea to analyze the remaining service life of essential components such as casings, shafts, valves and piping. A service life analysis reveals the current condition of a turbine`s components and systems and can point out areas where low-stress operation may enhance longevity or where new or refurbished components might make the most performance and efficiency improvements.
Another way to enhance service life and availability is to exchange (during scheduled or required maintenance) worn or broken parts with improved replacements (e.g., valve stems with flame-sprayed shafts and stellite bushings or low pressure rotor blades with hardened leading edges). These and other options should be discussed on a case-by-case basis with the turbine`s manufacturer.
The upgrading options described have already been implemented on numerous turbine generators and in practice have proved successful. Available modernization options afford aging steam turbine generator operators a viable alternative to new plant construction.
Figure 1. Optimized T4-Profile for cylindrical blades.
Figure 2. Last-stage low-pressure section stationary blade ring with curved blades.
Figure 3. Computed Mach number distribution in the tip cascade of a last-stage blade.
Figure 4. Relative effectiveness of clearance seal designs.
Figure 5. Admission section with loss-preventing shield.
Figure 6. Condenser upgrade at Unterweser nuclear power plant.
Dietmar Bergmann studied mechanical engineering at the University of Hannover. After graduating in 1962, he began employment with Siemens in the steam turbine division`s thermodynamic department. Since 1992, he has been responsible for the Siemens/KWU thermodynamic calculation department.
Dr. Michael Jansen studied mechanical engineering at the University of Hannover. After graduation, he earned his doctorate from his work researching radial compressors. He is now manager of all steam turbine analytical and blade designs.
Dr. Heinrich Oeynhausen studied mechanical engineering at the State Engineering Academy in Essen. After graduation in 1971, he joined Kraftwerk Union where he worked as an applied mechanics engineer. He received his doctorate from the Ruhr University in 1981. Since 1992 he has been senior director of the steam turbine division.