Design 185
Fig. 3–55. Isotachs for the tip, median, and root sections of a typical modern LSB
Source : F. P. Borisov, M.Y. Ivanov,A. M. Karelin, et al. 85
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186 Wet-Steam Turbines for Nuclear Power Plants
Fig. 3–56. Isotach fields on the surfaces of the vane and bucket profiles
(a) and in the interprofile channels (b) for the last stage of Turboatom’s 1,000-MW low-speed turbine
Source :A.V. Shcheglyaev86
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Design 187
Fig. 3–57. Calculated isotach field for the steam path of three last LP stages with 1,143-mm titanium LSB of MHI’s 3,600-rpm turbine
Source : By courtesy of Mitsubishi Heavy Industries
Vanes and buckets of axial turbine stages were traditionally configured radially; that is, the projection of the line through the centers of gravity of their profiles onto the plane perpendicular to the turbine axis was radial. Along with this, it has been known that some corotational incline of the vanes provides the steam flow with a favorable sweep in the root zone, increases the root reactivity, reduces the end (root) loss, and decreases the stage efficiency reduction when the steam flow amount varies. However, such an incline can be unfavorable for the peripheral stage sections.That is why, as long ago as the early 1960s, G.A. Filippov and Huang Chungchi of MEI proposed and investigated what they called saber-type vanes with a variable incline (that is, with the angle of inclination decreasing from the stage root to its outlying sections).87 Currently, the inclined and curved (bowed) vanes and blades are widely used by almost all steam turbine manufacturers.88 Analysis shows that the use of such blades is most effective in the last LP stages. Replacement of traditional (radial) blades and/or vanes by advanced, saber-type for these stages brings a great effect and is especially advisable at retrofitting the turbines in service.89 Such an inclined and curved vane of the last LP stage developed by
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188 Wet-Steam Turbines for Nuclear Power Plants
ABB and its influence on the meridional steam flow stream lines are shown in Figure 2–25. An advanced LP cylinder with a last stage of this type is presented in Figure 3–58.The field of relative steam velocities (Mach numbers) for a similar last LP stage developed by LMZ for reconstructed and newly designed turbines is shown in Figure 3–59b. It can be seen that the use of saber-type vanes makes the steam velocity field more uniform as compared with a conventional stage (Fig. 3–59a). The resultant velocity field also favorably influences the exhaust port characteristics, thereby making it possible to exclude a local supersonic zone in the divergent section of the port, even with relatively high average Mach numbers.
Fig. 3–58. ABB’s advanced LP cylinder with highly three-dimensional geometry in the last stage vanes
Source : A. P.Weiss90
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Design 189
Fig. 3–59. Isotach fields for the last LP stages of LMZ’s newly designed and refurbished turbines with conventional (a) and saber-type (b) vanes
Source : F. P. Borisov, M.Y. Ivanov,A. M. Karelin, et al.91
The rapidly increasing specific volume of the steam flowing in the LP steam path requires a sharp increase in the annular outlet area of the stages and leads to a large casing pitch angle. Modern large turbines, including wet-steam ones, feature high length-to-mean diameter ratios for the LSBs.The value of this ratio can be considered an indicator of the three-dimensionality for the steam flow through the last stage, as well as through the LP steam path as a whole. This ratio (l2/d2) commonly varies in a range from 0.33 to 0.41 for modern high-speed turbines and from 0.31 to 0.37 for low-speed turbines. The higher the ratio value, the more complicated is the aerodynamic design of the steam path. For high-speed turbines with steel LSBs, in order to achieve the maximum annular exhaust area with regard to the limits for tensile stress, this ratio should be equal to approximately 0.35–0.4 (Fig. 3–39).
According to a classic approach to the LP cylinder design, the root (hub) diameters of the stages are either constant (as in Fig. 3–58) or slightly decrease toward the last stage (as in the cases with most of the previously mentioned turbines). In these cases and in situations with high values for the length-to-mean–diameter ratio for the LSBs, the peripheral stream line pitch angles can be as high as 60°. Under such circumstances, transition from a broken, piece-linear peripheral outline of the steam path
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190 Wet-Steam Turbines for Nuclear Power Plants
to a conical meridional profile provides a noticeable gain in the local stage efficiency for the tip sections.This also refers to integrally shrouded rotating blades and requires a corresponding arrangement of the blade shrouds (Fig. 3–60).
Substantially lowering the stage hub diameter toward the last stage reduces the peripheral pitch angle, as shown in Figure 3–57. In this case, the designers increased the length-to-mean-diameter ratio for the LSBs up to the highest value of as much as 0.41 and even more.
Fig. 3–60. Two different types of shrouds for LP stages with conical meridional profiles
Last stage blade protection against WDE
The circumferential speed of modern steel LSBs used in highspeed turbines reaches 700 m/s ( 2,300 ft/s) and can reach as much as 800–830 m/s (2,620–2,730 ft/s) for titanium LSBs (Tables 3–3 and 3–4). Even for low-speed turbines, the circumferential speeds reach 500–530 m/s (1,640–1,740 ft/s).Although the mean steam wetness at the LSBs is never more than about 12–14%, the local steam wetness at the stage periphery can be significantly greater and more dangerous, because the water drops are mainly concentrated in the tip region. The coarse-grained water, which presents the main threat of blade erosion, lags behind the steam stream and, as a result, impacts the blade back at the row inlet (see Fig. 2–17).
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Design 191
In order to protect steel LSBs against WDE, the blade surfaces are covered by Stellite laminas (shields or strips) brazed to the blade surface (Stellite is a cobalt-based, 60–65%, alloy also containing 25–28% chromium and 4–5% tungsten. It has a high hardness and a very high resistance to erosion wear.). 92 The erosion resistance of Stellite exceeds that of stainless steel by 8–9 times and is 5–6 times higher compared with titanium alloys.Water drop erosion mainly affects the upper third of the LSB back near the leading (inlet) edge, and the Stellite laminas are attached only at this surface.This method is commonly used for steam turbines of both fossil fuel and nuclear power plants. Its disadvantage is that it creates some discontinuity in the blade profile, resulting in performance losses. In addition, during turbine operation, individual Stellite laminas sometimes break away from the blade surface, causing local abrasion of the blade profile, as well as changes in the blade’s vibrational characteristics. Nevertheless, Stellite laminas have remained the most common measure of protecting steel blades against WDE.
However, the use of Stellite is unacceptable for turbines operating with radioactive steam, because cobalt is washed out of the Stellite and brought into the reactor, where it is activated and then is deposited on the equipment surfaces in the forms of various oxides. Because Co-60 has a relatively long half-life period, in this case all of the equipment pieces would have to be thoroughly deactivated before repairs could be attempted. These deactivation processes would take much time. For this reason, instead of covering with Stellite laminas, the blade surfaces are often hardened with elec- tric-spark machining. (This method is also applied to the first stage blades of fossil fuel turbines to protect them against solid particle erosion. 93) In the course of machining, numerous micro discharges between the blade surface and the electrode result in the electrode material being deposited on the blade surface. Because the applied electrodes contain ferrochrome, nickel-boron, or other erosion-resis- tant materials, they shape the layer of protective alloy on the blade surface. Another promising method of protecting the LSBs against WDE is coating them with clustered chromium.94
Instead of Stellite strips, the leading edges of steel blades can be protected by a welded bar-nose, which combines an erosion-resistant cobalt-free insert with a weld filler of Inconel® 82, which is a very
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192 Wet-Steam Turbines for Nuclear Power Plants
ductile material with high fracture toughness. This method has been well proven for years in plant operational practice, including English Electric’s 1,060-MW wet-steam turbine at Fermi Unit 2 with BWR.95 Welding allows complete restoration of the airfoil profile, leaving no surface discontinuity. The use of ductile weld filler also protects the blade parent material from crack propagation through the leading edge in the event of extensive water impact.
Titanium alloys are less prone to WDE than are stainless blade steels, even though titanium is inferior to Stellite in this regard.The first commercially employed titanium-alloy LSBs were originally protected by a nithinol coating, but long-term experience has proven the possibility of their use without antierosion protection. 96 Nevertheless, for the newest shrouded LSBs with tip circumferential speed of 700 m/s and more, it is recommended to protect the shroud and the very tip portion of the blade with an erosion shield made of Ti-15 Mo-5 Zr-3 Al, which is electron-beam welded to the leading edge.An example of such a shield is shown in Figure 3–61 as applied to Hitachi’s 1,116-mm titanium LSB. 97 Although the operational experience for these titanium LSBs is not as long as that for conventional 12% Cr steel blades with Stellite protection, the first operational data match the predicted erosion rate for more than 120,000 operation hours (Fig. 3–62).
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Design 193
Fig. 3–61. Protection against WDE in the tip section and shroud of Toshiba’s 1,016-mm titanium LSB
Source :T. Suzuki, M.Watanabe, and M.Aoyama98
Fig. 3–62. Estimation of WDE for Toshiba’s 1,016-mm titanium LSB
Source :T. Suzuki, M.Watanabe, and M.Aoyama99
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194 Wet-Steam Turbines for Nuclear Power Plants
The erosion rate at the leading edge of LSBs can also be influenced by certain design measures. For example, according to Toshiba,100 the erosion rate noticeably reduces with an increase in the gap between the stationary and rotating blades, because this gives more space for the water drops leaving the nozzles from the steam flow. This effect was well confirmed by experiments on model turbines.The more effective the water separation and removal from the steam flow is, the less the erosion rate (as shown, for example, in Fig. 2–38c).
Along with the leading edge in the tip zone, LSBs also suffer from WDE at the trailing (outlet) edge in the lower (root) and median zones. This phenomenon is mainly stipulated by reverse motion of steam and vortices in the root stage portion under low-flow transient operating conditions (Fig. 3–52). These steam streams commonly bring large-sized water drops that erode the LSBs, especially their thin outlet edges. All of these unfavorable processes are additionally promoted by steam discharged into the condenser from the cold reheat and main steam-lines through the turbine steam bypasses, which come into operation exactly at low-flow transients. Sprays cool the discharged steam, and the resultant steam-water mixture is often injected by the reverse steam streams to the LSBs.That is why the correct location of these devices in the condenser port and the correct design of the turbine exhaust hood are so important.
Water Removal from Turbines
For wet-steam turbines, with the large moisture content in the operating steam, it is desirable to remove as large a portion of this moisture from the steam flow as possible. Primarily, this is necessary to reduce the rate of erosion damage to the turbine components and secondarily to improve the blading efficiency. The water extraction devices employed in wet-steam turbines could be subdivided into two functional classes: 1) those designed to remove potentially hazardous water drops directly from the blade path and 2) more sophisticated separators intended to dry the steam flow altogether by removing the free water.101 However, in practice, it seems more convenient and obvious to classify the water extraction devices not by their functional purposes, but by their designs. On this basis, it makes sense to consider separately two completely different methods of water separation and
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