Design 175
So the newest Siemens titanium LSBs, with annular exit areas of 16 m2 and 11.1 m2 per flow (for the rotation speeds of 3,000 and 3,600 rpm, respectively), are characterized by an interlocked design and feature an integral shroud, as well as a mid-span snubber.
Fig. 3–46. Families of Siemens LSBs and selection diagram for 3,600-rpm turbines
Source : M. Gloger, K. Neumann, D. Bermann, and H.Termuehlen73
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176 Wet-Steam Turbines for Nuclear Power Plants
Unshrouded blades allow a more precise determination of their vibrational characteristics and thus a more reliable tuning out of the blades. In addition, for unshrouded LSBs, it is easier to arrange their noncontact continuous vibrational monitoring.74 Such a non-contact vibration measuring system developed by Siemens is shown in Figure 3–47. Figure 3–48 gives an idea on how the tip of a vibrating free-standing blade is moving, and Figure 3–49 presents a Campbell diagram for a 965-mm (38-in) LSB (for the rotation speed of 3,000 rpm) with the plotted results of actual measurements from a typical 1,000-MW highspeed wet-steam turbine. An operation principle of a similar optical measuring system developed by ABB for a 1,200-mm free-standing steel LSB is shown in Figure 3–50.The calculated vibrational characteristics were confirmed by bench tests in a vacuum chamber.
Some institutions also managed to develop noncontact vibration measuring systems as applied to shrouded LSBs, too. In this case, the primary sensors are installed at the wheel side.75
Fig. 3–47. Schematic diagram of a dual-sensor, proximity-type blade vibration measuring system developed by Siemens
Source : M. Gloger, K. Neumann, D. Bermann, and H.Termuehlen76
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Design 177
Fig. 3–48. Movement of a tip of a vibrating free-standing LSB
Source : M. Gloger, K. Neumann, D. Bermann, and H.Termuehlen77
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178 Wet-Steam Turbines for Nuclear Power Plants
Fig. 3–49. Campbell diagram (see note) for a 965-mm LSB according to measurements at a Siemens 1,000-MW-class 3,000-rpm wet-steam turbine.
Source : M. Gloger, K. Neumann, D. Bermann, and H.Termuehlen78
Note: A Campbell diagram presents the dynamic frequency of certain tones for the blade, blade packet, or entire stage wheel, plotted against the turbine rotational speed. It also includes lines from the coordinate origin showing changes in rotational speed of the perturbing force frequency for different harmonics. The abscissas of the intersection of these lines with the blades’ dynamic frequency lines (or ranges) determine the resonant rotational speeds for each perturbing harmonic.
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Design 179
Fig. 3–50. Operation principle of ABB’s optical blade vibration measuring system (a), optical probe (b), and its installation at the LP exhaust (c)
Source : E. Krämer and E. Plan79
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180 Wet-Steam Turbines for Nuclear Power Plants
Aerodynamics of LSBs
The LSB geometry is derived from complex aerodynamic calculations. The aerodynamic design is confirmed by experiments at test cascades and model turbines with model and actual buckets. If the geometry of the developed actual blade is completely similar to that of the model bucket and their dimensions are in inverse proportion to the rotation speed, all of the aerodynamic, vibrational, and strength properties of both the model and actual blades are the same.This enables creating families of standard LSBs for different rotation speeds (1,500 rpm, 1,800 rpm, 3,000 rpm, and 3,600 rpm) based on a single model version to cover a wide range of output and vacuum values. Several families of Siemens LSBs are presented in Figure 3–46.Similar LSB families are commonly developed and employed by all of the turbine manufacturers.
The advent of newer, longer, more efficient LSBs allows replacement of older ones at steam turbines in service in the course of their refurbishment. Thus, for example, in the process of refurbishing the 1,300-MW low-speed wet-steam turbine (similar to that shown in Fig. 3–8) at the German Unterweser nuclear power plant, the existing LSBs were replaced by 1,422-mm (56-in) buckets of blade family 23 (see the table in Figure 3–46); and at the Spanish Trillo nuclear power plant, new 965-mm (38 in) LSBs of blade family 32 were installed at the 1,000MW, high-speed wet-steam turbine (similar to that shown in Fig. 3–7), producing a considerable gain in the turbine output (see chapter 5). The energy losses with the exit velocity for a full-speed 60-Hz (the rotation speed of 3,600 rpm) turbine, depending on the volumetric exhaust steam flow amount and the number and size of the LP exhausts, are plotted in Figure 3–46b. So, for a wet-steam turbine with an exhaust steam flow amount of 539 kg/s (9,945 m3/s) and the backpressure of 8 kPa (the volumetric exhaust steam flow amount in this case will be equal to 9,945 m3/s), transitioning from TC-6F30 to TC-6F32 (replacement of 760-mm buckets by 815-mm LSBs in three double-exhaust LP cylinders) reduces the exhaust losses by approximately 14.5 kJ/kg, which is equivalent to a gain in the turbine output of approximately 7.8 MW (see the flow chart diagram in Fig. 2–1).
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Design 181
When designing the last stages, it is especially important not only to obtain the high efficiency under the nominal (rated) operating conditions, minimizing the energy loss with the exit velocity. It is also necessary to ensure stable stage operation at reduced volumetric flow amounts, as well as to maintain the stage efficiency as high as possible under these variable conditions. Significant changes in the calculated steam flow patterns, as applied to the 1,450-mm LSB developed for Turboatom’s series of 1,000-MW low-speed wet-steam turbines while operating under load in the range between 84% and 24% MCR, are shown in Figure 3–51. Under no-load operating conditions and minimal loads, these changes become even more significant with the appearance of reverse vortex motion in the tip and root sections, capturing even the nozzle row and the preceding stage blade (Fig. 3–52).
Fig. 3–51. Changes in a streamline pattern with variations in volumetric steam flow amount through the last stage
Source :Y. I. Shnee,Y. F. Kosyak,V. N. Ponomarev, et al.80
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182 Wet-Steam Turbines for Nuclear Power Plants
Fig. 3–52. Appearance of reverse vortex motion in the last two LP stages at low (14%) volumetric steam flow amount
Source :A.V. Shcheglyaev81
Besides the streamline pattern, with the steam flow amount there also change the ratios of the steam velocities to the current acoustic velocity, that is, the Mach numbers. Because the steam velocity diagrams, steam flow patterns, and Mach numbers essentially vary lengthwise of the stage height, the applied blading profiles also must be different for different sections.The diagrams shown in Figure 3–53 demonstrate the profile types proposed to obtain minimum profile losses in the wide range of Mach number values for the different LSB sections. Development of new, often nontraditional, profiles (especially for LSBs operating with high values of the Mach number and their sharp variations along the stage height) produces considerable gains in the stage efficiency, as seen, for example, in Figure 3–54, which compares the stage efficiencies for conventional and newly developed Hitachi LSBs with the length of 660 mm (26 in) for the rotation speed of 3,600 rpm. 82
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Design 183
Fig. 3–53. Typical blade rows and dependencies of profile losses on Mach number for the LSB’s root section (a) (1: row with convergent channels; 2: purely impulse blading; 3: row with divergent channels), mean section
(b)(1: common row; 2: row with double-convex profiles), and tip section
(c)(1: row with divergent channels and common profiles; 2: row with convergent channels and common profiles; 3: row with convergent channels and a ridge on the back of the profile)
Source :A.V. Shcheglyaev83
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184 Wet-Steam Turbines for Nuclear Power Plants
Fig. 3–54. Comparison of stage efficiencies for conventional and newly developed, advanced Hitachi 26-in LSBs
Source : M. Machida, H.Yoda, E. Saito, and K. Namura84
The world’s leading turbine producers presently conduct 3-D aerodynamic calculations of LP steam paths with regard to the steam viscosity, taking into consideration the complex shape of the row profiles, the appearance of local supersonic velocities, and the wave phenomena that accompany them. Some computer programs have also been developed to solve the reverse problem—to obtain row profiles on the basis of the set stream lines and distribution of the steam flow conditions. Because of the extreme complexity of such a problem, all the applied approaches and computer programs unavoidably adopt some more or less serious assumptions. Nevertheless, they allow researchers and designers to obtain detailed space nets of meridional stream lines for the given boundary conditions, find lines of constant relative velocities, λ or M (isotachs) and pressure values (isobars) for different sections, distribution of energy losses along the stage height, and so on. Three-dimensional computations also result in a field of velocities at the exit edge that lead to the possibility of optimizing the exit edge shape. The relative steam velocity fields for the tip, median, and root sections of a typical modern LSB are presented in Figure 3–55. Figure 3–56 illustrates some results of such calculations for the last LP stage of Turboatom’s 1,000-MW low-speed turbines, and Figure 3–57 shows the isotach field for the LP steam path of three stages developed by MHI with 1,143 mm (45-in) titanium LSBs for the rotation speed of 3,600 rpm.
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