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removal: 1) internal—that is, directly from the steam path and 2) external, with the use of special MSRs placed outside (or, more accurately, beside or alongside) the turbine. 102 The measures for internal water extraction include devices of peripheral moisture separation and removal, intrachannel water extraction, and special stage separators.
Peripheral moisture separation and removal between the stage rows
Some water is separated and removed from the steam path due to the circumferential component of the wet-steam motion—under action of centrifugal forces drawing the water drops out to the stage periphery. This separation takes place within the stages, in the gaps between the stationary and rotating blade rows.The separated water can be captured and gathered in water trap belts (as shown, for example, in Figure 2–26 as applied to the Turboatom K-220-44 turbine’s HP cylinder), but the efficiency of this natural separation is not great.The separation efficiency, , is defined as the ratio between the amount of the removed water and the total water contents in the steam flowing through the stage. Its value depends on the steam wetness, the steam velocity and circumferential rotation speed at the blade tip, the portion of the coarse-grained water drops in the wet steam flow, the shape, size, and location of the water-taking channels and water traps, as well as numerous other factors.
A major contributor to the moisture separation process is the centrifugation of water from the surface of the rotating blades (see Fig. 2–41). To intensify this process, turbine manufacturers such as GE, Hitachi, and Toshiba have sometimes furnished the rotating blades (mainly in the HP stages) with special lengthwise (radial) furrows on the back surface near the leading edge, which in this case is not covered by the shroud (Fig. 3–63). According to GE, based on their tests at Dresden Unit 1, such a design decision effectively prevents erosion of the blades in subsequent stages. The experimental data showing the separation efficiency for the furrowed rotating blades, as dependent on the steam pressure and wetness, are presented in Figure 3–64. Moisture removal can be additionally enhanced by suction of the steam-water mixture from the water trap chambers to the lower pressure steam chambers. An example of such a design decision with furrowed rotating blades and the water trap belts situated after the stationary blade rows is presented in Figure 3–63.
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Fig. 3–63. LP stages with furrowed rotating blades and water traps between the nozzle and blade rows connected to the condenser (A: radial furrows on the back surface near the inlet edge)
Source : B. M.Troyanovskii103
Fig. 3–64. GE experimental data on separation efficiency for furrowed rotating blades
Source : B. M.Troyanovskii104
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Some turbine designers place the water trap belts between the stationary and rotating blades of the stage, as shown in Fig. 3–63; others prefer to position them after the rotating blade rows, as shown in Figure 3–65.The water amounts to be captured and removed come from three sources:
Wc — water that enters the stage from the preceding stages (having not been removed by their separation devices) and moves mainly along the peripheral side of the nozzle;
Wn — water that is deposited on the stationary blade surfaces and joins the Wc flow; and
Wr — water that is centrifuged by the rotating blades.
Fig. 3–65. Water trap positioned after the stage
Source:W. P. Sanders105
All of the variables in Figure 3–65, characterizing the position, shape, and size of the water trap (Ca V S A D, and R), are to be optimized to increase the separation efficiency. To capture more of the water flow (Wr ), it is rather advisable, as distinct from the picture of Figure 3–65, to have the meridional profile of the casing ring across
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the rotating blades and near the water trap belt’s inlet sloping and the trailing edge of the rotating blades overlapping both the shroud and the water trap belt’s inlet edge. Positioning the water trap belt after the stage is also preferable for stages with unshrouded rotating blades.
Along with the captured water, the water trap belts also withdraw some amount of steam, usually estimated to be as much as 0.5% of the total steam flow through the stage. The energy of this steam can be utilized if the withdrawn water–steam mixture is forwarded into the regenerative feed-water heaters.The turbine’s regular steam extraction chambers for steam bleedings also play a role of water traps.The HP steam path of an impulse-type low-speed wet-steam turbine with the water trap belts located after the rotating blade rows and connected to the lower pressure steam bleedings is shown in Figure 3–66.
Fig. 3–66. Water removal from HP steam path of an impulse-type wet-steam turbine
Source : B. M.Troyanovskii106
Open chambers of the water trap belts such as those shown in Figures 3–63 and 3–65 slightly impair the aerodynamic properties of the steam path and increase the energy losses. For these reasons, some
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researchers propose not using these chambers (apart from those used for steam bleedings) and substitute them with specially fluted surfaces above the rotating blades for the deposition and subsequent removal of water.A possible appearance of such a surface-type water trap is shown in Figure 3–67. The captured water is to be gathered and drained from the casing ring.
Fig. 3–67. Possible design for a surface-type water trap for LP steam path
Source : I. I. Kirillov and G.G. Shpenzer107
Intrachannel moisture separation
The diaphragms of the last and next-to-last LP stages of modern wet-steam turbines are often made with hollow nozzle vanes, which are either stamped and welded, or solid and drilled (Figs. 3–68 and 3–69).Apertures or slots connect the nozzle surface with the internal vane space, the ends of which are drained to the turbine condenser. As a result, the water film is withdrawn from the nozzle surface instead of being pulverized into drops and eroding the subsequent rotating blades.
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200 Wet-Steam Turbines for Nuclear Power Plants
Fig. 3–68. Turboatom’s hollow stationary blades with intrachannel water removal (1: water-taking apertures; 2: water-taking internal channels; 3: suction slots)
Source : B. M.Troyanovskii,Y. F. Kosyak, M.A.Virchenko, et al.108
Fig. 3–69. Typical last stage of LMZ with intrachannel water separation and removal
Source : I. I. Pichugin,A. M.Tsvetkov, and M. S. Simkin109
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Different designers locate the water-taking suction slots in different areas of the nozzle surface: on the back surface near the inlet, on the face surface where the steam flow turns (see Figs. 3–68 and 3–69), directly in the exit (trailing) edge, and so on. To increase the separation efficiency, it is reasonable to have at least two slots in different areas. Under conditions of common internal space of the vane, to avoid water being pumped from one slot to another, these slots should be located at places on the nozzle surface with equal steam pressure. For example, in Figure 3–70, the suction slot pairs could be positioned at points 1 and 2 or 3 and 4 along the nozzle profile. Suction slots on the back and face surfaces of the nozzle vane can be seen in Figure 3–71, as applied to Toshiba’s 1,016-mm (40-in) titanium LSB (similar to points 3 and 4 in Fig. 3–70). The combined influence of peripheral and intrachannel water separation and removal on the blade erosion rate is shown in Figure 2–38c. The intrachannel water extraction decreases the relative erosion rate by as much as a factor of two, and additional peripheral water extraction with an increased distance between the stationary and rotating blades further decreases this rate by another 0.1–0.2.
The separation efficiency of the intrachannel water removal also significantly depends on the amount of steam withdrawn along with the water—the separation efficiency increases with the steam amount until a certain threshold, and then remains almost invariable. The quantitative characteristics of these dependencies substantially change with the shape of the suction slots and their position on the nozzle surface.
Fig. 3–70. Possible position of suction slots for intrachannel water removal
(a) and distribution of specific steam pressure along the vane profile (b)
Source :V. I. Kiryukhin, G.A. Filippov, O.A. Povarov., and V. I. Dikarev110
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202 Wet-Steam Turbines for Nuclear Power Plants
Fig. 3–71. Toshiba’s last stage with intrachannel water separation and removal
Source :T. Sakamoto, S. Nagao, and T.Tanuma111
The water separation factor, for hollow nozzle vanes with intrachannel moisture separation can be estimated with the use of a diagram shown in Figure 3–72 as a function of the steam pressure at the stage inlet, p0 , and the Mach number for the midsection exit of the nozzle row, M1 = (c1 /a 1 )m. It is assumed that the nozzles have two suction holes with a slot width of approximately 0.7–0.9 mm (28–36 mil): on the profile back and in the trailing edge (points 1 and 2 in Fig. 3–70) along the entire vane height for relatively short stages or in the upper third or half of the stage height for relatively long stages (l/dm > 0.17).There are many other factors influencing the separation efficiency (such as the geometric forms of the nozzle vanes and the slot, the moisture dispersion, the velocity ratio, the Reynolds number, and so on), but they can be considered only as applied to the actual geometry and operating conditions of specific stages.
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Fig. 3–72. Estimation of the water separation factor for hollow nozzle vanes with two suction slots
Source : B. M.Troyanovskii, G.A. Filippov, and A. E. Bulkin112
The LP steam path of a Siemens low-speed (1,500 rpm) wet-steam turbine, with a 1,365-mm (54-in) LSB and intrachannel water removal from the last stage nozzle surfaces, is shown in Figure 3–73. In addition to (or instead of) the intrachannel water removal, for cases with very high erosion coefficients, Siemens also has proposed heating the last LP stage stationary nozzle vanes with steam taken from one of the LP steam extractions (Fig. 3–74).
Fig. 3–73. Steam path of Siemens’ low-speed wet-steam turbine with water removal from last stage nozzle surfaces
Source : B. M.Troyanovskii113
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Fig. 3–74. Heating hollow nozzle vanes for the last stages of Siemens’ turbines
Source : H. Oeynhausen, G. Roettger, J. Ewald, et al. 114
Moisture separating stages, or stage-separators
The quest for more effective water separation in the blade rows has led to the creation of special moisture separating stages, or moisture stage-separators (MSSs), which have been proposed at the Moscow Power Engineering Institute (MEI).115 Some of the previously mentioned turbine stages do have a somewhat increased separation capacity thanks to certain design features such as radial furrows and a shortened shroud of the rotating blades (Fig. 3–63). Nevertheless, these stages do not differ in principle from the adjacent ones.The specially designed MSSs provide increased separation efficiency at the expense of the stage efficiency. In the ultimate scenario, MSSs do not produce any useful work at all and can be completely removed from the turbine, resting on independent bearings. In this case, such an MSS, called a rotor-type separator, turns into an external MSR.
An MSS, if it remains in the steam path along with the other,“regular” stages, features a low enthalpy drop, an increased axial clearance between the nozzle and blade rows, a low pitch-to-chord ratio for the rotating blade rows, and special profiling and machining of both the stationary and rotating blades. Some results of experimental investigations for a few different types of MSSs are presented in Figure 3–75. In the experiments, it was found that the moisture separation efficiency varies differently with the speed to velocity ratio, u/c0 , for chambers positioned across the leading (inlet) or trailing (outlet) edges of the rotating blades.The total separation efficiency (for both the chambers),
however, remains virtually invariable, accounting to over 60%.
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