Design 205
Fig. 3–75. Efficiency of moisture removal for different types of MSSs depending on the speed-to-velocity ratio, u/c0 (1: for the chamber across the leading edge of the rotating blades; 2: for the chamber across the trailing edge of the rotating blades; 3: total for both chambers)
Source :V. I. Kiryukhin, G.A. Filippov, and O. I. Nazarov116
www.EngineeringBooksPdf.com
206 Wet-Steam Turbines for Nuclear Power Plants
MSSs can be designed either unshrouded or with a specially drilled roof-shaped shroud (Fig. 3–75c). The separation efficiency of MSSs in the power plant conditions was investigated at several turbines manufactured by Kaluga Turbine Works to drive the feed water pumps of nuclear power units with VVER-1000 reactors. The MSSs, made of titanium alloy, replaced the seventh stages in the turbines. The driving turbines have 450-mm (17.7-in) LSBs made of 13% Cr steel with the tip circumferential speed of up to 339 m/s (1,111 ft/s). The flow path has an advanced system of peripheral separation, and the diaphragms of the last two (ninth and tenth) stages are made hollow, with intrachannel water separation. Eight turbines operated in total for more than 70,000 combined hours with the seventh stages replaced by the MSSs. The rate of erosion in the LSBs for these turbines was about 2.5 times lower than that for similar turbines without the MSSs.The MSSs themselves were not affected by erosion.
External moisture separators and reheaters
Almost all modern wet-steam turbines are equipped with external MSRs, which are usually combined in common vessels, two or four MSRs per turbine.The MSRs are located after the turbine’s HP cylinder (section), before the IP section (if it exists) or the LP cylinders. Strictly speaking, external MSRs are not a portion of wet-steam turbines themselves, but rather represent a kind of turbine auxiliaries—along with the condenser, feed-water heaters, and so on—and so are not considered here in detail. More thorough information about external MSRs can be found in various sources.
Traditional external MSRs are made up of three parts: 1) an outer shell (a cylindrical vessel, often with formed heads welded to each end), 2) the moisture separator’s packing, (usually with chevron plates, or merely chevrons, which are also called louver-plated or corrugated-plate separator, or knitted wire mesh, which is also called a demister), and 3) one or more tube bundles for reheating the working (cycle) steam by the heating steam of a higher pressure. Different versions of MSRs have been designed with heating steam flowing either within the tubes or between them. Steam for reheating is commonly taken from the main steam-lines. For wetsteam turbines with two-stage steam reheat, the first stage is fed with the heating steam taken from the HP cylinder (see Fig. 2–1).
www.EngineeringBooksPdf.com
Design 207
As a rule, both the separating and reheating surfaces of the MSR are composed as separate modules to make their replacement easier. Some turbine producers (for example, ALSTOM, GE, Hitachi, MHI, and Westinghouse) use horizontal MSRs exclusively (Figs. 3–3, 3–10b, and 3–12b). Other manufacturers (such as Turboatom) prefer vertical MSRs (Fig. 3–27b). Some use both types, such as Siemens (Figs. 3–9 and 3–76). Vertical MSRs appear to be more compact, although in some cases, especially for single-circuit nuclear power units with hermetically sealed turbines boxes, it is difficult to fit large vertical MSRs in the turbine enclosure.
Fig. 3–76. Front views of Siemens’ 1,200-MW wet-steam turbines with horizontal (a) and vertical (b) MSRs
Source : By courtesy of Siemens Power Generation
www.EngineeringBooksPdf.com
208 Wet-Steam Turbines for Nuclear Power Plants
Both types of traditional external MSRs (with chevron plates and knitted wire mesh) can be called inertial separators, because it is just the inertia of water drops that prevents the water from following the steam streamlines. As the steam-water flow passes through the separator’s chevron plates or knitted wire mesh, the moisture particles drop out of the flow, and the impinged water then passes to the feed water system through drains. The separation efficiency improves with the increase of water drop size and steam velocity and with the decrease in the size of the collection elements (mesh or chevrons). For the knitted wire mesh, this size is determined by the wire diameter, which is about 0.1 mm (4 mil), and for chevron plates, the effective element size is equal to a half corrugation wavelength, which is normally not less than 10 mm (0.4 in).With increasing steam velocity, a breakthrough point is reached at which the deposited water is ejected by the steam flow again.This breakthrough velocity for the knitted wire mesh is significantly less than that for the chevron plates.
A principle scheme of horizontal MSR developed by Westinghouse is presented in Figure 3–77. It is considered to be the third MSR generation.117 The first-generation MSR had a demister section of stainless steel wire mesh.While mesh is able to effectively remove moisture, large areas placed in a horizontal configuration are needed. Because of this, meshes were replaced by vertical chevrons in secondgeneration MSR designs, allowing 45% more steam to pass through the unit. Third-generation units shown in Figure 3–77 were actually made up of two second-generation MSRs connected by a common inlet section.To improve the thermal cycle efficiency, two-stage steam reheat was used instead of single-stage reheat. The working steam from the HP cylinder exhausts passes to the MSR’s steam inlet manifold and is directed to the chevron plate separators (Fig. 3–78). The dried saturated steam passes over the outside of low-profile, integrally finned tubes.The heated steam leaves the MSR through outlets on top of the vessel and flows into the LP cylinders.
www.EngineeringBooksPdf.com
Design 209
Fig. 3–77. Principle schematic of Westinghouse’s third-generation MSR
Source : R. L. Coit, P. D. Ritland,T. F. Rabas, and P.W.Viscovich118
Fig. 3–78. Part of a chevron-plate separator
Source : R. L. Coit, P. D. Ritland,T. F. Rabas, and P.W.Viscovich119
www.EngineeringBooksPdf.com
210 Wet-Steam Turbines for Nuclear Power Plants
Other turbine producers went through similar design stages, Therewith, MSRs of different manufacturers and even of the same manufacturer but different generations may differ significantly in the arrangement of the steam and water flows and packing of the internal space. By way of illustration, Figure 3–79 demonstrates two types of horizontal MSRs developed by Stein Industrie for French nuclear power units with individual capacities of 1,000 and 1,350 MW. Vertical MSRs also feature many different designs. Figure 3–80 shows a vertical MSR developed for Turboatom’s K-220-44 turbines (Figs. 3–4 and 3–6). Figure 3–81 depicts another vertical MSR used in Siemens’ 1,300-MW turbines.
Fig. 3–79. Stein Industrie’s horizontal MSRs for French nuclear power units of 1,000-MW and 1,300-MW output
Source : J. C. Franc and D. Gilchrist120
www.EngineeringBooksPdf.com
Design 211
Fig. 3–80. Vertical MSR for Turboatom’s K-220-44 turbines (1: first steam reheater stage; 2: second steam reheater stage; 3: separator; 4: steam distribution chamber; 5: steam reheater’s tube assembly; A: working steam inlet; B: superheated steam outlet; C: heated steam supply for the first steam reheater stage; D: heated steam supply for the second steam reheater stage;
E: heated steam condensate outlet from the first steam reheater stage;
F:heated steam condensate outlet from the second steam reheater stage;
G:separated water outlet; H: drain)
Source : By courtesy of Turboatom
www.EngineeringBooksPdf.com
212 Wet-Steam Turbines for Nuclear Power Plants
Fig. 3–81. One of two vertical MSRs for Siemens’ 1,300-MW wet-steam turbines (1: working steam inlet; 2: superheated steam outlet; 3: steam reheater; 4: heated steam condensate outlet from steam reheater; 5: separator’s modules; 6: separated water outlet; 7: preseparator; 8: drain; 9: working steam inlet)
Source : By courtesy of Siemens Power Generation
www.EngineeringBooksPdf.com
Design 213
Designers always quest to make their MSRs more compact. Hitachi, for example, reduced the overall length of its horizontal MSRs by 28% by means of increasing the steam flow velocities in the MSR reheater tubes and removing the drain tanks from the MSR shell.121
Some turbine and MSR manufacturers have experimented with different nontraditional design decisions for moisture separation. In 1989, ABB developed and tested a new MSR system built in the LP crossover pipes between the HP and LP cylinders. This system was implemented at several nuclear power units with the outputs ranging from 720 to 1,100 MW (Fig. 3–82). 122 This system consisted of four moisture preseparators (MOPSs), one at each HP turbine exhaust, mainly separating the moisture flowing along the turbine walls, and special cross-under pipe separators (SCRUPS) installed in the crossover pipe elbows after the MOPSs.The main SCRUPS components are turning vanes installed in the internal chamber at the 90° elbows. In particular, the system was installed and tested at the Swiss nuclear power plant Leibstadt. The steam quality was measured using the sodium tracer method. These tests showed a separation efficiency of approximately 97% downstream of the SCRUPS and verified the predicted performance of the system. In eight of 10 other nuclear power units, measurements showed total separation efficiencies of more than 98%, with decreases in the pressure drop along the crossover (cross under) pipes. Observed increases in turbine output varied from 3 to 10 MW. In all cases, the installation of the MOPS/ SCRUPS system significantly reduced erosion-corrosion effects in the crossover pipes and the separators themselves. At the Swiss nuclear power plant Mühleberg, the wear on these carbon steel components decreased from 95 kg/yr to 3.1 kg/yr. Nevertheless, this system has not found wide acceptance.
www.EngineeringBooksPdf.com
214 Wet-Steam Turbines for Nuclear Power Plants
Fig. 3–82. (a) General view of ABB’s three-cylinder wet-steam turbine with “distributed” MSR system (a) (1: preseparators; 2: separators at the crossover pipe elbows; 3: steam reheaters); (b) preseparator with phase separation;
(c) preseparator without phase separation; (d) and separator (4: working steam flow; 5: steam extraction to feed water heater; 6: separated water extraction; 7: drain)
Source : P. von Boeckh, M. Stiefel, and U. Frick123
www.EngineeringBooksPdf.com