Design 145
to the tip circumferential blade speed and for nontight flange joints, Kf = 0.08, with the steam flow velocity calculated from the pressure drop.This approach gives rather qualitative estimates. It is clear from Siemens’ data presented in Figure 3–26b, where the expected, calculated ECW rates are plotted against their actual measured values for several wet-steam turbines.
Experimental and calculated data show that for stator elements fabricated from carbon steels the ECW rate can reach as much as 4–5 mm/yr ( 200 mil/year). The most characteristic places for ECW in the HP cylinder of the Turboatom K-220-44 turbine are pointed out in Figure 3–27. The maximum ECW occurs with a steam wetness of over 5% and a temperature range of 160–200°C, which completely corresponds with the Siemens data, as well as the zones of most intense ECW outlined in the Mollier diagram in Figure 2–3.
Along with ECW, crevice erosion is prevalent in the flange joints of casings, rings, and diaphragms, and its rate often exceeds the mean rate expected based on local steam conditions. For these elements, it is especially important to prevent a through steam leakage across their joints that bears a positive feedback; the greater the leakage, the more abrasion there is due to wear. Leakage through the HP cylinder’s outer casing flanges is especially unacceptable for turbines working with radioactive steam. To enhance the tightness of the flange joints in their K-220-44 turbines, Turboatom had to increase the tightening force for the bolts in the rigid belt zone and bolt the diaphragm halves together. 43 Some other wet-steam turbine producers also had to increase the number of bolts in the flange joints of HP cylinder casings.
Intense ECW could be prevented by using high-alloy chromium steels for endangered elements or by plating the exposed areas with similar materials. Erosion-corrosion wear of elements composed of 2.5% chromium steel drops to approximately one quarter of that expected with carbon steel, and elements made from 12–13% chromium steel suffer almost no ECW at all. Comparative calculations for the LMZ K-1000-60/3000 turbine showed that it is finally more profitable to fabricate the HP cylinder casings upon the whole from stainless steel (Fig. 3–14).44 The HP cylinder casings of some ALSTOM’s 1,000-MW and 1,350-MW turbines are made from 13% Cr steel, with
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146 Wet-Steam Turbines for Nuclear Power Plants
stainless steel piping. Along with this, many turbine manufacturers prefer to protect the endangered elements with special shields or stainless steel deposits (Figs. 3–23 and 3–28). In addition, the surfaces of flange joints are built up by welding or spraying with highly resistant alloys.
Fig. 3–27. Typical ECW locations in the HP cylinder (a) and MSR (b) of
Turboatom’s K-220-44 turbine (the black dots denote areas of the most intense ECW, with their sizes corresponding to the relative ECW rate)
Source : O.A. Povarov, G.V.Tomarov, and V. N. Zharov45
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Design 147
Fig. 3–28. Areas of corrosion-erosion protection in the turbine steam path and gland seals of ABB’s (a) and Siemens’ (b) wet-steam turbines
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148 Wet-Steam Turbines for Nuclear Power Plants
Steam admission elements
Huge steam flow amounts for large wet-steam turbines require special attention to their steam admission elements. With raising the turbine output, the size of the turbine valves and the energy amounts dissipated in them also increase. Simultaneously, it becomes more difficult to provide their tightness. Increased valve size results in lower natural frequencies of the moving elements (the valve itself and the valve stem).This also increases a risk of high-amplitude vibrations of the valves. There is a rule of thumb that calls for limiting the steam velocity in HP valves and their inlet pipes to 75 m/s ( 250 ft/s).This standard has stood the test of time, but designers of large wet-steam turbines are often forced to ignore it.
To reduce pressure drops, some turbine manufacturers integrate the HP stop and control valves into a combined unit using sometimes even a common saddle.An example of such a combined valve unit developed by Brown Boveri is shown in Figure 3–29a. It resembles the HP valve unit developed by the same company for the supercritical 1,300-MW turbines manufactured for U.S. fossil fuel power plants.46 In particular, the valve unit shown in Figure 3–29a is applied to the 1,160-MW wet-steam turbine installed at the Donald C. Cook nuclear power plant; the turbine is furnished with four such units (Fig. 3–10b). A more traditional design of the stop and control valves integrated in a unit with the common saddle is presented in Figure 3–30a, as developed for Turboatom’s 1,000-MW-class low-speed wet-steam turbines. Another design, this one used in the HP steam admission unit (one of four) in LMZ’s 1,000-MW wet-steam turbine, is presented in Figure 3–31. In this case, a traditional stop valve is replaced by a turngate, a kind of butterfly valve.
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Design 149
Fig. 3–29. One of four combined HP stop and control valve units (a) and butterfly-type intercept valve (b) for Brown Boveri’s large wet-steam turbines
Source : B. M.Troyanovskii47
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150 Wet-Steam Turbines for Nuclear Power Plants
Fig. 3–30. Combined HP stop and control valve unit (a) (1: casing; 2: saddle; 3: control valve; 4: stop valve; 5: relief cylinder; 6: steam sieve; 7: stop valve stem; 8: control valve stem; 9: crossarm; 10: springs; 11: stop valve lever) and intercept valve (b) for Turboatom’s low-speed wet-steam turbines
Source : By courtesy of Turboatom
Fig. 3–31. HP control valve and stop turn-gate for LMZ’s K-1000-60/3000 turbine
Source : By courtesy of LMZ
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Two other design schemes for the HP stop and control valve units are presented in Figures 3–32a and 3–33. These are implemented in the GEC Alsthom’s 630-MW wet-steam turbine, and various Siemens turbines, respectively.
Fig. 3–32. HP stop and control valve unit (a) and LP butterfly valve (b) for GEC Alsthom’s 630-MW wet-steam turbine
Source : Data from J.A. Hesketh and J. Muscroft48
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152 Wet-Steam Turbines for Nuclear Power Plants
Fig. 3–33. HP stop and control valve unit for Siemens’ wet-steam turbines (1: actuator; 2: steam strainer; 3: actuator; 4: stop valve; 5: control valve)
Source : Data from U. Sill and W. Zörner49
Significant potential for improving valve aerodynamics by means of streamlining the valve’s geometric shape were revealed by ALSTOM while developing the control valves for their 1,500-MW Arabelle turbine. 50 The resultant design is presented in Figure 3–34. Serial wind tests were performed on scale models to classify the different geometric forms.These experiments were then completed by actualsize tests at one of the 1,000-MW nuclear power units and led to the design of the control valves mounted on the Arabelle
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Design 153
Fig. 3–34. HP control valve of ALSTOM’s 1,500-MW wet-steam turbine
Source: Data from ALSTOM
Most wet-steam turbines are designed with a throttle control, that is, with full-arc HP steam admission. In doing so, all of the HP control valves are handled simultaneously as if they were a single valve, and the steam flow entering the turbine is forwarded to the
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154 Wet-Steam Turbines for Nuclear Power Plants
entire nozzle arc of the first HP stage. When the turbine operates under a partial load, the entire steam flow is throttled in the control valves down to a pressure value approximately proportional to the current turbine load. Such a way of control, combined with a relatively small available heat drop for the turbine as a whole, leads to considerable efficiency losses under partial-load operating conditions. Thus, for the Siemens 1,300-MW turbine, the decrease of the load to 52% of MCR results in a 12% increase in the heat rate (Fig. 3–8). The lower the initial main steam pressure and available heat drop, the greater the efficiency loss.
It is well known that the nozzle-group control mode is more efficient for partial-load operating conditions. Nevertheless, the Turboatom K-220-44 turbine with its nozzle-group control is an exception (Fig. 3–4).Turbines with throttle control are simpler in design—they do not need a special nozzle box and a control stage with an enlarged diameter and greater enthalpy drop; the first HP stage does not differ from the other stages, and, as a result, the HP casing gains a more favorable outline. In addition, with everything else being equal, turbines with nozzle-group control are always a little less efficient under a full load because of the energy losses in the first stage related to its partial-arc steam admission. More importantly, nuclear power units are commonly assumed to predominantly operate under base-load conditions, producing the maximum possible output, because their power production costs are mainly defined by capital expenditures and are less dependent on fuel costs.This reasoning is absolutely correct, but in reality it is not always possible to keep nuclear power units operating under continuous full load, especially with the higher reliance of power systems on nuclear power. In many cases, individual nuclear power units ought to participate in covering the variable part of the power consumption graphs and operate under a lessened load. Nevertheless, most wet-steam turbine producers prefer to design them with throttle control.
Turbine manufacturers try to make the steam path between the HP and LP cylinders, including the MSR, as compact as possible to reduce the amount of latent energy stored in the enclosed water, and minimize turbine overspeeding after load rejections with switching off the generator.The overspeed tests performed by Siemens on their early wet-steam turbines for nuclear power plants (one with an output of 662 MW and rotation speed of 1,500 rpm
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