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Operation 347

dependencies between these steam temperatures and the turbine load. If the loading rate can be limited by two (or more) different leading indications for the HP cylinder, the admissible values of the heating steam temperature and load are defined for each element with inserting technological limitations for the absolute value of the current load and the rate of its change.The final admissible value is the minimum of the two (or more).The heating steam temperature values for each element are defined from the found load value, and these steam temperature values are used to calculate the temperature fields of the considered elements for the next moment in time. The static dependence, N(θ), and the reverse dependence, θ(N), are assigned based on design calculations or data of measurements at an actual turbine in service.

Depending on the turbine design features, the critical elements of the HP cylinder that determine the flexibility of a wet-steam turbine can be the rotor or the outer casing flange.The radial temperature difference in the rotor is limited by the value [∆tr] from the standpoint of low-cycle (thermal) fatigue of the rotor steel, and the admissible value of the temperature difference across the flange width, [∆tft ], is determined by the yield strength of the casing steel.The most-stressed sections of the rotor and flanges are swept with steam of different temperatures. Under the nominal load, these temperatures are tst(r) and

whereas the initial metal temperatures of the rotor and flange (before the turbine is started-up) are assumed to be equal: tin(r) = tin(ft) The flange of the HP cylinder can be eliminated as a critical element if its width, H, is smaller than the critical value, given as:

where ∆R is the thickness of the HP rotor in the most-stressed section; ar and aft are the temperature conduction of steel for the rotor and flange, respectively; and kr and kft are the shape influence factors, equal to the ratio of the temperature difference, ∆t, for the considered element to the temperature difference in a plate of the same thickness under quasi-stationary heating.This expression was applied to various types of wet-steam turbines. For turbines with a typical correlation of the flange sizes, kft varies in the range from 1.2 to 1.5.This value takes into consideration flange holes with an air gap between the flange and bolt, as well as a heat flux on the external flange surface outward through the thermal insulation. For forged rotors, kr is approximately 0.55–0.60.

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348 Wet-Steam Turbines for Nuclear Power Plants

As applied to a series of Soviet wet-steam turbines for nuclear power plants, the calculation results showed that the HP flange should be treated as the only critical element for such wet-steam turbine types as the K-220-44 (see Fig. 3–6) with the forged HP rotor and the K-500-60/1500 (see Fig. 3–18) and K-1000-60/1500 (see Figs. 3–13 and 3–16), with their welded drum-type, thin-walled HP rotors. For other turbine types (for example, the K-500-65/3000, with the welded disk-type HP rotor, or the K-750-65/3000 and K-1000-60/3000, with the forged HP rotors), both the rotor and flange could be considered as critical elements, alternating for different types of transients or their different stages (Fig. 4–14). In addition, a large factor is the quality of the HP casing’s thermal insulation.This quality can be characterized by the value of the temperature difference across the flange width under stationary operating conditions, and it significantly influences the values of allowable changes to the turbine load without rate limitations.

The critical elements for the cylinders downstream from the MSR are usually welded or forged LP rotors. Sometimes, however, if the turbine has a separate IP section (like some wet-steam turbines of ALSTOM, Siemens, and Turboatom), its rotor or casing flanges can become the critical elements.The heat transfer coefficients from superheated steam after the MSR to the metal of these elements (IP casing flanges, IP or LP rotors) are relatively low and vary appreciably with an increase in the steam flow through the turbine (Fig. 4–47b). Under these conditions, in order to heat these elements more efficiently, it is reasonable to raise the steam temperature after the MSR to the nominal level, while the turbine is rolling-up and idling and the heat transfer coefficients are quite low, then decrease this temperature when the generator is switched on to the grid and accept an initial load (with a significant increase in the heat transfer conditions in the cylinder), and, finally, gradually raise the steam temperature again to the nominal value during loading. This nonmonotonous steam temperature diagram is accomplished relatively easily, because the steam temperature after the MSR, tMSR , decreases with an increase in the heated steam flow rate if the valve governing the heating-steam flow is partially opened (Fig. 4–54).

The diagrams for raising the steam temperature after the MSR are also optimized on the basis of Equation 4.4. In doing so, the diagrams of turbine rolling-up and loading determine the change in the

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Operation 349

Biot number for the critical element(s). Results of such optimization as applied to a cold start-up of Turboatom’s K-500-60/1500 turbine with an integrated HP–IP cylinder are presented in Figure 4–15.The diagrams for both loading and raising the steam temperature after the MSR are determined by the temperature differences in the outer casing of the HP–IP cylinder.

Another example of the diagram for the steam temperature after the MSR being close to optimal at actual start-up under automated control as applied to the K-220-44 turbine can be seen in Figure 4–80. In this case, the diagram is conditioned by the thermal stresses in the welded LP rotors.

It becomes more difficult to optimize the diagrams for raising the steam temperature after the MSR if the individual capacity of the turbine is 1,000 MW or more. In this case, the long LSBs are intensively heated under no-load conditions and do not permit an increase in the steam temperature before the LP cylinder. On the other hand, large LP rotors of large turbines are heated with greater inertia and have no time to be heated effectively while the turbine is rolling-up, even if the heating-steam temperature were raised as much as possible. This can be seen, for example, with LMZ’s K-1000-60/3000 turbine (Fig. 4–64).The radial temperature difference in the LP rotors reaches its maximum value in 60–75 minutes after the steam temperature increase. This is commensurable with the length of the entire startup time. For the original design version of this turbine with welded LP rotors, it was necessary either to slow down the loading process at warm start-ups because of the thermal stress in the LP rotors or to allow accepting the nominal load while tMSR was far below the nominal value. Transition to forged, solid LP rotors without a central bore removed this problem, because the metal of these rotors permits greater thermal stresses (see Fig. 3–14).

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350 Wet-Steam Turbines for Nuclear Power Plants

Fig. 4–64. Calculation of heating for welded (1) and solid (2) LP rotors of LMZ’s K-1000-60/3000 turbine at warm start-ups (tLPR: LP rotor metal temperature at the axis in the section through the second-stage diaphragm seal; ∆ tLPR: entire temperature difference along the LP rotor radius in the same section)

Source :A. S. Leyzerovich, B. L. Levchenko, and V. B. Kirillov166

The described methodology was verified during field tests for different types of wet-steam turbines (Fig. 4–44). Results of start-up optimization for different turbines are presented in Figures 4–12 through 4–15. For some of them, the start-up rate is limited by the temperature differences across the HP casing flange width (Figs. 4–13 and 4–15); for others, the critical element is the HP rotor (Fig. 4–12); and sometimes, as with LMZ’s high-speed 1,000-MW turbine, the critical elements are alternately either the HP rotor or the outer casing flange.The diagram for raising tMSR can also be dictated by the heating of either the IP casing flanges or the LP or IP rotor(s).

The optimized start-up diagrams for different initial temperature conditions should be generalized and unified in their structure (Figs. 4–12, 4–13, and 4–65). This is important for constructing both start-up instructions for manual control and start-up programs for automated control.

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Operation 351

Fig. 4–65. Optimized calculated schedules of hot (1), warm (2), and cold (3) start-ups for Turboatom’s K-1000-60/1500 turbine (tflHP: metal temperature on the external HP flange surface; ∆tflHP : temperature difference along the flange width in the HP steam admission zone)

Source :A. S. Leyzerovich,V.B. Kirillov,V.A. Paley, and V.L.Yasnogorodsky167

For the no-load operating conditions of wet-steam turbines, the heated steam temperatures for the critical elements of the HP cylinder do not differ substantially from the initial metal temperatures, except for the very hot start-ups after the turbine trips with a load discharge,

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352 Wet-Steam Turbines for Nuclear Power Plants

when the turbines have no time to cool down.As a result, no undesirable temperature differences and thermal stresses can be expected in the HP cylinder elements while the turbine is running up. Raising the turbine rotation speed can be done in the way that will be most comfortable for the operator, and the running-up diagrams can be unified for the turbine start-ups from almost all the initial temperature conditions.This way, the running up diagrams can be reduced to four discrete operations: 1) rolling up the turbine from the turning gear to a certain intermediate rotation speed, 2) holding at this level while listening to the turbine, 3) raising the rotation speed to the synchronous level, and 4) holding at the synchronous (or subsynchronous) speed until the generator is ready to synchronize and accept the initial load. The rate of raising the rotation speed can be maintained constant in the entire range of the rotation speed changes and independent of the start-up type. The amount of time the turbine is held at the intermediate rotation speed level should not depend on the turbine temperature state, except for hot start-ups, when it is desirable to omit any delay before the turbine is brought up to the synchronous speed and the generator is switched on to the grid.

For cold start-ups, there is little point in slowing down the running up process to heat the IP or LP rotors at the intermediate rotation speed—it is too ineffective because of low heat transfer conditions. It is more reasonable to heat the rotors by steam passed to the end gland seals before the start-up begins. However, this process takes a significant amount of time and goes only up to the saturation temperature; that is why this prestart heating is advisable to conduct with a decreased vacuum in the condenser. Some calculated curves for prestart heating of welded LP rotors with an external body diameter of approximately 1,200 mm (47 in) are shown in Figure 4–66 (the metal temperature shown in the diagram refers to the rotor axis).These rotors are standard for Turboatom’s high-speed wet-steam turbines with individual capacities of 220, 500, and 750 MW.The cooldown characteristic of such a rotor is presented in Figure 4–56. For other rotors, the time taken for heating varies as the square of the rotor diameter.

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Operation 353

Fig. 4–66. Calculated temperature curves for prestart heating of standard welded LP rotors of Turboatom high-speed wet-steam turbines with different initial temperature conditions and vacuum in the condenser (1: for vacuum of 400 mm Hg [7.7 psi]; 2: 540 mm Hg [10.4]; 3: 650 mm Hg [12.5 psi])

The loading diagrams for start-ups from different initial conditions can also be subjected to unification. Optimizing the loading process allows developers to present it consisting of two parts: 1) the initial loading, with the maximum rate, up to a level depending on the initial temperature state of the HP cylinder and 2) subsequent loading, with the rate determined by the current load value. Examples of these dependencies for Turboatom’s K-220-44 turbines with nozzle-group and throttle control are presented in Figure 4–67.The same approach can also be used for other wet-steam turbines if their loading rate is determined by the only indication of the HP section’s temperature state (the temperature difference across the flange width or along the rotor radius). Loading diagrams constructed with this approach and laid in the basis of instructive start-up diagrams for the most characteristic initial temperature conditions of Turboatom’s low-speed 1,000-MW turbines are shown in Figure 4–65. 168 If the loading rate can be alternately limited by two or more indications (for example, the temperature difference across the flange width and along the rotor radius), the start-up diagrams take somewhat more complicated, individual forms (Fig. 4–14).

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354 Wet-Steam Turbines for Nuclear Power Plants

Fig. 4–67. Dependencies of the upper level of initial loading with the maximum rate on the initial temperature state (a) and the rate of further loading on the current load (b) for Turboatom’s K-220-44 turbines with nozzle group (1) and throttle (2) control (t flHPin : initial metal temperature of the HP casing flange)

Optimal control of the reheat steam temperature after the MSR for most wet-steam turbines during start-ups begins with opening the valve(s) on the steam lines feeding the steam reheater with heating steam up to an intermediate position while the turbine is rolling-up and until reaching the synchronous rotation speed (for turbines with two-stage steam reheaters, this refers to steam passed to the second stage; the steam-lines to the first stages should be completely open from the beginning). This makes the reheat steam temperature rise to near the rated value. Accepting the initial load after switching on

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Operation 355

the generator causes some decrease in the reheat steam temperature. While loading the turbine, the heating steam valve(s) should be gradually opened all the way. The resultant diagrams for the reheat steam temperature can be seen in Figures 4–15 and 4–80. Specific recommendations depend on the turbine design features (a danger of overheating the LSBs under no-load conditions, the presence or absence of an IP section, the LP rotor type, and so on).

For wet-steam turbines with combined LP rotors (with shrunk-on wheel disks), if the steam temperature at the LP cylinder entrance (after the MSR) is not controlled, a rapid increase in the metal temperature of the disks and the resultant high tensile thermal stress on the disk bore surfaces can unshrink the disks, and this factor alone can limit the rate of raising the reheat steam temperature after the MSR. If the maximum load achievable under the partial reheat steam temperature is limited, any delays in raising this temperature can hamper the loading process.

Optimizing the start-up loading diagrams for wet-steam turbines is based on two assumptions: 1) the presence of a single-valued dependence of the heating steam temperature for the critical HP elements on the turbine load and 2) diminutive, negligible differences between the metal temperatures on the heated surfaces and the heating steam temperatures (the saturation temperature for wet steam). The same principles are applicable to finding the range of admissible load changes without limitations to their rate when the turbine operates under load. The original temperature conditions determining these opportunities are characterized by the value of in Equation 4.4. For wet-steam turbines with bulky welded or forged HP rotors, these changes are limited by the thermal stress state of these rotors, and the characteristic metal temperature is the average integral rotor temperature in the most thermally stressed section (Fig. 4–11).

The admissible load change ranges for the Turboatom K-1000-60/ 1500 turbine are shown in Figure 4–68. For this turbine, the leading indication is the temperature difference across the HP outer casing flange width. Under the original load, N, the characteristic metal temperature on the external flange surface, tm, depends on the initial (stationary) value of this temperature difference.Therefore, the limitations on the turbine load changes are different depending on whether

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356 Wet-Steam Turbines for Nuclear Power Plants

they are to increase or decrease the turbine load.This difference also depends on the thermal insulation quality of the HP flange: the better this insulation, the less the initial (stationary) temperature difference across the flange width and the less the difference between the admissible load changes for loading and unloading.

Fig. 4–68. Range of admissible load changes for the Turboatom K-1000-60/ 1500 turbine with various qualities of thermal insulation of the HP casing

(1: steady-state temperature difference across the HP casing flange width equal to 40 °C; 2: steady-state temperature difference across the HP casing flange width equal to 20 °C)

Source :V.B. Kirillov and A. S. Leyzerovich169

Information Support for Operators

and Automated Control During

Turbine Transients

Control and instrumentation (C&I) systems for nuclear power plants should support the operational personnel in observing the state of equipment. These systems should provide the operator with proficient, timely, and comprehensive information about equipment condition during stationary and transient operating conditions, as well as well-grounded assistance in alarming or ambiguous situations,

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