семинары / семинар 4 / Wet_Steam_Turbines_for_Nuclear_Power_Plants_By_Alexander_S_Leyzerovich

Operation 357

relying on the properly processed measured data. In modern practice, these objectives are mainly attained with the help of highly computerized data acquisition and control systems (DACS). An example of such a relatively up-to-date system is a digital complex called the Nuclear Power Plant Control Complex with Advanced Man-Machine Interface 90 (NUCAMM-90), developed by Hitachi in the mid-1990s and adopted for nuclear power units with ABWR-type reactors. In particular, this complex is implemented at Japanese Kashiwazaki-Kariwa Unit 7 with the rated output of 1,356 MW. 170 The complex includes a main control panel that comprises an intensively arranged set of principal operation and observation devices and a wide display panel that provides the main bulk of information on the entire power unit, so as to make the man-machine interface obvious, comfortable, and practical (Fig. 4–69). The display panel contains 226 fixed mimic displays, 231 hard switches, and 170 annunciators arranged so as to make the current status of the power unit comprehensible at a glance. The number of operation devices on the operator console of the main control panel is minimized and concentrated so that they can be easily observed from a sitting position. This was done by incorporating touch-screen operation, positioning flat displays (FDs) and color cathode ray tubes (CRTs) for easy observation, and expanding the scope of automatic control.As a whole, the main control panel has 465 CRTs and 153 FDs available for playback, and includes 325 hard switches, 15 FD push buttons, and 235 CRT annunciators.The operator load is reduced to a large extent owing to automatic power control, particularly at start-ups, shutdowns, and other transients, including program and logic control of the turbine and its auxiliaries, such as motorand turbine-driven feed water pumps, and so on. The diagram of starting up the unit under automated control is shown in Figure 4–5.

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358 Wet-Steam Turbines for Nuclear Power Plants

Fig. 4–69. Main control board of NUCAMM-90 for the 1,356-MW KashiwazakiKariwa Unit 7

Source : F. Mizuki,Y. Miyamoto, and T. Seiji171

The continuing progress in information technology has opened the way for further development of supervisory and control systems, in particular, to extend their capabilities. 172 The core of these systems has become supervisory operation via CRT terminals. Owing to multiwindow displays, main parameters can be displayed and operations are performed without interrupting the monitoring process. Because the majority of technological operations are performed automatically (using sequential, or logic, control), the operator can focus his or her attention on continuous monitoring and control. For a clearer understanding of the unit’s equipment conditions, the operator can use numerous diagrams, charts, and trend graphs that show the current and forecasted (to a degree) operational status of the monitored objects. An example of a multi-window screen of a supervisory and control system for nuclear power plants is shown in Figure 4–70.

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Operation 359

Fig. 4–70. Multiwindow CRT operation display of Hitachi’s supervisory and control system for nuclear power plants

Source:T.Yamamori,T. Ichikawa, S. Kawaguchi, and H. Honma173

Along with this, data acquisition and control systems for many older nuclear power plants were designed based almost entirely on analog or electromechanical C&I techniques that have now almost exhausted their lifetime, becoming less reliable, requiring more maintenance, and looking rather obsolete and ineffective compared with modern systems. For these reasons, the owners of some nuclear power plants are striving to upgrade their surveillance and control systems. For example, the analog C&I system at the Ignalina nuclear power plant in Lithuania, with two RBMK-1500 power units, is being replaced by one of the largest digital-control systems to be applied to a nuclear power plant anywhere in the world; the renovation involves the entire data acquisition and presentation system.174 Similar projects are also under way at other nuclear power plants.175

New, up-to-date C&I techniques provide many possibilities for better information support of the operational personnel and advanced automated control, in particular as applied to steam turbines. At the

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360 Wet-Steam Turbines for Nuclear Power Plants

same time, even relatively out-of-date (from the standpoint of today) C&I means used at older nuclear power plants provide possibilities of significantly better information support.

In parallel with the aim of providing the operator with more detailed and complete information, there is the problem of presenting this information in more compact and obvious forms. An abundance of information on the control board and supervisory terminals can frequently overwhelm and irritate an operator. As a rule of thumb, any irrelevant, excessive information on the control board is detrimental to the operation quality and should be considered harmful, because it diverts the operator’s attention. Because of surplus flows of information on the control panel, in many cases the operator never uses most of the data presentation options, relying instead on just a few of the most informative and convenient windows with relatively limited informative capabilities. As a result, along with an abundance of information, the operator is often short of some specific data and timely relevant advice. This information should be presented to the operator preprocessed and in the most capacious and representative graphic forms. For example, as applied to possible needs of varying the current turbine load, the operator, instead of tracing the current values of the dispersed set of parameters and indications—such as turbine output, metal temperatures, and temperature differences with nothing that relates them to each other—would prefer to see combined images where all of these data would be presented with their interrelations and allowable change ranges.176

For wet-steam turbines with a single leading indication of temperature conditions limiting the load change rate, this problem can be solved by presenting both the load and the leading indication to be monitored in the form of an N tm diagram, where tm is the characteristic metal temperature of the critical element.The leading indication, by definition, is the temperature difference across the thickness of the critical element and can be seen in the same diagram. An example of such an N tm diagram is given in Figure 4–71 as applied to Turboatom’s K-220-44 turbine. In this case, the characteristic metal temperature is that on the external surface of the HP casing flange, tfl.ext, in the section of the greatest temperature difference, and the leading indication is the temperature difference across the flange width in this section, ∆tfl (Figs. 4–13 and 4–45).The diagram is supplied with lines of constant temperature differences (∆tfl - isolines). Because the metal temperature on the heated

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Operation 361

surface can be considered equal to the saturation temperature, which in turn is treated as unequivocally dependent on the steam flow amount through the turbine (see Fig. 4–48), the actual value of the temperature difference across the flange width at any given instant is defined by the current correlation of the measured coordinate values N and tfl.ext in the isoline field. The vertical distance from this point to the isoline of the upper admissible temperature difference gives the range of the permissible load step change. During transients, the optimal loading should be led in such a way that the trajectory of the condition point in the given coordinates would move along the upper admissible isoline, just as in the case of the automated start-up shown in Figure 4–71c.During the entire process of loading (except the beginning stage and with forced suspensions), the temperature difference across the flange width was held at the set admissible level of 80°C (144°F).

Fig. 4–71. Two-coordinate loading diagram for Turboatom’s K-220-44 turbine and its application to start-ups at the Kola nuclear plant (a: daily loading diagram with two start-ups [I: 3:40 A.M.—generator is switched on to grid after two-day outage; II: 5:30 A.M.—turbine reaches the final load of 210 MW; III: 11:40 A.M.—turbine unloading begins; IV: 12:00 P.M.—generator is switched off;V: 9:07 P.M.—generator is repeatedly switched on to the grid;VI: 10:45 P.M.—turbine reaches the final load of 220 MW]; b: loading diagram of automated start-up after two-day outage; c: N t diagram of the same automated start-up after two-day outage)

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362 Wet-Steam Turbines for Nuclear Power Plants

Such diagrams were first implemented at the Kola nuclear power plant using special two-coordinate analogous recorders. Similar diagrams for the K-500-60/1500 and K-1000-60/1500 turbines at the Novovoronezh and Zaporozhe nuclear power plants were later presented on CRT displays of the computerized DACSs.

If the critical design element of the turbine is its HP rotor and the leading indication of the turbine’s thermal stress state is the effective radial temperature difference (between the heated surface and the integral average metal temperatures in the most stressed section), this temperature difference should be monitored by means of mathematical modeling of heating the rotor. For the purpose of operational monitoring, a one-dimensional calculation model provides sufficient accuracy (as well as in the case with temperature monitoring for the HP and IP rotors for steam turbines of fossil fuel power plants), which makes it possible to rely on minimum input measurements.177 Because the metal temperature variations for wet-steam turbines are quite limited, in this case, the mathematical model can be linearized—that is, the thermal conduction of the rotor steel is taken as invariable. Due to the high heat transfer conditions from wet steam, the heated surface temperature can be considered equal to the saturation temperature; but if the turbine is shut down, the HP rotor temperature is close to the measured metal temperature of the HP casing near the steam admission zone. All of these suppositions significantly simplify the problem of modeling as compared to that for rotors of superheated-steam turbines.

For forged or welded HP rotors without a central bore and for a unit-step function, that is, for the boundary conditions of ts t(R, ) = 1 and t(r, 0) = 0, the average integral metal temperature variation over time can be described by the following expression:

where Fo = (a /R2) is the Fourier number, and µn (n = 1, 2, 3, …) are the roots of the characteristic equation for the simple geometric form modeling the rotor (for forged, welded, or monoblock rotors without a central bore, the characteristic equation is J0(µ) = 0, and Bn = 4/µn2 ). With an increase of n the values of µn rise rapidly and monotonously,

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Operation 363

and the values of Bn tend toward zero, thus for some numbers n > m the value of Bn can be considered equal to zero, and Equation 4.6 becomes:

Equation 4.7 with m = 3 corresponds to the functional chart of the device for the temperature monitoring of an HP rotor for wet-steam turbines shown in Figure 4–72. The device’s outputs

thermal stress on the heated surface in the considered section. In the same way, though with somewhat less accuracy for the same m, it is possible to derive the metal temperature at the rotor axes, t0 , and the full temperature difference across the rotor radius, ∆t = ts t0 178

Fig. 4–72. Functional chart of a device for temperature monitoring of the HP rotor for wet-steam turbines (1: nonlinear transducer; 2: logic element; 3: switchboard; 4: switch; 5: summators; 6: aperiodic elements; 7: amplifier)

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364 Wet-Steam Turbines for Nuclear Power Plants

For a welded disk-type HP rotor, Figure 4–73 compares the changes in the metal temperatures and temperature differences along the rotor radius calculated using different mathematical models: 1) a “precise” two-dimensional model, taking into consideration the axial temperature fluxes in the rotor metal (Fig. 4-12b); 2) a“precise” one-dimensional model, and 3) an approximate, simplified model, corresponding to the block chart of Figure 4–72 and used as the basis for the temperature monitoring device.The main source of inaccuracy lies in neglecting the two-dimensionality of the rotor temperature field, whereas the simplification of the one-dimensional model (rejecting the row terms with n > 3 in Equation 4.6, that is, transition to the form of Equation 4.7) does not affect the final result as much. The total modeling errors do not exceed 5–8°C, and that seems quite acceptable.

Fig. 4–73. Calculation of heating-up the HP rotor of a K-500-65/3000 turbine during start-up and subsequent load change using different mathematical models (1: precise two-dimensional model; 2: precise one-dimensional model; 3: approximate one-dimensional model simulating the device for temperature monitoring of the rotor)

A similar functional chart was also developed for the welded and monoblock LP rotors of wet-steam turbines without an IP section. 179 The model in Figure 4–74 is closer to those developed for the HP and IP rotors of large superheated-steam turbines for fossil fuel power plants and takes into consideration the variation over time of the heat transfer conditions from steam to the rotor surface, depending on the steam flow rate through the turbine. In this case, the heat flow from

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Operation 365

steam to the heated surface is determined as the difference between the heated steam and surface temperatures multiplied by the Biot number. In turn, the Biot number, Bi, is taken as a nonlinear function of the steam pressure at the cylinder inlet; this dependence is calculated using known dimensionless, criterial equations. Relying on results of experimental investigations, as shown in Figure 4–52, the heating steam temperature for the most thermally stressed section of the rotor is determined in reference to the measured reheat steam temperature after the MSR at the cylinder inlet, with some correction taken as a nonlinear function of the measured steam pressure at the cylinder inlet.The average integral metal temperature of the rotor section is determined by integrating the heat flow from the heated surface with the time constant, T0 = R2/2a (where R is the external radius of the rotor body, and a is the thermal conduction of the metal).The metal temperatures on the heated surface and the rotation axis are calculated by summing the outputs of the aperiodic elements with the time constant, Tn = R2/(a × µn2), where µn are the roots of the characteristic equation J1 (µ) = 0.As applied to the scheme of Figure 4–74,

m is taken equal to 2. For the external surface ( = r/R = 1) and the rotor axis ( = r/R = 0), the influence factors for the aperiodic elements are calculated as follows:

kn = 2 × J0 (µn )/[µn2 × J0 (µn)]

Unlike the HP and IP rotors, the initial, prestart metal temperature of the LP rotor cannot be set by measuring the metal temperature of the casing. So, when the turbine is shut down, the decrease of the average integral metal temperature of the rotor is assumed to follow an exponential curve, and the cooling time constant, Tc is established on the basis of experiments or calculations based on experimental data (Figs. 4–56 and 4–57).

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366 Wet-Steam Turbines for Nuclear Power Plants

Fig. 4–74. Functional chart of a device for temperature monitoring of LP rotors for wet-steam turbines (1: summators; 2: nonlinear transducers; 3: aperiodic elements; 4: multiplier; 5: switch; 6: amplifier; 7: integrator; 8: switchboard; tMSR: steam temperature after MSR; pMSR: steam pressure after MSR; ∆t: complete temperature differences along the rotor radius)

If the turbine’s most thermally stressed element is the HP rotor and the turbine is furnished with a device for the temperature monitoring of the rotor (or if the turbine’s critical element is the HP casing flange and the metal temperature on the external flange surface in the most stressed section can be reliably measured), the operator can be continuously provided with the data of the current upper and lower boundaries for admissible load changes.This can be done using Equation 4.4 and the characteristic metal temperature, tm which is set equal to either the calculated (modeled) average integral metal temperature of the HP rotor or the measured metal temperature on the insulated external surface of the casing flange—depending on the turbine design. For some turbines, the admissible load boundaries should be determined with regard to the current temperature state of both the HP rotor and flange.

The described devices for the temperature monitoring of the HP and LP rotors and determining the admissible load change boundaries were consecutively developed as applied to the use of different kinds of computational techniques, such as analogous devices, microprocessor-based programmable controllers, special microprocessors built into the turbine’s electrohydraulic governing system, computers of the power unit’s DACS, and PCs specially built

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