Operation 327
Fig. 4–53. Comparison of measured (I) and calculated (II) steam temperatures in the LP cylinder of K-220-44 turbine at start-up (numbering of temperature curves is given in accordance with designations in Fig. 4–50)
The obtained static and dynamic (Equation 4.1) characteristics refer to the steam temperature at the LP cylinder inlet, after the MSR (Fig. 4–52). However, this reheat steam temperature itself changes with the flow amounts of the heated and heating steam in the MSR. Knowledge of these dependencies is important to govern the reheat steam temperature at start-ups. As can be seen, for example, from results of heat-rate performance tests, with the full flow rate of the main heating steam to the MSR’s second stage, the reheat steam temperature slightly increases with a decrease in the turbine load.146 However, with partially opened valves at the steam-lines of heating steam, the pattern drastically changes. A qualitative pattern of static dependencies of the reheat steam temperature on the turbine load and position of the valve governing the heating steam flow is shown in Figure 4–54a.This graph is based on heat balance calculations and some experimental measurements (Fig. 4–54b). Switching on the generator to the grid and accepting the initial load sharply decreases the reheat steam temperature. The dynamics of these variations also substantially depend on the heated steam flow amount.According to
www.EngineeringBooksPdf.com
328 Wet-Steam Turbines for Nuclear Power Plants
the start-up tests for the K-220-44 turbines of the Kola nuclear power plant in Figure 4–55 and K-500-65/3000 turbines of the Chernobyl plant, the dynamics of the steam temperature after the MSR, regarding disturbances by the heating steam flow amount, is characterized by an initial delay (with a subsequent change in the exponential law) and can be approximated by the following transfer function:
W |
(s) = [exp(–sT1 )]/(1 + sT2) |
(4.3) |
For the K-220-44 turbine, the time constants are T1 |
1–2 min and |
|
T2 5–15 min, linearly decreasing with the increase in the turbine load. If the heating steam flow rate decreases, the reheat steam temperature changes are about 1.5 times more inertial than those with an increase in the heating steam flow rate, all else being equal.
Fig. 4–54. Qualitative static characteristics of the reheat steam temperature after the MSR and heating steam flow rate related to the turbine load and heating steam valve position (a) and variation of steam temperature after the MSR for the K-220-44 turbine at start-up (b), according to field start-up test data (G2 : heating steam flow rate to the MSR’s second stage; h : position of the valve governing heating steam flow rate to MSR; p2: steam pressure after the valve)
www.EngineeringBooksPdf.com
Operation 329
Fig. 4–55. Response of reheat steam temperature and heating steam pressure to disturbances by the valve governing the heating steam flow to the MSR of a K-220-44 turbine (1 and 2: steam temperatures after the MSRs on the left and right turbine sides, respectively; p2: steam pressure after the valve governing the heating steam flow amount to the MSRs)
In most cases, the initial, prestart temperature state of the turbine rotors can be taken relying on the measured casing metal temperatures, except for the LP cylinders.147 A relatively simple and original method of obtaining the cooling-down characteristics of the LP rotor was developed and employed in the previously mentioned start-up tests of the K-220-44 turbine at the Kola nuclear power plant. 148 The metal temperatures on the external surface of the LP rotor body at several characteristic sections were measured with special temperature probes at the stopped turbine (Fig. 4–50). The design features of the LP cylinder made it possible to use these probes in only three sections: in the steam admission zone and after the disks of the second and third stages.The probe was developed and manufactured as a long (about 3.5 m, or 11.5 feet) system of steel tubes, 14–40 mm in diameter (0.5–1.6 in). The thin, bottom end of the probe was furnished with a copper measuring head fixed in a fluoroplastic collar. The collar provided the head’s electric and thermal insulation from the main probe body. Measuring the rotor temperature was carried out by pressing the head to the rotor surface. To lead the probes to the measurement spots, three radial guide tubes were assembled in the bottom half of the cylinder. At the one end, these tubes were welded to the inner cylinder casing and entered to the level of the
www.EngineeringBooksPdf.com
330 Wet-Steam Turbines for Nuclear Power Plants
external diameter of the diaphragms. The opposite ends passed out of the outer casing via special glands that, being sufficiently tight, at the same time provided a freedom of thermal expansion for the tubes. The tubes were assembled so that the measuring heads of the probes fitted the smooth cylindrical portion of the rotor surface between the disk fillet and diaphragm gland seal, with regard to the relative thermal displacement of the rotor, and the probes were sunk in the radial grooves made in the diaphragm bodies. In the steam admission zone, the temperature probe reached the rotor surface through a special hole in the casing ring. Preliminarily, before installing at the turbine, the probe design and the procedure of measuring were worked up at a special bench, verifying the sensitivity of the measuring head. Measurements of the rotor surface temperatures were begun immediately after the turbine was shut down and the rotor ran down. Measurements were conducted approximately each hour, with the turning gear switched off for the period of measuring. Each set of measurements took no more than 4–6 min with steam supplied to the end glands and up to 10–12 min when sealing steam was not supplied. The sealing steam temperatures were measured in addition to the measurement set shown in Figure 4–50.
Results of measuring the LP rotor metal temperatures for two turbine outages are shown in Figure 4–56. Steam supply to the end seals during about one hour after closure of the turbine valves hardly affected the metal temperature near the first stages. The high cooldown rate of the rotor is noteworthy. The metal temperature on the rotor surface in the steam admission zone and in the vicinity of the first two stages decreased over 24 hours from 170–210°C to 85–90°C, with the cooling-down rate constant equal to 0.043 h-1 . (To compare, the cooling-down rate constant for the HP casing was approximately 0.012 h-1 .) In addition, an irregular nature of the cool-down process that manifests itself in a considerable difference of the cooling-down rate constants for different portions of the rotor is noteworthy.
www.EngineeringBooksPdf.com
Operation 331
Fig. 4–56. Cool-down characteristics of a welded LP rotor for the K-220-44 turbine (numbering of temperature curves [1, 2, 3, 4] follows notations in inset; I: experimental measured data for two outages; II: calculated data with no heat transfer from rotor surfaces within cylinder space; III: calculated data with regard to natural convection within cylinder space)
Source B. N. Lyudomirskii, A. S. Leyzerovich, and Y. N. Kolomtsev149
To analyze the influence of individual factors on the cooling-down process and connect the rotor’s surface metal temperatures with the temperature state in the rotor thickness, the cooling-down characteristics of the LP rotor were also calculated with the use of various models. The initial temperature field of the rotor was taken relying on the results of calculations for the stationary operating conditions (Fig. 4–10b). According to many traditional approaches and recommendations, the cooling-down process for LP rotors is mainly due to the heat transfer from the metal to the lubricating oil in the adjacent journal bearings, without any heat transfer from the rotor surfaces within the casing space. However, in this case, the calculation results considerably disagreed with the experimental data, and only an allowance of the natural convection heat transfer from the rotor surface to the wet air within the cylinder space yielded a satisfactory agreement with the experimental data.The intracylinder space is connected to the condenser and steam bleedings, except for the steam admission section shielded by the first stage’s casing ring.The heat transfer coefficients for the rotor surfaces were calculated on the basis of well-known criterial
www.EngineeringBooksPdf.com
332 Wet-Steam Turbines for Nuclear Power Plants
equations for free convection around a ribbed horizontal cylinder.The calculation results confirmed a considerable unevenness in the cool- ing-down rates for different parts of the rotor during virtually the entire cooling-down process.
The developed approach was later used in a more sophisticated computer program for calculating the cooling–down characteristics of turbine rotors.150 Results of two such calculations for the aforesaid LP rotor of the K-220-44 turbine and another welded LP rotor of Turboatom’s low-speed K-1000-60/1500 turbine (see Fig. 3–16) are shown in Figure 4–57. In the second case, the rotor cools down considerably more slowly because of its greater mass and greater thermal resistance to the axial heat fluxes with thermal conduction toward the journal bearings.
Fig. 4–57. Calculated cool–down characteristics of welded LP rotors for K-220-44 (1) and K-1000-60/1500 (2) turbines (first stage disks) (points denote experimental data from the Kola nuclear power plant)
Source :V. M. Kapinos, Y.Y. Matveev,V. N. Pustovalov, and V.A. Palei151
The measurements also showed that with a cooling duration of less than 1–2 days, the sealing steam cannot provide prestart warming of the LP rotors, but rather conserves the temperature state attained by the instance of giving the sealing steam to the end glands. Even for cold start-ups, prestart warming by sealing steam does not make it possible to reach rotor metal temperatures greater than the saturation
www.EngineeringBooksPdf.com
Operation 333
temperature corresponding to the back pressure in the condenser, and the warming process takes approximately 2–2.5 hours because of the great mass of the rotor.
More than 30 fast-response experimental temperature measurements were installed in the steam path of the LP cylinder at another K-220-44 turbine also operated at the Kola nuclear power plant to investigate the possibility of keeping the turbine rotating without steam, using the generator as a motor.152 As already said, such so-called motor operating conditions can be helpful to decrease the amount of time it takes to restart the power unit after shutting down the reactor for a relatively short term.These conditions are also inherent in the cases of emergency load discharges if the generator is not switched off from the grid. Unfortunately, the obtained results were not processed in full measure to extend them to other turbines and operating conditions. The main result of the experiments was the finding that all of the measured temperatures in the steam path remained much lower than the permissible level (230°C; 446°F), even with the turbine being rotated by the generator up to two hours with the synchronous speed, with all the HP and LP valves closed and steam in the turbine given only to the end seals to keep the back pressure in the condenser at a level of approximately 4.5–5.0 kPa (0.65–0.72 psia). With supplying the turbine with cooling steam passed into the LP crossover pipes, these operating conditions can be maintained for as long as necessary without violating the given limitations.
The main problem of all no-load conditions, that is, operating conditions with small (or relatively small) volumetric steam flow amounts into the condenser, is a possible overheating of the rotating blades, because of energy losses with friction and fanning. The temperature distribution along the height of the 1,200-mm (47-in) titanium LSB used, in particular, in LMZ’s high-speed 1,000-MW K-1000-60/3000 wet-steam turbine, in Figure 3–14, under no-load operating conditions is shown in Figure 4–58. Special experimental temperature measurements in the LP steam path at low steam flow conditions were also conducted by LMZ on a 1,200-MW supercritical-steam turbine also furnished with 1,200-mm titanium LSBs. 153
www.EngineeringBooksPdf.com
334 Wet-Steam Turbines for Nuclear Power Plants
Fig. 4–58. Measured temperature distribution along the height of LMZ’s 1,200-mm titanium LSB under no-load conditions (white points denote steam temperature measurements; black points denote metal temperature measurements; solid lines: temperatures at or near the leading edge; dashed lines: temperatures at or near the trailing edge)
Source :V.V. Malev and Y. N. Nezhentsev154
Along with temperature measurements, in situ investigations of LP steam paths can also include gas dynamic experiments under actual operating conditions, which are good supplements to bench tests on model turbines.The steam flow pattern is studied by means of special combined probes moved transverse to the steam flow over the stage radii (see Figs. 2–27 and 2–37).These probes allow measuring the main parameters of the steam flow with regard to its threedimensional nature: static and total pressure, stagnation temperature, and so on.These field tests are more expensive, more laborious, and allow less variability in operating conditions than the bench tests, but they provide the possibility to research the influence on actual steam flow patterns of many factors that cannot easily be reproduced in the laboratory.155
www.EngineeringBooksPdf.com
Operation 335
Field start-up tests often reveal some unexpected factors affecting turbine operation that cannot be foreseen prior to testing. During the start-up tests at the Kola nuclear power plant, for example, the fast-response steam measurements revealed some deep impulse-type decreases in the steam temperature at the HP cylinder entrance in the instants when the control valves were opened quickly—such as when the generator is switched on to the grid and accepts the initial load (Fig. 4–44).As this takes place, any possibility of water induction from the steam generators was excluded. Similar events were also observed at other turbines when the turbine rotation speed or load was sharply increased by opening the control valves.This was regarded as a sign of water induction into the turbine from the main steam-lines, and made the researchers assume the presence of water pools in the steam-lines that remained even with a full steam flow rate through the turbine, as well as when the turbine is stopped. For power units with superheated-steam turbines when they are stopped and cool down, these pools can be revealed by measuring comparatively the metal temperatures in the top and bottom outlying lines of the pipeline. For wet-steam lines, these comparative measurements cannot help, because both the top and bottom metal temperatures are practically equal and close to the steam saturation temperature, whether or not there is water in the bottom of the steam-line.
To prove the presence of such water pools and locate them, the main steam-lines were furnished with special metal temperature measurements (Fig. 4–59a).The pools were located by means of gradually reducing the steam pressure in the plugged steam-lines by opening the drainage valves with the MSGs and with the turbine stop valves closed. While the steam pressure is decreasing and the water pool is evaporating, the metal temperature at the bottom of the steam-line drops sharply, due to heat of vaporization, whereas the top of the steam-line continues to cool slowly. If the reduction of the steam pressure is stopped, the metal temperature at the steam-line bottom gradually levels with the temperature on top.This process continues until the water completely evaporates.To prevent forming such water pools, the steam-line routes were thoroughly checked and leveled to eliminate any sags and to supply necessary inclines toward the drain lines. Implementing these measures to a great degree prevented water induction into the turbines and the resultant erosion damages to the first-stage nozzles.156
www.EngineeringBooksPdf.com
336 Wet-Steam Turbines for Nuclear Power Plants
Fig. 4–59. Sketch of additional temperature measurements (a) and cool-down diagrams (b–d) for the main steam-lines of the K-220-44 turbine downstream from the main steam gates, according to field start-up tests at the Kola nuclear power plant (temperature curves are denoted according to measurement point numbers on the sketch;A: instants of opening drainage valves of steam lines between the MSG and stop valves; B: instants of closing drainage valves; p˝:
steam pressure after the MSG)
Source :V.B. Kirillov,A. S. Leyzerovich, and Y.V. Kolomtsev157
www.EngineeringBooksPdf.com