семинары / семинар 4 / Wet_Steam_Turbines_for_Nuclear_Power_Plants_By_Alexander_S_Leyzerovich

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mass and heat-transfer conditions for the rotors and casings make wet-steam turbines less prone to significant changes in RRE and, as a result, decreases in axial clearances in the gland seals, with possible brushing and rubbing as a consequence. Great rigidity of the rotors also diminishes the hazard of increased vibration and radial brushing. On the other hand, the operating reliability of wet-steam turbines is hindered by the fact that almost the entire turbine path is swept with wet steam featuring a high content of coarse-grained water, which promotes the intensity of corrosion-erosion processes. In addition, the combination of a large individual capacity and low steam parameters leads to the design of wet-steam turbines with a great number of cylinders and long LSBs, which can also lead to problems in operation.

The comparative influence of different damage mechanisms on the steam paths for different types of steam turbines is presented in Table 4–5.68 The specific focus of this table is damage to the fixed and rotating blades, as well as to the blade attachments (including both the blade roots and the attachment areas on the disk or shaft), lacing and tie wires, shrouds, tenons, and so on. The table does not cover specific damages to rotor bodies, casings, casing rings, seals, valves, pipes and other components beyond the steam path. Nevertheless, it is obvious that wet-steam turbines are not prone to many factors that are influential for superheated-steam turbines of high steam conditions. In particular, this refers to the phenomena of creep, creepfatigue, solid particle erosion, and copper deposition. If wet-steam turbines are operated in a base-load mode, it is understandable that they are not prone to low-cycle (thermal) fatigue. Thus, the primary causes of damages to wet-steam turbines are water erosion and corrosion, in all of their different forms. Considerations of damage that are common to steam turbines of both fossil fuel and nuclear power plants can be found in various editions. In addition, a general outline of operating experiences relating to damages of wet-steam turbines is presented in special articles. Some damages that are most generic to wet-steam turbines are considered below, although they can also be observed in LP cylinders of superheated-steam turbines.

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278 Wet-Steam Turbines for Nuclear Power Plants

Table 4-5. Relevance of steam path damage mechanisms for various steamturbine types

Notes: “xx” indicates damages commonly found or presenting a major problem when found;

“x” indicates damages that can be found but present a lesser problem;

“uc” indicates damages uncommon to have occurred in this type of turbines. Source: T.H. McCloskey, R.B. Dooley, and W.P. McNaughton69

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Stress-corrosion and corrosion-fatigue cracking

Corrosion-assisted fatigue has been responsible for the cracking and failures of many rotors and wheel disks of wet-steam turbines. Cracking of the LP rotors of AEG’s 660-MW low-speed (1,500 rpm) turbine at Würgassen is one of the best-known instances of this type of damage.70 Cracks were detected in the metal of both LP rotors in 1974 after 6,800 hours of operation.The turbine was stopped because of a high level of shaft and bearing vibration. Examinations revealed the cracks initiating from the axial stress-relief slot at the transition from the seating of the generator-side disk to the central thrust collar (Fig. 4–24).The crack in the LP-1 rotor reached a maximum depth of 245 mm (10 in), with the shaft diameter of 978 mm (38.5 in), and the crack in the LP-2 rotor had a maximum depth of 50 mm (2 in), at the same location on the shaft.

Fig. 4–24. Crack damage on the LP-1 rotor of AEG’s 660-MW wet-steam turbine at Würgassen

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280 Wet-Steam Turbines for Nuclear Power Plants

Subsequent analysis showed that the cracks were caused by highcycle fatigue under the action of altering bending stresses due to the incorrect fit of the shrunk-on wheel disks. Rotating the turbine with a high-speed (100-rpm) turning gear caused stresses that probably initiated the cracks, whereas the much smaller high-frequency dynamic stresses caused by operating at the nominal speed were responsible for their growth. Calculations showed that in an air or pure steam atmosphere, these alternating stresses were insufficient for the cracks to progress as far as they did.The residual time to final fracture of the rotor at the instant when the turbine was stopped was estimated to be 1.5 hours.Thus, the cracks were caused by the combined effects of high-cycle fatigue and corrosion.

Very similar cracks occurred on three LP rotors with shrunkon disks in the 500-MW turbines at the British fossil fuel power plant Ferrybridge “C”, and on the IP rotor’s coupling of BBC’s 1,300-MW cross-compound turbine at TVA’s fossil fuel power plant Cumberland. 71

Thereafter, most cases of damages with wheel disks have been experienced on wet-steam turbines and LP cylinders of superheatedsteam turbines.The damages sometimes led to failures of the turbine rotors. Except for the cracks due to creep-rupture in the blade grooves of the high-temperature stages, almost all of the disk damages and failures were caused by stress corrosion—that is, corrosion acting under high-stress conditions.As in all the previously mentioned cases, corrosion cracks most frequently originated in places where liquid concentrates of steam contaminants could gather and be stored for a long time. Corrosion is stimulated by such mechanisms as deposition, evaporation, and drying. Impurities are consolidated on the disk surfaces in various slots, recesses, grooves, and other design crevices

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Operation 281

or occasional fissures or pores in the metal, which simultaneously become places of stress concentration. This phenomenon was the subject of special consideration and analysis in numerous investigations. 72 According to specialists of ABB, for stress corrosion cracking (SCC) to occur, three conditions must be satisfied: a sufficiently high tensile stress must be applied to a susceptible steel in a corrosive environment 73 Combination of all three factors creates a real menace of the appearance and propagation of SCC.

There are over 150 various steam contaminants, but the most aggressive mediums promoting stress corrosion are such compounds as NaOH, NaCl, Na2 SO4 , Na3PO4, NaNO3, Na2SiO3 , NH4Cl, Na2CO3 , organic and inorganic acids, and ferric and cupric oxides.The most indicative factors of steam impurity are the steam pH level, conductivity, cation conductivity, and the concentration of such chemicals as oxygen and sodium chloride. The latter should be kept to concentrations of few parts per billion (ppb) or less. There were some accidents in which, when after a damaged rotor had been replaced, the stress corrosion cracks reappeared at the same wheel disks in the Wilson region. In these cases, the power plant operators managed to avoid the repeated appearance and propagation of cracks only by means of radical improvements in water treatment and steam chemistry monitoring. The formation of corrosive media and their deposition on steam path surfaces in the phase transition zone and the effect of these media on the SCC processes were the subject of special experimental and analytical investigations.74 Proper water treatment and proper material selection can diminish and even minimize corrosion attacks. The effect of the pH level of the steam/water cycle on the specific corrosion rate is shown in Figure 4–25; the rate drastically reduces with a pH level above 9.0.75 Replacing carbon steel components with 2.5% chromium steel components increases the resistance to erosioncorrosion from 0% to approximately 97–99%.

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282 Wet-Steam Turbines for Nuclear Power Plants

Fig. 4–25. Influence of pH level on the erosion-corrosion rate of various steels

Source :W. Engelke, K. Schleithoff, H.-A. Jestrich, and H.Termuehlen76

The most general explanation of SCC is given by its electrochemical theory. According to this concept, the main factor affecting crack propagation is anodal dissolving of metal in the crack root. Under certain circumstances, a protective film appears on the metal surface (a phenomenon known as passivation). If the passive layer is

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broken under action of, for example, mechanical stresses, the crack root turns out to be under conditions of chemical activity, and the metal begins to dissolve actively with continuous depassivation.This process is accompanied by hydrogenization of metal on the juvenile surface at the crack root and results in embrittlement of the metal. Some ideas exist that this process is intensified if the turbine operates under conditions of frequent load changes, with resultant shifts of the phase transition zone and variations of the steam pressure at the turbine stages working in this zone. In particular, this also means that the Wilson region at transient operating conditions embraces a greater number of stages, and more disks are involved in this process.

Along with the steam/water chemistry, the intensity of stress corrosion to a great degree depends on the composition and heat treatment of the steel applied to the disks, as well as their design features. So, stress corrosion of disks has been almost absent, not only at power plants in countries with a traditionally high culture of steam/water chemistry, but also at steam turbines made by ABB and Siemens, for example, even if these turbines were operated at power plants without high-quality water treatment. The only indication of cracks found in wheel disks of Siemens’ turbines was related to insufficient local heat treatment.77 In contrast, steam turbines manufactured by U.S. producers were prone to stress corrosion to a large degree. From the late 1970s to July 1980, inspections were carried out on 72 wet-steam turbines at 32 U.S. nuclear power plants; at 20 of these turbines, cracks were discovered in 70 disks of 36 different LP rotors. 78 By 1996, of 110 U.S. nuclear units surveyed, 33% of 290 LP rotors had to be retrofitted, including 18% of 119 LP rotors on the turbines manufactured by Westinghouse and 35% of 150 LP rotors produced by GE. 79 The cracks took place in the stages in the Wilson region and ranged in depth from 2.5 to 75 mm (0.1–3 in). The rate of crack growth varied from 0.1 to 10 mm/yr (4–400 mil/yr). Because of SCC in the shrunk-on wheel disk keyways, for example, of four 820-MW turbines at the nuclear power plants Dresden (Units 2 and 3) and Quad Cities, all of the 12 LP rotors with shrunk-on disks were replaced with rotors of the welded type delivered by another manufacturer.80 Similar damages were revealed in the LP cylinders of many turbines at fossil fuel power plants.The most typical zones of disk damages are shown in Figure 4–26. Among 131 damaged disks that were inspected, 38% had cracks in the blade attachment zone, 29% on the side surfaces, and 30% on the bore surface, including 26% with cracks in key slots.

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284 Wet-Steam Turbines for Nuclear Power Plants

Fig. 4–26. Typical zones of stress corrosion cracking in shrunk-on wheel disks of wet-steam turbines at U.S. power plants (late 1970s)

Cracks in the blade attachment zone can entail rupture of the disk rim and blade liberation, with the potential consequences of avalanche-shaped damages in the steam path, sudden rise in shaft vibration, damages to bearings, and so on (Fig. 4–27). Cracks on the bore surface can be equally or even more dangerous, because their propagation can cause fracture of the whole disk, resulting in subsequent destruction of the turbine. One of the most serious failures of this type happened at the British nuclear power plant Hinkley Point A,81 and presently similar failures bear this name. Three wheel disks on the LP rotor burst; the shaft of the 93-MW turbine was broken in three places; the entire steam path crashed, and two neighboring turbines were also severely damaged. The process originated from stress-corrosion cracks in the semicircular axial keyway in the shrunk-on disks and on their bore surfaces.The cracks propagated, remaining unnoticed, and resulted in brittle fracture when the turbine ran through an ordinary overspeed test and reached a rotation speed of 3,200 rpm (with the synchronous speed of 3,000 rpm) (Fig. 4–28). Immediately after this failure, 810 wheel disks on 102 rotors were inspected, and stress-corrosion damages were discovered on 124 disks of 50 rotors.

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Fig. 4–27. Stress corrosion cracks at a disk rim in the blade attachment zone

Fig. 4–28. Brittle fracture of shrunk-on disk due to propagation of stress corrosion cracks from the bore surface at the British nuclear power plant Hinkley Point A

If a turbine operates under conditions of aggressive steam (with undesirable impurities) and the disk steel with regard to its heat treatment is prone to stress corrosion, the probability of stress corrosion

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286 Wet-Steam Turbines for Nuclear Power Plants

damages for the wheel disks in the Wilson region practically approaches 100% (Fig. 4–29). These statistical data for Westinghouse turbines are very similar to those for turbines made by AEG, GE, NEI Parsons, and TMZ. On the basis of field measurements for 40 damaged wheel disks,Westinghouse researchers derived an empirical equation for crack growth, which can be recast as follows:

where v is the crack growth rate in m/s; t is the metal temperature in °C; and σ0.2 is the conventional yield limit in MPa.

Fig. 4–29. Probability of damages in the key slot (1) and on other surfaces (2) of wheel disks in the Wilson region

Source: O.A. Povarov and E.V.Velichko82

According to investigations of EPRI, the crack growth rate mostly depends on the steel composition and tempering temperature, yield limit, and disk metal temperature. For standard NiCrMoV steels used for LP wheel disks and rotors, the most influential components of the steel composition are manganese, vanadium, nickel, and sulfur. If the sulfur content rises to 0.01%, the crack growth rate increases, but any additional increase of sulfur content is accompanied by a decrease

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