408 Wet-Steam Turbines for Nuclear Power Plants
Replacing LP disk-type rotors with solid ones (experience of Westinghouse)
An alternative to replacing LP disk-type rotors prone to SCC with welded rotors is the use of monoblock rotors (solid forged rotors without a central bore). The absence of a central bore (as with welded rotors) considerably reduces the level of maximum centrifugal stresses in the rotor, and the absence of separate disks with their bore surfaces and keyways eliminates the danger of cracking in these areas. Lower maximum stresses allow the use of more corrosionand erosion-resistant materials for the rotors.
As mentioned above, GE replaced the repaired disk-type LP rotors with monoblock rotors on the 1,350-MW turbines at Palo Verde. As well, MHI replaced the original disk-type rotors with monoblock rotors on Westinghouse’s 566-MW low-speed turbine (TC4F-44) at Japanese Ikata Unit 2. In turn,Westinghouse accumulated its own experience in manufacturing monoblock LP rotors for large steam turbines, including low-speed rotors, and retrofitting LP cylinders with the use of such rotors. So, for example, six LP rotors were replaced by Westinghouse on two 400-MW turbines originally produced by English Electric at the Swedish nuclear power unit Ringhals with a double-turbine configuration (Fig. 5–6). 27
In 1986, Westinghouse introduced a new design concept, called a ruggedized LP turbine, especially for retrofit applications. 28 This design approach was originally conceived to improve turbine reliability by means of avoiding primarily SCC problems. However, the application of modern technologies in designing blade paths more efficiently, reducing exhaust and leakage losses, and a significant increase in the annular exit area have provided considerable performance improvement in addition to improved reliability. The ruggedized concept is characterized by the use of solid LP rotors with no central bore, freestanding LSBs, and integrally shrouded blades in the upstream rows. A typical ruggedized LP cylinder design with a 1,194-mm (47-in) freestanding LSB for retrofitting low-speed (1,800 rpm) wet-steam nuclear turbines is shown in Figure 5–7.The design decisions employed in this case provide greater resistance to SCC in the rotor attachment area because of the improved side-entry blade root design, which features lower peak stresses.The developed LSBs are intended to replace the original 44-in and 45-in LSBs in retrofit applications, offering improved efficiency due
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to their longer length and hence greater annular exhaust area, optimized profiling, and an advanced exhaust flow guide design.These LSBs also provide the turbine’s improved operational flexibility at low loads and high back pressure due to better airfoil design and increased structural stiffeners.The steam path has an enhanced resistance to moisture erosion because of increased axial spacing and improved wraparound Stellite strips. Some improved resistance to the blading WDE also came from the use of moisture trap grooves, moisture drainage slots in the honeycomb seals, and specially designed moisture drainage slot holes in the inner casing structure.
Fig. 5–6. LP cylinder of a low-speed turbine after (a) and before (b) retrofitting by Westinghouse
Source: J. C. Groenendaal, L. G. Fowls, R. Subbiah, et al.29
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Fig. 5–7. Ruggedized LP cylinder of Westinghouse for retrofitting nuclear steam turbines
Source: E.P. Cramer, J.A. Moreci, C.W Camp, et al. 30
A ruggedized LP design was used to retrofit the 1,154-MW lowspeed four-cylinder turbine (one double-flow HP cylinder and three LP cylinders) at the U.S. BWR-type Columbia nuclear power unit put into commercial operation in 1984. Originally, the turbine was delivered with disk-type LP rotors. During the second yearly refueling outage in April 1987, a single instance of a smallest detectable size crack initiation in a disk keyway was found on one of the LP rotors in the process of their first inspection. This entailed a decision to replace and retrofit all of the turbine’s LP rotors, and the proposal of Westinghouse was chosen as the best overall technical solution with regard to a cost/benefit analysis of the warranty, the guaranteed amount of the input increase, the required duration of the retrofit, and the recommended frequency of following inspections.The output increase was guaranteed to be 15.5 MW. Heat-rate tests of the unit proved the load improvement of 22.9 MW at the reactor’s rated thermal level, that is, exceeding the contract guarantee by 7.4 MW.31
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Another example is the upgrading of Prairie Island Units 1 and 2, with the net outputs of 530 MW each, and Point Beach Units 1 and 2, with the net outputs of 500 MW each. These units have essentially identical low-speed turbines consisting of one HP cylinder and two LP cylinders. The retrofit at the Prairie Island Unit 1 turbine took 28 days, with the contract outage window of 35 days.The preand post- heat-rate tests showed an increase in turbine efficiency and output of 2.7% that is, by approximately 0.5% above the guaranteed gain. 32
Retrofitting LP cylinders with disk-type rotors (experience of Siemens)
For large, high-speed (3,000 and 3,600 rpm) steam turbines for both fossil fuel and nuclear power plants, Siemens uses monoblock LP rotors (see Fig. 3–7), and LP rotors with shrunk-on disks are employed only for low-speed (1,500 and 1,800 rpm) nuclear turbines (see Fig. 3–8). Due to careful design decisions and proper choice of disk material and its heat treatment, Siemens’ disk-type LP rotors practically do not suffer from SCC. There was only one case of SCC found among 310 disks of Siemens wet-steam turbines inspected at nuclear power plants by the late 1980s,33 whereas similar disk-type LP rotors made by other turbine manufacturers were greatly affected by SCC.That is why Siemens’ developments of advanced LP cylinders, first, have relied on the well-proven concept of disk-type rotors for low-speed turbines and, second, have been aimed primarily at increasing turbine efficiency.These developments were intended particularly for retrofitting aging wet-steam turbines in service, and thus assumed replacement of both the LP rotors and inner casings to improve the steam path.Along with this, serious attention was paid to supporting the high reliability of the LP rotors and their high resistance to SCC. Siemens’ advanced LP rotor designs were developed to be fully competitive with any monoblock or other rotor designs.
Siemens’ original 10-disk rotor, which had been used since 1987, was redesigned for the eight-disk rotor, and then, in 1995, for the six-disk rotor, as shown in Figure 5–8. The disk forgings are thermally treated to provide the yield strength and fracture toughness levels well below the threshold values, so that even if SCC commenced, its rate would be small enough to reach the critical crack size for the time period significantly exceeding the
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412 Wet-Steam Turbines for Nuclear Power Plants
turbine’s lifetime. Because of the relatively wide disk forgings, a special treatment is applied to form compressive stresses at the disk surfaces, including the blade attachments, that increases the margin against potential SCC initiation (see Fig. 4–33).34
(a)
(b)
(c)
Fig. 5–8. Improvements of the LP cylinder design for Siemens’ low-speed wet-steam turbines (from a ten-disk rotor to a six-disk rotor)
Source: By courtesy of Siemens
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The advanced LP steam path includes improved, integrally shrouded blading for the first LP rows, increasing the stage group efficiency by approximately 1.0–1.5%.The improved performance is achieved owing to, in particular, a continuous curvature of the blade profile’s suction side, resulting in minimum velocity reduction and avoidance of flow separation. The blunt nose makes the profile immune to changes in the incident flow angle, and the narrow trailing edges minimize the exit losses.Another improvement in performance is achieved by utilizing double-strip interstage seals for the stationary and rotating blades, as shown in Figure 3–22b.These interstage seals minimize leakage losses, but allow relative axial expansion between the rotating and stationary components. A leakage flow reduction of up to 50% is expected as compared with previous designs. In order to achieve the speed ratio nearer to the optimum value, an extra stage is added to each flow, and this improves the stage group efficiency by approximately 1.5%. The internal efficiency of the last three stages has been proven to be as much as 7% better than that of the previously employed stages.The better stage efficiency is achieved with a relatively low reaction at the outer diameter to reduce the exhaust losses at the blade tip and a slight increase in the low reaction at the hub section to avoid flow separation at the blade root.This improvement in the last stage blading, with local transonic velocities and high supersonic exhaust velocities, resulted in the development of tapered and forward-curved, tangentially inclined stationary blades. In the outlet area, where the steam flow exits the optimized diffusers downstream of the last stages, flow baffles were positioned in the upper center of the enclosure and underneath the lower part of the diffusers to reduce vortices in the outflowing steam. The improved outflow to the condenser also reduces the local pressure drop. Steam stream lines with and without baffles are compared in Figure 5–9. This improvement was combined with an increase in the size of the inflow cross-section to even out the inlet steam flow along the steam admission arc. This resulted in an estimated increase in the turbine output of approximately 3 MW. The total results of retrofitting LP cylinders on Siemens turbines at various power plants are presented in Table 4–4. 35
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Fig. 5–9. Steam stream lines in the outflow to the condenser for old (a) and improved (b) Siemens turbine designs
Source: H. Oeynhausen, H.-P. Classen, and J. Riehl36
According to the heat-rate test conducted at the 1,300-MW Unterweser nuclear power unit, the initial gross efficiency of the turbine-generator was 35.01%; measurements before the retrofit (pretest) gave the value of 35.11%, and after retrofitting one of three LP cylinders (post-test) the net efficiency value was 35.6%, with the gross output at the generator terminals equal to 1,310.8 MW. Estimations allow assessing the gross efficiency of the turbine after retrofitting all the three LP cylinders as equal to 36.56%. 37
Special considerations were taken and studies performed to make sure that all of the LP cylinder’s inner components could be easily replaced during the annual refueling outage. As a result, the
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replacement of the LP rotor and inner casing at the Unterweser plant was accomplished within the preapproved scheduled period of one month,38 but similar works at the German nuclear power plant Emsland took only 15.5 days. 39
Complete Upgrading of Turbines
Including Both HP and LP Cylinders
The upgrading of wet-steam turbines at nuclear power plants mostly focuses on the refurbishment of LP cylinders, because they provide the largest part of the turbine output and are the turbine’s most vulnerable components. It can be said that their reliability determines the turbine availability more than any other component does. Along with this, as most paramount problems with the LP cylinders are solved, more attention is paid to retrofitting the HP cylinders.This is mainly tied with significant advances in the design of HP blading achieved in recent years. Upgrading the HP steam path with blading of modern design provides an additional substantial increase in the power output, with a relatively short payback period.40 If possible, the HP cylinder is retrofitted in parallel with retrofits of the LP cylinders during the same turbine outage; otherwise, it is commonly performed later, during the next outage.
Upgrade experience of Siemens
After the successful refurbishment of the LP cylinders at several German nuclear power units, as discussed above, a more complete upgrading with the retrofit of both the HP and LP cylinders was undertaken at the German nuclear power plants Grafenrheinfeld and Gundremmingen, both with low-speed (1,500 rpm) 1,300-MW turbines.41 The turbines include one double-flow HP cylinder and two LP cylinders, with 1,365-mm ( 54-in) LSBs.
A comparison of the previous and advanced HP cylinder designs can be seen in Figure 5–10. To improve the velocity ratio, the first stage is changed from a traditional 50% reaction-type blading to an impulse-type stage with tilted stationary blades (Fig. 5–10c).
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Fig. 5–10. Comparison of previous (a) and advanced (b) HP cylinders for retrofitted Siemens wet-steam turbines; steam inlet segment (c)
Source: K. D.Weschenfelder, H. Oeynhausen, D. Bergman, et al.42
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An inlet steam guide ring is added to optimize flow conditioning and eliminate any leakage losses around the first stage. An enhanced reaction profile is employed for the cylindrical intermediate stages. The leading edge, with a large radius of curvature characteristic for this profile, makes it relatively insusceptible to changes in the angle of the steam flow approach, and the slender trailing edge reduces the losses induced by trailing-edge wakes. Twisted integrally shrouded blades are used at the exhaust ends, considerably reducing the profile and secondary-flow losses.The interstage seal system features a larger number of labyrinth seal strips (see Fig. 3–22a). The inner casing’s exhaust configuration is modified to optimize the diffuser section. On the basis of all these improvements, an increase in the HP section efficiency of approximately 3% is expected.
At the Grafenrheinfeld nuclear power unit, the entire refurbishment, including the replacement of all three rotors and inner casings (in the HP and two LP cylinders) was performed during the regular refueling outage from May 29 to July 7, 1993, taking five-and-a-half weeks. The heat-rate test results showed an increase in the power output from 1,305.8 MW to 1,365.2 MW, that is, by 59.4 MW, or 4.5%. After applying all of the correction factors (especially the correction for the back pressure), the performance improvement was determined to be 45.3 MW, whereas the guaranteed figures were assessed equal to 7.8 MW for the HP cylinder and 32.6 MW for the two LP cylinders (an aggregate 40.4 MW). Even better results were achieved at the Gundremmingen nuclear power unit (see Table 4–4). 43
If Siemens’ original upgrade projects referred to their own turbines, later Siemens also accomplished the complete refurbishment of several turbines (both their HP and LP sections) produced by other manufacturers. One of the best known projects was the retrofit of two 1,160-MW low-speed (1,800 rpm) turbines at the U.S. nuclear power plant Limerick with BWR-type reactors.44 The original turbines were produced by GE and comprised one double-flow HP cylinder and three double-flow LP cylinders with disk-type rotors, 966-mm (38-in) LSBs, and steam flowing down into a multi-pressure condenser. The OEM’s LP rotors and inner casings were replaced because of SCC on the shrunk-on disks and severe crevice corrosion of the LP casings and diaphragms.The HP steam path was retrofitted only from considerations of efficiency improvement.
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