Show me the improvements
AmerenUE's Labadie Plant, 35 miles west of St. Louis, has been recognized by the U.S. EPA and the State of Missouri for its exceptional environmental performance. But plant management is equally well known for its commitment to continuously improving the production statistics of Labadie's four 600-MW (nominal) coal-fired units.
Over the past few years, plant staff—with the help of Alstom Power (Windsor, Conn.)—has raised maximum unit capacity by at least 10% (from 580 MW to 630 MW) while simultaneously increasing their availability (POWER, July/August 2003, pp. 58—61). Beyond boiler improvements necessitated by its switch to Powder River Basin (PRB) coal in the late 1990s, the Labadie plant also installed low-NOx burners, an overfire air system, and advanced computer controls to improve unit performance and significantly reduce emissions.
One of Labadie's greatest performance gains came from the upgrading of all four of its steam turbines. The four units were designed in the late 1960s and began commercial operation between 1971 and 1974. Westinghouse supplied the BB44-design turbine-generators of Units 1 and 2; General Electric followed up with a G-2 design for Units 3 and 4. Each turbine-generator is a tandem-compound unit with two double-flow LP sections. The upgrade program spanned several years; the HP/IP turbines of the Westinghouse units were replaced in 2002, and new HP/IP and LP turbines for the GE units were put in a year later.
The HP/IP performance records of Units 1 and 2 over their first 30 years of service were disappointing. Major-maintenance intervals remained stubbornly at around four years, unit efficiency continued to inch downward, start-up times kept rising, and operational problems grew in number and severity. Turbine efficiency would improve after maintenance overhauls but never returned to design levels and degraded rapidly between overhauls. Internal turbine problems such as thumbnail chipping of the nozzle block and steam seal degradation proved difficult to correct permanently. Other mechanical problems included non-uniform creep of rotors, causing them to bow and further extending start-up times and outage intervals. The end result—fewer megawatt-hours sold—dictated nothing less than replacement of both units' HP/IP turbines.
AmerenUE chose to replace the turbines as modules, with the new units featuring low-reaction (impulse) blading with full arc admission. Doing so cost more but gave engineers more freedom to maximize performance unconstrained by the units' existing outer casing (although the outer casing also was replaced during the retrofit to eliminate other maintenance problems). The modular approach also simplified the arrangement of the stages. Because the inlets of the new HP/IP sections are at the center of the turbine (along with single-flow HP and IP expansions), a large balance piston is no longer needed to offset the axial thrust that the old, reaction-design turbine used to produce. Removing the piston reduced the number of internal leakage paths from five to one.
A further performance improvement resulted from the revised steam sealing arrangement, which does more than significantly reduce leakage. It also uses cold reheat steam—rather than main steam—to seal the turbine. The beneficial trickle-down effect extends to reduction of gland steam spillover and reduction of the condenser's heat load.
Both turbine retrofits have produced the desired results. Units 1 and 2 (where only the HP/IP turbines were replaced) saw an increase in HP efficiency of 7% and LP efficiency of 5%. The total increase in the capacity of each unit was demonstrated at 27 MW, due to efficiency increases alone.
The aforementioned O&M issues also drove Ameren to do wholesale replacements of the turbines of Units 3 and 4. In this case, the goals were to improve unit efficiency and plant output and to lengthen the turbine's maintenance interval (Figure 4). The tandem-compound HP/IP turbine was replaced with a similar low-reaction turbine design (Figures 5 and 6). Replacement of the outer casing was not necessary, which saved capital cost and reduced piping and insulation rework.
4. Bird's eye view. Labadie Unit 3's high-pressure/intermediate-pressure turbine rotor-finishing assembly. Courtesy: AmerenUE
5. You make the call. Compare Labadie Unit 3 and 4's new and old high-pressure (HP) turbines. The old HP section (left) has two fewer rows than the new one (right). Testing found the new design to be 7% more efficient. Courtesy: AmerenUE
6. Which would you want? On the left is the old intermediate-pressure turbine blade/shroud design of Units 3 and 4. At right is the modern design. Courtesy: AmerenUE
The two double-flow LP turbines with 30-inch L-0 blades were replaced because stress corrosion cracking of the L-1 wheel dovetail area had accelerated enough to warrant either a major repair job or the blades' replacement. After running the numbers, Ameren chose the replacement option, which entailed the use of modern, 34-inch L-0 blades and a smaller blade path (Figure 7). This increased unit capacity by 10 of 14 MW.
Perhaps the most significant lesson Ameren learned was the need to consider upgrade projects holistically and coordinate their execution with other planned plant changes. Within a unit, any change made to any system almost always produces changes elsewhere. For example, putting in a new economizer and air preheater lowers the gas path pressure drop, decreasing the level of fan power needed. At Labadie, doing so saved 3 to 6 MW and created an ancillary benefit: improved precipitator performance due to the lower entering gas temperature. Similarly, modifying the superheater division panels eliminated a number of gas flow restrictions and fatigue-related tube failures, enabling operators to increase unit maximum power by another 12 MW. AmerenUE engineers made sure the new steam turbine had the margin to take advantage of other plant performance improvements.
7. Almost finished. Lengthening the blades of Unit 3's low-pressure turbine from 30 to 34 inches increased the low-pressure section's efficiency by 5%. Courtesy: AmerenUE
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