April 15, 2006

Steam turbine upgrading: Low-hanging fruit

—By Dr. Robert Peltier, PE

In 1500, Leonardo Da Vinci drew sketches of a device that rotated when hot air going up a chimney passed through a set of fan-like blades. Leonardo called his invention a "chimney jack," and although it only turned a roasting skewer, it gave birth to the idea of mounting blades on a shaft to convert thermal energy into mechanical energy. Sir Charles Parsons' improvements on the concept led to a patent on the first multistage reaction turbine in 1884 and a 4-kW prototype the following year. A century later, the technology has been refined to the point where modern ultra-supercritical plant efficiencies are approaching the magic 50% barrier.

 

In the U.S., market forces have driven some plant owners to continue to operate units whose heat rates are considered mediocre. Some 50-year-old best-of-class steam plants, which sported heat rates in the 9,000 Btu/kWh range when commissioned, continue to operate at similar efficiency today to supply a short-term need. The kicker, of course, is that the cost of operating a plant rises as it ages and becomes less reliable (Figure 1).

 

Keeping any plant on-line and profitable requires continuous investment. Not the mandatory sort—such as installing or upgrading emissions-control systems to meet stricter standards—but discretionary investment. It only seems reasonable to invest in areas where the return on investment will be biggest—such as upgrading a plant's workhorse, its steam turbine (Figure 2). The dollars are significant: Improving a baseloaded 500-MW coal plant's heat rate by 100 Btu/kWh could save as much as $10 million dollars annually in fuel costs alone.

 

Forward . . . thinking

Dairyland Power Cooperative (DPC) and Siemens Power Generation (Orlando, Fla.) recently completed a retrofit of the high-pressure (HP), intermediate-pressure (IP), and low-pressure (LP) steam paths of Unit 1 of Dairyland's J.P. Madgett Station in Alma, Wis. Madgett Station is a coal-fired subcritical steam plant that entered service in 1979 with a nominal rating of 365 MW. Retiring the unit wasn't an option, because DPC resource planners realized they need its capacity now and for another 30 years. Modernizing the steam turbine was the only feasible strategy. DPC initiated the retrofit project with three aggressive goals: improve the efficiency of the plant across the load range, boost unit capacity, and lengthen the turbine's maintenance period to 10 years.

Siemens provided a retrofit upgrade package that included new rotors, inner casings, and high-efficiency rotor and stator blades, as well as design and installation services. A 50-day outage was scheduled in the fall of 2004 to complete the retrofit work in parallel with major boiler maintenance, upgrading the control system to a modern distributed control system, and replacing the main transformer to handle the expected higher power output.

Siemens' BB44FA (full arc) HP/IP turbine retrofit package targets the existing fleet of Westinghouse building-block (BB) 44 turbines with inlet pressures up to 2,400 psig and an inlet temperature of 1,000F. BB44 turbines range in size between 350 MW and 680 MW. By using the package, engineers can add a full arc to the admission inlet section, eliminate the 180-degree steam turnaround to the HP blade path, eliminate the impulse control stage, add a fully integral inner casing, and improve steam sealing (Figure 2).

All internal stationary components were put within a single, fully integral casing—a design that minimizes parts count and decreases installation time and the duration of future outages. The BB44FA design duplicates the mating-flanges profile of existing units, enabling reuse of the outer casing and all the anchor points. This exemplifies Siemens' design philosophy for the retrofit package: to make it completely "plug-and-play."

The design of the HP/IP rotor design features fully integral, no-bore forging to shorten start-up times and lengthen fatigue life. Like the outer casing and anchor points, bearings can be reused because the new rotor matches the weight of the old one, reducing cost and installation time. Blade designs featuring integral shrouds were selected to optimize thermodynamic performance. The shroud design has two advantages: The shroud creates a circumferential boundary for the steam path, enabling the retrofit of more-efficient seals, and it provides individual blade tip supports between adjacent blades. The designs of the first-stage HP and IP blades reduce rotor inlet temperatures while providing favorable downstream flow conditions.

Unit 1's old, five-stage LP turbine was replaced with a new, seven-stage design that eliminates riveted shrouds on the front-end blading and riveted shrouds and lashing wires on the larger LP blades (Figure 3). A single inner casing with moisture removal features is included in the retrofit package. The LP turbine was designed to be a "drop-in" replacement to allow the reuse of many inner-to-outer cylinder connection points. Other features of the package include a monoblock rotor forging without thru-bore, reaction front-end blades with integral shrouds, 37.7-in freestanding last-row blades, and a single inner casing with an improved exhaust diffuser.

 

The upgraded turbine was successfully installed during the scheduled 50-day outage of J.P. Madgett Station. Test results were exceptional: The efficiency of the HP turbine increased by 8% to 10% over the load range while that of the IP section rose 2% to 4%. Total plant output went up 20 to 27 MW. Much of the measured 7- to 8-MW performance gain in the LP section was attributed to the improved front end blade design, the 25% increase in annulus area, and the more-efficient last three stages of the LP section.

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