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Turbine Retrofit Increases Output
Operating with a fleet of aging and inefficient plants, many electric utilities encounter problems in raising output. New plant construction to meet growing demand is almost nonexistent. More and more nuclear generating stations are reaching retirement age. The U.S. electric power reserve capacity is diminishing. The comfortable reserve of electric power of a few years ago has vanished. As a result, utilities seek alternative methods to maintain power production; updating older plants and installing gas turbines to add capacity. For example, to increase output and reduce operating costs, PECO Energy (formerly Philadelphia Electric Company) started in February 1997 to replace two aging nuclear-steam turbines at its Limerick Generating Station, (Pottstown, Pa.) Units 1 and 2. Unit 1 starts up in May 1998, and Unit 2 a year later. PECO contracted with Siemens Power Corporation, Milwaukee, to design and install the replacement turbines. Siemens Power Corporation and Siemens KWU, Germany, combined efforts to enginer the project. The Siemens service and repair shop in Milwaukee performed all welding for the job.
The Siemens rotor design, the heart of the turbine, specifies a deposit of a corrosion-resistant weld overlay on the shaft-seal area on each end of the rotor. The overlay improves the erosion- and corrosion-resistance properties of the seal area and reduces the effects of satu-- rated steam. It is a labyrinth seal, a series of grooves milled around the rotor, that uses multiple-pressure drops between the seal points and the welded area to reduce or eliminate flow.
Welding equipment
The new 32-ft-long alloy-steel rotor forging for the Unit 1 turbine weighs in at 73 tons, and measures 63 inches in diameter at the widest body dimension. Chemical composition includes nickel, chromium, molybdenum, and vanadium. To prepare for welding, Siemens developed extensive welding and inspection procedures. Laboratory research and testing answered metallurgical considerations and provided controls for the alloy properties.
Jetline Engineering Co. manufactured computer-controlled multiple-process welding equipment, designed to Siemens specifications. A Jetstar computer controller directs welding processes, using a library of weld procedures for recall on demand. During welding, a dataacquisition device monitors, records, and stores all weld parameters. An equipment car that moves along 57 feet of rail contains welding power sources, a boom structure with welding heads,...