Refurbishing steam turbines.
Steam-turbine operators are pressed with the challenge of keeping their aging machines functioning in the face of wear problems that are exacerbated by the demand for higher efficiencies. These problems include intense thermal cycling during both start-up and shutdown, water particles in steam and solid particles in the air that pit smooth surfaces, and load changes that cause metal fatigue.
Outright replacement of turbine components is a costly operation, in terms of the purchase of new parts and maintenance downtime. A more economical solution for many steam-turbine operators is to extend the life of worn parts by using wire-arc-coating or shot-peening technologies to refurbish aging components, or to replace obsolete bearings and seals with newer and more-efficient designs. Successful examples of each of these three retrofitting methods were presented at ASME's International Joint Power Generation Conference and Exposition, held Nov. 3-5 in Denver.
Interestingly, both wire-arc spray coating and shot peening were established metal-treating technologies borrowed from other applications to suit the power-generation field. For example, when engineers from Westinghouse Electric Corp. in Pittsburgh introduced the Spray-Pro wire-arc spraying process to utilities in 1994, they had already accumulated 18 years of experience using it to refurbish parts in the pulp and paper industry, according to Mike Metala, a mechanical engineer and manager of specialty services and development at Westinghouse.
The Spray-Pro process is designed to deposit molten, high-performance metal coatings on the walls of pressure vessels, piping, stationary blade assemblies, turning vanes and elbows, ductwork, boiler tubes, nozzle blocks, and other turbine-generator and balance-of-plant components, thereby providing corrosion and erosion protection.
The company sees potential for the technology in American and overseas utilities. Virginia Power has used Spray-Pro to refurbish parts at its Surry power plant, and Belgian, Spanish, and South Korean power producers have expressed interest in the technology.
Metala said that heat-exchanger rings can illustrate Spray-Pro's benefits: "These parts are typically replaced after seven years of service. According to data we have collected in the field, we believe Spray-Pro treatment can extend their life by 50 to 100 percent."
The Spray-Pro process can be performed on-site in the case of larger steam-turbine components, such as cross-under piping or turbine casings, or at a Westinghouse facility for smaller parts such as blade rings and nozzle blocks. In either case, the first step is cleaning the part to a white metal condition and profiling it. This is performed with a continuous direct-pressure nozzle that discharges a stream of abrasive material, such as aluminum oxide or steel grit, to provide the anchor profile required to mechanically bond the coating to the substrate.
A twin-wire electric-arc spray gun is then aimed at the target surface. Two electrically charged wires made of the desired coating material are mechanically fed from spools into the head of the gun where the arc is formed to melt the metal. Compressed air or inert gas at 30 to 50 pounds per square inch drives the molten-metal particles onto the prepared substrate. The operator manipulates the gun over the surface to create a series of uniform, overlapping layers of molten metal until the required thickness is achieved, typically 5 to 125 mils.
A vital consideration in the use of Spray-Pre is selecting the specific coating materials to match the thermal-expansion characteristics of the substrate and provide the wear characteristics desired by the end user's environment. "We build mock-ups of the part before we attempt to coat it, and conduct metallurgical and mechanical analysis to ensure that we have specified the optimum coating for the application," Metala said. Advanced nickel, chrome, or ceramic metal matrix coatings are used for their superior corrosion and erosion resistance in the face of such conditions as wet steam, hard particle erosion, elevated temperatures, and chemical attack. Westinghouse also conducts a regimented process and operator qualification on the mock-ups, he added.
After the coating process is completed, Westinghouse technicians perform an extensive visual inspection using a handheld optical comparator, which is a standard industrial magnifier. The coating thickness is then measured by two electronic means: One device measures the magnetic attraction between the sensor and the component base metal, then relates the force to thickness; the second uses an eddy-current sensor to sense the magnetic base metal and relates liftoff signals generated from the coating to a thickness value.
Westinghouse engineers recently developed a semiautomated version of Spray-Pro to coat the interior of piping or vessels 30 inches in diameter or larger. This model is designed to ensure both the safety of a technician using the spray gun manually inside a confined space and highly uniform coating quality. Components of the system fit inside a 12- by 18-inch standard manway access opening in the pipe. Westinghouse engineers designed mechanical "spider" supports with telescoping legs that extend to the pipe wall. The spray gun is mounted on a rotating head that runs on a variable-length bar mounted between the two sets of legs.
The gun is aimed at 90 degrees relative to the target surface but at a fixed stand-off distance from the pipe wall. The operator outside the pipe uses a computer console that is connected by multiconductor cable wire to the mechanical drive system inside the pipe. The control system allows the operator to control its movement and key operating parameters such as voltage, current, and speed.
Another traditional metal-treating technology used to prolong part life, shot peening, dates back to the 1930s, when technicians at the Buick Division of Detroit-based General Motors Corp. used the technique to increase the fatigue strength of valve springs for higher-horsepower engine blocks. Shot peening is a cold metalworking process that involves bombarding the part surface with small spherical shot (typically cast steel, but sometimes stainless steel, glass, or ceramics). Each piece of shot striking the metal acts as a tiny peening hammer, imparting a small indentation or dimple. To create these dimples, the material's surface fibers are yielded in tension. As a result, fibers located below the surface try to restore it to its original shape. This produces a hemisphere of cold-worked material, highly stressed in compression, below the dimple.
By overlapping dimples, shot-peening technicians develop a uniform layer of residual compressive stress in the metal. Cracks do not initiate or propagate in a compressively stressed zone; also, because nearly all fatigue and stress-corrosion failures originate at the part's surface, the compressive stresses induced by shot peening provide considerable increases in part life. Tests have demonstrated that shot-peened parts can withstand 20-percent-higher fatigue loads.
A leader in shot-peening industrial parts is Metal Improvement Co. in Paramus, N.J. The firm has been providing shot-peening services to industry since 1945, and currently operates 36 such plants in North America and Europe, with process licensees worldwide. "For example, we have sent teams of technicians to shot-peen steam-turbine equipment for Tokyo Electric Co. and for Westinghouse in Saudi Arabia," said Win Welsch, manager of metallurgy at Metal Improvement's Carlstadt, N.J., facility.
The company's peening engineers typically work with original equipment manufacturers (OEMs) such as ABB, General Electric, GEC/Alsthom, Siemens, and Westinghouse to develop shot-peening procedures for their utility and industrial customers. Parts that Metal Improvement has successfully shot-peened include rotor-disk fir trees, steam balance holes, stainless-steel low-pressure blades, blade disks, and airfoils.
Metal Improvement will peen steam-turbine parts on-site or at one of its own facilities if this is more practical. The process begins with the steam-turbine end user's engineers performing finite-element analysis to spot the highest load stresses on the part to be peened. "We use this information to select shot size that will provide the needed compressive stresses," Welsch said, "then build a prototype of the part, peen it, and send it to a laboratory that qualifies the depth of stresses using X-ray diffraction."
After prototype testing, Metal Improvement engineers measure the energy of impact to determine whether it is sufficient for the peening process. They accomplish this by exposing up to five mounted strips of 1070 spring steel called Almen strips to a stream of the selected shot at various angles, then measuring the arc height of the strips.
Metal Improvement engineers have added two refinements to their shot peening process in 1997 to serve their clients better. "We now subject MILAM strips, which are made out of the same material as the part to be peened and supplied by the OEM or part end user, to the shot stream to develop actual residual stress profiles produced by the controlled shot peening," Welsch said.
The company also offers chemically accelerated surface-engineering (CASE) polishing to its peening clients. "CASE combines vibratory finishing with a chemical accelerant to smooth the surface of the peened part," Welsch said. "The object is to control the polishing so as not to smooth away the benefits of the peening."
DESIGNING BETTER BEARINGS
Refurbishing worn parts is not a magic bullet for extending steam-turbine life. In some cases, replacing older bearings and seals with newly designed components is a cost-effective way to optimize turbine performance, according to ASME member Fouad Zeidan, president and director of engineering at Bearing Plus Inc. in Houston. Zeidan founded his company to design bearings and seals aimed at enhancing steam-turbine performance and reliability. He said that some indicators of less-than-optimal bearing and seal design were excessive vibration, unstable turbine operation, and shorter-than-expected bearing life.
Bearing Plus designs are based on proven technology and analysis techniques that are used to eliminate bearing deficiencies without compromise of the parts' desired rotor-dynamic characteristics. A case in point was the retrofit of the 120-megawatt steam-turbine generator at the Public Service Company of New Hampshire's Merrimack power plant in August 1996.
The power train consists of a high-pressure/intermediate-pressure rotor, a low-pressure rotor, and a generator rotor, all supported by six bearings. The original bearings were sleeve type and experienced subharmonic vibration during certain load conditions due to the partial steam-admission effect. The plant's operators replaced the Nos. 1 and 2 bearings with four-pad, tilt-pad bearings in 1986. Although the conversion improved the unit's stability, problems still persisted.
"The bearings ran hot and changes to oil flow did not result in the desired improvements," Zeidan said. "While increasing the bearing clearance did lower the bearing temperature slightly, this came at the expense of high vibration when the turbine was run above 90 megawatts, due to the loss of bearing damping."
In addition, the No. 1 bearing suffered severe vibration in the 90- to 100-megawatt range, caused by the partial steam admission, which resulted in the load direction being at 45 degrees directly in line with the pivot in a four-pad load between the pad bearing, Zeidan said.
David Gruwell, a rotating-equipment specialist at the Merrimack facility, conducted vibration analyses of the bearings using eddy-current proximity probes. This enabled Zeidan to determine the machine's critical speeds and performance prior to upgrading. Thermocouples in the bearings measured the temperature.
The geometric preload and pivot offset of five- and six-pad bearings were analyzed for the No. 1 bearing location. Four-pad, tilt-pad bearings were analyzed with different preloads and pivot offset for the No. 2 bearing location. Based on these analyses, a five-pad bearing with a load between the pad configuration was chosen for the first bearing location, and a four-pad bearing, again with load between the pad configuration, was selected for the No. 2 bearing location.
"We chose copper pad materials for the bottom two pads in both cases because of their superior heat dissipation characteristics, enabling the pads to provide lower temperatures and thus higher reliability than the steel pads that were previously used," Zeidan said. After the retrofit and rotor balancing were completed in September 1996, the unit's vibrations at the critical speed dropped to 20 percent of the previous values.
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|Date:||Dec 1, 1997|
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