The J-Series gas turbine by Mitsubishi Hitachi Power Systems Americas, Inc. (MHPSA) can produce over 327 MW of power in single cycle and 470 MW in combined cycle – it is the most powerful gas turbine in the world. Capable of operation at inlet temperatures of 1,600 °C, it operates with record-breaking efficiency. High-performance turbines such as the J-Series gas turbine are at the heart of power generating systems that provide millions of households with electric power – a great responsibility. Each of their components has to be manufactured to the highest quality standards. Since 2012, MHPSA operates ProfiTurn H lathes made by WaldrichSiegen to machine its gas and steam turbine rotors to extremely high standards.
MHPSA is a manufacturer of power generation systems for large utility companies, such as Southern Company, Georgia Power, and Florida Power and Light. A subsidiary of Mitsubishi Heavy Industries, Ltd. that later merged with Hitachi, Ltd., the company produces steam and gas turbines in Savannah, Georgia. With 150 years of technology The carriage is driven by a preloaded double pinion drive leadership in power generation solutions, MHPSA's systems guarantee reliable power supply for millions of households and companies. When MHPSA decided to acquire state-of-the-art lathes to machine its turbine rotors, it turned to the market-leading manufacturer of lathes for this application, WaldrichSiegen. Based in Germany, WaldrichSiegen is an expert in the design and manufacture of customized heavy-duty machine tools for complex turning operations with more than 175 years' experience in various industries. Power and precision in a single machine is the dual challenge in this application. Russell Gamblin, Maintenance and Facilities Manager and author of "Machine Tools: Specification, Purchase and Installation", summarizes MHPSA's expectations: "Quality and accuracy are the most important criteria when machining turbine rotors – workpieces that weigh up to 200 tons and rotate at 3,600 rpm during operation."
"In 2010, we received an order from MHPSA comprising two customized lathes to machine gas and turbine rotors. Our ProfiTurn H series seemed like the ideal solution for the task, and it was. The order for another lathe followed a year later," says Marco Tannert, President of WaldrichSiegen. The ProfiTurn H series has been developed to machine large and heavy rotationally symmetric workpieces. The largest of the three lathes built for MHPSA machines workpieces with a diameter of up to 205" (5,200 mm), lengths of up to 571" (14,500 mm) and weights of up to 200 t. It has a drive power of 310 kW and a torque of 200,000 Nm. In close cooperation with MHPSA, WaldrichSiegen tailored the lathe specifically to the task at hand. As the rotors have to be machined tovery tight tolerances at immense metal removal rates, the machine design combines great machining force with outstanding precision.
"MHPSA requested an axial runout of less than 7 μm and a radial runout of less than 15 μm," Marco Tannert recounts. "In order to meet these tolerances reliably, the lathe operates with an axial and radial runout tolerance of 5 μm. Combining these accuracies with a maximum cutting force of 12 tons and 10 tons at the two tools provides an unparalleled machining performance." To achieve such high operating standards, the entire ProfiTurn H machine structure is extremely rigid. Its X-axis and Zaxis are hydrostatically guided for precise and wear-free operation and excellent thermal characteristics. "The rigid hydrostatic design and the robust structure are definitely highlights of the machine," says Russell Gamblin. The carriage is driven along the Z-axis by a preloaded double pinion drive. The hydrostatic guideway system has been constantly refined by WaldrichSiegen over the years – today, its extremely high dynamic stiffness is unsurpassed in the market, an indispensable precondition for the complex machining tasks required by MHPSA. The rotors are turned in hydrostatic steady rests The ProfiTurn H operates with an axial and radial runout tolerance of 5 μm.
Turbine rotors pose a number of complex machining challenges. "Journal machining for steam and gas turbine rotors is one of the most critical tasks," says Russell Gamblin. "The required tolerance is +50 μm/-0 μm. During operation, the rotor rotates at 3,600 rpm. The surface must match the bearing surface on the pedestal exactly." In addition to the rigid, hydrostatic machine design, a key factor to achieving the extremely high runout accuracy and maximum performance necessary is the absolutely precise positioning of the workpiece. The ProfiTurn H lathe is equipped with a counterstay instead of a tailstock for turning the rotors in steady rests. The counterstay monitors the positioning of the rotor and prevents axial displacement. The hydrostatic steady rests add to the stiffness required for concentricity in μm range and the excellent damping characteristics of the entire machine. According to Russell Gamblin, the steady rests in particular are a major advantage with regard to the overall shop floor efficiency: "In comparison to other large lathes in operation at our facility, the ProfiTurn H lathes are set up more easily due to a better hydrostatic steady rest design," he says. The rotors are positioned in the lathe with maximum precision by the help of a hydraulically adjustable quill. Clamping spindles with quick-adjustment function allow for fast and easy clamping and adjustment of the workpieces. The optimized guidance of the clamping jaws and deflection protection of the clamping spindles further increase the workpiece clamping accuracy.
These features result in smooth and reliable running of the machines in the day-to-day business in Savannah: "When I talk to the machinists on the factory floor, they tell me they would much rather operate the WaldrichSiegen lathe when compared to the other lathes," Russell Gamblin reports. Two smaller lathes with a maximum workpiece weight of 100 tons, a maximum workpiece diameter of 87" (2,200 mm) and a maximum workpiece length of 413" (10,500 mm) and 354" (9,000 mm) respectively also took up operation in MHPSA's facility in Savannah in 2013. They machine gas and steam turbine rotors as accurately as the larger one, as well as performing repair and revamp tasks in MHPSA's service business. Operating ProfiTurn H lathes has strengthened the efficiency of the entire production: "They have the least amount of downtime. Due to that and the excellent support from KPM, the WaldrichSiegen service unit in Ford City, Pennsylvania, we have significantly improved our processes and efficiency." The superior quality of WaldrichSiegen lathes made the ProfiTurn H the first choice for MHPSA's demanding machining tasks and since taking up operation, they have proven themselves down to the last detail. Russell Gamblin puts it simply: "Our high expectations in terms of machining accuracy and minimal downtime have been met to our full satisfaction."
Said and done – shortly after, Mitsubishi Hitachi Power Systems in Japan purchased yet another lathe made by WaldrichSiegen.