MANNINGS U.S.A.'s process for loosening and tightening of turbine bolts utilizes a method of induction heating to perform the task of opening and closing the turbine. The process reduces the time to a fraction of that normally taken by conventional methods i.e. cartridge heaters or gas burners.
Electromagnetic induction heating utilizes the principle that when a mass of conductive material is exposed to an alternating magnetic flux, eddy currents are set up in the mass. The energy generated by the flow of these eddy currents through the conductive mass with a given resistance dissipates as heat and conforms to I2R losses. Induction heating of magnetic steel has an added benefit through the phenomenon known as hysteresis losses i.e. the power losses needed to change the direction of a magnetic flux.Induction heating has the capability of producing much greater watt densities to the inside of the bolts, on the order of five (5) times that of cartridge heaters.
Induction heating also overcomes the problems associated with film co-efficients related to cartridge and gas heating i.e. the capability of the surface of the bolt to transfer the heat to which it is exposed. With induction, the heat is generated within the bolt itself thereby having much more mass heated by which thermal conduction can take place. MANNINGS has used this induction method since 1993 on hundreds of turbines. On a typical HP turbine, the loosening and tightening of all the bolts can be completed within a ten-hour shift, reducing the outage schedule by days. This process has far more benefits than economics.
The issue of safety is a major concern in all industry and particularly associated with this task during an outage. With cartridge heaters, 240 volts is the industry standard whereas induction uses less than 100 volts at the inductor. Gas heating has its own inherent safety concerns.
The MANNINGS induction heating system expands the bolts so thoroughly that the need for slugging wrenches and twenty-pound sledgehammers is eliminated. The nuts can be removed with simple hand tools.The MANNINGS system utilizes a self-tuning variable frequency power supply to deliver power to an inductor, which is placed in the bore of the bolt. The inductor is specially designed to heat just the body of the bolt between the threaded areas. By minimizing heat in the threaded area of the bolt there is little or no elongation of the threads thereby reducing the chance of binding of the threads.Because the actual kW/hrs are reduced substantially for the overall project, the heat transfer to the turbine shell is minimal; thereby the cooling time before the "Stretched" bolts can be measured is greatly reduced. Any final adjustments can be made quickly.
MANNINGS manufacture a wide range of induction and resistance heating equipment and accessories and in addition, provides on-site technical support and personnel for thermal expansion applications.MANNINGS personnel were responsible for the development of the induction heating methods used for generator disassembly/reassembly.
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