Rotor Design for Large Industrial Gas Turbines

The overall designs of rotors for modern large industrial gas turbines are all very similar; however, the manufacturers have widely different internal designs. Building up the discs and drums to form the rotor is achieved either by a number of different mechanical joining techniques or by welding. In addition the rotor materials, the mechanical integrity, proof testing and critical rotor areas are also discussed. The conflicting design requirements, i.e. for supporting the centrifugal loads, protecting the rotor against the hot gases and warming the rotor during a start-up without excessive thermal stress, lead to very different design solutions. Thus in this paper the stresses, low cycle fatigue and the operational behavior of some typical designs are compared. The investigation was limited to turbines with moderate pressure ratio (<16) and low cooling air temperatures allowing the rotors to be made of high strength ferritic steels. The result shows a preference for welded designs with discs of constant stress which are free of holes in the center. Strength, low cycle fatigue and safety aspects, as well as operational advantages like avoidance of vibrations and ease of inspection speak for this solution. However, all three considered designs are presently being used by different manufacturers and meet all requirements for successful operation.