Ultrasonic Testing for Detection and Sizing of Circumferential Cracking in Turbine Rotors

In the last 30 years, the fossil fuel power generation industry has seen tremendous changes in operations, management, and adaptation to meet the requirements of accommodating renewable energy generation. All these changes have affected the integrity of turbine and generator rotors in many ways, especially when a unit designed for base load operation is changed to peak load operation. One of the major problems stemming from these changes is the fact that steam turbine and generator rotors will develop fatigue cracking caused by thermal and mechanical operational stresses. Service induced defects start at the surface of the rotors, either peripheral outside diameter (OD) areas, inside diameter (ID) bore connected areas, dovetail areas under blading, areas under shrunk-on disks, etc. These service-related defects will propagate perpendicular to the stress-induced during service. For example, bore circumferential or hoop stress caused by thermal and mechanical stresses during startup and operation will induce axially oriented bore surface connected defects. Axial and bending stress applied on the shaft by thermal and mechanical forces will induce surface connected circumferentially oriented defects. A sample of 12 rotors that have develop circumferential cracking during this transition period were evaluated using ultrasonic testing (UT) techniques applied from the OD surface of the rotors and from the ID surface of the rotor central bore. The UT techniques include longitudinal and shear waves generated by conventional and phased array ultrasonic transducers. The group of 12 rotors include 7 HPIP (High Pressure – Intermediate Pressure), 4 DFLP (Double Flow Low Pressure), and 1 generator rotor all having a central bore manufactured by various Original Equipment Manufacturers (OEMs). The circumferential cracking was sized from incipient at the OD surface to through wall to the ID bore surface and from the dovetail OD surface to up to 0.75 inches deep.

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