Field Measurements of Ultrasonic Stress in a Compressor Rotor
The run-out report for a compressor rotor showed an almost continuous bow between the two bearings, with the maximum run-out of 0.03 mm (0.0012 in)at the inlet to the fourth stage impeller. It had been run in service, and removed for normal maintenance, when the bow was observed. The seven stage compressor rotor was made of 34 Cr Ni Mo 6 steel, which is approximately a 4340 steel. It was just over 3 m in length, with diameters of approximately 265 mm at the disk mounting areas (Fig. 1). Each of the compressor stages was mounted on the rotor at the time of the stress measurement. An evaluation of the residual stress at these locations in the shaft was performed using a nondestructive technique involving critically refracted longitudinal, LCR, ultrasonic waves. The LCR stress measurement data showed compressive stress on the bowed side of the rotor, supporting the conclusion that residual stress is the cause of the bow. At the time that the data were taken, the rotor was horizontal, resting on two stands at the bearing ends. The following is a brief report of the test and the results. Ref. 1 should be consulted for additional detail.