Energy-Tech Magazine

System calibration
Calibration of the boresonic data acquisition system is rigidly controlled under formal specifications promulgated by QCI-1014, an internally drafted quality control instruction.

The calibrations of all applicable ultrasonic channels utilized during the bore examination are performed using custom calibration blocks fabricated from a material having similar acoustical properties to that of the turbine rotor under test.

Distance and sensitivity calibrations are achieved using echo responses obtained from various 1/16″ diameter side drilled holes (SDH) that are precisely located within the boresonic calibration standard.

Using the amplitude response obtained from each respective SDH reflector, distance amplitude correction circuits are adjusted to provide a normalized A-scan amplitude display from reflectors located at various increasing depths. Documentation of the system calibration is included in the formal written inspection report.

Boresonic visual inspection (VT)
The surface of

Advertisement

the rotor bore is first thoroughly examined to assess the quality of workmanship of bore preparation operations and identify surface anomalies that might interfere with subsequent NDT procedures. Typically, bore preparation consists of a power honing operation which yields a specified 63 Ra surface finish. Verification of an adequate surface finish is important since surface conditions will affect the sensitivity of the ultrasonic test, as well as affecting the sensitivity of the magnetic particle examination. A smooth surface finish provides a better material interface for maximizing the passage of acoustic energy into the test piece, as well as providing increased sensitivity in magnetic particle testing by providing increased particle mobility on the surface. The visual inspection is conducted using a ridged, whitelight borescope having a magnification of 2x or greater.

Magnetic particle inspection (MT)
Magnetic particle inspection (MT) of the turbine rotor bore surfaces and the immediate underlying subsurface volume is performed utilizing a circular magnetic field created by a current bearing central bar conductor (CBC) passing through the centerline of the bore cavity. A visible, wet method particle application is performed during the presence of a continuous magnetic field produced by electric current flowing through the CBC. The existence of discontinuities within the volume of material subjected to the magnetic field would cause disruptions in the normal flux distribution in the rotor material and produce a flux leakage field that would attract the magnetic particle medium that was sprayed on the bore surface.

Therefore, resulting surface anomalies are detected by the presence of many finely divided ferromagnetic particles that have been attracted to and trapped by the flux leakage field produced by the material discontinuity. Viewing of the bore surface using a ridged, white light, borescope with 2x or greater optical magnification is then performed.

Initial pre-honing measurements
Upon removal of the bore plug(s), and prior to the start of honing, internal bore diameter measurements are taken at 2″ axial intervals. Typically, measurements are made using a specially fabricated sled-type assembly that has been fitted with a precision dial indicator. After initial calibration of the gauge assembly, the sled is pulled through the bore and plus or minus deviations are recorded at 2″ intervals across the entire bore length. A best effort approach is made to obtain precise measurements in cases where excessive internal scale or corrosion is present on the surface of the bore.

Power honing
Power honing of all accessible sections of the bore surface are generally performed internally by trained field machinists.