Eddy current testing (ECT) offer distinct advantages over the conventional methods in the sense that they are faster and easy to automate, resulting in labour saving as well as data reliability. However, other than general guidelines from the inspection of turbines not much is known as to the selection of proper transducers or the parameters suitable for application to the turbine components particularly to the blades.

A few trials using existing Eddy current testing equipment and transducers yielded very promising results on the feasibility of the techniques and at the same time limitations of existing transducers were realized.

Results

• With the available probes of absolute coil design (50 KHz frequency) the defects open to the surface could be detected, where as subsurface defects could not be detected.

• While using the edge probes with differential coil arrangement (5 to 50 KHz frequency range) surface defects as well as subsurface defects upto a limited depth were detected. Using less than 10 KHz posed problems in equipment balancing.

• Using edge probes with differential coil arrangement (50 to 500 KHz frequency range) only the defects open to surface could be detected.

• Using differential probes with the flat surface (6 mm dia and 10 mm dia) could able to detect only those open to the surface. While scanning for subsurface defects using 5 to 50 KHz frequency range some signals were obtained but because of the interference signals from lift off, it was not possible to detect subsurface defects. For getting reliable and reproducible results and to eliminate the lift-off and wobble effects edge transducers with convex and concave sides were identified for testing of leading and trailing edges of blades. The scanning with these probes proved to be faster, reproducible and the effects due to lift and probe wobble were minimal.

Artificial defects were induced on dummy blades for the purpose of standardization and calibration of instruments. (Not writing the parameters and test results - space constraint).

Eddy current testing (ECT) was used for testing the blades of gas turbine HPCL CPP. The results indicate that it is feasible to carry out the eddy current testing of steam and gas turbines blades to detect very small defects by arriving at proper parameter settings. Thereby the advantages of the ECT like speed, reliability over conventional testing techniques can be realized in practice.

Also a large potential exists for automation and application of these techniques for testing turbo generator components such as rotor bores, retaining rings etc. Efforts are being made in this direction. Further work is required to be done in applying these techniques to other types of blades.