[April 2008] 

Phased array ultrasonic technology has greatly improved the critical inspection of complex geometry blade fixings, allowing this to be done without removing the blades and obtaining a permanent record of the inspection. 

The Problem

Steam turbine rotors, particularly in the low pressure stages, can suffer from stress corrosion cracking. This manifests itself in the serrated root fixings and in the corresponding disc steeple areas.  Because of the high stresses involved, the critical crack size is small and there have been some notable catastrophic failures.  This can have major cost and safety implications for the power station operators, so regular inspections are performed during planned overhauls.  These historically have involved removal of blades and inspection for surface and volumetric defects using combinations of Magnetic Particle, Eddy Current and manual Ultrasonic techniques.  This has proved to be very time consuming with the attendant risk of damaging the components during removal and replacement.

The Challenge

The challenge was to develop ultrasonic techniques, which could be deployed to inspect blade roots and disc steeples of various designs without removal of blades, and giving a high degree of confidence in the results, both in terms of minimum defects size and repeatability.

Our Solution

By way of combining flaw simulation using Electron Discharge Machining (EDM) and CAD modelling with ultrasonic beam tracing, we have been able to design scans which give full coverage of the required areas for inspection and are capable of reliably detecting flaws such as cracks well below the critical crack size for the component.  For the most complex inspections such as axially mounted blades with curved roots, a full inspection may involve numerous scans using phased array and manual pulse-echo probes.  Detection of flaws at all required positions is ascertained by validation using real or artificial reflectors.  This results in easily interpreted computer imaging of complex geometry, high cost turbine components; rapid validation using a combination of CAD modelling and creating flaws in realistic reference samples; comprehensive reports including a selection of representative images.

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