Independent qualification of phased array inspection of fillet welds

Magnox Generation has investigated whether it is feasible to carry out ultrasonic inspection of welds associated with boiler beam support brackets at various Magnox power stations. The welds are small, with a leg length typically of 6 to 15 mm, and they could not be properly inspected using conventional ultrasonic probes. A technique was therefore developed using a phased array system. This allowed the ultrasonic beam to be focussed electronically, thereby obtaining the required spatial resolution, and also allowed the beam angle to be varied in order to direct the beam into the weld volume between the cap and root. Following the development work, it was decided to apply the technique to some boiler beam support bracket welds at Sizewell A power station. This paper addresses the qualification of the inspection procedure, which TWI managed on behalf of Magnox Generation following the ENIQ methodology. The comprehensive technical justification document was supported by practical trials of the procedure, equipment and individual data analysts. The qualification was completed successfully and confirmed that the procedure was capable of reliably detecting crack-like defects in the weld or heat affected zone greater than 1.5 mm wide by 10 mm long. It also confirmed that, subject to certain provisos, the defect width would not be underestimated by more than 1.5 mm. This qualification involved a novel technique with particular challenges and this paper describes the way in which these were overcome.