Phased array inspection at elevated temperatures

Interruption of plant operation can be avoided if non destructive testing inspections are performed on-line at operating temperatures, which may be up to several hundred degrees Celsius in a petrochemical or electric power generating plant. However, there are operational temperature limits for the phased array transducers and associated plastic wedges used for ultrasonic inspections. In this paper, an ultrasonic phased array system is described for inspections at elevated temperatures of up to 350°C. Wedges are built from plastics resistant to high temperature degradation, and equipped with a cooling jacket around the array. A model of the ultrasonic beam skew pattern due to thermal gradients inside a wedge is developed. The model is used in a separate algorithm to calculate transmission and reception time delays on individual array elements for generation of plane waves in a hot test piece, while compensating for thermal gradient effects inside the wedge. The algorithm results for planar wave inspections of test pieces at 150°C demonstrate that application of conventional element time delays can lead to serious phase errors. Experimental trials indicate that plane waves can be generated in a hot test piece using the new focal law algorithm with appropriate timing delays applied to all active array elements.