High temperature flexible ultrasonic transducers for NDT of pipes

NDT of power, chemical and petroleum plants is an increasingly important element for safety improvement and extension of plant life span. Such plants contain numerous tubular structures and to obtain ultrasonic signals with sufficient signal-to-noise ratio (SNR), ultrasonic transducers (UTs) often need to conform to these structures and operate at elevated temperatures. Flexible UTs are suitable under such conditions and adaptable to different tube diameters because they ensure good self-alignment with the object\u2019s surface and a uniform couplant thickness. This results in good transmission of ultrasonic energy into the component and reduced noise. In this study flexible UTs, consisting of piezoelectric films with thicknesses larger than 40 \u3bcm deposited on a 75 \u3bcm thick metal membrane, were developed for NDT applications up to 500\ub0C. The piezoelectric films were made by a sol-gel spray technique and can be used for NDT of pipes of diameters larger than 25.4 mm. At room temperature the ultrasonic performances of flexible UTs were at least as good as commercially available 5 MHz and 10 MHz broadband UTs. At elevated temperatures accurate pipe thickness measurements were achieved because of the high SNR of the ultrasonic echoes obtained in the pulse-echo mode. For continuous NDT, an induction brazing technique which can be performed on-NDT-site was developed to braze flexible UTs to steel pipes. The brazing material serves as a high temperature ultrasonic couplant. Finally, the development of high temperature flexible UT arrays for NDT will be discussed.Peer reviewed: NoNRC publication: Ye