Experimental Research On Thick Wall Pipes Inspection Based On Circumferential Guided Wave Technology

Thick wall pipes are one of the core components of four-pipe system in thermalpower plants. Due to the long-term high temperature operation, creep, degradationand stress relaxation, these pipes are prone to thermal fatigue damage, cracks andcorrosion, resulting in partial wall thinning. Pipes under high pressure, hightemperature steam, easily lead to the serious accidents of the pipe axial burst leakage.The current thick wall pipe inspection methods are point-by-point scanning, largeworkload, low detection efficiency and high cost, therefore it is difficult to achievefast, reliable and complete thick wall pipe inspection. Circumferential guided wavetechnology is especially suitable for the inspection of large diameter pipes includingthick wall pipes. When scanning inspection is carried out in vertical direction of thepipes, the complete and rapid inspection of the pipes can be achieved.For the purpose of the application of circumferential guided wave technology forin-service thick wall pipe inspection, circumferential guided waves propagationcharacteristics in thick wall pipes were studied in this paper. Main research activitiesare given as follows.(1) Based on wave theory, propagation characteristics of different types ofcircumferential guided waves in thick wall pipes are investigated. The combinationof probe with0.5MHz excitation frequency and wedge with60°is optimized. Anglebeam probe in pulse-echo configuration was used for exciting circumferential Lambwaves. EMAT at the center frequency of310kHz is used for circumferential SHwaves inspection in pitch-catch configuration.(2) Experimental setup for circumferential Lamb waves and circumferential SHwaves inspection was established. Excitation energy distribution by using differenttransducers and propagation characteristic of circumferential guided waves wereinvestigated experimentally. According to displacement distribution of thecircumferential Lamb waves and circumferential SH waves in thick wall pipes,different types of defects which located at the inner and outer wall of the pipe wereinspected.(3) Circumferential Lamb waves and circumferential SH waves inspectionsignals were processed by using continuous wavelet transform (CWT). Signalwavelet coefficients maps are obtained and extract amplitude envelope diagram at specific frequency was extracted. According to the corresponding relation of thewavelet coefficients map and the dispersion curves, all contained modes of receivedinspection signals were analyzed and then an amplitude envelope diagram at aspecific frequency was used to analysis the other modes which contained in theinspection signals. Studies show that this signal processing method is suitable for themode analysis of circumferential guided waves inspection signals.(4) Circumferential Lamb waves and circumferential SH waves propagatingaround the circumferential direction of thick wall pipes exist strong directionalcharacters. Continuous wavelet transform is used to extract the amplitude envelopsof inspection signals at the frequencies of500kHz and310kHz. Image of axial defectwas achieved by using the amplitude envelopes of the inspection signals obtained atdifferent scanning locations.