Theory And Method For Pulsed Eddy Current Testing Of Wall Thinning In Insulated Pipes

Wall thinning caused by erosion or corrosion is most threatening to the reliability andsafety of insulated pipes which are commonly used in petroleum, chemical and powergeneration industries. Periodical nondestructive testing (NDT) is vital for continued safeoperation. However, owing to the presense of themal insulation, it is challenging to performNDT work without removal of the insulation. Pulsed eddy current testing (PECT) has becomea powerful candidate to address this challenge, because it is an in-service, safety andcost-effective method.The study in this dissertation is systematically and deeply developed around the theoryand method for pulsed eddy current testing of corrosion in insulated pipes. First, the insulatedpiping structure is considered as a four-layerd plate structure, and thus it is simplified to anaxial symmetry problem which is suitable for the Dodd and Deeds models. Based on theharmonic eddy-current model and fast Fourier transform, the time-domain analyticalexpression of the induction voltage in PECT probe is derived. The presented model is fast andaccurate in calculation and suitable for calculating the probe response due to arbitrary periodicexcitation. Second, a new optimal design method for PECT probes based on the distributioncharacteristics of the incident field is presented. The distribution function of the incident fieldis extracted from analytical model by performing a detailed dimensional analysis of theintegration variable, and the relational model between the distribution of incident field andcoil structure dimension is obtained. Via comparing three optimization methods in which thecoil inner radius, outer radius and height are respectively taken as the main variable, theoptimization method based on the outer radius is selected. In this method, the distribution ofincident field and the eddy current distribution in the specimen are synthetically optimized,making the probe’s overall performance best. Four probes at different levels are developedaccording to the common industrial pipe specifications and their performances are examinedby experiment. Results show that the detection range of wall thickness is3to65mm and themaximum insulation thickness is up to150mm, which can serve the testing needs of commonpressure pipes. Then, after observing that the turn on/off of practical square-wave excitationcurrent is not an ideal step function, the influences of duty ratio and edge time on the excitation frequency spectrum are analyzed and their influences on PECT signals are studied.Based on these analyses, the excitation parameters are optimized. Afterwards, aiming at thatin practical applications the ferromagnetic cladding would result in degradation inperformance of the PECT instrument, its shielding effect on transient electromagnetic field isanalyzed, and an improved PECT technique called saturation-magnetization PECT which isdedicated for pipes covered with ferromagnetic cladding is presented. In this technique, thecladding below the probe is magnetized to saturation and its permeability is reduced, and thusthe magnetic shielding effect is weakened. Contrast experiment results show that the probeperformance is improved significantly by applying this technique. Finally, in order to solvethe shortages of existing thickness evaluation methods in being used for insulatedferromagnetic pipes, the differential PECT signal characteristics are investigated and it isfound that the time-to-peak has a linear relationship with wall thickness. And therefore, anevaluation method for assessing the residual pipe wall thickness, based on the time-to-peak ofdifferential PECT signals, is presented. A systematic study shows that the time-to-peak isindependent of insulation thickness, probe lift-off and probe structure, and its linearity againstwall thickness only holds for the relative wall thinning less than60%, but it still can satisfyhigh-precision periodical testing of wall thickness of insulated pipes.All the work in this dissertation enriches the theoretical research of wall thinningdetection in insulated pipes, provides important support for the development of nationalstandard GB/T28705-2012and can help promote the application of PECT technique.