Wall thickness measurement sensor for pipeline inspection using EMAT technology in combination with pulsed eddy current and MFL
An electromagnetic acoustic transducer (EMAT) is presented for excitation and detection of linear polarized shear waves at normal incidence with the use of a horizontal magnetization of the specimen. The sound field is optimised for the measurement of the wall-thickness of steel plates and pipe walls with metal-loss and pitting due to corrosion. As an EMAT - insonifying a pipe wall from the id-surface - does not provide an incident surface echo signal, because the wave is directly excited in the material, the position of metal-loss (id- or od-surface) cannot be detected. To overcome the drawback, the EMAT is utilized also as eddy current sensor and the transmitting pulse for UT-excitation at very early receiving times is evaluated as eddy current signal in order to detect the transducer lift-off. So far the lift-off is > 0 and simultaneously the EMAT signal can be evaluated, id-metal-loss is detected. In the case of zero lift-off, the EMAT time-of-flight information describes the wall-thickness and - if reduced - od-metal-loss. In the case of larger lift-off (> 500 µm), i.e. larger id-metal-loss, the potential of the EMAT to excite and detect ultrasound is tremendously reduced. However, in that case the pulsed eddy current feature to measure the lift-off can be evaluated up to a lift-off of 8mm to size the id-corrosion. Furthermore, the EMAT as an inductive sensor can detect the magnetic flux leakage excited by the horizontal magnetic field and produced by the metal-loss so far the speed of the inspection tool is not too low. Because of the low frequency of the flux leakage signal compared with the high frequency EC or ultrasonic signal it is possible to separate and analyse the different signal parts. Due to the combination a probe was developed and optimized. The contribution reports to the obtained state of the art and the potential for gas pipe inspection.