EMAT Development And Application For Monitoring Corrosion On Line

Electromagnetic acoustic testing is a new non-destructive testing technology, playing an increasingly important role in monitoring pressure vessel operation with its unique advantage. Aiming at the key technology of monitoring pipeline corrosion below 200 ℃ on line, the research has been done, including the design of special EMAT and monitoring the variation of the thickness caused by corrosion.According to requirements of monitoring uniform corrosion thinning about pipeline, a new type EMAT is proposed. Based on the computer simulation, this EMAT is made up of bias magnetic field supplied by a permanent magnet, a self- excitation and self- receiving coil, providing normal beam of shear wave. The structure of EMAT with a spiral coil is designed and made, which is applied to measure the thickness of pipeline. The eddy current on surface of magnet produced by the alternating magnetic field of coil is prevent by using the combination of shield and gap, while the effective role of magnetic field on the surface is evenly distributed by this way. The effect of magnet self-oscillating on testing signal of EMAT is eliminated effectively.With the special EMAT, four kinds of material with 8mm thickness are measured. The result is showed that the accuracy of measurement is better than 1%. Under room temperature, 20# steel with the thickness range from 18 mm to 85 mm is measured. The thickness of the upper detection limit is up to 75 mm, while the accuracy of measurement is better than ± 1% in the range from 18 mm to 75 mm. Within the temperature from 26℃ to 200℃,the detection sensitivity of measurements about Q235 with 14 mm thickness is better than 0.65%. The experimental testing about evaluation of detecting area is made on the block of zirconium plate. The result is showed that the thinning area size of 15 mm × 15 mm is detected effectively. For 15 CrMOG pipe with a rough surface, the accuracy of measurement is 0.7%.The research results in this thesis can be used for monitoring pipeline corrosion, achieving the acquisition of wall thickness in real-time, providing important data to assess the life of the pipeline. This thesis will also provide a useful reference for the research and development of electromagnetic acoustic transducers.