The explosion and leakage of boiler tubes in power plant is a serious problem that often leads to unscheduled and costly outages. According to the defect characteristics and running condition of boiler water wall tube, this paper introduces the magnetic nondestructive testing (NDT) method of water wall tube. Based on the magnetic flux leakage technique, the defects on the surface, including hole, pit and crack, can be inspected, and the wall loss can be tested with the magnetic bridge principle. The magnetic probe was designed by the principle above-mentioned, and developed with Hall sensors. A NDT system, which includes the furnace wall cleaning apparatus, the sucking and crawling mechanism and safety, the inspection instrument, the controlling system and the signal processing software, is developed based on this principle. The apparatus can crawl along the tube automatically; hence the quantitative inspection of the defect about water-cooled wall tube in service was realized. The inspection experiment demonstrate that the apparatus was able to test the defect of through hole (bigger than diameter 1.6 mm) and wall loss.
