Özet:

This paper dedicated to electromagnetic acoustic (EMA) inspection of boiler tubes in the process of regular maintenance for prevention of tube rupture. This non-destructive testing method is developed to replace commonly used ultrasonic method of inspection which is costly and very labor intensive. During proposed EMA inspection surface of tubes does not require sandblasting and special coupling between measuring tool and tube wall. Commonly used ultrasonic method, based on detection of the wave generated by a piezoelectric effect of a crystal transducer, provides accurate but sparse data and takes long time to inspect healthy boiler condition. Electromagnetic acoustic transducer EMAT generates such wave directly in tube material by electromagnetic method. EMAT technology is well known and used in many nondestructive testing situations. This method relies on generating ultrasound wave directly in the metal in the form of elastic wave with ultrasonic velocity, characteristic for a given metal. There are two ways that EMATs can generate elastic energy directly in the boiler water-wall. The first is via the “Lorentz force” mechanism where interaction between an applied magnetic field and induced eddy currents produces an elastic wave. The second is through magnetostriction (MS), where an alternating magnetic field generates an alternating elastic force. Developed and fabricated with high energy Neodymium-Iron-Boron permanent magnets with magnetic energy over 56 MGsOe by M. Bergander [1] transducer is able to generate strong elastic wave using magnitostrictive properties of tube material for prediction of the tube thickness. In the special conditions of coal-fired boiler inside and outside surfaces of the tube are covered by erosive and corrosive layers. Those corrosive and erosive layers have good magnitostrictive properties which allow making water-wall thickness measurement without sand blasting and without necessity to have coupling between measurement tool and the tube material surface. This method allows inspecting boiler tubing much faster with the high quality of assessment. To make process of inspection faster claiming robot has been designed. This robot is able to deliver sensor of the measuring device to the surface of the tube and provide stable data reading. Using such robotic device it is possible to do automation of the inspection procedure.

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