Electromagnetic acoustic boiler tubes inspection with robotic device

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

Remaining life assessment for boiler tubes affected by combined effect of wall thinning and overheating

Boilers, the most troublesome components of electric power, chemical and processing plants generate high costs in unscheduled shutdowns, repairs and power replacement. Every occurrence of ruptured tubes leads to emergency shutdown of the entire plant. This paper describes the joint international effort to develop faster and more efficient methods for condition assessment and remaining life prediction for boiler tubes made of low-carbon steel. Authors have undertaken a systematic research with the major objective to correlate the results of combined nondestructive testing (NDT) with condition assessment of boiler tubes. The evaluation included non-contact wall thickness measurement with EMAT technology plus internal oxide layer measurement with specialized ultrasonics. While the first method shows the remaining tube wall thickness, thus allowing calculating total stress, the latter one has the potential to characterize microstructure degradation, which up to now could only be determined by destructive analysis. The special attention was directed towards identification and analysis of creep damage due to overheating. In recent years, techniques were developed to identify heat damage by measuring the thickness of internal oxide scale because even a thin scale can seriously impede heat transfer causing elevation of temperature in tube wall. A combined effect of wall thinning and the “degree of overheating” on tube remaining life was investigated. The uniqueness of this work lies in one of the first attempts to develop and validate a tool for methodology for condition assessment and remaining life prediction, for Steel20 tube material, while most of previous authors had concentrated on Cr-Mo steels. Another contribution is the combined treatment of two different damage mechanisms and practical utilization of two various NDT techniques. To-date, both results are treated separately, and consequently separate reject criteria exist for overheating and separate for wall thinning. As a result of work presented in this paper, a procedure was recommended to calculate the tube remaining life based on the results of two ultrasonic tests

Combination Non-Destructive Test (NDT) Method for Early Damage Detection and Condition Assessment of Boiler Tubes

Boilers, the most troublesome components of electric power, chemical and processing plants generate high costs in unscheduled shutdowns, repairs and power replacement. Every occurrence of ruptured tubes leads to emergency shutdown of the entire plant. This paper describes the joint international effort to develop faster and more efficient methods for condition assessment and remaining life prediction for boiler tubes. The work was performed under the grant from Kazakhstan Ministry of Education and Science. The authors have visited a number of coal-fired electric plants throughout Central Asia and found that a combination of wall thinning and overheating were major damage mechanisms contributing to boiler tube failures. The periodic inspection of boiler tubes include ultrasonic measurement of remaining wall thickness and in many cases, it involves cutting tube segments and performing metallurgical analysis for loss of original strength due to overheating. Systematic research was undertaken with the objective to correlate the results of combined non-destructive testing (NDT) with condition assessment of boiler tubes. The evaluation included non-contact wall thickness measurement with EMAT technology plus internal oxide layer measurement with specialized ultrasonics. The first method shows the remaining tube wall thickness, thus allowing to calculate total stress, and the latter one has the potential to indirectly characterize microstructure degradation, which up to now could only be determined by destructive analysis. The existing tube removal criteria are treating each damage mechanism separately while in reality, a combined effect of wall thinning and the “degree of overheating” decides about true condition of a tube. The procedure that utilizes the results of both described NDT methods was developed for improved methodology to assess tube condition and to predict its remaining life