By Ankit Vajpayee Feature Using a Robotic System to Inspect Boiler Tubes A robotic wall crawling system utilizing electromagnetic inspection techniques inspects boiler water walls from the outside of the tube Tube failures continue to be the leading cause of forced outages in boilers. To get your boiler back on line and reduce or eliminate future forced outages due to tube failure, it is extremely important to determine and correct the root cause. Detecting flaws before they cause failures is of critical importance in boiler maintenance. Localized wall thinning due to corrosion in boiler water-wall tubing is a significant inspection concern for boiler operators. The typical methods used for inspecting boiler water walls include spot check ultrasonic testing (UT), Ascan UT, electromagnetic acoustic transducer (EMAT) UT, and scanning thermograpy. Spot check UT only gives thickness readings and gets very minimal coverage of the total surface area of the furnace water walls; the chances of finding ID flaw mechanisms using spot check UT are minimal at best. If boiler water walls have been sandblasted, A-scan UT may be used to inspect larger areas of the furnace walls; in these cases, a steady flow of water is most often used as the couplant. The EMAT technique requires sandblasting of any boiler water wall surfaces, and does not inherently get good surface area coverage unless the inspection team performs multiple passes using the 24 Inspection Trends / April 2011 EMAT probe. Scanning thermography is the most recent development for the inspection of boiler water walls; however, it is not yet commercially available in enough capacity. This article explains the deployment of a robotic wall crawler using an electromagnetic technique to inspect boiler water walls from outside of the tube as well as the theoretical background of the technique, which explains the quantitative nature of the inspection. Further, a case study is presented for the technique that allows the extraction of thickness information from the inspection data. Theoretical Background of Remote Field Technology In the 1950s, the Shell Development Co. pioneered an electromagnetic nondestructive examination technique known as remote field testing (RFT). It was first used to inspect well casings for corrosion and wall thinning, and for a number of years was used primarily in the petroleum and pipeline industries. In the mid-1980s, this technology became a subject of sophisticated research, and the combination of basic research and industrial innovation has resulted in elegant theoretical models that eventually developed into strong analytical methods that enable a greater variety of anomalies to be detected and quantified. Remote field testing is now a well-established inspection method for condition assessment of ferromagnetic tubes. Principles of RFT Remote field testing is based on a through-transmission principle. The field passes from the exciter coil through the tube wall, along the outside of the tube, and back in through the tube wall at the location of the detector coil — Fig. 1. Metal loss causes the field to arrive at the detector coil with less travel time and less attenuation, resulting in a change in signal phase and amplitude. The signal values of phase (time of flight) and logamplitude (signal strength) are directly related to wall thickness in the area of the detector coil(s). The RFT technique can be used for all conventional carbon steel materials, diameters, and wall thicknesses. It is, therefore, used in many different types Fig. 1 — A typical RFT probe. In comparison with conventional eddy current, RFT coils are widely spaced in order to measure the through-transmission field. Fig. 2 — An inspector using UT to do thickness measurements on boiler water wall tubes.
Inspection Trends - April 2011 - Spring
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