ET04.pdf

The first part of the study dealt with radial buildup

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The first part of the study dealt with radial buildup of magnetite from the support plate toward the tube, or flushing of magnetite from the tube toward the support plate. Figure 38 shows the geometry involved and the impedance plane trajectories for magnetite buildup in layers of 0.08 mm (0.003 in.). The change in signal from a clean gap to one full of magnetite is quite dramatic, both in shape and amplitude. The second part of the study assumed an axial buildup of magnetite from the center of the support plate outward. Although this direction is not very likely during buildup, it is representative of chemical flushing, where the chemical agents attack the magnetite from both sides. Figure 39 shows this situation: the changes between a full gap and partially filled gap are dramatic. Another aspect of testing steam generator tubing is that of denting due to magnetite buildup. Similarly, monitoring the flushing of magnetite in this situation is more important to ensure complete cleaning of the magnetite in the gap. Figure 40 shows the impedance plane trajectories of axial magnetite buildup (or flushing) in various amounts from a clean 114 Electromagnetic Testing F IGURE 36. Experimental impedance plane trajectory at 100 kHz from 3 mm (0.12 in.) long coil at nominal spacing of tube sheet and rolling. Reactance, scalable (H) Resistance, scalable ( ) F IGURE 37. Geometry used to study effect of magnetite accumulation in crevice gap between tube and support plate. Dashed line represents area in which denting is modeled. Coils Support plate Tube F IGURE 38. Modeling of radial buildup of magnetite of thickness ω . ω = 0.08 mm (0.003 in.) ω = 0.15 mm (0.006 in.) ω = 0.23 mm (0.009 in.) ω = 0.3 mm (0.012 in.) ω = 0.4 mm (0.015 in.) Support plate Tube ω Coils ω = 0 No magnetite Full of magnetite F IGURE 39. Modeling of axial buildup of magnetite (of extent ω ) outward from center. ω = 10 percent ω = 30 percent ω = 50 percent ω = 70 percent ω = 90 percent ω = 0 percent ω = 20 percent ω = 40 percent ω = 60 percent ω = 80 percent ω = 100 percent Support plate Tube ω Coils No magnetite Full of magnetite
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gap to a gap full of magnetite. From these plots, it is evident that even small amounts of magnetite in the gap affect the impedance plane trajectory. In addition to the data presented above, studies were carried out (1) for axial buildup from one side of the support plate (Fig. 41), (2) for axial buildup from one side of the support plate with a portion of the gap clean on both sides of the magnetite band (Fig. 42) and (3) for denting of the tube without the presence of magnetite (Fig. 43). For comparison, the clean crevice gap trajectory is given again in Fig. 43. 115 Modeling of Electromagnetic Testing F IGURE 40. Modeling of magnetite buildup in presence of denting. ω = 10 percent ω = 30 percent ω = 50 percent ω = 70 percent ω = 90 percent ω = 0 percent ω = 20 percent ω = 40 percent ω = 60 percent ω = 80 percent ω = 100 percent Support plate Tube d ω Coils Legend d = 0.1 mm (0.004 in.) ω = magnetite No magnetite Full of magnetite F IGURE 41. Modeling of buildup of magnetite of extent ω from one side of support plate.
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  • Fall '19
  • Wind, The Land, Magnetic Field, Dodd, Modeling of Electromagnetic Testing

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