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Boiler Tube Inspection System

The field trials of boiler tube inspection with MsS system were conducted at an operating plant in Texas. The MsS guided-wave testing was conducted on horizontal reheater tubes in one of the boilers at the plant. The outer diameter of the tube was 2-1/8 inches. The overall horizontal length of the reheater tube was approximately 40 feet. Each tube line was made of five different materials (SA-210 A-1, SA-213 T-11, SA-213 T-22, SA-213 T-9, and SA-213 TP-304H), with thicknesses ranging from 0.148 to 0.300 inch. Each line also contained three U-bends and two or more 90° bends.

The MsS instrument was set up outside the boiler near a manway on the 7th floor of the unit, and the MsS coils installed on the tubes to be tested were connected to the instrument via a 100-foot-long system cable. Both 128-kHz torsional-mode wave and 128-kHz longitudinal-mode wave were used in this trial.

The longitudinal-mode probe and test data are shown in the below figure. In the figures, "W" indicates a signal from a weld and "S" from a vertical spacer between the tubes in adjacent rows. The locations of 90° and U-bends in the reheater tube network are indicated in the plot.

Based on the findings from these field trials, we conclude the following:
  1. The long-range guided-wave inspection technique has potential for comprehensive inspection of reheater boiler tube networks in the field
  2. The achievable inspection range in one direction was approximately 30 feet for both 128-kHz T-waves and 128-kHz L-waves
  3. All tube networks could be inspected with proper tooling from the accessible side of the tube bank without requiring scaffolding and tube removal
  4. Vertical spacers increased background noise level and, thus degraded inspection capability. The degradation was greater for the L waves than for the T waves
  5. Except for tight U-bends, other 90° bends and U-bends in the tube networks did not show any observable negative influence on wave propagation. Tight U-bends (bend radius estimated to be three tube diameters or less) appeared to impair wave propagation and, thus, prevented inspection of the area beyond
  6. The entire length of the tube network between the inlet and outlet headers could be examined from two to three test locations, depending on the number of tight U-bends in the network.

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