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Improving Guided Wave Testing of Pipelines with Mechanical Attachments

Long-range guided wave testing techniques are widely used to detect corrosion under insulation in refinery piping systems. Along the piping systems, many mechanical features, such as clamps, hangers and pipe supports, are found that are in direct contact with the pipe outer diameter surface. Signals produced from the interactions with these features can disrupt the guided wave propagation, increasing background noise level, masking discontinuity signals, reducing the test range, and, if the presence of such a mechanical feature is unknown to the tester, leading to misinterpretation of signals. This paper examines the effects of mechanical clamps on the guided wave propagation and their dependence on the guided wave frequency. The effects are highly sensitive to the wave frequency and are greatly reduced at high frequencies. To show the degree of the sensitivity to wave frequency and reduction that can be achieved using high frequency guided waves, experimental data taken from laboratory samples with a mechanical clamp over frequencies of up to 200 kHz are presented. In long-range guided wave testing of piping systems with mechanical attachments, a marked improvement in test results can be gained by using a high frequency guided wave (above 100 kHz in torsional wave mode). Examples of actual field data are also presented that demonstrate a markedly improved test result and the need to employ high frequency waves for testing of refinery piping systems.

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