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High Frequency Guided Waves

    The high-frequency operation of MsS guided-wave system includes much merit compared to piezoelectric systems like GUL or Teletes in piping inspection and monitoring. MsS system operates at wide frequency range of 5 kHz to 250 kHz with pure torsional wave mode. The high-frequency operation allows finding smaller defects in short (less than 10 meters along one direction) distance for insulated pipeline in refinery, chemical and petrochemical facilities. It also has less interaction with attached geometric features such as clamp and pipe support as shown in the example below.

 

Example 1: Clamp on pipe


     The following figure shows inspection report plot in dike penetration region (top graph), photo of a clamp (left bottom), and amplified plot of clamp signal (right bottom). The inspection report was made with 64- and 150-kHz torsional mode data. The clamp signal at about 2 m was shown with 64-kHz data. The video signal in the dotted line was plotted at the right bottom with rf signals. The clamp signal is strong at 32 kHz data, and it decreases as the frequency increases as shown in the amplified plot. The clamp signal at 150 kHz is not shown because the guided wave signal at high frequency has less interaction with the clamp. If there is no defect in pipe clamp, no signal is reflected from it. Therefore, 150-kHz signal can indicate a defect under the clamp if the reflection from the clamp is strong at 150-kHz data. The following field testing shows that the inspection range at 150 kHz is longer than that at 32 and 64 kHz if the pipeline includes tightened U-clamps. If the inspection was performed at only low frequency, the trailing signals of clamp would’ve made a false call or difficult to find defect at the clamp or behind it.

 



Example 2: Unwelded pipe support 


     The following figure shows inspection report plot (top graph), a photo of pipe support (the left bottom), and amplified plot of pipe support signal (the right bottom). The big pipe was supported by a small diameter pipe on concrete block. The signal from a pipe support is shown at 32 and 64 kHz. But the pipe support signal at 128 kHz does not show because the guided wave signal at high frequency has less interaction with an attachment. If there is no defect in pipe support, no signal is reflected from it. Therefore, 128-kHz signal is much easier than low-frequency signal in finding defect in pipe support.

 


Example 3: Welded pipe support 


     The following figure shows inspection report plot (top graph), photo of welded pipe support in the insulation (left bottom), and amplified plot of pipe support signal (right bottom). The inspection report was made with 128-kHz torsional mode data. The welded pipe support has strong interaction with low-frequency guided wave as shown in the amplified plot of pipe support signal (right bottom). The amplified plot was marked with red-dotted line in the inspection report plot. The 32-kHz torsional data shows the strong trailing signal following the pipe support signal. If this inspection is performed at less than 60 kHz, the big trailing signal at low frequency causes a false call. Therefore, high-frequency inspection of pipeline is necessary for testing a pipeline having welded pipe support.

 

     The trailing signal is strong if the circumferential length is less than about 5 times of operating wavelength. Because 128-kHz torsional mode has 4 times shorter wavelength than that of 32 kHz, 128-kHz torsional mode does not interact with the pipe support and its signal is much better in finding any defect at the region following a pipe support.

 

Example 4: Pitting Corrosion under Insulation 


     The following photo shows CUI (corrosion under insulation) guided wave testing. The MsS probe was installed after removing the insulation and tested at 32, 64, and 128 kHz. But the small surface corrosion was sensitive to high frequency of 128 kHz (Wave length is 1 inch). We found pitting corrosion in the elbow region and verified by client after removing the insulation.