||Guided wave method is the most recommended non-destructive technique in the industry for pipelines condition inspection. Its characteristics are that it can speedy and widely examine the overall pipelines, and it can estimate the pipelines’ condition and lifespan. However, it is very common using elbow to promote the pipeline system’s application efficiency in refining, petrochemical, power plant and gas industries, but the geometry of the elbow changes the guided wave transporting path from symmetric to asymmetric, it could lead to mode conversion. The higher order asymmetric guided waves because of mode conversion influence each other, and the detection signal becomes complex and hard to predict. It would seriously influence the result of guided wave detection. In the previous studies, when the guided wave passes through the elbow, the energy focus on the outer side of the elbow leads to misjudging the seriousness of the defect. The energy of the other elbow areas is too small to detect the defect, and these areas become the more dangerous blind-area. This research firstly uses finite element method to simulate T(0,1) torsional guided wave pass through the defects which in the same geometry shape but at different elbow and straight pipes’ positions, and using the reflected signals to calculate the entire elbow area’s compensation coefficient map. Let the reflected signals of defect at the elbow return to the original condition. The results are that the compensation coefficients are larger than 1 at inner, top and bottom side of the elbow, and these compensation coefficients become larger as the defects’ positions approach the end of the elbow. The outer side compensation coefficients are smaller than 1. Besides, the top and bottom side compensation coefficients are symmetric. The compensation coefficient trends are different when the guided wave frequency is different. For example, the inner side compensation coefficients increase first and then decrease with the 30 kHz guided wave, but they simply increase with the 45 kHz guided wave. These results seriously increase the difficulties of making compensation coefficient map. This research also discusses the relationship between different geometry parameters (wall thickness and pipe outer side radius) and guided wavelength. The result shows that the compensation coefficients are very similar at 4 in. and 6 in. pipes when the guided wave’s wavelength ratio is identical to the outer side radius ratio, and this result can solve many difficulties when establishing common compensation coefficient map.|
The experiment pipeline setup of this research is identical to the numerical setup, and we use guided wave measurement system to detect the reflected signals. We find that the numerical results are larger than experiment results of the inner and top side area. The reason of this results is that the defects are too close to welds, so the defects’ reflected signals are affected by the welds’ reflected signals. However, it can also reach the benefit of preventing miss detection by using more large-numbered compensation coefficient to correct the reflected signals of the inner and top side defects. Consequently, we can apply the numerical compensation coefficient on the practice guided wave detection, and we can correct the reflected signals of defects at the elbow to promote the accurate and reliability of using guided wave to examine the elbow.