Guided waves testing allows a long-range screening in pipes of different types and represents an effective and powerful non-destructing technique for defect detections using a limited number of points of measures. After the characterization through a general theoretical analysis, the focus is set to a real steam discharge pipe with a high mechanical complexity used for many years in a research plant now dismissed. The experimental method applied here is the pitch-catch configuration of two magnetostrictive sensors. The objective of this paper is to establish a strong theoretical background to pave the way for a robust experimental investigation. Preliminary experimental results are consistent with the theoretical analysis.