Applications of Magnetostrictive Sensors for Guided Wave Screening of Heat Exchanger Tubing

Introduction

Screening of heat exchanger tubing using guided waves has been rapidly developed over the last decade due to the high effectiveness of the technology in screening 100% of the tube walls at high speed. The screening is made possible by the capability of guided waves to travel long distances in components without excessive attenuation. The waves can be generated from a single location, typically within 10 cm (4 inches) to 30 cm (12 inches) from the near end of the tube, by inserting a probe into the tube. The guided waves can be introduced into the tube wall either by first generating vibrations in a probe that is mechanically coupled to the tube wall or by generating the guided waves directly in the tube wall using electromagnetic coupling. In order to minimize variations of the results from different operators, training specialized to guided wave heat exchanger tube testing is required.

Figure 1a shows an illustration of probe insertion and the type of anomalies that can be tested including external and internal wall loss as well as tube support plate (TSP) attachments to the tube. These types of anomalies could also be detected using conventional eddy current testing (ECT), remote field eddy current (RFEC), and internal rotary inspection system (IRIS) methods. There are also several types of anomalies that are more difficult to detect using conventional methods. These include anomalies located close to or under TSPs, near the tube sheet, or in U-bends. In cases where the tube is obstructed on the inside due to deposits or dents, guided wave testing is the only method that can deliver information about tube condition.

Figure 1b shows the probe positioned in the tube during data acquisition. No probe pulling is needed to cover the entire length of the tube.

Even though guided waves seem to be a promising approach, the number of publications about heat exchanger tube inspection is still small compared to the number of publications about guided wave inspection of pipes. This is likely an indication of the difficulty of designing probes for small tube sizes and challenges in data interpretation. Another reason is the well-established presence of conventional inspection methods based on pulled probes. This article reviews the capabilities and limitations of guided waves that have been learned after more than 20 years of research and multiple field tests.