Inspecting and characterizing defects in finned ferromagnetic heat exchanger tubes using tube to tube through transmission technique.

摘要

Finned ferromagnetic tubes are used in chemical processing or air conditioner heat exchangers. The type of flaws which occurs in plain ferromagnetic tubes also occurs in finned tubes, however, conventional electromagnetic methods are rarely used to inspect finned tubes because of the interference caused by the fins. In this thesis a recently developed method, Tube to Tube Through Transmission (T4), has been used to inspect and characterize the defects in finned tubes. A T4 probe consists of two coils in a send-receive configuration, which are aligned in the different (usually neighboring) tubes running side by side. Because of the radial propagation of the field, T4 allows examination in areas and conditions where other techniques are less sensitive, including near baffle plates and finned tubing. As this technique is new, little signal interpretation exists for it. Three new methods were developed to analyze T4 defect signals based on the magnetic field study and skin depth model. The experimental results show T4 is able to identify and characterize partial circumferential defects as well as full circumferential grooves in finned ferromagnetic heat exchanger tubes. Good sizing results were acquired for 50% or deeper flats using the Voltage Plane Polar Plot (VPPP) and Multi-Frequency methods. Some tests were conducted to investigate the effects of bent fins and neighboring tubes and it was found that there is little effect on defect depth sizing. This makes it possible to apply our laboratory results to T4 inspection jobs.