Nondestructive Testing Method And Tools For In-Service Ferromagnetic Heat Exchanger Tubes

Ferromagnetic heat exchanger tubes are employed commonly in industries, corrosion in tube is the main failure reason of shell and tube heat exchangers and it usually cause large economic loss. The in-service inspection of ferromagnetic tube needs special technology for the limitation of tube's high relative permeability and structure restriction. The inspection of ferromagnetic tube is carried by imported foreign instrument, which is very expensive. So it is meaningful to take a study on the in-service inspection method of ferromagnetic tube。Two magnetic testing methods are put forward which are called magnetic flux leakage (MFL) and opened magnetic circuit based on the characteristics of ferromagnetic heat exchanger tube. Finite element method (FEM) by ANSYS software is used to analyze the magnetic circuit and configuration of MFL probe. How the design changes effects testing signal of the MFL assembly and the principle of opened magnetic circuit testing method in ferromagnetic heat exchanger tube inspections are also studied. The FEM analysis shows that the cross section area of the permanent magnets is the key factor of pipe magnetization. Magnet length, material and dimension of armature also effect the MFL signal. MFL and opened magnetic circuit probes are designed by the guidance of theoretical analysis. Experiments were taken by these probes. The experiment results show that it is applicable to use magnetic testing method in ferromagnetic heat exchanger tube inspection. The MFL probes are able to detectφ1.6mm through hole onφ25mm×2mm tube.φ1.6mm through hole and outside crack whose depth is about 20% wall thickness onφ38mm×3mm tube are also detectable by MFL probes. The MFL probes are able to detectφ3.2mm through hole onφ51mm×5mm as well. According to the research results, a magnetic testing tool is designed and the MFL probe design is discussed in detail. The tool was tested in TianJin petroleum and chemical mechanism Research Institute and the inspection result shows that both of the two kinds of magnetic probes are able to detectφ2mm through hole and taper blind hole which is 20% of the pipe's wall thickness depth. The result proves that the magnetic testing method and tool meet field demands.The experiment and application results show that the magnetic testing method and tool have advantages of high sensitivity, good stability, easy-handling and low-cost, which exhibit a bright application future.