Studies On MFL Internal Inspection For Heat Exchange Tube And The Influence Of Support Plate

As the core component of heat exchanger，the quality of heat exchange tube determines whetherthe exchanger can run safely and effectively. It is always in high temperature and high pressureenvironment. It may not work due to corrosion，impact and vibration. So it is necessary fornon-destructive testing of the heat exchange tube. Magnetic flux leakage（MFL）testing，which is anefficient non-destructive testing method，has been widely used in pressure vessel and pipe innerinspection. It needs inside detection for the structure limited. In China，it is just beginning to useMFLT for the detection of heat exchange tube. Therefore，it has great theoretical importance andguidance value to analyze the mechanism of heat exchange tube MFL testing method.At first，the paper analyzes the feasibility of small diameter ferromagnetic heat exchange tubewithin MFL testing method. The common failure forms of heat exchange tube are cracks and corrosionpit. A detection scheme is designed aiming at the standard defects on16x1.5mm tube. A permanentmagnet is used for axial local magnetization and the detecting coil picks up leakage magnetic field.The results show that the probe can finish N20defect holding in consistency and repeatability.Then，the paper analyzes the influence of support plate on defect detection. Statistics show that90%of the defects occur near the support plate. They are both ferromagnetic materials. The MFLsignal will reduce obviously when the defect is under the support plate. For this phenomenon，thepaper studies the influence of support plate on the pipe magnetization state，new distribution ofleakage magnetic field and the disturbance background magnetic field of support plate using finiteelement simulation. From these aspects，it analyzes the reason of support plate’s influence on MFLsignals and validates by experiment. Next，it researches the influence when the relative positionbetween support plate and the defect changes.Finally，the paper provides corresponding elimination and compensation measures to solve theeffect of support plate. A circumferential differential sensors layout is presented to eliminatebackground magnetic field of support method. It studies the impact of the fit clearance between theplate and tube on the detection signal. An optimized fit mode is proposed to weaken this impact. At theend of the paper，a probe with sensor array is advanced to confirm the position of the support plate.Next，it set different alarm threshold while there is a supporting plate to avoid omission factor.