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为增加仿真结果的可信度,模型选用与实际实验所用管道相同的参数进行搭建。其中,铁磁金属管道的尺寸和电磁参数如表1所示。
表 1 管道尺寸及电磁参数
Table 1. Pipe dimensions and electromagnetic parameters
管道参数 长度/mm 壁厚/mm 外直径/mm 相对磁导率 电导率/(S·m−1) 取值 368 13 180 150 4×106 在Maxwell中分别建立同轴式、垂直式单线圈无缺陷管道仿真模型,如图1和图2所示。求解域尺寸为500 mm×300 mm×400 mm(长×宽×高),将模型完全包裹其中,由于包覆层多为不导电不导磁的绝缘物质构成,因此求解域内选择使用空气填充管道外部空间。线圈采用圆柱线圈的形式,由于核电厂在役管道包覆层厚度一般为60 mm,为了模拟测试结果更真实,建立提离高度为60 mm的脉冲涡流无损检测模型。其中,将提离高度定义为线圈探头边缘与管道外壁之间的最短距离。
为方便区分,激励线圈外观选择用绿色表示,检测线圈颜色选为红色。激励和检测线圈的参数如表2所示。
表 2 线圈参数
Table 2. Coil parameters
mm 线圈参数 匝数/个 内直径 外直径 提离高度 线圈高度 检测线圈 900 36 38 60 15 激励线圈 300 40 42 60 15 由文献[12]可知,若铁磁材料的相对磁导率远大于1时,其涡流扩散时间常量为:
$$ {\tau _{\text{d}}}{\text{ = }}\dfrac{{\mu \sigma {d^2}}}{{{{\text{π }}^2}}} $$ (1) 根据式(1)计算可得,仿真模型中的涡流扩散时间常量为4.89 ms。在实际检测中,通常采用脉冲电流下降沿结束后的一段时间为研究对象,因此激励电流关断时刻为脉冲电流上升沿产生的涡流衰减到接近于0的时刻。由于脉冲涡流衰减规律为呈指数衰减,在4倍时间常数之后涡流幅值衰减为峰值的1.8%,可忽略其影响,脉冲持续时间取整后选为20 ms。脉冲激励电流选用梯形波,其中电流峰值取为1.5 A,上升沿和下降沿的持续时间均为2 ms,峰值持续时间为20 ms,波形如图3所示。
设置好激励电流后对模型进行网格剖分,网格剖分越精细所得结果可信度越高,但相应的仿真时间也会成倍增加。为在保证仿真结果可信度的基础上加快仿真速度,本文将仿真模型分为两部分,由于涡流主要分布于线圈探头正下方管道,因此对这部分管道进行精密剖分,设置其网格最大边长为4 mm,其余部分管道剖分单元最大长度为10 mm,网格剖分效果如图4所示。
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选取直径180 mm,12 mm厚的20#钢阶梯样管为检测对象,将保温层厚度设定为150 mm,铝皮厚度为0.5 mm,用垂直式线圈探头分别对A0、A1两处检测点进行检测,其中在A1位置处加工缺陷直径为32 mm的平底孔,深度为5 mm,将得到的壁厚检测结果与超声测厚结果记录到表3中。
表 3 保温层为150 mm下脉冲涡流与超声波测厚对比
Table 3. Comparison of pulse eddy current and ultrasonic thickness measurement at 150 mm insulation layer
mm 相对壁厚 均值 超声测厚 误差 A0 100.8 99.7 99.3 99.9 100.8 99.9 100 - A1 87.8 88.7 88.8 86.5 90.9 88.5 90.1 1.6 注:A0−标定点;A1−标检测点。 将保温层厚度设定在100 mm,带0.5 mm铝皮,用探头对A0、A1两处检测点进行检测,得到的壁厚结果,如表4所示。
表 4 保温层为100 mm下脉冲涡流与超声波测厚对比
Table 4. Comparison of pulse eddy current and ultrasonic thickness measurement at 100 mm insulation layer
mm 相对壁厚/% 均值 超声测厚 误差 A0 99.2 100.8 99.8 99.5 99.6 100 100 - A1 91.2 90.7 88.7 92.6 87.8 90.1 90.1 0 注:A0−标标定点;A1−标检测点。 上述实验结果对比可知,相同条件下,当提离距离增大,脉冲涡流对加工缺陷的检测分辨率降低,且随着提离距离增加,脉冲涡流所测数据与超声测厚数据存在较大误差,实验结果与仿真结果基本一致。
选取直径180 mm,12 mm厚的20#钢阶梯样管为检测对象,将保温层厚度设定为150 mm,铝皮厚度为0.5 mm,采用同轴式线圈探头分别对A0、A1两处检测点进行检测,其中在A1位置处加工缺陷直径为32 mm的平底孔,深度为5 mm,将得到的壁厚检测结果与超声测厚结果记录到表5中。
表 5 保温层为150 mm下脉冲涡流与超声波测厚对比
Table 5. Comparison of pulse eddy current and ultrasonic thickness measurement at 150 mm insulation layer
mm 相对壁厚/% 均值 超声测厚 误差 A0 99.3 98 98.8 99.8 98.7 98.8 100 - A1 77.6 76.3 77.4 75.9 76.7 76.5 73.2 4.2 注:A0−标定点;A1−检测点。 保持其他条件不变,采用同轴式线圈探头所得结果与垂直式线圈探头相比,PECT检测数据及超声测厚数据均存在较大误差,这是由于同轴式线圈在管道产生的涡流环面积较大,无法进行有效聚焦,相对于垂直式线圈而言,无法对缺陷进行精准识别及检测,所得结论与ANSYS仿真结果相吻合。
Analysis of the Influence of Coil Placement on Pulsed Eddy Current Detection
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摘要:
目的 核电厂汽水管线一般在管道外壁加装保温层,从而提高换热效率。目前对于铁磁性管道的检测手段主要为常规超声及超声导波,检测前需要将管道外壁保温层拆除,导致检测工期延长,人力成本增加,无法达到核电厂高质量发展的要求。核电厂脉冲涡流技术的应用可以省去保温层的拆装,实现不停机在线筛查。检测线圈的放置方式对缺陷的检出能力是脉冲涡流技术重要指标。 方法 文章利用ANSYS中的Maxwell模块进行管件建模及仿真,分别设计同轴式与垂直式检测线圈,保持提离距离、材料一致及其他条件一致下,模拟脉冲涡流对平底缺陷的检测能力。选取核电厂样管进行同轴式与垂直式脉冲涡流测试,将脉冲涡流(Pulsed Eddy Current Testing,PECT)测试结果与超声测厚进行复核,对比两种线圈放置方式对脉冲涡流检测的影响。 结果 研究表明:垂直式线圈相对于同轴式线圈对缺陷检出效果更佳。 结论 核电厂脉冲涡流技术的应用对脉冲涡流技术在核电领域实施具有重要意义。 Abstract:Introduction In the nuclear power plant, the steam pipeline is generally installed with an insulation layer on the outer wall to improve heat transfer efficiency. Currently, the main detection means for ferromagnetic pipelines are conventional ultrasound and ultrasonic guided waves. Prior to the inspection, the insulation layer on the outer wall of the pipeline needs to be removed, leading to extended inspection time, increased labor costs, and an inability to meet the requirements for high-quality development in nuclear power plants. The application of the pulsed eddy current (PEC) technique for nuclear power plants can eliminate the need for insulation layer removal, enabling non-stop online screening. The defects testing by coil placement is an essential indicator of the PEC technique. Method In this paper, the modeling and simulation of the pipelines was conducted by applying ANSYS Maxwell, coaxial and vertical detection coils were designed respectively to simulate the detection capability of PEC on flat bottom defects with consistency in the lift-off distance, materials and other conditions. Sample pipes were selected from the nuclear power plant for coaxial and vertical PEC testing. The pulsed eddy current testing (PECT) results were cross-validated with ultrasonic thickness measurement, and the effects of two coil placement methods on PECT were compared. Result The results show that vertical coils are more effective in defect detection compared to coaxial coils. Conclusion The defects testing by coil placement has great significance for implementing PEC in the nuclear power sector. -
Key words:
- pulsed eddy current testing /
- ferromagnetic pipe /
- ANSYS simulation /
- coil placement method /
- model
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表 1 管道尺寸及电磁参数
Tab. 1. Pipe dimensions and electromagnetic parameters
管道参数 长度/mm 壁厚/mm 外直径/mm 相对磁导率 电导率/(S·m−1) 取值 368 13 180 150 4×106 表 2 线圈参数
Tab. 2. Coil parameters
mm 线圈参数 匝数/个 内直径 外直径 提离高度 线圈高度 检测线圈 900 36 38 60 15 激励线圈 300 40 42 60 15 表 3 保温层为150 mm下脉冲涡流与超声波测厚对比
Tab. 3. Comparison of pulse eddy current and ultrasonic thickness measurement at 150 mm insulation layer
mm 相对壁厚 均值 超声测厚 误差 A0 100.8 99.7 99.3 99.9 100.8 99.9 100 - A1 87.8 88.7 88.8 86.5 90.9 88.5 90.1 1.6 注:A0−标定点;A1−标检测点。 表 4 保温层为100 mm下脉冲涡流与超声波测厚对比
Tab. 4. Comparison of pulse eddy current and ultrasonic thickness measurement at 100 mm insulation layer
mm 相对壁厚/% 均值 超声测厚 误差 A0 99.2 100.8 99.8 99.5 99.6 100 100 - A1 91.2 90.7 88.7 92.6 87.8 90.1 90.1 0 注:A0−标标定点;A1−标检测点。 表 5 保温层为150 mm下脉冲涡流与超声波测厚对比
Tab. 5. Comparison of pulse eddy current and ultrasonic thickness measurement at 150 mm insulation layer
mm 相对壁厚/% 均值 超声测厚 误差 A0 99.3 98 98.8 99.8 98.7 98.8 100 - A1 77.6 76.3 77.4 75.9 76.7 76.5 73.2 4.2 注:A0−标定点;A1−检测点。 -
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