奥氏体锅炉管弯头氧化皮堆积厚度脉冲涡流检测理论与方法研究

Austenitic boiler tubes are widely used in supercritical and ultra-supercritical power units, but often suffer from blockage in the elbow which can lead to unit accidents due to accumulation of oxide scales exfoliated from the inside tube wall. In order to ensure the safety of unit operation, it is crucial to conduct nondestructive testing (NDT) of the accumulated oxide-scale thickness. However, due to the influences caused by incompact accumulation of oxide scales and magnetic transformation of tube wall, it is difficult to quantify the accumulated thickness by existing NDT methods. Aiming at the above questions, in this project, pulsed eddy current (PEC) testing method is presented to quantitative test the accumulated oxide-scale thickness. The main research contents are listed as follows: (1) Based on the effective medium theory, the medium equivalent method for accumulated oxide scales is proposed, the corresponding theoretical model of PEC testing is established, and thus fast simulation of the PEC testing signal is realized. (2) Based on the target field approach, the exciting and focusing methods for enhancing eddy currents in oxide scales, which can fit well with the structure features of elbow surface, are investigated and, consequently, the signal to noise ratio and spatial resolution of PEC testing of accumulated oxide-scale thickness are increased. (3) Based on the self-referencing method, the feature identification method for PEC testing signal of accumulated oxide-scale thickness is investigated and the characteristic which is not associated with the electromagnetic properties of tube wall is extracted. The research of this project will provide theoretical basis for the application of PEC testing technique for thickness measurement of oxide scales accumulated in austenitic boiler tube elbow.

奥氏体锅炉管被广泛用于超（超）临界机组，经常因内壁脱落的氧化皮在弯头堆积发生堵塞而造成机组事故。为保障机组安全运行，对氧化皮堆积厚度进行无损检测至关重要，而现有方法不能很好地解决由氧化皮松散堆积和管壁磁性转变带来的难以定量问题。针对上述问题，本项目提出用脉冲涡流检测方法实现氧化皮堆积厚度的定量检测，就以下几个方面开展研究工作：（1）基于等效媒质理论，提出堆积氧化皮媒质等效方法，建立相应的脉冲涡流检测系统理论模型，实现对脉冲涡流检测信号的仿真；（2）基于目标场法，研究与弯头曲面结构特点相适应的增强氧化皮中涡流强度的激励方法和聚焦方法，实现氧化皮堆积厚度检测信噪比和空间分辨率的提高；（3）基于自参考对比法，研究氧化皮堆积厚度脉冲涡流检测信号特征的辨识方法，提取与管壁电磁属性无关的特征量。研究成果将为奥氏体锅炉管弯头氧化皮堆积厚度脉冲涡流检测技术的应用提供理论基础。

奥氏体锅炉管是现代燃煤电厂的关键部件，但由于汽侧氧化皮的堆积而经常发生堵塞甚至破裂。现有的无损检测方法难以对剥落氧化皮的堆积厚度进行定量。本项目致力于利用脉冲涡流检测技术解决这一问题。通过三年研究工作，基本实现了预期研究工作目标，主要研究成果包括：.1）测试了TP347H锅炉管氧化皮的形貌参数和磁特性参数，应用有效介质近似法将堆积氧化皮等效为连续媒质，建立了氧化皮堆积厚度脉冲涡流检测有限元模型，研究了脉冲涡流检测信号随堆积厚度的变化特征，发现脉冲涡流检测信号峰值与堆积厚度存在线性的定量关系。.2）设计了结构灵活、易装拆且通用性好的新型脉冲涡流传感器，搭建了基于LabVIEW的脉冲涡流检测系统，证实了利用脉冲涡流方法检测氧化皮堆积厚度的有效性。.3）实验研究并证实了不同影响因素和检测条件下，应用脉冲涡流检测方法检测锅炉管内氧化皮堆积厚度的有效性。传感器检测能力受传感器偏离堆积区距离的影响较大，检测中需要根据实践经验估计易堆积区域；信号峰值随传感器提离增加而线性减小，有助于进行提离补偿或消除；传感器采用线圈阵列模式可有效识别出堆积区域最严重部位；将初始检测信号与无堆积位置的参考信号进行差分，可消除管壁磁性转变带来的测量不准确的问题。.4）提出了一种基于维纳自适应滤波和主成分分析的脉冲涡流信号降噪方法。运用该方法能够有效的消除强噪声对检测信号的干扰，大幅提高信噪比，解决了强背景噪声下微弱脉冲涡流信号的特征量难以准确提取的问题。.5）针对压力管道在役检测的需求，设计了一种在管外放置的远场涡流检测探头。该探头可在管道周向产生远场涡流效应，对轴向缺陷的敏感性高，有效弥补了传统远场涡流探头的不足。