为能更好地保证核电站辅助工艺管道系统中的小口径薄壁管在役时的运行状态良好，有效检出其内外壁轴向缺陷十分重要。介绍了小口径薄壁管在役超声波检测的系统组成、技术方法、超声波探头参数的选择、设计原理、在役检测时局部水套的设计制作基本原理以及检测实施过程中的要点。经过大量的试验和现场检测，证明采用聚焦探头和局部水套的水浸聚焦法检测小口径薄壁管内外壁的轴向缺陷能够达到理想的检测效果。

To make sure that small diameter thin-walled pipes, which are in the aided process piping system of nuclear power plants, run well in in-service condition, we should find an effective way to inspect axial defects on inside and outside the wall of small diameter thin-walled pipe. This paper introduces the in-service UT inspection for small diameter thin-walled pipe. In the paper, we discuss the system composition, technical method, parameter selection for UT transducer and design principle of the UT inspection. Also, we study how to design and manufacture the local water jacket for the in-service inspection. And the main points of the inspection process are emphasized. By a great deal of testing and field inspection, it can be proved that the immersion focusing method is an effective way, which uses focusing ultrasonic probe and local water jacket, to inspect axial defects on inside and outside the wall of small diameter thin-wall pipe. And the test result is good.