Detection of hot water boiler tubes inner surface damages at early stage of their development using the metal magnetic memory method

Abstract

Operational reliability of boilers and heat exchangers strongly depends on reliability of heating surface tubes. A wide range of non-destructive testing (NDT) technologies applied for their diagnostics includes various methods of eddy current testing, magnetic stray fields, an ultrasonic immersion echo method, laser optics, and visual inspection. However, all the above methods have limitations in their application. Moreover, conventional NDT methods are aimed at detecting only the existent defects. One of the solutions to prevent unexpected damages of heating surface tubes is the application of stress–strain state (SSS) control methods. This approach allows detecting the stress concentration zones (SCZs) as the main sources of the future defects initiation. Therefore, it becomes possible to identify defects at an early stage of their development, and monitor their propagation. The metal magnetic memory (MMM) method is a passive technique of magnetic testing. It is based on the self-magnetic stray field analysis that allows assessing the SSS of tested objects. The MMM method is used as a screening tool for diagnosis of impending failure of critical industrial equipment. The article discusses the MMM method application for the inspection of the inner surface of the boiler heat exchanger steel tubes. The examples of the SCZs detection and the identification of defects in these zones at an early stage of their development are presented. The influence of the revealed SCZs on the strength characteristics of the tube metal is shown. The relationship between the material strength characteristics and the magnetic characteristics of MMM testing is presented calculating the magnetic strain index m.