Estimation of Blast Furnace Refractory lining thickness using Non Invasive Techniques 

Blast furnace, a high-energy efficiency counter-current packed bed reactor, is being successfully used in all integrated Iron & Steel plants of the world to produce pig iron.  The condition of refractory lining of a blast furnace play a very important role to determine its campaign life and is the major controlling factor for determining the actual timing of its overhaul. As the refractory lining is gradually worn out during the operation, prediction of remaining line thickness of a blast furnace is very essential to avoid any mishap. An effort has been made to develop a suitable non-invasive NDT technique for the measurement of blast furnace lining thickness. Three techniques: low frequency ultrasonic, impact-echo and thermography have been explored. A mock up facility has been set-up at NML to study and optimize the NDT parameters under cold and hot conditions.  As the hearth lining is the most critical area in the whole Blast furnace, a full-scale model of 9⁰ sector of hearth zone of designated Blast Furnace of Bokaro Steel Plant, SAIL was considered for the mock up facility (Fig1). The facility was built with identical constructional items of the designated Blast Furnace. The results of this study have indicated low frequency ultrasonic test technique can be employed to measure the refractory lining thickness of blast furnace hearth when the hearth is empty.

 Ultrasonic testing is one of the versatile methods, which is applicable to most materials. Ultrasonic testing of refracting lining utilises use of low frequency acoustic waves generated by piezoelectric transducers. Frequencies ranging from 50KHz to 1MHz are generally used. The resultant wavelength of the acoustic waves, which depends on the ultrasonic wave velocity in the material, is of the order of few centimeters. A directional sound beam is transmitted to the test piece through a suitable couplant, usually grease or oil like material. While various types of instrumentation and display modes are feasible, suitable technique for wear monitoring in blast furnaces, where only on side (shell side or outer side) is accessible is the pulse echo technique. Ultrasonic velocity, attenuation and spectral characteristics of back wall echo signals are to be used for evaluation. A typical view of the ultrasonic signal obtained during testing of mock-up facility at   1140⁰C (Fig.2). 

Fig 1: Schematic view of the mock-up facility	Fig 2: Ultrasonic signal obtained during testing