The Advancement of a Non-Destructive Testing Method for Cooling Staves Thicknesses
The cooling system of the modern blast furnaces incorporates staves that are either made of cast iron or copper or copper plates. In some cases water spray cooling is employed to cool blast furnace hearth. Currently majority of modern blast furnaces are equipped with a combination of copper and cast iron staves. In most of the new blast furnace designs, cast iron staves are used on the less active areas of the furnace such as upper stack or hearth while the copper staves are used at more active and hot areas of the furnace such as bosh, belly and lower stack. To perform a non-destructive measurement of the remaining stave thickness, signals have to transmit through layers of different material: steel furnace shell, castable or ramming and stave. The thickness of the castable, such as silica carbide, may vary from 100 to 280 mm. These materials significantly attenuate ultrasonic signals, and thus conventional ultrasonic techniques cannot detect the remaining stave thickness by sending signals on the furnace shell.
Furthermore, the resolution of the measurements has to be high (about +/- 2 mm) in order to identify the difference between the rib and root regions of the stave. Owing to these stringent requirements, traditional ultrasonic techniques were unable to achieve the objectives, unless installation of ultrasonic probes1, or copper/composite rods2 was made on the furnace. These ultrasonic techniques are also not flexible due to the limitation of measurement locations.
In view of the conventional ultrasonic test limitations, a technique known as the low frequency pulsed ultrasound (LFPU) was developed in 2013. This technique was verified on site by comparing the results with the conventional ultrasound measurements at the pre-installed copper rod locations. The advantage of LFPU is that measurements can be done on the shell of the furnace without pre-installation of rods and probes. It is a broadband echo system that is capable of measuring thicknesses in a multi- layered, thin (less than half meter), and composite furnace walls from the cold side. The system is designed to generate the frequencies and power required for the signals to measure stave thicknesses3,4, with an accuracy of about +/-2 mm.
The focus of LFPU measurements is to determine the thickness of the stave. Staves can be in four possible physical conditions: at full thickness and protected by the refractory, partially worn rib and refractory, completely worn rib and refractory, and wear of the root (Figure 1)4.