Observations and improvements on the impact-echo method used for blast furnace hearth refractory measurement

The refractory thickness of the blast furnace hearth is a critical parameter for the safe operation of a blast furnace. A replacement of the refractory comes with high costs and, therefore, unnecessary repairs should be avoided. A promising method to estimate the refractory thickness is the impact-echo method. For the interpretation of impact-echo, often, the peaks in the spectrum of the measured signal are associated with the echo from an interface. In this paper, it is shown that this method should be used with care when applied to multi-layered structures. An alternative approach is presented for the calculation of the thickness of multi-layered structures. This method solves the wave equation based on transmission line theory. The formula is checked against 2D finite element analysis and for simple layered structures, good agreement is found. Three cases are simulated using finite element models. Both the impact-echo method and the proposed method overestimate the refractory thickness.