Supercritical boilers use austenitic stainless steel (SS) boiler tubes in the high temperature regions (mainly as superheater and reheater tubes) and oxide layers (mainly magnetite) are formed on the internal surface of these tubes during operation. When there are load changes or fluctuations in temperature during operation, there is possibility of exfoliation of the internal scale due to large difference in coefficients of thermal expansion of the base SS metal and the oxide. These exfoliated oxides accumulate at the lower bends of the tubes and may cause blockage and failure of the boiler tubes in steam path. While these tubes are non-magnetic, their oxides are magnetic. Some methods based on estimation of only amplitude of magnetic field are available for qualitative information on blockage. A few equipment analyzing amplitude and phase both, claim for quantification of blockage, but they are rather expensive. Hence, there is need for some simple, economic, easily deployable non-destructive method for detection as well as quantification of such blockage. Quantification helps power utilities in taking timely decision for cleaning or replacement of the blocked tube bends. Use of Radiographic Testing is inconvenient as it hampers parallel working in nearby areas. A method based on the estimation of Magnetic Coercive Force (MCF) has been explored. Experimentation shows that the method gives good detectability as well as quantitative correlation with the exfoliation build up inside the austenitic stainless steel tubes. The method is non-destructive, fast and easily field deployable with no hampering of any parallel activities during overhauling of boiler.
- Tube blockage
- Magnetic coercive force