Evaluation of ceramic candle filters degradation and damage location using four-point bending tests

The structural integrity of ceramic candle filters is a key element for hot gas cleanup systems. Ceramic candle filters used in demonstration plants have experienced degradation and fracturing. Preliminary examination of these ceramic filters indicated that damage of the filters may have resulted from strength degradation due to persistent high temperature operation, thermal transient events, excessive ash accumulation and bridging, and pulse cleaning. This study proposes a nondestructive evaluation of the structural properties of these ceramic filters. The ceramic candle filter is a slender structure made of layers of porous materials. The structure has high acoustic attenuation, which has greatly limited the conventional acoustic emission or acoustic-ultrasonic detection capability. A virgin Refractron filter and twelve Schumacher filters which were previously tested under high pressure and high temperature at a demonstration power plant were evaluated. The acoustic signatures of the ceramic filters are established using a transient impact response technique. Results indicate that the natural frequencies of the filters can be used as an index to quantify the damage condition of the filters (Chen and Parthasarathy, 1996).;In order to confirm the dynamic response analysis, bending tests were performed to estimate the Young's modulus from the load-deflection curve and compared the results with the predicted values from vibration analysis. Four-point bending tests were conducted on seven intact filters, which included five used Schumacher filters, one new Schumacher filter and one new Refractron filter. Each ceramic candle filter was subjected to an applied load up to 40.7 lb. and the center deflection was measured using a digital transducer. The deflection was used to reflect the stiffness of ceramic filters. The bending test results can reflect the change of the stiffness among these filters and this change of stiffness matches with the observation from the vibration tests. To measure the deflection of ceramic candle filters one should conduct the tests in a quiet environment. Any small tremble will lead a large difference between theoretical and experimental results.