Stiffness evaluation and damage detection of ceramic candle filters

This paper presents a nondestructive evaluation method to identify the stru ctural stiffness of ceramic candle filters. A ceramic candle filter is a ho llow cylindrical structure made of a porous ceramic material used in advanc ed, coal-fired power generation systems. The candle filters need to sustain an extreme thermal and chemical environment over a great period of time to protect the gas turbine components from exposure to particulate matter. A total of 92 new candle filters and 29 used candle filters have been tested nondestructively using a dynamic characterization technique. All filters we re subjected of an excitation force, and the response was picked up by an a ccelerometer in a free-free boundary condition. The frequency response func tion and vibration mode shapes of each filter were evaluated. Beam vibratio n equations and finite-element models were built to calculate the filter's dynamic response. Results indicate that the vibration signatures can be use d as an index to quantify the structural properties of ceramic candle filte rs. The results also show estimations of the overall bending stiffness valu es for four different types of candle filters. The used filters show a tren d of stiffness degradation, which was related to the filter's exposure time . Damage detection procedures using modal strain energy and finite-element simulation were studied for detection of a localized damage in the candle f ilter. The location and the size of the damaged section can be identified u sing the measured model strain energy.