Evaluation of vibration response of ceramic candle filters using an accelerometer and a laser vibrometer
Ceramic candle filters are key components for a successful coal-based power-generation system. The ceramic candle filter is a hollow cylindrical structure made of a porous material. One end is closed with a cap and the other end is open. Ceramic candles are one of the few clean-up technologies that can protect the heat exchanger and consistently meet the gas turbine manufacturer’s inlet particulate requirement to prevent erosion.
A total of three ceramic candle filters were tested nondestructively to investigate the boundary conditions of the filters using a laser vibrometer and an accelerometer. The study includes evaluation of vibration data from a laser-based sensor as compared to the traditional contact sensor; assessment of vibration data quality excited by laser pulses as compared to the traditional hammer excitation, and, examination of the symmetry of the filter structures. These filters include one used and one unused Shumacher-Dia-Schumalith, and one new Refractron 442T. All filters were subjected to an external force and the response was picked up by an accelerometer or a laser vibrometer. Beam vibration equations and the finite element method are used to predict the dynamic behavior for various boundary conditions of each filter. Finite element models were built using eight nodes and three-dimensional isotropic solid elements. The boundary elements using elastic-translational springs were imposed in order to simulate a fixed-free boundary condition on the filters.
Results from this study show that the contact and laser-based non-contact measurements are fundamentally similar. The modal frequencies of filters with a fixed-end mounting setup fall between fixed-free and hinge-free boundary condition assumptions. From both contact and non-contact tests, the Young’s modulus and boundary condition of a candle filter can be predicted using the second and the third bending modes. The modal frequency values of the candle filters increase with the increase of their boundary restraints. The vibration frequency of a candle filter also increase with the increase of its Young’s modulus. Both phenomena were confirmed by numerical and theoretical analyses as well as by experimental observation. The differences of dynamic responses from two perpendicular axes are small. However, an engineer may need to inspect on the same axis of a candle filter in order to accurately assess the structural properties of the filter.