Numerical and experimental identification of vibration convection chamber of fluid power boiler

On the housing of convection chamber vibrations were measured, using acceleration sensors in 16 measurement points, connected to a multichannel recorder. The measurement data were recorded for different states of boiler load, in a wide range of time - 4 hours of measurement. The measurement results indicated that the most important vibrations frequencies of side walls were in the band 30- 30.5 Hz, appeared in block during increase of the power and operation under full load. High amplitudes were also recorded in the band 7-9Hz with a load of 260 MW unit after 2 hours of steady work. Convection chamber was at resonance for these frequencies [1]. In order to identify the vibration of the actual object it was necessary to identify the form of natural frequency of the convection chamber. As first, was discrete model of convection chamber created, and then modal analysis was made, using the Lanczos algorithm. Results of the modal analysis pointed out that due to the complexity of the convection chambers geometry there was a big number of local and global modal shapes. We could see big amplitudes of convection chamber shell displacements in the ranges of structural mode bands: 7-9 Hz, 1618 Hz, 30-30.5 Hz. If the excitations frequencies in the example from flowing gas are in the same range as natural shapes, the resonance may occur. We cannot determine actual displacements and stress using numerical modal analysis. We have to identify the source of variable strength, to obtain values of time-varying stresses. If we compare the results of modal analysis and measurement at the real object, we can select the areas of construction, in which there is a high probability of resonance. The cause of the convection chambers resonance was probably force created by turbulent boundary layer of gas. The gas flows along the inside of the convection chamber walls [6]. Resonance can be prevented by increasing the stiffness of the chamber walls in areas where there are large vibration amplitude. In this way, we can increase local natural frequency of the chamber. A good method to increase stiffness of the chamber walls is the distribution of bracing beams between the existing stiffeners [2].