Modal Analysis and Optimization of Fluid-Structure Coupling for Rotor and Inner Cylinder of Vertical Condensate Pump

Abstract

Low-frequency resonance is one of the common issues encountered during the variable-frequency operation of condensate water pumps. There have been numerous patents and papers proposing solutions to address the low-frequency resonance problem in condensate water pumps. However, the solutions for resonance problems often need to be tailored to specific circumstances. Condensate pump is one of the important auxiliary equipment of thermal power plant, which consumes a lot of electric energy in the production process. Improving the efficiency of condensate pump can save energy and reduce consumption. Variable frequency operation is an effective method to improve the efficiency. However, the condensate pump is prone to excessive vibration in the process of frequency conversion operation, which affects the safe operation of the condensate pump Based on the acoustic method, the dynamic model of the rotor and inner cylinder of Jiangsu Guohua Chenjiagang Power Plant 2B condensate pump is established to compare the difference between dry modal and fluid-structure coupling modal, the influence of perpendicularity, concentricity and bearing wear on the natural frequency of rotor is studied. In order to systematically understand the modal characteristics of the condensate pump rotor and inner cylinder under the fluid-structure coupling condition, a three-dimensional finite element model of the 2B condensate pump rotor and inner cylinder in Jiangsu Guohua Chenjiagang Power Plant was established. The rotor is rigid under normal conditions. When the bearing is worn, the frequency of the rotor will be greatly reduced and may fall into the frequency conversion operation range to excite resonance. The deviation of perpendicularity and concentricity will not directly lead to the decrease of rotor modal but will lead to the increase of bearing stress, aggravate bearing wear, and then affect the rotor modal. As the inner cylinder only relies on the fixed support at the top, the structure stiffness is low, which may lead to low-frequency resonance. By adding two support structures at the guide vane, the first-order modal frequency of the inner cylinder can be increased from 3.29 Hz to 28.88 Hz, effectively avoiding the operating frequency range of the system. The fluid-structure coupling modal of the rotor was calculated based on acoustic method, and the difference between the dry modal and the fluid structure coupling modal was compared. At the same time, the influence of the rotor perpendicularity, concentricity related installation parameters and bearing wear on the modal was studied. This study can guide the optimization of similar pump structures. 1. The fluid-structure coupling modals of both the rotor and the inner cylinder are much lower than the dry modal. Compared with the dry modal, the first two orders of fluid-structure coupling modals of the rotor are reduced by 8.11%, while the first two orders of fluid-structure coupling modals of the inner cylinder are reduced by more than 25%, which is mainly caused by the additional mass of the fluid. Therefore, when conducting modal analysis, it is necessary to consider the effects of fluid forces. The frequency conversion operation range of the condensate pump is 15-25 Hz. In this operation range, there is no resonance point for the rotor, which is rigid under normal working conditions. The 3rd and 4th modals of the inner cylinder are located within the range of variable frequency operation, which may cause resonance during variable frequency operation of the condensate pump. 2. The deviation of perpendicularity and concentricity will not cause the modal frequency decrease of the rotor, on the contrary, it will cause the frequency increase of some modals. The stress on the bearing is very small when the rotor is mounted without deviation. When the perpendicularity and concentricity of the rotor is deviated, the stress on the bearing increases dramatically, thus accelerating the wear on the bearing system. The influence of different bearing wear on the rotor modal varies greatly. Generally speaking, the rotor modal will be reduced. Some have even been reduced to the frequency conversion operation range, so it can be seen that although the perpendicularity and concentricity deviation does not directly affect the modal frequency, it will accelerate the wear of the bearing, and then lead to the reduction of the modal frequency. 3. The first four modals of the inner cylinder are all below 25 Hz. By finite element calculations, it was found that after adding two supports, all modals of the inner cylinder increase above 25 Hz. No additional remarks provided.