ANALYSIS OF THE DYNAMIC STIFFNESS OF A CENTRIFUGAL PUMP BY ODS

Transcription

1 The 21 st International Congress on Sound and Vibration July, 2014, Beijing/China ANALYSIS OF THE DYNAMIC STIFFNESS OF A CENTRIFUGAL PUMP BY ODS Karllyammo Lennon de Souza, Fabíola Fernandes da Silveira, João Bosco da Silva, Wandicler Marcos Souza Silva and Mário César de Oliveira Spinelli Department of Mechanical Engineering, Federal University of Rio Grande do Norte, Natal-Brazil karllyammo@gmail.com Defects in rotating machinery or on its structure generates high levels of vibration, which is one of the major causes of premature degradation of components of machines and may cause unexpected stops of production, increasing costs and reducing the operational reliability. To determine the causes of these defects exist techniques of analysis of machines such as ODS (Operating Deflection Shape) utilized to analyze the dynamic behavior of the machine or structure, by determining the existing strains and their probable causes, providing concrete data to diagnose and correct the defect. This paper was used to analyze ODS, to determine the vibrational behavior of a centrifugal pump of a diesel processing unit. For the experiment was used a specific software for analysis of ODS. Were carried measurements of phase and of the amplitude of vibration at predetermined points. Based on the results obtained it was determined the cause of high levels of vibration of the centrifugal pump and the recommendations for correcting the problem. 1. Introduction Centrifugal pumps are hydraulic machines, which transfer energy to the fluid, with the purpose of transporting it from one location to another. Are powered from a source motor, and transmit some of that energy to the fluid in the form of kinetic energy that is converted into potential energy 1. Figure 1 shows a schematic drawing of a centrifugal pump and its components. Figure 1. Centrifugal pump and its components 2 ICSV21, Beijing, China, July

2 High levels of vibration can cause excessive strain on the machines or in its structure, making a major factor for premature degradation of its components and therefore can cause to equipment failure, thereby reducing the average time between failures and operational reliability. Conventionally, analytical modal analysis is used to characterize resonant vibration in machinery and structures from a theoretical point of view. However, it is often required to study a structure under one or few specific conditions. For particular scenarios it has been proven that direct measurements are faster, simpler and more accurate than analytical predictions 3. Modal analysis identification techniques give useful information on modal parameters to understand the dynamic behavior of a structure 4,5. Experimental modal analysis can be performed by measuring Operational Deflection Shapes (ODS), and then interpreting or post processing them in a specific manner to define mode shapes 6,7. The goal of modal analysis is to measure frequency response and mode shapes 8. In this paper the technique of Operational Deflection Shapes (ODS) was used to determine the dynamic behavior in real operation conditions of a centrifugal pump a processing unit diesel. 2. Operating Deflection Shapes The ODS (Operating Deflection Shape) is used to understand the dynamic behavior of the equipment or structure existing visualizing the deformations and their probable causes, providing information for the diagnosis and correction of problems associated with these deformations. Human understanding in mainly based on seeing hence the visualization of the vibration behavior by ODSs may lead to discover which optimal modifications should be made in order to control noise and vibrations, lessen fatigue, reduce wear or solve related problems 3. Operating Deflection Shape may also be predicted by analytical models (modal analysis), defining the boundary conditions. If, however, the objective is to study a particular structure under one or a few specific conditions, a direct ODS measurement is faster, simpler, and more accurate than analytical predictions 3. ODS can be set from a forced movement or at a specific frequency or in any one moment in time. In most cases, structural responses at or near a resonant (modal) frequency are dominated by the mode, and the ODS closely approximates the mode shape 6. Operating Deflection Shape are displayed from response only measurements taken from two or more degrees-of-freedom (DOFs) of a machine or structure 9. The modal superposition theorem shows the response Y versus the modal parameters as in Eq. (1): Where: Y is the displacement vector of the structure; F is the force vector applied on the structure; Φi and ωi are respectively the vectors of the modal deformation of mode and the natural frequency of the structure; Τ denotes the transposed vector; ξi is the damping of the mode; ω is the frequency of the applied force. ICSV21, Beijing, China, July

3 3. Materials and methods In this paper the technique of ODS was used in the analysis of the dynamic behavior of a centrifugal pump operating in real conditions of the process, Figure 2. The pump runs at a speed of 3500 RPM (58.3 Hz), is powered by an electric motor with flexible coupling and is used in a diesel processing unit. For the creation of ODS model, measurements were made of amplitude and phase of vibration, collected in electric motor bearings and centrifugal pump in the horizontal, vertical and axial, according to Figure 3. Where it was used for the measurements a collector and vibration analyzer SKF Microlog GX an accelerometer with sensibility of 100 mv/g and a phase sensor. The collected data were inserted into software used for the analysis of ODS. Where it was obtained modeling the dynamic behavior of the centrifugal pump. Figure 2. Centrifugal pump Figure 3. Points of measurement 4. Experimental Data Table 1 show the values of vibration amplitude and phase for each measured point of the machine. We can observe that the largest vibration amplitudes are the points of the electric motor in the horizontal direction and that all points are in phase. Vibration levels collected at points of the pump are within normal parameters. Table 1. Values of amplitude and phase of vibration DOFS MAGNITUDE mm/s PHASE 1XA 3,2 15 1YH 10, ZV 9,5 5 2XA 3,4 18 2YH 13, ZV 6,9 3 3XA 0,3 20 3YH 0,8 10 3ZV 0,9 7 4XA 0,2 20 4YH 0,7 7 4ZV 0,4 3 ICSV21, Beijing, China, July

4 4.1 Data Vibration The spectrum of vibration were collected in speed (mm/s RMS) in the range Hz. Figure 4 and 5 shows the spectrum of vibration point 1H and 2H respectively, indicating high amplitude at the frequency of 117 Hz, approximately the second harmonic of the rotation of the equipment (116.6 Hz). Figure 4. Spectrum of vibration point 1H Figure 5. Spectrum of vibration point 2H Figure 6 and 7 shows the spectrum of vibration point 1V and 2V respectively, indicating high amplitude at the frequency of 117 Hz and its second harmonic. Figure 6. Spectrum of vibration point 1V Figure 7. Spectrum of vibration point 2V 4.2 ANALYSIS BY ODS Based on the data of amplitude and phase of vibration that were collected, was generated the ODS model. The deflections caused to equipment are shown as Figure 8. Figure 8. ODS model ICSV21, Beijing, China, July

5 Figure 9 shows to 3D view of the ODS model, where large deformations are observed at the base of the electric motor. Figure 9. 3D view of the ODS model Observe in the dynamic behavior of the equipment deflections at the base of the electric motor in the horizontal and vertical directions, according to Figures 10 and 11 respectively. Figure 10. Horizontal direction Figure 11. Vertical direction Analyzing the dynamic behavior of the pump, it is noticed that the vibrational behavior of the equipment is critical at some points, causing deformation of large amplitudes at the base of the electric motor. It is concluded that the deflections generated in the equipment are caused by low stiffness of the base electric motor, excited frequency of 117 Hz, which is close to second harmonic of rotation equipment (116.6 Hz) generating high vibration amplitudes at the points of the electric motor, being necessary perform a reinforcement in the motor base in order to increase their stiffness. Figure 12 shows the base electric motor before realization the reinforcement. Figure 13 shows the base of the electric motor after the realization of reinforcement to increase the stiffness. ICSV21, Beijing, China, July

6 Figure 12. Base electric motor before realization the reinforcement Figure 13. Base of the electric motor after the realization of reinforcement 5. Results Table 2 show the values of vibration amplitude and phase after the realization of reinforcement base of the electric motor. The vibration levels were significantly reduced, confirming that the deflections in the structure of the machine were caused due to low stiffness of the motor base. Table 2. Values of vibration amplitude and phase after the realization of reinforcement DOFS MAGNITUDE mm/s PHASE 1XA 0, YH 2,0 2 1ZV 0,8 7 2XA 0,3 9 2YH 1, ZV 0,7 10 3XA 0,3 11 3YH 0,5 10 3ZV 0,8 9 4XA 0,2 11 4YH 0,6 7 4ZV 0,4 358 ICSV21, Beijing, China, July

7 The ODS model analyzed after the realization of reinforcement at the base of the electric motor indicates significant reduction in deflections the structure of the equipment, as shown in Figure 14. Figure 14. ODS model after the realization of reinforcement The Figure 15 shows the vibration levels in 1H and 2H points before and after the reinforcement at the base of the electric motor. Figure15. Vibration levels in 1H and 2H points ICSV21, Beijing, China, July

8 6. CONCLUSIONS In this paper it was possible to observe the application of ODS in the analysis of the dynamic behavior of a centrifugal pump. Based on data collected in real operating conditions of the equipment it was possible to diagnose a structural defect at the base of the electric motor. It was found that after the realization of structural reinforcement, vibration levels and deformation of the equipment have been reduced. Thus, we conclude that the use of ODS technique is a very effective tool to understand the dynamic behavior of structures and equipment, providing important data that contribute to the early diagnosis of defects. REFERENCES KSB Bombas Hidraulicas S.A. Manual de treinamento, 3 edição, (2003). Craig Sever, D., Charles, T.H. Centrifugal Pumps: Overview of Design, Operation and Malfunctions. Bently Nevada, United States, (1999). Dossing, O. Structural stroboscopy - measurement of operational deflection shapes. Sound and Vibration Magazine, 1:18 24, (1988). Maia, N.M.M., Silva, J.M.M., Modal analysis identification techniques. Royal Society, No. 359, 29 40, (2001). Thomas, V.H., Vu, M., Lakis, A.A., Marcouiller, L., Operational modal analysis by updating autoregressive model. Mechanical systems and signal processing (MSSP), Elsevier, 25(3): , (2011). Richardson, M. H. Is it a mode shape or an operating deflection shape Sound and Vibration Magazine, 31(1):54 61, (1997). Schwarz, B. J., Richardson, M. H., Experimental modal analysis. In CSI Reliability Week, (1999). Avitabile, P., "Experimental modal analysis," Sound & Vibration Magazine, 1-15, (2001). Schwarz, B. J., Richardson, M. H., Introduction to operating deflection shapes. In CSI Reliability Week, (1999). ICSV21, Beijing, China, July