finite element analysis for structural modification and control resonance of a vertical pump

Alexandria Engineering Journal (2017) xxx, xxx–xxx H O S T E D BY Alexandria University Alexandria Engineering Journal www.elsevier.com/locate/aej www.sciencedirect.com ORIGINAL ARTICLE Finite element analysis for structural modification and control resonance of a vertical pump Dalia M El-Gazzar Mechanical & Electrical Research Institute, National Water Research Centre, Ministry of Water Resources & Irrigation, Delta Barrage, Egypt Received 23 October 2016; revised 22 January 2017; accepted 13 February 2017 KEYWORDS Vibration; Vertical pump; Modal analysis Abstract The main objective of this research was to evaluate and enhance dynamic performance for a vertical pumping unit The original electric motor of the pump unit had been replaced by another one different in design and weights Vibration has been increased greatly after installing the new motor Consequently, it is necessary to estimate the change in the vibration characteristics owing to the difference in the boundary conditions of the new motor Measured vibration levels and frequency analysis were dangerous at 1Â due to resonance problem Finite Element Analysis was used to model the motor structure in order to find its natural frequencies and mode shapes The results confirm that the third natural frequency is very close to 1Â operating speed with deviation about 1% To solve the resonance problem, it was recommended to increase the structure stiffness The results after modifications confirmed that the overall vibration level decreases by 89% Ó 2017 Faculty of Engineering, Alexandria University Production and hosting by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Introduction The pumping system is the most important water system supplying water for irrigation and removing subsurface water for drainage purposes There are more than 2000 large scale irrigation and drainage pumping stations in Egypt operating under different conditions Pumping stations in Egypt are subjected to many problems Podugu [1] indicated that the sources of vibration in pumps can be categorized into three types such as mechanical, E-mail address: dalia_engdalia@yahoo.com Peer review under responsibility of Faculty of Engineering, Alexandria University hydraulic and peripheral causes Imbalance and misalignment are the major reasons for mechanical problems Peripheral causes of vibration include harmonic vibration from nearby equipment or drivers, operating the pump at critical speed Problems with any of these issues will show up as symptoms, showing higher than normal vibration at certain key frequencies Redmond and Hussain [2] analyzed the vibration resulting from a simple linear rotor model on isotropic supports and showed the dominant response to be similar to that resulting from a shaft bow The predicted vibration response did not contain any second-harmonic content The reliability and performance of any pump system can be directly affected by its dynamic characteristics Sinha and Rao [3] conducted Modal http://dx.doi.org/10.1016/j.aej.2017.02.018 1110-0168 Ó 2017 Faculty of Engineering, Alexandria University Production and hosting by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Please cite this article in press as: D.M El-Gazzar, Finite element analysis for structural modification and control resonance of a vertical pump, Alexandria Eng J (2017), http://dx.doi.org/10.1016/j.aej.2017.02.018 D.M El-Gazzar Figure Measurement locations at the pump unit analysis on the complete assembly of pumps and piping layout and identified resonance as the root cause for pump failure DeMatteo [4] stated that modal techniques are powerful tools that enhance an analyst’s ability to understand the sources of vibration A case history of the vertical pump was investigated Testing progression from problem identification in route vibration measurements to resonance testing was presented Resonance problems are difficult to solve Modal Analysis gives a clear picture of the machine’s motion; however, neither tool has the capability to solve resonance problems Marenco [5], has experimentally investigated the effect of bearing housing design will influence the dynamic characteristics of the system In this paper, an attempt was to study the effect of the base plate stiffness on improving the dynamic Table characteristics of the pump assembly Scheffer [6], conducted an experiment to monitor pump condition through vibration analysis This research illustrates the typical steps required to solve resonance problems This paper describes the use of operational deflection shape (ODS) and modal analysis testing for problem-solving Kumatkar and Panchwadkar [7], carried out a modal analysis of a vertical turbine pump to determine its dynamic characteristics such as its natural frequencies and corresponding mode shapes They have analyzed the rotor assembly of VT Pump theoretically, numerically and experimentally The system is modeled as a lumped mass structure to theoretically determine its torsional natural frequencies and as a continuous system to determine its transverse natural frequencies The numerical model is validated with the results of the theoretical analysis Nikumbe et al [8] discussed the modal analysis of vertical turbine pump Natural frequencies of a vertical turbine pump are calculated by performing a modal analysis using the Finite Element Method (FEM) They founded total six modes of vibration for this analysis Experimental analysis is defined as the study of dynamic characteristics of a mechanical structure Experimental analysis is done by using Fast Fourier Transform (FFT) analyzer During this analysis, exciter mechanism is done by using an instrumental hammer, as this mechanism requires a minimum amount of hardware and provides shorter measurement times Comparison between natural frequencies with an operational frequency of vertical turbine pump ensured the safe working of the pump de Souza [9], has used Operating Deflection Shape (ODS) technique to analyze the dynamic behavior of the machine or structure, by determining the existing strains and their probable causes Measurements of phase and of the amplitude of vibration at predetermined points were carried The cause of high levels of vibration of the centrifugal pump was determined and the recommendations for correcting the problem were achieved Dupac and Rahman [10], have developed and used ODS procedure to monitor relative in-service planar or orbital displacements of vertical pump for any signs of excess or incompatible displacements In such cases, the system is taken out of Overall vibration measurements on the pump unit Measurement locations Overall velocity before supporting Overall velocity after supporting Full load test No load test without coupling Full load test No load test without coupling Motor Upper Bearing (MUB) Axial Vertical Horizontal Point (1) Point (2) Point (3) 1.87 6.91 9.84 0.398 5.21 4.9 2.80 35 19.7 0.669 14.9 8.11 Motor Lower Bearing (MLB) Axial Vertical Horizontal Point (4) Point (5) Point (6) 1.5 5.27 4.72 0.394 0.838 0.96 1.45 7.54 8.22 0.670 3.94 4.78 Pump Bearing (PB) Axial Vertical Horizontal Point (7) Point (8) Point (9) 2.19 1.57 1.87 2.75 2.32 1.40 Please cite this article in press as: D.M El-Gazzar, Finite element analysis for structural modification and control resonance of a vertical pump, Alexandria Eng J (2017), http://dx.doi.org/10.1016/j.aej.2017.02.018 Structural modification and control resonance of a vertical pump Motor upper bearing Motor lower bearing Figure Vibration spectrum measured during full load at motor upper and lower bearings before supporting service for full dynamics analysis and subsequent design modification A combination of ODS, Modal analysis and FEA has been used to verify any design improvements The undesirable structural dynamics is the root cause of poor reliability Prajapati [11], found out the solution for reducing vibration in vertical turbine centrifugal pump He enumerated some methods of identifying vibration in pump and some possible cause of vibration He found out that the possible cause of vibration in pump is due to its structure and it is due to either weight of the motor placed at higher cause maximum vibration or due to improper misalignment between upper and lower base part of pump Problem description & task In this research, dynamic measurements are evaluated for an axial pumping unit at El-Shabab Pumping Station in the area of El-Salhiya This pumping station is used to irrigate 9000.5 feddans and consists of pump units; each pump unit is of discharge 1.5 m3/s, head 11.6 m, 992 rpm, and motor power 1000 kW The vibration problem on Pump unit (1) began after its original motor (a 1000 kW, BROWN BOVERI Type: SOV560wb) was replaced with a new Hungarian motor as shown in Fig The old motor was in service for many years The old motor is 987 RPM and weighs 6200 kg The new motor is 992 RPM and weighs 8600 kg Since the new motor was installed, the vibration on the machine has been extremely rough Therefore, the decision was to modify the motor setup by supporting its base plate with reinforcing concrete supports at the sides of the base After supporting, the measured vibration level had been duplicated especially on the motor upper and lower bearings and reached a danger value So the task was to determine the source of the high vibrations and the method to overcome this problem in the final motor setup, a part of metal has four webs of 15 mm thickness and each is added by welding to the lower motor Please cite this article in press as: D.M El-Gazzar, Finite element analysis for structural modification and control resonance of a vertical pump, Alexandria Eng J (2017), http://dx.doi.org/10.1016/j.aej.2017.02.018 D.M El-Gazzar Motor upper bearing Motor lower bearing Figure Vibration spectrum measured during full load at motor upper and lower bearings after supporting base to increase the stiffness to overcome the problem of high vibration due to resonance Results of vibration measurements tests Overall vibration levels and vibration spectra are measured at the motor running speed (16.53 Hz) at different locations on the whole pumping unit parts to determine the dynamic performance for such huge machines installed at the heavy base plate and located at class I according to ISO 1-10861 This class defines that up to 2.8 mm/s is a good level, up to 4.5 mm/s is an allowable level, up to 11.2 mm/s is just tolerable level and what exceeds this value is not permissible and dangerous Measurements were done during no load condition where the motor was disconnected completely from the pump via the coupling and full load condition Vibration measurements were done on locations on the motor upper and lower bearings and pump bearing in the axial, vertical, and horizontal directions as shown in Fig Vibration measurements were taken before and after supporting the motor base at full load and no load conditions as shown in Table Firstly the measurement results before Please cite this article in press as: D.M El-Gazzar, Finite element analysis for structural modification and control resonance of a vertical pump, Alexandria Eng J (2017), http://dx.doi.org/10.1016/j.aej.2017.02.018 Structural modification and control resonance of a vertical pump Motor upper bearing Motor lower bearing Figure Vibration spectrum measured during no load at motor upper and lower bearings after supporting supporting during full load condition indicated that the overall vibration levels are extremely rough On the motor upper bearing, the overall vibration level reached 6.91 and 9.48 mm/s in vertical and horizontal directions On the other hand, the overall vibration level reached 5.27 and 4.72 mm/s vertical and horizontal directions at the motor lower bearing These levels of vibration are a danger to the machines and structures where it transmits to the foundations and structures After that, the motor was disconnected completely from the pump via the coupling to check and confirm that the main source of high vibration level is from the motor The overall vibration levels are decreased on the motor upper bearing about 24% in the vertical direction and 50% in the horizontal direction, while they decreased on the motor lower bearing about 84% in the vertical direction and 79% in the horizontal direction Frequency analysis is used to define the exciting frequencies and determine the level of vibration at each specific frequency Also, it is used to determine the sources of vibration, to control vibration levels and solve vibration problems Results of frequency analysis shown in Fig indicated that, there is a high vibration occurs from the mechanical defect at 1Â rotational speed Structural modifications were done by adding reinforced supports to the motor base in order to decrease and control the vibration level Please cite this article in press as: D.M El-Gazzar, Finite element analysis for structural modification and control resonance of a vertical pump, Alexandria Eng J (2017), http://dx.doi.org/10.1016/j.aej.2017.02.018 D.M El-Gazzar Figure Vibration level during full load and no load conditions Results of the overall vibration level after supporting are extremely increased and reached a danger level according to ISO 10816 In the case of full load, the overall vibration level at the motor upper bearing increased about 400% in the vertical direction and 100% in the horizontal direction On the other hand, the overall vibration level at the motor lower bearing increased about 43% in the vertical direction and 74% in the horizontal direction Results of frequency analyses in the case of full load indicated that the vibration amplitude reached a danger value about 26.3 and 15.7 mm/s in the motor upper bearing in the vertical and horizontal directions respectively as shown in Fig In the case of no load, the overall vibration level at the motor upper bearing increased about 180% in the vertical direction and 65% in the horizontal direction as shown in Fig On the other hand, the overall vibration level at the motor lower bearing increased about 300% in the vertical direction and 500% in the horizontal direction Results of frequency analyses in the case of no load indicated that the vibration amplitude reached a danger value about 11.32 and 6.75 mm/s in the motor upper bearing in the vertical and horizontal directions respectively Vibration level change during full load and no load conditions before and after supporting is shown in Fig Run-up test A common Way to identify resonances empirically is to operate the equipment across its range of operating speeds while measuring the vibration it exhibits Run-up or coast-down tests, which monitor vibration from standstill to maximum speed and back down, are a quick way to see whether troublesome resonances are present in the system As shown in Fig 6, a waterfall plot of the spectral data is used to identify a peak vibration level at a certain speed during the run-up a test of the pump unit This plot consists of the 1Â vibration amplitude being collected simultaneously with a 1Â rpm phase reading as the machine coasts to a stop running speed It could be seen that during the pump starting up, there is a discrete peak in the magnitude at 1Â and it’s a good indication that a resonance exists there The speed range includes 10 Hz, 12.5 Hz, 13.75 Hz, 15 Hz, 16.5 Hz, and 17 Hz, 18 Hz and the amplitude at this speed frequency is 0.0479 mm/s, 0.2077 mm/s, Please cite this article in press as: D.M El-Gazzar, Finite element analysis for structural modification and control resonance of a vertical pump, Alexandria Eng J (2017), http://dx.doi.org/10.1016/j.aej.2017.02.018 Structural modification and control resonance of a vertical pump Figure Run-up test data 0.488 mm/s, 0.659 mm/s, 0.17 mm/s, 0.301 mm/s, 0.15 mm/s It could be seen that the amplitude increases until the rotor reaches its critical speed (16.5 Hz) and then decreases to the normal level as the speed continues to change So the speed at 16.5 Hz which the amplitudes decrease is a possible natural frequency The run-up data identified a natural frequency 1Â Please cite this article in press as: D.M El-Gazzar, Finite element analysis for structural modification and control resonance of a vertical pump, Alexandria Eng J (2017), http://dx.doi.org/10.1016/j.aej.2017.02.018 D.M El-Gazzar  The foundation is not rigid enough (weak) to support the new motors  The new motors are heavier than the old one affected the natural frequency of the foundation leading to high vibration levels due to resonance  Vibration from a mechanical defect at 1Â rotational speed is exciting a natural frequency of the pump structure Therefore it is mandatory to perform dynamic structure study to determine the exact stiffness needed The study determines the type and the method of re-enforcing the structure The study includes the following: build a numerical model of the structure,  determine its dynamic characteristics,  Design of the re-enforcement to ensure safe and normal operation of the pumps Finite element analysis Figure structure Table Geometrical dimensions of motor and base plate Predicted natural frequencies Modes Description Frequency (Hz) 1st bending 1st bending 2nd bending 2nd bending 2nd bending 2nd bending Horizontal extension Horizontal extension 7.908 7.908 18.35 18.35 50.71 50.71 35.47 35.47 Finite element analysis (FEA) was used to model the motor structure to estimate the dynamic characteristics using ANSYS WORKBENCH 14.5 The FEA model was built for the original motor structure and simulation is made to find its natural frequencies and mode shapes The model consists of a motor weighing 8700 kg, a steel base, and a concrete foundation Concrete is assumed to be a homogeneous and isotropic material and to behave in a linear elastic manner The mechanical properties of the concrete are assumed to be modulus of elasticity: 32 GPa, Poisson’s ratio: 0.2, and weight density: 2400 kg/m3 Steel is assumed to be a homogeneous and isotropic material behaving in a linear elastic manner The mechanical properties are assumed to be modulus of elasticity: 200 GPa, Poisson’s ratio: 0.3, and weight density: 7800 kg/ m3 A simple geometrical structure was designed using SOLID WORKS 2010 to simulate the motor structure as shown in Fig When a 3D model of solid volumes is generated, solid modeling is generally more convenient compared to direct generation Solid modeling is tedious and too much time consuming The boundary conditions assumed that the concrete foundation is fully clamped in the two cases including modified and final motor structure setup Model results at 16.5 Hz The resonance is coincident with 1Â rotational speed Vibration from this mechanical defect at 1Â rotational speed is exciting a natural frequency of the pump structure As a result of operating the pump with its 1Â at a resonant frequency, there was an experience excessive vibration and it will wear prematurely Corrective actions should be taken to move the resonant frequency away from the 1Â frequency, through the addition of stiffness (to increase the resonant frequency) or mass (to lower the resonant frequency) to the structure Another solution is to use a ‘‘tuned absorber” or a ‘‘tuned mass damper” These devices greatly reduce the amount of vibration observed at the natural frequency All the previous results confirmed that characteristics of the problem are listed below: Natural frequencies of a symmetric structure occur in orthogonal pairs as shown in Table The physical significance is that the motor can actually vibrate (bend) in any direction based on the direction of the applied excitation So it is recommended to increase stiffness results in the high natural frequencies The results of the model indicated that the low stiffness of the motor base contributed to the relatively high oscillatory motion of the motor Add stiffeners as proposed will decrease the amplitude of vibration; however, it will not eliminate the source of vibrations The predicted mode shapes are shown in Fig The possible cause of this problem is due to that, all these motor applications have a high thrust bearing design consisting of angular contact bearings (2 down and up) Please cite this article in press as: D.M El-Gazzar, Finite element analysis for structural modification and control resonance of a vertical pump, Alexandria Eng J (2017), http://dx.doi.org/10.1016/j.aej.2017.02.018 Structural modification and control resonance of a vertical pump Mode Mode Mode Mode Mode Mode Figure Normal mode shapes of motor and base plate structure Please cite this article in press as: D.M El-Gazzar, Finite element analysis for structural modification and control resonance of a vertical pump, Alexandria Eng J (2017), http://dx.doi.org/10.1016/j.aej.2017.02.018 10 D.M El-Gazzar Figure Table A four web model added to the lower motor base Normal mode shapes of motor and base plate structure Modes Description Frequency (Hz) before adding stiffness Frequency (Hz) after adding stiffness 1st bending 1st bending 2nd bending 2nd bending 2nd bending 2nd bending Horizontal extension Horizontal extension 7.908 7.908 18.35 18.35 50.71 50.71 35.47 35.47 13.51 13.51 33.12 33.12 61.47 61.47 72.21 72.21 Lack of precision of fits at upper thrust bearing could create high unbalance due to eccentricity, besides that insufficient down thrust causing improper loading of the bearing and affecting stiffness Modified model Since the natural frequency at 16.5 Hz is at the upper end of the motor at 1Â operating speed, it is recommended increasing the machine stiffness Increasing stiffness results in a higher natural frequency To accomplish the change, a part of metal has four webs of 15 mm thickness and each is added by welding to the lower motor base as shown in Fig The modified model produced acceptable results Increasing stiffness by adding the four web model leading to alters the natural frequencies of the motor This modification increases the frequency of the first pair and second bending modes The change at each natural frequency based on adding the four webs model is shown in Table Normal mode shapes of motor and base plate structure after modifications are shown in Fig 10 Results of vibration measurements after modifications After achieving the new modifications to the pump unit, a new measurement was done to indicate the effect of this modifica- tion on the performance of the pump unit Overall vibration levels and vibration spectra are shown in Table indicating that the vibration level on the pump unit is extremely reduced The overall vibration level reached 3.6 and 2.631 mm/s on the motor non-drive end in the horizontal and vertical directions The overall level decreases about 89% and 86% on the motor drive end in the horizontal and vertical directions respectively The frequency spectrum showed that the measured vibration amplitude is obviously decreased The maximum vibration amplitude reached 0.227 and 0.294 mm/s on the motor nondrive end in horizontal and vertical directions respectively as shown in Fig 11 The results after achieving the new modifications indicated that the overall vibration level is in the permissible zone of ISO 10816-1 Conclusion Dynamic behavior of the pumping system is affected greatly by the changing in the motor weight The low stiffness of the motor base contributed to the relatively high oscillatory motion of the motor Adding stiffeners as proposed decreased the amplitude of vibration The results confirmed that how the dynamic characteristics of the pump structure are improved after applying modifications The increase in the stiffness of the motor base moves the natural frequencies away from Please cite this article in press as: D.M El-Gazzar, Finite element analysis for structural modification and control resonance of a vertical pump, Alexandria Eng J (2017), http://dx.doi.org/10.1016/j.aej.2017.02.018 Structural modification and control resonance of a vertical pump 11 Mode Mode Mode Mode Mode Mode Figure 10 Normal mode shapes of motor and base plate structure after modifications Please cite this article in press as: D.M El-Gazzar, Finite element analysis for structural modification and control resonance of a vertical pump, Alexandria Eng J (2017), http://dx.doi.org/10.1016/j.aej.2017.02.018 12 Table D.M El-Gazzar Overall vibration level after new modifications Measurement locations Overall velocity (mm/s) Motor None Drive End (MNDE) Axial Horizontal Vertical Point (1) Point (2) Point (3) 3.74 3.65 2.631 Motor Drive End (MDE) Axial Horizontal Vertical Point (4) Point (5) Point (6) 2.285 2.279 0.300 Pump Upper Bearing (PUB) Axial Horizontal Vertical Point (7) Point (8) Point (9) 1.165 1.987 1.854 Figure 11 Vibration spectrum measured in horizontal and vertical direction after modifications Please cite this article in press as: D.M El-Gazzar, Finite element analysis for structural modification and control resonance of a vertical pump, Alexandria Eng J (2017), http://dx.doi.org/10.1016/j.aej.2017.02.018 Structural modification and control resonance of a vertical pump forced vibrations FEA is a useful tool to evaluate the effectiveness of potential structural modifications The results confirmed that the overall vibration level decreases about 89% after achieving structural modifications 13 [6] [7] References [1] Ramana Podugu, Modal approach for vibration analysis and condition monitoring of a centrifugal pump, Int J Eng Sci Technol (IJEST) (8) (2011) 6335–6344, ISSN: 0975-5462 [2] I Redmond, K.M Hussain, Misalignment as a source of vibration in rotating shaft systems, in: Proc Intl Modal Analysis Conf (IMAC) XIX, Orlando, 2001, pp 116–123, ISSN 0277786X [3] Jyoti K Sinha, A Rama Rao, Vibration diagnosis of failure of mechanical coupling between motor and pump rotors, Int J Acoust Vibr 10 (2) (2005) 89–92 [4] T DeMatteo, Operational Deflection Shape and Modal Analysis Testing to Solve Resonance Problems, Consultant Emerson Process Management/CSI Division, CSI RBM University, 2001 [5] Giacomo Marenco, Alessandro Nicchio, Alberto Pivo, Dynamic improvement of an overhung single stage pump, in: Proceedings [8] [9] [10] [11] of the Twenty-Fifth International Pump Users Symposium, 2009, pp 33–38 Cornelius Scheffer, Pump Condition Monitoring through Vibration Analysis Pumps: Maintenance, Design, and Reliability Conference, IDC Technologies, 2008 R.R Kumatkar, A.A Panchwadkar, Modal analysis of rotor assembly of vertical turbine pump, Int Eng Res J (IERJ) (Special Issue 2) (2015) 325–333, ISSN: 2395-1621 A.Y Nikumbe, V.G Tamboli, H.S Wagh, Modal analysis of vertical turbine pump, Int Adv Res J Sci Eng Technol (5) (2015) 117–221, ISSN: 2393-8021 (Online) Karllyammo Lennon de Souza, Analysis of the dynamic stiffness of a centrifugal pump by ODS, in: 21st International Congress on Sound and Vibration, Beijing/China, 13–17 July, 2014 S.N Dupac, A.G Rahman, Dynamic design verification (DDV) of vertical pump using, combined modal analysis (MA), finite element analysis (FEA) & operation deflection shape (ODS), Recent Res Appl Mech (2016), ISBN: 978-1-61804-078-7 (19 Nov 2016) H Prajapati, Vibration Vertical Centrifugal Pump, IJARIIEISSN (O)-2395-4396, 2(6) (2016) Please cite this article in press as: D.M El-Gazzar, Finite element analysis for structural modification and control resonance of a vertical pump, Alexandria Eng J (2017), http://dx.doi.org/10.1016/j.aej.2017.02.018