High Vibration Analysis of Eddy Current Drum Coupled Motor to a Vertical Centrifugal Pump Solution Based on EMA, ODS and FEA By:

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1 Juan Gamarra Mechanical Solutions, Inc. High Vibration Analysis of Eddy Current Drum Coupled Motor to a Vertical Centrifugal Pump Solution Based on EMA, ODS a FEA By: Jody Barksdale, PE Juan R. Oqueo, PE Gresham, Smi, a Partners Jack Ferras, PE James Hennessy, PE City of Tampa Florida

2 Biography Juan D. Gamarra Mechanical Solutions, Inc. Assistant Manager of Turbomachinery Testing Senior Staff Engineer Focus on new rotating machinery development projects, as well as resolving difficult field machinery problems. B.S. & M.S., Mechanical Eng., Stevens Institute of Technology Co-Auor of Effect of Component Interference Fit a Fluid Density on e Lateral a Torsional Natural Frequencies of Pump a Turbomachinery Rotor Systems - Proceedings of e Twenty- Nin Pump Symposium Co-Auor of Drive Shaft Failure Analysis on a Multistage Vertical Turbine Pump in River Water Supply Service in a Nickel a Cobalt Mine in Madagascar - Based on ODS a FEA - Proceedings of e Thirtie Pump Symposium 2

3 Problem Statement The 35-year old pump station moves up to 43 MGD of raw wastewater to e city s treatment facility via two 1,500-hp high-capacity pumps for wet weaer flows. After e motors were refurbished in 2011, excessive motor vibration occurred at certain pump speeds, ereby restricting e pumping system s ability to hale various flow ranges. After multiple field balances to e motors a coupling drums, e City of Tampa set out to fix e vibration issue, instead of putting on a Ba-Aid (field balancing) every time e vibration levels exceeded eir limits. Prior to any finite element analysis (FEA), cladding a extra bracing were added to supporting I-beam structure to reduce vibration. This only made matters worse. The goal became to use FEA to identify a implement a practical fix at worked over e pump operating speed range. 3

4 Outline Drawing Motor HP: 1000 or 1500 Volts: 4160 Phase: 3 Hertz: 60 RPM: 294 or 592 AMPS: 137 or 187 Weight: 44,000 lbs Pump Type: Application: Speed Range: Centrifugal Gray Water 228 to 590 RPM * Eddy current coupling allows e constant dual speed motor to operate at pump range shown. 4

5 Photos Top: Support added at motor base Left: Motor to Junction Box Connection Weld Crack 5

6 Analysis Meod a Steps Taken Experimental Modal Analysis (EMA) data was collected to fi natural frequencies of e different system components. RF telemetry strain gauges measuring torque were installed on e driveshaft of one pump. Time-transient vibration testing results on e motor, pump, a drive shaft were collected using accelerometers a proximity probes. An Operating Deflection Shape (ODS) test was performed to reveal dynamic behavior of e entire system. A test-calibrated FEA-based solution design approach was used to develop a fix wi a high degree of confidence of success. 6

7 Experimental Modal Analysis (EMA) [g/lbf] 1m 300u 5.5 Hz Frequency Respo nse(signal 1,Signal 17) - M ark 1 (M agnitude) Working : m1_para_rec_0 : Input : Enhanced [g/lbf] 1m 300u 8.25 Hz Frequency Respo nse(signal 2,Signal 17) - M ark 1 (M agnitude) Working : m1_perp_rec_1 : Input : Enhanced 100u 100u 30u 30u 10 u 3u 1u 300n Parallel to Pump Discharge 10 u 3u 1u 300n Perpeicular to Pump Discharge 100n 100n [Hz] [Hz] [g/lbf] 1m 300u 100u 30u 10 u 3u 1u 300n 100n 12.3 Hz Frequency Respo nse(signal 3,Signal 17) - M ark 1 (M agnitude) Working : m1_vertical_rec2 : Input : Enhanced Vertical [Hz] *Directions struck at e top of e motor wi a modally tuned impact hammer. FRFs are of accelerometers located near a inline wi impacts. 7

8 Coition Monitoring Plots Resonance Pump 1xN Motor 1xN 305 RPM (5.1 Hz) 0 RPM (7.5 Hz) ~11 mils pk-pk Overall ~10.5 mils 1x rpm The radial proximity probes were located at e motor output shaft near e coupling. 8

9 FFT Spectra of Torque Strain Gauge 17.5 Hz 1 st Torsional Natural Frequency of e System. Since e torsional natural frequency was calculated to be 17.5 Hz, it was ruled out as a possible source of e elevated vibration. 9

10 Top of e Motor Vibration vs. Speed Motor Lateral Natural Frequencies in Resonance This speed vs. vibration tre plot shows at as e speed approaches e natural frequencies, e vibration gradually increases a vice versa. 10

11 Computer Model Using ME scopeves Software Plotting ODS Test Results Bolted Connection Loose The ODS animation at 10 Hz iicated a strong motion of e motor swaying a jumping. It also iicated a loose connection between e junction box a motor. 11

12 Based on e motor rotor weight a speed, e radial force due to residual imbalance using ISO G2.5 or four times API (16W/N), e radial dynamic force to be supported was calculated. Modifications 3.0 Sch. 80 Pipe 250 lb-ft torque on union (turn-buckle) The struts were to be anchored at e existing concrete supports a welded to a fabricated ring arou e motor. Turn-buckle pre-load in compression (pushing e bracket a e ring outwards) was produced by e torque applied at e coupling. 375 lb-ft City of Tampa support structure reinforcement 1/4 ick 7 wide ring (collar) 12

13 Finite Element Analysis Perpeicular Mode: 714 CPM Parallel Mode: 738 CPM The lowest predicted FEA separation margin from e full motor speed of 600 rpm (10 Hz) was approximately 19%. This iicated e suggested modifications would increase e motor natural frequencies enough to prevent resonance. 13

14 Vibration Response Before a After Modifications Blue: Before Red: After Wall Anchor Turn-Buckle 14

15 Conclusions / Observations 1. The testing performed showed at e rocking structural natural frequencies of e motor were 5.0 Hz a 7.5 Hz in e parallel a perpeicular to e discharge pipe direction, respectively. 2. These natural frequencies were in resonance when e pump operated at speeds of 300 cpm a 0 cpm. There was no separation margin since e pumps operated from a minimum of 228 cpm to a maximum of 590 cpm. Bo of ese natural frequencies were determined to be in resonance wi e running speed forcing function. 3. The vibration levels were not greatly excited due to e currently well-balanced rotor. From experience, is vibration would be amplified in e future as e imbalance load increases. 15

16 Recommeations/ Results 1. Test results were used to calibrate an FEA model to determine e effectiveness of various meods to increase e structural stiffness of e top of e motor in order to shift e problematic natural frequencies up to a safer separation margin of 15% above e running speed frequency. 2. Results of e analysis predicted at e addition of two steel struts (turn-buckles), going from a steel ring at e top of e motor to mounting plates on e concrete pillars, would shift e structural natural frequency above e acceptable 15% separation margin from e pump running speed. 3. Iepeent verification of e modifications by e City of Tampa before a after eir installation showed a clear decrease in vibration roughout e pump operating speed range. The recommeed fix worked as inteed. Note: The City of Tampa decided to weld e struts to motor, eliminating e need for a steel ring arou e top of e motor. 16