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ODS Modal Case Histories

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ODS Modal Case Histories

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    1. ODS & Modal Case Histories Barry T. Cease Cease Industrial Consulting February 20th, 2009 1

    2. ODS & MODAL CASE HISTORIES   BARRY T. CEASE, CEASE INDUSTRIAL CONSULTING FEBRUARY 20TH, 2009   INTRODUCTION What is ODS analysis and why do we need it? What is Modal analysis and why do we need it? When should either technique be used? Example of how to collect ODS & Modal data (test unit)   CASE HISTORY#1 – ACCEPTANCE TESTING OF AHU FAN Equipment & problem description Route data, coastdown data & determination of “offending frequencies” Modal analysis of fan, motor & base Conclusions & recommendations   CASE HISTORY#2 – ACCEPTANCE TESTING OF WATER PUMP Equipment & problem description Route data results versus standards & determination of “offending frequencies” ODS analysis of pump, step 1 (baseline) ODS analysis of pump, step 2 ODS analysis of pump, step 3 Conclusions & recommendations   QUESTIONS & CREDITS “Modal Testing”, Robert J. Sayer, PE, Vibration Institute 31st Annual Meeting, June 19th, 2007 “Applied Modal & ODS Analysis”, James E. Berry, PE, 2004 “Machinery Vibration Analysis 3, Volume 2”, Vibration Institute, 1995 “Mechanical Vibrations, 2nd Edition”, Singiresu S. Rao, 1990 2

    3. What Is ODS? ODS stands for operating deflection shape. ODS analysis generates a computer model of your machinery that depicts its motion while running at operating speed & load. You literally “see” how your machine is moving as it operates. This modeling can be extremely useful to illuminate an otherwise elusive solution to machinery vibration problems. First, a CAD model of the machine or mechanical system is created (structure file). Second, detailed & meticulous vibration measurements are made on the machine typically during normal operation. These measurements consist of both the amplitude & phase of vibration at one or multiple frequencies of interest all referenced to a common point. Finally, these field measurements are imposed on the model to generate visible animations of the model/machine at the distinct vibration frequencies of interest (typically the “offending frequencies”). 3

    4. What Is Modal Analysis? Modal analysis identifies the frequencies & shapes your machine “likes to vibrate at” (natural frequencies) and compares these to the normal forces present on the machine to see if a match exists that produces an undesirable resonant condition. If a resonant condition is identified, common solutions involve the following: force reduction (ie: reducing the vibration forces present in the machine), tuning of the mechanical system (ie: adding or reducing mass or stiffness to the system at the right spots), or force “movement” (ie: changing the machine speed as possible to avoid the condition). The actual process of modal analysis is similar to that of ODS analysis except measurements are made while the machine is not running typically using a force hammer and one or more sensors. The hammer provides the input (force) and the sensor(s) measure the response (motion) at multiple points on the machine. These modal measurements are then processed thru a technique known as curve-fitting and then like ODS measurements, imposed on the model to produce animations that are analyzed. 4

    5. Vibration Spectra .vs. Modal Data 5

    6. When Should ODS or Modal Analysis Be Used? When standard vibration analysis techniques have failed to determine the exact problem. When resonance is suspected. An ODS or Modal job begins best with a determination of the “offending frequencies of vibration” usually made using standard, route vibration spectra. 6

    7. Example: Collecting ODS Data From CMS Test Rotor Kit Machine operating. Determine reference point (typically use route data point with strong vibration at all “offending frequencies”). First roving point collected at reference point (ie: 1Y:1Y). Continue collecting other points all along machine at predetermined points. Both the total number of points collected as well as the point locations are key to how accurate the model animation will represent reality (ie: spatial aliasing). 7

    8. Example: Collecting Modal Data From CMS Test Rotor Kit Machine not operating. Determine reference (driving) point. Like ODS analysis above, we want to use a point with strong vibration at all “offending frequencies”, but for modal analysis, we must be even more “picky” by applying the impact & measuring the response at many points until good representation of all offending frequencies is found (“driving point”). First roving point collected at driving point (ie: 1Y:1Y). Usually, we rove around with the sensor(s) and apply impact at the driving point, but this isn’t necessary. We could also rove around with the hammer with similar results although getting a good impact at all points is typically difficult. Continue collecting other points all along machine at predetermined points. Like ODS analysis, both the total number of points collected as well as the point locations are key to how accurate the model animation will represent reality (ie: spatial aliasing). 8

    9. Case History#1: Acceptance Testing Of AHU Fan Equipment & Problem Description Newly installed AHU Fan operating at medical facility. Vibration acceptance testing required for all rotating equipment at facility. Fan OEM contacted for vibration specifications - maximum acceptable vibration at 0.35 ips-pk. Isolated, center-hung, centrifugal fan driven thru v-belts by a 4-pole induction motor operating on a variable speed drive. Entire machine supported by 4-ea spring isolators mounted on floor arranged per diagram at right. Two spring isolators are also mounted between the fan frame and wall to counter fan thrust. 9

    10. INITIAL DATA & FINDINGS, PART 1 Initial vibration data was collected on both fan & motor at 100% speed and overall levels were compared to OEM specifications. Because this machine operated on a variable speed drive with normal operation anywhere between 50 and 100% full speed, coastdown data was collected between this speed range. Unfortunately, this machine failed to stay within OEM specs both at 100% speed and at many points between 50 & 100% speed. Maximum vibration levels occurred not at 100% speed, but at lower speeds suggesting possible resonance problems. “Offending speeds/frequencies” were identified from coastdown data at approximately 1,500, 1,800 & 1,900 cpm. Field observations noted the entire machine visibly “jumped” when the machine speed was set to 90-95% and motion at the motor outboard isolator seemed worst. 10

    11. INITIAL DATA & FINDINGS, PART 2 11

    12. SPECTRAL DATA AT FULL SPEED 12

    13. FAN COASTDOWN DATA, BODE PLOTS 13

    14. INTERFERENCE DATA (MOTOR & FAN SPEEDS) 14

    15. INTERFERENCE DIAGRAM 15

    16. MODAL ANALYSIS OF AHU FAN A Simple CAD model of the fan, motor & base was created and modal data collected. This modal data was imposed on the model appropriately to identify the natural frequencies of the mechanical system. The known offending frequencies were compared with natural frequencies found to identify a match that would result in resonance condition. Two natural frequencies (modes) were identified which most likely are being excited by the fan speeds as: 26.1 & 31.1 Hz or 1,566 & 1,866 cpm. Both these modes involve distortion of the machine base near the motor. 16

    17. MODAL ANALYSIS – 26.1 Hz Mode 17

    18. MODAL ANALYSIS – 31.1 Hz Mode 18

    19. CONCLUSIONS & RECOMMENDATIONS, AHU FAN This fan failed OEM vibration specifications due primarily to resonances identified in the machine frame at 26.1 & 31.1 Hz. Unbalance may exist in the fan, but it’s contribution is minor by comparison to the resonances identified. If balancing is done to reduce forces, perform at 1,200 rpm fan speed or lower to avoid resonances and associated balance difficulties. The isolator near the motor outboard may be loose with the floor. Please inspect & repair as needed. Resolving the resonance issues will likely involve either adding an additional pair of isolators between the fan & motor or stiffening the machine frame near the motor or both. Stiffening the machine frame might be accomplished by welding either “X” bracing inside the base near the motor or welding plate onto the machine frame for the motor base to rest on. A slightly larger AHU fan of similar design with six isolators instead of four was also tested as part of this job – this six isolator fan passed acceptance testing at all speeds. These conclusions were presented to the customer along with documentation. Months later I checked with plant personnel who informed me my customer had opted to balance the fan with disappointing results. 19

    20. CASE HISTORY#2 - ACCEPTANCE TESTING OF HIGH PRESSURE WATER PUMP Equipment & Problem Description Newly installed critical high pressure water pump at plant. Plant vibration specs called for maximum vibration levels of 0.10 ips-pk. At first glance, many problems were seen with the design & layout of the pump & piping. What follows are vibration spectral & ods data at progressive stages of our attempt to bring this pump into plant specs. 20

    21. BASELINE OVERALL LEVELS 9/16/08 Plant vibration specs called for overall levels no greater than 0.10 ips-pk. Both the motor & pump failed specs during baseline measurements taken on 9/16. Highest levels were seen at pump with much higher than expected thrust levels. Movement could be felt at the floor while collecting data. 21

    22. BASELINE SPECTRA 9/16/08 Pump spectra from 9/16/08 shows dominant vibration at the vane-pass frequency (4x rpm) of the pump. A higher than normal vibration level at this frequency generally indicates flow problems of some sort with the pump. From the photo earlier, what did you see that could be causing flow problems at this pump? Horizontal measurement shows high 1x & 2x rpm vibration as well as vane-pass. Thus, our offending vibration frequencies are primarily 1x, 2x & 4x rpm for this machine on 9/16/08 (baseline). 22

    23. BASELINE ODS 9/18/08 – MOTION @ 1xRPM (3,590 cpm) Pump maximum vibration at this frequency occurred at the pump, inboard, horizontal measurement (PIH) at 0.05 ips-pk. Note 180 degree radial motion across the coupling at this key frequency. Shaft alignment & soft foot are suspect. Note movement of both machine pedestal & surrounding floor suggesting significant problems with this machine foundation. 23

    24. BASELINE ODS 9/18/08 – MOTION @ 2xRPM (7,180 cpm) Pump maximum vibration at this frequency occurred at the pump, inboard, horizontal measurement (PIH) at 0.08 ips-pk. Note vertical movement of entire pedestal & surrounding floor at this frequency (120 Hz) again suggesting significant problems exist with this machine foundation. 24

    25. BASELINE ODS 9/18/08 – MOTION @ 4xRPM (14,400 cpm) Pump maximum vibration at this frequency occurred at the pump, inboard, vertical measurement (PIV) at 0.24 ips-pk. Note thrusting of both pump suction area and entire pump rotor. I suspect this is due in part to turbulence at the pump suction from “elbow entry”. Note continued pedestal & foundation movement. Note little movement at motor. 25

    26. BASELINE ODS 9/18/08 – MOTION @ 8xRPM (28,800 cpm) Pump maximum vibration at this frequency occurred at the pump, inboard, axial measurement (PIA) at 0.04 ips-pk. Note continued thrusting of pump & pump suction at this frequency. Note relatively little motion of the motor or pedestal at this frequency. 26

    27. CONCLUSIONS & RECOMMENDATIONS, 9/18/08 CONFIGURATION The suction piping entering the pump requires modification to allow for a minimum of 5 pipe diameters of straight length before entering the pump (10 diameters length preferred). The presence of the elbow at the pump suction is no doubt causing excessive turbulence in the fluid flow as it enters the pump which in turn is exciting the pump vanepass frequency. The shaft alignment is questionable due to the 180 degree radial motion across the coupling. Please recheck shaft alignment & soft foot and correct as necessary to plant specs. The machine pedestal & surrounding floor appear loose from the ground. Movement of the pedestal & floor were clearly seen in ODS at both 1x & 4x rpm. Both motor & pump were hot to touch and low air flow was noted at the motor. Uncertainty exists as to the existing & proper lube for pump. Install larger fan at motor endbell & change oil to OEM specs. 27

    28. PUMP & PIPING CONFIGURATION 10/2/08 The suction piping was modified per 9/18/08 suggestions. The alignment was checked & reportedly corrected to plant specs. Soft feet were reportedly identified and corrected. A new pump rotor was installed with an impeller reportedly balanced to plant specs. A recirculation line was added. A larger motor fan was added. The pump oil was changed to an ISO 68 weight per OEM specs. 28

    29. OVERALL LEVELS, 10/2/08 Unfortunately, both motor & pump vibration levels actually increased with the 10/2/08 modifications. Motor vertical measurements were the only ones that decreased. Both motor & pump remained out of plant vibration specs. 29

    30. SPECTRAL DATA 10/2/08 High vibration at 1x, 2x & 4x rpm (vane-pass) remained in all pump spectra. New appearance of vibration at 3x rpm with 10/2/08 modifications not seen in baseline data. Highest vibration levels remain at 4x rpm (vane-pass). 30

    31. ODS 10/2/08 – MOTION @ 1xRPM (3,590 cpm) Pump maximum vibration at this frequency occurred at the pump, inboard, axial measurement (PIA) at 0.12 ips-pk. Note the excessive horizontal movement of the newly modified pump suction piping at this key frequency. Notice how much more the piping is moving when compared to motion at either the pump or motor. Could the solution to the bad actor at your plant be found at the piping or ducting? 31

    32. ODS 10/2/08 – MOTION @ 2xRPM (7,180 cpm) Pump maximum vibration at this frequency occurred at the pump, inboard, axial measurement (PIA) at 0.08 ips-pk. As in the earlier ODS at 1x rpm, note how piping motion is much greater than that seen at either the pump or motor. Note the near perfect 2nd mode motion (sinusoidal) of the horizontal run of discharge piping. Note the excessive vibration of both the vertical run of discharge piping as well as the newly installed recirculation line (1st mode). Note how total motion of the discharge piping seems to “pull the pump” in the axial or thrust direction. 32

    33. ODS 10/2/08 – MOTION @ 3xRPM (10,800 cpm) Pump maximum vibration at this frequency occurred at the pump, inboard, horizontal measurement (PIH) at 0.16 ips-pk. Again, notice how the piping motion dwarfs that seen at either pump or motor. Note how excessive motion of the recirculation line (1st Mode) is “pulling the pump”. Note how excessive motion of the suction line is also “pulling the pump”. Note the excessive motion in the short section of discharge piping between the recirculation line & pump discharge. 33

    34. ODS 10/2/08 – MOTION @ 4xRPM (14,400 cpm) Pump maximum vibration at this frequency occurred at the pump, inboard, horizontal measurement (PIH) at 0.36 ips-pk. Again, notice how much more the piping is moving (vibrating) compared to either the pump or motor. Note how motion of the recirculation line at this key frequency is by far the most and resembles a possible 2nd mode. Note how motion at the suction piping remains high as well. 34

    35. ODS 10/2/08 – MOTION @ 8xRPM (28,800 cpm) Pump maximum vibration at this frequency occurred at the pump, inboard, axial measurement (PIA) at 0.05 ips-pk. Again, notice how motion of the piping dwarfs that seen at either motor or pump. Note the excessive motion of the discharge piping here. Note how motion at the recirculation line is relatively small when compared to earlier frequencies. 35

    36. INSPECTION RESULTS 10/2/08 A close inspection of the piping found a broken discharge pipe hanger just above the horizontal pipe run in the ceiling. It was unknown how long this hanger had been broken, but it’s absence no doubt added flexibility to the discharge piping run. 36

    37. CONCLUSIONS & RECOMMENDATIONS, 10/02/08 CONFIGURATION Remove the recirculation line if possible. The addition of the new recirculation line has had a negative effect on machine vibration levels due to multiple suspected resonances occurring there. Like the recirculation line above, motion of the discharge piping at multiple frequencies is having a negative effect on machine vibration. Add additional support to the discharge piping at the points where high motion is observed in the ODS animations. If possible, try adding support from at least two additional points. Repair or replace the broken discharge hanger found at the ceiling. Provide additional support (if possible) under the suction piping as excessive motion continues here. No soft foot records were identified from the alignment job performed since 9/16/08 on this machine. Please perform another soft foot and alignment check on this machine, make corrections as necessary and document results. 37

    38. PUMP & PIPING CONFIGURATION 10/16/08 A new, larger standpipe was added under the suction piping for better support. A new support was added to the discharge piping at the nearby wall. A new stiffening “connection” was added between the discharge & suction piping above the recirculation line. The recirculation line was not removed. The broken discharge hanger was not repaired or replaced. Pump soft foot corrections were made and documented. Machine alignment is documented to plant specs. 38

    39. OVERALL LEVELS, 10/16/08 Both motor & pump overall vibration levels dropped significantly with the 10/16/08 modifications. Motor overall vibration levels are now below plant spec at every measurement point. Pump overall vibration levels are much better, but remain out of plant specs with highest levels being seen at the pump horizontal measurement. 39

    40. SPECTRAL DATA 10/16/08 Significantly reduced vibration levels at all offending frequencies is seen in all pump spectra. Remaining vibration still occurring at 1x, 2x, 3x & 4x rpm (offending frequencies). Vibration at 4x rpm (vane-pass), although reduced, remains the dominant vibration frequency in most measurements. Vibration at 3x rpm, although reduced, is the highest single source of vibration in all three pump measurements. 40

    41. ODS 10/16/08 – MOTION @ 1xRPM (3,590 cpm) Pump maximum vibration at this frequency occurred at the pump, inboard, axial measurement (PIA) at 0.12 ips-pk. Notice how motion of the piping is much greater than that observed at either the pump or motor. Note how now both the discharge & suction piping are flexing in the axial (thrust) plane and are “pulling the pump” with them. Notice how the motor is virtually “still” at this key frequency – any alignment or coupling problems are now unlikely here. Both the addition of the new “stiffening connection” between suction & discharge as well as the continued existence of the recirculation line appear to have negative effects on machine vibration (transmission path). 41

    42. ODS 10/16/08 – MOTION @ 2xRPM (7,180 cpm) Pump maximum vibration at this frequency occurred at the pump, inboard, horizontal measurement (PIH) at 0.05 ips-pk. The newly installed discharge piping support at the wall was found both loose from the wall and the piping. This looseness is at least partly to blame for the excessive motion of the discharge piping seen at or near the location of this new support. Both the newly installed stiffening connection between discharge & suction piping as well as the recirculation line must be eliminated to reduce machine vibration levels. 42

    43. ODS 10/16/08 – MOTION @ 3xRPM (10,800 cpm) Pump maximum vibration at this frequency occurred at the pump, inboard, horizontal measurement (PIH) at 0.14 ips-pk. Note the excessive horizontal motion of the discharge piping at this frequency with maximum deflection occurring somewhere between the recirculation line & discharge valve. The recirculation line should be removed. Horizontal bracing of the discharge line somewhere between the recirculation line & discharge valve may be necessary to eliminate this vibration. Only consider this modification after the glaring problems mentioned earlier are corrected and high vibration levels persist. 43

    44. ODS 10/16/08 – MOTION @ 4xRPM (14,400 cpm) Pump maximum vibration at this frequency occurred at the pump, inboard, horizontal measurement (PIH) at 0.12 ips-pk. Both the recirculation line as well as the newly installed “stiffening connection” continue their negative effects on machine vibration levels. Again, notice how little both the motor & pump are moving when compared to the piping. 44

    45. ODS 10/16/08 – MOTION @ 8xRPM (28,800 cpm) Pump maximum vibration at this frequency occurred at the pump, inboard, axial measurement (PIA) at 0.02 ips-pk. From the earlier spectral plots, vibration at this frequency is small when compared to the others. Machine vibration levels at this frequency could be reduced by removing the “stiffening connection” between discharge & suction lines. The suction pipe stand is vibrating excessively at this frequency in the horizontal direction. 45

    46. CONCLUSIONS & RECOMMENDATIONS, 10/16/08 CONFIGURATION Remove the recirculation line & stiffening connection. Excessive motion (vibration) was seen at both the recirculation line & newly installed “stiffening connection” at many frequencies. Remove these two piping components to reduce machine vibration levels. Tighten up the newly installed support between the discharge piping and wall. Repair the broken discharge hanger located in the ceiling. 46

    47. QUESTIONS & CREDITS “Modal Testing”, Robert J. Sayer, PE, Vibration Institute 31st Annual Meeting, June 19th, 2007 “Applied Modal & ODS Analysis”, James E. Berry, PE, 2004 “Machinery Vibration Analysis 3, Volume 2”, Vibration Institute, 1995 “Mechanical Vibrations, 2nd Edition”, Singiresu S. Rao, 1990 47

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