ODS & Modal Case Histories

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

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 – HIGH VIBRATION AT REFINER Equipment description Problem description Comparative analysis of similar machines Route data results & determination of “offending frequencies” ODS analysis of refiner & motor Modal analysis of motor Conclusions & Recommendations CASE HISTORY#2 – ACCEPTANCE TESTING OF AHU FAN Equipment description Problem description Route data, coastdown data & determination of “offending frequencies” Modal analysis of fan, motor & base Conclusions & recommendations CASE HISTORY#3 – ACCEPTANCE TESTING OF WATER PUMP A) Equipment description 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

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”).

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.

5. Vibration Spectra .vs. Modal Data PLOT 1: Vibration data measured during normal operation. Dominant vibration at 1,789 cpm or 1x RPM of machine (“offending frequency”). PLOT 2: Modal data measured while machine down. Note the strong response at 1,837 cpm which is near 1x RPM.

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.

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).

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).

9. Case History#1: High Refiner VibrationEquipment & Problem Description • Wood products refiner, overhung rotor design driven by a 3,000 HP, 4-pole induction motor (~ 1,790 rpm). • Machine mounted to a rigid base resting on a concrete pedestal. • Both refiner & motor run in anti-friction bearings. • Refiner has 12-ea plates. • Customer complaining of high machine vibration and repeat motor outboard bearing failures. • Three identical machines at plant site running at identical speeds under similar loads.

10. Comparative Analysis Of Refiners, Overall Vibration PLOT 3: Note how overall levels at the #1 refiner are consistently higher than the others.

11. Comparative Analysis Of Refiners, Waveform Levels PLOT 4: Compare waveform levels among all refiners. Note how levels at the #1 motor, outboard are much higher than the others. Looseness at the motor outboard bearing was suspected due to impacting in the waveform and a large number of harmonics in the spectra.

12. Conditional Analysis, #1 Motor • On 5/1/08 the #1 motor was run uncoupled, coupled & unloaded, and coupled & loaded. Vibration data was collected at each step and differences & changes were noted. • First, radial vibration levels rose dramatically throughout the motor when it was coupled to the refiner versus uncoupled. This was strong evidence for a large part of the vibration problem originating at the refiner or at the coupling. • Second, axial or thrust vibration levels at the motor actually dropped when the motor was coupled to the refiner. This was strong evidence against the presence of either coupling misalignment, coupling problems, or bent shafts. • Third, vibration levels generally dropped when the motor was loaded. • Fourth, no significant beating vibration was noticed at the motor when it was loaded up. This was noteworthy as beating vibration is usually seen when common motor rotor problems such as an eccentric rotor or cracked rotor bars are present. PLOT 5: Conditional analysis of #1 motor, 5/1/08 data. Black = Uncoupled, Red = Coupled & Unloaded, Yellow = Coupled & Loaded

13. Conditional Analysis, #1 Refiner • As mentioned previously, on 5/1/08 the #1 motor was run uncoupled, coupled & unloaded, and coupled & loaded. Vibration data was collected at the motor & refiner with each step and differences & changes were noted. • Changes in vibration levels at the #1 refiner were also noticed as follows. • First, overall vibration levels were highest at the #1 refiner when it was unloaded. • Second, the highest vibration levels occurred at the refiner outboard bearing in the radial directions (ie: horizontal & vertical directions). PLOT 6: Conditional analysis, #1 refiner, 5/1/08. Red= Unloaded, Yellow= Loaded.

14. Spectral Analysis, Part 1 • Observations from comparing motor, outboard, axial spectra from all three motors shows peaks at the #1 motor are much larger than those seen at the other two. • Second, peaks seen at the #1 motor are multiples of the motor speed of 1,795 cpm. The large number of multiples occurring here at great amplitude suggests looseness of some type is present at the motor outboard bearing. This looseness could be either between bearing & housing or bearing & shaft. • The three dominant frequencies of vibration seen here are 1,800, 3,600 & 7,200 cpm (1x, 2x & 4x rpm). PLOT 7: Compare motor, outboard, axial spectra from all three refiners. Note how #1 motor (top) has much higher vibration levels at all offending frequencies. Many multiples of running speed.

15. Spectral Analysis, Part 2 • Compare vibration spectra from all three refiners at the refiner, outboard, horizontal measurement. • First, the dominant frequency of vibration at all three refiners is the running speed of 1,795 cpm, not at 3,580 cpm, or 7,180 cpm as in the motor. • Second, both the overall vibration and the vibration at 1x rpm are highest at the #1 refiner. • If unbalance were present at the #1 refiner, it would appear as high radial vibration at 1x rpm (1,795 cpm) - this is exactly what is occurring here. PLOT 8: Compare vibration spectra at refiner, outboard, horizontal among all three refiners.

16. Spectral Analysis, Part 3 • Coupled versus uncoupled data were compared at the motor, inboard, axial measurement and changes were noted. • First, vibration at 1x rpm dropped from 0.230 to 0.004 ips-pk (98% drop) when the motor was uncoupled. Thus, the overwhelming source of vibration at 1x rpm vibration must be coming either from the refiner or due to being coupled to the refiner. It cannot be in the motor itself. • Remaining lower vibration at 2x rpm & 4x rpm are at least in part related to 1x & 2x electrical line frequency. PLOT 9: Compare coupled (top) to uncoupled (bottom) vibration data at the motor, inboard, axial measurement.

17. Waveform Analysis • Vibration waveform data from the motor, outboard were compared with that at the refiner, outboard and differences were noted. • First, the motor, outboard waveform is more impact-like in nature when compared to the refiner, outboard waveform. Impact-like waveforms are expected when looseness is present. • Second, the dominant period of vibration seen at the refiner, outboard waveform is at the running speed of 1,795 cpm. This would be expected if the source of vibration was unbalance. • Third, some clipping of the refiner, outboard waveform was seen. PLOT 10: Compare waveform data from motor, outboard, axial (top) to refiner, outboard, horizontal (bottom).

18. 1X Peak/Phase Analysis • With the discovery of high 1x rpm vibration levels throughout the machine, it was decided to collect 1x peak & phase data. Observations & conclusions from this data are presented below: • Both axial & radial phase relationships across the coupling (ie: compare MIH to RIH and compare MIA to RIA, etc) are nowhere near 180 degrees. This fact argues against a bent shaft or significant alignment or coupling problems here. • The refiner axial to axial relationship is near 0 degrees which is what we expect for an overhung rotor out of balance. • The refiner radial relationships are also nowhere near 180 degrees. • The horizontal to vertical phase relationships at each bearing are near 90 degrees except at the motor outboard where looseness is suspected. TABLE 1: 1x Peak/Phase Data from motor & refiner

19. ODS Analysis Of #1 Refiner1,780 CPM Motion, Part 1 • A model of the motor, refiner, baseplate, pedestal & floor was created and ODS vibration data was collected and imposed on the model. Observations from animations of the model at the running speed of ~ 1,780 cpm follow: • Excessive horizontal, in-phase swaying of motor and its pedestal. • Excessive circular-like motion of the refiner, outboard bearing. This motion is in keeping of what would be expected with unbalance at the refiner. • Much more motion of both the machine pedestal & surrounding floor than expected. This was strong evidence that the machine’s foundation & surrounding floor were soft and not firmly fastened to the ground. We have a very flexible base supporting this machine.

20. ODS Analysis Of #1 Refiner1,780 CPM Motion, Part 2

21. ODS Analysis Of #1 Refiner1,780 CPM Motion, Part 3 One of the refiner outboard feet appears loose or soft relative to the others.

22. ODS Analysis Of #1 Refiner3,580 CPM Motion, Part 1 • Machine motion at 3,580 cpm shows continued signs of softness & flexibility at the foundation & floor. • Machine motion at 3,580 cpm shows some thrusting at the motor bearings.

23. ODS Analysis Of #1 Refiner3,580 CPM Motion, Part 2

24. ODS Analysis Of #1 Refiner7,180 CPM Motion, Part 1 • Machine motion at 7,180 cpm shows severe thrusting at the motor bearings. This type of thrusting at the motor isn’t normal and prompted the motor modal measurements made later. • Motion of the pedestal & surrounding floor is relatively low at 7,180 cpm.

25. ODS Analysis Of #1 Refiner7,180 CPM Motion, Part 2

26. Modal Analysis Of #1 Motor, 124 Hz 3D View From Motor Outboard 3D View From Motor Inboard Is the frame of your induction motor resonant to 120 Hz (2x line frequency) in the thrust/axial direction?

27. Modal Analysis Of #1 Motor, 138 Hz 3D View From Motor Outboard 3D View From Motor Inboard Is the frame of your induction motor resonant to 120 Hz (2x line frequency) in the thrust/axial direction?

28. Modal Analysis, #1 Motor, Conclusions • At least two natural frequencies exist which are within 20% of 120 Hz (2x line frequency): 124 & 138 Hz. • Both modes appear “dish or bowl-like” with the center thrusting out and relatively little motion at the corners or perimeter of the motor frame. • Simple modal measurements were also taken on the nearby #2 motor with similar results leaving me to conclude this is a possible design flaw in the motor frame.

29. CONCLUSIONS & RECOMMENDATIONS: CASE HISTORY#1 - HIGH VIBRATION AT REFINER

30. Case History#2: Acceptance Testing Of AHU FanEquipment & 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. 4-ea Floor Isolators 2-ea Wall Isolators Fan Motor

31. 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.

32. INITIAL DATA & FINDINGS, PART 2 PLOT 11: Plot of overall vibration levels at all measurement points at full speed.

33. SPECTRAL DATA AT FULL SPEED PLOT 12: Spectral data from points of high vibration at full speed (MOH, MOV, FIH & FOH). Dominant vibration in all spectra occurs at top fan speed of 1,987 cpm or 33.1 Hz.

34. INTERFERENCE DATA (MOTOR & FAN SPEEDS) TABLE 3: Interference data table. Motor & fan forcing frequencies .vs. suspected natural frequencies.

35. INTERFERENCE DIAGRAM PLOT 13: Interference diagram of fan & motor speeds .vs. suspected natural frequencies.

36. COASTDOWN DATA, BODE PLOTS PLOT 14: Coastdown data at fan, inboard, horizontal (FIH) position in Bode format shows suspected natural frequency at approximately 1,900 cpm (31.667 Hz). The highest vibration level on the fan was measured at this point at 1,903 rpm at 3.11 ips-pk!! PLOT 15: Coastdown data at fan, outboard, horizontal (FOH) position in Bode format shows suspected natural frequency at approximately 1,500 cpm (25 Hz). The highest vibration level measured at this point occurred at 1,495 rpm at 2.60 ips-pk!!

37. 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. PLOT 16: Simple CAD Model of AHU fan.

38. MODAL ANALYSIS – 26.1 Hz Mode PLOT 17: Modal animation at 26.1 Hz of AHU fan & motor inboard. Note distortion of machine frame near motor. PLOT 18: Modal animation at 26.1 Hz of AHU fan & motor outboard. Note distortion of machine frame near motor.

39. MODAL ANALYSIS – 31.1 Hz Mode PLOT 19: Modal animation at 31.1 Hz of AHU fan & motor inboard. Note distortion of machine frame near motor. PLOT 20: Modal animation at 31.1 Hz of AHU fan & motor outboard. Note distortion of machine frame near motor.

40. 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 a ½” 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.

41. CASE HISTORY#3 - ACCEPTANCE TESTING OF HIGH PRESSURE WATER PUMPEquipment & 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. Initial state of newly installed water pump. What is wrong with this design & layout?

42. 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.

43. 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).

44. BASELINE ODS 9/18/08 – MOTION @ 1xRPM (3,590 cpm) • 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.

45. BASELINE ODS 9/18/08 – MOTION @ 2xRPM (7,180 cpm) • Note vertical movement of entire pedestal & surrounding floor at this frequency (120 Hz) again suggesting significant problems exist with this machine foundation.

46. BASELINE ODS 9/18/08 – MOTION @ 4xRPM (14,400 cpm) • 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.

47. BASELINE ODS 9/18/08 – MOTION @ 8xRPM (28,800 cpm) • Note continued thrusting of pump & pump suction at this frequency. • Note relatively little motion of the motor or pedestal at this frequency.

48. 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.

49. PUMP & PIPING CONFIGURATION 10/2/08 • The suction piping was modified per 9/18/08 suggestions. • The alignment was checked & reportedly corrected to mill 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.

50. 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.