dynamic vibration absorber - the seasoned analyst...dynamic vibration absorber distributing energy...
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Dynamic Vibration AbsorberDistributing energy in line with the First Law of Thermodynamics
James SylvesterParsons Peebles Service
THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
History & Background
High lift pumps at the pumping station facility have historically suffered from low reliability and required many expensive overhauls with new bearing sets (Babbitt/White Metal)
Various attempts to resolve the root cause have not been successful
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Bearing Inspection
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Bearing Location
Tilting pad bearing location
Bed Plate – view from underneath
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Bearing Inspection: Oil
Microscopic inspection of the oil, debris is present
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Bearing Inspection: Tilting Pad
The pads vary from no contact to full contact and cracking of the tilting pad, indicating eccentric (non‐orbital) motion
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Bearing Inspection: Tilting Pad
Microscopic image of a pad with no wiping, and an image of a pad with wiping damage
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Bearing Inspection: Tilting Pad
Microscopic image of the cracked pad surface displaying fatigue cracking
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Bearing Inspection: Tilting Pad
Microscopic cross sectional image of the cracked pad
Bond line intact
Failure is at the weakest point, the pivot point
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Vibration Inspection
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Velocity Spectra
Forcing function 1 Order Directional vibration Vibration changes with speed
Dial Number
RPM 1Order Overall
0 662.51 30.25 35
1 679.34 26.94 28.96
2 694.75 20.98 21.22
3 707.68 15.87 18.00
4 721.29 12.77 15.02
5 740.79 11.63 13.69
Horizontal
Vertical
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Adding Mass to the system The addition of weight moved
the resonant frequency down to speed range 2
The weight (additional mass 10Kg) dampened the vibration motion and lowered the resonant frequencies
Conformation of a resonant condition
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Overall Vibration vs Speed: Mass added
• The peak vibration is now occurring at speed set point 2
• As the speed increases the vibration reduces
• This type of vibration and speed relationship indicates a resonant issue
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Vibration vs Speed: Strip Chart
Speed 3 high fluctuation, indicating a critical speed / resonance / beat frequency
Speed 5 lower levels and more constant
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Orbit Data
Raw Data 1 Order Filtered
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Resonance Test ‘Bump Test’
Bump test indicating a structural resonance
Bed Plate
537.58 CPM
650.50 CPM
375 CPM to 860 CPM
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Floor Structure
Bed Plate
False Floor
False Floor
Bed Plate – view from underneath
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Workshop: Amplitude and Phase
One order tracked
Set up on a test bed no load
Accelerometer location on the top of the motor (NDE)
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
On‐Site: Amplitude and Phase
One Order tracked
Set up on foundation under normal operational load
Note how the structure changes the motion
Resonance
Running speed range on site
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Why?
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Motor bearing failures, concentrated at the motor drive end
THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
The ‘Five Whys’
The motor Babbitt (white metal) tilting pad bearings fail
There is metal to metal contact
Overload fatigue ‐ Increased dynamic load on the bearing
High directional vibration causing the coupling and prop shaft to overload the bearing
On site structural resonance amplifying the motion of the motor. This creates highly stressed/torsional load at the drive end of the motor
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Conclusion
There are no issues with the motor rebuild, the balanceand dynamics of the motor when operated unloaded inthe test bay, >4mm/s RMS
A structural resonance on site is causing an amplificationof vibration levels
This resonance increases the dynamic loads on the driveend bearing
This then causes Babbitt fatigue, cracking, and Babbittwiping due to rotor to stator contact
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Visualising the issue
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Side by Side
Standard Video Amplified Video
Higher movement this direction
Horizontal Measuring plane
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Bed Plate
False Floor
Motor mounting frame
Floor in resonance
Bed Plate
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Motor mount
False Floor
Standard Video
Motor mounting frame
Bed Plate
Amplified Video
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Support Steel Work: Modification
A decision to stiffen the structure was made All good in theory but where does the energy go?
2.5m
0.38m
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Support Steel Work: Vibration Results
Stiffening the structure increased the vibration levels to 35mm/s RMS
Usual operating range around 10 mm/s RMS – 14 mm/s RMS
Additional steel work 35mm/s RMS
Still high directional motion
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Dynamic Vibration Absorber
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
What is a DVA?
It is a tuned spring mass system which reduces or eliminatesthe vibration of a harmonically excited system
All rotating machines can induce vibration due to structuralresonances. A dynamic absorber can be tuned to excite insuch a way exactly counteracts the force from the structuralresonance
Properly implemented, a dynamic absorber will neutralise theundesirable vibration, which would otherwise reduce servicelife or cause mechanical damage
Dynamic absorbers differ from tuned mass dampers in thatdynamic absorbers do not require any damping to functionsatisfactorily
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
DVA Design: Dunkerly Method
“a”L
D
hw
LumpedMass M2
Bar propertiesrho, E
Motion
• Bar Cross Section• Length • Free Distance to weight• Bar diameter • Bar width • Bar height • Density • Young's Modulus• Target resonant frequency
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
DVA Site Installation
Safety tethers
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
DVA Site Installation: Video Normal Speed
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
DVA Site Installation: Video
Slow Speed showing thepoint of flex
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Vibration Results: Overall Levels
Structural modifications 35mm/s RMS
Normal operating range After fitment of DVA 6.5mm/s RMS
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Location
Radial
(Deg
.)
Normal
Ope
ratio
nal
Structural
Mod
s
DVA
NDE (TOP)0 10.86 35 5.41
90 8.21 18.17 5.82
DE (BOTTOM)
0 2.41 9.55 1.27
90 2.11 5.92 1.57
THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Vibration Results: Overall With Speed
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Table of Directional Ratio Vibration Data
This table is the difference in the radial vibration velocity in mm/s RMS (Lower the number the less stress to the bearings)
LocationNormal Operational
Structural Modifications
Vibration Dynamic Absorber
NDE (TOP) 2.65 16.83 0.41
DE (BOTTOM) 0.30 3.63 0.30
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
12 Month Inspection
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Follow Up
THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
12 Month Follow up on Trial
The fatigue rate for the main arms of the Dynamic Vibration Absorbers are 11/12 months
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Installation of replacement DVA Arms One order
vibration reduced from 13.7 to 4.5 mm/s RMS after the DVA Arms were replaced
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
DVA: Summary
The Dynamic Vibration Absorbers have been successful inabsorbing the vibration energy from this asset ensuringlow vibration levels and correct dynamic motion
This will enable the asset to run longer in operationbefore maintenance/repair increasing the functionabilityof the pump and reducing the overall operational lifecosts of the asset
The average costs to change the motor bearings is£15,000, if the motor failed the average cost for a fulloverhaul and rotor rewind is £55,000. This excludes thecosts of loss of production and associated financial andreputation costs.
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THE WORLD’S CONDITION MONITORING CONFERENCE | MANCHESTER, UK 2019
Thank you ‐ CM Pearls of Wisdom The most important part of any program is the person performing the data collection and
analysis. The second most important part of any program is the training given to the person
selected. 5 years of experience is not the same as 1 year of experience 5 times. The most important question you can ever ask is “why”. It is important to understand the values of the numbers you are using. Physics of the machine is really important. You can’t analyse what you don’t know or understand. A person may not be stupid, they may just not understand what you are saying. 1 times RPM is not always imbalance. There are no universal vibration severity limits. Absolute amplitude in the frequency domain is relatively useless. Don’t forget the time
domain & phase. There are no ghost frequencies or unknown frequencies but only frequencies not enough
analysed. What was the last maintenance, modification or process change performed on the asset. Don’t ignore the potential benefits of chit chat in the crib/break room with the operators
and maintenance teams. They know their machines! When all else fails, leave the air conditioning, and go examine the operating equipment.
Go look, touch, feel, smell and listen to the machinery
https://theseasonedanalyst.guru/
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