structural vibration absorber ppt
TRANSCRIPT
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Structural vibration problem in power station (Yallourn)
Annoying low frequency vibration:
In Chem Lab, glassware would vibrate across bench, crash to floor!
Control room shake - disturbed operators!
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Steel structure 79m tall
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Chem Lab and control room on this level
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Vibration would come and go over 4 minutes
Frequency found
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Where from and Why?
Read that natural frequency (i.e.
resonance) of steel structures typically
6- 10 Hz
Initiating force? Coal mills rotate at 8Hz…
Much of structure is invisible to all but civil
engrs!
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Two vibration velocity transducers were installed on adjacent mills. Output connected To 2-beam CRO
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Vibrations both in-phase: high resulting
vibration
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• 2 minutes later, vibrations were anti-phase: resulting vibration low.
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2 minutes later : Vibrations both in-
phase: high resulting vibration
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Know cause, how to fix?
Balance mills in field
Reduce exciting
force
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Reduce response
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Stiffening the structure
•Can be easy and cheap to experiment
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•Adding mass (300kg) to tune out resonance in vertical pump motor system
Adding mass
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Changing mass or stiffening not suitable here
Vibration absorber?
… convert a system to get two natural frequencies – one above, one below the original
New frequencies must be away from an exciting frequency.
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•Primary system
•Absorber mass
•Spring
•Absorber mass and spring are tuned to natural frequency of primary system
m
kf
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• Calculation method in vibration textbooks
• Absorber mass usually 5% - 25% of primary mass – not realistic here
• Started with experiment in one area
• It WORKED! Vibration amplitude of floor was reduced to much less than problem level
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• Action repeated at selected parts of structure
• Success: absorbers set up in centre of several floor beams under control room.
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•Compression springs
•Any mass will do – scrap mill liners
used
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Problem solved
• Absorbers have been in service for some years without any further problems
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New designs
• New plant extension had completely different steelwork structure
• Mills were to be variable speed, so harder to ensure that vibration problem could be avoided
• So, new mills were mounted on isolating spring supports (specialist supplier) and used later at Loy Yang.
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Sometimes the problem is unusual and
needs deeper investigation