Structural vibration absorber PPt

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!

Steel structure 79m tall

Chem Lab and control room on this level

Vibration would come and go over 4 minutes

Frequency found

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!

Two vibration velocity transducers were installed on adjacent mills. Output connected To 2-beam CRO

Vibrations both in-phase: high resulting

vibration

• 2 minutes later, vibrations were anti-phase: resulting vibration low.

2 minutes later : Vibrations both in-

phase: high resulting vibration

Know cause, how to fix?

Balance mills in field

Reduce exciting

force

Reduce response

Stiffening the structure

•Can be easy and cheap to experiment

•Adding mass (300kg) to tune out resonance in vertical pump motor system

Adding mass

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.

•Primary system

•Absorber mass

•Spring

•Absorber mass and spring are tuned to natural frequency of primary system

m

kf

• 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

• Action repeated at selected parts of structure

• Success: absorbers set up in centre of several floor beams under control room.

•Compression springs

•Any mass will do – scrap mill liners

used

Problem solved

• Absorbers have been in service for some years without any further problems

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.

Sometimes the problem is unusual and

needs deeper investigation