2. Investigation of vibrations at two Pilbara mines
Mr Peter Airey
BE GradDipAdmin FIEAust CPEng RPEQ
Managing Director - Airey Taylor Consulting Engineers & Scientists Chairman – Advanced Substructures Limited

3. Structural investigation of wet screening and scrubbing building

4. Dynamic machine installation/frequencies
2 double deck screens at level 3 : 900 +/- 10% rpm, 13.5Hz considered
2 single deck screens at level 6 : 802 +/- 10% rpm, 12.0Hz considered
2 scrubbers at level 11 – Drive : 1500rpm, 25Hz considered (Max Charge)
Pinion : 281 rpm, 4.68Hz considered (Max Charge) Mill : 15.19rpm, 0.25Hz considered (Max Charge)

5. Working scenario analysis required
50% of plant is in operation while other equipment is maintained. Thus one set of product line (Scrubber, Single Deck Screen, Double Deck Screen) is in operation while other set is removed.
All 6 machines may start together (in Phase) or with some time lag (Out of Phase) – different scenarios are thus feasible
During pre-commissioning phase machines and static equipment are in place
Since majority of machines operate in 13-15Hz range, the less the weight of the structure-machine system, the greater the peak velocities will be

6. Three analysis models were generated
Operational – machine lower operating frequency with operational mass source (0.9 dead load + 0 live load material load)
Commissioning – full machine frequency range to upper operating frequency with commissioning mass source (0.9 dead load live load + 0 material load)
Seismic – machine lower operating frequency with seismic mass source (1 dead load live load material load)
The measured damping ratio of 3.3% adopted instead of advised damping ratio of 2% by “Civil Design Criteria”

7. Measurements confirmed analysis prediction
Many of the locations have a vibration velocity in excess of 5 mm/sec
This is above the Design Criteria value

8. Structural natural frequency is close to forcing frequency of main equipment
Velocities are then magnified due to resonance – in accordance with Civil Design Criteria, the ratio of forcing frequency to resonant frequencies of structure must be 0.5 to Small periodic driving forces have the ability to produce large amplitude oscillations. This is because the system stores vibrational energy.

9. The structure is Low tuned
Natural frequency of the structure < operating frequency of main machines Low tuned structures should be avoided Resonance can occur momentarily during start-up - for longer periods during shut down, or if equipment operating below normal or peak speed
Direction/Velocity
Min. Site Measure
Max. Site Measure
Max. Prediction
Vertical
-5.4 mm/s
5.7 mm/s
5.8 mm/s
Horizontal
-4.9 mm/s
4.9 mm/s
4.6 mm/s

10. Conclusion
Vibration problem results from incorrect design philosophy and geometry
If a rigid concrete structure had been used up to level 3 (double deck screen supporting level) or level 9 (single decks screen supporting level) – almost none of the defects would exist from both operating and start-up/shut-down conditions – structure is entirely composed of steel

11. Conclusion
In Crusher Building, vibration mainly from Primary, then Secondary Crusher
Actual installation includes :
Run of Mill Bin
Rock Breaker
Primary Sizer Crusher
Apron Feeder
Apron Feeder Feed Chute
Dribble Chute
Primary Sizer Feed Chute
Primary Sizer Travelling Chute
Secondary Sizer Discharge Chute
Secondary Sizer Maintenance Overhead Chute
Structure responds to their activity by vibrating

12. Conclusion
The Primary Crusher (165 t.) receives feed from raw ore bins Feed allegedly limited to 3 tonnes - Control limited, leads to “marbling”
Marbling : Flicking tonne lumps of ore into air via crusher prongs Owner object : restriction of vibration to less than 5mm per second not met
Vipac : measured vibration acceleration during “marbling” - 12 G’s

13. Conclusion
The greater mass as achieved by a concrete, rather than steel, structure is a far better design solution for the accommodation of Primary and Secondary Crusher – with the subordinate associated structure of steel