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

Structural investigation of wet screening and scrubbing building

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)

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

Three analysis models were generated Operational – machine lower operating frequency with operational mass source (0.9 dead load + 0 live load + 0.3 material load) Commissioning – full machine frequency range to upper operating frequency with commissioning mass source (0.9 dead load + 0.3 live load + 0 material load) Seismic – machine lower operating frequency with seismic mass source (1 dead load + 0.3 live load + 0.6 material load) The measured damping ratio of 3.3% adopted instead of advised damping ratio of 2% by “Civil Design Criteria”

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

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 1.5. Small periodic driving forces have the ability to produce large amplitude oscillations. This is because the system stores vibrational energy.

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

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

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

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 3 - 5 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

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