1. Vibration in SAG Mill Foundation of Stream 3 - Chronological Sequence & An Update

2. SAG Mill of Stream 3 Make: CITIC Heavy Machinery Company, China Physical dimensions  Length: 3.35 Mtr  Diameter : 6.1 Mtr  Design RPM -10.5-14.0 (60 % -80 % of critical speed)  Motor kW – 2200 kW  Design charge : Ball charge -7-12 % Total charge – 25-40 % (66+62 =128 MT) Initially the Mill was operated between 12.5-14.0 RPM with more water to avoid jamming of feed chute Feed chute was replaced in April’09. Since then Mill is running between 9.5-11.0 RPM till March’10 Presently there is a perceptible vibration if the speed is increased beyond 8.50 ( at 586 RPM corresponding to 60 % of Motor RPM)

3. Vibration Problem of SAG Mill – Chronology Since beginning there was a perceptible vibration on SAG Mill floor Major changes in magnitude starts from Nov’09. Substantial increase in vibration amplitude (Displacement & velocity) in all points ( from motor NDE to pinion bearings). Change in amplitude observed only in Horizontal direction. From Nov’09 to Feb’10 there was continuous increase in vibration level. It reaches in the range of 7 to 9 mm/sec from 1 to 3 mm/sec. BET representative from South Africa visited site during Feb’10 shutdown, measured driveline alignment & gear backlash & root clearance. Visit concluded that there is no mechanical fault with the mill or the alignment of the driveline. During March’10 vibration level reaches in the range of 16 to 18mm/sec. Which is defined as Critical (>11.2 mm/sec) as per ISO 10816 Standard. Also vibration level varies with Mill RPM. There is an appreciable change in vibration level if the mill speed exceeds 8.5 RPM

4. Trend of SAG Mill Vibration

5. Vibration at SAG Mill – Outcome of Investigations SPM, Hyderabad – carried out vibration analysis of all dynamic components from Motor to Mill trunion of SAG Mill. Their report suggests that :  Low frequency peak of 525 CPM (cycles per minute) was observed at all locations which is not coinciding with natural frequency of any of the rotating component. This rules out possibility of sources of vibration from any of the rotating component  The RCC Structures has to be checked for any internal cracks generated over a period of time. Smart NDT system, Vadodara : It carried out R C C Analysis consisting of following  Rebound Hammer test. (for surface quality of concrete.)  Ultrasonic Pulse Velocity Test. (for inner quality and integrity of concrete)  Pull (CAPO) test. ( for in situ compressive strength near to the surface) Their report suggests that  The surface hardness as measured by rebound Hammer test on all surfaces varies from 30 to 40 Mpa. This indicates a good potential strength of concrete as against design value of 25 Mpa.  Strength of concrete is further confirmed by Pull out (CAPO) test results.  Stability of foundation may be an issue. Stability is affected by horizontal displacement caused by high frequency vibration  Dynamic analysis of existing mass of concrete is suggested to avoid resonance in concrete in view of the increased vibrations.

6. Vibration at SAG Mill – Outcome of Investigations( Contd.) ACME, Hyderabad : It carried out dynamic & Finite Element Analysis (FEA). Prof Ramamurti, Ex Prof – IIT- Chennai has visited the site & collected data which was later analyzed through FE Modelling. Findings of ACME are:  Excitation frequency of vibration is very close to the running speed of the motor. The drive unit support experiences the same excitation frequency.  Natural frequency of Drive unit (7.3 cps )& Mill foundation(2.9 cps) is within the excitation frequency (16.5 cps corresponding to 100 % Motor RPM). This is a case of resonance.  In order to eliminate near resonanent condition, fundamental natural frequency of these three foundations are to be increased to values more than 20% of excitation frequency (16.3 cps).  Accordingly various options of re-balancing of foundation was analyzed through FE modelling.  H Z L engineers visited Chennai to analyze different options & for detail drawing of final solution  The final option selected on the basis of field conditions involves enveloping RCC foundation of drive unit with 25 mm thick MS plate with suitable anchoring & enveloping RCC foundation of driven unit with 25/50 mm thick MS plate with suitable anchoring

7. Present Status In the first phase, enveloping of drive unit RCC foundation by 25 mm steel plate is completed. M/S ACME has visited site on 12.06.10 & again vibration analysis has been done. Observations are as follows –  No change in vibration amplitude all along drive unit.  Vibration level at Mill Trunion pedestals have increased substantially & reach upto 35mm/sec & displacement in the tune of 2000 micron at a Motor RPM of 580.  A new dimension has been added in the total problem which is observed for the first time. From Nov’09 till the strengthening of driveline foundation, at all points dominant peaks were observed at 0.8X (i.e 80% of Motor RPM) on frequency plot, but now dual peak at 0.8 X and 1X are started appearing at all point from Motor to Pinion end with “DOMINANT PEAK AT 1X COMPONENT”.  While at driven end Trunion pedestals, “DOMINANT PEAK STILL AT 0.8X COMPONENT”.  Amplitude of 0.8X component all along the drive line foundation is reduced by around 40% from earlier level, but appearance of 1X dominant peak, overall amplitude of vibration not being improved. Looking to the dominance of 1X component, it was decided to replace the Motor so that possibility of prime source of unbalance force can be eliminated. In addition, driveline alignment & tightening of all foundation bolts were checked. (A)In alignment part, “ PINION SHAFT PART IS FOUND 0.8MM DOWN VERTICALLY WHERE AS READING WAS NOT DISTURBED AT ALL IN ACROSS”. Same was corrected & keep the alignment within +0.25 mm Important observation here is, on 24.04.10 same was observed vertically down by 0.5mm & was already corrected. (B)All six foundation bolts of Gearbox (between Gearbox sole plate & RC) got tightened by one round but there is absolutely no change in alignment.

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