Norbar Case Study
Grinding Mills Structural Critical Fasteners
USING ULTRASONIC MEASUREMENT TECHNOLOGY TO IMPROVE TIGHTENING ACCURACY AND BOLT LIFE
Why flange bolt testing is vital for mill operators:
Since the mid 1990’s mill manufacturers have been using Ultrasonic Bolt Tension Monitoring to ensure that critical flange joints don’t fail. A few disastrous breakdowns and subsequent litigation lead to a drive to eliminate these failures.
The use of the technology has proven to be successful and in the main is a requirement for all new constructions. It is also part of the operating manual to annually re-measure the ongoing bolt tension. This non-invasive technique allows operators to make regular minor corrections. Mostly this happens once per year but some sites do need this more often.
So why is this necessary? In simple terms if it’s not carried out the risk of major breakdown is very high. The images below are from a mill that wasn’t re- monitored for 3 years.
Clearly this flange is at a very high risk of failure. It may need to be stripped down, re-machined and re-assembled. Recorded cases of this have caused 1-3 months out of action.
We have over 32 years’ experience with the technology and 18 years specifically dealing with Critical Mill Flange Bolts. During this time, we have history that shows some typical results, in summary:
1. After initial construction and shortly after commissioning we typically see that around 20% of the bolts have dropped below the OEM’s tolerance band. Early correction is vital.
2. For the first 2-3 years mill components bed in and relaxation of bolts continue to relax. Again, early correction is vital to mill health & reliability.
3. At around the 6-8 year mark, we have seen many bolts developing defects (cracks) just under the bolt heads. Although our technology is primarily designed to measure bolt tension, thanks to its very high definition, we detect defects very early. Long before the joint starts to separate, we replace these defect bolts avoiding major un-planned breakdowns.
We have over 32 years’ experience with the technology and 18 years specifically dealing with Critical Mill Flange Bolts.
More recently we have developed special bolting systems that improve stress concentrations. To date this has solved significant joint failure problems at 2 Australian and one USA mine site. It’s early days to determine the longevity of these but one case has lasted 3 years, another 2 years and the third has only recently been installed.
This design type has also proven successful on the critical slewing ring bearing bolts on Stackers, Re-Claimers and Mobile Plant.
If we were involved in the construction of the grinding mill, we would already have the baseline data and re-monitoring the bolt tension will be a quick and easy task of putting the transducer on the bolt. From there we can carry out any adjustments/re-tightening needed.
If we do not have the baseline data we have a few options, we can carry out crack testing on all critical fasteners to determine if there are any flaws or cracks evident.
We can also loosen each structural bolt one at a time, take an ultrasonic baseline (loose) reading and re-tighten to the correct OEM elongation target. This will ensure all the mill critical fasteners have the required bolt load and will ensure good mill health.
If loosening each fastener is not deemed an option, we can also take ‘as found’ ultrasonic measurements. We can then use this in 6 – 12 months’ time when another ultrasonic measurement is taken and we can compare to obtain if there is any bolt relaxation occurring in that timeframe. The downside to this option is that we do not know the original bolt load of that fastener.
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