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Testing the waters

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Mining operations rely on the continued safe operation of vehicles and machinery to maintain profitability.

Any equipment failure can be expensive in terms of lost production and cost of repairs. An estimate for the cost of lost production for a single dragline is $8000 per hour, making any breakdown costly when it might take weeks for a replacement part to be available.

One way to minimise losses is to continually monitor plant and machinery using non-destructive testing (NDT) methods to ascertain component health, and monitor mechanisms for damage.

Phased array ultrasound can enhance day to day inspection of surface mining equipment, improving safety and keeping critical equipment operational.

However, it is important that inspections are performed with minimal down time, so need to be planned and co-ordinated to ensure that critical parts are available when the maintenance is carried out.

As part of the technical programme at the recent Australian Institute for Non-Destructive Testing (AINDT) Conference in Brisbane, Nicholas Bublitz, a global product support specialist with Olympus Scientific Solutions America (OSSA), explained how phased array ultrasonics could improve mine profitability.

NDT is a group of techniques used to detect discontinuities in materials or components without causing damage or permanently altering the article being inspected.

The AINDT Conference provided opportunities for NDT and maintenance professionals to meet and discuss the latest advances in NDT from around the world. Leading corporate members of the Institute displayed and demonstrated some of their innovative, precision analysis instrumentation and applications supporting NDT.

"NDT methods like ultrasonics, eddy current and radiography - along with condition monitoring techniques such as oil and vibration analysis - play an important part in predictive maintenance programs and help reduce unexpected expenses," Bublitz said.

"Outage and emergency repair costs can be a significant percentage of the total operating cost of a mine. Finding a crack and repairing it before failure, or monitoring it until it needs replacement and ensuring that parts are ordered to arrive in time for the maintenance work, saves both time and money."

Conventional ultrasonic testing methods use a single element transducer that produces a beam either at a fixed angle or perpendicular to the surface.

This method is sufficient for many general weld and component tests with unobstructed access and relatively simple geometry to allow probe movement.

Phased array ultrasonics began with the development of transducers with multiple elements that can be individually excited but work as a group.

The sound waves from each element join to form a dominant wave, the direction of which is controlled by the firing sequence and timing.

In this way it is possible to electronically sweep through a range of angles and display the results as a composite picture--this is known as phased array and is a similar technique to ultrasound used in medicine.

Since being introduced in the 1990s, the cost of phased array inspection tools has steadily decreased making the technique accessible to more companies and industrial sectors.

As technology has advanced, the capabilities of the equipment have increased, while set-up and operation has been dramatically simplified.

Machinery used to access, remove and haul away coal and other mineral resources at mine sites is often used during every shift and is under constant operational stress due to the sheer weight or the rotation and vibration of components and engines.

Draglines and shovels are two key pieces of operating equipment where there are many beneficial applications for phased array inspection and analysis.

There are many shafts, pins, bores and lugs on these machines, all of which present excellent opportunities to benefit from phased array inspection.

In addition, the benefits of phased array in weld inspection are well documented and are equally applicable in mining applications.

One example Bublitz presented at the AINDT conference was the checking of dragline swing shafts, which are often long with variable outside diameters. Cracks commonly occur at the transition or taper down areas.

Often two or more ultrasonic angles are chosen and multiple manual conventional ultrasonic inspections are performed where the probe can be applied to the shaft. He showed that the application of phased array simplifies the inspection, and provides better identification and sizing.

Phased array imaging helps identify geometric landmarks to help define the location and importance of any discontinuities.

The overall probability of detection is greatly increased using the comprehensive visual representation.

Data capture even allows for further off-line analysis.

The result is more dependable, accurate and faster assessment of potential failure sites.

The benefits of phased array have been proven by more than twenty years of use in other industries and can now be shown to benefit typical mining applications.

"The use of multiple angles and imaging increases coverage, reduces scanning times, and provides better detection and characterisation capabilities especially when monitoring crack growth," Bublitz stated.

"Phased array can add a huge benefit to mine predictive maintenance programs."  

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