Sound performance

Acoustics emissions monitoring is the only tried and tested way of evaluating the condition of critical, slow-moving high-capital plant and machinery, according to engineers at

(CNES). Though rather overlooked, they claim its time may now have arrived.

CNES has been using this non-intrusive monitoring technique at its steelmaking plants for the past 20 years but only made it commercially available to other industries recently.

Acoustic emissions are the high-frequency stress waves generated by the rapid release of strain energy that occurs within a material during crack growth, plastic deformation or phase transformation. Monitoring systems use surface-mounted transducers to detect the waves, which lie within the 25kHz to 1MHz frequency range.

Ian Taylor, business development engineer, plant condition monitoring at CNES, said this type of monitoring is beneficial when there are fluctuating load conditions and where a breakdown would impact production. Still, many companies use vibration monitoring systems to check critical plant machinery. 'Vibration monitoring is fine for high-speed motors, fans and pumps,' said Taylor. 'Anything less than 80rpm, that's our cut-off point for vibration against acoustic — it becomes difficult to interpret with vibration monitoring.'

Acoustic emission monitoring equipment has a high sensitivity to machine faults, but is also immune to audible noise and low-frequency background vibration. Taylor said the reason more companies do not use acoustic emission monitoring systems is because they do not possess the in-house skills to interpret the data it produces.

'You can get more information from vibration testing [on fast-rotating machines],' he said. 'You can tell what the faults are and if it's a problem with the alignment. With acoustic, it might just tell you there is a change in the bearing of the structure, so then you will have to do further investigation.'

Taylor said engineers have to learn to fit a testing technique to a particular application. 'There is not one technique that will fit all applications,' he said, adding that CNES also offers thermal imaging, vibration analysis, balancing and laser alignment, contamination monitoring of hydraulic systems, rotor bar testing, remote visual inspection and debris analysis of lubricants and hydraulic fluids.

Although acoustics testing has been around for almost two decades CNES' method for testing rotating machinery has yet to gain wide popularity.

Taylor said there are few specialist companies in the UK that produce equipment to look at rotational acoustics. 'Most people use acoustics for looking at structures, looking at cracking and things like that,' he said. 'We've taken it one stage further.'

CNES has proved acoustic emissions testing is effective and can reduce plant maintenance costs and improve machine availability, claimed Taylor. It recently helped the Corus Group's Scunthorpe plant save at least five weeks' production time, he said.

The plant uses three basic oxygen steelmaking plant vessels, each weighing 1,300 tonnes, which convert molten iron from the blast furnaces into steel. Each vessel is supported on two, 1.75m diameter spherical roller bearings and can tilt to angles up to 150° from the vertical for charging and discharging purposes. They generate high local contact loads that, over time, generate high residual stresses in the tilt bearings.

'It would take us two years to get another bearing because they have to be made from scratch, so we have to keep an eye on the ones we have,' said Taylor. 'As soon as we see signs of deviation in any of them we had better start ordering new ones because they cost £60,000 to £80,000 a unit.'