NDT keeps plant on line

Engineers could be able to test the integrity of components and plant that operate at high temperatures without shutting down operation, using new monitoring technology developed by a British research team.

The joint effort between Warwick University and University of the West of Scotland has the potential to save millions of pounds for the oil refining, power generation and petrochemical industries.

The academic team, supported by industrial partners, including BP, RWE nPower and Shell, will produce preliminary results when its EPSRC-funded study ends in 2011.

Current non-destructive testing is carried out at lower or ambient temperatures, which means plant — including turbines, high-temperature processing pipework and boilers — must be at least partially shut down to be examined.

Steve Dixon, the principal investigator from Warwick, said conventional ultrasonic testing involves placing a transducer on the surface of the plant; this transducer then sends short ultrasonic pulse-waves through the material. The intensity of the returning waves and the time it takes for them to return give engineers enough information to determine imperfections within the object. The technique can also be used to determine the thickness of an object, meaning it can also monitor pipework corrosion.

The problem, Dixon added, is that these ultrasonic tests usually rely on transducers made of piezoelectric material that operates only at relatively low temperatures.

'The standard material that people use is lead zirconium titanate,' he said. 'It's a highly efficient, fairly inexpensive material, but it can't operate much above 100^oC without suffering degradation.'

Oil refining, power generation and petrochemical industrial plant operate at temperatures sometimes reaching 800^oC. Dixon said the transducers, which are bonded to the test material, can lose performance and contact at high temperatures. 'So even if you identify a material that can survive and do functions at high temperatures, making sure that it stays well bonded to the sample is a challenge in itself, because the temperature could go from ambient temperature to more than 500^oC,' he said. 'Now if you have a transducer that is able to operate at high temperatures reliably and you can leave it in position, then that means you don't have to wait for shut-down periods to test the plant.'

The Warwick and West of Scotland team is meeting the challenge with a two-pronged approach. The first is to develop a transducer material that performs at high temperatures and stays well bonded. The second is to develop non-contact methods of generating and detecting ultrasound in the sample.

Dixon said one option for a transducer material is aluminium nitride. This material could be directly deposited as thin films on the surface of turbines, pipework, boilers, reactors and other areas of plant that need monitoring.

'These thin-film transducers are actually very hard wearing and because they're thin they will be less prone to damage during thermocycling,' Dixon explained.

He said the group will look at ways of depositing the transducer material so it can be retrofitted onto existing plant or installed on new sections. He suggested the group may try methods such as sol-gel, a wet-chemical technique used to deposit thin films on substrate.

In terms of non-contact methods, the research group will look into the use of pulse lasers for generating ultrasonic waves. Dixon suggested this could be accomplished by installing a catheter-like device into the pipe so that a fibre optic could shoot a laser beam onto the sample surface.

Other non-contact devices could use magnetic fields to detect or generate ultrasound in a sample without touching it.

While the Warwick and West of Scotland team are looking into both contact and non-contact methods of ultrasonic generation and detection, Dixon does not believe either technique alone will be the solution.

'The thin-film transducers are very sensitive detectors but they don't make great ultrasonic generators,' he added. 'So we might consider using a non-contact method to do the generation and one of these thin-film approaches to do the detection.'

Current industrial partners include Tenaris, an international steel-tube manufacturer, and major players in oil refining, power generation and petrochemical industries, but Dixon sees applications for the technology extending beyond these areas.

'This could potentially be very useful for the nuclear power industries,' he said. 'You've got an extra level of safety and confidence you want to attain there so the idea of being able to have inspection technology in situ is a real benefit.

'Although fossil-fuel power stations are the immediate problem we're going to address,' he added, 'the applications of this technology go beyond the current industrial collaborators.'

Siobhan Wagner