Acoustic emission: a powerful non-destructive tool

Acoustic emission: a powerful non-destructive tool

By Paul Grad, engineering writer

A highly sensitive non-destructive testing technique called acoustic emission has proven successful in detecting and locating defects or discontinuities in power station assets across Australia, improving and lowering the costs of maintenance.

The acoustic emission (AE) technique has been applied to monitor hot and cold piping at Loy Yang Power’s Loy Yang A power station, in an ongoing project which started in 2007 and has been planned to 2047.

Loy Yang A, located near the city of Traralgon, in south-eastern Victoria, has four generating units with a combined capacity of 2200MW. The technique has also been used to monitor high-pressure piping, and testing of tanks for corrosion, at AGL in South Australia. It has also been applied locally to test for partial discharge within oil-filled transformers, allowing for early detection of insulation breakdown.

AE is the phenomenon of radiation of acoustic waves in solids when a material is subjected to stresses such as crack formation, slip and dislocation, melting, twinning, temperature or pressure gradients, mechanical loading, corrosion or phase changes. AE can be detected in frequencies ranging from 1kHz up to 100MHz, but is most commonly detected within the range of 1kHz to 1MHz.

Detection and analysis of AE signals can provide valuable information on the origin and importance of a discontinuity in a material, in industrial applications such as assessing the structural integrity of various types of equipment, detecting flaws, testing for liquid or gas leaks, or monitoring weld quality.

Equipment where AE testing can be valuable include storage tanks, fibreglass reinforced plastic tanks, large pressure vessels, pipelines, spherical tanks, and power transformers.

There are a lot of advantages of AE over conventional non-destructive testing, including: the ability to monitor equipment in real time, significant cost reduction because scaffolding and stripping of lagging are not required, time reduction over conventional non-destructive testing, high sensitivity, the possibility of fairly accurate defect localisation, the ability to cover areas of difficult access, identification of areas with potential defects for follow up inspection, the possibility of periodic condition assessment, and the ability of combining with other non-destructive testing techniques.

AE also presents limitations, however. It is possible for flaws to go undetected if the loading on the equipment under test is not high enough to produce an acoustic event. Systems can only qualitatively indicate how much damage is contained in a structure. Other non-destructive testing methods are necessary to obtain quantitative information on a structural flaw or defect.

Another drawback of AE is its sensitivity to outside sources of noise, as in a loud serviced environment.

To detect the AE signals, sensors such as accelerometers are placed onto the structure, typically at a maximum of 5m intervals – 2.5m in high noise environments – to pick up frequencies usually between 30kHz and 300kHz. Frequencies measured are typically 30kHz for tank bottom monitoring, 60kHz for fibre composites, 150kHz for metallic structures, and 300kHz for high-pressure piping. The acoustic waves generated by defects or discontinuities stimulates piezoelectric sensors mounted – either directly to the equipment’s surface, or using waveguides – which convert the energy in the waves into a voltage, which is then amplified and sent to a digital processing unit.

AE testing should be performed across a range of operating conditions. It should be carried out during plant start-up, from ambient pressure and temperature conditions; during plant cool-down; during normal changes in power load changes; and during the plant’s full power output, when temperature and pressure variations are kept to a minimum.

The first trial of acoustic emission monitoring of piping in Australia was carried out for Loy Yang Power’s Loy Yang A, which was chosen for the trial because it has Australia’s largest boilers. The project was carried out via the Australian Power Technology Group, consisting of 10 power companies organised by WTIA (Welding Technology Institute of Australia). Group members were AGL Torrens Island, Delta Electricity Australia, Eraring Energy, Loy Yang Power, Stanwell Corporation, Tarong Energy Corporation, Verve Energy, Loy Yang B, and NRG Gladstone.

Physical Acoustic Ltd of Cambridge, UK, was chosen as the technology provider due to the company’s extensive experience in AE testing of thermal power stations. Physical Acoustics transferred its capabilities to Australian company HRL Technology, of Morwell, Victoria. Alinta Energy, of Melbourne, and Attar Pvt Ltd (Advanced Technology Testing and Research), of Melbourne, also participated in the project.

A total of 52 welds were covered. According to the project participants, traditional inspection costs almost five times as much per weld as the AE testing, although it should be borne in mind the project focused on easy to access pipe locations, whereas the traditional inspection includes difficult to access locations. According to the project participants, the project was not only cost-effective but also technically successful and the technology can be used across all coal-fired power stations in Australia.

HRL Technology is now offering consulting services for applying the AE technology across Australia.

Loy Yang Power has major maintenance outages every six years and these will be carried out in the years to 2047. Loy Yang Power uses a targeted inspection program based on information from stress analysis, engineering assessment and metallurgical reports. As the plant ages, risk grows and the planned number of inspections increases. According to the project participants, there are significant cost benefits in the use of AE.

AE monitoring offers extended coverage over a larger quantity of pipe at the same cost as traditional inspections. Therefore, the likelihood of detecting a defect increases, and the risk of future failures decreases.

The technology has been used for a much longer time in some other countries. In the US, a meeting that led to the Acoustic Emission Working Group (AEWG) was held in Alcoa, Tennessee, in November, 1967. The first meeting of the AEWG was held in Idaho Falls in February 1968. The 57th AEWG Conference will be held in Chicago, on May 13-15, this year.

The Latin American Acoustic Emission Group was formed in Buenos Aires, Argentina, in April 1997. The European Working Group on Acoustic Emission will hold its 32nd European Conference on Acoustic Emission Testing in Prague, Czech Republic, on September 7-9, 2016.

The US Journal of Acoustic Emission has been published for some 30 years. The AEWG endorses the journal, but the journal is not part of the AEWG. Its editorial board includes members from Australia, the US, UK, France, Germany, Greece, Japan, Austria, Belgium, Canada, Korea, Czech Republic, India, and Poland.

Though Australia has been a bit late in adopting AE, the technology has been quite successfully applied here and will now be an important part of the operation and maintenance of power station and power assets.