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Acoustic Emission Testing (AE)
Acoustic Emissions are sound waves produces by movement within the material due to stress experienced by them. Emissions from the material could arise from a multitude of sources including flaw growth and plastic deformation.
For pressure vessels, this method involves pressurizing the vessel with a medium, usually a gas, typically Nitrogen, and observing the vessel for acoustic emission through 2 or more piezo electric sensors (transducers) which are connected to an acquisition hardware that filters & amplifies signals. These signals are transmitted to a computer system that converts analog signals to digital signals and displays them on the monitor. The transducer bandwidth and its characteristics are carefully selected based on the frequency spectrum of the source of interest which is dependent on many variables including the thickness of the material. The frequency bandwidth and the type of filtration determine the spectrum of noise and emissions collected by the test. There are 2 different approaches to this test method as detailed below.
Conventional Acoustic Emission test system, the more commonly used variant of this test method employs resonant sensors which are coupled to a computer that runs complex algorithm and displays statistical data in the form of charts & graphs (plots). Resonant sensors are highly sensitive to a very narrow band of frequencies. Frequencies outside this bandwidth mostly go unpicked by the sensor. The waveform, qualitative (un-measurable) part of the signal, displayed in such system remains more or less the same irrespective of the emission source and hence seldom displayed. It relies on collection of different variables of the wave form like rise time, duration, max amplitude that can be quantified (measured) & statistically analyzed and plotted. All of the emissions captured are deemed as valid signals from the material resulting in the lack of necessity to resolve actual emissions of interest from noise. Since most of the interpretation is done through in built algorithms, operator training is very minimal.
Modal Acoustic Emission test system is another less used variant of the same testing method which employs wide band sensors and relies more on the qualitative (non-measurable) aspects instead of the quantitative (measurable) aspects of the waveform. The wider band while ensuring the whole spectrum of the signal and a close to actual waveform is captured for qualitative analysis, it also increases the collection of noise. These systems also collect quantitative (measurable) data for analysis and display, but are employed to a lesser degree during inspection due to the quality of results from qualitative analysis of near to raw waveforms. Use of wider band sensors also aids in capture of emissions of interest from a wide range of thickness. Due to the complexities in instrumentation set up, noise filtration and the protracted training requirements, modal acoustic emission test systems are employed by researchers and are less favored by commercial establishments.
Irrespective of the type of test system chosen, Acoustic Emission testing primarily aids in locating the area (zone) from which the emission of interest (such as crack growth) occurred. To confirm the location and to interpret and evaluate the emission source, a follow up Ultrasonic Testing (Circumferential Shear scan) is required.
The need to locate the zone of emission with reasonable accuracy by the acoustic emission test system is quintessential for the success of employing this combination of AE & UT methods. Errors in locating the zone for follow up ultrasonic testing can lead to the emission zone left without an Ultrasonic test, thereby making the whole effort of inspection meaningless.
CMW has experience with both systems but employs Modal Acoustic Emission test system for testing pressure vessels due to the relatively higher degree of accuracy in identifying and locating the emission zone.
While Acoustic Emission Testing is a good method to identify and locate an active flaw (a growing crack) in a pressure vessel, it has certain limitations when it comes to pressure vessels in corrosive or erosive services. Vessels in corrosive & erosive services are inspected better with Hydrostatic Testing.
Although damage due to trans-granular and inter-granular corrosion may be identified by Acoustic Emission Testing, the normal follow up Ultrasonic Testing of such zones are not likely to yield any useful result since the ultrasonic test system in follow up UT is geared towards fatigue crack detection and not for cracks whose orientation and dimensions are not similar to a fatigue crack. Such pressure vessels are better inspected by submitting the vessels to acoustic emission as well as hydro test or at minimum alternate between these two test methods.
Non Destructive Testing methods are indirect way of measuring certain properties and characteristics of a material. Each one focuses on a small set of variables and the results are based on the changes to those variables. When it comes to testing, there is no one method that is complete or fool proof, even if they are the touted to be the latest and the greatest.
Though the ocean of rules and regulations may permit multiple methods to certify a pressure vessel’s fitness, it is essential to understand the method, the assumptions, capabilities and more importantly their limitations prior to deciding on a particular test method for your pressure vessels. Users are encouraged to gain knowledge from the suggested reading material. If you have questions or require further clarification, please feel free to contact us.
Compressed Gas Association (CGA) Pamphlet C 18 - Methods for Acoustic Emission Requalification of Seamless Steel Compressed Gas Tubes
American Society for Non Destructive Testing (ASNT) Handbook Volume-6 Acoustic Emission Testing