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Technical malfunctions and failure caused by leakage, cracking, corrosion or defects pose serious risks for managers and operators of plants, pressure vessels and machines. Acoustic emission testing (AET) is a non-destructive test (NDT) method that reliably recognizes impending problems and defects before they become serious.
Acoustic emission testing is not only ideal for localizing cracks and leakage and for identifying corrosion and faults. It is also recommended for mandatory periodic technical inspections of pressure vessels and tanks in accordance with the German Regulation on Health and Safety in the Use of Work Equipment (BetrSichV), and in particular as a special test method replacing
Acoustic emission testing can generally be performed using the fluid stored in the tank. This has the advantage that pressure vessels or tanks need not be opened and drained prior to testing, so that AET is faster as well as more cost-effective and involves less organizational effort than conventional test methods.
Acoustic emission testing is ideal for simple pressure vessels, including:
AET offers the following advantages:
Complex vessels are the other major type of pressure vessel. Complex pressure vessels can include various fittings and installations, geometries and diameters, many nozzles or an interstitial space, and are generally characterised by complex geometry.
AET is also recommended for the periodic technical inspections required under the German BetrSichV.
The sensors are fastened to the tank’s outer wall in two rows. The sensor positions are defined before the test using the available drawing and must be safely accessible on site. The sensors are fastened to the bare metal surface.
For the measurement process itself, the tank must be filled to a certain minimum level. The tank also needs to be taken out of service before measurement, i.e. no filling or draining processes may be performed. Any tank heating units or agitators must be switched off. If possible, all machines, pumps and devices in the direct vicinity of the tank should be switched off while measurement is performed.
Once all preparations have been taken, measurement can be started. In addition to other criteria, the length of measurement also depends on the weather conditions. For example, measurement must be interrupted if rain starts, as even small droplets hitting the tank wall might cause sensors in the measurement chain to overload.
Once measurements are completed the test equipment is removed from the tank, which is then reset to its baseline condition and returned to service. The measured data are then evaluated over the following days and documented in a report.
Flat-bottom tanks with interstitial space have a leak monitoring system. Detection of fluid in the interstitial space suggests leakage in the upper bottom, whereas collapse or dissipation of the existing vacuum without fluid ingress into the interstitial space indicates leakage in the lower tank bottom, which is not accessible for inspection. Given this, acoustic emission testing aims at localising leakage in the lower bottoms of flat-bottom tanks with interstitial space.
For this type of application the sensors must be placed directly onto the upper bottom inside the tank. To do so, the tank must be taken out of operation, emptied and drained before the test.
Measurement can be started as soon as all test preparations have been completed. In order to localise the leak, leakage must first be induced, for example by activating the available vacuum pump. The attempt to build up a vacuum state causes a flow at the point of leakage; in this case the external air is sucked inside, producing acoustic signals that can be localised using the AET method.
Once measurement is completed, the test equipment is removed from the tank and measurement evaluated. The measurement results are presented in the report and any suspicious areas indicated.
Generally, the test report is prepared according to the requirements of the DIN EN ISO/IEC 17025 standard; it outlines the general details of the tested equipment, the test results and the required actions.
Acoustic emission testing is also suitable for high-pressure tanks and tanks used in road carriage on trucks.
Pipelines are also used to transport pressurised natural gas, biogas, liquefied gas and hydrogen, particularly over longer distances. These pipelines are subject to mandatory testing and inspection, for which acoustic emission testing is ideal. The AET advantages outlined above also apply to this use case.
Increasing use is being made of advanced materials, which offer certain advantages but also involve new challenges. Composite pressure vessels for example, have the advantage of resistance to corrosion. Equipment made from these types of materials is likewise suitable for acoustic emission testing.
If a defect or damage has been found on your equipment but delivery of the replacement vessel will take a few months, the defect can be monitored with the help of acoustic emission testing to ensure any propagation of the defect will be detected immediately.
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