The “Law on Systems Requiring Monitoring” obliges operators of pressure equipment to carry out regular inspections. The Industrial Safety Ordinance (BetrSichV) regulates the use of work equipment to protect employees and people in the area – this results in requirements for the recurring inspections of printing systems and their parts.

Visual, ultrasound, X-ray, pressure and leak tests are among the common non-destructive testing methods for pressure equipment. In addition, a sound emission test (SEP) can be carried out: It allows material fatigue in the form of cracks and cracking, corrosion, leaks and plastic deformation as well

Detect and localize friction. This means that damage can be detected particularly early and counteracted before it reaches a critical stage.

Sound waves in the material of the pressure vessels

Unlike ultrasonic testing, where a sound wave is sent into the material, in acoustic emission testing the material of the container is placed under load: As a rule, the internal medium – such as gas or a liquid – is introduced up to the desired test pressure. The devices often do not have to be emptied, which minimizes the effort – in terms of time, organization and money. Damage such as cracks or leaks emit sound waves in the ultrasonic range under the pressure in the wall of the container, which can be detected by piezoelectronic sensors. Their piezo crystals are made of special materials such as quartz or ceramic and have the necessary piezoelectric properties: They generate an electrical voltage when even the smallest mechanical influences such as pressure or tension are exerted on them. The voltage generated in this way is converted into a measurement signal in the sensor. The measuring system filters out information from these signals: the maximum amplitude of the sound wave, i.e. the moment of highest sound pressure, its energy, the time from detection to maximum deflection and the duration. Signals are assessed and evaluated based on these characteristics.

Because the sound reaches the sensors at different times, signals can be precisely located based on the difference in transit time. Knowing the origin of the noise is important because it provides information about the meaning: During the pressure test, the container expands – the movement can, for example, cause rubbing on its supports. The sensors also detect this noise. If it can now be located, it quickly becomes clear that it is not a crack in a critical location such as a weld seam. During the test, the container can be monitored 100 percent through the placement of the sensors, which allows an accurate assessment.

Norms and limits

For ultrasonic testing, standards specify when an error display is still permissible. Factors here include wall thickness or echo or amplitude height. There are no specific limit values ​​for sound emission testing; the standard simply specifies which values ​​must be recorded: signal rate as a function of time and load as well as the amplitude as the maximum deflection of the voltage signal over time. The sources of the noise must be located and assigned. Another standard specifies the parameters based on which the signals are assessed: Here, too, there are no limit or threshold values; instead, the standard gives example values. An important clue is high amplitudes, which correlate with a lot of energy, which requires close inspection.

Every testing organization that carries out noise emission tests must develop specific guidelines themselves and thus develop internal criteria.

Measurement and results of the SEP

Ambient noise, material and wall thickness, geometry and medium are important influencing factors for sound emission tests.

Testers must take into account that noise from leaks may be present right from the start of a test or may arise as a result of the pressure applied. The inspectors must also be able to recognize and differentiate between support noises and the noises from nearby machines. Possible damage also varies in noise: In order to be able to detect cracks, for example, valves and seals must be checked before the test and tightened or replaced if necessary – otherwise possible leaks would simply drown out the noise of the crack growth. Detecting corrosion is also challenging: the chemical process and the flaking of material layers are audible but quiet. In order to record the accompanying noise, the sensors require a low resonance frequency – and are therefore sensitive to ambient noise, such as wind and rain.

Another factor is the wall thickness: In a thin material, sound can only propagate in two dimensions, in the X and Y directions; In a container with a wall thickness of several centimeters, a more voluminous spread in three directions is possible – the sound then travels faster through the material; In addition, reflections and interference can occur. More sensors are necessary here and their placement must be checked carefully. If there is a liquid medium such as water or oil in the container, the sound wave is attenuated compared to a gaseous content; more sensors are also necessary here.

It is rare for containers to no longer be allowed to operate after sound emission tests. The containers are usually designed in such a way that they can withstand higher loads than usual in daily operation, for example due to corrosion additions in the wall thickness. Sometimes there is even an all-clear signal, for example when a container shows signals in the area of ​​a sensitive circular seam and a follow-up ultrasound examination then detects that these are still permissible inconsistencies, for example from production.

Setting up the software

At the approved testing company TÜV Hessen, a model of the container is created in the testing software for the measurement, on which the position of the sensors is noted. This allows the inspectors to see exactly where in the container a signal is generated.

The software settings always have to be adjusted for a test, although the effort varies: The adjustment is for a standardized, cylindrical pressure vessel

executed quickly; The number of sensors depends on the circumference and diameter and settings can often be adopted from previous tests of similar containers. However, the geometries of systems are often complicated and have several cylindrical containers. Then setting up the software becomes more complicated.

Approved testing company as a partner

The SEP can be used as a special test procedure for the internal inspection as part of the internal inspection or for the hydrostatic strength test for pressure equipment that must be tested regularly in accordance with the Industrial Safety Ordinance.

It makes sense for an approved testing company such as Technische Monitoring Hessen GmbH to organize, coordinate, carry out and document the tests. The staff is qualified according to EN ISO 9712:2022 and has the necessary experience and expertise in the area of ​​non-destructive testing. It is also an advantage if the partner can cover all the necessary areas and offer them from a single source; TÜV Hessen, for example, can also certify the strength test and internal test and, if necessary, initiate and carry out follow-up tests, such as ultrasonic tests on weld seams. This means that no additional company has to be brought on board, which avoids delays such as waiting for documents from an expert. If necessary, TÜV Hessen also explains the possible testing techniques and special procedures, such as the use of an ultrasound camera to detect leaks, which is also carried out in addition to the sound emission test.

Companies thus reduce costs and time for preparing and carrying out the audit. The state of the art technology is always used, which delivers valid results. Sound emission testing is a suitable procedure not only for pressure equipment, but generally for stressed components and structures. It is also possible to set up monitoring using sound emission measurement in order to be able to permanently monitor a structure.

Conclusion

With acoustic emission testing and its sensitive piezoelectric sensors, growing cracks in the material of pressure vessels can be detected early. The SEP is therefore suitable as a supplement and, if necessary, a replacement for recurring non-destructive tests and can be carried out without the container having to be emptied. It is recommended to have the SEP carried out by an approved testing company that has the appropriate know-how and the necessary resources to carry out the complete testing program smoothly

Author:

Benedict LorsbachTÜV Hessen