The “Law on Systems in need of surveillance” obliges operators of pressure devices for the recurring examination. The Operational Safety Ordinance (BetrSichV) regulates the application of work equipment to protect employees and people in the area – this results in requirements for the recurring exams of printing systems and their parts.

View, ultrasound, X-ray, printing and leaking tests are common destructive test methods for printing devices. In addition, a sound emission test (SEP) can be carried out: It allows material fatigue in the form of cracks and crack formation, corrosion, leakage and plastic deformation as well as

To detect and localize friction. This allows damage to be recognized and counter -controlled particularly early before reaching a critical stage.

Sound waves in the material of the pressure vessel

In contrast to ultrasound tests, where a sound wave is sent into the material, the material of the container is set under load during the sound emission test: As a rule, the self -medium – such as gas or a liquid – is initiated to the desired test pressure. The devices often do not have to be emptied, which minimizes the effort – in terms of time, organization and monetary. Damage such as cracks or leaks release sound waves in the ultrasound area in the wall of the container, which can be detected by piezoelectronic sensors. Their piezo crystals consist of special materials such as quartz or ceramics and have the necessary piezoelectric properties: they create an electrical voltage if even the smallest mechanical effects such as pressure or train are exerted on you. The voltage created in this way is implemented as a measuring signal in the sensor. The measuring system filters information from these signals: the maximum amplitude of the sound wave, i.e. the moment of the highest sound pressure, its energy, the time from detection to the maximum rash and duration. Using these characteristics, signals are assessed and evaluated.

Because the sound reaches the sensors at different times, signals can be precisely located using the term difference. The knowledge of the origin of the sound is important because there is information about the meaning: The container stretches during the printing test – the movement can lead to scratch on its supports, for example. The sensors also perceive this sound. If it can now be located, it quickly becomes clear that this is not a crack in a critical area like a weld seam. During the examination, the container can be monitored 100 percent by placing the sensors, which allows an exact assessment.

Standards and limit values

In the case of ultrasound tests, standards determine when an error display is still permitted. Factors include wall thickness or echo or amplitude height. There are no concrete limit values ​​in the sound emission test; The standard only specifies which values ​​need to be recorded: signal rate as a function of time and load and the amplitude as the maximum rash of the voltage signal over time. The sources of the noises must be located and assigned. Another standard specifies which parameters the signals are assessed: Here, too, there are no border or threshold values, instead the standard calls sample values. An important clue are high amplitudes that correlate with a lot of energy, which requires precise inspection.

Each test organization that carries out sound emission tests must develop concrete guidelines themselves and thus develop internal criteria.

Measurement and results of the SEP

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

Examiners must take into account that noise from leakage noise is available from the start or can only arise from the pressure. Likewise, the examiners must be able to identify and differentiate the sounds of support and the noises from the machines adjacent. Possible damage is also different: In order to be able to recognize cracks, for example, valves and seals must be checked or replaced if necessary – otherwise possible leaks would simply drown out the noises of crack growth. The detection of corrosion is also challenging: the chemical process and flutting out material layers are audible, but quiet. In order to absorb their accompanying noises, the sensors need a low resonance frequency – and are therefore sensitive to noise from the surrounding area, such as wind and rain.

Another factor is the wall thickness: in a thin material, the sound can almost only spread to two dimensions, in the X and Y direction; With a container with a wall thickness of several centimeters, a more voluminous spread in three directions is possible – the sound is then faster in the material; Reflections and interference can also occur. More sensors are necessary here and your edition must be checked carefully. If there is a liquid medium such as water or oil in the container, the sound wave is steamed compared to gaseous content, and more sensors are also necessary here.

It is rare that containers may no longer be operated according to sound emission tests. The containers are usually constructed in such a way that they can withstand higher loads than in daily operation, for example by corrosion surcharges in the wall thickness. Sometimes there is even an all -clear, for example when a container shows signals in the area of ​​a sensitive traits and is then recognized by a follow -up examination with ultrasound that it is still permissible unpleasant, for example from the production.

The furnishings of the software

In the approved TÜV Hessen test mode, a model of the container is created for measurement in the test software, on which the location of the sensors is noted. The examiners thus recognize where a signal is created in the container.

The software settings must always be adjusted for an examination, whereby the effort varies: The adjustment for a standardized, cylindrical pressure tank is

quickly executed; The number of sensors results from the scope and diameter and often settings can be taken from previous tests of similar containers. However, the geometries of systems are often complicated and have several cylindrical containers. Then the setting up of the software is more complex.

Approved test mode as a partner

The SEP can be used as a special test procedure for the internal examination or for the hydrostatic strength test in printing devices that must be checked in accordance with the Operating Safety Ordinance.

It makes sense that an approved test mode such as Technical Monitoring Hessen GmbH organizes, coordinates, carries out and documented the exams. The staff is qualified according to EN ISO 9712: 2022 and has the necessary experience and expertise in the area of ​​the non -destructive examination. 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 inner examination and, if necessary, to initiate and carry out follow -up exams, such as ultrasound tests on weld seams. This means that no further company has to be brought on board, which avoids delays such as waiting times for documents from an expert. If necessary, TÜV Hessen also explains the possible testing techniques and special procedures, such as the use of an ultrasonic camera for detection of leaks, which is also carried out in addition to the sound emission test.

Companies thus reduce costs and time to prepare and carry out the exam. The state of the art always uses the state that provides valid results. The sound emission test is not only a suitable process for pressure devices, but also for stressed components and structures. It is also possible to set up monitoring with the sound emission measurement in order to be able to permanently monitor a structure.

Conclusion

With the sound emission test and its sensitive piezoelectric sensors, growing cracks in the material of pressure containers can be recognized early. The SEP is therefore suitable as a supplement and possibly replacement to recurring non -destructive tests and can be carried out without the container having to be emptied. It is advisable to have the SEP carried out by an approved test mode, which brings the corresponding know-how and the necessary resources to carry out the full test program with it smoothly

Author:

Benedikt LorsbachTÜV Hessen