Acoustic Emission Testing: Early Detection of Damage to Pressure Vessels

Pressure vessels, pipework and equipment in the process industry are constantly subjected to mechanical, thermal and chemical stresses. Operators of pressure equipment are therefore required to carry out periodic inspections: The Act on Installations Requiring Monitoring and the Industrial Safety Regulation (BetrSichV) require that defects and material fatigue be detected in good time and that both employees and the surrounding area be protected. These provisions give rise to specific requirements regarding the scope of testing, testing intervals and documentation for pressure equipment and its components. In this context acoustic emission testing is playing an increasingly important role.

Established non-destructive testing methods include visual, ultrasonic, X-ray, pressure and leak tests, each of which covers specific types of defects and component areas. Their common drawback is that they assess the condition only at specific points or in sections and often require the plant to be drained, cleaned and made accessible.

Sound emission testing (SET) is therefore gaining in importance as a complementary method, because it not only detects cracks and crack formation, corrosion, leaks, plastic deformation as well as friction, but also pinpoints their location on the component – across the entire structure under monitoring. This enables damage to be identified before it reaches a critical stage.

For plant operators, this provides additional scope for action: refurbishments can be planned rather than carried out reactively, downtime is reduced, and the assessment of the component’s condition is based on measurable parameters rather than on empirical values.

Principle of operation: The component generates the sound itself

Unlike ultrasonic testing, where a sound wave is specifically introduced into the material, SEP subjects the vessel’s material to stress. As a rule, the vessel’s own medium – approximately a gas or a liquid – is introduced up to the desired test pressure. The vessel often does not need to be emptied for this purpose, which significantly reduces the time, organisational and financial costs involved.

Under pressure, damage such as cracks or leaks emits sound waves in the ultrasonic range into the vessel wall. Piezoelectric sensors detect these signals: their quartz or ceramic crystals generate an electrical voltage even in response to the slightest mechanical stresses caused by pressure or tension, which the sensor outputs as a measurement signal. The measurement system filters out characteristic features from these signals – the maximum amplitude as the moment of peak sound pressure, the signal energy, the time from detection to maximum deflection, as well as the signal duration. The signals are assessed and evaluated on this basis.

As the sound reaches the individual sensors at different times, the source can be precisely located using the difference in transit time. Localisation is crucial for the assessment: a vessel expands under test pressure and may rub against its supports in the process – the sensors also detect this noise. If it can be clearly attributed, it quickly becomes apparent that there is no crack at a critical point such as a weld seam. By positioning a sufficient number of sensors, the vessel can be monitored 100 per cent during the test.

Influencing factors, standards and evaluation criteria

Ambient noise, material, wall thickness, geometry and the contained medium have a direct impact on the test result. Interference noise from leaks may be present from the outset or only arise under pressure; likewise, support noise and the sound from neighbouring machinery must be detected and distinguished. Before the test, valves and seals must therefore be checked and, if necessary, retightened or replaced – otherwise, escaping media will drown out the significantly quieter sounds of crack propagation.

Detecting corrosion is also challenging: whilst the chemical process and the flaking of material layers are audible, they are faint. Sensors with a low resonance frequency detect these accompanying noises, but are consequently more sensitive to environmental influences such as wind and rain. Wall thickness also determines how sound propagates: in thin material, sound propagates almost two-dimensionally in the X and Y directions; in material several centimetres thick, however, it propagates three-dimensionally. The sound then dissipates more quickly, whilst reflections and interference increase – requiring more sensors and carefully calibrated coupling. Liquid media such as water or oil attenuate the sound wave more strongly than gaseous contents and also increase the number of sensors required.

No Specific Benchmarks

Thus, SEP differs significantly from ultrasonic testing, in which wall thickness and echo or amplitude level determine whether a defect is detected. There are no specific limit values for sound emission testing. The standards specify which values are to be recorded and the parameters according to which the signals are to be assessed. Instead of threshold values, they provide example values; high amplitudes with high energy are regarded as an important indicator and require closer examination.

Each inspection organisation must therefore develop its own specific guidelines and internal evaluation criteria. An accredited inspection body, whose staff are qualified in accordance with EN ISO 9712:2022, organises the inspection, inputs a vessel model with the sensor positions into the inspection software and documents the results. SEP is used as a specialised testing method for internal inspection as part of internal testing or for hydrostatic strength testing – and, furthermore, for the continuous monitoring of stressed components and structures.

Source: Trade journal ‘Chemie Technik’

Foto: Mey (AI)