TÜV SÜD applied a state-of-the-art test method to confirm the integrity of weld seams at Zolling power station near Munich
Innovative test methods enable increasingly exact assessments of power station components to be made. However, high-resolution imaging sometimes also produces false positives that are not caused by actual inhomogeneities in the component, but by test artefacts. In case of doubt, as in our case study at Zolling power station in the district of Freising near Munich, Southern Germany, the expertise of independent test engineers provides certainty.
GDF SUEZ Kraftwerk Zolling GmbH is a member of GDF SUEZ Group, an internationally leading energy supplier. During the 2011 turnaround, the power-station engineers replaced a control valve in one of the pipes. Within the scope of this measure, the existing welds were renewed and subjected to ultrasonic testing. The result of ultrasonic testing could be interpreted either as an echo caused by an inhomogeneity in the material or a false positive caused by component geometry. Echoes caused by geometry, e.g. the welding root, are permissible since they do not impact on component integrity.
Ultrasonic testing with phased array
GDF SUEZ commissioned the experts at TÜV SÜD to investigate the cause of the test results. The TÜV SÜD professionals used the state-of-the-art phased array method for this purpose. This multi-channel ultrasonic testing method provides for continuously adjustable angles of incidence. In addition, the experts used an ultrasound probe with which individual vibration elements can be successively actuated, producing a sector image that supplies far more information than the conventional ultrasonic test method.
"Evaluation of the welds based on the sector image revealed that the echo was caused by geometry, i.e. that it was a test artefact", says Hans-Jürgen Cramer, Head of Nondestructive Testing at TÜV SÜD Industrie Service. "The weld root broke up a sonic wave which was reflected back to the capping layer and projected an artefact in the area surrounding the weld."
The more detailed phased array testing revealed that all of the examined welds were intact. "The TÜV SÜD experts not only confirmed component integrity but also prevented unnecessary repairs, thus not least increasing plant availability", says Werner Bienert, project engineer at the Zolling power station.