The starting position of the project: A power plant operator experienced boiler failures in one of its coal-fired power plants due to leakages in the waterwall tubes. These failures were due to longitudinal cracks on the inner wall at the fireside, initiated by hydrogen damage. The power plant was searching for a relevant non-destructive examination technique able to detect such inner defects and to inspect a wide part of the boiler in the timeframe of the annual outage.
The Vallourec solution: Experts from Vallourec proposed a solution based on ultrasonic phased array technology. This non-destructive testing technique has been implemented in some tube manufacturing plants for more than ten years now with good results. It is suitable for the detection of thin inner cracks in a large range of material and tube thicknesses, and it offers the possibility to do a large and fast angular covering of the fireside thanks to electronic scanning along the Phased Array probe.
On-site testing with Boiler Field Service International (BFSI)
Following a qualification process, an inspection campaign was planned in the relevant coal-fired power plant. The maintenance team prepared for the on-site testing by installing the required scaffolding and cleaning the tubes using high-pressure water in order to remove ash deposits.
The on-site testing was performed over two weeks in order to cover the maximum area during the short period of the boiler shutdown. More than 8,000 meters of tubes were tested during these two weeks.
The BFSI experts classified the indications that were found according to their ultrasonic maximum amplitude. The amplitudes were compared to those of notches of different depths. Tubes where indications were found were marked during the on-site testing. The power plant maintenance management team was able to take quick decisions before restarting the boiler. They decided to cut and replace 10 tubes that exhibited the highest signal amplitudes. Some of these tubes were analyzed by micrographic analysis and the presence of longitudinal defects was confirmed. An example of such a defect is shown in the figure.