Long term coupling of AE sensors in high temperature environment

Abstract

Acoustic emission monitoring took place in a long-term test in a coal fired power plant in Germany for 4 years. The test object was a steel pipe with a ceramic matrix composite (CMC) jacket, which was installed in a bypass of a live steam line. The peak operating temperature was approx. 530 °C. The aim of the AE monitoring was to investigate damage evolution in the CMC jacket. The test pipe was removed from the power plant in 2021. Due to several reasons, the jacket did not experience high loads during operation and therefore information about damage mechanisms in the AE data is rare. However, a lot could be learned from this project concerning long term stability of the equipment in rough environment, data storage and filtering and about sensor long term coupling of AE sensors at high temperatures. The high temperature AE sensors were applied directly onto the surface of the steel pipe and the CMC jacket using a ceramic glue. Additionally, the sensors were held by stainless steel bands which were acoustically decoupled by mineral wool. The application of the sensors took place at a temperature of about 30 °C. It was not possible to check on the quality of the coupling during operation as the whole pipe was covered by insulation, including all sensors. Also, the sensors did not allow pulsing as test for the coupling quality. Effects like thermal expansion of sensors, CMC jacket and ceramic glue had to be considered. Additionally, the power plant adepts its electrical output to the demands by flexible operation. This leads to cyclic changes in temperature and a high amount of heating and cooling cycles, which affects the coupling condition of the AE sensors. During dismantling of the test pipe from the power plant, a visual inspection of the sensor coupling was possible. The condition of the sensor coupling was documented before and after the sensors were removed. Pencil lead breaks were performed to quantify the quality of coupling as well. It could be shown that long term coupling of AE sensors is possible even in high temperature environment combined with cyclic temperature changes.