Condition monitoring of exchange towers of heavy water plant during hydrotesting by acoustic emission technique

This paper presents results of acoustic emission (AE) testing carried out during hydrotesting of cold and hot exchange towers of a heavy water plant. Point type defect indications were observed in certain locations during ultrasonic inspection of the exchange towers. Acoustic emission monitoring during hydrotesting was carried out to determine the nature of these defects, ie whether these defects are growing or not. The cold and the hot towers were pressurised to a maximum pressure of 26.5 kg/cm 2 and 27.8 kg/cm 2 , respectively, with holds at different pressures. Acoustic emissions were picked up using sensors placed selectively at areas of interest where ultrasonic testing showed defect-like indications and at some critical areas. AE events were observed during the first pressurisation of both the towers and were attributed to micro yielding. No significant acoustic emissions were detected from either tower during the hold periods or the repressurisation. This confirmed that there is no extraneous noise and there are no severe defects in the regions monitored by the sensor arrays and nearby regions monitored by individual sensors. The combined results from AE and ultrasonic techniques confirmed the structural integrity of different regions of the towers and supported returning the towers to service for continued operation.